Mechanical Behaviour and Destructive Testing

Learning outcomes

The learner should be able to define fundamental mechanical properties, interpret a tensile curve, compare impact, bend, hardness and fracture tests, distinguish macro- from micro-examination and connect test sampling to NDT conclusions.

13.1 Stress, strain and elastic response

Engineering normal stress is applied force divided by original cross-sectional area:

sigma = F / A0

Engineering strain is change in gauge length divided by original gauge length:

epsilon = Delta L / L0

Within a linear elastic region, Young's modulus is:

E = sigma / epsilon

Stress has units of pressure, commonly MPa. Strain is dimensionless but may be reported as a percentage. Modulus describes elastic stiffness, not yield strength. Two steels can have similar modulus while their yield strength differs greatly.

As a ductile tensile specimen is loaded, it first deforms elastically, then yields and accumulates plastic strain. Strain hardening raises the required load until maximum engineering stress. Local necking follows, and fracture occurs. Yield strength, tensile strength, elongation and reduction of area describe different parts of the response. A brittle material may fail with little plastic deformation and no obvious neck.

13.2 Toughness, fatigue and fracture

Toughness is the ability to absorb energy through deformation and fracture. Charpy impact testing breaks a notched specimen with a pendulum and reports absorbed energy at a stated temperature and orientation. It is valuable for transition behaviour and quality control but is not a direct component fracture-toughness value.

Fracture-mechanics tests estimate resistance to crack extension using controlled specimen geometry and validity rules. Fatigue tests apply cyclic load and may produce stress-life, strain-life or crack-growth data. Results depend on stress ratio, environment, surface, frequency and specimen preparation.

13.3 Bend and hardness tests

Bend tests assess ductility, soundness or the behaviour of a weld face, root or side under imposed bending. As the specimen bends around the former, its resulting outer convex surface is placed in tension, making opening discontinuities visible. Test angle, former diameter, specimen orientation and acceptance criteria must be controlled.

Hardness tests measure resistance to indentation or rebound under specified conditions. Brinell uses a ball and measures indentation diameter. Vickers uses a diamond pyramid and measures diagonals. Rockwell derives depth change under minor and major loads. Portable rebound or ultrasonic-contact-impedance methods require their own controls. Hardness conversions are approximate and material dependent; report the measured scale rather than laundering it through a convenient chart.

13.4 Macro and micro examination

Macroscopic examination uses unaided or low magnification on a prepared section to show weld penetration, fusion, segregation, flow or large discontinuities. Microscopy at higher magnification reveals phases, grains, precipitates and small cracks after suitable preparation and etching. Sampling is destructive and local. A clean section does not prove the entire component sound, while NDT coverage does not provide the same microstructural detail.

13.5 Linking destructive and non-destructive evidence

Mechanical specimens must have traceable location and orientation. A transverse weld tensile test, longitudinal plate Charpy specimen and surface hardness check answer different questions. NDT can map larger volumes and locate conditions, then destructive testing can characterise selected regions. Correlation requires coordinate control and awareness that sectioning may relax stress or miss the peak response.

Practical checkpoint

Given a tensile curve and specimen dimensions, calculate engineering stress and strain at two points and identify modulus region, yield, ultimate strength, necking and fracture. Then match five NDT findings to suitable confirmation methods without assuming that confirmation is always required or authorised.

Common errors and misconceptions

Several common errors recur in this topic: confusing stiffness with strength; treating Charpy energy as fracture toughness; converting hardness values without material-specific validity; generalising one metallographic section to the whole item; and ignoring test orientation and temperature.

Chapter summary

Mechanical tests quantify how a material responds to controlled load, while macro and micro examination reveal structure locally. Stress, strain, strength, ductility, toughness, hardness and stiffness are distinct. Sampling and orientation control are essential when destructive evidence is correlated with NDT.