OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.4 Materials - laminates – Page 57
Guidance note:
In general, the frictional force is associated with the expenditure of energy in the contact region, and it is the process
of energy dissipation that may lead to destruction of the surface layers and to the eventual wearing of the material.
While both friction and wear are the result of surface interaction, there is often no absolute correlation between the
two. Especially the rate of wear may change by several orders of magnitude by varying certain factor of the wear
systems and the material properties, yet the friction force remains nearly constant. However, frictional forces are a
prerequisite for wear of materials.
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4.6.4 In most polymer sliding systems, wear takes place by one of the following processes or a combination
of them: adhesive wear, abrasive wear, fatigue wear and corrosive wear. In practical situations, the four types
of wear interact in a complex and unpredictable way. In addition, the wear process may modify the contact
surfaces and thus change the relative importance of the separate mechanisms:
— Adhesive wear arises as a result of a process by which isolated spots on two sliding surfaces adhere together
momentarily, weld or stick together, removing a wear particle. It often involves the transfer of material
from one surface to the other. It is the only wear mechanism that is always present and, unlike the others,
cannot be eliminated. Friction is not involved in adhesive wear.
— Abrasive wear occurs especially when the surface of a material is loaded by hard and sharp mineral
particles. In addition, abrasion can be effective when relatively soft materials slide against rough metallic
counterparts. In that case, the abrasive wear is usually highest for the softest material. Abrasive wear is the
most destructive wear mechanism and produce the highest material loss in the shortest time. Abrasive wear
may be the result of a two body abrasion (e.g. a surface against sandpaper) or a three body abrasion (e.g.
sand particles between two moving surfaces).
— Fatigue wear arises from cyclic loading of surface layers with repetitive compressive and tangential
stresses. Material is removed after fatigue crack growth in and below the surface by producing spalled
particles. Fatigue wear is extremely small compared with adhesion or abrasion.
4.6.5 The following wear properties are defined to characterise the properties of a wear system:
•
— The length related wear rate w designates the ratio between the wear depth dy (thickness of removed
material) and the sliding distance dx. It is dimensionless.
•
dy
w =
dx
(m/m)
•
•
— The specific wear rate w S
designates the ratio between the wear rate and the contact pressure p between
the two surfaces. It has the dimension of a (stress) -1 w
.
•
•
w
w S = (m3/Nm)
p
•
— The wear factor k* is numerically the same as the specific wear rate w . It has the dimension of a (stress) -1 .
•
S
— The time related wear rate w t
designates the ratio between the wear depth dy (thickness of removed
material) and the sliding time dt. It has the dimension of a speed.
4.6.6 The wear properties are related together according to the following equations:
where: v = dx / dt - is the sliding speed.
•
dy
w t =
dt
k* = w
4.6.7 The (pv) factor is used as a performance criterion for bearings. The (pv) factors are widely quoted in the
literature and may take one of the two forms:
— the “limiting” (pv) above which wear increases rapidly either as a consequence of thermal effects or stresses
approaching the elastic limit
— the (pv) factor for continuous operation at some arbitrarily specified wear rate.
Guidance note:
In neither case is the (pv) factor a unique criterion of performance because the assumptions made in the derivation of
the equations are usually valid over only a very restricted range of p and v (see Figure 4-4 below). At low speed, the
maximum pressure that can be used is limited by the strength of the material, and, as this pressure is approached, the
specific wear rate no longer remains independent of load but begins to increase as a result of possible changes in the
wear mechanisms. At high speeds, the generation of frictional heat raises the temperature of the surface layers and
tends to increase the specific wear rate.
(m/s)
• •
•
w wt
S
= =
p pv
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