A diesel engine flywheel was failing around the bolt circle. Photo A shows a flywheel coated with PhotoStress plastic, and then bolted to the diesel engine for dynamic testing. When the bolts were tightened, very high stresses appeared, which were well above the design limit of the material as shown in Photo B. Superposition of forces due to dynamic testing caused premature fatigue failure. The major problem was thus defined by PhotoStress analysis as one of assembly-induced stresses. Redesign of the flywheel (where it mated to the shaft of the diesel engine) significantly reduced the initial assembly stresses as shown in Photo C.

A metal fan hub was failing in service where the hub shaft support was welded to the flange. Analytical studies predicted low stress levels during the dynamic loading sequence. Strain gage measurements near the weldment supported this prediction. Several of the fan hubs were fabricated for test purposes, and PhotoStress coatings were contoured and bonded over the surface area. After application of the coating, the hubs were sawed through, releasing the internal forces (residual stresses) developed by nonuniform heating during the welding process. The fringe patterns in the PhotoStress coating shown in Photo A revealed locked-in residual stresses, which were of very high magnitude in the welded area. The modest cyclic stresses, superimposed upon the high residual mean stresses, were sufficient to produce field failures. Subsequent test samples were stress-relieved after fabrication, and PhotoStress analysis of the stress-relieved hub showed no evidence of residual stress after cutting, as shown in Photo B.

PhotoStress® is a complimentary technique to FEA for achieving optimal and verifiable stress analysis of a design. In this example, an FEA analysis of an automotive steering knuckle was conducted, and after manufacturing the actual part, PhotoStress® testing was chosen to verify the FEA results. Photo A shows an illustration of the steering knuckle and how the directional loads were applied. Photo B shows the FEA results indicating that the highest stresses are located in the fillet area of the protruding spindle. Photo C shows a physical model of the actual part in the test rig for PhotoStress® testing. Photo D shows the results of PhotoStress® analysis confirming the general location of the significant stresses revealed on the FEA model. PhotoStress® measurement, however, showed that the peak stress magnitudes were approximately 20 percent higher than the computer solution.

Strain Gage Knowledge Base

The Strain Gage Knowledge Base is a compendium of technical information related to the application of Vishay Micro-Measurements strain gages, installation accessories and instruments in the making of precision strain measurements.

Publications

• Strain Gage Technology (Tech Notes & Papers)

• Application Techniques (Tech Tips & Application Notes)

• Product Usage (Instruction Bulletins)

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PhotoStress Knowledge Base

The PhotoStress Knowledge Base is a compendium of technical information related to use of the Vishay Micro-Measurements PhotoStress Plus system for optical measurement and analysis of stresses/strains in test parts and structures.

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