ASTM G40-22a

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Designation: G40 − 22 G40 − 22a
Standard Terminology Relating to
Wear and Erosion
This standard is issued under the fixed designation G40; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 The terms and their definitions given herein represent terminology relating to wear and erosion of solid bodies due to
mechanical interactions such as occur with cavitation, impingement by liquid jets or drops or by solid particles, or relative motion
against contacting solid surfaces or fluids. This scope interfaces with but generally excludes those processes where material loss
is wholly or principally due to chemical action and other related technical fields as, for instance, lubrication.
1.2 This terminology is not exhaustive; the absence of any particular term from this collection does not necessarily imply that its
use within this scope is discouraged. However, the terms given herein are the recommended terms for the concepts they represent
unless otherwise noted.
1.3 Certain general terms and definitions may be restricted and interpreted, if necessary, to make them particularly applicable to
the scope as defined herein.
1.4 The purpose of this terminology is to encourage uniformity and accuracy in the description of test methods and devices and
in the reporting of test results in relation to wear and erosion.
NOTE 1—All terms are listed alphabetically. When a subsidiary term is defined in conjunction with the definition of a more generic term, an
alphabetically-listed cross-reference is provided.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C242 Terminology of Ceramic Whitewares and Related Products
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
3. Terminology
abradant, n—a material that is producing, or has produced, abrasive wear.
This terminology is under the jurisdiction of ASTM Committee G02 on Wear and Erosion and is the direct responsibility of Subcommittee G02.91 on Terminology.
Current edition approved June 1, 2022Nov. 1, 2022. Published July 2022November 2022. Originally approved in 1973. Last previous edition approved in 20212022 as
G40 – 21a.G40 – 22. DOI: 10.1520/G0040-22.10.1520/G0040-22A.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G40 − 22a
abrasion, n—in tribology, the process by which relative motion between a surface and hard particles or protuberances on an
opposing surface produces abrasive wear of that surface. (See also abrasive wear.)
abrasive wear, n—wear due to hard particles or hard protuberances forced against and moving along a solid surface.
abrasion-corrosion, n—a synergistic process involving both abrasive wear and corrosion in which each of these processes is
affected by the simultaneous action of the other and, in many cases, is thereby accelerated.
abrasivity, n—the ability of a material or substance to cause abrasive wear.
absolute impact velocity—See impact velocity.
acceleration period, n—in cavitation and liquid impingement erosion, the stage following the incubation period during which
the erosion rate increases from near zero to a maximum value. (See also erosion rate-time pattern.)
accumulation period, n—in cavitation and liquid impingement erosion, a less-preferred term for acceleration period.
adhesive wear, n—wear due to localized bonding between contacting solid surfaces leading to material transfer between the two
surfaces or loss from either surface.
angle of attack, n—in impingement erosion, the angle between the direction of motion of an impinging liquid or solid particle
and the tangent to the surface at the point of impact.
angle of incidence, n—in impingement erosion, the angle between the direction of motion of an impinging liquid or solid particle
and the normal to the surface at the point of impact.
apparent area of contact, n—in tribology, the area of contact between two solid surfaces defined by the boundaries of their
macroscopic interface. (Contrast with real area of contact.)
asperity, n—in tribology, a protuberance in the small-scale topographical irregularities of a solid surface.
attenuation period, n—in cavitation and liquid impingement erosion, a less-preferred term for deceleration period.
average erosion rate, n—a less preferred term for cumulative erosion rate. (See also interval erosion rate.)
Beilby layer, n—an altered surface layer of supposedly amorphous material formed on a crystalline solid during mechanical
polishing, whose existence was proposed in Sir George Beilby’s writings. The existence of such a layer is not supported by
recent research, and the use of this term is therefore considered archaic and is strongly discouraged.
bio-tribocorrosion, n—in the field of medical devices and dental restoratives, a form of tribocorrosion in which biological
substances make up one or more of the triboelements or are present in the contact region between them.
break-in,n—See run-in.
break in,v—See run in.
brinelling, n—damage to a solid bearing surface characterized by one or more plastically formed indentations caused by static
or impulsive overloads, especially as found in rolling contact bearings. (See also false brinelling.)
