R.A. Smith

Fatigue crack growth under compressive loading

Abstract Fatigue cracks were grown in centre-notched sheet made from BS4360 50B structural steel. Despite the fact that loading was fully compressive, cracks initiated and grew in regions of residual tensile stress at the notch roots. It was observed that crack growth rates decreased with increasing crack length until arrest occurred. Near tip strain gauges were used to monitor crack closure; closure readings agreed well with those deduced from growth rates. The residual stress ahead of the slit tip and normal to the slit plane, σres, was estimated from the crack growth rate response. It was found that σres scaled with distance, x , from the slit tip as σ res αx −0.56 .

Fatigue crack growth in a fillet welded joint

Abstract Existing theories for the growth of cracks at weld toes have proved difficult to verify because of a lack of experimental proof at short crack depths and slow growth rates. Arbitrary initial defect sizes have been employed in life calculations coupled with approximate two-dimensional stress analyses. In this study, the fatigue performance of a stress relieved fillet weld is determined by both theory and experiment. Crack growth results for shallow (less than 1 mm depth) elliptical cracks at weld toes are used to test an elastic expression for stress intensity using a correction factor from a three-dimensional stress analysis. No evidence of higher than expected growth rates, observed by others for very short cracks and cracks in notch plastic zones, is apparent. Integration of a growth law that includes the threshold stress intensity factor provides fatigue life predictions for various stress ratios and from experimentally measured defect depths. Needle peening the weld toe improves the fatigue life by retarding crack growth up to 1 mm below the weld toe.

Fatigue crack growth at stress concentrations—the role of notch plasticity and crack closure

Abstract Fatigue crack growth and closure behaviour in sharply notched specimens, made from AISI 316 stainless steel, BS 4360 grade 50B structural steel and a commercially pure aluminium, was monitored under constant amplitude cyclic loadings. The initial notch crack growth rates were in general high but decelerating. These observations are in contradiction with the predictions of linear elastic fracture mechanics. It is argued from experimental observations that plasticityinduced crack closure was mainly responsible for the initial deceleration of the notch crack. The effective stress intensity range, a parameter that has taken crack closure into account, was able to correlate most but not all of the notch crack growth data with the long crack data. When the applied loading is sufficiently high, a strain controlled condition exists just ahead of the notch tip. In this case, crack growth rates were better predicted by a shear decohesion model for high strain fatigue crack growth. Neither the nominal stress intensity range, a strain based intensity factor, a cyclic J integral nor a total shear parameter could bridge the gap between notch and long crack behaviour.

Criteria for brittle fracture in biaxial tension

Abstract The criteria of maximum tangential stress, maximum tangential principal stress, maximum tangential strain and strain energy density are applied to the problems of slit and elliptical cracks under remote uniform biaxial tension. The predicted direction of crack extension and the critical load are compared with experimental results reported by other investigators. The unstable crack paths are determined. The four criteria differ in the case of unequal tension; the strain energy density criterion is the least satisfactory. The criteria of maximum tangential strain and strain energy density can be modified to give a good prediction of critical load.

Fatigue crack growth from sharp notches

Abstract Notch-like stress raisers occur widely in engineering components. They are preferred sites for crack initiation when the components are subjected to cyclic loadings. Thus the growth of cracks initiated from notches is very relevant to design against fatigue failures. Schematic models proposed to explain the departure of notch crack growth from linear elastic fracture mechanics predictions are briefly reviewed. Different methods of measuring crack closure are compared. It is found that the commonly employed notch-mouth clip-gauge method is not sensitive enough to detect the closure of short cracks in regions of notch plasticity. Various mechanics parameters have been claimed to be able to bring the notch crack and long crack growth rate data to a single base. In the present work on double-edge notched AISI 316 stainless steel specimens, it is found that none of them is able to correlate satisfactory all the experimental data.

Crack closure - is it just a surface phenomenon?

Abstract The fatigue crack closure response in the plane strain regions of a BS4360 50B steel specimen is evaluated. Compliance measurements using a new type of closure gauge, crack-mouth clip gauge and back face strain gauge show that the fatigue crack is closed for a significant portion (20%) of the load cycle. A sectioning technique is used to confirm that the fatigue crack is closed along the whole crack front, at zero load.

THE MEASUREMENT OF FATIGUE CRACKS AT SPOT-WELDS

Abstract Current fabrication changes in the automotive industry have stimulated interest in fatigue avoidance at spot-welded joints. Basic endurance data are available only for specific specimen types; a generalized fracture mechanics approach has been impeded by lack of crack propagation data. This paper describes a direct current potential difference technique for the continuous measurement of crack dimensions in spot-welded mild steel specimens under fatigue loadings. Initial results indicate that macrocrack propagation is the dominant fatigue mechanism for specimens with lives of the order 2 × 10 6 cycles.

Measuring fatigue cracks in fillet welded joints

Abstract Accurate measurement of short ( Preliminary results have indicated the relative importance of stress ratio, defect size and material variation on the growth of these short elliptical cracks. When the weld toe is subject to high stress ratios the phenomenon may be considered propagation dominated whereas low stress levels increase the influence of threshold and initiation mechanisms.

Upper and lower bounds for the lengths of non-propagating cracks

Abstract The problem addressed is one of deciding whether or not a crack of known length, growing from a notch root, will stop or continue to failure. A simple theory employing upper and lower bounds for the lengths of non-propagating cracks is proposed. The method, which uses the minimum amount of material data, is shown to work for a wide variety of geometrical shapes. The present theory might be used as the first step towards deciding the future of a cracked component.

Fatigue life prediction of a structural steel under service loading

Abstract Fatigue life prediction techniques for variable amplitude load histories are reviewed. The fatigue crack growth rate and crack closure responses of BS4360 50B steel are determined for a service load history experienced by a gas storage vessel. Crack propagation rates are found to be independent of specimen thickness. Crack growth is successfully predicted by linear summation using the Paris law; no significant improvement is achieved by incorporating crack closure into the analysis. The particular choice of cycle counting technique is also found to have an insignificant effect on the predicted fatigue life. The load-interaction model proposed by Willenborg et al correctly indicates the absence of retarded growth, whilst the Wheeler and Fuhring models erroneously predict retarded crack growth.