Session: 03-01-01: Mechanics of Penetration, Shockwaves, and High-Strain-Rate Events: Modeling and Experiments

Paper Number: 97030

97030 - Improving Low Temperature Signal Quality in Ir Thermography Kolsky Bar Experiments Through Coating Design 

Kolsky bar experiments are widely used to investigate the dynamic mechanical properties of materials. During these experiments, infrared (IR) thermography is often conducted to measure the temperature rise of the plastically deforming material. Combined Kolsky bar and IR thermography data are utilized to assess the efficiency of the conversion of mechanical work to heat. This efficiency is commonly known as the Taylor-Quinney factor. Since most metals and alloys have low emissivity in the infrared range, determining low-temperature rises (<10°C) during Kolsky bar experiments is extremely difficult. Hence, the recorded signal becomes comparatively small for materials such as OFHC Cu 110 and Mg AZ31B. The low IR signal-to-noise ratio will decrease the precision of the calculated Taylor-Quinney factors, particularly those measured during the early stages of plastic deformation. One methodology to increase the emissivity of the surface is to deposit a thin sub-micron thickness coating layer on top of these material systems. The coating should cope with the high ductility of these samples and bond firmly to the substrate as the inertia forces are significant in Kolsky bar experiments. Based on analysis of room temperature emissivity and IR detector sensitivity, a thin layer of pure titanium was selected for deposition to increase the surface emissivity. The high energy of the particles in the sputtering technique provided strong bonding between the coating and the substrate. Also, finite element simulations were implemented to ensure the validity of the temperatures at the coating surface. It was shown that for the 250 nm thickness of pure titanium, the coating does not serve as a thermal barrier. The coating temperature matches the substrate temperature over the short timescales of a Kolsky bar experiment. The calibration experiments revealed that the coating successfully improved the emissivity and IR response of the IR detector. Also, the calibration curves of coated Mg AZ31B and Cu OFHC 110 were nominally identical, which proved the designed Ti thickness was sufficient to create an opaque coating. Three tensile Kolsky bar experiments were conducted on coated Mg AZ31B and OFHC Cu 110 samples, and the Taylor-Quinney factors of these samples were calculated. It was shown that the results were repeatable, and the Ti coating successfully increased the produced IR signal. SEM and EDS analysis on the gauge section revealed that more than 90% of the area was covered with Ti coating for more than 40% strain. Experiments supported by high-speed photography showed the Ti layer does not debond during the first loading cycle, which guarantees the validity of the results.

Presenting Author: Seyyed Danial Salehi University of Utah

Presenting Author Biography: Danial received his bachelor and masters from Iran University of Science and Technology. He continued his research in the field of high strain rate deformation of materials under extreme conditions at the University of Utah. He is currently a PhD candidate at the high strain rate mechanics of materials lab working under the supervision of Dr. Kingstedt.

Authors:

Seyyed Danial Salehi University of Utah
Owen Kingstedt University of Utah