Glossary of Terms
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Abrasion:
A process where hard particles are forced against and moved along a solid surface.
Abrasive:
Hard particles, such as sand, rocks or fragments of certain hard metals, that wear away a material's surface when they move across it under pressure.
Abrasive Wear:
Abrasive wear occurs when a hard material scratches or gouges the surface of a softer material. The abrasive material may be one of the sliding surfaces or particles between the two surfaces. When the contact stress are too low to crush the abrasive particles, the cutting action is defined as low-stress scratching abrasion. This usually results in surface scratches with little sub-surface deformation.
Adhesive Failure:
A break in an adhesive bond such that the separation appears at the adhesive-adherend interface. Often termed failure in adhesion.
Adhesive Wear:
Wear caused by localized bonding between solid contacting surfaces leading to material transfer between the two solid surfaces or loss from either surface.
Anelastic Deformation:
Any amount of the total deformation of a body occurs as a function of time when a load is applied and disappears completely after a period of time when the load is removed.
Anode:
(1) The term anode refers to the electrode in an electrolytic cell where oxidation takes place. In the external circuit, electrons move away from the anode. Typically, this electrode is where corrosion happens and metal ions dissolve into the solution. (2) It is the positively charged, electron-deficient electrode in an electrochemical setup. This is in contrast to the cathode.
Anode Corrosion:
The dissolution of a metal acting as an anode.
Anodizing:
Depositing a type of conversion coating on a metal surface via anodic oxidation. Anodizing is most commonly applied to aluminum substrates as most conventional coatings adhere poorly to the base metal. ALCOAT is a thin dense chrome technology specifically engineered for aluminum and offers improved performance over traditional anodizing.
Applied Stress:
A degree of stress applied to a component or assembly resulting from external load or force.
Asperity:
In tribology, a protuberance found in the minute-scale topographical irregularities of a solid surface.
Atmospheric Corrosion:
The alteration or gradual degradation of a solid material by contact with substances found in the atmosphere, such as oxygen, water vapor, carbon dioxide, and sulfur and chlorine compounds.
Atomic Wear:
Wear between contacting surfaces in relative motion is attributed to the migration of individual atoms from one solid surface to the other.
Austenite:
Austenite refers to a solid solution comprised of one or more elements within the face-centered cubic structure of iron (known as gamma iron). In instances where specific elements are mentioned, like nickel austenite, the solute is typically understood to be carbon unless stated otherwise.
Bainite:
Bainite is a metastable combination of ferrite and cementite formed when austenite transforms at temperatures falling below the pearlite range but above the martensite start temperature (Ms). Upper bainite is characterized by parallel lath-shaped ferrite units, giving rise to a "feathery" appearance under optical microscopy, and typically forms above roughly 350°C (660°F). Lower bainite, displaying an acicular structure akin to tempered martensite, develops at temperatures below approximately 350°C (660°F).
Beach Marks:
Macroscopic features refer to visible lines found on a fatigue fracture, indicating the position of the fatigue crack tip at a particular moment. It's important to distinguish these from striations, which are much smaller and formed through a different process.
Bearing Steels:
Bearing steels are alloy steels employed in the fabrication of rolling element bearings. Traditionally, bearings have been crafted from both high-carbon (1%) and low-carbon (0.20%) steels. High-carbon steels are utilized either in a through-hardened state or subjected to surface induction hardening. Conversely, low-carbon bearing steels undergo carburization to achieve the required surface hardness while retaining desirable core properties.
Bearing Stress:
Bearing stress denotes the shear load experienced by a mechanical joint, like a pinned or riveted joint, divided by the effective bearing area. In the case of a riveted joint, for instance, the effective bearing area is calculated by multiplying the sum of the diameters of all rivets by the thickness of the loaded member.
Biofouling:
Metal deterioration as a result of the metabolic activity of microorganisms. More commonly known as biological corrosion.
Biological Corrosion:
Metal deterioration as a result of the metabolic activity of microorganisms. Also known as biofouling.
Blister (metals):
A raised area, usually dome-shaped, resulting from loss of adhesion between a deposit or coating and the base metal.
Brinelling:
Indentation found on the surface of a solid body by repeated local impact, or static overload.
