In this paper, the stress sensing performance of two well-known mechanoluminescence (ML) sensing materials, (1) SrAl2O4:Eu (SAOE) and (2) SrAl2O4:Eu, Dy (SAOED), has been experimentally studied. Under the same input loadings and strain rates, changes of the light intensity have been characterized in terms of sensitivity, repeatability and linearity. Effects of the strain rate on the light intensity changes have also been investigated for both ML sensing materials. SAOED appears to perform better as an ML stress sensor than SAOE because it shows higher sensitivity and no saturation of light during the loading history. Although SAOE showed saturation of light emissions, its initial sensitivity to loading was higher than that of SAOED. Therefore, SAOE appears to be more suitable for sensors for monitoring dynamic active cracks.

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This Letter reveals for the first time, to the best of our knowledge, the effects of stress-free persistent luminescence (PL) decay on the mechanoluminescence (ML) phenomena and the effects of stresses and strain rates on the PL decay of SrAl(2)O(4):Eu(2+), Dy(3+) (SAOED) materials. Previous research on ML phenomena in this material has focused on the effects of strain rates and stress variations on ML light intensity. However, experimental evidence provided herein shows that the ML light emission is also related to the PL decay time elapsed until the onset of stressing and the PL decay rate is dependent on the stress, strain rate, and the stress-free PL decay time interval. For quantitative stress measurements using SAOED materials, understanding of ML light sensitivity and its dependence on critical factors (strain rate, stress-free PL decay time interval, photoexcitation time, instantaneous PL decay rate, etc.) is crucially important. This Letter provides new and important perspectives that are essential for developing predictive models and/or calibration procedures for ML stress sensors.

In this paper, a new calibration method for mechano-luminescence (ML) thin film sensors was proposed to enable quantitative full-field strain measurements in pixel-level resolution for the first time along with two standard reference test methods. The proposed method has a distinct advantage of its facet-free full-field strain sensing capability with pixel-level resolution. For the ML sensor, standard reference test methods were proposed for developing calibrated relationships between ML light intensity and effective strains: (1) uniaxial tensile reference test and (2) non-uniform strain reference test. From the reference tests, two different calibration models were developed in a recurrence equation form and validated measuring general strain distributions on different experimental specimens. Verified finite element (FE) simulation results were compared with ML effective strains to confirm its accuracy. The comparisons of the ML effective strains with FE simulation results showed that the calibration models can acceptably measure full-field strains. Limitations, sources of errors, suggestions for improving accuracy and practical considerations were also discussed. A conclusion of this research is that the proposed method enables ML sensing films to measure quantitative full-field strain distributions.

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