A survey is made of analytic, experimental and numerical techniques in the field of the dynamic initiation, and early propagation, of cracks. As no closed form analytic solutions exist for finite geometries, even in the elastic case, numerical and experimental techniques have to be developed. In instrumented impact tests plasticity often occurs. The specific problem of the Instrumented Charpy Test is discussed in detail by virtue of its technological significance and the extensive literature available for the test. Although a standard for the test has been proposed there are still outstanding questions to be answered, for which the techniques described above can be used. The problem of the dynamic calibration of various notched geometries is addressed in the original work of the thesis. The Charpy, Izod, Slender Cantilever and Double Notched Bar geometries are studied using dynamic photoelasticity and 8000 fps photography. It is shown that the response of the DNB is more straightforward than the Charpy geometry. Further photoelastic study of the latter two geometries, using epoxy model material and 10⁶ fps photography, gives a quantitative measure of the growth of stress intensity factor at the notch tips and hence a dynamic calibration is deduced. An explicit finite difference code is used to supplement photoelastic data. Having achieved progress in the derivation of the dynamic calibration of the two selected geometries, corresponding instrumented impact tests are then undertaken. The Hopkinson Pressure Bar method of loading is used. It is concluded that the proposed standard for the Instrumented Charpy Test is valid within limits but that there is a requirement for a dynamic calibration. Such a calibration is complex in the case of the Charpy geometry whereas a simpler geometry, viz. DNB, could prove to be more amenable to analysis and hence be more practical from the technological point of view.

The current practices of the UK steel foundry industry have been described and a review has been made of the fracture characteristics and mechanisms which commonly occur in steel castings. Emphasis has been placed on intergranular embrittlement phenomena caused by segregation and precipitation effects, and in particular the role of aluminium nitride has been considered. The problem of A1N-induced intergranular fracture (I. F.) has been examined using experimental 0.30C-1.50bh castings with varying Al and N contents, and commercially-produced material. The mechanism of fracture has been studied using various optical and electron optical techniques and the existence of two forms of A1N (a plate-like form and a dendritic form) has been confirmed. Directional solidification work has shown that the dendritic A1N forms in the residual liquid at a late stage of solidification. The plates are precipitated at a later stage when the casting is fully solidified. The plates nucleate at the grain boundaries and grow into the adjacent austenite grains with an orientation relationship of the form <1010> (0001)// // <110> {111} Y A1N Y. The effects of some processing variables on the susceptibility to IF have been examined. IF is favoured by a high Al and N content and a slow cooling rate. The state of heat treatment was also found to be important, with quenched and lightly-tempered samples being more susceptible to IF. A model has been proposed which is based on a comparison of the relative yield strength, cleavage strength and intergranular strength of a casting. This can be used to explain the variation in fracture behaviour with testing temperature. A programme of mechanical testing has shown that different testing techniques and conditions can influence the severity of IF observed quite significantly. Although there may be a considerable decrease in Charpy impact energy as a result of intergranular embrittlement, there is little effect on the initiation value of the crack opening displacement (Si). The C. O. D. at maximum load, (ö), however, max is decreased and the implications with regard to engineering design practices are considered.

This project is concerned with the fatigue response of roller quenched and tempered (RQT50I) steels. Three closely related aspects are considered: (i) the relationship between the microstructure and fatigue properties, (ii) the influence of the environment on fatigue properties, (iii) theoretical modelling of fatigue crack growth. The material used is an RQT501 grade steel. Two steels have been chosen: (A) without Mo and V additions, (B) with Mo and V additions. The second condition is chosen to evaluate the role of these elements, as hydrogen traps, on the resistance to corrosion fatigue crack growth. In order to evaluate these effects, tests have been carried out under freely corroding conditions and applied cathodic polarisation (-9 . SO mV/SCE). Metallographic examination has been carried out to evaluate grain size and inclusion distribution, size and morphology. To enable a comparison between short and long fatigue crack growth, smooth shallow hour-glass specimens have been used to study initiation and growth of short fatigue cracks, and compact tension specimens were used to monitor propagation of long fatigue cracks. The fatigue response (S - Nf curve) and fatigue crack growth rate behaviour has been studied in air (5 - 10 biz) and in 3.5%NaCI solution (0.2 Hz) or artificial seawater (0.2 Hz) with an applied stress ratio of 0. I and sinusoidal waveform. The generation and growth behaviour of cracks from smooth hour-glass specimens has been investigated through successive observation using the plastic replication method. Long fatigue crack lengths were measured using a Direct Current Potential Drop (DCPD) technique and monitored via a chart recorder. Based upon the results of these tests a fatigue crack growth model was developed based on non-equilibrium statistical theory incorporating a consideration of the influence of microstructure, e. g. grain boundaries and hydrogen embrittlement. A stochastic theory of corrosion has also been developed for the use with corrosion fatigue test data.

