Papers by Author: George D. Quinn

Abstract: The theta test specimen is a versatile tool for evaluating the strength of extremely small structures. Round and hexagonal rings are compressed vertically on their ends creating a uniform tension stress in the middle gauge section. The simple compression loading scheme eliminates the need for special grips. A conventional nanoindentation hardness machine with a flat indenter applied load, monitored displacement, and recorded fracture loads. Prototype miniature specimens with web sections as thin as 7.5 m were fabricated by deep reactive ion etching (DRIE) of single crystal silicon wafers. The strength limiting flaws were 200 nm to 500 nm deep surface etch pits.

Abstract: The dental community is using a variety of ceramic restorative materials such as porcelains (leucite or alumina based), glass-ceramics (leucite, mica, lithium disilicates), alumina-glass infiltrated, and CAD-CAM ceramics including pure alumina and zirconia (3Y-TZP) core materials. Polycrystalline ceramics such as alumina and zirconia serve as substructure materials (i.e., framework or core) upon which glassy ceramics are veneered for an improved appearance. Under masticatory loads, sudden fracture of the full-thickness restoration or of the veneering ceramic (chips) may occur. Stereomicroscope and scanning electron microscope analyses were used to perform qualitative (descriptive) fractography on clinically failed dental ceramic restorations. The most common features visible on the fracture surfaces of the glassy veneering ceramic of recovered broken parts were hackle, wake hackle, twist hackle, arrest lines, and compression curls. The observed features are indicators of the local direction of crack propagation and were used to trace the crack’s progression back to its initial starting zone (the origin). This paper presents the applicability of fractographic failure analyses for understanding fracture processes in brittle dental restorative materials and it draws conclusions as to possible design or processing inadequacies in failed restorations.

Abstract: The evolution of the science of fractography of brittle materials initially was driven by failure analysis problems. Early analyses focused on general patterns of fracture and how they correlated to the loading conditions. Many early documents are simply descriptive, but the curiosity of some key scientists and engineers was aroused. Scientific or engineering explanations for the observed patterns gradually were developed. Advances in microscopy and flaw based theories of strength and fracture mechanics led to dramatic advances in the state of the art of fractographic analysis of brittle materials. Introduction: This author was drawn backwards in time as he researched the current state of the art of fractographic analysis of brittle materials for his fractography guide book.[ ] Others have written about how the fractographic analysis of metals evolved (e.g., [ , , , ]), but there is no analogue for ceramics and glasses. The key scientists, engineers, and analysts who contributed to our field are shown in Fig. 1. Other work done by industry workers who were unable or loathe to publish is now lost, inaccessible, forgotten, or even discarded. It is the goal of this paper to review the key publications and mark the noteworthy advances in the field. Some deem fractography as the study of fracture surfaces, but this author takes a broader view. Fractography is the means and methods for characterizing fractured specimens or components and, for example, a simple examination of the fragments and how they fit together to study the overall breakage pattern is a genuine fractographic analysis.

Abstract: Direct tension strength tests were conducted on chemical vapor deposited silicon carbide microspecimens. Three types of specimens were used: straight gage section, tapered gage section, and notched gage section. The average strengths and standards deviations were: 0.42 GPa ± 0.13 GPa; 0.47 GPa ± 0.16 GPa; and 0.68 GPa ± 0.19 GPa, respectively. The fracture origins were identified by fractographic analysis and were cracks in large grains next to surface grooves from the deep reactive ion etch (DRIE) process used to fabricate the specimens.

Abstract: Grinding may create flaws that control strength and limit the performance of finished ceramics. Machining cracks sometimes have been difficult or impossible to find especially in toughened ceramics with interlocking grain microstructures that create rough fracture surfaces. Our fractographic examinations show that machining damage leaves telltale markings on fracture surfaces that may be easily detected using common fractographic techniques. A comprehensive study with over 400 ground rods and rectangular bars was conducted on several commercial silicon nitrides to study the effects of various machining conditions. Similarities and differences in behavior were observed. A paradoxical finding was that tougher silicon nitrides developed deeper grinding cracks. Machining crack size and shape strongly depended on the grinding wheel grit size.