Written by Matthew Spiret
This guide is designed to introduce you to the basic elements of an ASTM E8 / E8M tensile test and will provide an overview of the materials testing equipment, software, and tensile specimens needed. However, anyone planning to conduct ASTM E8 / E8M testing should not consider this guide an adequate substitute for reading the full standard.
ASTM E8 / E8M measures the tensile properties of metallic materials in any form at an ambient temperature between 10 – 38 degrees Celsius (50 – 100 degrees Fahrenheit). Though ASTM E8 / E8M measures many different tensile properties, the following are the most common:
- Yield Strength - The stress at which a material becomes permanently deformed
- Yield Point Elongation - The phase during which a material is transitioning from elastic behavior to plastic behavior
- Tensile Strength - The maximum force or stress that a material is capable of sustaining
- Reduction of Area - A measurement of the ductility of a material
Because ASTM E8 / E8M testing is conducted on such a wide variety of metals, system force requirements can differ greatly. The Instron® 6800 Series offers test frames suitable for testing sheet metal (10kN) all the way up to steel plate (600kN). The 6800 Series load frame includes pre-loaded bearings, precision ball screws, a thick crosshead and base beam, and low-stretch drive belts. These features contribute to better performance by producing more accurate results and minimizing the energy stored during a test, which is especially evident when testing high-strength materials such as aerospace composites, metal alloys, and crystalline polymers.
ASTM E8 / E8M allows for many different specimen types and defines suitable geometries and dimensions for each one. Bars, tubes, sheets, pin-loaded specimens, round specimens, and powdered metallurgy products are some of the many options for testing to this standard. However, the most common specimen is a dogbone-shaped rectangle with a width of 12.5 mm (0.5 in) and gauge length of 50 mm (2 in).
As with the most common metals testing standards, there are three types of test controls described: Stress Rate Control, Strain Rate Control, and Crosshead Displacement Control. For ASTM E8 / E8M, these types of controls are referred to as Methods A, B, and C. The video below discusses the differences between test control methods in greater detail.
While there are many different gripping technologies suitable for E8 / E8M testing (wedge, side-acting, hydraulic, pneumatic, etc.) they can all be classified as either proportional or non-proportional according to the way in which they exert clamping force on the specimen.
With proportional grips, the force exerted on the specimen is proportional to the tensile load being applied. As tensile load increases during a test, so does the gripping force on the specimen. Wedge grips are a popular option for proportional gripping and come in manual, pneumatic, and hydraulic varieties to suit a wide range of testing applications. The shape of a wedge grip is what allows it to exert proportional pressure: as tensile force is applied to the specimen, the specimen is pulled more tightly into the narrowest area of the wedge, increasing the gripping pressure.
With non-proportional grips, the clamping force on the specimen remains consistent and is independent of the tensile load being applied. This is typical of side-acting grips and fatigue-rated hydraulic wedge grips where the clamping force is generated by a power source that is not directly associated with the tension loading of the specimen. This source is typically a high pressure (210 bar/3000 psi or higher) hydraulic supply. One benefit of non-proportional grips is that the clamping force is typically adjustable, which offers more potential application advantages.