Determination of plastic collapse load of pre-cracked Small Punch Test specimens by means of response surfaces

In a previous research paper, a new methodology was developed to estimate the fracture toughness of 15.5PH stainless steel, one that combines the Failure Assessment Diagram (FAD), a procedure for evaluating the structural integrity of cracked components, along with the load–displacement curve obtained from pre-cracked specimens using the Small Punch Test (SPT), a non-standard test that is commonly used when there is not enough material for conducting standard tests in order to obtain the mechanical and fracture properties of materials. One of the steps in this methodology is the determination of the plastic collapse load PU of the pre-cracked specimen by carrying out a complicated elastic–plastic finite element analysis (FEA). In order to avoid the FEA in this step, an expression for obtaining PU in pre-cracked SPT specimens was developed in this paper. This expression can be used without the need for numerical simulation and is calculated by applying response surface techniques based on the design of experiments for materials whose behavior can be described by a Ramberg–Osgood type equation. The expression yields the plastic collapse load as determined by the specimen dimensions and the material’s elastoplastic parameters and encompasses the typical behavior range of steels.