Brittle Erosion Behavior:
Erosive behavior having distinct properties (e.g., crack formation, little or no plastic flow) that may be associated with brittle fracture of the exposed metal surface. Maximum volume reduction occurs at an approximate angle of 90°.
Brittle Fracture:
The separation of a solid material accompanied by little macroscopic plastic deformation, if any at all. Contrast with ductile fracture.
Brittleness:
The potential for a material to fracture without first undergoing extensive plastic deformation.
Buckling:
A failure mode generally characterized by an unstable lateral material deflection due to compressive action on the structural element involved.
Catastrophic Wear:
Sudden deterioration, surface damage, or change of shape to a component induced by wear to such an extent that the life of the component is appreciably shortened or action is impaired.
Cathode:
Cathode refers to the negative electrode within an electrolytic cell where reduction is primarily observed. (Electrons move towards the cathode in the external circuit.) Common cathodic processes involve cations accepting electrons and undergoing discharge, reduction of oxygen, and reduction of an element or group of elements from a higher to a lower valence state. This is distinct from anode.
Cathodic Disbondment:
The destruction of adhesion between a substrate and its coating by products of cathodic reaction.
Cathodic Protection:
Cathodic Protection is a corrosion mitigation technique aimed at reducing the corrosion rate of metal structures by shifting their electrode potential towards a less oxidizing state through the application of an external electrical current. This method helps safeguard metals from corrosion damage, either partially or completely, by rendering them cathodic, typically achieved through either galvanic means or by applying an impressed current.
Caustic Cracking:
A form of stress-corrosion cracking frequently encountered in iron-chromium nickel alloys or carbon steels that become exposed to concentrated levels of hydroxide solutions at temperatures of 400 to 480°F (200 to 250°C).
Cavitation:
The formation and collapse of numerous small bubbles during turbulent flow. The ultrasonic shock of the collapsing bubbles scrubs the metal surface. This can cause long term surface loss.
Cavitation Damage:
Degradation of a solid body resulting from its exposure to cavitation. Cavitation damage may include surface deformation, loss of material, or changes in appearance or properties.
Cavitation Erosion:
Progressive loss of material from a solid surface due to continuous exposure to cavitation.
Chafing Fatigue:
Fatigue formed in a surface body damaged by rubbing against another body.
Charpy Test:
The Charpy test involves an impact examination where a specimen, featuring V-notches, keyhole-notches, or U-notches and supported at both ends, is struck behind the notch by a striker attached to the lower end of a swinging bar, operating as a pendulum. The energy absorbed during fracture is determined by comparing the striker's potential height in the absence of a specimen to its actual height following the specimen's fracture.
Cleavage Fracture:
A fracture, typically of a polycrystalline metal, in which most grains have failed by cleavage, resulting in radiant reflecting facets.
Cohesive Failure:
A failure of an adhesive joint mainly occurring in an adhesive layer.
Cold Cracking:
Cracks that form in cold or nearly cold cast metals due to excessive internal stress brought on by contraction. Cracking typically occurs below 400°F (205°C). Cracking may also occur during or after cooling the cast metal to room temperature, occasionally with a significant time delay.
Compressive Strength:
Compressive Strength refers to the highest compressive stress a material can withstand, determined by its original cross-sectional area. When a material fails due to compression via a shattering fracture, the compressive strength is precisely defined. However, if failure occurs without shattering, the compressive strength value becomes somewhat arbitrary, contingent upon the extent of distortion considered indicative of complete material failure.
Compressive Stress:
A stress causing an elastic material body to deform in the direction of the applied load.
Concentration Cell:
In an electrolytic setup, the electromotive force arises from divergent concentrations of a constituent in the electrolyte. This discrepancy initiates the creation of separate cathode and anode zones within the cell.
Contact Fatigue:
Cracking and successive pitting of a material surface subjected to alternating stresses like those produced under rolling contact or combined sliding and rolling. Contact fatigue is encountered most commonly in rolling-element bearings or gears.
Contamination:
A foreign substance or impurity in a material or environment that affects one or more properties of the material.
