Substantiation of the predicted fatigue curve for structural elements of aluminum alloy

Abstract

The paper provides a more specific definition of the method for calculating fatigue life of aluminum alloy structural elements by a local stress-strain state in pulsating regular loading. The calculated results are compared to the test data for specimens with a free and filled holes and for lugs. Recommendations on the method application for calculating fatigue life by nominal stresses are given.Introduction. The method for calculating fatigue life by nominal stresses [1] is based on experimental fatigue curves for specimens with a free hole. These specimens with a ratio between the width and hole diameter B d = 6 have been standardized in aircraft industry. The stress concentration factor in elastic deformation is 2.6 for net stresses and 3.12 for gross stresses. The gross stresses are found without allowing for weakening of the specimen cross section near the hole. The hole diameter depends on the fasteners to be used and is usually 6 to 8 mm. Standard plate specimens were tested by means of a testing machine with pulsating loading. The fatigue curve plotted by the test results is called the basic curve. The fatigue life of structural elements is calculated by using an effective stress concentration factor which is determined either experimentally or by empirical relations based on test results for specimens with a free hole or for lugs. In specimens with a hole, the stress concentration is due to the stresses that pass round the hole. For lugs a contact problem of interaction between the bolt and the specimen is realized; a load is transmitted to the specimen via bearing stresses, the maximum stresses arise at the contact arc break points. The tests are performed until the specimen fails, and a relation between the number of cycles and the maximum nominal stress is found. The fatigue curves are approximated by exponential equations.The method of the fatigue life calculation by a local stress-strain state (SSS), which is based on the energy criterion for fatigue failure [2][3][4], makes it possible to theoretically predict the fatigue life till the formation of microcracks on the hole surface and macrocracks in the stress concentrator. For input data on the material properties this method uses cyclic deformation and fatigue characteristics as determined from the test results for smooth specimens.The objective of the present work is to specify the method for calculating fatigue life of aluminum alloy structural elements by the local SSS under pulsating regular loading and to provide recommendations for the method application to fatigue life calculation by normal stresses.The Method for Fatigue Life Calculation by Local SSS. The analysis for the pulsating regular loading case consists in the following.1. The Calculation of Local Notch-Tip Stresses and Strains at Maximum Nominal Stress s n max in a Loading Cycle. The local stress can be found through the solution of a nonlinear deformation problem by the finite-element method or by the refined Neuber formula [5]