Embedment is a phenomenon in
mechanical engineering
Mechanical engineering is the study of physical machines and mechanism (engineering), mechanisms that may involve force and movement. It is an engineering branch that combines engineering physics and engineering mathematics, mathematics principl ...
in which the surfaces between mechanical members of a loaded joint embed. It can lead to failure by fatigue as described below, and is of particular concern when considering the design of critical
fastener joints.
Mechanism
The mechanism behind embedment is different from
creep
The Committee for the Re-election of the President (or the Committee to Re-elect the President, CRP, but often mocked by the acronym CREEP) was, officially, a fundraising organization of United States President Richard Nixon's 1972 re-election ...
. When the loading of the joint varies (e.g. due to
vibration
Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...
or
thermal expansion
Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions).
Substances usually contract with decreasing temp ...
) the protruding points of the imperfect surfaces will see local stress concentrations and yield until the stress concentration is relieved. Over time, surfaces can flatten an appreciable amount in the order of thousandths of an inch.
Consequences
In critical fastener joints, embedment can mean loss of
preload. Flattening of a surface allows the strain of a screw to relax, which in turn correlates with a loss in tension and thus preload. In bolted joints with particularly short
grip lengths, the loss of preload due to embedment can be especially significant, causing complete loss of preload. Therefore, embedment can lead directly to loosening of a fastener joint and subsequent
fatigue
Fatigue is a state of tiredness (which is not sleepiness), exhaustion or loss of energy. It is a signs and symptoms, symptom of any of various diseases; it is not a disease in itself.
Fatigue (in the medical sense) is sometimes associated wit ...
failure.
In bolted joints, most of the embedment occurs during torquing. Only embedment that occurs after installation can cause a loss of preload, and values of up to 0.0005 inches can be seen at each surface mate, as reported by SAE.{{Citation needed, reason=the numbers, also use ft to m, so the numbers are given in international recoginzed format, date=July 2008
Prevention and solutions
Embedment can be prevented by designing mating surfaces of a joint to have high surface
hardness
In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...
and very smooth
surface finish
Surface finish, also known as surface texture or surface topography, is the nature of a interface (matter), surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a ...
. Exceptionally hard and smooth surfaces will have less susceptibility to the mechanism that causes embedment.
In most cases, some degree of embedment is inevitable. That said, short grip lengths should be avoided. For two bolted joints of identical design and installation, except the second having a longer grip length, the first joint will be more likely to loosen and fail. Since both joints have the same loading, the surfaces will experience the same amount of embedment. However, the relaxation in strain is less significant to the longer grip length and the loss in preload will be minimized. For this reason, bolted joints should always be designed with careful consideration for the grip length.
If a short grip length can not be avoided, the use of conical spring washers (
Belleville washer
A Belleville washer, also known as a coned-disc spring, conical spring washer, disc spring, Belleville spring or cupped spring washer, is a conical shell which can be loaded along its axis either statically or dynamically. A Belleville washer is ...
s or disc springs) can also reduce the loss of bolt pre-load due to embedment.
See also
*
Stress relaxation
In materials science, stress relaxation is the observed decrease in stress in response to strain generated in the structure. This is primarily due to keeping the structure in a strained condition for some finite interval of time hence causing som ...
References
* Comer, Dr. Jess. (2005); "Source of Fatigue Failures of Threaded Fasteners"
* T. Jaglinski, et al. (2007); "Study of Bolt Load Loss in Bolted Aluminum Joints"
External links
SAE Fatigue Design and Evaluation Committee
Mechanical engineering
Reliability engineering
Fasteners
Materials degradation