Corrosion:
The destruction or deterioration of a material, typically a metal, due to its interactions with the surrounding environment. Corrosion is a natural process and occurs when the substance is in contact with air, water, chemicals like acids, etc.
Corrosion Embrittlement:
The significant loss of ductility in a metal caused by a corrosive attack, typically intergranular and often not apparent to the naked eye.
Corrosion Fatigue:
A process where metal fractures prematurely under circumstances of concurrent corrosion and repeated cyclic loading at fewer cycles or lower stress levels than would be needed in the absence of the corrosive environment.
Corrosive Wear:
Wear of a material in which there is significant chemical or electrochemical reaction with the environment.
Crack Branching:
The splitting of a material into two or more segments.
Crater Wear (tooling):
Wear occurring on the rake face of a cutting tool caused by contact with the material in the chip that is sliding along that face.
Craze Cracking:
Atypical cracking of a metal surface associated with thermal cycling. The term craze cracking is more common in the United Kingdom than in the United States, where the term heat checking is used instead.
Creep:
A Time-dependent strain that occurs under stress. The creep strain occurring at a minimum and nearly constant rate is called tertiary creep; that occurring at a diminishing rate is called primary creep.
Creep-Rupture Embrittlement:
Embrittlement under creep conditions of steels and aluminum alloys, creating abnormally low rupture ductility. In steels, the mechanism is related to the number of impurities present, such as sulfur, copper, arsenic, phosphorus, and tin. In aluminum alloys, iron in quantities above the solubility limit is a common cause of such embrittlement. In either scenario, failure ensues by intergranular cracking of the embrittled material.
Crevice Corrosion:
Localized corrosion of a metal surface at, or adjacent to, an area that is covered from exposure to its environment due to close proximity between the metal and the surface of another metal or material.
Crystalline Fracture:
A pattern of reflecting crystal facets on the fracture surface of a polycrystalline metal occurs from the cleavage fracture of multiple individual crystals.
Cyclic Load:
Repetitive loading on a part that sometimes leads to fatigue fracture. Cyclic load may also be defined as loads that change value by following a regular repeating sequence of change.
Dealloying:
Also known as selective leaching or parting, dealloying is caused by the selective corrosion of one or more components made up of a solid solution alloy.
Decarburization:
Depletion of carbon from the surface layer of a carbon-containing alloy caused by a reaction with one or multiple chemical substances in a medium that contacts the surface.
Decobaltification:
Corrosion where cobalt is selectively leached from cobalt-base alloys.
Decohesive Rupture:
A form of brittle fracture displaying little or no plastic deformation and does not occur by cleavage, fatigue, or dimple rupture. This fracture type is usually the result of a reactive environment or a unique microstructure and is associated nearly exclusively with rupture along the grain boundaries of a solid surface.
Denickelification:
Corrosion where nickel is selectively leached from alloys containing nickel. A common occurrence is found in copper-nickel alloys after extensive service under water.
Deposit Corrosion:
Corrosion occurring around or beneath a discontinuous deposit on a metallic surface body. Also termed poultice corrosion.
Dezincification:
Corrosion where zinc is selectively leached from alloys containing zinc. This corrosion process leaves behind a moderately weak layer of copper and copper oxide. A common occurrence of dezincification is found on copper-zinc alloys possessing less than 85% copper after a long period of service in water containing dissolved oxygen.
Droplet Erosion:
Wear of erosive nature created by impingement of liquid droplets onto a solid surface.
Ductile Crack Propagation:
Gradual crack propagation accompanied by apparent plastic deformation. Energy must be supplied from outside the body for ductile crack propagation to occur. Contrast with brittle crack propagation.
Ductile Fractures:
A fracture characterized by the tearing of a metal material followed by appreciable plastic deformation and expenditure of substantial energy. Contrast with brittle fracture
Ductility:
The ability of a material to deform plastically without fracturing.
Dynamic Creep:
Creep occurring under conditions of fluctuating temperature or fluctuating load.
Elastic Deformation:
A shift in dimensions directly proportional to and in phase with a decrease or increase in applied force.
Elastic Limit:
The highest stress level that a material can endure without retaining any permanent strain or deformation once the stress is completely relieved. When the applied load is adequate to induce plastic, or irreversible, deformation, the material is considered to have exceeded its elastic limit.
