[pt] O presente trabalho avalia a evolução da microestrutura e as propriedades mecânicas devido à influência do aporte de calor exercido por um procedimento de soldagem que utilizou sequencialmente dois processos de soldagem sobre um tubo de aço API 5L X80, fabricado pelo processo UOE, de um aço produzido por laminação controlada sem resfriamento acelerado. A soldagem foi realizada em um tubo de 20 de diâmetro nominal e 3/4 de espessura, fixado na posição horizontal simulando condições de campo, usando o processo MAG de curtocircuito de corrente controlada com gás de proteção CO2 (100%) para o passe de raiz e o processo por Arame Tubular com proteção gasosa Ar - CO2 (80% - 20%) para os demais passes. As propriedades mecânicas foram avaliadas segundo os ensaios mecânicos exigidos na norma API 1104, além dos ensaios de microdureza Vickers e de impacto Charpy V. As mudanças microestruturais na Zona Afetada Termicamente e Material de Solda foram avaliadas por microscopia eletrônica de varredura (MEV) e microscopia óptica. A avaliação mecânica segundo a norma API 1104 foi reprovada, onde os resultados dos ensaios de tração e Nick-Break foram aceitos e o ensaio de dobramento lateral um corpo-de-prova apresentou uma trinca superior ao comprimento máximo aceitável. Os resultados da microdureza foram aceitáveis e o resultado do impacto Charpy V, segundo a norma DNV-OS-F101, para a temperatura de 0 °C foi insatisfatório na região do metal de solda dos passes de acabamento. A região da ZTA apresentou maior energia de impacto quando comparado com o material de base à temperatura de 0 °C, embora com presença do microconstituinte A-M. / [en] The present work evaluates the changes in the microstructural and mechanical properties of an API 5L X80 steel tube due to the influence of heat input exerted during a welding procedure that used two sequential welding processes. The tubes were manufactured using the UOE process, from steel that was produced by controlled rolling without accelerated cooling. The welding was carried out on a 3/4 thick and 20 nominal diameter pipe, while it was held in a horizontal position in order to simulate field conditions, using a controlled short circuit GMAW process with CO2 (100%) gas shielding for the root pass and a flux cored arc welding process with Ar-CO2 (80% - 20%) gas shielding for the other passes. The evaluation of the mechanical properties was done by means of mechanical tests according to the API 1104 standard, in addition to the Vickers microhardness and Charpy V-notch tests. The changes in the microstructure of the Heat Affected Zone (HAZ) and the welded metal were evaluated by means of scanning electronic microscopy (SEM) and optical microscopy. The mechanical evaluation was unsatisfactory according to the API 1104 standard, while the tensile and Nick-Break test results were acceptable. The side bend test showed a crack in a specimen that exceeded the maximum acceptable value. The Vickers microhardness results were acceptable and the Charpy V-notch result, according to the DNV-OS-F101 standard, at a temperature of 0 °C, was unsatisfactory in the weld metal region of the over cap. The HAZ region showed greater energy of impact absorption compared to the base metal, at a temperature of 0 °C, even with existence of the microconstituent M-A.

The fractal-like finite element method (FFEM) is developed to compute stress intensity factors (SIFs) for isotropic homogeneous and bi-material V-notched plates. The method is semi-analytical, because analytical expressions of the displacement fields are used as global interpolation functions (GIFs) to carry out a transformation of the nodal displacements within a singular region to a small set of generalised coordinates. The concept of the GIFs in reducing the number of unknowns is similar to the concept of the local interpolation functions of a finite element. Therefore, the singularity at a notch-tip is modelled accurately in the FFEM using a few unknowns, leading to reduction of the computational cost.The analytical expressions of displacements and stresses around a notch tip are derived for different cases of notch problems: in-plane (modes I and II) conditions and out-of-plane (mode III) conditions for isotropic and bi-material notches. These expressions, which are eigenfunction series expansions, are then incorporated into the FFEM to carry out the transformation of the displacements of the singular nodes and to compute the notch SIFs directly without the need for post-processing. Different numerical examples of notch problems are presented and results are compared to available published results and solutions obtained by using other numerical methods.A strain energy approach (SEA) is also developed to extract the notch SIFs from finite element (FE) solutions. The approach is based on the strain energy of a control volume around the notch-tip. The strain energy may be computed using commercial FE packages, which are only capable of computing SIFs for crack problems and not for notch problems. Therefore, this approach is a strong tool for enabling analysts to compute notch SIFs using current commercial FE packages. This approach is developed for comparison of the FFEM results for notch problems where available published results are scarce especially for the bi-material notch cases.A very good agreement between the SEA results and the FFEM results is illustrated. In addition, the accuracy of the results of both procedures is shown to be very good compared to the available results in the literature. Therefore, the FFEM as a stand-alone procedure and the SEA as a post-processing technique, developed in this research, are proved to be very accurate and reliable numerical tools for computing the SIFs of a general notch in isotropic homogeneous and bi-material plates.

