Investigation of Modes of Thermal Fracture

I~VESTI~ATION OF MODES OF THERMAL FRACTURE OF SOME BRITTLE MATERIALS by CHARLES R. NELSON Submitted to the De~artment of Civil Engineering on September 2, 1969, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. An analysis of the temperatures and the thermal stresses in a detailed and accurate manner with the finite element technique resulted in the following advancements in the understanding of the natureof the thermal spalling problem. Thermal spalling due to local heating on the surface· of briLtle materials is the result of the comple~e propagation to total failure of a fracture induced by tensile stresses acting on a plane parallel to the surface and just under the heated area. If the material at th15 surface fails in compression before the tensile stress induced fracture occurs, spal11ng is prevented. There must be sufficient strain energy released by the fracture process to form the fracture surface required for complete failule. Experiments conducted on granite, marble and porcelain corroborate the theoretical expectations. The granite and porcelain specimens spalled at the heating time which produce calculated tensile stresses equal to the tensile strength. Crushing of the surface was observed on the marble specimens as predicted from the calculated stresses. Convex surfaces and a high ratio·of compressive strength to tensile strength increase the likelihood that spalling will occur in a given situation. Thesis Supervisor: .Fred Moavenzadeh Title: Associate Professor of Civil Engineering