Skin is a highly non-linear, anisotropic, rate dependent inelastic, and nearly incompressible material which exhibits substantial hysteresis even under very slow (quasistatic) loading conditions. In this paper, a series of uniaxial cyclic loading tests of porcine and rat skin at different strain rates and with samples oriented in different directions (with respect to the spine) were conducted to study the effect of strain rate and samples orientations with respect to spine on Mullins-type softening and skin inelastic response. A noteworthy feature of skin is that, similar to certain filled rubbers, its mechanical response shifts after the first extension and exhibits softening and hysteresis when loaded under cyclic tension and Mullins-type softening is observed. The results of these strain-controlled cyclic loading tests also indicated that the extent of softening is different for different strain rates and orientations. Also, a substantial hysteresis persists even at very low strain rates indicating inelastic behavior beyond the rate sensitive viscoelastic response. Through this series of experiments, by investigating the effect of strain rate on pig skin and rat skin, we conclude that the skin response is rate dependent but inelastic and shows irreversible changes in fiber orientation which are observed in histology results. Also, skin shows persistent deformation that is only partially recovered even after a long period of unloading.
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The authors gratefully thank Dr. Terry Creasy from Texas A&M University for allowing us to use his facilities.
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In order to compare the Instron cross-head strain and the strain measured by DIC, the strain from grip displacement and the strain from image correlation along the sample from different positions of the sample were calculated, it was seen that the difference between both measurement was less than 5 percent.
Samples with a width of 10.4 mm, thickness of 1.65 mm and initial length of 54 mm were tested under a simple tension test. The measured engineering strain from crosshead displacement was 1.85 (εc). Also, engineering strain was measured by DIC; 5 different lines were chosen from different positions of the sample Fig. 14. The engineering strain (εd) was calculated as the differences between displacements of the top and bottom point of the line (y1 and y2) and divided by the difference between positions of these points on the line (u1 and u2). Table 1 shows strains measured by both DIC and Instron cross-head. The differences between εd and εc are less than 5 percent on each line and by getting the average of strain on all 5 lines, the difference is 2.7 percent which conforms to the strain measurement by the Instron cross-head.
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Afsar-Kazerooni, N., Srinivasa, A. & Criscione, J. Experimental Investigation of the Inelastic Response of Pig and Rat Skin Under Uniaxial Cyclic Mechanical Loading. Exp Mech 60, 535–551 (2020). https://doi.org/10.1007/s11340-019-00556-6
- Mullins effect
- Uniaxial cyclic tension testing
- Partially unloading
- Fiber orientation and strain rate