Electrical Pitting:
Surface cavities created by the removal of metal due to an electrical discharge across an interface.
Electrochemical Corrosion:
Corrosion accompanied by a flow of electrons between anodic and cathodic areas on a metallic surface.
Electrocorrosive Wear:
Wear of a solid surface accelerated by the existence of a corrosion-inducing electrical potential across the contact interface.
Electrode:
An electrode is a conductor that facilitates the transfer of electric current into or out of a medium via electrochemical reactions. It is characterized by its material properties, including electrical conductivity, electrochemical activity, and structural integrity. Electrodes are integral to devices such as batteries, fuel cells, and sensors, serving as sites for oxidation (anode) or reduction (cathode) reactions that involve electron transfer.
Electrolyte:
A chemical substance or solution typically in liquid form, containing ions that move under the influence of an electric field. Additionally, it denotes a chemical compound or a combination of compounds that, when melted or dissolved, has the capacity to conduct an electric current.
Electrolytic Corrosion:
Corrosion by way of mechanical or electrochemical action.
Environmentally Assisted Cracking:
Brittle fracture of a typically ductile material by which the corrosive effect of the environment is a causative factor.
Erosion:
Material loss from the abrasive action of moving fluids on a component.
Erosion Corrosion:
A combined action consisting of corrosion and erosion in the presence of a moving corrosive fluid, leading to an accelerated loss of material.
Etch Cracks:
Shallow cracks in hardened steel containing high residual surface stresses, produced by etching in an embrittling acid.
Exfoliation Corrosion:
Corrosion that moves laterally from the areas of initiation along planes parallel to the solid surface, typically at the grain boundaries, creating corrosion products that force metal away from the body of the solid material, giving rise to a layered appearance.
False Brinelling:
Damage to a solid bearing surface characterized by indentations not brought on by plastic deformation resulting from overload but thought to be caused by other failure mechanisms such as fretting corrosion. False brinelling may also be defined as local spots appearing when the protective film on a metal material is damaged continually by repeated impacts, usually in the presence of a corrosive agent.
Fatigue Failure:
Failure that occurs when a material undergoing fatigue fractures into two parts and has softened or been otherwise significantly reduced in toughness by thermal cracking or heating.
Fatigue Limit:
The highest stress level that is expected to cause fatigue failure within a defined number of stress cycles. It is essential to specify both the maximum stress value and the stress ratio associated with it.
Ferrous:
Metallic materials in which the primary component is iron, such as steel, stainless steel, and cast iron.
Fibrous Fracture:
Corrosion occurs under the surface of some coatings that takes the form of randomly distributed, threadlike filaments. Sometimes referred to as underfilm corrosion.
Filiform Corrosion:
Corrosion occurs under the surface of some coatings that takes the form of randomly distributed, threadlike filaments. Sometimes referred to as underfilm corrosion.
Flake (metals):
A short, sporadic internal rupture in ferrous metals attributed to stresses created by hydrogen-solubility effects and localized transformation during cooling of the metal after hot working.
Flank Wear:
Loss of relief on the flank of a tool behind the cutting edge caused by rubbing contact between the work piece and the tool during cutting.
Flexural Failure:
Failure of a material caused by repeated flexing.
Flow Cavitation:
A form of cavitation caused by a reduction in static pressure caused by changes in the velocity of a flowing fluid.
Fracture Mechanics:
Fracture Mechanics involves a quantitative examination used to assess the structural performance concerning applied stress, crack length, and the geometry of the specimen or machine component.
Fretting:
Material loss that occurs between tight-fitting surfaces that are subject to vibrational movements (such as riveted, other fastened joints and electrical connections). Material loss is from a combination of oxidative and abrasive wear. The oscillation of the two surfaces causes the formation of oxide films that are then abraded away by oxidized wear debris. The affected surfaces sometimes look as if they were mechanically deformed, so this wear is sometimes called false brinelling. It is also referred to as fretting corrosion, friction oxidation, chafing fatigue, and wear oxidation.