This thesis provides a structural and materials engineering explanation for many of the running fractures that occurred in steel structures during the destructive Kobe and Northridge earthquakes in the mid 1990s. A method is developed that allows the ductile endurance of structural steel members subjected to cyclic plastic deformation during earthquakes to be assessed and for pre-necking running fractures to be avoided. The study commenced following the 2000 World Earthquake Conference in Auckland. The conference brought together the findings of the huge research effort, in America, Japan, Europe and New Zealand, that followed the Kobe and Northridge earthquakes. The running fractures that had occurred in steel structures represented an unpredicted failure mode that structural engineers have not known how to predict or suppress through the engineering design process. A clear fundamental understanding of the causes and how to prevent the fractures did not arise from the conference. In fact apparently conflicting results were reported. Full scale cyclic tests in New Zealand on structural assemblies had not resulted in running fractures, whereas tests in American and Japan had. Structural engineers designing earthquake resistant structures rely on constructional steel to be materially homogeneous and nominally tri-linear in behaviour. Steel is expected to behave elastically under regular in-service loading, have a reliable and flat yield stress-strain characteristic, and under overload then develop predictable levels of strain-hardening in conjunction with significant plastic elongation up to its ultimate tensile strength. Steel is expected to eventually fracture after further plastic elongation and necking. Ductile design strategies and methods utilise the plastic elongation characteristics of steel to protect structures in earthquake. Plastic deformation is considered to beneficially dissipate energy generated in the structure by a severe earthquake and also dampen the structure’s response. The occurrence of running fracture without significant cyclic plastic deformation and before section necking in steelwork, therefore undermines the basis of the ductile seismic design approach. The initial part of the thesis is devoted to bringing together the fundamental aspects of materials engineering related to fracture of constructional steel. This is intended to provide a bridge of knowledge for structural engineering practitioners and researchers not fully conversant with materials engineering aspects of fracture. Fracture behaviour in steel is a broad and complex topic that developed rapidly in the twentieth century driven by the demands of technological growth. The unexpected fracture of welded liberty ships at sea in World War 2; the need for reliable long term containment for the nuclear reactors in the 1950s and 1960s; and prevention of fatigue failures in aircraft frames since the 1950s all drove engineering research into steel fracture behaviour. There are many subtle variations in definitions in the published literature on fracture that can be confusing. Therefore an attempt has been made to clarify terminology. The term brittle fracture in particular is only used in this thesis as applying to running fracture when the general or far field tensile stresses are below the yield stress of the steel. The term pre-necking or running fracture is preferred to describe the condition more broadly which may occur prior to and also after general yielding, but before section necking. Running fracture is a manifestation of pre-necking fracture in which insufficient plastic flow is available in the assembly to absorb the energy released upon fracture. The experimental studies investigated the behaviour of constructional steel commonly used in New Zealand, at various levels of plastic strain. This started with Charpy V-Notch (CVN) testing which revealed that a significant transition temperature shift and curve shape change occurs with increasing plastic strain and the associated strain-hardening. This showed that the ability of steel to avoid pre-necking or running fracture reduces as the level of plastic strain-hardening increases. Temperature controlled Crack Tip Opening Displacement (CTOD) testing was then undertaken. The setting of testing temperatures for the CTOD tests were guided by review of the CVN test results, using published CVN to fracture toughness correlation methods. However running cleavage fractures developed in the CTOD specimens at higher than predicted temperatures of 10 oC and 20 oC. These are typical service temperatures for structures in New Zealand and so are very likely to occur at the time of an earthquake. The implication from this is that there are levels of strain-hardening and conditions of material notching constraint that can lead to pre-necking and running fracture in New Zealand fabricated steel structures, under severe earthquake loading. Care was taken in the CTOD testing to monitor and maximise the capture of data electronically using a specially developed Direct Current Potential Drop method. This allowed the test results to be analysed and considered in varying ways, leading to a consistent assessment of the CTOD, crack growth, and the specific work of fracture in each test piece. While CTOD test results have sometimes been published by structural and welding engineering researchers in the wake of Kobe and Northridge, the results were typically of little use for this study as the CTOD initiation point was generally not identified effectively. The effect of remote plastic flow in the specimens was also not adequately accounted for. The CTOD test results were often simply used to help correlate other factors observed by the researchers. Side-grooving of specimens was not reported as having been used in any of the published results reviewed. When conducting CTOD test with highly ductile constructional steels it is very difficult to get useful CTOD results if the specimens are not side-grooved, as significant necking and tunnelling will otherwise occur and limit the usefulness of the results. Work by Knott and also by McRobie and Smith was seminal in terms of identifying some critical aspects of plane strain development in CTOD tests, and the links to non-metallic particle density with respect to fracture toughness and CTOD at initiation. Some of their findings with regards to the effect of pre-strain on CTOD initiation were subsequently found to confirm the experimental findings in this study. No effective methodology for prediction of pre-necking or running fracture in a structural member or assembly when subjected to gross plastic cyclic deformation was found to exist in the literature. It was concluded however that the principles of specific work of fracture, and monotonic and cyclic fracture similitude were particularly relevant. These were therefore utilised in the development of the design method proposed in this thesis. The CTOD test results were reviewed, isolating the remote plastic flow component, to determine the critical specific work of fracture property Rc of the steels tested. A meeting with Professor Kuwamura at the University of Tokyo was providential, allowing discussion of his similitude principle, and observations in person of some of the fractured specimens developed during his full scale test series’. Running fractures with cleavage were evident in the specimens, with their tell-tale chevron markings. He had predicted running fracture problems in structures in Japan ahead of the Kobe earthquake and been largely ignored. His insights were subsequently seriously considered in Japan after the earthquake. He and his colleagues developed the principle of structural similitude that relates monotonic fracture displacement ductility to cyclic fracture displacement ductility for a particular assembly. This arose from their observation that running fractures developed from ductile crack formation at blunt notches in structures. The similitude principle has echoes of the Coffin-Manson approach to ductile crack initiated low cycle fracture. The principle of similitude has a log–log relationship as does the Manson-Coffin relationship. So where notch plasticity controls the initiation of fracture in a structural assembly it is conceptually reasonable to expect that the number of cycles to initiation of fracture from a notch will have a log–log relationship to the amplitude of the cyclic strain developed in the notch. Kuwamura found that steel assemblies with lower CVN energy had reduced cyclic fracture endurance than the same assemblies made with steel with higher CVN impact energy. However no method of predicting performance of any particular assembly could be developed from his observations. The benefit of his method primarily relates to the minimising of testing necessary to assess the fracture limited cyclic displacement ductility of a structural assembly. However it doesn’t provide a means for designing a structural assembly to achieve specific levels of ductile endurance other than clearly identifying the need to use steel with good CVN characteristics. The most significant development arising from this thesis is therefore the development of a design method to assess cyclic ductile endurance. The method utilises the specific work of fracture properties obtained from CTOD specimens of the steel in conjunction with a relatively simple fracture mechanics assessment and an elasto-plastic finite element analysis (FEA). The FEA model is used to determine the displacement ductility of the assembly at the calculated onset of pre-necking fracture. The elasto-plastic stress–strain properties of the steel in various pre-strain states required for the FEA may be derived from tensile testing. Kuwamura’s similitude principle is then used to predict cyclic plastic endurance at various constant displacement ductility amplitudes. The method is extended using Miner’s rule to allow for the effects of increasing variable amplitude cyclic plastic loading. In summary the thesis explains why pre-necking and running fractures occur in steel members subjected to cyclic plastic deformation during a severe earthquake. In addition a method for consistently assessing the ability of structural steel assemblies to achieve a specified level of ductile endurance during earthquakes is proposed. The method is verified against published results for a cyclic test of a simple steel member with a crack at mid-span. / Whole document restricted, but available by request, use the feedback form to request access.

