This compact stress waveguide enables the delivery of long-duration impact pulses or transfer stress signals in a shorter footprint. Additionally, the waveguide is applicable for designing high-frequency fatigue testing equipment, where large displacements are needed at one end while applying long-duration stress at the other. It can also test equipment for obtaining the stress-strain response of materials in the low-to-intermediate strain-rate range. Strain rate refers to the deformation a material accumulates over a specific time interval. Knowledge of the strain-rate dependent stress-strain response of materials is a fundamental requirement to assess the suitability of materials in many impact applications.
Quasistatic or low-strain deformation tests are often needed to assess the suitability of materials in many engineering applications. However, numerous applications, such as crashworthiness of vehicles, impact, ballistics, and high-speed machining, require the high strain-rate response of materials. The split Hopkinson pressure bar (SHPB) is conventional equipment for testing materials in the intermediate to the high strain-rate range but not suitable to test materials in the low-to-intermediate strain-rate ranges. It features longer rods to accommodate longer stress wave durations, translating into lower strain rates, thus fulfilling this drawback. Unfortunately, manufacturing and housing the apparatus is space and cost-prohibitive, requiring several tens of meters of length in floor space and tens of thousands of dollars. Similarly, drilling and piling equipment, and fatigue testing machines are also currently designed with long rods to deliver impact or high-frequency stress pulses.
Researchers at the University of Florida have developed the Millipede Bar, a compact stress waveguide with a folded-bar design, enabling longer-duration stress waves to transfer undisturbed or obtain a stress-strain response in materials at lower strain rates.
Compact stress waveguide for the propagation of long-duration impact pulses and for testing the stress-strain response of materials in the low-to-intermediate strain-rate range
The Millipede waveguide is a folded-bar design, allowing a longitudinal stress wave to “flow” through a 180-degree bend junction if the stress wave duration is sufficiently high. The folded-bar design acts as a compact stress waveguide for use in various drilling tools and testing equipment. For example, it is applicable in piling or drilling equipment to reduce the length of the tool without losing the ability to deliver a long-duration impact stress pulse. Similarly, it can be used in Hopkinson Bar for testing materials in the low to intermediate strain-rate ranges. Traditional split Hopkinson pressure bars use 1-dimensional stress wave propagation principles to determine the stress-strain response of materials in the intermediate range of strain rates (10^2/s-10^4/s). Achieving longer-duration stress waves and lower strain rate requires longer rods. The Millipede Hopkinson Bar features a folded-bar design, including any number of folds to accommodate longer-duration stress waves. By enabling the generation of long stress waves, the apparatus makes achieving strain rates in the 10^1/s-10^2/s range possible. Additionally, each folded bar maintains the length-to-diameter ratio of at least 20 to obtain 1-dimensional stress wave propagation characteristics.