Low-Velocity Impact Testing

IMPACT TESTS are used to study dynamic deformation and failure modes of materials. Low-velocity impact techniques can be classified as plate-on-plate, rod-on-plate, plate-onrod, or rod-on-rod experiments. Two types of plate-on-plate impact tests have been developed: wave propagation experiments and thin-layer high-strain-rate experiments. The plate-on-plate experiments are further classified as nonrecovery or recovery experiments. The focus of this article is on plate-on-plate experimental techniques. At the end of this article, rod-on-plate and plate-on-rod experiments are briefly examined. Observation of plane waves in materials provides a powerful method for understanding and quantifying their dynamic response (Ref 1–9) and failure modes (Ref 10–29). Plate impact experiments are used to generate such plane waves (Ref 30–32). These experiments provide controlled extreme stress-state loading conditions, involving one-dimensional stress-pulse propagation. The recovery configurations in plate-on-plate impact experiments are performed with the objective of examining the microstructural changes in the specimen after it is subjected to loading under a uniaxial strain condition. The experiments are designed to achieve a controlled plane-wave loading of the specimens. In practice, this is limited by the finite size of the plates employed, which generate radial release waves. This has the potential for significant contribution to the damage processes by introducing causes other than the uniaxial straining of the material. Hence, this aspect of the plate impact experiment has been a subject of considerable research in the past (Ref 11, 13, 33–39). The plate impact experiments are performed in two main modes: normal impact and pressure-shear, or oblique, impact. Both modes have been specialized to several new configurations to achieve different aspects of control over the imposed loading. In these experiments, the time histories of the stress waves are recorded and used to infer the response of the specimen with the goal of constitutive modeling. To enable the formulation of correct constitutive behavior for the considered material, knowledge of the micromechanisms of deformation that occur during the passage of the stress waves is necessary. Such knowledge is also necessary for damage-evolution studies. Hence, it is important that the specimen is recovered after it is subjected to a well-characterized loading pulse so that it can be analyzed for any changes in its microstructure. This is achieved in the normal plate impact mode by using an impedance-matched momentum trap behind the specimen (Ref 1, 7, 11). Ideally, the momentum-trap plate captures the momentum of the loading pulse and flies away, leaving the specimen at rest. Initially, the recovery technique was developed for the normal plate experiments (Ref 1, 38, 39), and it has been implemented in the pressure-shear mode to study shear stress-sensitive, high-rate deformation mechanisms. The difficulty in conducting pressure-shear recovery experiments stems from the fact that both the shear and longitudinal momenta must be trapped and that there is a large difference in the longitudinal and shear wave velocities for any given material. To overcome this problem, one idea that had been proposed was to use a composite flyer made of two plates of the same material that are separated by a thin layer of a low shear resistance film, such as a lubricant (Ref 40, 41). This design would enable the shear pulse to be unloaded at the interface, while the pressure pulse would be transmitted to the next plate. The pressure pulse would return to the specimen momentum-trap interface as an unloading wave after the unloading of the shear wave has taken place. The thickness of the momentum-trap plate is chosen such that the normal unloading wave from its rear surface arrives at this interface much later, and hence, the momentum trap would separate just as in the normal recovery experiment, but after trapping both the shear and normal momenta. The plate impact experiments can be performed at different temperatures by providing temperature-control facilities in the test chamber. This may consist of a high-frequency (0.5 MHz) induction heating system, for high-temperature tests, or a cooling ring with liquid nitrogen circulating through an inner channel, for low-temperature experiments (Ref 42–44). Confined and unconfined rod experiments have been performed (Ref 45, 46) with the aim of extending the uniaxial strain deformation states imposed in the plate impact experiments. The bar impact and pressure-shear experiments provide a measurement of yield stress at rates of 103 to 105/s−1. They also allow the experimental verification and validation of constitutive models and numerical solution schemes under two-dimensional states of deformation. In-material stress measurements, with embedded manganin gages, are used to obtain axial and lateral stress histories. Stress decay, pulse duration, release structure, and wave dispersion are well defined in these plate and rod experiments.