This thesis is devoted to the study and designs of two important proportional weirs having the discharge-head characteristics of Q α H 2/3 In the first design a geometrically simple weir in the form of a rectangular weir over a inverted V-notch (Chimney weir) is presented. This weir gives for all flows above a threshold depth a discharge proportional to H 2/3 within a maximum percentage error of ±1.5, (measured above a reference plane) within certain limits of head. Second design is concerned with the self-basing weir in which a portion of the weir above the crest acts as a base. This design is achieved by using the complementary weir profile of a Quadratic weir above the parabolic base which has the significant property of fast convergence. This weir gives discharge for all flows above the threshold depth, proportional to (head)2/3 measured above a reference plane, with increasing accuracy as head increases. Experiments with these two weirs confirm the theory by giving a constant average Coefficient of Discharge (Cd) of 0.62. The importance of these weirs as a sensitive discharge measuring device in field and laboratory is highlighted.

No description available.

For the manufacture of higher strength pipelines to be feasible, a better understanding of the effects of welding on toughness is necessary. Bevel submerged arc welds were performed on X80 grade steel. The subsequent Charpy V-notch (CVN) test results indicated that the notch placement in the various heat affected zone regions, and hence the bead geometry, affected the test results. A series of bead-on-plate (BOP) submerged arc welds then were performed on X70 grade steel plate to determine the effects of current, voltage, heat input, polarity, and waveform manipulation (i.e., balance, offset, and frequency) on both single and tandem weld bead geometry. A new bead profile characteristic, the SP ratio, is proposed to describe weld bead geometry, and its relationship with welding parameters is discussed. Sub-size CVN specimens, pulled from many of the BOP weld coupons, were then tested. The greatest subsize CVN fracture energies were achieved when the bead was produced using lower heat input, and when the bead profile possessed a greater SP ratio. / Materials Engineering

This thesis provides a structural and materials engineering explanation for many of the running fractures that occurred in steel structures during the destructive Kobe and Northridge earthquakes in the mid 1990s. A method is developed that allows the ductile endurance of structural steel members subjected to cyclic plastic deformation during earthquakes to be assessed and for pre-necking running fractures to be avoided. The study commenced following the 2000 World Earthquake Conference in Auckland. The conference brought together the findings of the huge research effort, in America, Japan, Europe and New Zealand, that followed the Kobe and Northridge earthquakes. The running fractures that had occurred in steel structures represented an unpredicted failure mode that structural engineers have not known how to predict or suppress through the engineering design process. A clear fundamental understanding of the causes and how to prevent the fractures did not arise from the conference. In fact apparently conflicting results were reported. Full scale cyclic tests in New Zealand on structural assemblies had not resulted in running fractures, whereas tests in American and Japan had. Structural engineers designing earthquake resistant structures rely on constructional steel to be materially homogeneous and nominally tri-linear in behaviour. Steel is expected to behave elastically under regular in-service loading, have a reliable and flat yield stress-strain characteristic, and under overload then develop predictable levels of strain-hardening in conjunction with significant plastic elongation up to its ultimate tensile strength. Steel is expected to eventually fracture after further plastic elongation and necking. Ductile design strategies and methods utilise the plastic elongation characteristics of steel to protect structures in earthquake. Plastic deformation is considered to beneficially dissipate energy generated in the structure by a severe earthquake and also dampen the structure’s response. The occurrence of running fracture without significant cyclic plastic deformation and before section necking in steelwork, therefore undermines the basis of the ductile seismic design approach. The initial part of the thesis is devoted to bringing together the fundamental aspects of materials engineering related to fracture of constructional steel. This is intended to provide a bridge of knowledge for structural engineering practitioners and researchers not fully conversant with materials engineering aspects of fracture. Fracture behaviour in steel is a broad and complex topic that developed rapidly in the twentieth century driven by the demands of technological growth. The unexpected fracture of welded liberty ships at sea in World War 2; the need for reliable long term containment for the nuclear reactors in the 1950s and 1960s; and prevention of fatigue failures in aircraft frames since the 1950s all drove engineering research into steel fracture behaviour. There are many subtle variations in definitions in the published literature on fracture that can be confusing. Therefore an attempt has been made to clarify terminology. The term brittle fracture in particular is only used in this thesis as applying to running fracture when the general or far field tensile stresses are below the yield stress of the steel. The term pre-necking or running fracture is preferred to describe the condition more broadly which may occur prior to and also after general yielding, but before section necking. Running fracture is a manifestation of pre-necking fracture in which insufficient plastic flow is available in the assembly to absorb the energy released upon fracture. The experimental studies investigated the behaviour of constructional steel commonly used in New Zealand, at various levels of plastic strain. This started with Charpy V-Notch (CVN) testing which revealed that a significant transition temperature shift and curve shape change occurs with increasing plastic strain and the associated strain-hardening. This showed that the ability of steel to avoid pre-necking or running fracture reduces as the level of plastic