Three-dimensional Fatigue Crack Growth Under Spectrum Loading

Fatigue crack growth life assessment of cracked structures under spectrum loading is a key task of damage tolerance analysis, and is not solved so far. Fatigue crack growth usually shows strong three-dimensional effect, three-dimensional stress state near the crack tip affect fatigue crack closure and growth deeply. Based on the theory of three-dimensional fatigue fracture, the following points about fatigue crack growth behavior have been done:1. For the fatigue crack growth rate testing curve, which is used for fatigue crack growth life prediction, depends on the stress ratio and specimen thickness, this paper processes the fatigue crack growth rate data using the three-dimensional fatigue crack closure model under constant amplitude loading, and obtain the objective fatigue crack growth rate benchmark curve of the material.2. Based on the theory of three-dimensional fatigue fracture, this paper presents a three-dimensional fatigue crack growth life model under spectrum loading. It is a crack closure model based on the Dugdale model, which used for plane stress state is extended to general three-dimensional stress state. So the model can reflect the three-dimensional stress state near the crack tip on fatigue crack closure and growth.3. Using the fatigue crack growth life model under spectrum loading, this paper simulates the fatigue crack growth life of the 7475-T7351 aluminum alloy center crack tension specimens under variable amplitude loading, the test results carried out by CSAA's discussion about fatigue life model, the fatigue crack growth life of 2024-T3 aluminum and 7075-T6 aluminum alloy center crack tension specimens under the TWIST spectrum, as well as the fatigue crack growth life of 2024-T3 aluminum alloy center crack tension specimens under the FALSTAFF spectrum, most of the prediction results agree very well with the test results. In addition, this paper also simulates the fatigue crack growth life of 1.6 mm and 3.1 mm thick 2024-T3 aluminum alloy center crack tension specimens under the TWIST spectrum using the fatigue crack growth life model under spectrum loading, the prediction results show that the present model based on the theory of three-dimensional fatigue fracture has the capacity to predict the thickness effect of fatigue crack growth.