Critical examination of experimental data on strain bursts (pop-in) during spherical indentation

Pop-in during indentation testing is a term used to indicate the sudden displacement burst during loading. Experimental data are measured during an indentation pop-in event, using displacement sensors with 20 µs time constant at 100 kHz data acquisition rate. The load–depth response during the pop-in event that occurs within 160 µs is determined after accounting for the instruments’ dynamic response. Unlike the response reported in the literature for force-controlled tests, wherein the load on the sample remains constant during the pop-in, a steep load drop is observed after the onset of pop-in, followed by a significant increase in the load well beyond the load at the onset of pop-in. A model for the material and instrument’s dynamic response is presented that agrees well with the experimental observations. The implications of these findings for determination of pop-in length or velocity and for performing displacement-controlled testing involving closed loop control are discussed.