strain-hardening increases. Temperature controlled Crack Tip Opening Displacement (CTOD) testing was then undertaken. The setting of testing temperatures for the CTOD tests were guided by review of the CVN test results, using published CVN to fracture toughness correlation methods. However running cleavage fractures developed in the CTOD specimens at higher than predicted temperatures of 10 oC and 20 oC. These are typical service temperatures for structures in New Zealand and so are very likely to occur at the time of an earthquake. The implication from this is that there are levels of strain-hardening and conditions of material notching constraint that can lead to pre-necking and running fracture in New Zealand fabricated steel structures, under severe earthquake loading. Care was taken in the CTOD testing to monitor and maximise the capture of data electronically using a specially developed Direct Current Potential Drop method. This allowed the test results to be analysed and considered in varying ways, leading to a consistent assessment of the CTOD, crack growth, and the specific work of fracture in each test piece. While CTOD test results have sometimes been published by structural and welding engineering researchers in the wake of Kobe and Northridge, the results were typically of little use for this study as the CTOD initiation point was generally not identified effectively. The effect of remote plastic flow in the specimens was also not adequately accounted for. The CTOD test results were often simply used to help correlate other factors observed by the researchers. Side-grooving of specimens was not reported as having been used in any of the published results reviewed. When conducting CTOD test with highly ductile constructional steels it is very difficult to get useful CTOD results if the specimens are not side-grooved, as significant necking and tunnelling will otherwise occur and limit the usefulness of the results. Work by Knott and also by McRobie and Smith was seminal in terms of identifying some critical aspects of plane strain development in CTOD tests, and the links to non-metallic particle density with respect to fracture toughness and CTOD at initiation. Some of their findings with regards to the effect of pre-strain on CTOD initiation were subsequently found to confirm the experimental findings in this study. No effective methodology for prediction of pre-necking or running fracture in a structural member or assembly when subjected to gross plastic cyclic deformation was found to exist in the literature. It was concluded however that the principles of specific work of fracture, and monotonic and cyclic fracture similitude were particularly relevant. These were therefore utilised in the development of the design method proposed in this thesis. The CTOD test results were reviewed, isolating the remote plastic flow component, to determine the critical specific work of fracture property Rc of the steels tested. A meeting with Professor Kuwamura at the University of Tokyo was providential, allowing discussion of his similitude principle, and observations in person of some of the fractured specimens developed during his full scale test series’. Running fractures with cleavage were evident in the specimens, with their tell-tale chevron markings. He had predicted running fracture problems in structures in Japan ahead of the Kobe earthquake and been largely ignored. His insights were subsequently seriously considered in Japan after the earthquake. He and his colleagues developed the principle of structural similitude that relates monotonic fracture displacement ductility to cyclic fracture displacement ductility for a particular assembly. This arose from their observation that running fractures developed from ductile crack formation at blunt notches in structures. The similitude principle has echoes of the Coffin-Manson approach to ductile crack initiated low cycle fracture. The principle of similitude has a log–log relationship as does the Manson-Coffin relationship. So where notch plasticity controls the initiation of fracture in a structural assembly it is conceptually reasonable to expect that the number of cycles to initiation of fracture from a notch will have a log–log relationship to the amplitude of the cyclic strain developed in the notch. Kuwamura found that steel assemblies with lower CVN energy had reduced cyclic fracture endurance than the same assemblies made with steel with higher CVN impact energy. However no method of predicting performance of any particular assembly could be developed from his observations. The benefit of his method primarily relates to the minimising of testing necessary to assess the fracture limited cyclic displacement ductility of a structural assembly. However it doesn’t provide a means for designing a structural assembly to achieve specific levels of ductile endurance other than clearly identifying the need to use steel with good CVN characteristics. The most significant development arising from this thesis is therefore the development of a design method to assess cyclic ductile endurance. The method utilises the specific work of fracture properties obtained from CTOD specimens of the steel in conjunction with a relatively simple fracture mechanics assessment and an elasto-plastic finite element analysis (FEA). The FEA model is used to determine the displacement ductility of the assembly at the calculated onset of pre-necking fracture. The elasto-plastic stress–strain properties of the steel in various pre-strain states required for the FEA may be derived from tensile testing. Kuwamura’s similitude principle is then used to predict cyclic plastic endurance at various constant displacement ductility amplitudes. The method is extended using Miner’s rule to allow for the effects of increasing variable amplitude cyclic plastic loading. In summary the thesis explains why pre-necking and running fractures occur in steel members subjected to cyclic plastic deformation during a severe earthquake. In addition a method for consistently assessing the ability of structural steel assemblies to achieve a specified level of ductile endurance during earthquakes is proposed. The method is verified against published results for a cyclic test of a simple steel member with a crack at mid-span. / Whole document restricted, but available by request, use the feedback form to request access.

