Some of the fatigue tests performed using the standard compact tension (CT) and a non‐standard specimen made of rolled 7075 aluminium alloy exhibit fatigue crack growth (FCG) lagging in a small region along the crack front. Through‐thickness microstructural evaluation shows that material grains in this region did not flatten as much as other regions. In the non‐standard specimen, surface cracks are either grown under fatigue loading or broken under monotonically increasing quasi‐static loads at different crack sizes. The aforementioned lagging also exists in a narrow region of 3‐D FCG for specimens with microstructural through‐thickness non‐uniformity. A more important feature for this type of specimen with surface crack is the deflection of fast fracture direction into the grain interfaces, namely from L‐T orientation to S‐L and S‐T directions. It is proved that this is due to significant levels of second principal stresses near the free surface for small cracks and lower fracture toughness of the material in S‐L and S‐T directions.

中文翻译：

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显微组织厚度不均匀性和裂纹尺寸对轧制Al-7075合金疲劳裂纹扩展和断裂的影响

使用标准压紧张力（CT）和由7075轧制铝合金制成的非标准试样进行的某些疲劳测试显示，疲劳裂纹扩展（FCG）沿裂纹前沿的一小部分滞后。整个厚度的显微组织评估表明，该区域的材料晶粒不像其他区域那样扁平。在非标准试样中，在不同的裂纹尺寸下，表面裂纹要么在疲劳载荷下生长，要么在单调增加的准静态载荷下断裂。对于微观结构贯穿厚度不均匀的标本，上述滞后现象也存在于3D FCG的狭窄区域中。这类具有表面裂纹的样品的一个更重要的特征是快速断裂方向向晶粒界面的偏转，即从LT方向到SL和ST方向。