Fracture toughness of a sintered A12O3 was determined with four‐point‐bend specimens having either straight‐through or chevron notches. For the straight‐through notched specimens, measured KIc decreased with decreasing notch width. For the smallest notch width (66 μm) KIc= 3.42±0.13 MN m−¾. For specimens with chevron notches, a crack initiates and extends from the tip of the notch under increasing load. KIc is calculated from the maximum load without measuring crack length, under the assumption that the derivative of the compliance is the same as that for a specimen with a straight‐through crack. A refined calculation accounts for the truncated chevron crack shape at maximum load using Bluhm's slice model. For the chevronnotch configuration, a value of KIc= 3.49±0.11 MN m−¾ was measured, which appears to be independent of the initial notch length a0 (distance from the crack mouth to the tip of the triangular notch). An effect of a1 (length of the chevron notch at the surface) on KIc was observed, independent of whether the calculation of KIc was based on the straight‐through crack assumption or on the slice model.

No abstract

Bisphosphonates are used worldwide as a successful treatment for people with osteoporosis, which is the major underlying cause of fractures in postmenopausal women and older adults. These agents are successful at increasing bone mass and bone trabecular thickness, decreasing the risk of fracture, and decreasing bone pain, enabling individuals to have better quality of life. Bisphosphonates are also used to treat multiple myeloma, bone metastasis, and Paget's disease; however, bisphosphonate treatment may result in negative side effects, including osteonecrosis of the jaw (ONJ). ONJ involves necrotic, exposed bone in the jaw, pain, possible secondary infection, swelling, painful lesions, and various dysesthesias, although less-severe cases may be asymptomatic. First-generation bisphosphonates, which do not contain nitrogen, are metabolized into a nonfunctional, cytotoxic analogue of adenosine triphosphate and cause osteoclast death by starvation. Second-generation bisphosphonates are nitrogen-containing agents; these inhibit osteoclast vesicular trafficking, membrane ruffling, morphology, and cytoskeletal arrangement by inhibiting farnesyl diphosphate synthase in the mevalonate pathway. Physicians treating older adults with osteoporosis and cancer should work together with dental practitioners, pharmacists, and other clinicians to inform individuals receiving bisphosphonates of their possible side effects and to suggest precautionary steps that may minimize the risk of osteonecrosis, particularly of the jaw. These include practicing good oral hygiene; scheduling regular dental examinations and cleanings; and cautioning people who are scheduling treatment for periodontal disease, oral and maxillofacial therapy, endodontics, implant placement, restorative dentistry, and prosthodontics. Recommendations for management of people with ONJ include an oral rinse, such as chlorhexidine, and antibiotics.

The short rod chevron-notch specimen has the advantages of (1) crack development at the chevron tip during the early stage of test loading and (2) convenient calculation of Ktc from the maximum test load and a calibration factor which depends only on the specimen geometry and manner of loading. For generalized application, calibration of the specimen over a range of specimen proportions and chevron-notch configurations is necessary. Such was the objective of this investigation, wherein calibration of the short rod specimen was made by means of experimental compliance measurements converted into dimensionless stress intensity factor coefficients. List of symbolsa crack length a0 initial crack length (distance from line of load application to tip of chevron) al length of chevron notch at the surface (distance from line of load application to point of chevron emergence at specimen surface) a,~ crack length at minimum of Y* A constant of proportionality between Kit and Pmx used by Barker and Guest [4] C specimen compliance (load line displacement divided by load) C' = E' DC Ct, compliance of specimen with trapezoidal crack C~, = E'DC,, D specimen diameter E Young's modulus E' = E for plane stress, = E/(I -v 2) for plane strain KI mode I stress intensity factor Ktc plane-strain fracture toughness P load Pmax maximum load Y dimensionless stress intensity factor coefficient for a straight-through crack, = KDX/-W/P stress intensity factor for a trapezoidal crack, = KDX/-W/P,= y(~)l,2--y* dimensionless Y* minimum of Y* a = a/W ao = ao/ W al = adW am = am/W v Poisson's ratio

The crack growth resistance of a textured, extruded alumina body was compared with that of an isotropic, isopressed body of similar grain size, density, and chemistry. R-curve levels reflected the preferred orientation; however, R -curve slopes (dK, / d A a ) were the same in all instances, implying a similar crack growth resistive mechanism. Three orthongonal orientations of crack growth in the two structures exhibited similar forms of KIR versus Aa curves, for which a schematic diagram for polycrystalline ceramics is proposed. [