Brands Vishay Measurements Group Sampling of Industrial Case Histories

Sampling of Industrial Case Histories Presented
Case histories show how PhotoStress testing is being used today to solve a wide variety of stress-related design problems. Click below on an application for a case history example. Attendees will receive a booklet illustrating these and many other case history applications.

 •   Assembly Stress Analysis
 •   Residual Stress Analysis
 •   Composite Material Testing
 •   Yield Detection
 •   Biomechanic Applications
 •   Relationship between PhotoStress and Finite-Element Analysis

Assembly Stress Analysis -- Diesel Engine Flywheel Case History
Photo A
Photo B
Photo C
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.

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Residual Stress Analysis -- Metal Fan Hub Case History
Photo A
Photo B
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.

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Composite Material Testing -- Effect of Fiber Reinforcement on Strain Distribution Case History
Photo A
Photo B
Photo C
A fiberglass plate and an aluminum plate of similar dimension were coated with PhotoStress plastic and tested in uniaxial tension. The resulting strain patterns that developed around the holes in both plates were similar in geometry, demonstrating a definite correspondence in the gross strain distribution in homogeneous and heterogeneous materials. However, the fringe patterns appeared as smooth unbroken lines for the homogeneous material (aluminum) as shown in Photo A, while for the heterogeneous material (fiberglass), they were discontinuous, with a more-or-less scotch plaid appearance as shown in Photo B. In another example, Photo C shows the strain pattern on a simply loaded fiberglass/honeycomb beam.

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Yield Detection -- Materials Testing Case History
Photo A
Photo B
PhotoStress was used to study the post-yield strain behavior on "notched" tensile test specimens. Photo A shows a broad plastic strain field developing over most of the surface of one test sample, while Photo B shows initial yielding occurring in the form of Lueder's lines (slip planes) in the other.

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Biomechanic Applications -- Hip Replacement Case History
Photo A
Photo B
Photo C
Photo D
Among the more intriguing and significant applications for PhotoStress testing are those in the field of biomechanics. Areas of application, to name a few, include stress analysis of skeletal parts such as the femur, pelvis, and skull; knee, elbow, and other joint replacements; dental implants and bridges; and mechanical medical aids such as forceps and surgical staplers. PhotoStress analysis of the proximal femur was undertaken to evaluate the stress transfer for total hip replacement. Photo A shows the fringe pattern on the femur before the implant was inserted. Photo B shows the implant in place, and Photo C and D show the change in strain distribution on the surface of the femur when compared to the photo before implant.

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Relationship between PhotoStress and Finite - Element Analysis -- Steering Knuckle Case History
Photo A
Photo B
Photo C
Photo D
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.

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