RESONANCE FATIGUE TESTING--SHAKEN, NOT STIRRED
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Resonance testing machines are one of the most commonly used systems for fatigue testing of material coupons and components. Other from what some people might expect, resonance testing does not mean that the specimen is forced into its resonance frequency. In reality the whole system – a 2-mass oscillating system - consisting of the specimen, its fixture and the machine are excited into resonance.
The resulting force from the dynamic drive (load amplitude) follows a sine wave and when being paired with the static drive (mean load) any desirable load ratio can be achieved. In this setup, the specimen acts as a very stiff spring and experiences loading equivalent to that in a servo-hydraulic system. However, resonance testing provides major benefits over servo-hydraulic systems: Resonance testing machines are usually excited by electro-magnetic actuators - so there is hardly any infrastructure or maintenance required. Which means you can focus on shaking the specimen rather than stirring the oil of your hydraulic system. The high test frequencies of up to 1000 Hz allow for very high throughput and reduced test time, especially important for large test series. Even though “resonance testing” sounds complicated, the testing process itself is quite simple. In combination with a sophisticated software program the machine detects the resonance frequency of the system and starts to excite it until the desired load amplitude is reached. Once the machine has been brought to this frequency (within a few seconds) it only has to overcome minimal friction losses, which explains the extremely low energy consumption. Compared to servo hydraulic test systems, resonance testing uses typically less than 1% of the energy – another strong argument that ensures an incredible ROI. A further big benefit is the automatic and reliably/repeatable crack detection. A crack initiating/propagating in the test specimen changes its stiffness and hence affects the resonance frequency of the system. This precise abort criteria allows for proper analysis of the fracture surface by avoiding secondary damage. These systems are used for testing of material coupons to engine component, drive train component, fasteners, chains, couplings, exhaust components, implants, etc. Summing up, resonance fatigue testing for material coupon and component testing is efficient, quick, reliable, intelligent and inexpensive to operate. |
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