Effect of constraint on cyclic plastic behaviours of cracked bodies and the establishment of unified constraint correlation

Constraint effect existing in the cracked structures plays a vital role in monotonic loading conditions and affects the material fracture behaviour, it is not clear whether the constraint affects the cyclic plastic responses in the cyclic loading condition. In this research, the laboratory compact tension (CT) and central-cracked tension (CCT) specimens with different in-plane and out-of-plane constraints were selected to explore the constraint effect on the ratchet limit and cyclic plastic behaviour numerically by the Linear Matching Method (LMM). The results show that the plastic shakedown load domain and low cycle fatigue (LCF) life are compressed dramatically by increasing the in-plane constraint effect, while the out-of-plane constraint effect elevates the capability to resist ratcheting failure. In addition, the ratcheting boundary of the cracked specimen is much more sensitive and vulnerable under the influence of the high in-plane constraint effect. Moreover, the unified constraint parameter A _p is suitable to measure the strength of compound constraint under cyclic loading conditions, and there is a salient linear relation between constraint parameter A _p and cyclic plastic responses (including ratchet limit and alternating plastic strain range) of the cracked specimen. This correlation is very meaningful for evaluating the ratchet limit and low cycle fatigue (LCF) life of cracked structures in terms of different levels of constraint conditions (including different in-plane, out-of-plane, and compound constraint conditions), where the constraint effect is calibrated based on the unified constraint parameter A _p.