Three important points collectively highlight the need for high-speed testing:
- Tensile properties and fracture behavior change with strain rate.
- Conventional tensile tests using standard dogbone shapes take on the order of 1 to 2 minutes depending on the grade.
- An entire automotive crash takes on the order of 100 milliseconds, with deformation rates 10,000 to 100,000 faster than conventional testing speeds.
Characterizing the response during high-speed testing provides critical information used in crash simulations, but these tests often require upgraded equipment and procedures. Conventional tensile testing equipment may lack the ability to reach the required speeds (on the order of 20 m/s). Sensors for load and displacement must acquire accurate data during tests which take just a few milliseconds.
Higher speed tensile and fracture characterization also aids in predicting the properties of stamped parts, as deformation rates in stamping are 100 to 1,000 times higher than most testing rates.
Steel alloys possess positive strain rate sensitivity, or m-value, meaning that strength increases with strain rate. This has benefits related to improved crash energy absorption.
Characterizing this response requires use of robust testing equipment and practices appropriate for the targeted strain rate. Some techniques involve a tensile or compressive Split Hopkinson (Kolsky) Bar, a drop tower or impact system, or a high-speed servo-hydraulic system. Historically, no guidelines were available as to the testing method, specimen dimensions, measurement devices, and other important issues which are critical to the quality of testing results. As a result, data from different laboratories were often not comparable. A WorldAutoSteel committee evaluated various procedures, conducted several round-robins, and developed a recommended procedure, which evolved into what are now both parts of ISO 26203, linked below.
Published standards addressing tensile testing at high strain rates include:
- International Organization for Standardization (ISO), “ISO 26203-1: Metallic materials — Tensile testing at high strain rates — Part 1: Elastic-bar-type systems.”
- International Organization for Standardization (ISO), “ISO 26203-2: Metallic materials — Tensile testing at high strain rates — Part 2: Servo-hydraulic and other test systems.”
- Stahl-Eisen-Prüfblätter (SEP) Standard, “SEP 1230: The Determination of the Mechanical Properties of Sheet Metal at High Strain Rates in High-Speed Tensile Tests.”
The specific response as a function of strain rate is grade dependent. Some grades get stronger and more ductile as the strain rate increases (left image in Figure 1), while other grades see primarily a strength increase (right image in Figure 1). Increases are not linear or consistent with strain rate, so simply scaling the response from conventional quasi-static testing does not work well. Strain hardening (n-value) also changes with speed in some grades, as suggested by the different slopes in the right image of Figure 1. Accurate crash models must also consider how strain rate sensitivity impacts bake hardenability and the magnitude of the TRIP effect, both of which are further complicated by the strain levels in the part from stamping.