Case Studies

The PhotoStress® Analysis System has been used extensively for various purposes in many different applications:

• Aerospace
• Composite materials
• FEA validation
• Dynamic cycling / vibration tests
• Fatigue Tests
• Cycling, rotating & vibrating parts
• Residual stress analysis
• Assembly stress
• Yielding
• Biomechanical Tests
• Laboratory classes for educational settings

Aerospace — Landing Gear:

Many aircraft and aerospace parts and structures have been stress analyzed with the PhotoStress Analysis system under both static and flight conditions. The method is very well suited for such applications since it provides full field stress analysis and distribution on prototype structures. The landing gears for nearly all modern aircraft have been stress analyzed by covering the entire gear surface with PhotoStress photoelastic coatings. Landing gears are fabricated from forged and machined high strength steel. The gear is a complex assembly of part subjected to various static and shock loadings. Occasionally, certain parts are exposed to as many as six different loading conditions. Because the landing gear is used only twice during a flight and represents dead weight for the reminder of the time, any weight reduction is of great benefit. At the same time, safety is obviously of paramount importance; therefore, extensive safety factors must be employed unless the stress distribution is accurately known for all significant modes of loading.

Assembly Stresses: Diesel Engine Flywheel Case History:

One of the most frequently ignored situations that can produce unexpected high stresses occurs during the assembly of components which make up the whole part or structure. It is not unusual for local yielding to occur when bolts are tightened, or when parts are pressed into place. And, although such yielding may not necessary impair the safety of the structure, experience has proven in countless instances that fatigue cracks often develop in regions where alternating service stresses are superimposed upon high assembly stresses. If they exist, assembly stresses become immediately apparent when a PhotoStress coating is applied to the part prior to assembly.

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.

B

A

C

Biomechanical Tests:

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 shows the change in strain distribution on the surface of the femur when compared to the photo before implant.

A

B

C

Composite Materials

PhotoStress coatings can be applied to almost any material; this includes applications to composite materials such as reinforced plastics, carbon fibers, concrete, wood, and metal-matrix composites. Due to their inhomogeneity, most composite materials have mechanical properties that vary from point to point. Very commonly, such materials are also anisotropic in their mechanical properties and the magnitudes of the properties (elastic modulus, Poisson’s ratio, ultimate strength, etc.) at each point vary with the direction at the point. As a result, the stress and strain measurements may be seriously misleading.

Because of its full field stress analysis capability, PhotoStress is ideally suited for preliminary stress analysis to test objects made from composite materials. It reveals the detailed strain distribution and the principal strain directions over the entire coated surface of the part. As exemplified by some of the illustrations shown here, the coating also tends to display the underlying structure of the inhomogeneity.

As shown below, 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 scotch-plaid appearance as shown in Photo B.

Dynamic Cycling / Vibration Tests:

Cycling, Rotating & Vibrating Parts:

When dealing with rotating parts, PhotoStress Analysis system can work together with a stroboscopic light source (or a high speed camera) that gives the system the ability to inspect any kind of a moving part, rotating or reciprocating devices and vibration tests. By adjusting the frequency of the light source (or camera) to that of the moving part, we can create the impression that the part is still and analyze it easily.

Dynamic Cycling / Vibration Tests:

Fatigue Tests:

The PhotoStress Analysis System reacts instantly to any kind of a live load.

This feature makes a perfect tool to inspect structures under cyclic load, and to perform fatigue tests.

In this case, we see a complete transmission box that was coated with a photoelastic coating and then mounted on a slow cycling fatigue machine. Colors indicating of stress are appearing and disappearing according to the applied load.

FEA Validation

PhotoStress is a complementary technique to FEA for achieving optimal and verifiable full field 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 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.

A B C

FEA plot (right) of a human hip-bone replacement
validated by a PhotoStress analysis test (left)

Laboratory Classes:

PhotoStress Analysis System is often being used as a very powerful teaching aid at universities and colleges throughout the world. Many different laboratory courses with an infinite variety of techniques and models have been developed using the system’s ability to exhibit full field stress analysis in vivid colors and in a straight forward manner. This quality makes it very popular among undergraduate students that have found a way to view stresses and strains in colors and not only in numbers and equations.

Shown are specimens that demonstrate tension (pure and with different stress concentrations) and bending (3- and 4-point). Two similar C-shaped beam specimens, one having a hole; the stress distribution remains almost the same.

Pure Tension and Tension with Different Stress Concentrations

C-shaped Beam With and Without Hole

3-Point Bending

Residual Stress Analysis:

The detection and measurement of residual or “locked-in” stresses in a part has long been an important, and often elusive, problem for the design engineer. Residual stresses are usually introduced in a material during manufacturing processes such as casting, welding, machining, molding heat treatment, etc.

The effects of residual stresses may be either beneficial or detrimental, depending on the distribution, magnitude, and sign of the stress with respect to the load induced stresses. In many cases, residual stress is detrimental, and is the predominant factor contributing to the structural failure. There are several practical methods used to detect and measure residual stress, each having advantages and disadvantages. With PhotoStress, the principal advantage is that the presence of residual stress is revealed everywhere it occurs on the surface of the part. The disadvantage is the part must be cut or sectioned to reveal any residual stress present.

Yielding:

PhotoStress was used to study the post-yield strain behavior on "notched" tensile test specimens. Bottom photo shows a broad plastic strain field developing over most of the surface of one test sample, while the upper photo shows initial yielding occurring in the form of Lueder's lines (slip planes) in the other.

Relations between a notch and a crack in an aluminum part under tension (using the MNC-1 PhotoStress Monochromator)