The purpose of this study was to determine if calcium hydroxide, mineral trioxide aggregate, or sodium hypochlorite caused a change in the force required to fracture root dentin. Ten bovine central and lateral incisors were machined using various saws and drills to produce a cylinder of dentin with a 6.0-mm outer diameter 3.5-mm inner diameter and a length of 10 mm. The cylinders were cut lengthwise into four symmetrical pieces. The canal sides of the sections were then placed into Petri dishes containing a 1-mm depth of calcium hydroxide, mineral trioxide aggregate, sodium hypochlorite, or physiologic saline (control). The samples remained in the dishes for 5 weeks and were then shear tested by using an Instron machine. Data were analyzed using an ANOVA test for comparison of the groups as a whole, and a t test was used to compare each quarter section with its control from the same tooth. A 32% mean decrease in strength was discovered for calcium hydroxide, a 33% decrease in strength for mineral trioxide aggregate, and a 59% decrease for sodium hypochlorite. All decreases in strength were statistically significant: p < 0.001 for calcium hydroxide, p = 0.027 for mineral trioxide aggregate, and p < 0.001 for sodium hypochlorite. Results indicated that root dentin was weakened after 5 weeks of exposure to calcium hydroxide, mineral trioxide aggregate, or sodium hypochlorite.

A study was made of the residual stresses caused by weld shrinkage in a plate containing a central longitudinal weld. A theoretical analysis was developed in which the weld was treated as a moving concentrated heat source, through-thickness variations in temperature and stress being ignored. The equations were solved in finitedifference form and the stresses were computed at discrete stages during welding and cooling. Allowance was made for the variation of yield stress with temperature. Computed results were obtained for the bead-on-plate and butt welded cases.Residual stresses were measured in 40 flat plate specimens containing a central butt weld, with thicknesses ranging from 6 to 16 mm. The results led to proposed values for the factors H and C in the simple Moxham formula, which are in reasonable agreement with theoretical predictions.

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