Initiation, Growth, and Arrest of Cracks in Constrained Sintering Materials
Carazzone, Joseph Reid
Cordero, Zachary C
Doctor of Philosophy
This thesis examines the long-standing problem of cracking during constrained sintering of a powder aggregate, a process called sinter-cracking. The problem has gained new context due to additive manufacturing (AM) techniques like binder jet 3D printing that rely on sintering as an essential post-processing step. The advantage of AM is to enable production of geometrically complex objects not easily attained by traditional means, but such complexity causes problems when met with the high strains and internal stresses typical of constrained sintering. The goal of this thesis is to understand sinter-cracking by two approaches: experiments using in situ monitoring to directly observe binder jet 3D printed samples during sintering, and simulations using the discrete element method to assess particle-scale phenomena. The effects of varying the size and shape of the flaw, the 3D print build direction, and the initial relative density are considered. Sinter-cracking is found to experience incubation and arrest phases, with the possibility of crack shrinkage, and to exhibit important analogies to creep-cracking. The stress field ahead of a sinter-crack is found to be characterized by the net section stress, which correlates with the sinter-crack growth rate. Finally, guidelines for design and processing are suggested for suppressing sinter-cracking, leading to quality sintered products.