Search

Fracture of glass and ceramics is essentially brittle and a probabilistic phenomenon. Acoustic emission (AE) is a phenomenon due to microfractures before the final fracture. We have considered the models for microfracture process and proposed the method to analyze the obtained experimental data from AE measurement. We consider two models for microfracture, (1) microfracture occurs independently before the final fracture, and (2) a crack propagates from a microfracture as an origin. We obtain the probability distribution functions for microfracture stress and for microfracture location. The borosilicte glass composites were used in experiments, which have participated anorsite around Al 2 O 3 particles. The four point bending tests were carried out in both air and vacuum by wing the specimens of 3 by 4 by 40 mm. AE sensors were attached at the both ends of specimen and AE waveforms of two channels were recorded. One dimensional locations of AE signals were analyzed. Microfracture process of borosilicte glass composites was analyzed by these models. The analysis of microfracture stress and location from AE data demonstrates that the increase of humidity changes the microfracture process of the materials from the independent microfracture type to crack growth type. The experimental results of microfracture stress and location could be explained by the proposed models.

A silicon carbide ceramic composite material toughened by the incorporation of 20 vol% spherical alumina particles (SiC-P) and a monolithic reference material (SiC-M) were prepared by hot-pressing at 2000° C for 6 hours. SiC-P exhibited a fracture toughness as high as 6 MPa√m, whereas SiC-M showed a fracture toughness of 2.8 MPa√m. R-curve- and cyclic fatigue behavior were investigated for both materials. Flaw tolerance and R-curve behavior were evaluated using the indentation-strength technique. SiC-P exhibited enhanced flaw tolerance and rising crack resistance with crack extension, properties that were attributed to the presence of internal residual stresses from thermal expansion mismatch and crack wake bridging by both second phase particles and interlocking matrix grains. Because of the lack of toughening mechanisms, SiC-M exhibited a flat R-curve. Cyclic bending tests with beam shaped specimens and cyclic crack propagation tests using compact-tension specimens were performed at a load ratio of 0.1 in order to investigate the fatigue behavior. Strength degradation with increasing number of cycles and true cyclic fatigue-crack growth were observed for SiC-P, whereas SiC-M as a typically brittle ceramic was not susceptible to cyclic fatigue degradation. Fractographical results suggest that the mechanisms that enhance the R-curve of SiC-P are degraded when subjected to cyclic loads, resulting in amplified susceptibility to cyclic fatigue.

SiC particulate reinforced glass matrix composites were fabricated by hot pressing. The volume fraction of SiC was changed from 0% to 30%. The elastic modulus of the fabricated composites was consistent with a law of mixture based on the micromechanics. In order to evaluate the mechanical properties of the materials we measured four-point bending strength and fracture toughness (K IC ) by Single Edge Pre-cracked Beam (SEPB) method. Acoustic emission (AE) tests were performed during four-point bending and fracture toughness tests We observed fracture surfaces after bending tests. The bending strength increased with increasing volume fraction of SiC particle. The bending strength of glass was 96MPa and that of the composite reinforced with 30 vol % SiC particle was 144 MPa. The fracture toughness K IC also increased with increasing volume fraction of SiC particle. The fracture toughness K IC of the composite reinforced with 30vol% SiC particle was 2.1 MPam 1/2 . The number of AE events increased with increasing volume fraction of SiC particle. However AE behavior during bending tests is different from that during fracture toughness tests The total number of AE during fracture toughness tests is more than that during bending tests.

To study mechanical properties of fiber, matrix and their interface in C/C composites, fiber push-out test has been carried out. Where, to decompose the deformation, measured from indenter displacement, into local deformation is essential for mechanistic understandings. But up to now, there has been no systematic study on this effect. In this study, as the preliminary analysis of the push-out tests, those with a variety of groove width on specimen holder were performed and the elastic deformations of specimen were analyzed by a finite-element method (FEM). Materials used were unidirectionally reinforced C/C composites. Fiber push-out tests were carried out by utilizing dynamic ultra micro indentation test machine. For the cases of 20 and 100 μm groove, plateaus which indicate the occurrence of fiber push-out could be seen in Load-displacement curves ( indentation curves) and there was a difference in the duration of plateau. On the other hand, for all the cases, any difference could not be seen before plateaus. Now by using axis symmetrical FEM model, elastic deformation of C/C composites before push-out was calculated. From the results of calculation, the effect of groove width on indentation curves within elastic regime was considered to be little. But there was a large difference in the distribution of the stress transverse to carbon fiber direction on fiber-matrix interface between 20 and 100 μm, and this was considered to be the origin of the difference of the duration of plateau.

Interaction of microfracture and lamellar microstructure has been studied using acoustic emission method and TEM observation. TiE-34Al-1Mn 2 , -3Mn(wt%) alloy was prepared by arc-melting in Ar atmosphere. During the fracture toughness test microfracture behavior was monitored by acoustic emission method. Tests were stopped before the unstable fracture to facilitate a detailed study of the microfracture process. Then the specimens were sectioned at the plane in the thicltness direction and examined using optical microscope. TEM foils were prepared by mechanical grinding to 0.1mm thickness, ultrasonic cutting, dimpling, and finally ion milling. Then the cracks formed in the foils were observed. AE behavior was found to vary with the orientation relationship between crack propagation and lamellar interfaces. Crack propagation behaviors parallel and across the lamellar interfaces were observed by TEM. A close-up view of the crack across the lamellar interfaces showed that the crack went approximately straight in the lamellae, but was deflected by lamellar interfaces. In the ligaments region remarkable plasticity was found.

