Material Defects And Failures In Service
(toc)
1: Introduction
The response of metals to various stages of manufacture,
construction or service life can vary widely depending upon chemical
composition, heat treatment, mechanical working, surface conditions, presence
of discontinuity and other material characteristics.
2: Causes Of Material Failure
Products and .structures may be subjected to a number of
service conditions, as mentioned below, which may result in discontinuity.
a. stationary load.
b. dynamic load, unidirectional or multidirectional,
multi-directional - more serious.
c. high temperature.
d. pressure creating stress above a material's elastic
limit.
g. corrosive environment.
f: vibrations.
g: excess loading.
h. improper maintenance.
i. ageing.
3: Types Of Material Failure
There are two generally accepted types of material failure
in service: one is the easily recognized 'FRACTURE' or separation into two or
more parts, the second is the less easily recognized "EXCESSIVE PLASTIC
DEFORMATION" or change of shape and/or position.
4: Service Conditions Leading To Material Failures
4.1: Corrosion
Corrosion is the deterioration of metals by the chemical
action of some surrounding or contacting medium which may be liquid, gas or
some combination of the two. This deterioration can be either uniform or
localized. To some degree, corrosion can influence all metals, but the effect
varies widely depending upon the combination of the metal and the corrosive
agent..
4.2: Fatigue
Failures occurring under conditions of dynamic loading are
called "FATIGUE FAILURES". Most service failures occur as a result of
tensile stress.
4.3: Wear
Wear is probably the most important factor in the
deterioration of machinery with moving components, often limiting both the life
and the performance of such equipment. Wear is the loss of material from the
surface. Wear is affected by a variety of conditions, such as the type of
lubrication, loading, speed, temperature, materials, surface finish and
hardness.
4.4: Overstress
It may happen when a part is accidentally exposed to a load which is much greater than its design load. When this happens, the component may undergo plastic deformation or fracture, to relieve the high stress within.