Fracture resistance measurement in small punch test supported by optical monitoring device

Small punch test (SPT) is a conventional technique to determine material properties. Crack tip opening displacement (CTOD) and J-integral methods are two methods applied to pre-notched SPT specimens for fracture resistance estimation. In the CTOD method, the test is interrupted at certain forces less than the maximum force, and then, the specimen is put under microscopes to study the specimen notch status and measure the crack opening. This interruption leads to changes in deformations due to removing of forces. This study aims to remove the interruption problem. To this end, a new setup is introduced to embed an optical microscope under SPT specimens, thereby enabling online monitoring and real-time deformation measurement. This online deformation measurement where the specimen is under force can reduce the error in CTOD measurement. It helps to achieve more accurate results in fracture resistance estimation. To evaluate the method, a few SPT specimens were prepared from an aged compressor named Nevski. Mechanical properties were first conventionally determined by analyzing the SPT force-displacement curves according to standards. Then, prenotched SPT specimens with lateral notches and 4.5 mm and 5 mm notch lengths were employed for fracture energy estimation. Fracture energy was calculated according to the J-integral method that its accuracy is proven. Then, CTODs were measured at various forces using the images taken during the test, and the fracture energy was calculated from the CTOD method. There was a great agreement between the fracture energy calculated by the CTOD method and the proven J-integral method. The accuracy of online measurement was better than interruption measurement in previous studies, proving that the new approach is useful.