Standard Test Method for Measuring Forces During Normal Impact of a Soft Projectile on a Rigid Flat Surface
This document describes a method for measuring forces during an impact between a soft or frangible projectile and a relatively rigid flat normal surface. The document describes the hardware and instrumentation required, as well as the processing and data reduction required to compute force.
In this test, a projectile impacts one end of a long cylindrical bar with flat ends, called a Hopkinson bar. The impact occurs on the centerline of the bar in the axial direction. The diameter of the cylindrical bar is large compared to the lateral dimension of the projectile so that, during and after the impact, the projectile material moves radially or backward, rather than extruding around the perimeter of the impact surface.
The bar is instrumented with strain gages at some distance from the impacted end to measure the longitudinal strain in the bar. The bar must be sufficiently long so that the duration of the impact is less than the time it takes for the transient stress wave generated by the impact to travel to the end of the bar and back to the location of the strain gages.
This test is intended for measuring the transient force generated by the projectile when it impacts the cylindrical bar. Projectile breakup pattern data may also be captured during this test. The projectile material is expected to be soft compared with the bar material. Impact stresses are expected to be small compared with the yield strength of the bar material so that all deformations in the bar are elastic.
The values stated in either SI units or inch-pound units are considered separate standards. The values stated in each system may not be exactly equivalent; therefore, each system must be considered as independent.
This standard does not address all of the safety concerns associated with its use. It is the responsibility of each user of this standard to ensure that any safety issues are properly addressed.
Measurements of forces generated from a soft object impacting a relatively rigid stationary object can be difficult due to the very short time duration of the event and limitations on the dynamic response of standard transducers. This test method overcomes some of these limitations. The method can be used to generate data useful in developing and validating computational material models for soft or frangible objects such as ice, soft polymers, and materials used to simulate birds used for aircraft certification testing. The method can also be used to provide comparison data in the development of physical simulants for impact and ingestion certification testing for aircraft.