Determination Of Fracture Toughness Parameter Form Small Punch Test Using Inverse Finite Element Method

The fracture toughness is most important reference index in safety assessment, life prediction of pressure equipment, and risk assessment of pressure equipment containing defects. Conventional mechanical test specimens require a large number of material and cannot be carried out on in-service pressure equipment. The specimen of small punch test (SPT) is much smaller than conventional one, and can be sampled from the surface of pressure equipment. Repair is not acquired after sampling. Many mechanical properties parameters of the material can be determined from the load-displacement curve of test. SPT makes up the deficiencies of conventional mechanical properties test. In this paper, an innovative approach, inverse finite element method (IFEM), is proposed to deal with the load-displacement curve. The procedure of IFEM can be divided into3steps. Step1, a database containing a variety sets of material parameters and their corresponding load-displacement curve will be built using FEM. Step2, a set of matched material parameter of test load-displacement curve will be selected from the database by artificial neutral network (ANN). Step3, a numerical simulation of tensile test and fracture toughness test will be performed and yield strength, ultimate strength, percentage of elongation, percentage of area reduction and fracture toughness JIC will be determined from the result of simulation. Compared with empirical correlation method, IFEM is much more theoretical and needs not to perform a large number of small punch tests and conventional mechanical tests to create a correlation equation between the mechanical property and load-displacement curve. So IFEM is more efficiency and accurate. Meanwhile, this paper takes A106steel for example, mechanical parameters are determined successfully. The deviation of yield strength is6.3%, ultimate strength is1.1%and fracture toughness JIC is23.4%. In addition, the contact pressure distribution during SPT is first studied and a method to define the friction efficient using specimen with central hole is proposed. The concept of central thickness-displacement curve is first proposed which is used to determine the displacement of crack initiation.