New representation of the diagrams of ultimate stresses for the fatigue analysis of aluminum structures
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NEW REPRESENTATION OF THE DIAGRAMS OF ULTIMATE STRESSES FOR THE FATIGUE ANALYSIS OF ALUMINUM STRUCTURES V. I. Kyr’yan and V. A. Shonin
UDC 621.791.052:539.43
We propose a new analytic representation of the diagrams of ultimate stresses for constant fatigue life. The diagrams are constructed according to limited amount of experimental data obtained as a result of fatigue testing of smooth specimens of the metal of welded joints of aluminum alloys. In determining the “hot-spot” stressed state, these diagrams enable us to perform the fatigue analysis of welded aluminum structures with regard for the stress concentration, the level and character of distribution of residual stresses, misalignment in the zone of the joint, and other factors.
The numerical analysis of welded metal structures subjected to the action of alternating loads is based on the experimental characteristics of fatigue resistance of welded joints presented in the form of the diagrams of ultimate stresses for constant fatigue life [1–3]. These diagrams are plotted according to the results of cyclic testing of specimens of the same type simulating structural elements under loads corresponding to the actual operating loads and specified in the form of the nominal maximum ( σmax ) or amplitude ( σ a) stresses for different load ratios ( Rσ ). In this approach to the evaluation of ultimate stresses, all structural and technological factors affecting the fatigue resistance are directly taken into account. As a serious disadvantage of this approach, one can mention the necessity of testing of large numbers of actual welded joints with different values of the stress concentration factor aσ , levels of residual stresses, and their distributions. The experimental and numerical data on the fatigue of welded joints in the intact state are generalized for aluminum welded structures in the new European Code (Eurocode-9) and IIW documents [4]. The computed characteristics of the fatigue resistance of welded joints are depicted in the form of a unified fatigue curve (convenient for applications) as follows: N Δ σm = A, where Δ σ is the maximum amplitude of nominal stresses, N is the fatigue life (cycles), A is a constant, and m is the exponent of the power dependence ( m = 3 or 5). For different types of welded joints, these curves are plotted according to the lowest values of the experimental data independently of the load ratio and denoted as FAT x, where x is the value of Δ σ (in MPa) for a basic fatigue life N = 2 ⋅ 106 cycles. Actually, the indicated fatigue curves take into account the influence of the shape of welded joints and the maximum residual welding stresses. The outlined simplified procedure of evaluation of fatigue resistance inadmissibly underestimates the computed values of stresses and blocks the implementation of new technologies aimed at increasing the durability of joints, e.g., the technology high-frequency mechanical forging of the surface layers of the metal on the boundaries of the weld used to decrease the stress concentration and promo
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