The effect of artificial ageing on tensile and fatigue properties along with resistance to fracture of aluminium alloy 2198-T3 was investigated. The fatigue and damage tolerance capability of the alloy was studied under different artificial ageing conditions, simulating the natural ageing process which takes place during an aircraft lifespan. Four (4) different ageing conditions corresponding to under-aged (UA), peak-aged (PA) and over-aged (OA) conditions based on the initial, T3 temper were considered. In the PA condition, the conventional yield stress increased by more than 50 % with a simultaneous 46 % decrease in tensile ductility and 44 % decrease in the critical stress intensity factor . The high cycle fatigue results showed that with increasing artificial ageing time, the fatigue life continuously decreased until PA condition, and substantially increased in the OA. Normalized fatigue stress-life diagram over conventional yield stress underlined the suitability of the T3 temper in terms of constant amplitude fatigue testing. Fatigue crack propagation (FCP) characteristics at low Δ levels were superior in T3 temper, while at medium to high Δ values the UA condition was more damage tolerant, due to the presence of higher precipitates volume fraction and crack propagation promotion by precipitate bypassing mechanism. On the other hand, precipitates shearing at the crack path was observed in the PA condition and it was linked with higher FCP rates. Resistance to fracture evaluated by quasi-static tests, showed similar behaviour to the FCP tests, and was found to be worst in the PA condition. A small recovery of all damage tolerance properties was noticed for the OA condition.

中文翻译：

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(Al-Cu-Li)AA2198在不同时效条件下的疲劳和断裂行为

研究了人工时效对 2198-T3 铝合金拉伸性能、疲劳性能以及抗断裂性能的影响。研究了合金在不同人工时效条件下的疲劳和损伤容限能力，模拟飞机寿命期间发生的自然时效过程。基于初始 T3 回火，考虑了四 (4) 种不同的时效条件，分别对应于欠时效 (UA)、峰值时效 (PA) 和过时效 (OA) 条件。在PA条件下，常规屈服应力增加了50%以上，同时拉伸延展性下降了46%，临界应力强度因子下降了44%。高周疲劳结果表明，随着人工时效时间的增加，疲劳寿命持续下降直至PA工况，而在OA工况下则大幅增加。与传统屈服应力相比的归一化疲劳应力-寿命图强调了 T3 回火在恒幅疲劳测试方面的适用性。低 Δ 水平下的疲劳裂纹扩展 (FCP) 特性在 T3 状态下优越，而在中到高 Δ 值下，由于存在较高的析出物体积分数以及析出物旁路机制促进裂纹扩展，UA 条件具有更高的损伤容限。另一方面，在 PA 条件下观察到裂纹路径处的析出物剪切，并且它与较高的 FCP 速率有关。通过准静态测试评估的抗断裂性能，表现出与 FCP 测试相似的行为，并且发现在 PA 条件下最差。注意到 OA 条件下所有损伤容限特性都有小幅恢复。