Illustrative: Insufficient Instrumentation Masking the True Failure Mechanism
How a strain gauge placed away from the critical location produced misleading data that caused the engineering team to dismiss a real stress concentration.
Case type
Illustrative engineering case. This case demonstrates the importance of strain gauge placement and the danger of drawing conclusions from insufficient instrumentation.
1. The system or structure
A structural fitting tested in a static load test. The fitting had a fillet radius at a geometric transition. The FEA predicted a stress concentration at the fillet.
2. What failed?
No failure during the test. The problem emerged later: the fitting failed in service at the fillet radius. The test strain gauge, placed 15 mm away from the fillet, had recorded a strain of 1,200 µε — apparently confirming the design was adequate. The actual strain at the fillet was approximately 3,500 µε, consistent with the FEA prediction and above the fatigue limit.
3. The mechanism
The strain gauge was placed at a convenient flat surface 15 mm from the fillet, not at the fillet itself. The stress concentration at the fillet was not measured. The test report concluded that stresses were within allowables based on the remote gauge reading.
- FEA prediction: 3,500 µε at the fillet (Kt ≈ 3.0 location)
- Strain gauge location: 15 mm from the fillet, on a flat surface — outside the stress concentration zone
- Gauge reading: 1,200 µε — consistent with the nominal stress, not the peak stress
- Conclusion drawn: "stresses are within allowables" — based on the remote gauge, not the critical location
- Service failure: fatigue crack at the fillet after 30,000 cycles, consistent with the actual 3,500 µε peak strain
9. Engineering lessons
- Strain gauges must be placed at the critical location identified by FEA — not at convenient flat surfaces
- A strain gauge 15 mm from a stress concentration measures nominal stress, not peak stress
- Test conclusions must be based on the critical location, not on the most convenient gauge
- If the critical location is inaccessible (e.g. a fillet radius), consider alternative instrumentation: extensometers, DIC, or reposition the gauge as close as physically possible
- Correlate FEA and test at the same location — compare like with like