Fatigue Testing or Cyclic Testing

Fatigue Testing

Fatigue Testing PDF This is a preliminary summary report describing fatigue testing conducted by U.S. Bellows, Inc. The objective of the fatigue test was to detect fatigue crack growth generated by compressions during controlled cycling on a fatigue-testing machine.

U.S. Bellows, Inc. conducted a fatigue test on 12” diameter A240 TP 321SS bellows with 8 convolutions. The EJMA calculations for this test was based on compressing the bellows 2.3/4” 1000 times. This bellows was attached to each side of the cylinder of the fatigue test machine and pressurized to 25 PSIG for the purpose of leak detection. This bellows failed in the root of the convolutions at 1,285 cycles.

Figure 1 shows an overall view of the axial cycle fatigue test machine with bellows installed	Figure 2 shows a linear crack in the root of the convolution–about 3 inches long, which is typical of bellows low cycle fatigue)
This test has proven that U.S. Bellows Inc., EJMA cycle life calculation of 1,000 cycles is met. The actual cycle life of 1,285 cycles is higher than the EJMA Calculated cycle life.

 

Key reasons Cyclic Testing can be important in design and operational considerations: 

1. Validates design life – Confirms the joint meets its rated cycle life as calculated per EJMA formulas.
2. Ensures structural integrity – Verifies welds, convolutions, and materials won’t crack or fail under repeated flexing.
3. Confirms design assumptions – Demonstrates that analytical or computer-based life predictions match real-world performance.
4. Enhances safety and reliability – Prevents premature failure in high-pressure or high-temperature piping systems.
5. Spring rate check – When coupled with a load cell, it can verify that the spring rate of the expansion joint matches or exceeds theoretical calculations. Doing this will verify the assumptions done in a Pipe Stress Analysis (PSA).