Quick Look

Direct Measurement of Operational Induced Strain Using Laser-Embossed
Parts Marking, 18-R9549

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Principal Investigators
Laura L. Bequette
Peter C. McKeighan
John C. Simonis (SwRI retiree)
Ernest Franke
Michael P. Rigney

Inclusive Dates:  06/27/05 – 01/31/06

Background - Industry, like the military, uses measurement, data mining, and analysis tools to predict part failure and to avoid interrupting and impacting their operations. Both industry and the military lack the operational strain information necessary for prognostic failure analysis of operating parts. A method is required to non-invasively estimate the operational strain history of a part.

The purpose of the project is to evaluate the sensitivity, reliability, and accuracy of the strain measurement obtained from displacement measurements using embossed parts marking and a laser displacement measuring system. This quick-look project hypothesized that laser-embossed marking code on parts may be permanently deformed (strained) by the operational forces applied to the part and that these permanently induced strains could be used to estimate the stress induced in the laser-embossed part during its operation. The purpose for this program is to test this hypothesis.

Approach - Techniques using grids or patterns projected onto a surface have long been used to measure strain in a mechanical part. However, there is no reference research being done to measure the strain in parts using parts marking codes or other forms of laser-embossed markings. Strain in a part using full field techniques is evaluated by measuring the finite, but small, displacement of an embossed pattern caused by the applied loads. The displacements have in the past been determined by measuring the displacement of a regular geometric or dot matrix pattern embossed on the surface of a part before external loads strain the part, and then re-measuring this pattern after the external loads have been applied. The testing of this project's hypotheses were predicated on laser etching sample marking code on tension-compression coupons, statically and dynamically deforming the coupons in a materials testing machine, measuring the deformation of the pattern using available, high-resolution optical-displacement measuring equipment, and comparing the induced deformation of the laser etched matrix to strain measured by strain gages.

Accomplishments - Using the resulting data to quantify the deformation applied static or dynamic loads had on the laser-embossed matrix on "dog bone" type specimens, this project concluded that:

  • The trend of the strain data derived from the vision method was in agreement with the trend of strains measured by strain gages.
  • Plastic strains were not induced in the "dog bone" specimens for loading stress levels less than 55 percent of the yield strength of the material.
  • The optical displacement measuring method shows the feasibility for measuring the strain with this approach; however, the results clearly show that modifications are required to the available optical displacement-measuring device used in the program to improve the strain measurement technique. An additional equipment investment would be required to proof the method to the level required by the military,

Discussions are currently on-going with the U.S Army regarding the opportunity to implement this technique as an option to assess structural strains within the CH-47 platform. Implementing this technique will require additional cost for equipment and for assessment of the locations of the marking on the components. In an Army memo from the Aviation Engineering Directorate (AED) dated August 16, 2006, the AED stated "at this point in time, it does not recommend pursuing this proposal, but rather will wait and revisit it toward the end of FY07."

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