Fretting Corrosion:
The accelerated deterioration at the interface between two contacting surfaces resulting from corrosion and slight oscillatory motion between the contacting surfaces. Fretting corrosion is generally characterized by the subsequent formation of oxides and the removal of particles, which are often abrasive and thus increase wear. Fretting corrosion can involve other chemical reaction products, which may not always be abrasive.
Fretting Fatigue:
Progressive damage to a solid surface arising from fretting. If particles of wear debris are produced, the term fretting wear may be applied.
Fretting Wear:
Wear occurring as a result of fretting.
Friction:
Resisting force tangential to the shared boundary between two material bodies when, under the act of an external force, one body tends to move relative to the surface of the second body.
Galling:
A mechanism whereby excessive friction between high spots (asperities) results in localized welding with subsequent spalling and further roughening of the rubbing surface of one or both of two mating parts.
Galvanic Cell:
A cell in which electrical energy is generated directly from a chemical reaction. Typically, it includes two different conductors, each in contact with an electrolyte, or two identical conductors each interacting with different electrolytes. This setup facilitates a natural redox (oxidation-reduction) reaction that drives electrons to flow through an external circuit, harnessing this movement to produce electricity.
Galvanic Corrosion:
Corrosion associated with a galvanic cell's current consisting of two dissimilar conductors in an electrolyte or two similar conductors in dissimilar electrolytes.
Gaseous Corrosion:
A form of corrosion where gas is the only corrosive agent and there is no aqueous phase on the metal surface. Also known as dry corrosion.
General Corrosion:
Corrosion controlled by uniform thinning that proceeds without appreciable localized attack.
Gouging Abrasion:
A form of high-stress abrasion where observable gouges or grooves are created on a material surface.
Green Rot:
A high-temperature attack on stainless steels, nickel-chromium alloys, and nickel-chromium-iron alloys when subject to simultaneous carburization and oxidation. The attack appears first by precipitation of chromium as chromium carbide, followed by oxidation of the carbide particles.
Heat Checking:
A process in which fine cracks populate on the surface of a material body in sliding contact caused by buildup of excessive frictional heat.
High-Cycle Fatigue:
Fatigue that occurs at a relatively high number of cycles (10^5).
High-Stress Abrasion:
A form of abrasion where large cutting forces are imposed on the protuberances or particles causing the abrasion, producing significant cutting and deformation of the wearing surface. In most metals, high-stress abrasion can result in substantial surface strain hardening. High-stress abrasion is commonly found in agricultural and mining equipment and in highly loaded bearings where hard particles become trapped between mating surfaces.
High-Temperature Hydrogen Attack:
Loss of ductility and strength of steel via high-temperature reaction of absorbed hydrogen with carbides found in the steel resulting in internal fissuring and decarburization.
Holidays:
Discontinuities in a coating (such as gaps, cracks, porosity, and similar flaws) allowing areas of the base metal to become exposed to any corrosive environment that contacts the coated surface.
Hoop Stress:
Circumferential stress in a material of cylindrical form when subjected to external or internal pressure.
Hot Corrosion:
Accelerated corrosive attack of metal surfaces caused by the combined effect of oxidation and reactions with sulfur compounds and other contaminants, such as chlorides, to form a molten salt on a metal surface that disrupts, fluxes, or destroys the natural protective oxide layer.
Hot Cracking:
Formation of cracks in a weldment created by segregation at the grain boundaries of low-melting constituents in the weld metal. These cracks may cause grain-boundary tearing under thermal contraction stresses. Proper joint design accompanied by the use of low-impurity welding materials can reduce the likelihood of hot cracking.
Hydrogen Blistering:
The formation of blisters on or beneath a metal surface caused by excessive internal hydrogen pressure. Hydrogen may form during plating, cleaning, or corrosion.
Hydrogen Embrittlement:
A process resulting in a reduction of the ductility or toughness of a metal material caused by the presence of atomic hydrogen.
Hydrogen-Induced Delayed Cracking:
A term occasionally used to identify a condition of hydrogen embrittlement where a metal seems to fracture spontaneously under ongoing stress less than the metal's expected yield stress. There is often a delay between applying stress and the onset of cracking. The term is more commonly referred to as static fatigue.