This thesis provides a structural and materials engineering explanation for many of the running fractures that occurred in steel structures during the destructive Kobe and Northridge earthquakes in the mid 1990s. A method is developed that allows the ductile endurance of structural steel members subjected to cyclic plastic deformation during earthquakes to be assessed and for pre-necking running fractures to be avoided. The study commenced following the 2000 World Earthquake Conference in Auckland. The conference brought together the findings of the huge research effort, in America, Japan, Europe and New Zealand, that followed the Kobe and Northridge earthquakes. The running fractures that had occurred in steel structures represented an unpredicted failure mode that structural engineers have not known how to predict or suppress through the engineering design process. A clear fundamental understanding of the causes and how to prevent the fractures did not arise from the conference. In fact apparently conflicting results were reported. Full scale cyclic tests in New Zealand on structural assemblies had not resulted in running fractures, whereas tests in American and Japan had. Structural engineers designing earthquake resistant structures rely on constructional steel to be materially homogeneous and nominally tri-linear in behaviour. Steel is expected to behave elastically under regular in-service loading, have a reliable and flat yield stress-strain characteristic, and under overload then develop predictable levels of strain-hardening in conjunction with significant plastic elongation up to its ultimate tensile strength. Steel is expected to eventually fracture after further plastic elongation and necking. Ductile design strategies and methods utilise the plastic elongation characteristics of steel to protect structures in earthquake. Plastic deformation is considered to beneficially dissipate energy generated in the structure by a severe earthquake and also dampen the structure’s response. The occurrence of running fracture without significant cyclic plastic deformation and before section necking in steelwork, therefore undermines the basis of the ductile seismic design approach. The initial part of the thesis is devoted to bringing together the fundamental aspects of materials engineering related to fracture of constructional steel. This is intended to provide a bridge of knowledge for structural engineering practitioners and researchers not fully conversant with materials engineering aspects of fracture. Fracture behaviour in steel is a broad and complex topic that developed rapidly in the twentieth century driven by the demands of technological growth. The unexpected fracture of welded liberty ships at sea in World War 2; the need for reliable long term containment for the nuclear reactors in the 1950s and 1960s; and prevention of fatigue failures in aircraft frames since the 1950s all drove engineering research into steel fracture behaviour. There are many subtle variations in definitions in the published literature on fracture that can be confusing. Therefore an attempt has been made to clarify terminology. The term brittle fracture in particular is only used in this thesis as applying to running fracture when the general or far field tensile stresses are below the yield stress of the steel. The term pre-necking or running fracture is preferred to describe the condition more broadly which may occur prior to and also after general yielding, but before section necking. Running fracture is a manifestation of pre-necking fracture in which insufficient plastic flow is available in the assembly to absorb the energy released upon fracture. The experimental studies investigated the behaviour of constructional steel commonly used in New Zealand, at various levels of plastic strain. This started with Charpy V-Notch (CVN) testing which revealed that a significant transition temperature shift and curve shape change occurs with increasing plastic strain and the associated strain-hardening. This showed that the ability of steel to avoid pre-necking or running fracture reduces as the level of plastic strain-hardening increases. Temperature controlled Crack Tip Opening Displacement (CTOD) testing was then undertaken. The setting of testing temperatures for the CTOD tests were guided by review of the CVN test results, using published CVN to fracture toughness correlation methods. However running cleavage fractures developed in the CTOD specimens at higher than predicted temperatures of 10 oC and 20 oC. These are typical service temperatures for structures in New Zealand and so are very likely to occur at the time of an earthquake. The implication from this is that there are levels of strain-hardening and conditions of material notching constraint that can lead to pre-necking and running fracture in New Zealand fabricated steel structures, under severe earthquake loading. Care was taken in the CTOD testing to monitor and maximise the capture of data electronically using a specially developed Direct Current Potential Drop method. This allowed the test results to be analysed and considered in varying ways, leading to a consistent assessment of the CTOD, crack growth, and the specific work of fracture in each test piece. While CTOD test results have sometimes been published by structural and welding engineering researchers in the wake of Kobe and Northridge, the results were typically of little use for this study as the CTOD initiation point was generally not identified effectively. The effect of remote plastic flow in the specimens was also not adequately accounted for. The CTOD test results were often simply used to help correlate other factors observed by the researchers. Side-grooving of specimens was not reported as having been used in any of the published results reviewed. When conducting CTOD test with highly ductile constructional steels it is very difficult to get useful CTOD results if the specimens are not side-grooved, as significant necking and tunnelling will otherwise occur and limit the usefulness of the results. Work by Knott and also by McRobie and Smith was seminal in terms of identifying some critical aspects of plane strain development in CTOD tests, and the links to non-metallic particle density with respect to fracture toughness and CTOD at initiation. Some of their findings with regards to the effect of pre-strain on CTOD initiation were subsequently found to confirm the experimental findings in this study. No effective methodology for prediction of pre-necking or running fracture in a structural member or assembly when subjected to gross plastic cyclic deformation was found to exist in the literature. It was concluded however that the principles of specific work of fracture, and monotonic and cyclic fracture similitude were particularly relevant. These were therefore utilised in the development of the design method proposed in this thesis. The CTOD test results were reviewed, isolating the remote plastic flow component, to determine the critical specific work of fracture property Rc of the steels tested. A meeting with Professor Kuwamura at the University of Tokyo was providential, allowing discussion of his similitude principle, and observations in person of some of the fractured specimens developed during his full scale test series’. Running fractures with cleavage were evident in the specimens, with their tell-tale chevron markings. He had predicted running fracture problems in structures in Japan ahead of the Kobe earthquake and been largely ignored. His insights were subsequently seriously considered in Japan after the earthquake. He and his colleagues developed the principle of structural similitude that relates monotonic fracture displacement ductility to cyclic fracture displacement ductility for a particular assembly. This arose from their observation that running fractures developed from ductile crack formation at blunt notches in structures. The similitude principle has echoes of the Coffin-Manson approach to ductile crack initiated low cycle fracture. The principle of similitude has a log–log relationship as does the Manson-Coffin relationship. So where notch plasticity controls the initiation of fracture in a structural assembly it is conceptually reasonable to expect that the number of cycles to initiation of fracture from a notch will have a log–log relationship to the amplitude of the cyclic strain developed in the notch. Kuwamura found that steel assemblies with lower CVN energy had reduced cyclic fracture endurance than the same assemblies made with steel with higher CVN impact energy. However no method of predicting performance of any particular assembly could be developed from his observations. The benefit of his method primarily relates to the minimising of testing necessary to assess the fracture limited cyclic displacement ductility of a structural assembly. However it doesn’t provide a means for designing a structural assembly to achieve specific levels of ductile endurance other than clearly identifying the need to use steel with good CVN characteristics. The most significant development arising from this thesis is therefore the development of a design method to assess cyclic ductile endurance. The method utilises the specific work of fracture properties obtained from CTOD specimens of the steel in conjunction with a relatively simple fracture mechanics assessment and an elasto-plastic finite element analysis (FEA). The FEA model is used to determine the displacement ductility of the assembly at the calculated onset of pre-necking fracture. The elasto-plastic stress–strain properties of the steel in various pre-strain states required for the FEA may be derived from tensile testing. Kuwamura’s similitude principle is then used to predict cyclic plastic endurance at various constant displacement ductility amplitudes. The method is extended using Miner’s rule to allow for the effects of increasing variable amplitude cyclic plastic loading. In summary the thesis explains why pre-necking and running fractures occur in steel members subjected to cyclic plastic deformation during a severe earthquake. In addition a method for consistently assessing the ability of structural steel assemblies to achieve a specified level of ductile endurance during earthquakes is proposed. The method is verified against published results for a cyclic test of a simple steel member with a crack at mid-span. / Whole document restricted, but available by request, use the feedback form to request access.

