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We have constructed a facility for high-rate mechanical testing of materials based on the Kolsky bar geometry. Unlike ordinary facilities that operate only at room temperature, the NIST Kolsky bar facility is equipped with a pulse heating system that can heat the specimen under precisely controlled conditions to high temperatures. In addition, we are adding a three-dimensional strain mapping system to capture complicated deformation of test specimens during tests lasting less than a millisecond.
The mechanical properties of materials often change dramatically depending on the rate they are deformed. Developing models for the strength and deformation behavior at high strain rates requires accurate measurements of the parameters that make up those models.
The NIST Kolsky bar facility is unique in the nondefense research world. Like other similar facilities, it is capable of imposing strain rates up to 10,000 s-1. In addition, it is capable of heating the specimen resistively at rates up 2000 K/s, and then controlling the specimen temperature to ±1 K for the time necessary to complete the compression test. The system uses a near infrared thermal camera to image the specimen for temperature measurement. The high heating rates makes it possible to test nonequilibrium structures, such as are necessary for machining studies, in ways that would be impossible with a conventional mechanical test machine and furnace combination.
A new addition to the facility is a 3-d optical strain mapping system for imaging the specimen during the test. This system, when fully operational, will add two important capabilities to the Kolsky bar system. It will make it possible to measure the strain directly in the deforming specimen. The current system computes the stress and strain behavior from the shapes of the strain pulses recorded on the compression bars. Secondly, it will make it possible to test specimens with complicated geometries. Not only will this capability reduce the strain measurement error for cylindrical specimens, but it has already proven useful in analyzing the deformation of bullets, in support of the research on soft-body-armor standards.