Erik E. Nishida

Constant strain rate testing of a G10 laminate composite through optimized Kolsky bar pulse-shaping techniques

Pulse-shaping techniques have been used for many years now in Kolsky bar testing of brittle materials. The use of pulse shapers allow the experimentalist to conduct high strain rate tests on brittle materials while ensuring that the sample will achieve a state of dynamic stress equilibrium before it fails, as well as to achieve a constant strain rate loading state for a large portion of the test. The process of choosing the appropriate pulse-shaper system has typically been one of trail-and-error, sometimes requiring many experimental trails to achieve optimal results. Advances in analytic modeling of Kolsky bar tests now make it possible, in an a priori fashion, to design a pulse-shaper system to produce a known constant strain rate experiment. This article describes the approach of coupling these analytic models to an optimization technique to quickly find a pulse-shaper system that will produce an experiment at a known constant strain rate. Experiments were conducted and the model predictions compared to resulting strain rate histories for a G10 material. Stress–strain curves for G10 are presented at three different strain rates in both the in-plane and out-of-plane loading configurations with respect to the laminate plys. The G10 material is not found to be rate sensitive in either its strength or failure properties.

Comparative Shock Response of Additively Manufactured Versus Conventionally Wrought 304L Stainless Steel.

Gas-gun experiments have probed the compression and release behavior of impact-loaded 304L stainless steel specimens that were machined from additively manufactured (AM) blocks as well as baseline ingot-derived bar stock. The AM technology permits direct fabrication of net- or near-net-shape metal parts. For the present investigation, velocity interferometer (VISAR) diagnostics provided time-resolved measurements of sample response for one-dimensional (i.e., uniaxial strain) shock compression to peak stresses ranging from 0.2 to 7.0 GPa. The acquired wave-profile data have been analyzed to determine the comparative Hugoniot Elastic Limit (HEL), Hugoniot equation of state, spall strength, and high-pressure yield strength of the AM and conventional materials. The possible contributions of various factors, such as composition, porosity, microstructure (e.g., grain size and morphology), residual stress, and/or sample axis orientation relative to the additive manufacturing deposition trajectory, are considered...

EXPERIMENTAL EVALUATION OF LOW-PASS SHOCK ISOLATION PERFORMANCE OF ELASTOMERS USING FREQUENCY-BASED KOLSKY BAR ANALYSES

ELASTOMERIC MATERIALS ARE USED AS SHOCK ISOLATION MATERIALS IN A VARIETY OF ENVIRONMENTS TO DAMPEN VIBRATIONS AND/OR ABSORB ENERGY FROM EXTERNAL IMPACT FOR MINIMIZING ENERGY TRANSFER BETWEEN TWO OBJECTS OR BODIES. SOME APPLICATIONS REQUIRE THE SHOCK ISOLATION MATERIALS TO BEHAVE AS A LOW-PASS MECHANICAL FILTER TO MITIGATE THE SHOCK/IMPACT AT HIGH FREQUENCIES BUT TRANSMIT THE ENERGY AT LOW FREQUENCIES WITH MINIMAL ATTENUATION. IN ORDER TO FULFILL THIS REQUIREMENT, A SHOCK ISOLATION MATERIAL NEEDS TO BE CAREFULLY EVALUATED AND SELECTED WITH PROPER EXPERIMENTAL DESIGN, PROCEDURES, AND ANALYSES. IN THIS STUDY, A KOLSKY BAR WAS MODIFIED WITH PRE-COMPRESSION (UP TO 15.5 KN) AND CONFINEMENT CAPABILITIES TO EVALUATE LOW-PASS SHOCK ISOLATION PERFORMANCE IN TERMS OF ACCELERATION ATTENUATION THROUGH A VARIETY OF ELASTOMERS. THE EFFECTS OF PRELOAD AND SPECIMEN GEOMETRY ON THE LOW-PASS SHOCK ISOLATION RESPONSE WERE ALSO INVESTIGATED.

A Novel Torsional Kolsky Bar for Testing Materials at Constant-Shear-Strain Rates

Kolsky bars, also known as split-Hopkinson bars, have been widely used in the dynamic characterization of engineering materials for over 50 years. Kolsky bars can be made to test materials in compression, tension, or torsion, and until recently, have been generally employed in the testing of high-impedance ductile materials to collect rate-dependent stress-strain data during large-strain inelastic flow. The advancement of “pulse-shaping” techniques in the last decade has allowed Kolsky bars to be utilized for testing low-impedance and brittle materials as well. Pulse-shaping is a processes of tailoring the dynamic loading during a test, with consideration given to the material being tested, in order ensure that the sample achieves a state of dynamic stress equilibrium and constant strain-rate if desired. The most common design of torsional Kolsky bars currently in widespread use offer no way to incorporate pulse-shaping. This limits their use mostly to high-impedance, large-strain applications. A novel torsional Kolsky bar design is presented in this work, which allows for straightforward pulse-shaping, similar to the method employed in compression testing, that can be used to test brittle and low-impedance materials as well as to design experiments that ensure the sample is undergoing a constant-shear-strain-rate deformation. Details of the design as well as some preliminary data demonstrating the pulse shaping capabilities collected during tests are presented.

A Priori Pulse Shaper Design for Constant-Strain-Rate Tests of Elastic-Brittle Materials

Pulse shaping techniques have been used for many years now in Kolsky bar testing of brittle materials. The use of pulse shapers allow the experimentalist to conduct high-strain-rate tests on brittle materials while ensuring that the sample will achieve a state of dynamic stress equilibrium before it fails, as well as to achieve a constant-strain-rate loading state for a large portion of the test. The process of choosing the appropriate pulse shaper system has typically been one of trail-and-error, sometimes requiring many experimental trails to achieve optimal results. Advances in analytic modeling of Kolsky bar tests now make it possible, in an a priori fashion, to design a pulse shaper system to produce a known constant-strain-rate experiment. This paper describes the approach of coupling these analytic models to an optimization technique to quickly find a pulse shaper system that will produce an experiment at a known constant-strain-rate. Experiments were conducted and the model predictions compared to resulting strain-rate histories for a G-10 material.