Description

Fatigue crack test is one of the primary damage mechanisms of structural components. Fatigue cracking results from cyclic stresses that are below the static yield strength of a material. The name “fatigue” is based on the concept that a material becomes “tired” and fails at a stress level below the nominal strength of the material.

The facts that the original bulk design strengths are not exceeded and the only warning sign of an impending fracture is an often hard to see crack, makes fatigue damage especially dangerous.

The fatigue test embraces three basic domains of cyclic stressing or straining, differing distinctly in character. In each domain, failure occurs by different physical mechanisms

1. Low-cycle fatigue test
Where significant plastic straining occurs. Low-cycle fatigue involves large cycles with significant amounts of plastic deformation and relatively short life. The analytical procedure used to address strain-controlled fatigue is commonly referred to as the Strain-Life, Crack-Initiation, or Critical Location approach.

2. High-cycle fatigue test
Where stresses and strains are largely confined to the elastic region. High-cycle fatigue is associated with low loads and long life. The Stress-Life (S-N) or Total Life method is widely used for high-cycle fatigue applications—here the applied stress is within the elastic range of the material and the number of cycles to failure is large. While low-cycle fatigue is typically associated with fatigue life between 10 to 100,000 cycles, high-cycle fatigue is associated with life greater than 100,000 cycles.

3. Fatigue Crack Growth Rate Test Properties
For some components the crack propagation life is neglected in design because stress levels are high, and/or the critical flaw size small. For other components the crack growth life might be a substantial portion of the total life of the assembly. Moreover, preexisting flaws or sharp design features may significantly reduce or nearly eliminate the crack initiation portion of the fatigue life of a component. The useful life of these components may be governed by the rate of sub critical crack propagation.

Aircraft fuselage structure is a good example of structure that is based largely on a slow crack growth rate design. Many years ago, the USAF reviewed a great number of malfunction reports from a variety of aircraft.

The reports showed that the preponderance of structural failures occurred from
1) built-in preload stresses,
2) material flaws and
3) flaw caused by in-service usage.

These facts led to a design approach that required the damage tolerance analysis to assume a material flaw exists in the worst orientation and at the most undesirable location. The analysis helps to ensure that structures are designed that will support slow stable crack growth until the crack reaches a length where it can reliable be detected using NDT methods.

The rate of fatigue crack propagation is determined by subjecting fatigue-cracked specimens, like the compact specimen used in fracture toughness testing, to constant-amplitude cyclic loading. The incremental increase in crack length is recorded along with the corresponding number of elapsed load cycles acquire stress intensity (K), crack length (a), and cycle count (N) data during the test. The data is presented in an “a versus N” curve as shown in the image to the right. Various a versus N curves can be generated by varying the magnitude of the cyclic loading and/or the size of the initial crack.

The data can be reduced to a single curve by presenting the data in terms of crack growth rate per cycle of loading (Da/ DN or da/dN) versus the fluctuation of the stress-intensity factor at the tip of the crack (DKI). DKI is representative of the mechanical driving force, and it incorporates the effect of changing crack length and the magnitude of the cyclic loading. (See the page on fracture toughness for more information on the stress-intensity factor.) The most common form of presenting fatigue crack growth data is a log-log plot of da/dN versus DKI.

The fatigue crack propagation behavior of many materials can be divided into three regions as shown in the image. Region I is the fatigue threshold region where the Dk is too low to propagate a crack. Region II encompasses data where the rate of crack growth changes roughly linearly with a change in stress intensity fluctuation. In region III, small increases in the stress intensity amplitude, produce relatively large increases in crack growth rate since the material is nearing the point of unstable fracture.

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