I am planning to teach a new class and wanted to use this specimen. Note that the black-ish line on the specimen in the LHS picture was originally along the axis of the cylinder, indicating a bulk shear strain on the order of 1. The smooth failure surface at around 50 deg from the axis in the RHS picture is thought to occur first. Note the small similar failure surfaces initiating under the blue arrow in the LHS picture. Also note the this smooth surface is predominantly under compression with the torque direction indicated. I believe there is something about molecular alignment in the tensile direction creating a weaker material in the plane perpendicular...approx. corresponding to the smooth fracture surface.
https://www.sciencedirect.com/search?qs=fracture%20mechanism%20of%20PMMA%20in%20torsion&show=25&sortBy=relevance
Dr Akram:
Thank you so much for the search and references. I will read with interest.
tom.
PMMA is an amorphous polymer, often using heavily crosslinked formulations. The smooth section does not make sense. Was there some sort of discontinuity?
Hi Bart. Sorry for the delay in respons. I am still learning to use this...
There was no intentional discontinuity introduced. On a macro-scale the specimen appeared as a uniform cylindrical shape. It looks as though it was cut from a larger sheet and the turned with a lathe which would leave circumferential tool marks but these woul be perpendicular to the fracture surface.