Dj Smith

Effect of residual stress around cold worked holes on fracture under superimposed mechanical load

Abstract Cold working is one method used to enhance the fatigue life of holes in aerospace structures. The method introduces a compressive stress field in the material around the hole and this reduces the tendency for fatigue cracks to initiate and grow under superimposed cyclic mechanical load. To include the benefit of cold working in design the stress intensity factors must be evaluated for cracks growing from the hole edge. Two-dimensional (2D) finite element analyses have been carried out to quantify the residual stresses surrounding the cold worked hole. These residual stresses have been used in a finite element calculation of the effective stress intensity factor for cracks emanating from the hole edge normal to the loading direction. The results of the 2D analysis have been compared with those derived using a weight function method. The weight function results have been shown always to underestimate the stress intensity factor. A three-dimensional (3D) FEA has been carried out using the same technique for stress intensity factor evaluation to investigate the effect of through thickness variation of residual stress. Stress intensity factors calculated with the 3D analysis are generally higher than those calculated using the 2D analysis.

A finite element simulation of the cold working process for fastener holes

Two-dimensional axisymmetric finite element simulations have been conducted for the cold working of a fastener hole in an aluminium plate. The simulation models the actual cold working process where an oversize mandrel is pulled through the fastener hole. The results of the simulation are compared with a simplified finite element model where the cold working process is reduced to applying a uniform radial expansion to the hole edge. It is shown that substantial differences exist between the finite element simulations; specifically, the simulation of the actual process shows tensile residual radial stresses on the surface of the plate after cold working whereas the simplified simulation shows only compressive ones. Further comparisons are made for the axial deformation of the plate by using the results of an experimental measurement of the surface profile around a cold worked hole. There is good agreement between the finite element and experimental results. The results of this work show that accurate simulations of cold working are necessary if predicted residual stresses are to be used to assess fatigue life.

On the consequences of T-stress in elastic brittle fracture

By using a generalized maximum tensile stress (MTS) criterion to predict onset of brittle fracture, it is shown that the presence of T-stress can have a significant effect on mode I and mode II toughness. The most prominent influence of T-stress on toughness occurs for mode II conditions. However, earlier tests concentrated on near mode I and results were masked by scatter. New experiments, using combinations of mode II loading and T-stress, support conclusively the generalized MTS criterion. This criterion is shown to be very robust and applicable to predicting probability of brittle failure. The criterion is also relevant to other experimental results where combinations of mode II loading with high values of T-stress can lead to values of mode II toughness that are greater than mode I toughness.

Fatigue crack growth from plain and cold expanded holes in aluminium alloys

Fatigue crack growth in open holes in aluminium alloys 2024-T351 and 2650 was investigated. Tests were carried out using plates containing plain holes and cold expanded holes in aluminium. The tests explored the influence of the applied stress, the ratio of the minimum to the maximum applied stress, R, and crack closure. Longer fatigue lives of specimens with cold expanded holes were obtained provided that the applied load ratio was less than 0.7, and the maximum applied stress was less than 0.5 of the yield strength. The decrease in fatigue crack growth in cold-expanded specimens was related to higher crack opening stresses which is a consequence of the presence of compressive residual stresses arising from cold expansion. Fatigue crack growth rates were described as a function of an effective stress intensity factor, which was determined using measured crack opening stress. Measured crack opening stress was also compared with opening stress determined from fatigue crack growth rates.

Analytical and finite element predictions of residual stresses in cold worked fastener holes

Abstract Two-dimensional finite element (FE) studies, for plane stress, plane strain and axisymmetric conditions, were conducted to simulate 4 per cent cold working of a 6.35 mm diameter hole in a 6 mm thick plate of 2024 T 351 aluminium alloy. The simulations were used to assess the influence of strain hardening, the role of reversed yielding and through-thickness residual stress distributions. Experiments were also conducted to determine the tensile and compressive stress-strain response of the aluminium alloy, revealing a pronounced Bauschinger effect and non-linear strain hardening in compression. The FE simulations and results from several earlier analytical models were compared and substantial differnces found in the region of reversed yielding. Approximations used to model the compressive deformation behaviour of the material overestimate the compressive residual stresses at the hole edge. From the axisymmetric FE model a residual stress gradient through the plate thickness was found. The plane stress and plane strain assumptions used in the earlier analytical models did not satisfactorily approximate the three-dimensional residual stress fields obtained from the FE simulations.

