A theoretical model is presented that relates acoustic emission to fiber cracking which occurs during a rising load tension test on a fiber reinforced composite. The percentage of broken fibers in an Al3Ni fiber reinforced aluminum was measured as a function of tensile strain by optical inspection of the polished surface of strained specimens. This information was used in conjunction with the proposed model to predict the acoustic emission response of the composite material. These predictions were compared with experimental observations, and a good agreement was obtained between the two sets of results. These results indicate that it is possible to relate acoustic emission quantitatively to the micromechanics of the deformation processes occurring within fiber reinforced composites, thereby demonstrating the applicability of acoustic emission to materials studies and also to nondestructive evaluation of the integrity of composite materials.
The various stages of microcrack initiation, microcrack growth, and final fracture are reviewed and the relation between their processes and acoustic emission is discussed. A theoretical model relating acoustic emission to the microcrack density of low carbon steel is presented, based upon experimental results of the microcrack density in low carbon iron. The model predicts that the total number of acoustic emissions increases rapidly with applied stress, above a threshold value.
The effect of radiation on tensile properties, notch bend properties, and fracture toughness was determined on A212B steel from the Pathfinder reactor surveillance program and on A533B steel from the U.S. AEC Heavy Section Technology program. Impact tests were performed on an instrumented Charpy machine which provided load-deflection data in addition to energy absorption data. Valid fracture toughness values were obtained from precracked Charpy specimens. The results of the notch bend tests on the irradiated steels indicated that the radiation induced increase in the ductile-brittle transition temperature was mainly due to the large radiation induced increase in the friction stress. Radiation reduced the strain rate sensitivity of the yield stress but did not change the temperature dependence of the yield stress. The microscopic cleavage strength was essentially unaffected by irradiation. The relationships between the metallurgical fracture parameters (Cottrell-Petch) and the ductile-brittle transition temperature (DBTT) and fracture toughness (KIc) were established. These relationships were used to predict the radiation induced change in fracture toughness (both DBTT and (KIc) from a knowledge of the effect of radiation on metallurgical fracture parameters.
The process of fracture in homogeneous and anisotropic composite materials involves three steps : (1) the initiation of a microcrack, (2) the stable growth of this microcrack under increasing load, out to macrocrack size, and (3) the unstable propagation of this crack at a critical stress level. The conditions for each of these processes are reviewed for homogeneous materials and then discussed in detail for fiber composite materials. The conditions for fracture initiation by fiber cracking depend primarily on the length and diameter of the fiber, their perfection, the fiber modulus, the test temperature, and the degree of interaction of the fiber with the surrounding matrix. Matrix and interface strength depend primarily on temperature, strain rate, and interfacial void content (for resin matrices). Unless the matrix is extremely strain-rate sensitive, premature fiber fracture does not lead to instability, and the composite strength is determined primarily by the average fiber strength. Several mechanisms for increasing the matrix toughness are discussed. The toughness associated with unstable, longitudinal crack propagation in composites then is considered in terms of fiber content, matrix toughness, interfacial bond strength, and matrix shear strength. It is shown that in certain composites the toughness decreases with increasing fiber content, and consequently maximum load carrying capacity is achieved at a particular fiber content Vf*, that increases with decreasing crack length. In other systems, particularly those containing cold-drawn metal fibers, both toughness and ultimate strength increase with fiber content.
A method is described whereby on-load values of crack tip crack opening displacement (COD) can be measured at the midsection of precracked three-point bend specimens by infiltration of silicone rubber. A calibration curve relating midsection COD to clip gage displacement was derived from measurements on the silicone rubber “castings.” This calibration curve can be used to calculate midsection COD from on-load clip gage displacement and specimen geometry only, without further infiltration measurements. These values of COD have been shown to be simply related to the stress intensity factor, independent of material, as theoretically predicted. The central region of a Charpy specimen in three-point bend has been found to remain in plane strain until well after general yield. Thus, plane strain values of COD at fracture initiation, (COD)c, can be determined from small specimens. Two initiation detection methods are described whereby (COD)c can be determined. These values of (COD)c can be used to accurately predict KIc values which agree with data obtained on larger, more expensive valid ASTM KIc specimens.
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