A pulsed laser loading technique for controlled dynamic loading of nanostructred materials

摘要

This thesis presents an improved laser loading technique for controlling and calibrating the stress waves created from this ultra-high loading rate technique. In the high-power pulsed laser loading technique, a laser produced compressive stress pulse passes through a substrate and reflects at a traction free surface as a tensile wave of opposite sign and direction. During the reflection process the traction free surface moves out-of-plane, allowing for measurement of surface displacement that can be related to the stress wave profile through the use of one-dimensional wave mechanics. In this work we improved upon this traditional pulsed laser loading technique by adding a set of optical components that can control the laser power deposited onto the sample down to 2% (of full power) increments. We also performed parametric studies that precisely quantified the resulting laser loading pulse based on experimental parameters such as the energy absorbing and the confining layer thicknesses. (Both these parameters are aspects of the layered substrate sample used to generate the stress wave.) Through the testing of precisely manufactured calibration samples with aluminum energy absorbing layers of 500 nm and waterglass confining layers of 7.62 ??m, which were found to be the best combinations of these thicknesses, stress wave profiles are obtained over a nearly continuous laser energy spectrum at the 2% power increments. Repeatable stress wave profiles showing consistent rise times and decay times were obtained at the selected energy levels. The stress wave profile peaks that were measured ranged from 1101.3 MPa ?? 270 MPa to 1731.3 MPa ??237 MPa, as the laser power varied from 30% to 50% of peak. Below 30% laser power no analyzable signal was obtained, and above 50% power the Si substrates failed at every loading. With the improved methodology developed in this work it will be possible to employ the laser loading power as a calibration for the resulting stress wave in each case, and therefore use calibrated curves as input to numerical simulations.