Materials Testing Methods: Destructive and Non-Destructive Tests

Destructive Tests

Static Destructive Tests

Destructive tests (static) are used to evaluate the mechanical properties of materials. These tests are performed on samples of the material, and the results are used to determine the behavior of the material under load.

Hardness Tests

Hardness tests measure the resistance of a material to localized plastic deformation, such as indentation or scratching.

• Scratch hardness (Martens): Uses a diamond point to create a groove on the surface of the object. The depth of the groove is measured to determine the hardness of the material.
• Durability and static penetration (Brinell): A hardened steel or carbide ball is pressed into the surface of the material under a specified load. The diameter of the resulting indentation is measured and used to calculate the hardness.
• Vickers method: A variant of the Brinell method, in which the ball is replaced by a square-based diamond pyramid. The diagonals of the indentation are measured.
• Rockwell method: A minor load of 10 kg is applied first, followed by a major load. The depth of the permanent indentation is measured after the major load is removed. Hardness depends on the material being tested.

Dynamic Hardness Tests

• Impact method: A 5 mm diameter ball is struck against the surface of the material with a defined momentum. The diameter of the resulting mark is measured.
• Shore method: The hardness is determined by the rebound height of a hammer dropped from a fixed height onto the specimen. A Shore scleroscope uses a glass tube with a hammer that slides inside it without friction. Durometer variants use a pendulum instead.

Tensile Testing

Tensile tests apply a tensile force to a specimen until it fails, in order to determine the stress-strain behavior of the material. For materials that behave elastically, the modulus of elasticity can be obtained from the data, considering the unit tension and unit elongation.

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Compression Testing

In compression tests, metals are typically tested as cylinders with a height equal to the diameter to avoid buckling effects that depend on the compression chamber.

Shear Testing

Care must be taken to ensure pure shear. Applications include bolts, rivets, and pins.

Buckling Testing

Buckling occurs when a long, slender specimen is subjected to a compressive force along its axis, causing it to bend laterally rather than deform plastically.

Torsion Testing

Torsion tests are used on shafts, axles, and springs. Specimens are typically tubular in cross-section. The machines used consist of two heads, each with grips to hold the specimen. One head is driven by a motor and the other is fixed and measures the torque.

Flexure Testing

In flexure tests, a rectangular or square specimen is supported freely at both ends, and a load is applied at the center, causing progressively increasing stress until deformation occurs at the point of application.

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Destructive Dynamic Tests

Resistance to Impact (Charpy Test)

The Charpy test uses a pendulum of different masses, with three hammers developing energies of 10, 30, and 300 kgm. The process: the hammer is raised to a set height and released, striking the test specimen. The hammer rises after impact to a height determined by the energy absorbed in breaking the specimen.

Fatigue Testing

Many machine parts and structures are subjected to variable stresses that repeat with a certain frequency. Examples include drive shafts, connecting rods, springs, bearings, and engines. Fatigue occurs when stresses vary in magnitude and direction and the material eventually breaks under normal loads. The fatigue limit is the maximum load a material can withstand indefinitely without failing, regardless of the number of repeated cycles.

Destructive Technological Tests

Spark Test

Differences in the shape or color of sparks produced when a material is pressed against a grinding wheel help identify the material.

Bend Testing

Bend tests reveal the plasticity of the test material. The bend is performed once or twice, and the outer surface is observed for possible cracks. Good plastic characteristics imply the absence of cracks.

Cupping (Erichsen) Test

In a cupping test, a sheet is formed by pressing it into a die. The test is performed on an Erichsen machine. The depth of the cup at the moment the first crack appears on the surface indicates the formability of the material.

Non-Destructive Tests

Macroscopic Tests

Used to locate cracking and porosity defects. They are typically performed using liquids.

Optical Tests

Provide information about the alloy's constituents, the shape and size of grains, porosity, and similar features.

Magnetoscopic Tests

Applicable to materials that are good conductors of magnetic fields. Defects are detected by spreading magnetic powder over the part, which accumulates at the defect locations under a magnetic field.