Material Mechanical Properties: Key Testing Methods

Compression Testing

Compression is a sustained effort, similar to traction but in the opposite direction. It is the type of stress characteristic of columns or pillars that bear weight in the same direction. Compression is significantly influenced by the length of the bar in relation to the dimensions of its cross-section.

For stone materials (e.g., concrete, rocks), compression test specimens are typically cylindrical, with a height twice their diameter. Tests are also performed with prism-shaped or cube-shaped specimens. The mode of failure differs from that in tension. In metals, the compression test is less common.

In the case of ductile metals, the test is similar to the tension test, exhibiting three distinct periods: proportionality, yield, and fracture. In soft metals (e.g., lead, copper), the first period (proportionality) is almost nonexistent, and sometimes the flow (plastic deformation) is so significant that it leads to large deformations without rupture.

Shear Strength Testing

Shear strength determines the behavior of a material under a constant strain, gradually increasing until fracture occurs. This is relevant for components like keys, rivets, screws, and bolts.

Punching Test

This is a cutting test performed on a cylindrical surface. It can be performed with any universal testing machine, provided the appropriate accessories are used.

Impact Testing

Impact testing, also known as shock testing, determines the energy absorbed by a specimen of a certain size when broken in a single blow. It is crucial for understanding the material's behavior, especially for manufacturing machine parts and components that will be subjected to dynamic stresses.

Specimens are standardized, typically 55 mm long with a 10 mm square cross-section. The notch can be V-shaped or U-shaped. The most commonly used machine is the Charpy pendulum.

Fatigue Testing

Fatigue testing is a test in which a part is subjected to stresses that vary in magnitude and direction, repeated with a certain frequency. Many materials, especially those used in the construction of machines or structures, are subjected to frequently repeated variable stresses. This includes components such as shafts, axles, wheels, rods, bearings, and springs.

When a material is subjected to continually varying stresses in magnitude and direction, it can fracture at loads less than the normal fracture load for a constant static stress.

Understanding the Fatigue Limit

There is a ΔσF value below which no fatigue failure occurs. This is known as the fatigue limit. Fatigue loading is repetitive (cyclic), characterized by a maximum and minimum stress in each cycle. The difference between these two values (ΔσF) represents the fatigue limit, regardless of the number of repetitions.

Stress Cycle Representation

The graph illustrates a cycle of variable stress to which the material is subjected. These cycles are repeated, though they are not necessarily identical. If the difference between the maximum stress (σmax) and the minimum stress (σmin) experienced by the component in a cycle exceeds the ΔσF value, then there is a risk of fracture if the phenomenon is repeated over several cycles.