Dental composites are being increasingly used to restore occlusal surfaces ofposterior teeth. If these restorations are not to fracture in service, an understanding of their fracture behavior is essential. The fracture parameter used is the stress-intensity factor at crack instability (Kic). This has been calculated by means of the double-torsion test in a mechanical testing machine.The fracture behavior of the composite was studied by varying: the amount offiller (7%, 15%, 26%, and 41% by volume); the surface treatment of the filler (coated or uncoated); the environment (air and water); and the cross-head rate (0.05, 0.5, 5, and 50 mm/min).Fracture was found to occur in either a continuous (stable) or stickslip (unstable) manner. The stick-slip behavior was due to blunting of the crack tip, which was controlled by the yield behavior. When there was no significant blunting, continuous crack growth occurred. An unique relationship was found to exist between the yield stress and the stress-intensity factor for all conditions of testing.

The fracture behavior of a bis-GMA resin was studied by means of the double-torsion test. The fracture parameter measured was the stress-intensity factor. Fracture occurred in either a stick-slip (unstable) or continuous (stable) manner, depending upon the test conditions. When stick-slip propagation occurred, the fracture surfaces showed characteristic crack-arrest lines. The fracture surfaces were examined by use of a reflected-light optical microscope. The stress-intensity factor for crack initiation was found to be related to the size of the crack-arrest line which, in turn, could be related to the Dugdale model for plastic zone size. The evidence supported the concept that the behavior of the crack during propagation was controlled by the amount of plastic deformation occurring at the crack tip.

The corrosion-fatigue crack growth properties of several types of HY-100 steel weldments were studied, primarily in an environment of seawater with cathodic protection by zinc anode. Shielded metal-arc (SMAW), gas metal-arc (GMAW) of both pulsed and spray are processes, and submerged-arc (SAW) weldments were included. The fatigue crack growth tests were conducted on compact specimens removed from the weldments to confine the path of the growing crack in all-weld metal. Constant-amplitude sinusoidal loads were applied at a frequency of 0.17 Hz with a load ratio (minimum-to-maximum) of 0.10. In general, fatigue crack growth in weldments was considerably slower than that in HY-100 plate under the same conditions of load and environment, where the applied potential accelerated crack growth rate. The results showed minor differences among the weldments. It was suggested that the residual stress state along the weld centerline, the inhomogeneity of the weld metal with respect to the crack path, defects, and minor porosity all act to retard crack growth, especially at lower stress intensity levels. These factors tend to mask the environmental effects. However, weldments prepared with high heat input showed greater resistance to fatigue crack propagation than weldments prepared at low heat input. The corrosion fatigue crack growth exhibited by the HY-100 weldments was similar to that of HY-80 and HY-130 weldments where weld process, environment, and loading conditions were similar.

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