What is cyclic fatigue?

What is cyclic fatigue?

Cyclic fatigue can be defined as the stress, strain, and deformation induced in a material by cyclic loading. Cyclic fatigue life is the number of loading cycles which produce a rupture or breakage in the material.

Why the fatigue is important in engineering application?

Fatigue is very sensitive to stress, and ensuring the accuracy of load and stress estimates is an important step in fatigue engineering. The magnitude and distribution of stresses caused by applied loads provide input to fatigue analysis and also reveal opportunities for design improvements.

What is fatigue in engineering materials?

Fatigue is defined as a process of progressive localized plastic deformation occurring in a material subjected to cyclic stresses and strains at high stress concentration locations that may culminate in cracks or complete fracture after a sufficient number of fluctuations. From: Metal Fatigue Analysis Handbook, 2012.

What is the importance of fatigue testing in civil engineering field?

Fatigue testing is a crucial procedure used by engineers and technicians to help predict the durability of a part or component under its operating conditions.

What is the ratio of stress?

The stress ratio is defined simply as the amount of stress a component is under compared to the allowable stress that the code allows for the material. It varies from 0 to 1.0, i.e. 0 % to 100 %, and in an exam question is normally given.

How does mean stress affect fatigue life?

Since the mean stress raises the maximum stress and induces additional plastic strain, the fatigue life may be extended due to the reduced strain range for a given stress amplitude. Therefore, changes in the fatigue life due to the mean stress should be compared for the same strain range.

What is reverse bending fatigue?

When you bend it in one direction, you are applying a high tensile stress to the fibers on one side of the OD, and a high compressive stress on the fibers on the opposite side. When you bend it the other way, you reverse the stresses (fully reversing fatigue). It will break in about 25 cycles.

What is a SN curve?

A SN-Curve (sometimes written S-N Curve) is a plot of the magnitude of an alternating stress versus the number of cycles to failure for a given material. Typically both the stress and number of cycles are displayed on logarithmic scales.

What is fatigue How is an SN curve created?

Fatigue properties of materials are often described using the fatigue limit or the S-N curve (fatigue curve, Wöhler curve). The S-N curve describes the relation between cyclic stress amplitude and number of cycles to failure. Those curves describe the fatigue properties of a material.

How do you prevent material fatigue?

Fatigue Reduction

1. Use stronger, more capable materials.
2. Reduce the margin of errors in assembly and manufacture.
3. Avoid, soften when inevitable, stress concentrations.
4. Keep residual stress at surface, if any, in compression.
5. Take service environment into account.
6. Schedule routine maintenance, firm and thorough.

Does frequency affect fatigue life?

Yes, in general the frequency can affect fatigue behaviour of a material but then to produce a reasonable influence in fatigue life the frequency needs to changed in order of magnitudes (from Hz to kHz to MHz).

When fatigue occurs above what cycle it is called high cycle fatigue?

When the fatigue occurs above 103 cycles (usually 104 or more), it is usually called High-cycle fatigue. The material is subject to lower loads, usually less than 2/3 of the yield stress. The deformation is in elastic range. The fatigue life is “high-cycle” (103 ~ 106).

Do all materials have a fatigue limit?

I think the endurance limit and fatigue limit are not same..all materials do have a fatigue limit if the high cycle fatigue tests at that frequency can be run for 10^8 cycles but this does not ensure the flattening of S-N curve.

Is fatigue failure ductile or brittle?

Fatigue failure is brittle-like (relatively little plastic deformation) – even in normally ductile materials. Thus sudden and catastrophic! Applied stresses causing fatigue may be axial (tension or compression), flextural (bending) or torsional (twisting).

How do you know if its ductile or brittle?

Typically brittle materials have a fracture strain less than 0.05 (∊f < 0.05) and ductile materials have a fracture strain greater than or equal to 0.05 (∊f ≥ 0.05). Ductile materials deform much more than brittle materials. Brittle materials fail suddenly, usually with no prior indication that collapse is imminent.

What is the typical laboratory test used for fatigue?

There are several common types of fatigue testing as well as two common forms: load controlled high cycle and strain controlled low cycle fatigue. A high cycle test tends to be associated with loads in the elastic regime and low cycle fatigue tests generally involve plastic deformations.

What is the difference between brittle and ductile failure?

Brittle fractures occur with no apparent deformation before fracture; ductile fractures occur when visible deformation does occur before separation. Fracture strength or breaking strength is the stress when a specimen fails or fractures.

Why is ductile failure preferred?

In almost all material design situations, materials that exhibit ductile failures or fractures are preferred for various reasons, such as: Ductile materials deform plastically, slowing the fracture process and allowing more time to correct problems.