PSI - Issue 13

A. Spagnoli et al. / Procedia Structural Integrity 13 (2018) 137–142

141

A. Spagnoli et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 3. Tearing test on silicone rubber.

The final stage of the curves in Fig. 4 is that of steady state propagation of cut, where small changes of the penetration force are measured. During steady state of cut propagation, considering the compressive stresses due to blade thickness onto the target plate yields a strain energy derivative per unit thickness ( dU s / dD ) / t which tends to be independent on D . On the other hand, a linear dependence occurs for the derivative of energy dissipation due to friction dU f / dD / t . From the same elementary calculations considered in Sec. 2.2, considering uniaxial compression in the plate portion encompassing the penetrating blade for strain energy calculation, and constant friction τ f along the blade surfaces in contact with the target material for frictional energy dissipation, one can work out a constant value of the order of 1 × 10 − 3 N / mm for the former, and a value of the order of 0.1 N / mm when D = 1 mm for the latter. Bearing in mind the energy balance G = F t − dU s tda − dU f tda , these values show that the contribution of strain energy is neglegible in comparison to friction in the penetration curves of Fig. 4, simularly to the studied case of polystirene. Based on a few FE calculation, it turns out that the SIF value of full penetration of the blade in a cut of length a is K D = a = 2.11 × 10 − 3 MPa √ m and hence K c / K D = a 1. This confirms the experimental trend of the blade tip reaching the cut end before its propagation. The theoretical estimate of the force jump shown in Fig. 4 is related to the fracture energy G c = 1.02 N / mm of the material. An experimental campaign on cutting has been carried out with reference to the steady state propagation of a cut in glassy and soft polymeric plates through a steel blade. The steady state propagation has been investigated by applying various insertion velocities of a commercial blade. The force vs displacement curves of blade penetration have been recorded and discussed in terms of energetic contributions. It has been observed that the cutting resistance depends on the mechanical properties of the target material and on the sharpness of the cutting tool. Di ff erent responses in terms of steady state cut propagation emerged from the tests, if either a relatively blunt or a relatively sharp blade penetrates in the material, where blade sharpness has been shown to depend on the geometry of the blade profile as well as on the fracture toughness and the rigidity of the target material. 3. Concluding remarks

References

Atkins, A.G., 2009. The science and engineering of cutting: the mechanics and processes of separating, scratching and puncturing biomaterials, metals and non-metals. Butterworth-Heinemann / Elsevier, Oxford. Brighenti, R., Spagnoli, A., Carpinteri, A., Artoni, F., 2017. Defect tolerance at various strain rates in elastomeric materials: An experimental investigation. Engineering Fracture Mechanics 183, 79–93. doi:10.1016 / j.engfracmech.2017.05.001.