HKIE Professional Assessment - Materials Discipline key terms

The stress at which a material begins to deform permanently, usually measured as the 0.2% offset proof stress for metals that show no distinct yield point. It sets the limit for design against plastic collapse.

The increase in strength and hardness when a metal is plastically deformed at a temperature below its recrystallisation point, caused by rising dislocation density and interaction. It raises strength but lowers ductility and toughness.

A heat treatment that heats metal to a chosen temperature, holds it, then cools slowly to soften it, relieve internal stress, and restore ductility. Full annealing also refines grain structure after cold working.

A hard, brittle phase formed when austenite is cooled so fast that carbon cannot diffuse, forcing a diffusionless lattice shear. Its hardness rises with carbon content, and it is usually tempered before service.

A steel's ability to form martensite at depth during quenching, governed mainly by alloy content and austenite grain size. It is assessed by end-quench or Jominy testing, not by the maximum hardness achieved.

Reheating quenched steel to a temperature below the lower critical temperature to reduce brittleness, relieve quenching stress, and trade some hardness for toughness. The tempering temperature is chosen against the required strength-toughness balance.

Heating steel above its upper critical temperature and cooling in still air to produce a fine, uniform ferrite-pearlite structure. It refines grains, improves machinability, and gives more predictable properties than as-rolled stock.

The capacity of a material to be joined by welding into a sound joint with adequate properties under a specified procedure. It depends on composition, especially carbon equivalent, thickness, and restraint, and on preheat and post-weld control.

A single index that combines the carbon and alloying element contents of steel to indicate its susceptibility to cold cracking and the preheat needed for welding. Higher values mean slower cooling and greater hardening risk.

The region of parent metal next to a weld that is thermally altered but not melted, so its microstructure and properties differ from the original plate. It often becomes the weakest or most crack-prone part of a joint.

The temperature range over which an amorphous polymer changes from a hard, glassy state to a rubbery, flexible one. Above it, stiffness and creep resistance fall sharply, so it bounds the service temperature of many plastics.

The behaviour of a material that shows both elastic and viscous response, so its stress-strain relation depends on time and loading rate. Polymers exhibit it through creep, stress relaxation, and hysteresis.

Time-dependent permanent deformation under sustained stress, significant above roughly 0.4 of the absolute melting temperature for metals and at room temperature for many polymers. It governs design of high-temperature plant and prestressed elements.

The gradual loss of stress in a material held at constant strain, as occurs in bolted joints, gaskets, and prestressing tendons over time. It is a viscoelastic or creep-driven effect and is checked by sustained-load testing.

The stress amplitude below which a material can endure an effectively unlimited number of load cycles without failure, shown as a horizontal asymptote on an S-N curve. Steels often show one; aluminium alloys generally do not.

A material's resistance to propagation of a sharp crack under load, expressed as a critical stress intensity factor. It lets designers size allowable defects and is central to fitness-for-service assessment.

A structured engineering assessment of whether a component containing a known defect or damage can safely continue in operation until the next inspection. It uses fracture mechanics, remaining-life calculation, and code acceptance criteria.

A systematic investigation to establish how and why a component failed, using site evidence, fractography, metallography, and mechanical testing. Its purpose is to find the root cause and prevent recurrence, not merely to assign blame.

The examination and interpretation of fracture surfaces to identify the failure mode and its origin. Features such as striations, dimples, cleavage facets, and beach marks distinguish fatigue, ductile, and brittle mechanisms.

Loss of ductility and premature cracking caused by hydrogen absorbed into a metal, often during plating, welding, or cathodic protection. High-strength steels are most vulnerable, and baking after plating is a common safeguard.

Crack growth caused by the combined action of a sustained tensile stress and a specific corrosive environment, often with little visible surface attack. It can cause sudden brittle failure of normally ductile alloys.

Localised breakdown of a passive film that produces small deep cavities, frequently on stainless steels in chloride-bearing environments. Pits act as stress raisers and initiation sites for cracking and perforation.

The formation of a thin, stable oxide film on a metal surface that greatly reduces further corrosion, as on stainless steel, aluminium, and titanium. Damage to the film locally can trigger pitting or crevice attack.

Accelerated attack of the less noble metal when two dissimilar metals are coupled in an electrolyte. Its severity depends on the potential difference, area ratio, and conductivity of the environment.

A technique that makes a metal structure the cathode of a corrosion cell, using sacrificial anodes or an impressed current, so that dissolution stops. It is widely applied to buried pipelines, ship hulls, and reinforced concrete.

Depassivation and subsequent corrosion of steel reinforcement caused by chloride ions penetrating concrete, leading to section loss and expansive cracking. It differs from carbonation-induced corrosion in mechanism and threshold.

The reaction of atmospheric carbon dioxide with calcium hydroxide in concrete, which lowers pore-solution pH and removes the passivity of embedded steel. Depth advances roughly with the square root of time and is checked by phenolphthalein testing.

A reaction between reactive silica in certain aggregates and alkalis in concrete pore solution, forming a gel that swells and cracks the concrete. Control relies on non-reactive aggregates, low-alkali cement, and supplementary cementitious materials.

Expansion and softening of hardened concrete caused by sulfate ions reacting with hydration products to form ettringite and gypsum. It is resisted by low-permeability concrete and sulfate-resisting or blended cements.

The ability of a material or structure to retain its required performance over its design life under its actual exposure conditions. It is assessed through deterioration mechanisms, cover and quality of concrete, and planned inspection and maintenance.

A notched-bar test in which a swinging pendulum fractures a specimen and the absorbed energy is recorded, indicating toughness and notch sensitivity. Results are comparative and depend on temperature, so transition curves are used.

Inspection methods that reveal surface or internal defects without damaging the component, including ultrasonic, radiographic, magnetic particle, dye penetrant, and eddy current techniques. Method choice depends on material, defect type, and geometry.

A test in which a specimen is pulled to fracture to determine yield strength, ultimate tensile strength, elongation, and reduction in area. These properties underpin design allowables and material certification.

Measurement of resistance to local indentation, using Brinell, Vickers, Rockwell, or Leeb methods. Hardness correlates approximately with tensile strength and is used for acceptance, heat-treatment control, and weld verification.

The systematic choice of a material for a component by screening candidates against functional requirements, then ranking survivors on objectives such as mass, cost, or stiffness. Property charts and performance indices support the process.

A statistically justified property value that designers may use for a material in a given product form and specification, derived from test data with an agreed margin. It is not simply the average from a mill certificate.

A structured method to quantify environmental impacts of a product or material across raw material extraction, manufacture, use, and end-of-life. It supports comparing options on a common basis rather than on single indicators.

The greenhouse gas emissions associated with extracting, manufacturing, transporting, and installing a material or product, expressed as carbon dioxide equivalent. It excludes operational emissions and is increasingly a basis for structural comparison.

A situation where a professional's personal, financial, or other interest could improperly influence engineering judgement or duty to a client. It must be disclosed promptly and managed, not merely avoided in private.

The standards of conduct expected of an engineer, covering competence, honesty, public safety, confidentiality, and fair dealing. Breach can lead to disciplinary action by the institution or the registration authority.