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Strain Gage Identification of the Correction Procedure for Quantitative Thermoelastic Stress Analysis under Non-adiabatic Conditions

Thermoelastic stress analysis (TSA) can achieve high spatial resolution, capable of measuring very localized stress concentrations, but it can be affected by a systematic error if adiabatic conditions are not present. On the other hand, strain gages are not influenced by heat exchange of localized thermoelastic sources, but their physical dimensions limit their spatial resolution to a few millimeters. The measurement of the phase shift of the thermoelastic signal in a non-adiabatic test can be used for the recovery of the adiabatic temperature distribution if an assumption on the kind of stress distribution is correctly made. This study is aimed at the quantitative measurement of the stress distribution in an aluminum alloy component during a fatigue test in non-adiabatic conditions, using the result of a strain gage measurement for the correct assumption on the kind of stress distribution. Real aluminum or metal components seldom reach adiabatic conditions in fatigue tests because of the limits of the fatigue machine, which cannot apply and control the load at a sufficiently high frequency. In the present work the correction procedure to recover the adiabatic temperature was applied to a helicopter aluminum alloy component with a cross shape. Two strain gages were placed on one of the arms’ roots in the region of maximum stress value, but because of their size the gages could only measure the average stress value in the region of contact. However, the results helped in choosing the kind of correction to apply to the raw TSA data to recover the adiabatic temperature and the correct stress value. A finite element model was also used to evaluate the maximum stress, which was localized in a small region at an edge of the arm root. TSA proved to be a reliable technique, able to quantitatively measure stress concentrations with a spatial resolution much higher than that of a strain gage, also in non-adiabatic fatigue tests.

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