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The role of homogeneous and heterogeneous toughening mechanisms on DGEBA fracture
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The role of homogeneous and heterogeneous toughening mechanisms on DGEBA fracture

Amanda Bellafatto
Master of Science (M.S.), Drexel University
May 2018
DOI:
https://doi.org/10.17918/tjn1-pa21
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Abstract

Composite materials--Mechanical properties Composite Materials Materials Science Mechanical Engineering
As composite materials become increasingly utilized in engineering applications, it is imperative to understand the role composition has on their fracture and failure behavior. Epoxies, a group of thermosetting polymers, are commonly used in composites as a polymeric binding or matrix material due to their chemical stability and relative material strength. At the same time, their rate-dependent behavior is comparatively unexplored. Moreover, the mechanical properties of all polymers are highly dependent on their chemical makeup, including crosslinking and polymer chain organization. Consequently, this study investigates homogeneous and heterogeneous toughening mechanisms on variants of the epoxy DGEBA under quasi-static and dynamic fracture. Heterogeneous toughening mechanisms were explored via the addition of glass inclusions at three volumes: 5, 15, and 30 percent. Inclusions 7-10 [mu]m in diameter were investigated as this size modifies the polymer structure via crosslinking density and chain organization, and inclusions 30-50 [mu]m in diameter were examined as this size can introduce potential crack bridging and crack path obstacles. Homogeneous toughening mechanisms via diamine hardeners were examined in PACM, D230, and D400. PACM and D230 have similar diamine lengths, but vary in rigidity, and D230 and D400 have the same rigidity and structure, but vary in length. In all cases, quasi-static and dynamic fracture experiments using Single Edge Notch Tensile (SENT) specimens were conducted, and x-ray computed tomography (microCT) was used to investigate samples both pre- and post-mortem. Ultra-high-speed (UHS) imaging along with digital image correlation (DIC), a full-field imaging technique that tracks 2D surface kinematics during loading, were employed to extract fracture behavior. Overall, an increase in fracture loading from quasi-static to dynamic decreased the dependency on bead size for heterogeneous toughening mechanisms, but highlighted the effects of clumping in high volume percent samples; meanwhile homogeneous toughening mechanisms appeared to have higher rate dependency in fracture properties of amines with longer chains and lower rigidity.

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