Poisson's ratio is a material constant representing compressibility of material volume. However, when soft, hyperelastic materials such as silicone foam are subjected to large deformation into densification, the Poisson's ratio may rather significantly change, which warrants careful consideration in modeling and simulation of impact/shock mitigation scenarios where foams are used as isolators. The evolution of Poisson's ratio of silicone foam materials has not yet been characterized, particularly under dynamic loading. In this study, radial and axial measurements of specimen strain are conducted simultaneously during quasi-static and dynamic compression tests to determine the Poisson's ratio of silicone foam. The Poisson's ratio of silicone foam exhibited a transition from compressible to nearly incompressible at a threshold strain that coincided with the onset of densification in the material. Poisson's ratio as a function of engineering strain was different at quasi-static and dynamic rates. The Poisson's ratio behavior is presented and can be used to improve constitutive modeling of silicone foams subjected to a broad range of mechanical loading.
Understanding the dynamic behavior of geomaterials is critical for refining modeling and simulation of applications that involve impacts or explosions. Obtaining material properties of geomaterials is challenging, particularly in tension, due to the brittle and low-strength nature of such materials. Dynamic split tension technique (also called dynamic Brazilian test) has been employed in recent decades to determine the dynamic tensile strength of geomaterials. This is primarily because the split tension method is relatively straightforward to implement in a Kolsky compression bar. Typically, investigators use the peak load reached by the specimen to calculate the tensile strength of the specimen material, which is valid when the specimen is compressed at quasi-static strain rate. However, the same assumption cannot be safely made at dynamic strain rates due to wave propagation effects. In this study, the dynamic split tension (or Brazilian) test technique is revisited. High-speed cameras and digital image correlation (DIC) were used to image the failure of the Brazilian-disk specimen to discover when the first crack occurred relative to the measured peak load during the experiment. Differences of first crack location and time on either side of the sample were compared. The strain rate when the first crack is initiated was also compared to the traditional estimation method of strain rate using the specimen stress history.
In this study, a Johnson–Cook model was used as an example to analyze the relationship of compressive stress-strain response of engineering materials experimentally obtained at constant engineering and true strain rates. There was a minimal deviation between the stress-strain curves obtained at the same constant engineering and true strain rates. The stress-strain curves obtained at either constant engineering or true strain rates could be converted from one to the other, which both represented the intrinsic material response. There is no need to specify the testing requirement of constant engineering or true strain rates for material property characterization, provided that either constant engineering or constant true strain rate is attained during the experiment.
To understand interfacial interaction of a bi-material during an impact loading event, the dynamic friction coefficient is one of the key parameters that must be characterized and quantified. In this study, a new experimental method to determine the dynamic friction coefficient between two metals was developed by using a Kolsky tension bar and a custom-designed friction fixture. Polyvinylidene fluoride (PVDF) force sensors were used to measure the normal force applied to the friction tribo pairs and the friction force was measured with conventional Kolsky tension bar method. To evaluate the technique, the dynamic friction coefficient between 4340 steel and 7075-T6 aluminum was investigated at an impact speed of approximately 8 m/s. In addition, the dynamic friction coefficient of the tribo pairs with varied surface roughness was also investigated. The data suggest that higher surface roughness leads to higher friction coefficients at the same speed of 8 m/s.
Silicone foams and rubber are used in a variety of applications to protect internal components from external shock impact. Understanding how these materials mitigate impact energy is a crucial step in designing more effective shock isolation systems for components. In this study, a Kolsky bar with pre-compression and passive radial confinement capabilities was used to investigate the response of silicone foams and rubber subjected to impact loading at different speeds. Using the preload capability, silicone foam samples were subjected to increasing levels of pre-strain. Frequency-based analyses were carried out on results from silicone foams and rubber to study the effect of both pre-strain and material processing conditions on the mechanism of energy dissipation in the frequency domain. Additionally, effects of impact speed on energy dissipation through silicone foams and rubber were investigated.
Steel grades such as A572 and AISI 4140 are often used for applications where high rate or impact loading may occur. A572 is a hot-rolled carbon steel that is used where a high strength to weight ratio is desired. A grade such as AISI 4140 offers decent corrosion resistance due to higher chromium and molybdenum content and is commonly used in firearm parts, pressurized gas tubes, and structural tubing for roll cages. In these scenarios, the material may undergo high rate loading. Thus, material properties including failure and fracture response at relevant loading rates must be understood so that numerical simulations of impact events accurately capture the deformation and failure/fracture behavior of the involved materials. In this study, the high strain rate tensile response of A572 and 4140 steel are investigated. An increase in yield strength of approximately 28% was observed for 4140 steel when comparing 0.001 s-1 strain rate to 3000 s-1 experiments. A572 showed an increase in yield strength of approximately 52% when the strain rate increased from quasi-static to 2750 s-1. Effects on true stress and strain at failure for the two materials are also discussed.
Low carbon, high strength steel alloys such as Vascomax steels are used in a wide variety of extreme environments due to their high strength, high fracture toughness, and stability over a wide range of temperatures. In this study, Vascomax® C250 steel was dynamically characterized in compression using Kolsky compression bar techniques at two strain rates of 1000 and 3000 s-1. A pair of impedance-matched tungsten carbide platens were implemented to protect damage to the bar ends. The tungsten carbide platens were experimentally calibrated as system compliance which was then properly corrected for actual specimen strain measurements. In addition, elastic indentation of the high-strength compression sample into the platens was also evaluated and showed negligible effect on the specimen strain measurements. The Vascomax® C250 steel exhibited strain-rate effects on the compressive stress-strain curves. The dynamic yield strength was approximately 18% higher than quasi-static yield strength obtained from hardness tests. The dynamic true stress-strain curves of the Vascomax® C250 steel in compression were also computed and then compared with the previously obtained true tensile stress-strain curves at the same strain rates. The Vascomax® C250 steel exhibited a reasonable symmetry in dynamic compression and tensile stress-strain response. However, the fracture strains in dynamic compression were smaller than those in dynamic tension probably due to different fracture mechanisms in the different loading modes.