Determination of Fracture Toughness From Small Punch Test Using Inverse Finite Element Method for In-Service Equipment

Fracture toughness is the most important reference index in safety assessment, life prediction of nuclear reactor, pressure equipment, and risk assessment of pressure equipment containing defects. Conventional mechanical test specimens require a large amount of materials 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 into 3 steps. Step 1, a database containing a variety set of material parameters and their corresponding load-displacement curve will be built using FEM. Step 2; a set of matched material parameter of test load-displacement curve will be selected from the database by artificial neutral network (ANN). Step 3, a numerical simulation of fracture toughness test will be performed and fracture toughness J_ic 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.