Measuring Metal Strength (Tensile & Impact Strength)

When selecting a metal for a specific project, one of the most important considerations – especially when it comes to structural applications – is tensile strength and impact strength. Knowing what these mechanical properties are and how to evaluate them is essential to selecting the correct metal for your project.

What Is Tensile Strength?

Tensile strength is a value that indicates a metal’s ability to resist deformation and failure when loads are applied that pull it apart (known as tensile loads). Tensile strength is typically quantified through units of pounds per square inch (PSI) or pascals (Pa).

There are 3 different types of tensile strengths:

• Tensile yield strength
• Ultimate tensile strength
• Fracture tensile strength

The yield strength is the strength a metal has before it begins to plastically deform. The ultimate tensile strength is the maximum tensile strength a metal has, and it typically is found after plastic deformation has begun to occur. The fracture tensile strength is the strength of the metal at the point of final failure.

Testing Tensile Strength

A tensile testing machine is required to properly test the tensile strength of a material. This machine consists of two sets of jaws, a control unit, and cylinders that create the force of a tensile load.

To conduct the test, a metal specimen is loaded into the jaws. In most instances, the metal is machined so that it is thicker where it is clamped by the jaws than in the center. This helps ensure that fracture will not occur at the jaws and will not be a result of the stress concentration from the jaws. The cylinders are activated using the control unit and one or both sets of jaws begin to move away from one another which applies a tensile stress on the metal. Once the material reaches its point of failure, data is retrieved from the control unit on how much force was used. The force data is then used in conjunction with the area of the cross section of the metal to calculate the force over area unit such as PSI or Pa. The results of the tensile test can be displayed on a stress-strain curve.

Tensile strength is such a frequently documented mechanical property because so many metals are required to undergo a tensile load during their service life. Some practical examples where tensile strength is an important consideration include chains for lifting or towing, fasteners when tightened down, or structural metals in a skyscraper as the wind applies force to the building.

What Is Impact Strength?

Impact strength is the ability of a metal to absorb collision energy while resisting cracking or fracturing. Impact strength is also known as toughness and is usually expressed in foot-pounds (ft-lbf) or Joules per meter (J/m).

Testing Impact Strength

There are several ways to test impact strength. One of the most widely used tests is the Charpy V-notch test. To perform a Charpy V-notch test, a metal specimen is machined to a size defined by a standard and has a notch precision machined into the center. This acts as a geometric stress concentrator so that the material will fracture in the desired location during the testing. It is imperative that this notch be machined with extreme precision as it is very important for retrieving accurate test results.

The metal sample is then loaded in to a Charpy V-notch tester. The Charpy V-notch tester consists of a vice for the specimen and a pendulum with a known weight in a horizontal position. During the test, the pendulum is released, and the energy absorbed by the metal when the pendulum impacts and deforms it is recorded. The amount of energy absorbed is then used with the dimensional values of the metal to calculate the impact strength of the metal. This test is often done at several different temperatures since the temperature can have a great effect on the impact strength of metal. An alternative test to determine toughness is the Izod impact strength test.

Impact strength is such an important mechanical property to know because metals are subjected to collisions and impacts – even if it is unplanned – in so many applications. Everyday examples of impact stresses being applied to metals include a hammer head colliding with an object, stamping dies and chisels.