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brittle erosion behavior, n—erosion behavior having characteristic properties that can be associated with brittle fracture of the
exposed surface; that is, little or no plastic flow occurs, but cracks form that eventually intersect to create erosion fragments.
(See also ductile erosion behavior.)
DISCUSSION—
In solid impingement an easily observable aspect of erosion helps to distinguish brittle from ductile behavior. This is the manner in which volume
removal varies with the angle of attack. With brittle erosion the maximum volume removal occurs at an angle near 90°, in contrast to approximately
25° for ductile erosion behavior.
carrier fluid, n—in impingement or slurry erosion, fluid medium that transports impinging solid or liquid particles and that gives
the particles their momentum relative to the solid surface on which they are impinging.
catastrophic period, n—in cavitation or liquid impingement erosion, a stage during which the erosion rate increases so
drastically that continued exposure threatens or causes gross disintegration of the exposed surface. This stage is not inevitable;
it is observed most commonly with some brittle materials. When it does occur, it may begin during any stage of the more
common erosion rate-time pattern.
catastrophic wear, n—rapidly occurring or accelerating surface damage, deterioration, or change of shape caused by wear to
such a degree that the service life of a part is appreciably shortened or its function is destroyed.
cavitating disk device (or apparatus), n—a flow cavitation test device in which cavitating wakes are produced by holes in, or
protuberances on, a disk rotating within a liquid-filled chamber. Erosion test specimens are attached flush with the surface of
the disk, at the location where the bubbles are presumed to collapse.
cavitating jet, n—a continuous liquid jet (sometimes submerged) in which cavitation is induced by the nozzle design, or
sometimes by an obstruction placed in the center of the flow passage.
cavitating wake,n—See flow cavitation.
cavitation, n—the formation and subsequent collapse, within a liquid, of cavities or bubbles that contain vapor or a mixture of
vapor and gas.
DISCUSSION—
Cavitation originates from a local decrease in hydrostatic pressure in the liquid, usually produced by motion of the liquid (see flow cavitation) or of
a solid boundary (see vibratory cavitation). It is distinguished in this way from boiling, which originates from an increase in liquid temperature.
DISCUSSION—
The term cavitation, by itself, should not be used to denote the damage or erosion of a solid surface that can be caused by it; this effect of cavitation
is termed cavitation damage or cavitation erosion. To erode a solid surface, bubbles or cavities must collapse on or near that surface.
cavitation cloud, n—a collection of a large number of cavitation bubbles. The bubbles in a cloud are small, typically less than
1 mm (0.04 in.) in cross section. A surface that is being eroded by cavitation is usually obscured by a cavitation cloud.
cavitation damage,n—See damage.
cavitation erosion, n—progressive loss of original material from a solid surface due to continued exposure to cavitation.
cavitation erosion test, n—a procedure whereby the surface of a solid is subjected to cavitation attack under specified, or
measurable, or at least repeatable conditions.
DISCUSSION—
Such tests can be divided into two major classes depending on whether flow cavitation or vibratory cavitation is generated.
cavitation number, σ, n—a dimensionless number that measures the tendency for cavitation to occur in a flowing stream of
liquid, and that is computed from the equation:
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σ5 P 2 P / ρV (1)
~ !
o v o
where:
P = vapor pressure,
v
P = static pressure in the stream in an undisturbed state,
o
V = undisturbed stream velocity, and
o
ρ = liquid density.
