Technical report | Investigation of Stress Intensity Factor for Overloaded Holes and Cold-Expanded Holes

Abstract

 For life assessment of airframe components, it is important to understand how the residual stresses due to material yielding affect crack initiation or crack growth at holes. A key step in understanding such effects is the quantification of the Mode 1 stress intensity factor for cracks that may initiate subsequent to overloading. Here, a two-dimensional weight function approach is used to determine stress intensity factors for cracks in either tensile or compressive stress fields due to one of three mechanisms: remote tension overload, remote compression overload or hole cold expansion. The effect of subsequent remote loading is also considered. The key input is the stress distribution in the corresponding uncracked body along the prospective crack path. The key trends are investigated through many numerical examples for symmetrically-cracked holes in large steel and/or aluminium alloy plates. Cold expansion of finite-width plates representative of C-130 wing skin locations is also studied. For both remote overload cases, it is shown that, once the crack length is the same or larger than the initial yield zone, the stress intensity factors are the same as for the case without the initial overload. However, for cold-expanded holes, the beneficial reduction in stress intensity factor extends beyond the initial yield zone. Hence, the present work provides a greater depth of understanding of how typical residual stresses can affect key inputs into airframe life assessment.

Executive Summary

Significant work undertaken in Air Vehicles Division involves providing advice on through-life support of Australian Defence Force aircraft. For accurate life assessment of airframe components, it is important to have an understanding of how the residual stresses due to material yielding affect crack initiation or crack growth at holes in metallic components. A key step in understanding such effects is the quantification of the Mode 1 stress intensity factor, for cracks which may initiate subsequent to the overloading. The weight function method is a useful approach to determine Mode 1 stress intensity factors, since the crack does not need to be explicitly modelled. Hence, in the present investigation, a two-dimensional weight function approach is used to determine stress intensity factors for cracks in either tensile or compressive stress fields, due to one of three mechanisms: remote tension overload, remote compression overload or hole cold expansion. An important analysis input is the stress distribution in the corresponding uncracked body along the prospective crack path, obtained from either finite element analysis or theoretical methods.

In the present work, analyses are undertaken to consider key trends and limitations of current prediction methods, with an emphasis on the use of weight function methods. The main areas addressed are as follows. Initially, a detailed literature survey is presented on the use of weight function approaches in the context of three key types of overloads: remote tensile overload, remote compressive overload and local cold expansion. As an example for the generation of stresses along the prospective crack path, finite element analysis of an uncracked hole in a remotely-loaded steel plate is given. Stress intensity factor results for three sets of numerical test cases are then determined. Two of these involve cold-expanded holes in large D6ac steel plates and large aluminium alloy plates. The third test case geometry corresponds to a fatigue test coupon consisting of a finite-width aluminium alloy plate, which is representative of locations on the lower wing skin of the C-130 aircraft. Finally, results and comparison of generic cases for a D6ac steel plate for all three overload cases are given.

For both remote overload cases, it is shown that, once the crack length is the same or larger than the initial yield zone, the stress intensity factors are the same as for the case without the initial overload. However, for cold-expanded holes, the beneficial reduction in stress intensity factor extends beyond the initial yield zone. A set of Fortran computer programs has been written to generate the results in this report. These programs are given on an enclosed Compact Disc for future reference and use. The present work has provided a greater depth of understanding of how typical residual stresses can affect key inputs into airframe life assessment. The use of these techniques can assist in providing more timely and accurate prediction of fatigue life, for Australian Defence Force platforms.