Testing of Materials Notes

Last reviewed 16 Sept 2026 · 10 min read

Purpose of testing

Destructive testing (DT)	Non-destructive testing (NDT)
Specimen is damaged or destroyed	Specimen/structure remains usable
Gives direct, quantitative properties (strength, elongation)	Often indirect — needs correlation/calibration
On samples (cubes, coupons)	On actual components in situ
Tensile, compression, bend, impact, fatigue, core tests	Ultrasonic, radiography, rebound hammer, magnetic particle, dye penetrant

Hydraulic or electromechanical machine for tension, compression, bending and shear tests.
Components: loading unit, load measuring system (load cell/pressure gauge), grips/platens, extensometer for strain measurement, control and data acquisition.
Machines must be calibrated periodically against reference standards.

Standard specimen (round or flat) with a gauge length; load increased gradually to fracture.
Results: stress–strain curve, modulus of elasticity, yield strength / 0.2% proof stress, ultimate tensile strength, percentage elongation and reduction in area (ductility), fracture type (cup-and-cone for ductile metals).

FormulaTension test relations

σ = \frac{P}{A _{0}}, ε = \frac{Δ L}{L _{0}}, % elongation = \frac{L _{f} - L _{0}}{L _{0}} \times 100, % reduction in area = \frac{A _{0} - A _{f}}{A _{0}} \times 100

True stress $= P / A_{in s t an t an eo u s}$ ; true strain $= ln (L / L_{0})$

For reinforcement bars, elongation measured on a gauge length related to bar diameter (e.g. $5.65 A_{0}$ proportional gauge length).

For brittle materials — concrete cubes/cylinders, bricks, stone, cast iron, mortar cubes.
Platen friction confines ends; specimen shape and size (cube vs cylinder, height/diameter ratio) affect results — cube strength is generally higher than cylinder strength.

Test	Load arrangement	Modulus of rupture
Three-point bending	Central point load on simply supported beam	$f_{r} = \frac{3 P L}{2 b d ^{2}}$
Four-point bending (third-point loading)	Two loads at one-third span points — constant moment in middle third	$f_{r} = \frac{P L}{b d ^{2}}$ (failure within middle third)

Used for concrete flexural strength (pavements), timber, bricks/tiles, FRP.

Direct shear (single/double shear of rivets, bolts, pins); torsion test on shafts — shear modulus $G = \frac{T L}{J θ}$ and shear strength.

Test	Indenter / method	Result
Brinell (HB)	Hardened steel/carbide ball (commonly 10 mm) under load (e.g. 3000 kgf); measure indentation diameter	$H B = \frac{2 P}{π D ( D - D ^{2} - d ^{2} )}$ — suits castings, soft/medium metals
Rockwell (HRB, HRC …)	Steel ball (B scale) or diamond cone (C scale); depth of penetration under minor + major load	Quick direct reading — hardened steels (HRC)
Vickers (HV)	Square-based diamond pyramid (136°); diagonal of indentation	Wide range, thin sections; microhardness
Knoop	Elongated diamond	Microhardness of brittle/thin materials
Shore (scleroscope / durometer)	Rebound height or indentation of elastomers	Rubbers, plastics (Shore A, D)
Mohs scale	Scratch hardness 1–10	Minerals

Izod — cantilever notched specimen struck by a pendulum.
Charpy — simply supported notched specimen (V- or U-notch) struck at mid-span.
Energy absorbed (J) indicates toughness; tests at various temperatures reveal the ductile–brittle transition temperature — important for steels in cold regions and welded structures.

Specimens subjected to cyclic stresses (rotating bending, axial loading) until failure → S–N (Wöhler) curve; endurance limit for steels.
Relevance: bridges, crane girders, rail components, wind turbine towers, offshore structures, welded connections.

Constant load at constant temperature; strain measured over long periods → creep curve (primary, secondary, tertiary); creep rupture strength.
Relevance: prestressing losses, high-temperature components; concrete creep in long-term deflection.

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