Abstract
Strain gauges are ordinary transducers for strain measuring; their operation relies on the electrical resistance, which varies as the underlying substrate is subjected to mechanical deformation. The mechanical strain can be obtained by converting the electrical signal through a gauge factor, provided by the manufacturer. It was demonstrated that its values are not unique; it may be influenced by the geometrical characteristics of both the specimen and the strain gauge and by their respective moduli of elasticity. This can be extremely dangerous when low modulus materials are studied. This study confirms that even with commercial strain gauges specifically designed for low modulus materials the effect might be present. An experimental method for its evaluation is discussed; tensile specimens are used as a test bench and their modulus is determined using both strain gauges and a non-contact method (Digital Image Correlation). The results show that a local reinforcing effect is present and a higher tensile modulus is obtained when contact transducers are bonded to polymeric specimens. The amplitude of this effect is predicted with established methods available in the literature and through a simple 2D Finite Element (FE) model. All these models require the elastic modulus of the strain gauge to be considered; a digital procedure to estimate it for any wired transducer is therefore proposed. The predicted results were found to be consistent with those experimentally measured; this validated the method, thus advising on how to evaluate the phenomenon also when this information is not available.
Highlights
Stress analysis is mostly performed by measuring the strain field over the component under investigation
From Run(A) specimens, it was determined that the actual tensile modulus of 3D printed Poly Lactic Acid (PLA) can be evaluated in 3358 MPa; there is no discernible difference with Run(B) specimens, for which the mean value μ stabilized at 3353 MPa
Considering the mean values of the elastic modulus for this run, the local stiffening effect was quantified in +1.8%
Summary
Stress analysis is mostly performed by measuring the strain field over the component under investigation. A strain field evaluation is necessary to perform material characterization. In those circumstances, it is vital that precise measuring is performed. In the frame of contact-devices, some authors proposed alternative instruments. Embedded Fiber Bragg Grating (FBG) were considered in (Pereira et al, 2016) for polymer characterization; in this study, the author used those sensors in tensile tests of dog-bone specimens and proposed a calibration protocol using extensometers. FBG own the advantage to not influence the material resistance and characteristics, but their installation can not be done on existing components and has to be considered before manufacturing (Montazerian et al, 2020). Non-contact-systems such as laser extensometers, video extensometers (Tian et al, 2018) and Digital
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