Meaning
Experimental apparatus designed to measure the dynamic mechanical response of materials under high strain rates provides the data needed to model battery pack behavior during crashes. A hopkinson pressure bar consists of two long metal rods with a small specimen of the test material sandwiched between them. This system uses the propagation of elastic waves to determine the stress and strain in the specimen at speeds that traditional testing machines cannot reach.
The data collected from these tests is essential for the design of the structural cages that protect battery cells from high speed impacts. It is primarily used for metal and polymer characterization and is not suitable for testing full battery assemblies.
Experimental Setup
Loading is initiated by a striker bar that hits the end of the incident bar, creating a compressive pulse that travels towards the specimen. The hopkinson pressure bar relies on strain gauges mounted on the bars to record the passage of the incident, reflected and transmitted waves. These signals are then converted into a stress strain curve for the material under dynamic conditions.
Precision alignment of the bars is required to ensure that the waves travel without distortion.
Data Acquisition
High speed digitizers capture the voltage changes from the strain gauges at sampling rates of several million points per second. Analysis of the reflected wave provides information on the strain rate, while the transmitted wave reveals the stress level within the specimen. A hopkinson pressure bar can achieve strain rates up to ten thousand per second, which is typical of the forces seen in an automotive collision.
This information allows engineers to understand how the strength of a material changes when it is deformed very quickly.
Material Modeling
Dynamic properties measured by this apparatus are used to populate the mathematical models in crash simulation software. Many metals used in battery enclosures exhibit strain rate hardening, which means they become stronger as the speed of the impact increases. Knowing the exact magnitude of this effect allows for the optimization of the material thickness to save weight.
If the model is inaccurate, the battery protection frame might be either too heavy or dangerously weak. The final results inform the selection of the best alloy for the specific energy absorption requirements of the vehicle.
