This paper presents a unified analysis of isotropic out-of-plane shear problems with concentric circular elastic inclusions. The applied disturbances considered in this paper are longitudinal shear stress at infinity, analysis is based on the complex variable method using the Möbius transformations by Honein and Herrman. Several numerical examples are given by many graphic representation.
Simulated results of structural change, residual stresses and distortion are presented for carbon steel cylinder in the scanning-type induction hardening process by a CAE system “HEARTS (HEAt tReaTment Simulation system)”. The system HEARTS was developed to simulate heat treatment processes based on “metallo-thermo-mechanics” available for describing the coupling effect between metallurgical change due to phase transformation, temperature and inelastic stress/strain. A steel cylinder is treated as an axisymmetric model with scanning internal heat generation and convection boundary. The results under different scanning velocity as well as the magnitude and pattern of the heat source from induction coil are compared with experimental data of distortions, volume fraction of metallic phases as well as residual stresses.
High temperature fatigue crack growth behavior of silicon nitride has been investigated. Fatigue crack growth resistance decreased with increasing temperature and with decreasing loading rate at high temperature above the glass softening temperature. Microvoids were nucleated at triple points due to grain boundary sliding in the near crack tip region. The microvoid grew along a grain boundary accommodated by sliding of the adjacent boundaries to form a microcrack. A fatigue crack propagated by coalescing to the microcracks. The stress shielding due to bridging formed in the crack wake had also a significant influence on the fatigue crack growth behavior at high temperature.
Internal damage evolution of thin tubular specimens of CFRP was studied by means of AE (acoustic emission) measurement. The AE signals were detected first in the tension tests of CFRP tubular specimens of 0°, 90° and ±45° fiber orientation, and three damage modes induced by the tests, i.e., matrix crack, delamination and fiber breakage were identified by the frequency analysis of AE signals. Then, tension-torsion tests of specimens of ±45° were performed, and the behavior of damage development in every damage stage was analyzed from the frequency analysis of AE signals. AE count rate was also measured in order to detect the onset and the development of material damage. The change in the elastic properties were related to the AE count rate. Then, by performing damage tests under biaxial loading, the initial and subsequent damage surfaces were identified by use of AE count rate.
Metal matrix composites (MMCs) reinforced with continuous fibres have received considerable interests as the materials under severe service conditions, because they generally possess many attractive characteristics: higher specific strength and higher specific stiffness, compared with traditional monolithic materials. While these materials are on the way of improvements and development, there remain many works to be understood, or clarified. In this work thermal fatigue tests of an unidirectional SiC fiber reinforced Ti-24Al-11Nb matrix composite were carried out under various conditions with maximum temperature and temperature range without external load in air. The fiber employed was a β-SiC fiber of 140μm diameter with a carbon rich graded silicon carbide coating on the surface: SCS-6. During the study, special attentions were paid to understand what damage is produced by the application of thermal cycle, how the damage evolves, and how the thermal fatigue life should be evaluated, taking account of the change of reaction zone near the fiber/matrix interface. Based on the experimental observations, a simple method how to assess the thermal fatigue damage was discussed and proposed.
Thermal fatigue tests of a unidirectional SiC fiber reinforced Ti-24Al-11Nb matrix composite were carried out without external load in air, where the fiber was a β-SiC fiber of 140μm in diameter with a carbon rich graded silicon carbide coating on the surface: SCS-6. By employing the composite specimens to which the thermal cycle had been previously given, both the change of elastic properties of the composite and that of the resistance to stable crack growth were evaluated, compared with those of the specimens subjected to long term isothermal exposure at high temperatures in air. Summarizing the experimental results obtained and the findings of the previous work, discussions were made on the mechanics and mechanisms of thermal fatigue damage.
