Advances in Fracture and Failure Prevention

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Authors: B. Gomez, Yukio Miyashita, Yoshiharu Mutoh, M. Takahashi
Abstract: The crack propagation along the bond coat / topcoat interface of a TBC system has been studied in a previous work under a certain mode ratio (K2/K1). In order to discuss a parameter that describes the crack propagation, the growth behavior of a crack has to be studied under various K2/K1 ratios. In this work, BEM was used to determine the modifications that change K2/K1 ratios of the specimen employed in the previous study. Six characteristics of the specimen were modified: location of the applied load; height; length, position and direction of the notch, and its separation from the bond coat / topcoat interface. Among these modifications, the change of location of load point can effectively change K2/K1 ratio to values greater and smaller than those of the previous study. Crack propagation tests with three different positions of load point were performed using specimens with topcoat porosity of 5%, 15% and 20%. The results indicated that, independently of K2/K1 ratio, the parameter Ki dominated the crack growth behavior for an interface crack under mixed mode.
Authors: Cheng Wei Li, Masahiko Kato, Keijiro Nakasa
Abstract: Decarburization treatment was carried out on SKD5 tool steel by heating at 1323K for maximum 5.4ks and quenching followed by tempering at 873K for 3.6ks. This resulted in decarburized layer of maximum thickness 0.45mm. After the treatment, WC-Co cermet of 0.2mm thickness was deposited by high-velocity oxygen fuel thermal spraying. Tensile tests were carried out on the specimens, where the load was applied parallel to the specimen. The results show that the presence of decarburized layer increases the interfacial fracture toughness. The reason seems to be that the large plastic deformation of decarburized layer relieves shear and tensile stresses at crack tip. Edge indent tests were also conducted, where shear delamination of coating occurred during the penetration of indenter. The result is that the delamination energy decreases due to the presence of decarburized layer, which is contrary to that of the tensile test. The shear fracture of soft decarburized layer may be the reason for the smaller delamination energy of decarburized specimen.
Authors: Hiroyuki Waki, Izuru Nishikawa, K. Ogura
Abstract: A surface strain measurement approach to understanding of delamination processes of thermal barrier coatings (TBCs) under heat cycle conditions was described in this paper. Heat cycle tests between the high temperature ranged from 1473K to 1073K and the relatively low temperature (573K) was carried out on thermal barrier coated type 304 stainless steel specimens. 8mass%Y2O3- ZrO2 and Al2O3 coatings were used for the TBCs. The surface strain behavior during the heat cycle test was measured using a laser speckle strain/displacement gauge (SSDG). It was found that the thermal expansion of a substrate was almost reflected on a surface strain if a delamination wasn't initiated, while the value of a surface strain decreased to the value of the thermal expansion of a ceramics-coating if the delamination of the ceramics-coating was initiated. The state of a subsurface delamination was able to be nondestructively inferred by the surface strain behavior. The delamination life of a ceramics-coating in the specimen with a low-pressure-plasma-sprayed (LPPS) bond-coating was found to be longer than that with an atmospheric-plasma-sprayed (APS) bond-coating. The large roughness of a bond-coating was also found to be effective in improving the delamination life of a ceramics-coating owing to the restriction of a crack propagation parallel to the interface between the ceramics-coating and the bond-coating.
Authors: Somsak Leungvichcharoen, Anil C. Wijeyewickrema
Abstract: In this analysis, the effect of an imperfect interface on the stress singularity of an orthotropic elastic bimaterial wedge subjected to traction free boundary conditions, is investigated, where the planes of symmetry are aligned, and one symmetry plane is along the interface and another symmetry plane coincides with the cross-sectional plane. The Stroh formalism with the method of separation of variables are used to obtain the relevant expressions for displacements and stresses. At the interface, only the interfacial tractions and the displacement normal to the interface are assumed to be continuous. The imperfect bond is modeled using a local coordinate system, where each tangential traction component in this local coordinate system is directly proportional to the corresponding displacement discontinuity and inversely proportional to the distance from the wedge apex. The order of singularity is computed numerically for graphite/epoxy wedges and presented for various imperfect interface conditions. The numerical results agree with the available results for the fully bonded case. It is expected that when the axes of local and global coordinate systems are aligned, that the in-plane and antiplane problems are uncoupled, and this feature also can be seen in the numerical results.
