Embedment
Encyclopedia
Embedment is a phenomenon in mechanical engineering
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.
. When the loading of the joint varies (e.g. due to vibration
or thermal expansion
) 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.
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.
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 washers
or disc springs) can also reduce the loss of bolt pre-load due to embedment.
Mechanical engineering
Mechanical engineering is a discipline of engineering that applies the principles of physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the...
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 creepCreep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
. When the loading of the joint varies (e.g. due to vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
or thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
) 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 fatigueFatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
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.
Prevention/Solutions
Embedment can be prevented by designing mating surfaces of a joint to have high surface hardness and very smooth surface finish. 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 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 type of spring shaped like a washer. It has a frusto-conical shape which gives the washer a spring characteristic. The Belleville name comes from the inventor...
or disc springs) can also reduce the loss of bolt pre-load due to embedment.