Research On The Mechanical Properties And Damage Mechanism Of Thick Plates Used In Steel Structure

Components fabricated from thick steel plates have been extensively used in large engineering structures because of their light weight and high strength. But owing to the internal defects, brittle fracture may occur to the components in the welding and serving process. During the smelting, rolling and processing, many defects such as non-metallic, micro cracks and micovoids will stay in the thick steel plates. The microcavities become macroscopic cracks with the growth and coalescence of their nucleation resulting from the welding residual stresses or the external loads. These macro cracks weakening the material performance would lead to brittle failure or lamellar tearing. At present, the fracture phenomenon of thick plates has been studied by researchers, but the essential reason of the accumulative damage has not been clearly. In this paper, mechanical properties and accumulative damage of thick plates are studied to reveal the process of defect formation and development to provide data support on damage mechanics of thick plates.Based on the continuum damage mechanics and material tests, damage accumulative process and the mechanical properties of thick plates under static or cyclic loads based are studied. Then, a stratification damage model and a simplified single-layer damage model for thick steel plates are proposed according to the analytical results of the damage accumulative process in thick steel plates. The main content consists of four parts:(1) Develop elastic-plastic constitutive equations in which damage evolution is included. Both the damage evolution and bilinear mixed hardening are included in the constitutive relationship by applying Lemaitre’s damage model. The corresponding user subroutine material program is developed according to the interface UMAT in ABAQUS. Two benchmark examples are analyzed with the proposed model and their results are compared with the experimental results in the literature to verify the proposed model (i.e. reversed loading test of thick aluminum ring and hysteretic test of box-section steel column).(2) Study the mechanical properties of thick steel plates. Four steel plates with60mm,80mm,100mm and120mm thickness are studied. Mechanical properties in different locations along the thickness direction (e.g. outer surface,1/4thickness away from outer surface and mid-depth) and through-thickness direction under static and cyclic loads are experimentally studied. The experimental results show that the yield strength decreases with the increase of plate thickness. Reduction of cross-section area and elongation after fracture decrease with the increase of the distance away from the outer surface and reach the minimum at mid-depth of a plate.(3) Determine the damage parameters of thick steel plates. Based on the elastic modulus method, repeated tensile tests are conducted and damage parameters of the specimens are obtained. Since the elastic modulus method can only provide a mid value of the damage parameters and the obtained parameters need to be corrected, an approach is proposed to verify and correct the damage parameters. All the parameters are determined by the numerical simulation of monotonic tensile tests. The obtained damage parameters show that damage accumulates slower in horizontal locations than in through-thickness direction; damage accumulates slower at outer surface and1/4thickness in horizontal locations than at mid-depth.(4) Study the damage mechanics in thick steel plates. Damage accumulative process of specimens under cyclic loads is studied and the sensitivity of damage parameters is also analysed. Based on the mechanical properties of thick plates, the stratification damage finite element model is developed to substitute the tests of large components fabricated from thick steel plates according to stratification hypothesis of thick plate. At last, in terms of finite element analysis cases, damage parameters from1/4thickness layer are suggested to represent those of the whole plate and then a simplified single-layer damage model is developed, which provides an efficient approach for FE analysis of thick steel plate components.