How do different specimen geometries affect tensile test results?

Tensile test results include the ultimate tensile strength, yield strength, Young’s modulus, ductility, and the strain hardening exponent. All these properties can be calculated using a universal testing machine equipped with the right controller, software, grips, and accessories. Grip selection may vary depending on the material type, geometry, and dimensions. In many cases, the specimen sizes and geometries are dictated by ASTM standards.

This blog will discuss whether tensile properties are affected if the same standard material is being tested in different geometries or dimensions. The short answer is that it depends on the tensile property and the characteristics of the material being tested. For a given cross-sectional area and for any gauge length, different specimen geometries have no effect on the ultimate tensile strength and the yield strength of standard materials. However, different gauge lengths and cross-sectional areas will have altering effects on certain properties, described below.

1- Effect of Different Gauge Lengths

Let’s compare two specimens, made of the same material, with two different gauge lengths: Figure 1. Two dogbone specimens of different gauge length

Specimen A gauge length > Specimen B gauge length

When the tension test is started and Specimen A or Specimen B is pulled, the strain is uniform along the gauge length up to the point at which the maximum force is reached and the onset of necking occurs. The stretch in each material is uniform up to this point. The force will then start to drop, as shown in the stress-strain curve below, and the reduction of area will no longer be proportional to the amount of stretch in the material. Figure 2. The shape of a ductile specimen changes during tensile testing

The necking region will occupy a much larger portion of the 1-in gauge length of Specimen B compared to the portion occupied on the 2-in gauge length of Specimen A. When the test is over and the two fractures of the specimens are fitted together, the measured percent elongation of Specimen B with the smaller gauge length will be greater than the percent elongation of Specimen A with the larger gauge length.

Equation 1:

Percentage Elongation = ∆L/L0 x 100

Where:

• L0 is the original gauge length
• L is the change in length of the original gauge length. Measured after the specimen fractures and the specimen are fitted together (see Figure 2)

As the gauge length increases, the percent elongation decreases.

2- Effect of Different Cross-Sectional Areas

This time, Specimen A and Specimen B, made of the same material, have identical gauge lengths; yet, the cross-sectional area of Specimen A is greater than the cross-sectional area of Specimen B. Similar to the concept with the gauge length and the portion occupied by necking, the necking region will occupy a much larger portion of the smaller cross-sectional area of Specimen B compared to the portion occupied on the larger cross-sectional area of Specimen A.

The cross-sectional area of a specimen has a significant effect on elongation measurements. Slimness ratio is measured by the gauge length divided by the square root of the cross-sectional area, therefore is inversely proportional with the cross-sectional area.

Equation 2:

Slimness Ratio = L0/√A0

Where:

• L0 is the original gauge length.
• A0 is the original cross-sectional area of specimen

As the slimness ratio increases and the cross-sectional area decreases, the percent elongation decreases.