PSI - Issue 13

A. Spagnoli et al. / Procedia Structural Integrity 13 (2018) 137–142 A. Spagnoli et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 1. Geometry of specimen for cutting testing with details of the blade shape.

dent on the fracture toughness of the target material (e.g. see Brighenti et al. (2017) for an account of defect tolerance and fracture resistance in elastomeric polymers) and on the sharpness of the cutting tool. Di ff erent scenarios of steady state cut propagation are observed if either a relatively blunt or a relatively sharp blade penetrates in the material. Al ternative blade sharpness parameters (e.g. see the parameter recently defined by the present authors according to some classical fracture mechanics-based arguments (Terzano et al., 2018) or that proposed in McCarthy et al. (2007, 2010)) can be used to discriminate the di ff erent conditions of cut propagation observed in the experiments. A theoretical interpretation of the experimental outcomes is finally provided.

2. Experimental cutting tests

2.1. Set-up

A large experimental campaign on di ff erent materials (silicone rubber, polypropylene, polystyrene) has been car ried out. Tensile tests on coupon specimens were performed to determine tensile strength and Young’s modulus of the material. Fracture mechanical tests were performed to calculate fracture toughness. Finally, cutting tests were performed on plate specimens with or without an initial edge cut. Blade was held orthogonally to and moved parallel with the plate, measuring penetration force against penetration displacement. Surface full-field displacement maps were acquired by means of a digital correlation technique on some cutting test specimens (Fig. 1). Blade geometry is that of a commercial cutting tool made of stainless steel. The measured profile of the blade is: thickness s = 0.5 mm, width b = 18 mm, tip angle α = 20 ◦ . Some cutting tests were performed by lubricating blade-material interface. In a few cutting tests blade tip was manually blunted by means of abrasive paper. Finally, in a few cutting tests the blade was turned around so as to have a square edge penetrating the material (angle α = 180 ◦ ). In total 48 specimens were tested under cutting.

2.2. Glassy polymer

As a representative glassy polymer, featuring a hard and rather brittle mechanical behaviour, polystyrene, which is a thermoplastic polymer made from a monomer styrene, was considered. Polystyrene sheets of thickness t = 1.2 mm were used. A dog-bone shaped tensile specimen (length = 60 mm, width = 10 mm, thickness = 1.2 mm) was tested under tensile load under displacement control with rate of 10 µ m / s, showing a nearly linear response up to failure. The