The fracture surfaces of un-notched tensile specimens prepared from HDPE biaxially rolled at room
temperature and drawn to failure in tension were analyzed using scanning electron microscopy (SEM). The
HDPE sheets were reduced to a thickness of about 80% the initial during the rolling process and the tensile
test was conducted at -40 degrees Celsius and at a strain rate of 100%/min. In comparison to a melt processed sheet of
the same material and thickness, the rolled material exhibited greater work hardening capacity,
homogeneous yield behavior, and improved elongation to failure. The fracture surface manifested in a plane
roughly 45 degrees to the draw direction, and revealed three distinct zones: 1) the damage zone, 2) a fracture
surface associated with slow crack propagation, and 3) a fracture surface associated with rapid crack
propagation. The cross-sectional dimensions of sub-microlayers observed from the fracture surface
suggested that they could have resulted from the affine deformation of spherulitic crystals during the rolling
process.
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Parts made of polymers play an ever increasing role in many different industries (i.e. aerospace, medical, automobile, etc…), which are attracted by their very interesting material properties. Therefore, there is a need to understand why and how these parts fail to prevent incidents, reduce cost, and move toward a more sustainable approach to the dimensioning of structures made of this type of material. Here, we seek to apply the statistical fractography method to polymers to achieve this goal. This quantitative approach of the field is based on a deep understanding of the non-linear damage mechanisms at play at the crack tip during propagation, and that is expressed through a model used to bridge the measured fracture surface’s roughness and the fracture properties of the material, such as its toughness Kc. We show that our fractographic approach provides reasonable estimate of the fracture toughness, paving the way for the application of statistical fractography to the failure analysis of polymeric parts.
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Designed cellular lattice structures can be used in many engineering applications. While typically the viscoelastic deformation behavior (stiffness and damping) is utilized in many of these applications, the strength and the fatigue behavior plays an important role for components which are exposed to long-term cyclic loading. Selective laser sintered (SLS) polyamide 12 (PA12) and thermoplastic polyurethane (TPU) materials were investigated in two various lattice structures. A bistable structure based on curved bending beams (BB) and another structure with the combination of bending and torque of the trusses (USF) was designed and produced. To cope with the complexity of the SLS generated structure, three specimen configurations with different printing directions (0° and 90°) were used. To study the bulk behavior cylindrical hollow and notched round-bar specimens, to study the cellular behavior specimens consist of single trusses and knots and specimens contain multiple lattice cells were investigated under both uniaxial and axial/torsional, monotonic and cyclic loading conditions. The monotonic tests provided not only the strength values but relevant material models for subsequent simulations. The cyclic tests were performed at low strains for a comprehensive viscoelastic characterization and at higher strains for fatigue characterization in terms of conventional and strain based S-N curves.
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