21-Mat-A3 Structure and Characterization of Materials · Dec-12-Mtl-A3 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2018 — 12-Mtl-A3, Structure and Characterization of Materials. Three hours, open book, any non-communicating calculator permitted. Eight questions constitute a complete exam paper; all eight are solved here.
Reference texts. The answers below are keyed to the standard undergraduate materials-science references recommended for this syllabus code:
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Semicrystalline polymers — e.g. polyethylene, polypropylene — consist of alternating crystalline lamellae (folded-chain platelets) and amorphous regions, often organized into larger spherulites, with some chains threading between adjacent lamellae as tie molecules. Plastic deformation proceeds through a sequence of structural changes at this lamellar scale, well beyond the initial elastic (chain-stretching/uncoiling) response:
1. Elastic deformation and amorphous-phase stretching. At small strains, the amorphous tie chains between lamellae stretch elastically and the lamellae themselves may bend slightly; this is recoverable.
2. Adjacent slip within lamellae. As stress increases, chain-folded segments within a lamella begin to slip past one another along the chain-fold (slip) planes, similar in spirit to crystallographic slip in metals but occurring between folded polymer chain segments rather than atomic planes.
3. Lamellar separation. Amorphous tie chains connecting adjacent lamellae stretch further and the lamellae begin to separate from one another, opening voids/microcracks in the amorphous interlamellar regions.
4. Lamellar tilting and block segmentation. The lamellae tilt so that the chain-fold (slip) planes rotate to align more closely with the tensile axis, and continued slip breaks the lamellae into smaller, rigid "blocks" of folded chain segments, still connected to one another by the surviving tie-chain molecules.
5. Block/tie-chain alignment (fibrillar structure). With further straining, these blocks and their connecting tie chains rotate and align parallel to the tensile (draw) axis, producing a highly oriented fibrillar microstructure in which the polymer backbone chains themselves are nearly parallel to the applied load — this is the microstructural origin of the strong, stiff, oriented material produced by cold drawing (e.g. drawn polyethylene fiber or the necked region of a stretched polymer bar), and why such drawn regions are markedly stronger along the draw direction than the undrawn polymer.
