Materials Engineering Department
BS in Materials Engineering
We designed a tensile test fixture for a 112 lbf capacity Instron load frame that imparts a normal force on the face of a button epoxied to an investment casting shell sample, delaminating the shell area attached to the button. Using a green standard shell (Group 1), a partially fired standard shell (Group 2), and a green shell with a different third coat (Group 3), we verified that the fixture can measure differences in strength between sample groups. We attached steel buttons to leveled samples with 0.05 mL of Hysol-Loctite 9340 epoxy, let it cure for 48 hours, and tested them at 0.05 in./min. Most shells failed below the face coat, instead of spalling. Groups 1 and 2 failed in a backup layer, or at the larger stucco beneath it (0.035-0.044″ deep). Group 3 failed in the face coat (0.010″), flat in a backup layer (0.033″), or in rounded craters through several layers (0.064″). We measured fracture areas in Photoshop to calculate failure stresses, which averaged 116.21 psi for Group 1, 179.42 psi for Group 2, and 141.99 psi for Group 3, with respective standard deviations of 21.78 psi, 30.84 psi, and 31.21 psi. Two-sample t-tests showed statistically valid distinctions between each group’s results, indicating that this fixture could be used to further investigate designing a stronger shell to mitigate face coat spalling.
Available for download on Wednesday, June 10, 2020