Thermography detection on the fatigue damage of the specimen alloy - Essay Example
Fatigue occurs when a material is subjected to periodic stress which is below its tensile breaking or yield stress but which is enough to cause permanent damage. The formal definition of fatigue as given by Wikipedia (n.d.1) is:
[F]atigue is the progressive, localized, and permanent structural damage that occurs when a material is subjected to cyclic or fluctuating strains at nominal stresses that have maximum values less than (often much less than) the static yield strength of the material.
It is because fatigue happens so quietly and insidiously that makes it very dangerous even resulting in loss of life. Sadananda et al (2003) assert that "Fatigue is the principal cause of premature failure of engineering components".
Many structures such as aeroplanes, oil rigs and bridges, to name just a few, are exposed to fluctuating stresses. The engineering approach falls into two broad categories of dealing with stress induced fatigue. The first part is prediction of the lifetime of a material under stress. This model will provide recommendations on how frequently materials have to be replaced. The second approach is to predict how far a crack can grow before failure happens. Detection and prediction of failure of cracks can mean the difference between life and death of users of these facilities.
"The basic method o
"The basic method of presenting engineering fatigue data is by means of the S-N curve, a plot of stress S against the number of cycles to failure N." ( Key to Steel, n.d.) The S-N curves enable prediction of how long a material will last in terms of cycles of loading.
Figure 1. A S-N Plot for an aluminum alloy (Kelly, 1997)
Kelly (1997) explains that cracks go through three stages of formation, propagation and failure.
Stress Intensity factor K
"Stress Intensity, K, is a parameter that amplifies the magnitude of the applied stress that includes the geometrical parameter Y (load type)" (Wikepedia, n.d.2). This factor measures the degree to which stress is magnified around a crack. The loading around a crack falls into three modes I, II and III.
Figure 2. Three loading modes (Key to Steel, n.d.)
The three modes are:
opening or tensile mode (the crack faces are pulled apart)
sliding or in-plane shear (the crack surfaces slide over each other)
tearing or anti-plane shear (the crack surfaces move parallel to the leading edge of the crack and relative to each other)
(Key to Steel, n.d.)
The most common mode is mode I and this is what is used in most calculations. The intensity factor, K, determines the rate at which a crack will propagate and hence the lifetime of the material. The mathematical relationship is defined by Callister (1994, cited by Kelly(1997)) as:
This equation relates the rate of growth of a crack to the change in intensity factor K. In this equation A and m are dependent on the materials and da is the change in crack length while dN is the change in number of stress cycles. The change in K is defined by:
Where Kmax and Kmin are the maximum and minimum intensity factors respectively, Y is a constant dependent geometry of the material and is the applied stress on the material. When this equation is re-arranged and integrated it becomes:
This equation gives Nf, the estimated number of cycles before
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