Mechanical Properties of Materials

Tensile Strength

Definition: The maximum stress a material can withstand while being stretched or pulled before breaking.

Key Points

• Measured in units of stress (MPa or N/mm²).
• Determined from the highest point on the engineering stress–strain curve.
• Indicates a material’s ability to resist failure under tension.

Testing Method

• Tensile Test: A standardized specimen is pulled in a universal testing machine until fracture.
• Data obtained: yield strength, tensile strength, elongation, reduction of area.

Factors Affecting Tensile Strength

• Material composition and microstructure.
• Heat treatment and work hardening.
• Temperature and strain rate.

Applications

Design of load-bearing structures, cables, fasteners, and pressure vessels.

Impact Strength

Definition: The ability of a material to resist fracture under sudden loading or shock.

Key Points

• Reflects toughness — the capacity to absorb energy before fracturing.
• Important for materials used in dynamic or impact-prone environments.

Testing Method

• Charpy V-notch Test: Measures energy absorbed by a notched specimen broken by a swinging pendulum.
• Izod Test: Similar principle, but specimen orientation differs.

Factors Affecting Impact Strength

• Temperature (ductile-to-brittle transition in some steels).
• Notch sensitivity and stress concentrators.
• Microstructure and grain size.

Applications

Automotive crash components, helmets, pressure vessels, and structural steels in cold climates.

Fatigue Strength

Definition: The maximum stress a material can withstand for a specified number of cycles without failure under fluctuating or cyclic loading.

Key Points

• Fatigue accounts for the majority of mechanical failures in service.
• Failure occurs at stress levels below the tensile strength due to crack initiation and propagation.

Testing Method

• Rotating Bending Test / Axial Fatigue Test: Specimens are subjected to repeated loading until failure.
• Results plotted as S–N curves (stress vs. number of cycles).

Factors Affecting Fatigue Strength

• Surface finish and residual stresses.
• Environmental effects (corrosion fatigue).
• Mean stress and load ratio.

Applications

Aircraft components, springs, rotating shafts, and bridges.

Creep

Definition: Time-dependent, permanent deformation of a material under constant stress, occurring at high temperature relative to its melting point.

Key Points

• Significant in metals at ≥0.4 T_m (absolute temperature).
• Critical for components under load for long periods at elevated temperatures.

Stages of Creep

1. Primary (transient): Decreasing creep rate due to strain hardening.
2. Secondary (steady-state): Constant creep rate; longest stage.
3. Tertiary: Accelerating creep leading to rupture due to necking, grain boundary voids, or cracks.

Testing Method

• Creep Test: Specimen under constant load at constant temperature; strain measured over time.

Applications

Turbine blades, boilers, pressure vessels, and high-temperature piping.

Hardness

Definition: Resistance of a material to localized plastic deformation (e.g., indentation, scratching, or abrasion).

Key Points

• Correlates with wear resistance and, in metals, often with tensile strength.
• Different hardness scales exist for different materials and applications.

Testing Methods

• Brinell Hardness Test (BHN): Spherical indenter; measures indentation diameter.
• Rockwell Hardness Test (HR): Conical or spherical indenter; measures penetration depth.
• Vickers Hardness Test (VHN): Diamond pyramid indenter; measures diagonal length of indentation.
• Mohs Scale: Scratch resistance scale from 1 (talc) to 10 (diamond).

Factors Affecting Hardness

• Material composition and heat treatment.
• Work hardening and surface condition.

Applications

Tool steels, wear-resistant coatings, bearings, and cutting tools.