Though a tensile test is relatively simple and has been around for a very long time, some thought and consideration must be done to ensure that the test will have valid results. Factors involved are the specimen shape and dimensions, the choice of grips and faces, and many more.
The specimen's shape is usually defined by the standard or specification being utilized, e.g., ASTM E8 or D638. Its shape is important because you want to avoid having a break or fracture within the area being gripped. So, standards have been developed to specify the shape of the specimen to ensure the break will occur in the "gage length" (2 inches are frequently used) by reducing the cross sectional area or diameter of the specimen throughout the gage length. This has the effect of increasing the stress in the gage length since stress is inversely proportional to the cross sectional area under load, .
Grip and Face Selection
Face and grip selection is a very important factor. By not choosing the correct set up, your specimen may slip or even break inside the gripped area ("jaw break"). This would lead to invalid results. The faces should cover the entire tab or area to be gripped. You do not want to use serrated faces when testing materials that are very ductile. Sometimes covering the serrated faces with masking tape will soften the bite preventing damage to the specimen.
Vertical alignment of the specimen is an important factor to avoid side loading or bending moments created in the specimen. Mounting the specimen in the upper grip assembly first then allowing it to hang freely will help to maintain alignment for the test.
Typical Materials & Standards
- Ceramics - ISO 15733, ISO 15490, ISO 17561
- Composites - MIL-HDBK-17, ISO 527 (Parts 4 & 5 on FRP composites)
- Elastomers & Rubber - ASTM D412, ISO 37
- Metals - ASTM E8 (at room temperature, E21 (high temperature, BS EN 10002, ISO 6892 (at ambient temperature), ISO 783 (elevated temperature), ISO 15579 (at low temperature)
- Paper - ASTM D828, ISO 1924 (Parts 1 & 2), ISO 3781
- Plastics - ISO 527, ASTM D638
- Textiles & Yarns - ASTM D76, D3822, D2256, D2653, ISO 9073 (Part 3 on nonwovens), ISO 13934, ISO 13935
- Wood - ISO 9086, 3345, 3346