Research

Dynamic thermomechanical testing

The dynamic mechanical properties are characterized under tension, compression and dominant shear in our laboratory, at strain rates of up to 50.000s-1. This is achieved, thanks to our sets of Kolsky (split Hopkinson bars), for compression (3 sets, including a miniature 6mm diameter bar), and a tension bar. Shear dominant response, including shear localization studies, is carried out using the SCS (shear compression specimen), co-developed with Prof. G. Ravichandran and Dr. S. Lee (Caltech, USA). Various materials are (have been) investigated, including steels (1018, maraging), Ti and its alloys, Aluminum alloy, OFH Copper, and various polymeric materials as well.
Our investigations include the characterization of transient temperature changes, as a result of various thermomechanical couplings active during the high rate loading. This is achieved thanks to our 8 channel infrared radiometer (liquid nitrogen cooled linear MCT array). Emphasis is put on measurement of the Taylor-Quinney factor in metals and polymers.

Dynamic fracture properties of materials

Our dynamic fracture studies emphasize the subject of dynamic crack initiation and the development of fracture criteria.
We have developed specific tools and techniques to assess the dynamic fracture toughness (KId) of a variety of metallic and polymeric materials. Our expertise relies on specifically developed specimens (e.g. CCS, Compact Compression Specimen, co-developed with Dr. H. Maigre (Ecole Polytechnique/ INSA Lyon France) and techniques, such as one-point impact. The one-point impact technique has been extended and coupled to numerical simulations and high speed photography. As of today, we routinely use it to characterize the impact fracture toughness and the dynamic tensile strength of brittle materials (A. Belenky).
We have also developed a framework for the testing of dynamic mode II fracture, based on one-point-impact testing, with emphasis on the failure mode transitions and shear localization associated with thermomechanical couplings. Here too, several experiments have been accompanied by real time monitoring of the crack-tip temperature.

Dynamic behavior of soft matter

We have an ongoing research project on the characterization of the dynamic mechanical properties of gels (ballistic gelatin) with emphasis on the mechanical strength and attenuation properties. Y. Rothbaum investigates the behavior of inverse freezing gels for his PhD research, in collaboration with chemists (Dr. G. Garvari and prof. Y. Eichen).

Scaling structural response for large close-range explosions

Structural response characterization in the case of large scale, close-range explosions is a complex problem which requires large-scale costly experiments. Together with Dr. A. Neuberger and Dr. S. Peles, we have developed a methodology for scaling down the problem while preserving the reliability of the results. We are now interested in the final failure processes and their potential scalability.

Dynamic shear localization

An active subject of investigation over the past 6 years, for which significant progress has been achieved with the help of Dr. Z. Wang, Dr. A. Merzer, Dr. A. Venkert, Dr. P. Landau and S. Osovski. This failure mechanism is extensively characterized in its microstructural aspects, the latter being introduced into dedicated finite element simulations whose goal is to improve material impact toughness. Additional research at the atomic scale is under current development.

Hysteretic heating of polymers

Cyclically loaded polymers tend to develop self-heating leading to ultimate failure. We are investigating this phenomenon for the case of relatively high cyclic stresses in PMMA and PC. The evolution of the temperature is continuously monitored as well as the dissipated mechanical energy. A simple numerical model can be used to calculate the temperature rise by converting the dissipated mechanical energy into heat.

Non destructive testing and Structural Health Monitoring

Structural reliability is of interest to our group. Together with Dr. H. Saguy, we have extended the Potential Drop technique to its AC configuration (ACPD). As a result, we can now characterize the size and location of a hidden flaw in a electrical conductor with a good accuracy, this providing complementary data to that obtained using traditional techniques. Our new ACPD approach has been patented.
The degradation of end conditions in a structure is of primary concern to maintenance engineers. Together with Dr. B. Karp and Prof. D. Durban, an evanescent-wave methodology has been developed to characterize those unwanted changes in structural members. This methodology has been patented.

Ultra high strain rate collapse of thick walled cylinders

The ultrahigh strain-rate collapse of thick walled cylinders is provoked by an electromagnetic device which produces extremely short intense pulses. The material experiences strain-rates in the range of 105-106s-1. This work, headed by Z. Lovinger, provides unique information about spontaneous nucleation of multiple adiabatic shear bands leading to final collapse of the cylinder.

Ballistic properties of lightweight transparent materials

Our helium gas gun allows launching of slender projectiles at velocities of up to 350 m/s. Those projectiles impact and sometimes perforate lightweight transparent polymers. The experiments are filmed and modeled numerically (Dr. A. Dorogoy) in order to better comprehend the applicability of such materials to protection systems. This work is the continuation of the thermomechanical characterization of those materials at high strain-rates.

Failure of dental implants

This subject is investigated by Dr. K. Shemtov-Yona. In addition to the classical metal fatigue considerations, emphasis has been recently put on the potential influence of the saliva environment and its PH on the mechanical degradation and failure of dental implants.

Numerical modelling

Numerical modelling is underlying every experimental research topic in our group. Dr. A. Dorogoy is our specialist and aside from his own research work, he advises almost every graduate student in the group regarding numerical issues.