Low Cycle Fatigue Study Of Double-side Cracked Hull Plate Based On Closure Effect

Ships often encounter harsh sea conditions during navigation.This results in a more pronounced stress concentration phenomenon in the local structure of the hull,which causes the local material to quickly enter the plastic state,which is prone to crack initiation and crack growth,leading to low-cycle fatigue failure.The large-scale development of ships has led to an increasing load on the hull structure,and the problem of low-cycle fatigue strength has become increasingly prominent,which has become a key problem to be urgently addressed.At present,the researches on lowcycle fatigue crack growth mostly focus on single-side crack,and rarely consider more complicated and more dangerous double-side cracks.The research that simulates double-side cracks growth is based on the extended finite element method（XFEM）,but XFEM cannot capture the cumulative plastic deformation and residual stress during the expansion process,nor can it well consider the plastically induced crack closure（PICC）.PICC is widely used to analyze the low-cycle fatigue expansion law of plastic reinforced materials.In this dissertation,the effects of cumulative plasticity are considered.Based on the theory of crack closure,the low-cycle fatigue propagation law of AH32 rigid double-side cracked hull plates and the main influencing factors are discussed in detail.The main research work is as follows:（1）The research status of low-cycle fatigue crack growth is summarized,and related theories are systematically explained,which lays a theoretical foundation for subsequent work.（2）A low-cycle fatigue crack dynamic growth model was established using the finite element software ABAQUS,and the Crack Tip Opening Displacement（CTOD）changes at different initial lengths of double-side cracks were discussed.The relationship between the double-side crack growth rates is analyzed,and the influence of the plate thickness and stress ratio on the double-side crack growth rates is discussed.The research shows that when the initial crack lengths of the double cracks are large,the short crack growth can be ignored,and only the long crack growth is considered.（3）The change of fatigue parameters such as closing parameters,residual compressive stress,and cumulative plasticity near the crack tip under different stress ratios and crack lengths was studied,and compared with the case of single-side cracks,the inherent nature of low-cycle fatigue in double-side cracks was revealed.（4）Based on the theory of cumulative plastic damage,the low-cycle fatigue life of double-side cracks under constant amplitude cycling was predicted.The effect of stress ratio on life was analyzed.The results show that the larger the stress ratio is,the larger the double-side crack life is.The double-side crack fatigue life are more sensitive to stress ratio changes than single-sided cracks.（5）Analyzed the change of double-side crack closure parameters and crack tip stress field under different overload ratios and compared with single-side cracks.The research shows that the double-side crack overload hysteresis is more pronounced and the tensile stress near the crack tip is greater at moments of overload.（6）The influence of welding residual stress on double-side crack closure parameters is analyzed.The results show that the welding residual stress has a greater effect on double-side cracks,and the welding residual stress release of double-side cracks is slower.