Estimating the yield stress of soft materials via laser-induced breakdown spectroscopy

Taking three typical soft samples prepared respectively by loose packings of 77-, 95-, and 109-mu m copper grains as examples, we perform an experiment to investigate the energy-dependent laser-induced breakdown spectroscopy (LIBS) of soft materials. We discovered a reversal phenomenon in the trend of energy dependence of plasma emission intensity: increasing initially and then decreasing separated by a well-defined critical energy. The trend reversal is attributed to the laser-induced recoil pressure at the critical energy just matching the sample's yield strength. As a result, a one-to-one correspondence can be well established between the samples' yield stress and the critical energy that is easily obtainable from LIBS measurements. This allows us to propose an innovative method for estimating the yield stress of soft materials via LIBS with attractive advantages including in-situ remote detection, real-time data collection, and minimal destructive to sample.