Stress analysis is carried out for a double cantilever beam (DCB) specimen of unidirectional carbon-fiber-reinforced plastic (CFRP) by means of finite element method (FEM). Three models are used in the analysis; (1) composite model I in which the specimen is composed of elastic fibers and elastic matrix, (2) composite model II in which the specimen is composed of elastic fibers and elastic-perfectly plastic matrix, and (3) homogeneous model in which the specimen is homogeneously orthotropic and its elastic constants macroscopically coincides with those of the composite model I. The distribution of normal stress, σy, ahead of the delamination crack tip in the composite model I can be divided into three zones. In the first zone (i.e., the nearest zone from the crack tip), the stress distribution is represented by a stress intensity factor, Kcom, evaluated by the energy release rate and elastic constants of the matrix. In the second zone, the stress distribution is undulated. The stress distribution in the third zone is represented by a stress intensity factor, Khomo, in the homogeneous model. The analysis of the composite model II shows that the stress distribution in the third zone is hardly affected by plastic deformation at the crack tip. Since the length of this zone in crack propagation direction is much larger than those of the other two zones, Khomo could characterize the crack propagation. However, when the composite is composed of very brittle matrix, Kcom could be applied in stead of Khomo.
This paper deals with the effect of hot water immersion on tensile and fatigue properties of long glass fibre-reinforced polypropylene (LGF/PP). Tensile tests and pulsating-tension fatigue tests have been carried out using smooth specimens of two grades of the material, with and without an acid modified polypropylene (APP). Specimens were immersed in distilled water at 80°C up to 6666h. In both materials, the weight increased rapidly and reached the maximum value, then decreased gradually with immersion time. The material without APP showed initially larger increase in weight, but over the immersion time of 2500h more remarkable decrease was observed. Since the weight gain of the resin itself was negligible, water entered into the interface between the fibres and the resin. Therefore, the weight loss was attributed to leaching of surface treatment agents and the dissolusion of glass fibres by hot water. The variation in weight with immersion time depended strongly on water temperature. The weight increased gradually with time in distilled water at ambient temperature and the material without APP showed larger weight gain than the material with APP, indicating that the addition of APP was effective in improving water absorption characteristics in LGF/PP. Tensile and fatigue strengths were considerably decreased after hot water immersion, but the material with APP still exhibited higher strengths than the material without APP. The length of fibres appeared on fracture surfaces tended to be shorter in immersed specimens, suggesting the degradation of the fibres.
There are two major fracture criteria for notched C/C (carbon/carbon) composite materials. One is that the fracture strength is determined by the fracture toughness criterion, and the other is that it is simply decided by the net section stress failure criterion. In the present study, effects of crack depth, specimen size, and the heat treatment temperature on the strength and fracture mechanism of C/C composites were investigated by using double edge cracked plate tension specimen in order to clarify the confusion on the fracture criterion. For the case of carbonized C/C composites (heat-treatment temperature=1600°C), the notched strength of wider specimens was smaller than the net section stress failure criterion, showing the notch effect. When the ligament length of the notch was shorter than about 8mm, the fracture strength of notched specimen was well correlated to the net section stress criterion. On the other hand, the notched strength of graphitized C/C composites (heat treatment temperature=2500 and 3000°C) was well estimated by the net section stress criterion without respect to the ligament length. The strength of smooth specimen for C/C composites was almost the same without respect to the heat-treatment temperature, while the fiber strength decreased by half with increasing the heat-treatment temperature. Thus, the contribution of the fiber strength to composites strength was much higher for the graphitized C/C composites. The change of fracture mechanism in macroscopic and microscopic scale was well correlated to the change of the fracture criterion.
Tensile deformation and ductile fracture process are investigated for titanium alloy matrix composite reinforced with TiC particles in comparison with Ti-6Al-4V alloy. Material used in this work is TiC/Ti-6Al-4V-11Cr composite produced by vacuum arc remelting process. TiC/Ti-6Al-4V-11Cr composite has strong interface between matrix and TiC particles because of in-situ reaction process. Tensile tests for smooth and 1mmR notched round bar specimens are performed to investigate ductile fracture process and influence of stress triaxiality on it. Degradation in ductility due to the stress triaxiality is more significant in TiC/Ti-8Al-0.5Mo-0.5V composite than in Ti-6Al-4V alloy. Most of micro-voids in composite are nucleated from cracking of TiC particles. Unit cell model combined with FEM analysis has revealed that the cracking of TiC particles are caused by mismatching with the matrix due to plastic strain and high stress triaxiality. Difference in process of micro-void coalescence between smooth and notched specimens has been demonstrated in fractographic observation of fracture surfaces.
