Material Testing Methods: Hardness, Toughness, Fatigue, Creep

Material Testing: Hardness, Toughness, Fatigue, and Creep

Hardness Testing

Static Methods

• Brinell: A steel ball indenter is used, and the area of the footprint is measured. Suitable for metals.
• Vickers: Employs a pyramidal indenter tip with a square base. The surface of the print is measured. Prints are comparable regardless of the load. Applicable to any material (soft or hard), and thin pieces can be measured with small loads. Surface hardness can also be measured.
• Rockwell: Uses either a steel ball (Rockwell B) or a diamond cone (Rockwell C) penetrator. Measures the depth of the trace. Offers universal application and direct reading.

Dynamic Methods

• Poldi Method
• Rebound (Shore or Hammer Scleroscope)

Toughness Testing

Toughness is the work or energy a material absorbs during deformation or breakage.

• Charpy Method
• Izod Method

Factors Influencing Toughness:

• Grain Size: Larger grain size decreases toughness.
• Tempering: Eliminates tension and increases elasticity.
• Fiber Effect
• Temperature
• Chemical Composition

Fatigue Testing

Fatigue refers to failure under cyclical effort.

Stages:

1. Incubation
2. Growth
3. Rupture

• Fatigue Limit: The stress a material can withstand indefinitely without breaking.
• Restricted Fatigue Limit: The stress a material can withstand for a determined number of cycles.

Creep Testing

Creep is a mechanical process consisting of high-temperature deformation and strain over time. Very large grains are preferred to reduce the grain boundary and increase resistance to flow.

Factors:

• Temperature
• Time
• Tension

The test is conducted using a universal machine, similar to a tensile test, but with the piece in a high-temperature compartment.

Non-Destructive Testing

Ultrasonic Testing

Uses acoustic waves with a frequency higher than audible waves for detecting internal defects. They propagate well in solids, less in liquids, and very poorly in air. They do not spread in a vacuum.

Methods:

• Transparency or Shadow
• Impulse-Echo

Piezoelectric Effect: When a piezoelectric material is placed between two electrodes and an electrical potential is applied, it changes shape.

Penetrant Testing

A simple test for detecting surface defects not visible to the naked eye. Effective only in compact, non-porous materials.

Properties of Penetrating Liquids:

• Wetting Capacity: Ability to wet the material's surface and form a uniform, continuous film.
• Penetration Power: Capillary action should allow the liquid to penetrate surface discontinuities.

Procedure:

1. Clean the surface.
2. Apply the penetrating liquid.
3. Eliminate excess liquid.
4. Apply developer.
5. Inspect for interpretation and evaluation of indications.
6. Final cleaning.

Advantages:

• Allows testing the entire surface of the piece.
• Applicable regardless of the piece's size or shape.
• Economical.
• Can be automatic or manual.
• Applies to many materials.
• Can be performed without electricity or water.

Magnetic Particle Testing

Detects surface and internal defects in ferromagnetic materials (steel, excluding aluminum, copper, zinc, or austenitic stainless steels).

Procedure:

1. Prepare the surface of the piece.
2. Magnetize the piece.
3. Apply magnetic particles.
4. Observe and interpret.
5. Final cleaning.
6. Demagnetize the part.

Metallography

The science devoted to the study of metallic materials. A flat surface is needed to reflect the light beam.

Metallographic Preparation Stages:

1. Sample selection
2. Cutting
3. Mounting
4. Identification
5. Grinding and polishing
6. Attack

Better mechanical properties are achieved with smaller grain sizes, except for flow.

Testing Equipment:

• Light Microscope
• Electron Microscope

Industrial Radiology

Requires:

• Radiation source
• Object to be radiographed
• Sensitive material (radiographic film)

Types:

• X-ray Scan