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Acoustic emission monitoring has been used to record the sounds which emanate from such diverse processes as deformation of structures [1], geologic movements [2] and crack propagation in metals [3]. In most cases the approach is to simply record the acoustic response and then attempt to establish some correlation between the number of acoustic events and the load applied. Very little has been done to correlate observed emissions with a well defined sound source resulting from a specific deformation mechanism.As an outgrowth of our continuing efforts to analyze failure mechanisms in composites we have been able to isolate three basic failure mechanisms by optical means. These mechanisms, which were caused by tensile load of epoxy specimens containing only a few fibers, are [4]: 1) Fiber Fracture 2) Matrix Cracking 3) Interfacial Debonding. It is interesting to note that the local heterogeneity of advanced composites where very strong and stiff fibers are incorporated in a relatively weak and low modulus matrix may be an advantage in acoustic emission analysis. Fiber fractures are clearly audible and have been observed in tests of boron-epoxy composites without the aid of acoustic monitoring equipment. In more homogeneous materials the ability to discriminate between specific events in the failure process is a good deal more difficult and often requires very sophisticated and sensitive instrumentation covering a broad range of frequency. It appears, on the other hand, that the ability to discriminate between sudden energy release from a fiber fracture and the more gradual debonding mechanism, for example, is not nearly so difficult from the standpoint of equipment and frequency range. The major obstacle is establishing a correlation between the specific event and its acoustic signature by simultaneous observation of the two. The transmission of sound from a specific local event through the composite is a complicated process involving mode changes and attenuation, with the result that the transducer may not experience a sound pulse exactly like that which occurred some distance away. However, if a specific kind of local event causes the same response each time it occurs, the value of the technique in identifying specific failure mechanisms within the composite is obvious.Since the means for controlling local failure mechanisms in composite materials was already available, it provided the basis for identifying the specific acoustic signatures associated with these events. The acoustic signals were detected by a

Practical use of coal-fueled diesel engines depends on the improvement of component durability. The wear characteristics of standard materials in the coal-fueled engine were studied first. Candidate wear-resistant materials were sorted by bench-scale tests. The best material combinations were applied to small-scale engine tests for operation on seeded diesel fuel. Components with hard materials and coatings were scaled up for coal-water mixture testing on the locomotive engine. Results indicate practical solutions for ring and liner wear and positive progress toward defining the material requirements for the fuel injection nozzle.

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Fig. 3. for 20 s from ( A ) TiB, and (B) TiB, Microstructures of etched surfaces of bodies sintered A COMMERCIAL si/sic compositeLs* is formed by infiltrating densely packed carbon fibers in preformed shapes with liquid Si. The carbon fibers are converted to polycrystalline S i c which retains the distribution of the original fibers. The resulting structure consists of oriented S i c crystallites in an Si matrix. In the present work, the strength of this material was examined, with particular attention to its behavior above the melting point of Si and also as a function of time-to-failure. The carbon used was in the form of cloth,+ rather than tow or felt.' This two-dimensional carbon cloth, which was 0.020 in. thick and had a starting density of 0.43 g/cm3, was formed into a plate 1 .O by 1.5 by 2.0 in. together with graphite powder to a final green density of 0.90 g/cm3. The block was infiltrated with Si,' and specimens were removed by diamond cutting. No grinding was used except to clean the cast surface. The resulting microstructure consists of =75 to 80 vol% Sic, 10 vol% Si, and 5 to 10 vol% unreacted carbon. Because the starting carbon cloth is isotropic in the X-Y plane, the resulting composite is also isotropic in this plane. During testing, the specimens were oriented such that the X-Y plane was perpendicular to the applied load.Bend strength was determined by loading specimens 0.1 by 0.1 in. in 3-point bending over a span of 0.9 in. using a testing machine,$ S i c loading fixtures, and a resistance-heated MoSi, furnace providing an air environment. The load-displacement curve was almost linear up to the fracture point for all specimens tested.The strength of Si/SiC at 1200" and 1370°C over a wide range of average times-to-failure is shown in Table I. Times-to-failure of 0.05 and 17 min correspond to loading rates of 0.05 and 0.0002 in./min, respectively. Times-to-failure of 420 min were obtained by modifying the testing machine to turn on and off automatically. C S .- I -