The effect of thermal annealing and quenching on the notched Izod impact strength of several polymers has been studied. Primary emphasis was placed on polycarbonate, but ABS, PVC, polysulfone, and polymethylmethacrylate were also studied. It was determined that residual stresses created by thermal quenching from above the glass transition temperature can have a great effect on impact strength for the polycarbonate, PVC, and polysulfone polymers studied. In fact, it is shown that the thickness transition observed in impact strength for polycarbonates is governed by the residual stresses and not by thickness. In polycarbonates, quenched sheets up to 3/8 in. in thickness have shown impact strengths of 18 ft‐lb/in. whereas sheets 1/8 in. in thickness can be embrittled by annealing, showing an impact strength of 2 ft‐lb/in. However, it has been shown that this embrittlement results from the absence of residual stress. Residual stresses having maximum values up to 3000 psi (in Compression) have been determined at the polycarbonate sheet surface using birefringence measurement techniques. The existence of these compressive stresses is postulated to restrict the extent of craze growth at the notch tip, and the impact specimen can yield rather than fail in a brittle manner if the stress state is sufficient.
The effect of cold rolling on the Izod impact strength of amorphous polycarbonates has been studied. The impact strength is a function of the roll reduction as well as the original sheet thickness. Sheets varying from 0.125 to 0.645 inches in thickness have been studied and roll reductions up to 50 percent have been utilized. It is shown that enhancement in impact strength occurs at very small percent roll reductions. The orientation release stress has been measured as a function of roll reduction and the internal stresses through the thickness of the sheets have been studied by birefringence methods. It is suggested that the residual stresses are responsible for impact enhancement rather than the molecular orientation.
A unique method was developed for mechanical testing of brittle materials to create an unlimited number of stress ratios in the tension-tension and tension-compression quadrants. The stress states are achieved by internal and external pressurization of tubular specimens in 10 special pressure vessel. Failure envelopes were determined for polycrystalline alumina and fine-grained isotropic graphite. The modified maximum strain energy and Coulomb-Mohr theories fit the data best; these theories should be plotted as bands representing probability of failure. The biaxial tensile strengths of alumina and graphite are lower than their uniaxial tensile strengths; the tensile strengths decrease as the compressive stress in the orthogonal directions increases.
SynopsisThe development of a unique test method for biaxial stress experiments on brittle material is discussed. Results of biaxial tension-tension and tension-compression experiments on poly-methylmethacrylate (Plexiglas) are presented. The strength results compare reasonably well with modified distortion energy theory in the tension-compression stress quadrant.
Several types of amorphous polymers have been cold‐rolled to various thickness reductions and the notched Izod impact behavior has then been studied. It has been determined that whereas some polymers exhibit large increases in impact strength with modest reductions in thickness other polymers only exhibit small increases in impact strength. For example, PVC, ABS, poly (phenylene oxide), and polysulfones show large increases in impact strength whereas rubber‐toughened acrylics show only a small increase in impact strength.
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