The conditions prevailing in the cohesive zone of the blast furnace have been determined from published data and applied in a study of burden materials on an experimental scale. The mechanism of softening and melting of burden materials has been evaluated and it was found that two opposing reactions are important, namely the liquid slag formation reaction and the gaseous reduction reaction. At high temperatures the permeability of burden materials was found to be strongly dependent upon the degree of reduction and the pertaining temperature. A concept, summarised by Temperature-Reduction-Isobar Diagrams, was developed to portray this relationship, each burden material having its own characteristic diagram which can be used to predict the permeability of materials at elevated temperatures. The influence of gaseous sulphur and alkali vapour in the reducing gas upon the softening and melting behaviour of acid pellets has been evaluated. Sulphur is detrimental because of the formation of a low melting point Fe-S-O eutectic phase which hinders gaseous reduction and lowers the melt-down temperature. Alkali vapours increased the reduction rate of acid pellets, creating a rise in the melt-down temperature. Although the pellets absorbed alkali from the gas phase it had no discernable influence upon the liquid slag formation process. A joint sulphur/alkali study showed that the detrimental influence of sulphur was capable of eliminating the beneficial effect of alkali vapour. The information accrued from the experimental studies has been used to explain the structure of the cohesive zone found in dissected blast furnaces and to suggest methods to improve the performance of existing furnaces.

The work presented covers numerical analyses of edge-cracked thin plates of austenitic stainless steel, AISI 316, loaded with transient thermal downshock. Further to this isothermal centre-cracked plates loaded under isothermal biaxial conditions are modelled. Crack growth rates are then applied to the models to determine a correlation between the two geometries. The work presented demonstrates that a correlation exists between thermal downshock and biaxial isothermal crack growth rates. For such a correlation to be determined, the thermal shock and isothermal biaxial loading must be studied with an elastic-plastic material response. It is also determined that biaxiality in thermal shock is a function of the shock localisation due to localised contraction of the specimen in the direction of heat flow, making it necessary to evaluate thermal shock stresses in a two-dimensional sense. The relationship between biaxiality and localisation is found to be non-linear. At shocked regions greater than 25% of the available area maximum stresses remain largely unchanged. Below 25% of the available area maximum stresses drop significantly, by up to 50%. Finally a method is proposed whereby thermal shock crack growth rates can be estimated by the bounding conditions of isothermal biaxial loading. This method uses an estimation of the heat transfer coefficient determined through a thorough analysis of a broad range of thermal shock cycles for AISI 316. Using the defining temperatures and time period of the cycle the h value can be estimated to within 3%. Once known the h value is fed into a simple log function describing maximum thermal stress, which can then be converted for any localisation present. It is also found that the correlating isothermal peak load is approximately equal to that of the thermal stress calculated. From this a modified Paris law is proposed to predict two sets of crack growth rates; one at equibiaxial conditions and the second at a biaxiality determined from the thermal shock load. This is shown to be a minimum of 0.35 at maximum localisation. This will calculate the upper, and lower, bounding limits of the thermal shock crack growth rates, providing a good estimation of the thermal shock crack growth rates.

No description available.

The initiation of fracture and onset of chipping at plastic-elastic indentations and scratches on near (111), (111) and (001) silicon surfaces has been studied by optical and high resolution scanning electron microscopy. It is shown that a pattern of surface cracks are initiated at a critical width of impressions or scratches. It is proposed that the observed size effect is governed by the strain energy criterion put forward by Puttick et al (1979) according to which the critical size of indentations in a highly brittle material should be alpha x (EGamma)/(y2). The critical size parameter is evaluated for near (111) silicon; good agreement is found between the theory and observations. Indentations and scratches on near (111) silicon appear to produce cleavage on planes close to {110} rather than {111} planes, together with a preference for crack propagation in the surface layer in the directions [112], [121] and [211]rather than the reverse directions. This pattern of fracture exerts a marked influence on the anisotropy of abrasion of these surfaces. Similar behaviour is found on (111) silicon. Indentations on (001) silicon also appear to initiate surface fracture on or close to {110} rather than {111} planes, with no sign of asymmetry in the crack pattern. The mechanics of fracture at such plastic-elastic indentations is discussed and it is proposed that {110} cleavage is initiated by a dislocation reaction similar to that associated with indentation fracture in ionic crystals. The coefficient of friction ? (the ratio of the applied normal load to the tangential force) has been measured as a function of load (1 to 30g) on near (111) and (001) surfaces using Vickers diamond indenters having sharp and blunt tips. It is found that at light loads no penetration of the slider occurs; u is low and the resultant damage is associated with the nucleation of a high density of dislocations in a thin surface layer only. At higher loads considerable penetration of the slider occurs; u rises to a higher value due to the ploughing. No anisotropy can be detected using the blunt indenter. A decrease in u at cracking is observed in [112] and [110] on (111) and (001) surfaces respectively using the sharp indenter.