Impact Wear:
Wear of a solid surface caused by repeated impacts between that surface and another solid surface body.
Impingement Attack:
Corrosion associated with the turbulent flow of a liquid. an impingement attack can be accelerated by entrained gas bubbles.
Impingement Corrosion:
A form of erosion-corrosion usually associated with local impingement of a high-velocity, flowing liquid against a solid surface.
Impingement Erosion:
Wear caused by suspended solid particles is referred to as impingement erosion. Material resistance to impingement erosion varies with the angle of particle impingement and material hardness.
Interdendritic Corrosion:
A corrosive attack that moves preferentially along interdendritic paths. This form of attack results from differences in local composition. A typical example would be coring often encountered in alloy castings.
Intergranular Corrosion:
Corrosion arising preferentially at grain boundaries, typically with subtle or negligible attack on the adjacent grains.
Intergranular Cracking:
Cracking or fracturing that ensues between the crystals or grains in a polycrystalline aggregate. Also termed intercrystalline cracking.
Intergranular Fracture:
Brittle fracture of a polycrystalline material where the fracture initiates between the crystals, or grains, that form the material. Also known as intercrystalline fracture.
Intergranular Stress-Corrosion Cracking:
Stress-corrosion cracking where the cracking takes place along grain boundaries.
Liquid Metal Embrittlement:
Catastrophic brittle failure of a ductile metal when in contact with a liquid metal and subsequently stressed in tension.
Localized Corrosion:
Corrosion formed at discrete sites on a material. Forms of localized corrosion include crevice corrosion, pitting, and stress-corrosion cracking.
Low Cycle Fatigue:
Fatigue occurring at moderately small numbers of cycles (<10^4 cycles). Some plastic or permanent deformation may accompany low-cycle fatigue.
Low-Stress Abrasion:
Abrasion where moderately low contact pressures on the abrading protuberances or particles cause merely fine scratches and microscopic cutting chips to be produced.
Martensite:
Martensite refers to specific microstructures created through a diffusionless transformation, maintaining a defined crystallographic relation between the original and new phases. This transformation produces a needle-like pattern in the microstructure of both ferrous and nonferrous alloys. The characteristics of martensite can vary depending on the placement of solute atoms within its lattice: when these atoms are in interstitial positions, like carbon in iron, the resulting structure is hard and highly strained. Conversely, when solute atoms are in substitutional positions, such as nickel in iron, the martensite is softer and more ductile. The proportion of the high-temperature phase that changes into martensite during cooling largely depends on the minimum temperature reached, with a distinct onset temperature (Ms) and a finishing temperature (Mf) where the transformation is nearly complete.
Metal Dusting:
Accelerated breakdown of a metal in carbonaceous gases and at elevated temperatures to create a dust-like corrosion product.
Microbial Corrosion:
See Biological Corrosion
Necking:
The decrease of the cross-sectional area of a metal in a localized area by uniaxial strain or by stretching.
Nonferrous:
The term nonferrous describes a metallic material where iron makes up less than 50% of its composition, such as aluminum, magnesium, copper, zinc, and zinc alloys. Contrast to ferrous, where a metallic material's primary composition is iron (more than 50%) such as steel, stainless steel, and cast iron.
Orange Peel (metals):
Roughening of a surface takes the form of a pebble-grained pattern occurring when a metal of unusually rough grain size stresses beyond its elastic limit. Sometimes referred to as alligator skin.
Oxidation:
A corrosion reaction where the corroded metal creates an oxide; usually applied by reaction with a gas composed of elemental oxygen, such as air.
Oxidative Wear:
A corrosive process of wear in which chemical reaction with an oxidizing environment prevails.
Pitting:
Formation of small sharp cavities in a surface by wear, corrosion, or other forms of mechanically assisted degradation.
Plastic Deformation:
The permanent deformation of materials under applied stresses that strain the material well beyond its elastic limit.
Porosity:
Intricate pores or holes within a solid material; the amount of pores is defined as a percentage of the total volume of the solid material.
Poultice Corrosion:
See Deposit Corrosion
Primary Creep:
The initial stage of creep, or time-dependent deformation.