DISCUSSION—
The cavitation number and the net positive suction head (NPSH) are related by the equation:
NPSH 5 ~σ11!V /2g (2)
o
where g is the acceleration due to gravity.
cavitation tunnel, n—a flow cavitation test facility in which liquid is pumped through a pipe or tunnel, and cavitation is induced
in a test section by conducting the flow through a constriction, or around an obstacle, or a combination of these.
closed tribosystem, n—a tribosystem, in which the control volume has been defined, where where the boundaries with its
surroundings have been defined, for which influences or substances external to said system can neither enter nor affect its
behavior. (See also open tribosystem and tribosystem.)
coefficient of friction μ or f,n—in tribology, the dimensionless ratio of the friction force (F) between two bodies to the normal
force (N) pressing these bodies together. (See also static coefficient of friction and kinetic coefficient of friction.)
μ 5 ~F/N! (3)
collection efficiency, n—in impingement erosion and particulate flows, the cross-sectional area of undisturbed fluid containing
particles that will all ultimately impinge on a given solid surface, divided by the projected area of the solid surface, where these
two areas are perpendicular to the direction of relative motion between the solid surface and the particles in the undisturbed fluid.
DISCUSSION—
“Undisturbed fluid” means fluid that is sufficiently ahead of the solid surface to be undisturbed by the flow around the solid surface. For example, the
particles could be carried in a stream of fluid moving toward a solid surface that is stationary, or the solid surface could be moving through a suspension
of particles. Not all of the particles that move in the direction of the solid surface or lie in its path will impinge upon it, since some will be carried
away in the fluid as it flows around the surface.
DISCUSSION—
A variety of terms having the same meaning can be found in the literature. These include “collision efficiency,” “capture efficiency,” “catchment
efficiency,” “impaction ratio,” and others. The term “collection efficiency,” being perhaps the most widely used, is preferred.
continuous jet,n—See liquid jet.
corrosive wear, n—wear in which chemical or electrochemical reaction with the environment is significant.
cumulative erosion, n—in cavitation and impingement erosion, the total amount of material lost from a solid surface during all
exposure periods since it was first exposed to cavitation or impingement as a newly-finished surface. (More specific terms that
may be used are cumulative mass loss, cumulative volume loss, or cumulative mean depth of erosion. See also cumulative
erosion-time curve.)
DISCUSSION—
Unless otherwise indicated by the context, it is implied that the conditions of cavitation or impingement have remained the same throughout all
exposure periods, with no intermediate refinishing of the surface.
cumulative erosion rate, n—the cumulative erosion at a specified point in an erosion test divided by the corresponding
cumulative exposure duration; that is, the slope of a line from the origin to the specified point on the cumulative erosion-time
curve. (Synonym: average erosion rate)
cumulative erosion-time curve, n—in cavitation and impingement erosion, a plot of cumulative erosion versus cumulative
exposure duration, usually determined by periodic interruption of the test and weighing of the specimen. This is the primary
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record of an erosion test. Most other characteristics, such as the incubation period, maximum erosion rate, terminal erosion rate,
and erosion rate-time curve, are derived from it.
cutting wear, n—in solid impingement erosion, the erosive wear associated with the dissipation of kinetic energy of impact
arising from the tangential component of the velocity of the impacting particles.
DISCUSSION—
Since erosion due to oblique particle impact inevitably involves deformation wear as well as cutting wear, the magnitude of the cutting wear can be
experimentally determined by conducting a separate test at normal impact to determine the deformation wear, and subtracting that from the total wear
at any angle of impact, where both tests are conducted with the same normal component of impact velocity and both results are normalized to the mass
of impacting particles. See also related terms deformation wear, ductile erosion behavior, and brittle erosion behavior.
damage, n—in cavitation or impingement, any effect on a solid body resulting from its exposure to these phenomena. This may
include loss of material, surface deformation, or any other changes in microstructure, properties, or appearance.
DISCUSSION—
This term as here defined should normally be used with the appropriate modifier, for example, “cavitation damage,” “liquid impingement damage,”
“single-impact damage,” and so forth.
debris, n—in tribology, particles that have become detached in a wear or erosion process.
deceleration period, n—in cavitation or liquid impingement erosion, the stage following the acceleration period or the
maximum rate period (if any) during which the erosion rate has an overall decreasing trend although fluctuations may be
superimposed on it. (See also erosion rate-time pattern.)
deformation wear, n—in solid impingement erosion, the erosive wear of a material associated with the dissipation of kinetic
energy of impact arising from the normal component of the velocity of the impacting particles. It is therefore the sole component
of wear for particles impacting at a 90° angle of attack.
