Tensile testing
Pic 1 : Tensile testing
Tensile testing, is also known as tension testing, is a fundamental materials science test in which a sample is subjected to a controlled tension until failure. The results from the test are commonly used to select a material for an application, for quality control, and to predict how a material will react under other types of forces. Properties that are directly measured via a tensile test are ultimate tensile strength, maximum elongation and reduction in area From these measurements the following properties can also be determined:Young's modulus, Poisson's ratio, yield strength, and strain-hardening characteristics. Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic materials. For anisotropic materials, such as composite materials and textiles, biaxial tensile testing is required.
Pic 2 : Tensile specimen
Tensile specimens made from an aluminum alloy. The left two specimens have a round cross-section and threaded shoulders. The right two are flat specimens designed to be used with serrated grips.
The shoulders of the test specimen can be manufactured in various ways to mate to various grips in the testing machine (see the image below). Each system has advantages and disadvantages; for example, shoulders designed for serrated grips are easy and cheap to manufacture, but the alignment of the specimen is dependent on the skill of the technician. On the other hand, a pinned grip assures good alignment. Threaded shoulders and grips also assure good alignment, but the technician must know to thread each shoulder into the grip at least one diameter's length, otherwise the threads can strip before the specimen fractures
In large castings and forgings it is common to add extra material, which is designed to be removed from the casting so that test specimens can be made from it. These specimens may not be exact representation of the whole workpiece because the grain structure may be different throughout. In smaller workpieces or when critical parts of the casting must be tested, a workpiece may be sacrificed to make the test specimens. For workpieces that are machined from bar stock, the test specimen can be made from the same piece as the bar stock.
A. A Threaded shoulder for use with a threaded grip
B. A round shoulder for use with serrated grips
C. A butt end shoulder for use with a split collar
D. A flat shoulder for used with serrated grips
E. A flat shoulder with a through hole for a pinned grip
The repeatability of a testing machine can be found by using
special test specimens meticulously made to be as similar as possible.
A standard specimen is prepared in a round or a square
section along the gauge length, depending on the standard used. Both ends of
the specimens should have sufficient length and a surface condition such that
they are firmly gripped during testing. The initial gauge length Lo is
standardized (in several countries) and varies with the diameter (Do) or the
cross-sectional area (Ao) of the specimen as listed.
Flat test specimen
Round test specimen.
The machine must have the proper capabilities for the test specimen being tested. There are four main parameters: force capacity, speed,precision and accuracy. Force capacity refers to the fact that the machine must be able to generate enough force to fracture the specimen. The machine must be able to apply the force quickly or slowly enough to properly mimic the actual application. Finally, the machine must be able to accurately and precisely measure the gauge length and forces applied; for instance, a large machine that is designed to measure long elongations may not work with a brittle material that experiences short elongations prior to fracturing.
Alignment of the test specimen in the testing machine is critical, because if the specimen is misaligned, either at an angle or offset to one side, the machine will exert a bending force on the specimen. This is especially bad for brittle materials, because it will dramatically skew the results. This situation can be minimized by using spherical seats or U-joints between the grips and the test machine. If the initial portion of the stress–strain curve is curved and not linear, it indicates the specimen is misaligned in the testing machine.
The strain measurements are most commonly measured with an extensometer, but strain gauges are also frequently used on small test specimen or when Poisson's ratio is being measured. Newer test machines have digital time, force, and elongation measurement systems consisting of electronic sensors connected to a data collection device (often a computer) and software to manipulate and output the data. However, analog machines continue to meet and exceed ASTM, NIST, and ASM metal tensile testing accuracy requirements, continuing to be used today.
Performing a Tensile Test
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.
Specimen Alignment
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
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