Study of residual stresses around cold-expanded holes

• Main Author: Amjad, K.
• Other Authors: Patterson, Eann ; Wang, Wei-Chung
• Published: University of Liverpool 2018
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.755575

Split sleeve cold expansion is one of the most widely used methods in the aerospace industry to enhance the fatigue performance of fastener holes in airframe structures. The initial motivation, which led to this research programme, was to develop an understanding of the behaviour of fatigue cracks emanating from cold-expanded holes, with a particular emphasis on the influence of these cracks on the surrounding compressive residual stresses. There are two strands of the research presented in this thesis: first being related to the study of hole deformation resulting from split sleeve cold expansion; and the second one focused on the fatigue behaviour of cracks emanating from cold-expanded holes. The strain fields developed from cold expansion were measured using stereoscopic digital image correlation (DIC) technique in aluminium specimens of two different thicknesses giving thickness to hole diameter ratio of 0.25 and 1. The capability of DIC in providing full-field strain data was exploited to determine the shape and size of the plastic zones developed from cold expansion. The results showed that the existing split sleeve cold expansion process is not as effective in creating an axisymmetric compressive residual elastic stress zone around the fastener holes in thin as it is in the thick specimens. The thin specimens used in this investigation were equivalent in thickness to sheet material commonly used in an aircraft fuselage or wing skins and the results indicate that there is a need to review the use of cold expansion process using a split sleeve and mandrel for holes in thin sheets. A simple approach utilising DIC was presented to analyse the strain fields resulting from cold expansion in stacked specimens. The results showed that stacking offers some improvement in the cold expansion of thin sheet components. They also demonstrated the workability of this approach which can be applied effectively to analyse cold expansion of fastener holes associated with a real joint configuration in an airframe. The propagation of fatigue cracks initiating from the cold-expanded holes was investigated by employing the thermoelastic stress analysis (TSA) technique and their influence on the surrounding residual stresses was determined using synchrotron x-ray diffraction (SXRD) technique. A long-standing ambiguity in the literature regarding the potential relaxation of beneficial compressive residual stresses, as a result of fatigue crack propagation, was addressed; and it was established, from TSA and SXRD results, that the formation or propagation of a fatigue crack does not cause any significant relaxation of these residual stresses. The results also clearly identify the loading conditions under which the residual stresses are expected to relax. This information is important in improving the theoretical models for fatigue life assessment of cold-expanded holes. The results should also be useful for the engineers in the aerospace industry to realise the full potential of the cold expansion process and to utilise it more effectively in the manufacturing of airframes leading to improved fatigue endurance under different loading conditions.