Fatigue

Fatigue is a form of failure that occurs in structures subjected to dynamic and flucating stresses. In these circumstnaces, it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for static load. Fatigue is important as it is the single lrgest cause of failure in metals, estimated to be involved in approximately 90% of all metallic failures; polymers and ceramics are also susceptible to this failure. Fatigue failure is catastrophic and insidious occurring very suddenly and without warning. It is brittlelike in nature. The process occurs by the initiation and propagation of cracks and typically the fracture surface is perpendicular to the direction of an applied tensile stress.

Applied stress may be axial(tension-compression), flexural(bending), or orsional(twisting) in nature. In general, three different fluctuating stress-time modes are possible. One is time dependence and regular sinusoidal where amplitude is symmetrical about a mean zero stress level, aternates between max tensile stress and min compressive stress. Referred to as reversed stress cycle. Another type is repeated stress cycle, where max and min stress are asymmetrical relative to zero stress. Also there can be random stress cycle where amplitude and frequency are random.

Mean stress= sigma max + sigma min/2

Range of stress = sigma max-sigma min

Sigma a=range of stress /2 or sigmamax-sigma min/2 (sigma a is one half of this range of stress)

Stress ratio R = sigma min/sigma max

Tesnile = + , compressive = -

A serties of tests can be conducted on a material where the specimen is rotated and servie stress conditions are duplicated to gain data. Tests are normally conducted using an alternating uniaxial tension-compression stress cycle. Specimen is usually subjected to stress cycling around 2/3’s of the static tensile strength and cycles are counted to failure. Data are plotted as stress versus the log of the number of cycles to failure. Two distinct types of S-N behavior are observed. The higher tha magnitude of stress the smaller is the number of cycles the material is capable of sustaining before failure. For ferrous and titanium alloys, S-N curve becomes horizontal at higher N values; this is a fatigue limit(endurance limit), below this limit fatigue will not occur. It represents the largest value of fluctuating stress that will not cause failure for essentially infinite number of cycles. These limits range between 35% to 60% tensile strength for steel.

Most nonferrous alloys do not have fatigue limit, the S-N curve for these materials continues to decrease at increasing N values. Thus fatigue will occur regardless of the stress. Fatigue strength is defined as the stress level at which fatigue will occur for some cycles.

Fatigue life-it is the number of cycles to cause failure at a specified stress level, as taken from the S-N plot.

Scatter often occurs when conducting tests. Consequently it leads to uncertainities about fatigue limit and life. Some factors that cause scatter include specifimen fabrication, surface preparation, fatigue sensitivity etc. P value associated with a probability curve repsent probability of failure.

Low cycle fatigue has plastic and elastic strain and only has 10^4-10^5 cycles. High cycle fatigue is when relative large number of cycles to failure.

Polyers also experience fatigue failure and have lower levels relative to yield strength. They are no as extensive in polymers but data is calculated and plotted in the same manner. High cycling frequency or large stress can cause localized heating, failure may be due to a softening of the material rather than a result of typical fatigue processes

Fatigue first has 1) crack initiation at some point of high stress concetration. 2) crack propagation, during which this crack advances incrementally with each stress cycle and 3) final failure, which occurs very rapidly once the advancing crack has been reached. Cracks from fatigue failure always initiate on the surface. Crack nucleation sites include, surface scratches, sharp fillets, keyways, threads, dents. The two types of markings formed are benchmarks and striations. Both indicate the position of the crack tip at some point in time and appear as concentric ridges that expand away from the crack intiation sites, frequently in a circular or semicircular pattern. Beachmarks are of a macroscopic dimensions and may be observed with the unaided eye. Stritations are microscopic in size and subject to observation with the electron microscope. Each striation is thought to represent the advance distance of a crack fron during a single load cycle. Both marks are fatigue surface features and different in origin and size.  The absence of these does not exclude fatigue as the cause of failure. Striation detectability decreases with passage of time due to formation fo corrosion products.

Mean Stress

The dependence of fatigue life on stress amptlidue is represented on the S-N plot. Mean stress will also affect fatigue life; increase in mean stress level will lead to decrease in fatigue life.

Surface effects

For many common loading situations, the maximum stress within a component or structure occurs at its surface. Consequently, most cracks leading to fatigue failure originate at surface positions, specifically at stress amplification sites. Therefore fatigue life is sensitive to configuration of component surface.