Tensile testing, also known as tension testing, is probably the most basic mechanical test you can perform on a material. Tensile tests are simple, relatively inexpensive and fully standardized. When you pull something, you can easily see how the material will react to the forces applied in tension. As the material is pulled, its strength is found by the amount of elongation.
Why is a Tensile Test or Tension Test Conducted?
When tension is continued until the material breaks, a good and complete tensile profile is obtained. The resulting curve indicates how the material responds to applied forces. The point of rupture is typically referred to as ultimate strength.
Hooke’s Law
In most materials during tensile testing, especially in the initial part of the test, it’s observed that the relationship between the applied force or load and the elongation of the specimen is linear. In this linear region, the ratio of stress to strain remains constant, which is defined as Hooke’s Law.
Elasticity Module
The modulus of elasticity is a measure of a material’s ability to undergo elastic deformation under load, but it is only valid within the linear region of the curve. If a specimen is loaded within this linear region, it will return to its original state when the load is removed. However, beyond the point where the curve is no longer linear and deviates from a straight line relationship, Hooke’s Law no longer applies, and some permanent deformation may occur in the specimen. This point is referred to as the modulus of elasticity or Young’s modulus. Beyond this point in a tensile test, the material responds plastically to each increase in load or strain. It does not return to its original stress-free state when the load is removed.
Yield Strength
Yield strength is defined as the stress applied to a material at the point where plastic deformation begins when subjected to a load.
Offset Method
For some materials (e.g. metals and plastics), the departure from the linear elastic region cannot be easily detected. Therefore, an offset method is permitted to determine the yield strength of the material under test. These methods are described in ASTM E8 (metals) and D638 (plastics). An offset is specified as a strain % value (usually 0.2% from E8 for metals and sometimes 2% for plastics). According to the offset method, the yield strength becomes the stress (R) determined from the intersection point when the line of the linear elastic region (with slope equal to the Modulus of Elasticity) is drawn through the offset.
Alternative Module
In some materials, there is no well-defined linear region in the tensile curves. In such cases, ASTM Standard E111 provides alternative methods such as secant modulus and tangent modulus for determining the modulus of a material and also the Young’s modulus.
Strain
The amount of elongation or stress experienced by the specimen during the tensile test is found. This is expressed as an absolute measure of the change in length or as a relative measurement called strain. Strain can be expressed in two different ways: Engineering strain and true strain. Engineering strain is probably the easiest and most common expression of strain used. This is the rate of change in length relative to the original length
Engineering strain formula,
but is based on the instantaneous length of the specimen as the test continues,
True strain formula
where Li is the instantaneous length and L0 is the initial length.
Ultimate Tensile Strength
One of the properties that can be determined about a material is the ultimate tensile strength (UTS). This is the maximum load that the specimen can withstand during the test. The UTS can be equal to or different from the strength at rupture. This depends entirely on whether the material under test is brittle, ductile or exhibits both characteristics. In some cases, the material may be ductile but exhibit a brittle structure when exposed to cold.
APPLICATION STANDARDS
TS EN ISO 6892-1 : Metallic Materials – Tensile Testing – Part 1: Test Method at Room Temperature
TS EN ISO 4136 : Destructive Tests on Welds of Metallic Materials – Transverse Tensile Test
ASTM E8/E8M : Standard Test Methods for Tensile Testing of Metallic Materials
API1104, ASME Sec. IX, AWS D1.1, AWS D1.5, ASTM A370
TS EN ISO 5178 : Destructive Testing on Welds in Metallic Materials – Longitudinal Tensile Testing on Welds in Fusion Welded Joints
TS EN ISO 9018: Destructive Inspections on Welding Seams in Metallic Materials – Tensile Test in Cruciform and Lap Joints
TS EN ISO 14555 : Tensile Tests for Stud Arc Welding of Metallic Materials
TS EN 898-1: Tensile Test for Bolts, Screws and Studs