Crack path is simulated in particle-dispersed composites along with the related variation of fracture resistance with crack extension. The direction of crack propagation is influenced by the geometrical crack shape and residual stresses due to thermal expansion mismatch between particle and matrix. The direction is determined by the criterion of maximum energy release rate at crack tip. The simulation is conducted on the SiC matrix composites dispersed with Al 2 O 3 particles, where the composites are assumed to be elastically isotropic. Due to the higher thermal expansion of Al 2 O 3 particles, residual tensile stresses in the radial direction of the particles are generated within the matrix, and cracks propagating near the particles have a tendency to be repelled. The fracture resistance increases with crack extension when the crack approaches the particle, and decreases when propagates in the tensile stress fields around the particle. The entire fracture resistance in the composites of 10 vol. % particles shows a lower value than that of matrices, due to the interaction with the residual tensile stresses in the radial direction of the particles.

The effect of reinforcement size and volume fraction on fracture toughness in powder metallurgy (P/M)- processed 6061Al alloy matrix composites with different kinds of reinforcements (Si 3 N 4 whiskers, and SiC particles) are analyzed. The acoustic emission ( AE ) activities has also been investigated during three point bending fracture toughness test. Comparing the fracture toughness results in MMCs containing different volume fractions (V f ) of whiskers and particles show higher fracture toughness in materials with lower V f . On the other hand, the fracture behavior in 10 percent reinforced composites regardless of the type of reinforcement, particle or whisker, is ductile fracture, but it is semi- brittle in the case of 30 percent reinforced materials. Regarding the amplitude and the number of AE events during three point bending fracture toughness test, 30 percent reinforced materials with brittle fracture mode show very high amplitude signals with less number of events in the onset of plastic deformation. 10 percent reinforced composites with a ductile mode fracture show lower AE amplitude and higher number of events. Totally, a kind of interaction between the different parameters like volume fraction, reinforcement type and reinforcement size on the fracture behavior of 6061 Al alloy matrix composites are observed and discussed.

Compressive tests are conducted on the laminated unidirectional C/C composite of two shapes, block and tab specimens. Acoustic emission (AE) is monitored during the tests to investigate microscopic failure mechanisms. Compressive strengths in block specimens are within the range between 70 to 180 MPa. The compressive strength has no significant influence of the height of the specimens, but is dependent on the failure procedure. Two types of stress-strain curves are observed. In type-1 of lower strength, initial deformation is linear elastic with AE signals of low amplitude ( 60dB), corresponding to the initiation of delamination are observed. Compressive strengths in the tab specimens are within the range between 260 to 300MPa, higher than that of the block specimens. Due to the constraint of edges by the tabs, delamination can not propagate to the edges, resulting in higher strengths. In the present C/C composites, main failure mechanism is found to be delamination, whereas kink-band formation is observed only partially.

Micro indentation test technique was applied to evaluate interfacial shear strength of uni-diredional C/C composites, and the effectiveness of this method was discussed. Interfacial sliding was detected in the load - displacement of indenter curve (indentation curve), and push-in and/or push-out fibers were observed. From the load displacement information of the micro indentation test, interfacial shear strength in C/C composites was defined. At lower heat treatment temperature (HTT) than 2000K, almost no temperature dependence can be recognized. At higher HTT, however, interfacial shear strength was increased with increasing HTT, which was supposed to be effected by the growth of graphite layer and increment residual compressive stress to fiber.

Subcritical crack growth behavior was examined in Al 2 O 3 matrix composites reinforced with 0, 10, 20 vol% SiC nanoparticles. Crack velocity exponent, was evaluated from two different methods. One is from the mean bending strength, and the other from the curve-fitting on 3-point bending strength data obtained under different testing conditions. A bending strength of the 20% SiC composite is about 970 MPa and shows no influence of both the environment and loading rate, while the monolithic Al 2 O 3 shows lower strength in air than in vacuum. The n value from the curve fitting on strength data is about 58 in the monolithic, 36 in the 10% SiC and 66 in the 20% SiC composite. Fracture toughness evaluated by the controlled-surface-flaw method show a similar tendency to the dependence of n value on the volume fraction of SiC particles. Taking the effect of grain size into consideration, it is expected that enough SiC particles can contribute to the improvement of both the fracture toughness and subcritical crack growth resistance.

A SiC/SiC composite formed from woven sheets of carbon coated SiC fibers (NICALOCERM) has been examined during the tensile loading of both unnotched and double-edge-notched specimens. As the specimens were deformed at a slow crosshead speed of 0.02 mm/min, the load, strain and acoustic emission (AE) were all measured and characterized. Analysis of AE amplitude distributions, coupled with the stress strain curves and SEM observations indicates that the fracture mechanisms of the SiC/SiC composite consist of matrix cracking, fiber pull-out and fiber breakage. The fracture surfaces of the unnotched specimens were characterized by fiber pull-out. The fiber pull-out was a result of the stress concentration due to pores located in the woven yarns which produced heavy matrix cracking leading to fiber bundle breakage. Different behavior was found for the notched samples which demonstrated quasi-yield behavior during tensile loading. The notch was found to produce a localized stress concentration inducing debonding of the 0° direction fibers. Then fiber pull-out occurred after the breaking of fibers without causing substantial matrix damage. In NICALOCERAM, pores located in the woven yarns act as an initial defect which causes matrix cracking. The resultant stress concentration from the matrix cracking causes numerous localized fiber breaks.