This thesis provides a structural and materials engineering explanation for many of the running fractures that occurred in steel structures during the destructive Kobe and Northridge earthquakes in the mid 1990s. A method is developed that allows the ductile endurance of structural steel members subjected to cyclic plastic deformation during earthquakes to be assessed and for pre-necking running fractures to be avoided. The study commenced following the 2000 World Earthquake Conference in Auckland. The conference brought together the findings of the huge research effort, in America, Japan, Europe and New Zealand, that followed the Kobe and Northridge earthquakes. The running fractures that had occurred in steel structures represented an unpredicted failure mode that structural engineers have not known how to predict or suppress through the engineering design process. A clear fundamental understanding of the causes and how to prevent the fractures did not arise from the conference. In fact apparently conflicting results were reported. Full scale cyclic tests in New Zealand on structural assemblies had not resulted in running fractures, whereas tests in American and Japan had. Structural engineers designing earthquake resistant structures rely on constructional steel to be materially homogeneous and nominally tri-linear in behaviour. Steel is expected to behave elastically under regular in-service loading, have a reliable and flat yield stress-strain characteristic, and under overload then develop predictable levels of strain-hardening in conjunction with significant plastic elongation up to its ultimate tensile strength. Steel is expected to eventually fracture after further plastic elongation and necking. Ductile design strategies and methods utilise the plastic elongation characteristics of steel to protect structures in earthquake. Plastic deformation is considered to beneficially dissipate energy generated in the structure by a severe earthquake and also dampen the structure’s response. The occurrence of running fracture without significant cyclic plastic deformation and before section necking in steelwork, therefore undermines the basis of the ductile seismic design approach. The initial part of the thesis is devoted to bringing together the fundamental aspects of materials engineering related to fracture of constructional steel. This is intended to provide a bridge of knowledge for structural engineering practitioners and researchers not fully conversant with materials engineering aspects of fracture. Fracture behaviour in steel is a broad and complex topic that developed rapidly in the twentieth century driven by the demands of technological growth. The unexpected fracture of welded liberty ships at sea in World War 2; the need for reliable long term containment for the nuclear reactors in the 1950s and 1960s; and prevention of fatigue failures in aircraft frames since the 1950s all drove engineering research into steel fracture behaviour. There are many subtle variations in definitions in the published literature on fracture that can be confusing. Therefore an attempt has been made to clarify terminology. The term brittle fracture in particular is only used in this thesis as applying to running fracture when the general or far field tensile stresses are below the yield stress of the steel. The term pre-necking or running fracture is preferred to describe the condition more broadly which may occur prior to and also after general yielding, but before section necking. Running fracture is a manifestation of pre-necking fracture in which insufficient plastic flow is available in the assembly to absorb the energy released upon fracture. The experimental studies investigated the behaviour of constructional steel commonly used in New Zealand, at various levels of plastic strain. This started with Charpy V-Notch (CVN) testing which revealed that a significant transition temperature shift and curve shape change occurs with increasing plastic strain and the associated strain-hardening. This showed that the ability of steel to avoid pre-necking or running fracture reduces as the level of plastic strain-hardening increases. Temperature controlled Crack Tip Opening Displacement (CTOD) testing was then undertaken. The setting of testing temperatures for the CTOD tests were guided by review of the CVN test results, using published CVN to fracture toughness correlation methods. However running cleavage fractures developed in the CTOD specimens at higher than predicted temperatures of 10 oC and 20 oC. These are typical service temperatures for structures in New Zealand and so are very likely to occur at the time of an earthquake. The implication from this is that there are levels of strain-hardening and conditions of material notching constraint that can lead to pre-necking and running fracture in New Zealand fabricated steel structures, under severe earthquake loading. Care was taken in the CTOD testing to monitor and maximise the capture of data electronically using a specially developed Direct Current Potential Drop method. This allowed the test results to be analysed and considered in varying ways, leading to a consistent assessment of the CTOD, crack growth, and the specific work of fracture in each test piece. While CTOD test results have sometimes been published by structural and welding engineering researchers in the wake of Kobe and Northridge, the results were typically of little use for this study as the CTOD initiation point was generally not identified effectively. The effect of remote plastic flow in the specimens was also not adequately accounted for. The CTOD test results were often simply used to help correlate other factors observed by the researchers. Side-grooving of specimens was not reported as having been used in any of the published results reviewed. When conducting CTOD test with highly ductile constructional steels it is very difficult to get useful CTOD results if the specimens are not side-grooved, as significant necking and tunnelling will otherwise occur and limit the usefulness of the results. Work by Knott and also by McRobie and Smith was seminal in terms of identifying some critical aspects of plane strain development in CTOD tests, and the links to non-metallic particle density with respect to fracture toughness and CTOD at initiation. Some of their findings with regards to the effect of pre-strain on CTOD initiation were subsequently found to confirm the experimental findings in this study. No effective methodology for prediction of pre-necking or running fracture in a structural member or assembly when subjected to gross plastic cyclic deformation was found to exist in the literature. It was concluded however that the principles of specific work of fracture, and monotonic and cyclic fracture similitude were particularly relevant. These were therefore utilised in the development of the design method proposed in this thesis. The CTOD test results were reviewed, isolating the remote plastic flow component, to determine