A combined theoretical and experimental study of the criteria governing the failure analysis of pressure vessels with defects has been performed. The fields of fracture mechanics (linear elastic and elastic-plastic behaviour), failure governed by large scale plastic deformation and energy balance methods are critically reviewed. All three approaches are shown to have relevance in the complete failure analysis of a structure with defects. An experimental study of the failure mechanisms of precracked polycarbonate and maraging steel plates and model polycarbonate vessels is presented. Room temperature, static loading tests are performed on 85 mm wide, 5 mm thick compact specimens of polycarbonate (0.013 andle; ^aandfrasl;_W andle; 0.810). For comparison, 51 mm wide maraging steel compact specimens are monotonically loaded at room temperature. The influence of through-thickness constraint on the fracture toughness and slow crack growth characteristics of the steel is investigated using plates of varying thickness (3.1 mm-25.4 mm) and initial crack length (0.386 andle; ^aandfrasl;_W andle; 0.766 for 3.1 mm thick sheet); various ancillary studies (scanning electron microscopy, surface deformation studies) complement the results. The crack growth behaviour of longitudinal through (0.704 ≤ c / andradic;‾DT/2</span> ≤ 1.434) and deep part-through (^dandfrasl;_T = 0.700 and 0.878) cracks in 50.8mm and 102mm diameter (5mm and 6mm wall thickness) polycarbonate cylinders is also studied. Bowling and Townley's two-criteria approach to failure i.e. LEFM on the one hand and limit analysis on the other, is shown to provide a useful method for assessing the relative importance of crack initiation, in the presence of limited crack tip plasticity and general yield as failure criteria for a given sized defect. Thus, for crack tip plasticity fully contained by an outer elastic field i.e. not general yield, the LEFM parameter, K_Ic (with the possibility of a plasticity correction factor for thin sheet) can be used to predict crack initiation. For the low strain-hardening maraging steel, Irwin's plane strain plasticity correction, ¹andfrasl;_6π (K_{Ic/σ_y})² is shown to be applicable to sheet thicknesses comparable to ¹andfrasl;_π (K_{Ic/σ_y})² i.e. twice Irwin's plane stress plastic zone radius.

The development of a high speed pin-on-disc wear testing rig has been undertaken and it has enabled the wear behaviour of zirconia sliding against silicon carbide to be examined. Sliding wear experiments were conducted for zirconia pins and silicon carbide discs under both water-lubricated and unlubricated conditions. Under water-lubricated conditions, tests at a relative sliding speed of 6 m s-1 showed that wear was geometry dependent and the exact alignment of the pin was crucial. At this speed it was possible, with pins polished in-situ on the rig, to achieve hydrodynamic lubrication (HDL) which resulted in low wear. Increasing the nominal contact pressure to 70 MPa resulted in the breakdown of the lubrication and led to high wear coefficients (e.g. 1.1 x 10-6 mm3 N-1 m-1). Under unlubricated conditions, the zirconia pin wore faster than with water lubrication present, the wear rate increasing with speed and nominal contact pressure in the range 1-6 m s-1 and 2-14 MPa. Wear coefficients ranged from 1.4 x 10-6 to 5.1 x 10 -5 mm3 N-1 m-1. The wear tests were followed by examination of the worn surfaces, using a variety of techniques including reflected light and scanning electron microscopy, in order to elucidate likely wear mechanisms. These techniques revealed that there was some degree of commonality between water-lubricated and unlubricated tests, suggesting a universal mechanism which operates over a broad spectrum of testing conditions. The electron microscopy study, combined with observations in the literature, led to the development of a physical model for the wear mechanism, including surface modification and material removal. During the initial stages of wear, and under mild testing conditions, grooves were formed on the surface by plastic deformation due to counterface asperities and trapped debris. The surface was further smeared and deformed as the test proceeded. A deformed surface layer built up which caused intergranular cracks parallel to the surface, at a depth of approximately 3 m. Wear occurred when these cracks linked up with the surface, a process which may be helped by the formation of a network of cracks parallel and normal to the sliding direction. The mechanism of formation of these crack networks remains controversial but is thought to involve thermal shock. Once material is removed from the surface, it is either thrown out of the contact or trapped in it causing further damage.