The microstructure and low-cycle fatigue properties of 20-25-Nb stainless steel in both the unirradiated and irradiated conditions have been studied over the temperature range 400 to 850C. The microstructure of the steel in the as heat-treated and aged, and neutron irradiated conditions is described, followed by a study of the effect of various test variables on the low-cycle fatigue endurance and fracture characteristics, together with some subsidiary work on tensile properties and fracture. A new type of elevated temperature low-cycle fatigue crack nucleation has been identified, and a quantitative model is presented to account for the increased low-cycle fatigue endurances associated with it. The influence of test variables on the phenomenon of square grain formation during low-cycle fatigue is studied, and interpreted in terms of a new model based on dislocation energetics. The low-cycle fatigue and tensile mechanical properties and fracture characteristics of the irradiated steel is next described and the deficiencies of the existing theory of elevated temperature embrittlement highlighted. The reasons for these deficiencies are discussed, and followed by a description of a qualitative model based on the existence of a damaged zone ahead of a graving fatigue crack, which it is believed offers an explanation of the observed mechanical and fracture characteristics of the irradiated steel.

This thesis describes a study of indentation fracture transitions in poly (methylmethacrylate) (PMMA). The objective of this study was to deteimine the ductile-to-brittle transition temperature (DBTT) and to investigate its correlation with indenter radius. For this purpose PMMA has been indented by ball indenters of radii ranging from 0. 5 mm to 8 mm over a temperature range of -80 C to 60 C with various testing speeds. It has been shown that at least two types of cracking can be formed, radial cracking and ring cracking. A fracture transition in a given test is defined as that temperature at which one of these crack types is observed to form. The DBTT obtained by observing both types of cracking are governed by a size effect related to the indenter radius in a form predicted by the Puttick theory of fracture transitions. In connection with the main investigation, subsidiary studies have been made of (a) the ball indentation hardness and (b) the extent of crazing around indentations as functions of temperature.

Damage development in transparent woven glass fibre reinforced epoxy laminates manufactured from two different commercial cloths has been investigated under quasi-static and cyclic loading. Of the two different cloths, one was woven using untwisted fibre bundles, the second cloth was woven using a fibre bundle which was formed by twisting three smaller bundles together. All laminates were fabricated using a wet lay-up process to impregnate two layers of cloth prior to curing. Uniaxial quasi-static tension and tension-tension fatigue tests were carried out on coupons from both sets of laminates. Observations of the damage caused by the different loading modes were made in two ways, In-situ observations of coupons held within the testing machine allowed the damage to be monitored during testing. Observations of metallographically polished edge- sections containing damage allowed the through-thickness characteristics of the damage to be observed. The major damage morphology observed m both laminates under both types of loading was matrix cracking. The crack morphology observed in laminates reinforced with the cloth woven using untwisted fibre bundles was similar to the cracks observed in cross-ply laminates. This similarity allowed the damage to be quantified by a line density measurement analogous to that used for cross-ply laminates. The damage observed in the laminates reinforced with the cloth woven using twisted fibre bundles was more complicated. This complex damage required quantification by measurements from both the plan view and the edge-section. Shear-lag analysis was used to model the stiffness reduction of the laminates due to cracking damage. Equivalent laminates based on a cross-ply lay-up were derived. The reduced stiffness of the region of the laminate affected by the cracking was calculated using shear-lag and the stiffness of this region was then combined with the stiffness of the rest of the laminate to give the reduced stiffness of the laminate as a whole. The test data and the model predictions showed good agreement for both laminates.

