The most effective method of verifying a product’s performance is via materials testing, which includes materials fatigue testing. Although analytical computational methods exist to simulate how materials react under fatigue, a materials fatigue test program is still the preferred method in understanding and therefore preventing product’s fatigue failures.
Materials Fatigue Test
We have an extensive materials and fatigue test laboratory capable of performing coupon, component as well as fatigue testing of metals (including aerospace alloys), and product strength and performance testing of various metals, plastics, composites and paper.
DTB’s materials testing experts are available to help you build a customized mechanical fatigue test program which includes one or more of the following methods:
- Crack Initiation
- Crack Growth
- Crack Fatigue (Fatigue Crack)
- Crack Propagation
- Crack Growth Rate (da/dN)
- Damage Tolerance
- Low Cycle Fatigue
- Fatigue Fracture
- Fatigue life prediction
- Fatigue failure analysis
- S-N curve
S-N curves are based on mean fatigue life with a given probability of failure and they can provide a high degree of certainty regarding cyclic performance of a product for expected fatigue loading conditions.
S-N tests focus on the nominal stress required to cause a fatigue failure in some number of cycles. This type of test results in data that is presented as a plot of stress (S) against the number of cycles (N) to failure, which is known as an S-N curve.
A log scale is normally used for N. Generating an SN curve for a material requires many fatigue test iterations performed to statistically vary the alternating stress, or stress ratio, and count the number of cycles.
Normally most DTB materials fatigue test programs have focused on situations that require more than 104 cycles (high-cycle fatigue testing) to failure where stress is low and deformation is primarily in the elastic region. However low-cycle test programs can be designed where the stress is high and deformation is primarily in the plastic deformation region.
The life of a fatigue crack has two parts, initiation and propagation. The most common reasons for crack growth initiation include:
- Notches, corners, or other geometric inconsistencies that act as stress risers. These features may be inherent in the product design or mistakenly caused during manufacturing.
- Material impurities, inclusions, or product usage causing material loss due to wear or corrosion.
- Mechanical or thermal fatigue encountered during the products normal life cycle.
Material fatigue fractures have common phases, such as crack initiation, crack growth propagation
(followed by) and a sudden failure. Once a crack has been initiated (crack initiation phase), repeated loadings can cause the crack to lengthen or propagate (crack growth phase). If fatigue loading is continued, crack growth will occur resulting in a final sudden fracture of the remaining cross section (overload failure).
By analyzing the material fatigue fractures of several iterations of mechanical fatigue tests, a crack growth curve can be formed to assist with fatigue life predictions. The crack growth rate (da/dN) is obtained by taking the derivative of the crack length, a, versus cycles, N. A well planned and executed DTB materials test program will be able to take into account the crack growth curves, obtained from the materials fatigue testing, and establish a fatigue life prediction for the product, as well as possibly provide damage tolerances for the product.
Damage tolerance is a property of a material or structure relating to its ability to sustain defects safely until repair can be made or the structure replaced. The DTB approach to damage tolerance in material testing is to account on the assumption that initial flaws exist in any structure or material and such flaws will propagate with usage. Our material test programs will locate the initial flaws, recommend the proper modification to correct the flaw, and then test again to prove that the recommendations worked.
Preventing fatigue failure is the most effective method of improving fatigue performance in designs. A DTB materials testing program will allow manufacturers to:
- Eliminate or reduce stress risers in the part.
- Improve manufacturing process by helping manufacturer avoid sharp surface tears resulting from punching, stamping, shearing, or surface finish processes.
- Help prevent the development of surface discontinuities during manufacturing.
- Eliminate or reduce tensile residual stresses caused by manufacturing.
Material fatigue is the progressive damage that occurs when a material or structure is subjected to cyclic loading. Fatigue failure is when the progressive damage caused by the cyclic loading leads to failure of the material or structure.
Fatigue failures are a significant problem for the reason that they can occur due to repeated loads below the static yield strength and can result in an unexpected and catastrophic failure causing monetary loss and/or even loss of life.
Many engineering designs include materials and structures that contain discontinuities or stress risers. Most metal fatigue cracks initiate from discontinuities in highly stressed regions of the structure.
The stress risers may be due the discontinuities in the material, inherent in the design, improper manufacturing technique or even improper maintenance. Our material
fatigue test program and comprehensive analysis can determine the cause of the fatigue failure.
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