The present work consists of both experimental and theoretical aspects of corrosion fatigue in a high strength steel. Fatigue tests were conducted under fully reversed shear loading in an aerated 0.6M NaC1 solution at pH6, for a silico-manganese spring steel (BS251A58) having a yield strength around 1400 MPa. The fatigue crack evolution process can be defined sequentially in terms of a pit, a short crack in stage I and a crack in stage II. It was observed that in the early stages pits developed at Mn-rich sulphide inclusions, from which short cracks developed and propagated in stage I; the crack growth rate of such cracks was dominated by microstructural features. Stage II, microstructure-independent crack growth was observed following a transition from stage I crack growth. In addition consideration was given to the influences of cyclic frequency, the effect of cathodic polarisation and the effect of electrolyte compositions on corrosion fatigue lifetime and crack growth behaviour. Mechanical properties, metallurgical and electro-chemical properties of the material used were also investigated. The failed specimens were examined using SEM and X-ray spectra analysis in order to study the failure mechanism. Using a dislocation based Navarro-de los Rios model to represent the crack and its associated plastic zone, the crack growth process is characterised by the interaction of dislocations associated with micro-cracks, or local damage, and microstructural barriers (grain boundaries, precipitates, dispersed particles, phase interfaces, etc.). Some modifications to the model were made to incorporate the corrosive effect on the early stages of crack initiation (i.e. pit growth), the transition of stage I crack growth to stage II crack growth, and strain hardening of a parabolic form in order to achieve a more accurate prediction. The validity of the model was justified by the good agreement achieved between predictions based on the model and the experimental results. Based on the present study the following conclusions were made: 1. The fatigue strength of BS251A58 steel, subjected to 107 cycles in a 0.6M NaC1 solution, at pH6 and at a frequency of 5Hz, is 98 MPa, and is significantly lower than the value of the fatigue limit in air, i.e. 457.5 MPa. 2. Corrosion fatigue crack propagation in the high strength BS251A58 steel can be described sequentially in terms of corrosion-induced pits, microcracks from pits, cracks in stage I (shear type cracks), and cracks in stage II (tensile cracks). 3. The number of cycles in the stage I-to-stage II transition, in relation to the whole fatigue life, varies from 20% to 60%, increasing as the applied load decreases. 4. Crack coalescence may occur in both the stage I and stage II regimes. In the stage I regime, microcracks with an effective tip-to-tip distance less than 1 to 2 prior austenite grains will coalesce, while those with distances greater than 2 to 3 prior austenite grains will change to stage II propagation. In the stage II regime, crack coalescence °ems after about 70% of the fatigue life Nf. 5. The lower the cycling frequency, the shorter will be the corrosion fatigue life. The combination of a high stress level and a low frequency can eliminate the in-air-fatigue micro structural barrier effect. 6. Cathodic polarisation (E=-1280mv (SCE)) can prevent the surface of the specimen from pitting, delay the stage I-to-stage II transition, and decelerate the early stage II crack growth rate. However, as a stage II crack grows, cathodic polarisation will accelerate the crack growth rate. 7. In a 0.6M NaC1 solution, the final stage I crack length increases from 50ium to 300 gm with increasing cyclic shear stress range (from 224 MPa to 926 MPa). This distance equals approximately 2 to 10 prior austenite grain diameters. The final stage I crack length is much longer under cathodic polarisation than for a free corrosion potential under the same stress range. There is no significant frequency effect on the final stage I crack length when the frequency is in the range of 2Hz to 12.51-1z. 8. The dislocation based Navarro-de los Rios model was employed in the present study to describe crack growth behaviour and to predict fatigue life. The pit growth behaviour, the crack transition from stage I-to-stage II, and a parabolic strain hardening law, were incorporated into the model to reflect more closely the actual fatigue behaviour of the material. The validity of the model was justified by the good agreement achieved between predictions based on the model and the experimental results.

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The nature and scope of this thesis relates to obtaining an understanding of the way in which sheet metal forming processes affect the residual stresses (plastic deformation) and the fatigue perfomiance of wheel centre discs made of a dual phase steel. Automotive wheels are mainly manufactured from sheet metal plate by forming processes using tools and dies. This manufacturing process is directly concemed with deforming the plate under predominantly applied tensile forces where the component acquires a sfress state which is multi-axiial. This stress state is both complex and difficult to assess from an analytical point of view and thus the work reported in this thesis is of an applied nature, and seeks to understand the results of this stress state on residual stresses, hardness and, hence fatigue behaviour. Components from each stage of the centre disc manufacturing process and of the assembled and painted wheel will be assessed and characterised. The first part of the work involved a literature review of the factors influencing sheet metal forming, i.e. elongation, anisofropy, grain size, springback, residual sfresses and fatigue. This review gave the researcher a background from which he could confidently approach the various investigations that were required for successfiil completion of this project. The residual stress analysis led to an investigation that provided a new technique for residual stress measurement using the high speed centre hole drilling method. This new measurement technique gave improved results when assessing the residual stress behaviour of textured sheet materials subjected to plastic deformation. It is felt that this technique will also minimise the inherent scatter that is perceived by many an analyst to be a fundamental part of residual stress assessment by the hole drilling method. The concept of this measuring technique has since been patented and is subject to a South African Patent Application No.2000/2042. The thesis then deals with the process analysis of centre disc manufactiure as well as the analysis of the fatigue properties of specimens removed from each stage of the manufacturing process and of the complete wheel. Here the complexity of the cenfre disc manufacturing process is discussed and assessed in terms of the process effects on microstructure, surface hardness, induced residual stresses and fatigue performance. The aim of this part of the work is to seek relationships with respect to surface hardness, induced residual stress and fatigue life in terms of the various production stage components including the centre disc of the complete wheel. The results showed that relationships do exist and that fatigue life prediction can be based on surface hardness as well as residual sfress assessments for centre discs manufactured from dual phase steel plate. The final part considers modelling the fatigue life of automotive wheels based on the mean stress models of Soderberg, Goodman and Gerber and incorporating the induced residual stresses into the equations. The Gerber model was found to give the better correlation with regard to fatigue life prediction when compared with the actual fatigue data. hi conclusion, this research has led to the development of a model showing the relation and impact of the centre disc manufacturing process on induced residual stress levels and fatigue properties of dual phase steel plate.