the critical specific work of fracture property Rc of the steels tested. A meeting with Professor Kuwamura at the University of Tokyo was providential, allowing discussion of his similitude principle, and observations in person of some of the fractured specimens developed during his full scale test series’. Running fractures with cleavage were evident in the specimens, with their tell-tale chevron markings. He had predicted running fracture problems in structures in Japan ahead of the Kobe earthquake and been largely ignored. His insights were subsequently seriously considered in Japan after the earthquake. He and his colleagues developed the principle of structural similitude that relates monotonic fracture displacement ductility to cyclic fracture displacement ductility for a particular assembly. This arose from their observation that running fractures developed from ductile crack formation at blunt notches in structures. The similitude principle has echoes of the Coffin-Manson approach to ductile crack initiated low cycle fracture. The principle of similitude has a log–log relationship as does the Manson-Coffin relationship. So where notch plasticity controls the initiation of fracture in a structural assembly it is conceptually reasonable to expect that the number of cycles to initiation of fracture from a notch will have a log–log relationship to the amplitude of the cyclic strain developed in the notch. Kuwamura found that steel assemblies with lower CVN energy had reduced cyclic fracture endurance than the same assemblies made with steel with higher CVN impact energy. However no method of predicting performance of any particular assembly could be developed from his observations. The benefit of his method primarily relates to the minimising of testing necessary to assess the fracture limited cyclic displacement ductility of a structural assembly. However it doesn’t provide a means for designing a structural assembly to achieve specific levels of ductile endurance other than clearly identifying the need to use steel with good CVN characteristics. The most significant development arising from this thesis is therefore the development of a design method to assess cyclic ductile endurance. The method utilises the specific work of fracture properties obtained from CTOD specimens of the steel in conjunction with a relatively simple fracture mechanics assessment and an elasto-plastic finite element analysis (FEA). The FEA model is used to determine the displacement ductility of the assembly at the calculated onset of pre-necking fracture. The elasto-plastic stress–strain properties of the steel in various pre-strain states required for the FEA may be derived from tensile testing. Kuwamura’s similitude principle is then used to predict cyclic plastic endurance at various constant displacement ductility amplitudes. The method is extended using Miner’s rule to allow for the effects of increasing variable amplitude cyclic plastic loading. In summary the thesis explains why pre-necking and running fractures occur in steel members subjected to cyclic plastic deformation during a severe earthquake. In addition a method for consistently assessing the ability of structural steel assemblies to achieve a specified level of ductile endurance during earthquakes is proposed. The method is verified against published results for a cyclic test of a simple steel member with a crack at mid-span. / Whole document restricted, but available by request, use the feedback form to request access.

This thesis provides a structural and materials engineering explanation for many of the running fractures that occurred in steel structures during the destructive Kobe and Northridge earthquakes in the mid 1990s. A method is developed that allows the ductile endurance of structural steel members subjected to cyclic plastic deformation during earthquakes to be assessed and for pre-necking running fractures to be avoided. The study commenced following the 2000 World Earthquake Conference in Auckland. The conference brought together the findings of the huge research effort, in America, Japan, Europe and New Zealand, that followed the Kobe and Northridge earthquakes. The running fractures that had occurred in steel structures represented an unpredicted failure mode that structural engineers have not known how to predict or suppress through the engineering design process. A clear fundamental understanding of the causes and how to prevent the fractures did not arise from the conference. In fact apparently conflicting results were reported. Full scale cyclic tests in New Zealand on structural assemblies had not resulted in running fractures, whereas tests in American and Japan had. Structural engineers designing earthquake resistant structures rely on constructional steel to be materially homogeneous and nominally tri-linear in behaviour. Steel is expected to behave elastically under regular in-service loading, have a reliable and flat yield stress-strain characteristic, and under overload then develop predictable levels of strain-hardening in conjunction with significant plastic elongation up to its ultimate tensile strength. Steel is expected to eventually fracture after further plastic elongation and necking. Ductile design strategies and methods utilise the plastic elongation characteristics of steel to protect structures in earthquake. Plastic deformation is considered to beneficially dissipate energy generated in the structure by a severe earthquake and also dampen the structure’s response. The occurrence of running fracture without significant cyclic plastic deformation and before section necking in steelwork, therefore undermines the basis of the ductile seismic design approach. The initial part of the thesis is devoted to bringing together the fundamental aspects of materials engineering related to fracture of constructional steel. This is intended to provide a bridge of knowledge for structural engineering practitioners and researchers not fully conversant with materials engineering aspects of fracture. Fracture behaviour in steel is a broad and complex topic that developed rapidly in the twentieth century driven by the demands of technological growth. The unexpected fracture of welded liberty ships at sea in World War 2; the need for reliable long term containment for the nuclear reactors in the 1950s and 1960s; and prevention of fatigue failures in aircraft frames since the 1950s all drove engineering research into steel fracture behaviour. There are many subtle variations in definitions in the published literature on fracture that can be confusing. Therefore an attempt has been made to clarify terminology. The term brittle fracture in particular is only used in this thesis as applying to running fracture when the general or far field tensile stresses are below the yield stress of the steel. The term pre-necking or running fracture is preferred to describe the condition more broadly which may occur prior to and also after general yielding, but before section