Epoxy resins for encapsulating integrated circuit (IC) devices are filled with silica particles to reduce the thermal expansion coefficient and to improve thermal conductivity. Recently, the size of chips mounted in a package has increased rapidly with advances in large-scale integration technology. This trend creates a problem: increased mechanical stress in the package, which sometimes causes cracking in the encapsulation resin under temperature cycling. Hence, evaluation of the fracture properties of these materials has become an important issue in package design. This study reports the effects of the filler particle volume fraction on the static strength of smooth specimens and fracture toughness of silica-particulate-filled epoxy resins for IC encapsulation. The smooth specimen strength and fracture toughness were measured, respectively, by the three-point bending test and the double torsion test. It was found that the fracture toughness increased with increasing volume fraction. The static strength of epoxy resins with a small amount of silica particulate was smaller than that of unfilled resins. The crack propagation in resins containing 30% volume fraction of large size particles was unstable. At larger than 45% volume fraction of particles, the crack propagation was stable. These crack propagation properties have been related to the plastic zone size of matrix and particle spacing.
A quantitative analysis of tensile fracture surface was performed on JIS FCD400 and FCD500 spheroidal graphite cast irons. The unnotched and circumferentially notched specimens with different notch acuity were employed to obtain a wide range of fracture strain. All specimens were loaded up to break under tensile loading at room temperature. The fracture strain decreased markedly with increasing stress concentration factor for both FCD400 and FCD500 cast irons. The relative amount of dimple area on the fracture surface was measured using a scanning electron microscope associated with fracture strain. Fractographic examination revealed the fracture mechanism of FCD400 with ferritic matrix to be significantly different from that of FCD500 with 42% pearlitic matrix, depending upon matrix structure. For FCD400, dimple fracture was predominant for a series of tensile specimens. Whereas, cleavage fracture predominates for FCD500, and the area fraction covered by dimples was about 30% for the unnotched specimen. On the other hand, the asperity on the fracture surface reduced significantly as the fracture strain decreased. Then, the roughness of the fracture surface, expressed in terms of the maximum height on the fracture surface, was examined with a laser microscope associated with fracture strain. As a result, the maximum height on the fracture surface reduced with decreasing fracture strain for both FCD400 and FCD500 cast irons, irrespective of matrix structure. Hence, the roughness of the fracture surface is expected to provide a more positive and quantitative information regarding the fracture behavior of ductile cast irons.
The Au-Ag heat-press bonding process was applied to the micro bonding process for Multi-LSI package. The Auplated Cu lead was bonded on the Ag-plated Cu frame. In the case of bonding temperatures above 325°C and pressures from 150MPa to 300MPa, the bonding strength was about 8N/mm and the fracture occured at the neck of Cu lead. However, when the bonding pressure was higher than 300MPa, the strengths were reduced. On the other hand, when the bonding pressure was less than 100MPa, the strengths were reduced rapidly and the fracture occured at the Au/Ag interface. By observing the structure on the cross section, several non-bonding areas were found at the Au/Ag interface. On the other hand, non-bonding areas were not found at the interface of Cu lead fractured bonding. According to the AES analysis of the surface of the bonding fractured at Au/Ag interface, it was assumed that this bonding was achieved by the Au-Ag slight mutual diffusion at the interface. In the case of the bonding done at more than 300MPa, deformation of Cu lead caused the reduction of the bonding strength.
This paper presents the strength of micro bonding with adhesive resin and solder under combined loads. The combined loading tests of micro butt joints between steel and piano wire bonded with resin or solder were carried out using a new testing machine of micro combined loads. The machine is available to measure the strength of microbonded parts under combined loads. In case of micro-bonded parts, the torsional failure stress of the specimen is stronger than the tensile stress. For adhesive bonding the tensile failure stress of the specimen as well as the torsional failure stress decreased with increasing the bonded area. For solder bonding, the tensile failure stress decreased with increasing the bonded area. But the torsional failure stress was not dependent on the bonded area in the experiments.