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Chipping caused by micrometric particles poses a threat to the structural integrity of modern dental prosthetic materials. It can degrade their fracture strength and cause wear of both artificial crowns and antagonist teeth. Here, surface chipping of the main types of commercial ceramic‐based dental materials at the microcontact/particle level is investigated by means of indentation tests. Conical tips of different sizes (radii 20 and 200 μm) under axial and sliding loading are employed to simulate individual microcontacts. Both decreasing particle size and adding a lateral contact force decrease the chipping load below typical bite forces. Specific damage mechanisms are identified as predominantly brittle fracture in ceramics with small, equiaxed crystals, with significant quasi‐plastic damage in ceramics containing large, elongated crystals and composites. Critical loads for the occurrence of chipping are quantified (lowest values in equiaxed glass–ceramics; greatest in zirconia) and analyzed within the framework of fracture mechanics. The brittleness index (BI) is proposed as a simple indicator of the resistance to chipping of dental materials—the lower the BI, the greater the resistance. Special attention is paid to the effect of the materials' microstructure, which can result in transformation toughening (as in zirconia) or quasi‐plastic behavior (as in lithium disilicate), both highly beneficial to increasing the chipping resistance. Finally, practical implications for the selection of current dental materials as well as for the development of novel materials with improved durability are discussed. [ABSTRACT FROM AUTHOR]

A study of the influence of water environments on the cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC-M) and without (PVC-U) chlorinated polyethylene (CPE) impact modifier was undertaken and compared with corresponding results in air. Two frequencies of 1 and 7 Hz were applied to assess the influence of frequency on the fatigue behavior; a higher fatigue resistance and threshold were obtained with increasing frequency. This trend is more significant in water. However, in this environment, the fatigue resistance deteriorated under conditions of higher stress intensity factor amplitude (ΔK) and frequency. The fatigue properties of PVC-U are the most affected by the presence of water, particularly at low frequency and higher ΔK. Examination of the fracture surface showed the interaction of water molecules and the PVC matrix with the formation of (1) a nodular structure, close to the fatigue threshold and (2) plasticized structures at high ΔK, which are associated with a greater threshold value and fatigue resistance. The absorption of the water retarded the fibrillation of craze and caused crack blunting effects. Water functions as a plasticizer, particularly at high ΔK, through the formation of the plasticized structures. Results are compared with those observed from an in-service failure. POLYM. ENG. SCI., 50:352-364, 2010. [C] 2009 Society of Plastics Engineers, INTRODUCTION The success of plastic pipes in various applications such as water and gas distribution and sewer and irrigation systems has led to increasing demand for these materials. In applications [...]

The cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC-M) and without (PVC-U) chlorinated polyethylene (CPE) impact modifier, was studied. The effect of impact modifier upon fatigue crack growth rate and threshold was evaluated at frequencies of 1, 7, and 20 Hz. It was shown that the addition of CPE lowered the threshold stress intensity factor amplitude for crack growth (Δ[K.sub.th]) of PVC-M compared to that of PVC-U at lower frequencies, and that the effect became more pronounced at lower frequency. At lower stress intensity factor amplitudes (below ΔK = 1 MPa*[m.sup.[1/2]]), there was a slight difference between the crack growth rates of U- and M-PVC. The crack advance mechanism is investigated by microscopic observation of the crack tip process zone. Although the zone is relatively large in PVC-M, associated with higher toughness, it did not improve the fatigue crack growth resistance significantly. Fracture surface observations reveal a higher density of fibrils on the fatigued surface of PVC-M with the density, relative to that observed in PVC-U, reducing with frequency. It is therefore hypothesized that accelerated fibril failure is a mechanism of fatigue. POLYM. ENG. SCI., 49:1299-1310, 2009. [C] 2009 Society of Plastics Engineers, INTRODUCTION The increased application of pipes in gas and water supply systems has had a significant impact on materials development. Initially, metals were mainly used but with time, iron pipes [...]