DISCUSSION—
This term is used for the erosion of brittle materials, even though plastic deformation is lacking. See also related terms brittle erosion behavior, ductile
erosion behavior, and cutting wear.
distributed impact test, n—in impingement erosion testing, an apparatus or method that produces a spatial distribution of
impacts by liquid or solid bodies over an exposed surface of a specimen.
DISCUSSION—
Examples of such tests are those employing liquid sprays or simulated rainfields. If the impacts are distributed uniformly over the surface, the term
“uniformly distributed impact test” may be used. (Contrast with repetitive impact erosion test.)
drop, liquid, n—see liquid drop.
drop size, n—the diameter of a liquid drop if it is approximately spherical; otherwise, the approximate shape and appropriate
dimensions must be described.
DISCUSSION—
In a spray or rainfall, there will normally be a spectrum of drop sizes, which can be presented by distribution curves or histograms, showing either
number of drops or combined volume of drops as a function of drop size. A representative drop size for a distribution is afforded by the sauter mean
diameter, or else by the size interval containing the largest total volume.
ductile erosion behavior, n—erosion behavior having characteristic properties that can be associated with ductile fracture of the
exposed solid surface; that is, considerable plastic deformation precedes or accompanies material loss from the surface which
can occur by gouging or tearing or by eventual embrittlement through work hardening that leads to crack formation. (See also
brittle erosion behavior.)
DISCUSSION—
In solid impingement, two easily observable aspects of erosion help to distinguish ductile erosion behavior. The first is the manner in which volume
removal varies with the angle of attack. Ductile materials show maximum volume removal for angles from approximately 20 ° to 30°, in contrast to
near 90° for brittle erosion behavior. A second indication of ductile behavior is the characteristic ripple pattern that forms on the exposed surface at
low values of angle of attack.
erodant, n—a material that is producing, or has produced, erosive wear.
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erosion, n—in tribology, progressive loss of original material from a solid surface due to mechanical interaction between that
surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles.
DISCUSSION—
Because of the broad scope of this term, it is recommended that it normally be qualified to indicate the relevant mechanism or context, for example,
cavitation erosion, liquid impingement erosion, solid impingement erosion, beach erosion, and so forth.
erosion-corrosion, n—a synergistic process involving both erosion and corrosion, in which each of these processes is affected
by the simultaneous action of the other, and in many cases is thereby accelerated.
erosion rate, n—any determination of the rate of loss of material (erosion) with exposure duration. (See also rationalized
erosion rate.)
DISCUSSION—
Erosion rate is usually determined as a slope on the cumulative erosion-time curve. Since in cavitation or liquid impingement this curve is generally
not a straight line, it is necessary to specify how any particular numerical value was determined from this curve. The following more explicit terms
may be used: average erosion rate, instantaneous erosion rate, interval erosion rate, maximum erosion rate, and terminal erosion rate. See individual
definitions of these terms.
erosion rate-time curve, n—a plot of instantaneous erosion rate versus exposure duration, usually obtained by numerical or
graphical differentiation of the cumulative erosion-time curve. (See also erosion rate-time pattern.)
erosion rate-time pattern, n—any qualitative description of the shape of the erosion rate-time curve in terms of the several
stages of which it may be composed.
DISCUSSION—
In cavitation and liquid impingement erosion, a typical pattern may be composed of all or some of the following “periods” or “stages:” incubation
period, acceleration period, maximum-rate period, deceleration period, terminal period, and occasionally catastrophic period. The generic term
“period” is recommended when associated with quantitative measures of its duration, and so forth; for purely qualitative descriptions the term “stage”
is preferred.
erosion resistance, n—a test and measurement dependent characteristic of a triboelement in a tribosystem that describes the
relative amount of erosive wear measured, or to be expected, in a test or application. (See also normalized erosion rate.)