necking. Running fracture is a manifestation of pre-necking fracture in which insufficient plastic flow is available in the assembly to absorb the energy released upon fracture. The experimental studies investigated the behaviour of constructional steel commonly used in New Zealand, at various levels of plastic strain. This started with Charpy V-Notch (CVN) testing which revealed that a significant transition temperature shift and curve shape change occurs with increasing plastic strain and the associated strain-hardening. This showed that the ability of steel to avoid pre-necking or running fracture reduces as the level of plastic strain-hardening increases. Temperature controlled Crack Tip Opening Displacement (CTOD) testing was then undertaken. The setting of testing temperatures for the CTOD tests were guided by review of the CVN test results, using published CVN to fracture toughness correlation methods. However running cleavage fractures developed in the CTOD specimens at higher than predicted temperatures of 10 oC and 20 oC. These are typical service temperatures for structures in New Zealand and so are very likely to occur at the time of an earthquake. The implication from this is that there are levels of strain-hardening and conditions of material notching constraint that can lead to pre-necking and running fracture in New Zealand fabricated steel structures, under severe earthquake loading. Care was taken in the CTOD testing to monitor and maximise the capture of data electronically using a specially developed Direct Current Potential Drop method. This allowed the test results to be analysed and considered in varying ways, leading to a consistent assessment of the CTOD, crack growth, and the specific work of fracture in each test piece. While CTOD test results have sometimes been published by structural and welding engineering researchers in the wake of Kobe and Northridge, the results were typically of little use for this study as the CTOD initiation point was generally not identified effectively. The effect of remote plastic flow in the specimens was also not adequately accounted for. The CTOD test results were often simply used to help correlate other factors observed by the researchers. Side-grooving of specimens was not reported as having been used in any of the published results reviewed. When conducting CTOD test with highly ductile constructional steels it is very difficult to get useful CTOD results if the specimens are not side-grooved, as significant necking and tunnelling will otherwise occur and limit the usefulness of the results. Work by Knott and also by McRobie and Smith was seminal in terms of identifying some critical aspects of plane strain development in CTOD tests, and the links to non-metallic particle density with respect to fracture toughness and CTOD at initiation. Some of their findings with regards to the effect of pre-strain on CTOD initiation were subsequently found to confirm the experimental findings in this study. No effective methodology for prediction of pre-necking or running fracture in a structural member or assembly when subjected to gross plastic cyclic deformation was found to exist in the literature. It was concluded however that the principles of specific work of fracture, and monotonic and cyclic fracture similitude were particularly relevant. These were therefore utilised in the development of the design method proposed in this thesis. The CTOD test results were reviewed, isolating the remote plastic flow component, to determine the critical specific work of fracture property Rc of the steels tested. A meeting with Professor Kuwamura at the University of Tokyo was providential, allowing discussion of his similitude principle, and observations in person of some of the fractured specimens developed during his full scale test series’. Running fractures with cleavage were evident in the specimens, with their tell-tale chevron markings. He had predicted running fracture problems in structures in Japan ahead of the Kobe earthquake and been largely ignored. His insights were subsequently seriously considered in Japan after the earthquake. He and his colleagues developed the principle of structural similitude that relates monotonic fracture displacement ductility to cyclic fracture displacement ductility for a particular assembly. This arose from their observation that running fractures developed from ductile crack formation at blunt notches in structures. The similitude principle has echoes of the Coffin-Manson approach to ductile crack initiated low cycle fracture. The principle of similitude has a log–log relationship as does the Manson-Coffin relationship. So where notch plasticity controls the initiation of fracture in a structural assembly it is conceptually reasonable to expect that the number of cycles to initiation of fracture from a notch will have a log–log relationship to the amplitude of the cyclic strain developed in the notch. Kuwamura found that steel assemblies with lower CVN energy had reduced cyclic fracture endurance than the same assemblies made with steel with higher CVN impact energy. However no method of predicting performance of any particular assembly could be developed from his observations. The benefit of his method primarily relates to the minimising of testing necessary to assess the fracture limited cyclic displacement ductility of a structural assembly. However it doesn’t provide a means for designing a structural assembly to achieve specific levels of ductile endurance other than clearly identifying the need to use steel with good CVN characteristics. The most significant development arising from this thesis is therefore the development of a design method to assess cyclic ductile endurance. The method utilises the specific work of fracture properties obtained from CTOD specimens of the steel in conjunction with a relatively simple fracture mechanics assessment and an elasto-plastic finite element analysis (FEA). The FEA model is used to determine the displacement ductility of the assembly at the calculated onset of pre-necking fracture. The elasto-plastic stress–strain properties of the steel in various pre-strain states required for the FEA may be derived from tensile testing. Kuwamura’s similitude principle is then used to predict cyclic plastic endurance at various constant displacement ductility amplitudes. The method is extended using Miner’s rule to allow for the effects of increasing variable amplitude cyclic plastic loading. In summary the thesis explains why pre-necking and running fractures occur in steel members subjected to cyclic plastic deformation during a severe earthquake. In addition a method for consistently assessing the ability of structural steel assemblies to achieve a specified level of ductile endurance during earthquakes is proposed. The method is verified against published results for a cyclic test of a simple steel member with a crack at mid-span. / Whole document restricted, but available by request, use the feedback form to request access.