A major disadvantage in strain gauge work on plastics is the effect of strain gauge reinforcement, which induces signifcant errors in the quantitative evaluation of strain derived from the plastic. The parameters influencing reinforcement are well documented and procedures have been developed that permit the identification and correction of this effect in certain situations. The principal causes, effects and corrective procedures are reviewed and where appropriate compared with results obtained by the authors of this article.
The embedding of three‐dimensional strain rosettes embedded into epoxy models provides an experimental technique for analysing complex structures; however, this technique has been known to produce data that were difficult to explain in terms of their physical significance. To gain a greater insight into the behaviour of a three‐dimensional strain rosette used in this way, a three‐dimensional strain rosette was embedded into each of two separate prismatic bars of square cross‐section and subjected to fundamental tests of compression and torsion in standard commercial testing machines. In initial tests on a bar containing a three‐dimensional strain rosette (Bar A) the data derived from the individual gauges sometimes departed from the theoretical values by more that 30 μe. After critical evaluation of the procedures used for making and testing Bar A, further tests were carried out on Bar B, which led to a reduction in the difference between theoretical and experimental data to 14 μe, acceptable for most practical purposes. The use of square plugs containing three‐dimensional strain rosettes which are embedded into square cavities in the model, and the measurement of the actual direction cosines of the gauges on the square plug prior to embedment is a distinct advantage over the use of cylindrical plugs. In addition, the use of testing machines with a fixed base as opposed to a floating lower platen is recommended.
Strain gradients give rise to a number of problems in the field of embedded threedimensional strain measurement. In order to avoid these problems a modular type three-dimensional strain rosette was embedded into known strain fields and the data from the individual gauges compared with theoretical predictions. Finally, the least squares strain tensor was predicted from experimental data analysed using the Monte-Carlo technique and the theoretical results forecast from finite element data taking into account the mechanical properties of the carrier, plug and prismatic bar. Some of the experimental results were found to correlate well with the theoretical values but some values in the least squares strain tensor, in particular under compression and torsional loading, departed considerably from the theoretical values. It was found that the effect of the measurement errors in the individual gauges combined with the matrix operations in the least squares strain tensor were responsible for biasing the resultant tensor data. However, the modular technique provided a solution to the problem of strain gradients.KEY WORDS: embedded strain gauge, Monte-Carlo, strain gradients, three-dimensional strain rosette NOTATION e strain l, m, n direction cosines [T] matrix of squares and products of direction cosines [c] least squares strain tensor
Two spheres were manufactured, each containing planes of embedded strain gauges that were arranged so that individual gauge readings could be combined in a variety of ways to calculate the least squares strain tensor. Each sphere was tested by loading along its axis of symmetry. It was found that the particular pattern of rosette chosen produced satisfactory predictions of the orthogonal strains but minor off-axis gauge misalignments combined with the matrix operations in the least squares strain tensor led to misleading forecasts of the shearing strains. The method of manufacture used to embed the gauges was largely responsible for these errors. The investigation confirmed that three-dimensional strain rosettes should be made from square plugs and the direction cosines measured on the plug prior to embedment in the model. KEY WORDS: embedded strain gauge, spherical strain field, three-dimensional strain rosette
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