The plane strain fracture toughness of seven different particle-filled epoxies has been measured using compact tension specimens. These toughened epoxies were based on 828 epoxy resin filled with three types of phenolic beads and four types of carbon beads. Significant increases in toughness were observed (up to about 50% with 30% volume fraction of bead) and the mechanisms of toughening have been studied using scanning electron microscopy. The suggested major toughening mechanisms are crack pinning, localized plastic deformation associated with particle-matrix debonding and transparticle fracture. The shape of the load-displacement records obtained during the compact tension tests have been correlated with the failure mechanisms and compared with other studies in the literature. Based on the results obtained from the fracture toughness testing of bead filled epoxies, a carbon bead filled epoxy was selected as the matrix material for a hybrid composite. A method of preparing glass fibre laminates using bead filled epoxy as matrix has been developed which results in the beads concentrating at the inter-ply regions. The interlaminar fracture behaviour of the hybrid composite has been investigated using DCB (double cantilever beam) and ENF (end notch flexure) specimens for Mode 1 and Mode 2 tests respectively. The hybrid composite shows an increase in both GIC initiation and GIIC values as compared to a GFRP laminate with pure epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15 and 20 %. However, the pure epoxy glass fibre composite shows a higher GIC propagation value than that of the hybrid composites due to fibre bridging which is less pronounced in the hybrids as the presence of the beads results in a matrix rich interply region. The relationship between the Mode 1 interlaminar fracture mechanics parameters (i.e. GIC and KIC values), obtained from DCB specimens, has been reviewed using orthotropic fracture mechanics. It has been shown that KIC values calculated using an isotropic analysis of the DCB specimen are an overestimate and that they can be corrected by a factor derived from orthotropic fracture mechanics.

A new finite element which adequately represents the character of the strains near the crack-tip is presented. The element is based on displacement functions, derived from, assumed simple strain functions, by integrating the strain - displacement equations. The resulting displacement functions contain terms associated with the rigid body modes. The element is 'trapezoidal' in shape with two straight and two circular boundaries. A 'triangular' crack-tip element based on assumed simple displacement functions is also presented. This element has two of its boundaries straight, meeting at the tip of the crack, and the other circular. When employed in the singular region of the crack-tip, using as few as 312 degrees of freedom, these elements give results for displacements and stresses whose accuracies are within l«57o and ~y/o , respectively, of 'exact' solutions. The elements are combined with Constant Strain Triangular elements to analyse practical geometries used in fracture toughness testing. Relatively coarse meshes are sufficient to obtain accurate estimates of stress intensity factors by a direct displacement method not requiring an extrapolation process.

Thermally formed oxides scales can protect metals from aggressive environments at high temperatures. However the barrier function is destroyed when the oxide fails. Therefore a new 4-point bend test technique has been developed to measure the failure strains and to study the failure mechanisms of brittle layers in tension and compression. Tests were made with iron oxide and nickel oxide at room temperature and 550 or 900 °C, respectively, using strain rates of 10~* and 10"5 s"1 . Brittle lacquer was used as a model layer. Acoustic emission (AE) was employed to monitor and interpret failure mechanisms together with post test metallography. Equi-distant cracks were formed during failure in tension. Further cracking was affected by elastic and plastic stress relaxation processes, and interface delamination only started after these processes were exhausted. The crack spacing increased with oxide thickness and the results indicated that plastic relaxation processes were dominant at growth temperature conditions. The shear strength of the interface was lower at growth temperature. Shear failure within the layer was found in NiO and brittle lacquer when tested in compression. Failure in iron oxide under compression always started at the interface. The failure mechanism and initiation in compression was determined by the relative shear strength of interface, the shear strength of the layer and the buckling stability of the layer. However, spallation always required crack growth at the interface. Measured failure strains in tension and compression agreed well with the predictions of a model incorporating the fracture mechanics condition for tensile cracking or interface crack growth respectively and factors like residual strains, oxide creep and lateral oxide growth which accounted for the behaviour of a thin growing scale on a thick substrate. The critical fracture mechanics parameter in tension was the composite void size. A K1C value of ~ 1.1 MN nv3/2 was obtained for iron oxide for room temperature and 550 °C. Values of 0.41 and -1.61 MN m3/2 were found for NiO at room temperature and at 900 °C, respectively. The residual growth stresses in iron oxide were determined as approximately zero and the cooling strain from 550 °C was -0.05 - 0.06%. The residual stresses in NiO were -175 MPa at room temperature. The strain energy release rate for interfacial failure in iron oxide was 27 J m-2 and the fracture surface energies were 3.4 and 0.8 J m-2 for iron and nickel oxide, respectively. AE was a useful tool for explaining the failure mechanisms and a numerical analysis showed a slight difference in the AE signal released during tensile and compressive failure.