Experimental stress analysis

The greatest uncertainty of a design is often whether it will withstand mechanical loads and displacements. Examining critical areas of high stress or strain is most precisely quantified using experimental stress analysis methods. 

Common techniques for experimental stress analysis are as follows, in descending order of the resolution and quantity of data they yield  

An inexpensive initial evaluation may be accomplished by placing a crackle-lacquer-coated part under typical environmental loads. After observing the result, the engineer can select from among the above techniques to develop more information. 

Full-field speckle interferometry is similar to holographic interferometry except that the speckles are observed on the specimen surface by illuminating it with laser light or by establishing an optically speckled surface (for example by painting it). 

Photoelastic stress analysis is a powerful full-field technique where a product part is first covered with a thin film of special, transparent plastic. The layer must be bonded on with a reflective adhesive. The part is then deformed under static conditions and illuminated with polarized light. Viewing the part with a special polarizing optic system 

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Appendix 6 contains requirements of experimental stress analysis. Appendix 8 has acceptance standards for radiographic examination. Appendix 9 covers nondestructive examination. Appendix 10 gives rules for capacity conversions for safety valves, and Appendix 18 details quahty-control-system requirements. 

Contents Introduction to Materials. Manufacturing Considerations for Injection Molded Parts. The Design Process and Material Selection. Structural Design Considerations. Prototyping and Experimental Stress Analysis. Assembly of Injection Molded Plastic Parts. Conversion Constants. 

Handhook of Experimental Stress Analysis , J. Wiley, NY (1950), p 982 (Fairly comprehensive description of y-ravs technique is given) 11) K. Siegbahm, "Beta and Gamma Ray Spectroscopy , Interscience, NY (1955) 12)W.J. Price, "Nuclear Radiation Detection ,... 

J.W. Dally and W.F. Riley, Experimental Stress Analysis, 2nd edn, McGraw-Hill, 1978. 

The hydrostatic test pressure is 1.25 times the design pressure corrected for temperature, rather than the usual 1.5. Division 2 establishes upper limits for the test pressure relative to the yield strength at test temperature. The pneumatic test pressure is 1.15 times the design pressure corrected for temperature rather than 1.25 required by Division 1. Division 2 has no provision for proof tests to establish the maximum allowable working pressure. But Appendix 6, Experimental Stress Analysis, provides for the determination of the critical or governing stresses for unusual geometries for which theoretical stress analysis is inadequate. 

The following sections review laboratory NDE, microscopy, and experimental stress analysis methods, which the engineer can use to obtain information about the presence and severity of flaws developed by test or parts when they are subjected to sustain mechanical loading. 

NDE methods have the advantage that they cause no harm to the specimen thus the same part can be nondestructively retested or subsequently tested destructively. However, NDE only reveals the location and severity of flaws. The experimenter must judge the importance of each particular flaw. Some times, the importance of a defect is obvious (harmless, or likely to shorten service lifetime, or likely to cause catastrophic failure) otherwise, experimental stress analysis will quantify the severity of the flaw. The defects that are relevant to the strength of short/long fiber RPs are as follows ... 

Strain birefringence - bi-ri- frin-j9n(t)s n. Double refraction in a transparent material subjected to stress and accompanying strain. One of the techniques of experimental stress analysis is based on this phenomenon. 

Peters WH, Ranson WF (1982) Digital imaging techniques in experimental stress analysis. Opt Eng... 

Whitney, J.M., Daniel, I.M., and Pipes, R.B., Experimental Mechanics of Fiber Reinforced Composite Materials, SESA Monograph No. 4, The Society for Experimental Stress Analysis, Brookfield Center, Connnecticut, 1982. 

Walrath, D.E. and Adams, D.F., Analysis of the Stress State in an losipescu Shear Test Specimen. Composite Materials Research Group, Department of Mechanical Engineering, University of Wyoming, Laramie, June 1983, Department Report UWME-DR-30I-I02-I. Whitney, J.M., Daniel, I.M. and Pipes, R.B., Experimental Mechanics of Fiber Reinforced Composite Materials. Society for Experimental Stress Analysis, 1982, Monograph 4. Brookfield Center, Connecticut, Prentice Hall, Englewood Cliffs, 1982. 

The above diseourse appeared in the October 2000 issue of Experimental Stress Analysis NOTEBOOK published by the Measurements Group, Inc. of Raleigh, NC, and included here with permission of the Editor, Dr. L. D. Lineback. 

A.210. This section should describe methods for design and analysis of systems and components, including design transients, computer programs used, experimental stress analysis, and any programmes for dynamic testing and analysis of the mechanical systems and components. Particular attention should be paid to items important to safety. 

Once the part failure resulting from poor molding practices or improper material usage through visual examination and material identification is ruled out, the next logical step is to carry out an experimental stress analysis. 

The photoelastic method for experimental stress analysis is very popular among design engineers and failure analysts and has proved to be an extremely versatile, yet simple, technique. 

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