PSI - Issue 34

Veronika M. Miron et al. / Procedia Structural Integrity 34 (2021) 65–70 Miron et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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at ambient temperatures can be of advantage for the prediction of the part behavior where the load case is not fully known or where different loading rates are expected. The average compressive strengths at 30% strain are 0.74 MPa for 100% filled Silastic, 0.13 MPa for Dragon Skin, and 0.02 MPa for 10% filled Silastic (Figure 2). That makes the 100% filled Silastic 37% stiffer than the 10% filled Silastic which shows the influence of the amount of infill for the compressive behavior of a part. Even though Dragon Skin is with the Shore A hardness of 10 softer than Silastic with Shore 50A, the apparent hardness of a Silastic part can still be lower than for the Dragon Skin part, as apparent hardness is a combined effect of the m aterial’s intrinsic hardness and its porosity. In our case, the 100% filled Shore A 10 material shows 6.5 times the compressive strength of a 10% filled Shore A 50 material. As the haptic experience of a part is influenced by the mechanical, thermal, surface properties, and purity of a material (Çakmak et al., 2011), the apparent hardness of a structure influences the user perception. This is relevant for the experience of anatomical models that should mimic the haptic properties of the human body to perform medical simulations, e.g. for neurosurgery. With brain being the softest material in the human body, with an elastic modulus from 0.5 kPa (Taylor and Miller, 2004) up to 16 kPa (Rashid et al., 2012) depending on the strain rates, the ability of adjusting the stiffness and apparent hardness of a brain model is very valuable as otherwise only soft hydrogels would show this behavior, which however, need special humid storage conditions. The compression tests of the 10% filled Silastic specimens strongly indicate the influence of print quality on the stress-strain curve. The specimens that showed stop and go extrusion at their walls, ended up with small holes in their walls. This results in consistently more compliant specimens due to open cell compression behavior where the air escapes the cells compared to the specimens with closed walls, where the air is trapped in the cells and gives resistance to the load which results in higher compressive loads at the same displacement. Buckling behavior is exhibited only by 10% filled specimens tested in z-direction with buckling strains between 5-25% and stresses between 0.01 0.22 MPa, depending on their wall quality. Overall, Silastic specimens tested in x-direction show higher stresses at 30% strain than specimens tested in z-direction. However, for the 100% filled Dragon Skin specimens it is opposite, suggesting further investigation of the orientation dependency with more specimens would be necessary (Figure 2). The print settings (P1, P2, P3) changed the stiffness and strength at 50% strain for the Dragon Skin samples only marginally, so that σ(P1) = 0.34 MPa > σ (P3) = 0.31 MPa > σ(P2) = 0.28 MPa.

Figure 1: Stress-strain curves of uniaxial and biaxial tensile tests as well as pure shear tests.

Figure 2: Average compressive strength at 30% strain for 3D printed Silastic and Dragon Skin cubes tested at 0.1 mm/s and 1 mm/s in z- and x direction with their minimum and maximum values.