The tensile test is the most fundamental and widely specified mechanical test. A machined specimen — round or flat — is gripped at both ends in a universal testing machine and pulled in tension at a controlled rate until it fractures. The machine continuously records the applied load and the extension of a defined gauge length, and from these two measurements, the complete engineering stress-strain curve is generated.

Specimen geometry is standardised: for round specimens, a 12.7 mm diameter with a 50 mm gauge length is the ASTM standard; flat specimens have a width-to-thickness relationship governed by the product form (plate, sheet, pipe wall). The gauge length is marked on the specimen before testing and measured after fracture to calculate elongation.

Hardness is a material’s resistance to permanent surface deformation under a localised indenting force. Unlike tensile or impact tests, hardness testing is essentially non-destructive (the indentation is very small) and extremely rapid, making it the workhorse of production quality control and post-weld inspection. Three primary hardness scales are used in engineering.

Hardness testing is also used to verify post-weld heat treatment (PWHT). After PWHT of alloy steels such as Cr-Mo grades, hardness traverses confirm that the tempered martensite microstructure has been achieved and that weld zone hardness has been reduced to within specification. Hardness correlates approximately with tensile strength — for carbon steel, UTS (MPa) ≈ 3.3 × HV — providing a rapid indirect verification of strength.

The impact test measures the energy absorbed by a notched specimen when fractured by a single blow from a swinging pendulum. This energy, measured in Joules (J), is a direct measure of the material’s toughness — its ability to absorb energy before fracturing. The crucial importance of toughness is that it determines whether a material will fail in a ductile manner (with significant deformation and warning) or in a brittle manner (suddenly, with little deformation and no warning). For pressure vessels and pipelines in cold-climate service, brittle fracture is the dominant concern.

Impact testing is also the key tool for characterising the ductile-to-brittle transition temperature (DBTT). Most ferritic steels exhibit a sharp drop in absorbed energy as temperature decreases below the transition temperature. Impact tests are therefore conducted at multiple temperatures (e.g., 0°C, −20°C, −46°C) and the absorbed energy plotted against temperature to determine the DBTT and confirm compliance with the required design minimum temperature.

The bend test is the primary test used in welding procedure qualification (PQR) to assess weld ductility and to reveal defects at or near the weld surface. A rectangular strip cut from the weld test coupon is bent over a mandrel of specified diameter. The mandrel diameter is typically 4t (where t is the specimen thickness) for carbon and low-alloy steels. The specimen must be bent through 180° without showing open discontinuities on the tensile surface exceeding 3 mm in length (ASME IX criteria).

The bend test is particularly useful for revealing lack-of-fusion defects that can pass visual examination and even radiography but that cause the specimen to crack during bending. It is one of the most reliable indicators of weld metallurgical quality — a weld that bends cleanly through 180° without cracking has demonstrated adequate ductility and fusion throughout its cross-section.

Fatigue failure is responsible for the majority of structural and mechanical failures in service — estimates range from 50% to 90% of all engineering failures. Fatigue occurs when a component is subjected to repeated cyclic stresses that, individually, are well below the material’s ultimate tensile strength or even its yield strength. Over time, microscopic cracks initiate at stress concentrations — notches, weld toes, surface defects, inclusions — and propagate under each cycle until the remaining cross-section can no longer support the applied load and sudden fracture occurs.

The S-N curve (Wöhler curve) plots stress amplitude on the vertical axis against cycles to failure on a logarithmic horizontal scale. For ferrous metals, the curve typically flattens at the endurance limit — the stress level below which the material can withstand an infinite number of cycles. For carbon steel, the endurance limit is approximately 40–50% of the UTS. Non-ferrous metals such as aluminium and copper do not exhibit a true endurance limit and continue to fail at decreasing stress as cycles increase.

Creep is the slow, time-dependent plastic deformation of a material under sustained stress at elevated temperature. It occurs even when the applied stress is well below the material’s room-temperature yield strength. For most engineering metals, creep becomes significant above a homologous temperature (T/T_melt) of approximately 0.3–0.4 — which for carbon steel corresponds to roughly 370–450°C, and for nickel superalloys extends to 900°C and beyond.

A creep test applies a constant tensile load to a specimen held at a constant elevated temperature in a controlled furnace. The elongation of the specimen is measured continuously over time — typically weeks or months. The resulting creep curve shows three distinct stages:

• Primary creep (transient): Initial rapid deformation that decelerates as the material work-hardens
• Secondary creep (steady-state): Constant minimum creep rate where work hardening and recovery are in balance — the most important region for engineering design
• Tertiary creep: Accelerating deformation due to void formation, grain boundary cracking, and neck formation, culminating in creep rupture

Wear is the progressive material loss from a solid surface as a result of relative motion against another surface or medium. It is the dominant failure mode for cutting tools, bearings, gears, pump impellers, valve seats, and any component in sliding or rolling contact. The wear test quantifies the rate at which material is removed from a specimen under controlled contact conditions.

The standard pin-on-disk test (ASTM G99) uses a stationary pin pressed against a rotating disk under a controlled normal load. The wear rate is expressed as volume loss per unit sliding distance (mm³/m). Surface coatings applied by GTAW or thermal spray are routinely evaluated using wear tests to confirm that the hardface deposit will perform adequately in service.

Fracture toughness quantifies a material’s ability to resist crack propagation when a pre-existing crack is present. Classical strength-based design assumes defect-free material. Fracture mechanics acknowledges that all real engineering components contain some cracks or crack-like defects — from weld discontinuities to material inclusions — and asks: how large can a crack be before it becomes unstable and propagates catastrophically?

The plane-strain fracture toughness K_Ic is measured by loading a pre-cracked specimen (compact tension or three-point bend geometry) and recording the stress intensity at the onset of crack extension. K_Ic is the critical value of the stress intensity factor K_I in mode I (opening) loading under plane strain conditions.

The shear test measures a material’s resistance to forces applied parallel and opposite to the cross-section being tested — forces that try to cause one part to slide relative to an adjacent part. Shear strength is distinct from tensile strength: for ductile metals, shear strength is approximately 0.6 × tensile strength (Von Mises yield criterion), while for brittle materials, shear strength can equal or exceed tensile strength.

Shear testing is critical for evaluating fasteners (bolts, rivets), welded connections, adhesive bonds, and composite material layers. In welding, lap joint and fillet weld shear tests confirm the weld throat strength. In structural engineering, the design of fillet welds under shear loading uses the shear strength of the weld metal deposited, which must meet minimum values specified in ASME Section IX or AWS D1.1.