Analysis of different techniques for obtaining pre-cracked/notched small punch test specimens

Nowadays, there are standards for determining the mechanical and fracture properties of a material. However, these standards require a sufficient amount of material to be tested, something that is not always possible or convenient. In those cases where there is not enough material for conducting conventional tests to determine these properties of the material analyzed, there are now several non-standard tests that will achieve this purpose. One of them is the small punch test (SPT), which basically consists of deforming a miniature specimen using a high-strength punch, while the sides of the specimen are clamped between two dies. One of the greatest challenges at present is to obtain the fracture properties of a material from this type of test using pre-cracked specimens. To achieve this initial crack in the SPT specimen prior to fracture testing, there are two techniques which are mainly being used at present. The first one uses high-precision micromachining (HPM), while the second relies on laser-induced micromachining (LIM). The main objective of this paper is to analyze the differences between these two techniques, taking into account the shape of the pre-crack obtained and the stress distribution at the pre-crack tip during the test. In this way, it is possible to determine which of them is the most appropriate for estimating the fracture properties of the material used.

► Two techniques used to cut the specimens (HPRD or WEDM) have no influence on results.
► HPM: good surface finish, uniform notch depth. Drawback: blunted notch.
► LIM: sharp realistic crack, resembles behavior at the crack tip of the CT specimens.
► LIM most appropriate technique for estimating fracture properties of material used.