Measurement of through-thickness stresses using small holes

Previously the deep-hole method was developed to measure residual stresses in thick-section engineering components. In this paper it is demonstrated that stresses arising from external applied loading can also be measured. A series of calibration studies using aluminium and steel samples, with thicknesses varying from 5 to 50 mm, are presented. The samples were subjected to tensions, bending and torsion. It is shown that the deep-hole method can measure linear and non-linear stress distributions.

The effect of cold expansion on fatigue crack growth from open holes at room and high temperature

Abstract In this paper a series of residual stress measurements and fatigue crack growth tests have been carried out using aluminium alloy 2650 specimens containing cold expanded and non cold expanded holes. Residual stress measurements have been done after cold expansion and after various loading and temperature conditions. In order to measure an angular variation of residual stresses, X-ray and a new technique called the Garcia–Sachs method have been employed. Results revealed that residual stress relaxation occurred as a result of exposure at 150°C. The magnitude of relaxation was shown to be dependent on the level and the sign of externally applied load. Fatigue crack growth tests have been carried out at 20°C and 150°C for both cold expanded and non-cold expanded conditions. Fatigue crack growth rates in specimens containing cold expanded fastener holes were affected significantly by elevated temperature exposure. Depending on the exposure time and loading conditions the fatigue life improvement was found to be between one and greater than 10 for tests at 20°C.

An assessment of residual-stress measurements around cold-worked holes

An X-ray diffraction technique was employed to determine the residual stresses introduced by cold working a fastener hole in a 6-mm thick 2024-T351 aluminum plate. The radial and tangenital residual stresses were measured at both faces of the plate and the measurements compared with the results from a two-dimensional axisymmetric finite-element model. The comparisons were favorable, although modifying the finite-element model to simulate the X-ray process provided better agreement. Experimental determinations of residual stresses showed differences between the two faces of the plate. This feature was attributed to the directional nature of the cold-working process.

The significance of prior overload on fracture resistance: A critical review

Abstract The review presented in this paper is limited to prior overload effects in pressure vessels. The incidence of pressure vessel failure is examined, and the effects of prior overloading on residual stresses and fracture toughness are reviewed. It is found that the effectiveness of the mechanical stress relief of residual stresses cannot be guaranteed as the level of relief is dependent on local geometry. Prior overload effects on fracture toughness are divided into proof testing aspects and warm prestress effects. In this context the effects of prestraining, ageing and cyclic loading are reviewed. In warm prestressing it is assumed that the existence of flaws is postulated, and some benefit may be obtained by operating at temperatures lower than the proof test temperature. The empirical evidence of the benefit of warm prestressing is reviewed, and examined in relation to the effects of loading and temperature cycling, and material and geometry effects. The models for predicting warm prestress effects are also reviewed. The models are found to be restrictive in their application, and the effects of warm prestressing cannot be adequately quantified without recourse to experimental work. There is little quantifiable evidence of benefits of proof testing on subsequent behaviour of large-scale pressure vessels. Progress in the last 20 years is considered, and although recommendations have been made in the past, there is little evidence that codes have implemented the more important elements. Finally, recommendations for future work are presented.

Type IV creep cavity accumulation and failure in steel welds

One region of concern for creep damage accumulation in steel welds is a narrow region adjacent to the parent material. This narrow region, called Type IV, consists of partially transformed material having a relatively small grain size. In this region creep arises from crack growth and creep cavitation. The conditions where one mechanism dominates failure are not clear. In this paper, results are presented from an extensive experimental programme using both cross-weld and simulated Type IV specimens, first to determine the mechanisms of strain accumulation in the narrow region, and second to determine creep crack growth characteristics. Creep cavitation occurred across the net section, and the numbers of cavities per unit area were quantified as a function of fractions of rupture life. When sufficient constraint was provided by cross-weld specimens, creep crack growth was promoted in the Type IV region. When constraint was low, the presence of a crack did not promote localised cavity accumulation and creep crack growth. It is suggested that cavitation is a consequence of grain boundary sliding leading to relaxation of constraint and multiaxial rupture governed by the von Mises stress.