Understanding Material Properties: Hardness, Toughness, Fatigue, Thermal

Material Hardness: Resistance to Deformation

Hardness is the resistance a material offers to being penetrated or scratched by another material. This property is primarily due to the cohesive forces between the atoms of the material. Hardness is compared and measured using various types of tests.

Generally, the more penetration achieved by applying the same force, the softer the material being studied will be.

Brinell Hardness Test

The Brinell hardness test uses a hard spherical indentor, typically made of tungsten carbide, which is situated above the sample material to be tested. A specific load is applied to the sphere for a defined period. After removing the load and the indentor, the diameter of the indentation left on the specimen is measured to determine the Brinell hardness value.

Relationship Between Hardness and Tensile Strength

An advantage of understanding this relationship is the ability to obtain very accurate approximations of a material's tensile strength without performing a costly tensile test. This correlation provides a practical and efficient method for material characterization.

Material Toughness: Impact Resistance

Toughness is defined as a material's ability to resist impact. It is the property contrary to brittleness. Tough materials are capable of absorbing significant energy during an impact, converting it into plastic or elastic deformation, and thereby preventing sudden breakage.

Resilience Test

Resilience is the energy required to break a material with a single impact. The higher the resilience value, the tougher the material tested will be.

Charpy Impact Test

The Charpy test is performed on a machine that incorporates a pendulum with a 22 kg mass located at its tip. At the vertical turning point of the pendulum, an anvil holds the fixed specimen. During the test, the pendulum is dropped from an initial position at a fixed height 'h'. Once the specimen is impacted, it breaks, and the pendulum continues its swing. The final height 'h'' reached by the pendulum will be lower than the initial height, due to the energy consumed in breaking the specimen. The difference in height (h - h') is directly proportional to the material's resilience.

Fatigue Testing: Cyclic Stress Resistance

Stresses that alternate their direction of application repetitively or cyclically over time are known as fatigue stresses. Fatigue tests aim to reproduce the real working conditions of materials to assess their durability under such conditions.

Fatigue strength is the value of the stress amplitude that causes material breakdown after a certain number of cycles.

Fatigue life is the number of working cycles a material can withstand for a given applied stress amplitude, represented by 'N'.

Two different behaviors are distinguished:

• Materials that will eventually break, regardless of the stress amplitude (if above a certain threshold).
• Materials that, if the applied stress amplitude does not exceed a certain value (the endurance limit), will not break regardless of the number of applied work cycles.

Thermal Properties of Materials

Thermal properties indicate the behavior of materials when exposed to heat.

Thermal Conductivity

Thermal conductivity is the ease with which a material allows heat to flow through it.

Thermal Expansion

Thermal expansion is the phenomenon that causes an increase in the size of a material, especially metals, when its temperature increases. Thermal expansion depends on:

• The specific material.
• The increase in temperature.

Based on the dimensions of the object where the increase is determined, different types of expansion are defined:

• Linear Expansion: Increase in length.
• Superficial Expansion: Increase in surface area.
• Cubic Expansion: Increase in volume.

Key Material Property Metrics

Here are some key metrics and their notations:

• Stress Unit: MPa (Megapascals)
• Uniform Elongation
• Brinell Hardness: XX HBW (D/C/T)
  • D: Indentor diameter (mm)
  • C: Load factor (0.102 * F / applied load in N)
  • T: Test duration (seconds)
• Resilience (K): J/mm²
  • Ec: Kinetic energy consumed in breaking (J)
  • Ao: Original breaking specimen section (mm²)