Finite element analysis was performed on an implanted femur, using loads of daily living activities, performed by total hip replacement patients. A probabilistic bone fatigue failure model was utilized to analysis the risk of post-operative femoral fracture in different patient groups, depending on bone fatigue strength for young and elderly patients, and activity levels for normal and active patients. Different bone-implant interface conditions were considered: after surgery, osseointegrated, fibrous tissue covering, and loose. For young patients, the probability of failure is less than 4% for all cases. While in elderly patients, high of 28.2% and 57.9% are reported for normal and active groups respectively. For both age groups and activity levels, loose stems had the highest probability of failure, while osseointegrated had the lowest.

No description available.

Damage to steel moment frames in the 1994 Northridge and 1995 Hyogken-Nanbu earthquakes subsequently motivated intensive research and testing efforts in the US, Japan, and elsewhere on moment frames. Despite extensive past research efforts, one important problem remains unresolved: the degree of panel zone participation that should be permitted in the inelastic seismic response of a steel moment frame. To date, a fundamental computational model has yet to be developed to assess the cyclic rupture performance of moment frames. Without such a model, the aforementioned problem can never be resolved. This dissertation develops an innovative way of predicting cyclic rupture in steel moment frames by employing artificial neural networks. First, finite element analyses of 30 notched round bar models are conducted, and the analytical results in the vicinity of the notch root are extracted to form the inputs for either a single neural network or a competitive neural array. After training the neural networks, the element with the highest potential to initiate a fatigue crack is identified, and the time elapsed up to the crack initiation is predicted and compared with its true synthetic answer. Following similar procedures, a competitive neural array comprising dynamic neural networks is established. Two types of steel-like materials are created so that material identification information can be added to the input vectors for neural networks. The time elapsed by the end of every stage in the fracture progression is evaluated based on the synthetic allocation of the total initiation life assigned to each model. Then, experimental results of eight beam-to-column moment joint specimens tested by four different programs are collected. The history of local field variables in the vicinity of the beam flange - column flange weld is extracted from hierarchical finite element models. Using the dynamic competitive neural array that has been established and trained, the time elapsed to initiate a low cycle fatigue crack is predicted and compared with lab observations. Finally, finite element analyses of newly designed specimens are performed, the strength of their panel zone is identified, and the fatigue performance of the specimens with a weak panel zone is predicted.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2000. / David L. McDowell. Part of the SMARTech Electronic Thesis and Dissertation Collection.

No description available.

Fatigue damage of aluminium alloys is one of the key concerns in transport industries, particularly in the aerospace industry. The purpose of the project is to develop new knowledge and techniques against fatigue failure for these industries through a systematic investigation of fatigue resistance and crack growth behaviours of aluminium alloys. Fatigue and fracture mechanics have been investigated analytically, numerically and experimentally in this project. Overload transient effect on fatigue crack growth has been examined by considering various parameters including crack closure, overload ratio (OLR), load ratio (R ratio), baseline stress intensity factor range, (∆K)_BL and geometry. It was found that crack closure can be correlated qualitatively and quantitatively to all other parameters associated with overload transient behaviour. It is proposed that the effect of crack tip plasticity on the non-linearity of the compliance curve can be separated to obtain reliable crack closure measurement. In this project, different methods are used to better understand the transient retardation process so that the damage tolerance design (DTD) of the components made of aluminium alloys can be enhanced. Another important parameter for fatigue and damage tolerance design (DTD) of engineering components is the threshold stress intensity factor range for fatigue crack growth, ∆K_th. A small variation in identification of ∆K_th can lead to a big change in overall estimation of fatigue life. In this project, an analytical model has been developed for aluminium alloys by fitting an analytical curve with raw crack growth data in order to identify the ∆K_th. This model has the capacity to identify ∆K_th for different aluminium alloys at various R ratios. There is a great demand for enhanced fatigue life of aluminium alloys in the transport industry. This project has carried out a detailed investigation of electromagnetic treatment (ET) in the form of electropulsing treatment to develop an efficient technique for fatigue resistance enhancement. ET parameters including the treatment intensity, treatment time and the number of applications have been optimised. It is suggested that the duration of ET treatment can be used as the main parameter among all these to control the fatigue resistance of the aluminium alloy. The improvement in fatigue resistance has been explained by the change in microhardness and conductivity of aluminium alloy due to ET. Additionally, the fracture morphology was analysed using scanning electron microscopy (SEM). The precipitates and dislocation characteristics were also studied using transmission electron microscopy (TEM). The outcomes of this investigation will help improve structural integrity by enhancing fatigue resistance of aluminium alloys.