Quasi-Cleavage Fracture:
A form of fracture that combines the traits of dimple fracture and cleavage fracture. This moderate form of fracture is found in particular high-strength metals.
Quenching Crack:
A crack developed in a metal due to thermal stresses created by rapid cooling from a high temperature.
Radial Crack:
A crack formed by a sharp, hard object when pressed onto the surface of brittle material. The crack will look semi-elliptical in shape and is typically perpendicular to the surface.
Rain Erosion:
A type of liquid impingement erosion caused by raindrops. Rain erosion is usually encountered on the external surfaces of fixed-winged and rotary-wing aircraft.
Ridging Wear:
A deep form of scratching in parallel ridges typically induced by plastic flow of the subsurface layer.
Rock Candy Fracture:
A fracture displaying separated grain facets. Rock candy fracture is commonly used to describe intergranular fractures occurring in large-grained metals.
Rolling Contact Fatigue:
The repeated stress of a solid surface caused by rolling contact between it and another solid surface. Continuous rolling contact fatigue of gear or bearing surfaces can result in rolling-contact damage in the form of subsurface fatigue cracks, material pitting and spallation, or both.
Rosette (star) Fracture:
Often occurring in temper-embrittled steels; rosette fractures, or star fractures, are a form of tensile fracture that exhibits a central fibrous zone, an outer circumferential shear-lip zone, and an intermediate region of radial shear.
Rouge:
Finely separated, hydrated iron oxide.
Rupture Stress:
The stress at which a material fails. Also termed fracture stress or breaking stress.
Rust:
An observable corrosion product consisting of hydrated iron oxides. Rust is applied solely to ferrous alloys.
Scabbing:
Scabbing may carry two meanings. In tribology, scabbing is a term that refers to the formation of bulges on the surface. In fracture mechanics, scabbing is simply an alternative word for spalling.
Selective Leaching:
A form of corrosion where one element is preferentially removed from an alloy, leaving a residue of elements that are more resistant to the particular environment involved. Commonly referred to as dealloying or parting.
Shear Bands:
Bands of significantly high shear strain are observed during sheet metal rolling.
Shear Fracture:
A form of fracture in crystalline materials resulting from translation along slip planes that are oriented preferentially toward the shearing stress.
Shelling:
An advanced stage of spalling in railways. Shelling is also a term used to describe failure caused by the deterioration of a coated abrasive component or product where the entire abrasive grains separate from the cement coating that held the abrasive to the support layer of the component.
Shock Load:
The sudden application of an external force creates a highly rapid internal stress build-up, such as piston loading in a combustion engine.
Shot Peening:
Shot peening is a cold working process that embeds compressive stresses into the surface layers of metal components through the high-velocity impact of shot. This technique is carried out under meticulously controlled conditions, differentiating it significantly from blast cleaning, which primarily aims to clean surfaces. In shot peening, while cleaning occurs, it is a secondary outcome; the primary objective is to enhance the metal's fatigue strength. The materials used for the shot in peening typically include iron, steel, or glass, selected to ensure durability and effectiveness.
Slant Fracture:
Slant fracture refers to a specific category of fracture in metals, often observed under plane stress conditions. This fracture features a separation plane that is angled, typically around 45 degrees, relative to the axis of the stress being applied.
Slurry Erosion:
Erosion created by the movement of a slurry along a solid surface.
Solder Embrittlement:
A metal's mechanical properties is reduced due to local penetration of solder along grain boundaries.
Solid-Metal Embrittlement:
Embrittlement occurring in a material below the melting point of the embrittling species.
Solidification Shrinkage Crack:
The formation of cracks, typically at elevated temperatures, due to the internal shrinkage that develops during the solidification of a metal casting. Hot crack is more often used to describe this phenomenon.
Spalling:
The breakup of particles from a material's surface in the form of flakes. Spalling typically occurs due to subsurface fatigue and is more extensive than pitting.
Step Fracture (metals):
Cleavage fractures that form on multiple parallel cleavage planes. Step fractures usually occur during the first stage of fatigue fractures, such as low-stress or high-cycle fractures.
Stray Current Corrosion:
Corrosion caused by a direct flow of current through paths other than the intended circuit.
