Evaluation And Relief Of Residual Stresses In Aluminum Alloys For Aircraft Structures

AL-Zn-Mg-Cu (7000 series) and AL-Cu-Mg (2000 series) high strength aluminum alloys have been widely used as the key materials of aircraft structures due to their high strength-to-weight ratio, excellent corrosion resistance, and high fracture resistance. These precipitation-hardened aluminum alloys gain their high strengths through heat treatment involving a severe quenching operation, which introduces a very high level of residual stresses. Residual stresses not only result in unexpected machining distortion, but also increases the possibility of unpredicted fatigue failures and stress corrosion cracks (SCC). Therefore, It is of interest to investigate residual stress measurement and prediction in aircraft aluminum parts.In this dissertation, systematical investigations are carried out on control and evaluation of residual stresses in aircraft aluminum parts. The main work done and the main results obtained are as follows.First, several residual stress measurement methods were summarized with overview of the history and the current development of residual stress research. The principles of incremental hole drilling method and X-ray diffraction method were analyzed to measure surface or subsurface residual stresses in engineering components. Based on fundamental theoretical relations of liner-elastic fracture mechanics, a novel method known as the crack compliance method was chosen for this research. Its basic theory, principle, and experimental implementation were comprehensively studied. Experimental results revealed that this proposed method can be used to evaluate through-thickness residual stress distributions in rolled 7075 aluminum plates procured in T73, T7351 , T7352 and T7353 temper designations. It further showed that measured subsurface residual stresses are more sensitive and accurate as compared with those obtained by incremental hole drilling method or X-ray diffraction method.Second, the quench-induced residual stresses in rolled 7075T73 aluminum plate and forging of bulkhead were investigated by theoretical analysis and experimentation. The heat equation for a simple geometric model, such as an infinite plate, was solved with an experimental surface conductance and a step-by-step method of determining the temperature field in the thickness of the plate. This temperature field was introduced as data for the uncoupled thermal elastic-plastic model for quenching. In the calculation of the plastic-strain path, the thermal and mechanical properties were considered as temperature dependent for a homogeneous and isotropic material. The quenching process was simulated by finite element analysis to predict the residual stress distributions that develop in aluminum plates and forgings. Experimentally, the through-thickness residual stress distributions in 7075T73 plate specimens were measured by the crack compliance method. Good agreement is found between theoretical predictions and experimental results.Third, we summarized current developments of three stress relief methods include Tx51 (stress relieved by stretching deformation), Tx52 (stress relieved by compressive in a separate die), and Tx54 (stress relieved by combined stretching compressive in a finish die). A systematic study was performed on effect of amount of stretching deformation on stress relief of 7000 series aluminum plates. The effect of...