One of the most prominent factors affecting the performance and longevity of risers is vortex induced vibration (VIV), which can cause severe fatigue damage, especially in risers used in deep waters. The available approaches for analyzing VIV induced fatigue in risers mainly focus on the VIV aspect of the problem; indeed less attention has been paid on the effect of VIV on a riser’s fatigue life and in prediction of fatigue life using various models. This dissertation first demonstrates how one can characterize fatigue of pipes and risers using an equivalent plate specimen as opposed to using a pipe specimen, thereby simplifying the task, yet obtaining good accuracy. Actual variable amplitude loadings (VAL) are used to study the fatigue crack growth in risers’ material with a focus on the various influencing parameters. Extensive experimental investigations are performed, followed by analytical and computational nonlinear finite element analyses. It is shown that the higher harmonics do cause significant fatigue damage, thus their influence should not be ignored. The influence of load interaction effects is also investigated, focusing on the fatigue crack growth retardation effects due to tension overloads, as well as the acceleration effects due to compression underloads. The crack closure concept is then used to explore into both the fatigue retardation and acceleration effects within a VAL scenario. An effective method for calculation of the stress intensity factor is proposed, which considers only the tensile portion of the stress range, while proposing another effective approach for accounting for the influence of compressive stress cycles. Moreover, a two-parameter approach is used in this dissertation, relating the fatigue crack growth rate (FCGR) to the crack tip opening displacement (CTOD). It is shown that the CTOD provides adequate information for calculating the FCGR under VAL, and it can be effectively used to account for the influence of the compressive stress cycles. The experimental investigation also considers the retardation effect resulting from the applied peak tensile overload cycles (TOLC) and the influence of various so-called “clipping” levels, demonstrating the significant influence of the TOLC on crack growth retardation in VAL.

Fatigue and fracture assisted by the environment are responsible for the majority of failures in implants. Due to low tribological properties of titanium alloys, the thermal oxidation technique has been evaluated to improve the surface hardness and consequently, to improves the tribological properties of Ti6Al4V alloy. However, despite improved tribological properties of the Ti6Al4V alloy, there is a tendency to reduction of the fatigue limit of the oxide layer. The combined action of body fluid and cyclic loads also need to be investigated. This study evaluated the effect of thermal oxidation in the fatigue limit of the Ti6Al4V in environment containing 0.9% NaCl. It was observed a reduction in the fatigue limit for thermally oxidized Ti6Al4V alloy and it is suggested that the reduction in fatigue properties of alloy is associated with the brittleness of oxide layer. / Os processos de fadiga e fraturas assistidas pelo ambiente são responsáveis pela maioria das falhas em implantes. Devido às baixas propriedades tribológicas do titânio e suas ligas, a técnica de oxidação térmica tem ganhado destaque por conferir um aumento da dureza superficial e consequentemente melhorar as propriedades tribológicas da liga Ti6Al4V. Entretanto apesar da melhoria das propriedades tribológicas existe uma tendência na redução do limite de fadiga quando há presença de uma camada de óxido na superfície da liga Ti6Al4V. Ainda a ação combinada de fluidos corpóreos e de carregamento precisa ser investigada. Neste estudo foi avaliado o efeito da oxidação térmica combinado a aplicação de carregamento cíclico ao ar e em meio contendo 0,9 % NaCl. Foi observada uma redução no limite de resistência à fadiga para a liga Ti6Al4V oxidada termicamente. O meio contendo 0,9 % de NaCl não influenciou significativamente a resistência em fadiga de corpos de prova oxidados termicamente e sugere-se que a redução nas propriedades de fadiga da liga está associada à fragilidade da camada de óxido.