Mechanical Testing: Fatigue, Creep, Compression, and Hardness

Posted on Apr 7, 2026 in Civil Engineering, Transportation, and Urban Services

Fatigue of Materials

Definition: Damage accumulated through the application of repeated stress cycles.

  • Variable amplitude loadings cause different levels of fatigue.
  • Fatigue is cumulative throughout the life of a component.
  • Fatigue: Failure due to dynamic or cyclic loading.

Examples: Aircraft, automobile parts (axles, transmission parts, suspension systems), turbine blades, and bridges.

Factors for Fatigue Failure

  • Maximum tensile stress.
  • Large variation or fluctuation in applied stress.
  • Large number of cycles of applied stress.

Other variables such as stress concentration, corrosion, temperature, overload, metallurgical structure, residual stresses, and combined stresses alter the condition of fatigue.

Key Fatigue Terminology

  • Fatigue Life (Nf): The number of cycles to cause failure at a specified stress level, as taken from the S–N plot.
  • Fatigue Strength: The stress level at which failure will occur for a specified number of cycles (e.g., 5×107 cycles).
  • Fatigue/Endurance Limit: The stress value below which fatigue failure will not occur.

Factors Affecting Fatigue Life

  • Loading Conditions: Type of stress, stress amplitude, mean value.
  • Condition of Specimen: Stress concentrations, surface finish.
  • Material: Thermal history (e.g., grain size in metals).
  • Environmental Conditions: Temperature, corrosion effects.

Creep in Materials

Creep occurs when a metal is subjected to a constant tensile load at an elevated temperature, resulting in a time-dependent increase in length.

Creep Temperature Threshold

Materials creep when the homologous temperature is greater than 0.5.

Testing Methods

  • Creep Test: Measures dimensional changes at high temperatures.
  • Rupture Test: Measures the effect of temperature on long-term load-bearing characteristics.

The creep test is carried out by applying a constant load to a tensile specimen maintained at a constant temperature (according to ASTM E139-70).

Three Stages of Creep

  1. Primary Creep: A period of transient creep where resistance increases due to material deformation. Predominates at low-temperature tests.
  2. Secondary Creep: Provides a nearly constant creep rate, known as the minimum creep rate.
  3. Tertiary Creep: Shows a rapid increase in the creep rate due to an effectively reduced cross-sectional area.

Compression Testing

Like the tensile test, the compression test is carried out on a Universal Testing Machine (UTM).

  • In a compression test, the material is subjected to end loading which produces a crushing action, whereas in a tension test, the piece elongates.
  • Specimens are limited in length to prevent bending due to column action.
  • Barreling: The bulging of the cylindrical specimen surface during compression.
  • A height-diameter ratio of 10 is suggested as a practical upper limit to avoid bending.

Brinell Hardness Test

The Brinell method consists of indenting the metal with a 10 mm diameter steel ball subjected to a load of 3000 kg. For soft materials, the load is reduced to 1500 or 500 kg.

Factors Influencing BHN

  • Magnitude of the indenting load.
  • Diameter of the ball indenter.
  • Elastic characteristics of the indenter ball.

Guidelines for Reliable Results

  1. Apply load gradually to avoid inertia errors.
  2. Ensure accurate measurement of the impression.
  3. Thickness (t) must be at least ten times the depth (h) of indentation (t > 10h).
  4. Spacing: Distance from the edge or another indentation should be at least 2.5 times the diameter of the indentation.
  5. Radius of curvature: Should not be less than 25 mm for a 10 mm ball.
  6. Load selection: Keep the ratio of indentation to ball diameter (d/D) between 0.25 and 0.60.

Rockwell Hardness Testing

The Rockwell test measures the additional depth to which a penetrator is forced by a major load beyond the depth of a previously applied minor load.

  • Minor Load: Establishes a datum line (zero setting).
  • Major Load: Applied to create the final indentation.
  • HR = K – e: The formula used to deduce the Rockwell Hardness Number.

Types of Rockwell Tests

  • Normal Rockwell Testing: Uses a 10 kg minor load (e.g., B and C scales).
  • Rockwell Superficial Testing: Used for thinner or sensitive materials (e.g., 30N and 30T scales).