Fatigue

Fatigue tests are typically illustrated using a  generally represented by Wöhler curve which shows the relationship between Stresses to Number of cycles to failure (hence SN relation). When plotted on a log-log scale¹, the Wöhler curve generally appears as a straight line, with Stress values² log-log , with Stresses² (σ_max or σ_amp) in Y-axis and Number of cycles to failure (N_f) in X-axis.

To establish an SN relationship, fatigue tests must be performed at several different stress levels, not including repetitions³, which are conducted at the same stress levels. Therefore a simple Design Of Experiments (DOE) is required for every fatigue test request (example request form here.)  

Example test request: Determine the SN relationship for a Syensqo grade polymer (ISO 527-1BA), using dry as molded (DAM) samples, tested at ambient temperature (23°C), with a frequency of 5 Hz, and a loading ratio of R = 0.1⁴. Tests should be conducted at five different stress levels (80%, 70%, ..., 45% of σ_UTS), with four repetitions at each level, except for the lowest stress level (45% of σ_UTS), where only two repetitions are needed.

Foot-notes:

1. Some data are also expressed in semi log scale (Y in linear & X in log), a common practice to make the Stresses scale visible. 
2. Stresses can be represented as both maximum stress (σ_max) or stress amplitude (σ_amp).
3. Repetitions between 2-5 (less repetitions at low stress levels to save time and cost) are chosen to account for variability which are inherent for any material (both plastics and metals). 
4. Loading ratio of R 0.1 is generally chosen as default loading case. Please be aware that a part can experience loading cases much more than mere R0.1.

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• Core Labs Capabilities

Electro-mechanical machines are generally used for static tests, as they provide better control over applied force or displacement. For polymers, these machines can also be used for mid-range fatigue tests; note that at Alpharetta, only a contact strain gauge is available for strain acquisition.

The film folding tester at Bollate is designed to evaluate the lifespan of films, but it is limited to film materials only and can only test thicknesses between 250 and 350 microns.

• Corporate Capabilities

Servo-hydraulic machines are well-suited for fatigue testing, with a dynamic testing range equipped with larger load cells. Strain measurements can be performed using Digital Image Correlation (DIC), or with contact / non-contact extensometers.

At the Brussels site, in-house developed demonstrator parts and testing setups enable us to conduct fatigue bending tests on L-shaped samples with various radii and on demonstrator parts with or without ribs and weld lines.

Contacts: Michel Houba, Dominique Delcourt

• ADL Capabilities

Although tooth bending is not a classic fatigue test, this can be used to validate the FE models, with capacity to test higher frequencies.

_{Tips: The current FE model for fiber filled polymers, can be applied only if the failure mode is slow crack growth controlled failure or brittle failure, which typically occurs at the root of the teeth in gear tooth bending.} 

Contacts: Matilde Scurria, Lorenzo Mancin

• Potential external partners

Syensqo has engaged Leartiker to generate fatigue test data (including hysteresis measurements), to support the creation of Digimat fatigue cards across different temperatures. Other capabilities include sample molding, creep analysis, advanced...