DISCUSSION—
Erosion resistance is typically reported in relative terms, for example, “Material A is more erosion resistant than Material B,” or “Material A is 3.5
times more erosion resistant than Material B.”
exposure duration, n—in erosion or wear, exposure time, or any other appropriate measure of the accumulation of exposure
to an erosion or wear environment.
DISCUSSION—
For impingement erosion, some alternative duration parameters are the number of impacts that have occurred on a given point, or the mass or volume
of particles that have impinged on a unit area of exposed surface. For wear, it may be the sliding distance traveled.
false brinelling, n—damage to a solid bearing surface characterized by indentations not caused by plastic deformation resulting
from overload but thought to be due to other causes such as fretting corrosion. (See also brinelling.)
fatigue wear, n—wear of a solid surface caused by fracture arising from material fatigue.
flow cavitation, n—cavitation caused by a decrease in local pressure induced by changes in velocity of a flowing liquid.
Typically, this may be caused by flow around an obstacle or through a constriction, or relative to a blade or foil. A cavitation
cloud or “cavitating wake” generally trails from some point adjacent to the obstacle or constriction to some distance downstream,
the bubbles being formed at one place and collapsing at another.
fretting, n—in tribology, small amplitude oscillatory motion, usually tangential, between two solid surfaces in contact.
DISCUSSION—
Here the term fretting refers only to the nature of the motion without reference to the wear, corrosion, or other damage that may ensue. The term fretting
is often used to denote fretting corrosion and other forms of fretting wear. Usage in this sense is discouraged due to the ambiguity that may arise.
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fretting corrosion, n—a form of fretting wear in which corrosion plays a significant role.
fretting wear, n—wear arising as a result of fretting (see fretting).
friction force, n—the resisting force tangential to the interface between two bodies when, under the action of an external force,
one body moves or tends to move relative to the other. (See also coefficient of friction.)
friction loop, n—in tribology, a graphical construct of data from a single forward and reverse stroke in a reciprocating sliding
or fretting test in which the magnitude of friction force or friction coefficient is plotted on the vertical axis and the relative
tangential displacement of the moving body is plotted on the horizontal axis.
DISCUSSION—
When presenting the results of fretting experiments, a sequence of friction loops is commonly stacked to create a quasi-three dimensional plot.
galling, n—a form of surface damage arising between sliding solids, distinguished by macroscopic, usually localized,
roughening, and the creation of protrusions above the original surface; it is characterized by plastic flow and may involve
material transfer.
hard particle erosion, n—deprecated term; use the preferred synonyms solid impingement erosion or solid particle erosion.
Hertzian contact area, n—the apparent area of contact between two nonconforming solid bodies pressed against each other,
as calculated from Hertz’ equations of elastic deformation.
Hertzian contact pressure, n—the magnitude of the pressure at any specified location in a Hertzian contact area, as calculated
from Hertz’ equations of elastic deformation.
impact angle, n—in impingement erosion, an angle that could be either the angle of attack or the angle of incidence, which
see. Because of this ambiguity, this term should be specially defined when used or, preferably, used only in contexts where the
ambiguity does not matter.
impact velocity, n—in impingement erosion, the relative velocity between the surface of a solid body and an impinging liquid
or solid particle.
DISCUSSION—
To describe this velocity completely, it is necessary to specify the direction of motion of the particle relative to the solid surface in addition to the
magnitude of the velocity. The following related terms are also in use:
(1) absolute impact velocity—the magnitude of the impact velocity.
(2) normal impact velocity—the component of the impact velocity that is perpendicular to the surface of the test solid at the point of impact.
impact wear, n—wear due to collisions between two solid bodies where some component of the motion is perpendicular to the
tangential plane of contact.
impingement, n—in tribology, a process resulting in a continuing succession of impacts between (liquid or solid) particles and
a solid surface.
DISCUSSION—
In preferred usage, “impingement” also connotes that the impacting particles are smaller than the solid surface, and that the impacts are distri
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