oxidation states of the metal are reduced by the hydride and reoxidized with the peroxide catalyst. The firm has not yet revealed the particular hydrazide used, which is said to be a purified grade of a compound Uniroyal has supplied for many years to the rubber industry. The company is a producer of benzene- and toluenesulfonylhydrazide, and oxybis(benzenesulfonylhydrazide). A second system based on liberation of carbon dioxide by reaction of modified methylenediphenyl isocyanate (MDI) or polymethylenepolyphenyl isocyanate with methyl ethyl ketone peroxide is now in field testing by the U.S. Peroxygen division of Witco Chemical and is almost ready for full-scale commercial introduction. Whitney & Co. also has introduced a method that uses an isocyanate and that may depend on reaction with a polyhydroxy compound. Unlike other techniques, the Whitney system is intended as a resin extender rather
than for making laminate foam cores or backups. Also, the Whitney formulation achieves densities as low as 6 to 10 lb per cu ft, compared with densities of 20 to 40 lb per cu ft for other systems. First in the field in the mid-1970's was Pennwalt's Lucidol division, with its 2-£er£-butylazo-5-methyl-2-hexanol. This method depended on residual acid in polyester compounds to catalyze liberation of nitrogen gas and generation of free radicals to initiate resin curing. Lucidol removed the compound from the market in November 1980, however, after workers at Lasco Industries allegedly suffered nerve damage. Lucidol spokesmen say workers did not follow recommended safety procedures. Lucidol chemists currently are working on other promising azo compounds whose foaming action may be triggered by polyester residual acidity or by peroxide catalysts used in resin formulations. •
Device tests composites nondestructively A portable device that tests composite fiber/resin materials nondestructively has been introduced by Acoustic Emission Technology Corp., Sacramento, Calif. Designated by the company as the 206 A/U acoustoultrasonic system, the instrument combines for the first time two materials testing techniques—acoustic emission and ultrasound—to exploit the best features of each. The device, rather than detecting individual flaws in a material, is capable of indirectly measuring the total effect of various flaws on the mechanical strength of a material. Acousto-ultrasonics was developed over the past three years by Alex Vary, materials research technologist at the National Aeronautics & Space Administration's Lewis Research Center in Cleveland. According to Vary, finding discontinuities in fiber composites is important, but just as important is the material environment in which they occur. Acoustoultrasonics provides information about how defects, both macroscopic and microscopic, interact in the material as a whole. Fiber/resin composites—materials composed of fibers such as glass, boron, carbon, or other materials for strength and held together with a resin such as epoxy—are replacing aluminum and steel in many aerospace, automotive, and petrochemical applications. Composite materials
provide the same strength and resilience as metal with significant savings in weight and sometimes in cost. Unlike metals, whose strength is largely a function of composition, the strength of composites is a function of both composition and engineering— that is, how they are put together. Because of this, composite strength and loss of strength during product life are somewhat more complex and far less well-characterized phenomena than in metals. According to Allen Green, president of AET, pure ultrasound techniques, although nondestructive, have drawbacks for testing composites. Ultrasound techniques involve introducing discrete waves into a material and measuring them after they are transmitted through the material or reflected off the back of the material. In either case, the amplitude of the wave decays only slightly if it encounters no flaws during its propagation through the material. If it does encounter a flaw, its amplitude decays sharply. However, according to Green, it is difficult to correlate a detected flaw to overall performance of a part made of a composite material. Additionally, evaluating strength loss after use, where changes may be quite subtle and distributed throughout the material rather than isolated in a single detectable flaw, is important. Ultrasound techniques alone cannot fur-
nish data to evaluate strength loss after use. Acoustic emission techniques, on the other hand, can be used to evaluate the integrity of a material as a whole. When put under stress, materials spontaneously produce ultrasonic emissions. A flaw in the material, Green says, "is a stress concentration point and is the source of the emissions." The signal that is detected is a product of all the flaws in the material and thus yields information on the material as a whole. The problem with acoustic emission is that a material must be put under stress to produce the emissions. "You have to bend, twist, differentially heat, apply pressure to, or otherwise apply stress to a material," says Green, to get a signal. Not much is known currently about the fatigue life of composites, Greeri says, so it is difficult to predict what such application of stress, often above the level the material is designed to withstand, does to the life of the item being tested. Also, applying stress to a composite material item is difficult, if not impossible, while the item is in use. The acousto-ultrasound technique used in the AET instrument overcomes the problems of the other two testing techniques. When ultrasound waves are introduced into a material not under stress, they imitate the waves that stress causes spontaneously and can be measured with an acoustic emission detector. Like acoustic emission techniques, acousto-ultrasound is sensitive enough to pick up the influence of microstructure, mechanical properties, and flaws on energy dissipation in the material. By the time the rather simple wave introduced into the material reaches the acoustic emission detector, it has become a complex jumble of waveforms. Using any one of a number of parameters to quantize this jumble, a "stress-wave factor" is assigned to the pattern. The stress-wave factor is a relative measure of the efficiency of energy dissipation in the material. It doesn't matter whether the stresswave factor deviates up or down from the norm along a specific propagation path; what matters is that a deviation indicates that some variation in the material exists along the path. "Think of it as a very sophisticated version of a tap test to determine the quality of a crystal goblet," Vary says. "The goblet, if the crystal is of high quality, rings true when tapped. What we're doing is tapping the material to see if it rings true." • March 9, 1981 C&EN
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