Strength Analysis Method Research Of Cracked Ship Structure

Crack damage is one of the inevitable damage forms in the large welding structure in ship and offshore engineering. The safety assessment for cracked damage component is quite important in ensuring the structural safty operation. It is necessary that the performance parameters of crack damage strength:crack-tip stress intensity factor, crack-tip plastic zone size and crack-tip opening displacement and the performance parameter of structure's fracture resistance:fracture toughness, are clearly mastered to accurately evaluate the state of cracked components. In this paper, based on theoretical analysis and finite element numerical calculation, the relationships between maximum crack opening displacement and the performance parameters of crack damage strength are established, respectively, and the method to determine fracture toughness of steel plate based on the absorbed energy from impact test is established. These efforts provide the basis for the evaluation of structural damage strength.The major contents and conclusions are surmmarised as follows:(1) For the surface cracked finite plate model, a simple method to calculate stress intensity factor at the crack tip of crack depth was proposed. Based on theoretical analysis, the function relationship between maximum crack opening displacement and stress intensity factor was determined. Finite element numerical calculation is adpoted to consider the influences of several key factors. By analyzing these results, a series of modified coefficients' expressions were fitted. The method is realized to calculate crack depth and the stress intensity factor at the crack tip of crack depth based on maximum crack opening displacement.(2) On the basis of the method to calculate stress intensity factor for the surface cracked plate model, a new method is further proposed to calculate stress intensity factor at the surface cracked weld toe in the complex components. The influences of welded toe sizes and tube curve surface effect on stress intensity factor are considered in the process. The expressions of stress intensity factor related to angel of welded toe, crack depth ratio and related to crack shape ratio, crack depth ratio are finally obtained. A simple and convenient method is thus established to calculate stress intensity factor for the typical plates welding components and the cracked tubular joints based on maximum crack opening displacement.(3) For the through cracked infinite plates, a conventional method is proposed to calculate crack-tip stress intensity factor. The function relationship between maximum crack opening displacement and stress intensity factor was derived firstly by theoretical analysis. The finite element simulation calculation is employed to analyse the influences of several typical factors on the relationship. This method avoids studying the crack-tip stress field and eliminates the effects of model size, crack shape, form of applied load, loading modes and material properties, and is suitable for through cracked finite plate with any material subjected to different load forms.(4) Bsed on the thought of stress intensity factor calculated in the single though cracked model, the interactions of multi cracks are analyzed, and a simple method was proposed to calculate stress intensity factor. In the collinear and parallel double cracks cases, the influences of crack vertical spacing ratio, crack longitudinal spacing ratio and crack lengths ratios on stress intensity factor were considered. The function relationships between crack length ratios, crack space ratios and modified coefficients of stress intensity factors were obtained. Also, the applicalities of modified coeffiecents in the abitrary form of double cracks cases are considered, and the composite method, the stress intensity factors of three and more than three cracks is able to determine.(5) Simple methods to calculate crack-tip plastic zone size and crack-tip opening displacement are proposed. Based on the analysis of elastic-plastic fracture theory, results show that the crack-tip plastic zone size and crack-tip opening displacement are directly related to maximum crack opening displacement, respectively. On the basis of finite element calculation results which considered the effect of several key factors, the dimensionless data processing was adopted to analyse these results, and the function relationships between maximum crack opening displacement and crack-tip plastic zone size or crack-tip opening displacement are established, respectively. The method eliminates the effects of model size, crack length, applied load, yield stress, plane state and material hardening, thus it is suitable for finite plate members with any material.(6) Based on the internal relations between impact toughness and fracture toughness, a method to determine fracture toughness of ship plates with different strength and different quality level was realized according to the absorbed energy of charpy v-notch impact test. The expressions of fracture toughness of ship plates related to steel plate thicknesses and service temperatures were established under different reliabilities, respectively. These expressions provide a basis to quickly calculate fracture toughness of ship plates. Crack arrest properties of combination steels with different quality level steel plates are also taken into account. The two reasonable combinations of A, D and A, E were proposed, which can be a reference to make reasonable arrangements of ship plate with different quality levels.