加载参数对2A12-T4铝合金高温多轴疲劳失效的影响

Effects of Loading Parameters on Multiaxial Fatigue Failure of 2A12-T4 Aluminum Alloy at High Temperature

关键词:  铝合金  热载荷  力学性能  裂纹  断裂  疲劳试验

Key words:  Aluminum alloy  Heat load  Mechanical performance  Crack  Fracture  Fatigue test

摘要:

为研究铝合金材料的高温多轴疲劳失效规律，在175℃温度环境下对2A12-T4铝合金实心圆棒试样进行拉扭复合加载试验。在相同的等效Von-Mises应力幅值下，通过观察并记录不同加载循环下的裂纹萌生与扩展情况，研究加载参数（应力幅比λ与相位差φ）对裂纹萌生及扩展影响。试验结果表明，当试样表面存在最大切应力平面时，裂纹优先在最大切应力平面附近萌生并传播，最大切应力平面为危险平面；在λ=0.5，φ=90°这种特殊情况下，试验表面各处切应力相同时，裂纹优先在最大正应力平面附近萌生并传播，最大正应力平面成为危险平面，当正应力主导裂纹传播时，由裂纹萌生至疲劳断裂这一过程仅占总寿命的16.2%，容易发生快速断裂；在λ=0.5，φ=0°和λ=0.5，φ=90°这两种加载条件下，裂纹的传播过程中存在第I阶段向第II阶段转变的过程，断口平整并且存在脆性条纹，断口表现趋向于脆性断裂；当λ=1，φ=90°和λ=3，φ=0°这两种加载条件下，没有明显的第I阶段向第II阶段的转变，断口存在明显的疲劳源区、扩展区和瞬断区，且凹凸不平，为典型的混合型断裂。

Abstract:

In order to study the multiaxial fatigue failure law of aluminum alloy at high temperature， tension-torsion fatigue tests on 2A12-T4 aluminum alloy round bar specimens were conducted at 175℃.Under the same equivalent Von-Mises stress amplitude， the cracks initiation and propagation under different loading cycles were observed and recorded， and the effects of loading parameters（amplitude stress ratio λ and phase angle φ） on the cracks initiation and propagation were studied.The results show that the maximum shear stress plane was a dangerous plane if it existed， and cracks initiated and propagated preferentially in this plane. When λ=0.5， φ=90°， the shear stress was the same everywhere on the test surface， the crack initiated and propagated preferentially near the maximum normal stress plane， which became the dangerous plane. When the normal stress dominated the crack propagation， the process from crack initiation to fatigue fracture only accounted for 16.2% of the total life， which was prone to generate rapid fracture. When λ=0.5， φ=0°and λ=0.5， φ=90°， there was an obvious transition from stage I to stage II in the process of crack propagation，the fracture tended to brittle fracture with brittle stripes and flat surface. When λ=1， φ=90°and λ=3， φ=0°， there was no obvious transition from stage I to stage II， the fracture surface was uneven， which was a typical mixed fracture， with visible fatigue source region，propagation region and final rupture region.