Stress Concentration:
Stress concentration refers to the phenomenon where the intensity of applied stress is amplified at the site of a notch, void, hole, or inclusion within a material, particularly observed at a macromechanical scale.
Stress Corrosion Cracking:
A cracking process requiring the simultaneous action of sustained tensile stress and a corrodent. Stress corrosion cracking does not include corrosion-reduced sections that fail by rapid fracture.
Stretcher Strains:
Elongated marks appearing on sheet material surfaces when deformed slightly past the yield point.
Striation:
Striation in metals is a characteristic feature of fatigue fractures, commonly visible in electron micrographs. It marks the location of the crack front following each stress cycle. The spacing between these striations reveals the progression of the crack front through the crystal during a single cycle of stress. Additionally, a line perpendicular to the striations shows the direction of the crack's local advancement. This feature is often discussed in conjunction with beach marks, which are related phenomena.
Subsurface Corrosion:
The formation of secluded particles of corrosive products under a metal surface. Subsurface corrosion results from the preferential responses of particular alloy constituents to the inward diffusion of nitrogen, oxygen, or sulfur.
Sulfidation:
A process where metals or alloys interact with substances containing sulfur, resulting in the formation of sulfur compounds either on or below their surface.
Sulfide Stress Cracking:
Brittle fracture caused by cracking under the conjoined action of corrosion and tensile stress in the presence of hydrogen sulfide and water.
Surface Distress:
Damage to the contacting surfaces of gears and bearings that occurs via intermittent contact involving a slight degree of surface fatigue, sliding, or both.
Tensile Stress:
Stress causing two parts of an elastic body, on either side of the typical stress plane, to separate.
Tension:
The load or force that produces elongation
Thermal Embrittlement:
A form of intergranular fracture that reduces the toughness of maraging steels due to imprecise processing after hot working.
Thermal Fatigue:
A fracture caused by the presence of temperature gradients that change with time in such a way as to produce cyclic stresses in a material's structure.
Thermal Shock:
The development of a steep temperature gradient and accompanying elevated stresses within a structure or material.
Thermal Stress Cracking:
The cracking and crazing of various thermoplastic resins caused by over exposure to elevated temperatures.
Thermal Wear:
Material removal caused by melting, softening, or evaporation during rolling or sliding contact.
Toe Crack:
A crack within the base metal appearing at the toe of a weld.
Torsion:
A twisting action causing shear stresses and strains.
Torsional Stress:
A twisting action causing shear stress on a transverse cross section of a material.
Transcrystalline Cracking:
Fracturing or cracking that occurs across or through a crystal. Also known as intracrystalline cracking.
Transgranular Cracking:
Cracking that occurs through or across a grain or crystal. Also known as transcrystalline cracking.
Transgranular Fracture:
Fracture across or through the grains or crystals of a material. Also termed transcrystalline fracture.
Tuberculation:
A term used to describe a form of biological corrosion in which the formation of localized corrosive products scatters over a surface in the shape of knoblike mounds called tubercles.
Underfilm Corrosion:
A form of corrosion occurring under organic films in randomly distributed spots or threadlike filaments. In most cases, underfilm corrosion is identical to filiform corrosion.
Uniform Corrosion:
A form of corrosion attack uniformly dispersed over a metal surface.
Uniform Strain:
The strain occurring before the beginning of necking (localization of strain).
Vibratory Cavitation:
Cavitation created by pressure fluctuations within a fluid or liquid caused by the vibration of a solid metal surface submerged in the liquid.
Viscous Deformation:
See Elastic Deformation
Warpage (metals):
Distortion that may occur during stress relieving, annealing, and high-temperature service.
Wear Scar:
The portion of a surface showing evidence that material has been removed due to the influence of one or multiple wear processes.
White Rust:
Another term for zinc oxide; is the powdery product of corrosion of zinc or zinc-coated surfaces.
Yield:
Plastic deformation in structural materials. Most often referred to as creep.
Zinc Worms:
Surface defects characteristic of high-zinc brass castings occur when vapor from the zinc condenses at the mold or metal interface, where it goes through oxidation and becomes entangled in the solidifying metals.
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