Authors: Zheng Hua Qian, Feng Jin, Zi Kun Wang, Kikuo Kishimoto
Abstract: Following the advances in structural applications, composite structures are being used commonly in transducer applications to improve acoustic, mechanical and electrical performance of piezoelectric devices. Functional composite transducers for sensors and actuators generally consist of ceramics and polymers, the disadvantage of the brittleness nature of the piezoelectric ceramics can be overcome and the structures especially good for sensing can be allowed for building up. Propagation behavior of horizontally polarized shear waves (SH-waves) in piezoelectric ceramic-polymer composites with 2-2 connectivity is taken into account. The multilayer structures are consisted of piezoelectric thin films bonded perfectly with polymeric thin films alternately. The phase velocity equations of SH-waves propagation in the piezoelectric ceramic-polymer composites with 2-2 connectivity are obtained for the cases of wave propagation in the direction perpendicular to the layering and along the layering, respectively. Filter effect of this kind of structure and the effect of volume fraction and shear modulus ratio of piezoelectric layer to polymer layer on the phase velocity are discussed in detail, respectively. One practical combination of piezoelectric thin film-polymer thin film multilayer system is chosen to carry out the numerical simulation, some basic properties of SH-waves propagation in above multilayered structures are revealed.
Authors: Shu Hong Liu, Z.Z. Zou, B.Q. Xu, Z.G. Zhang
Abstract: The problem of an interface edge crack between two dissimilar piezoelectric materials is analyzed under the conditions of anti-plane shear and in-plane electrical loading. The crack is considered to be traction-free, but electric permeable one across which the normal component of the electric displacement are continuous. A series form of electromechanical solution and field intensity factors are obtained. The results show that all fields including strain, stress, electric field strength and electric displacement are singular in the front of crack tip. At last, the stress intensity factor is solved by the boundary collocation method (BCM), numerical results are given and discussed.
Authors: Wen Jie Feng, Zhi Wen Zou, R.K.L. Su, Z.Z. Zou
Abstract: The linear piezoelectricity theory is applied to investigate the dynamic response of coplanar interface cracks between two dissimilar piezoelectric materials subjected to the mechanical and electrical impacts. The number of cracks is arbitrary, and the interface cracks are assumed to be permeable for electric field. Integral transforms and dislocation density function are employed to reduce the problem to Cauchy singular integral equations. Numerical examples are given to show the effects of crack relative position and material property parameters on the variations of dynamic energy release rate.
Authors: Masaki Omiya, Hisahiro Inoue, Kikuo Kishimoto
Abstract: Multi-layer thin films are widely used in micro-sensor and microelectronics products. These electronic devices contain several metal or polymer thin films and reliability of these systems is strongly dependent on the interfacial adhesion of these thin films. Due to the thermal stress, residual stress or elastic mismatch, the delamination between layers sometimes occurs. Therefore, it is important to evaluate the interfacial strength precisely. Peel tests are simple way to estimate an interfacial strength and, in fact, widely used in industrial field. Recently, a new simple but functional device for peel tests has developed in our group. This test method is called 'Multi-stages peel test'. There are two features in this device. At first, peeling tips can be observed continually and it becomes easy to measure a peeling angle. Second is that the peeling angle can be varied by attaching dead weights on the specimen. This dead weight works as a shear force at the peeling tip and the peeling angle can be changed variously. Therefore, the fracture tests under various phase angles are possible. In this paper, Multi-stages peel test is applied to the evaluation of interface strength of multi-layer thin films that are composed of Cu, Cr, PI and Si layers. By considering the energy balance during the peel test, the interfacial strength independent of the thickness can be obtained.
Authors: Meng Kao Yeh, C.Y. Tsai
Abstract: The material properties of underfill and substrate in flip chip package have temperature-dependent and moisture-sensitive characteristics. During the solder reflow process, the CTE mismatch in the package causes thermal stresses, which may reduce the reliability of the flip chip package. The package reliability can be improved by varying the die thickness, the fillet angle and the thickness of the underfill and by changing the underfill material. In this paper, the temperature- dependent properties of the underfill were established first. The flip chip reliability was then analyzed by finite element code ANSYS. Both underfill A and underfill B were used in the analysis. The results show that better reliability of the flip chip package was obtained for underfill A, for larger fillet angle of the underfill, for thinner die in the package, and for larger Young's modulus of underfill with linear elastic assumption. Also a hygrothermal preconditioning before thermal cycling reduces the reliability of the flip chip package.
Authors: Woon Tahk Sung, Hyung Taek Lim, Tae Sam Kang, Jang Gyu Lee, Young Jae Lee
Abstract: This paper presents a design and performance tests of in-plane gimbal-structured Z-axis gyroscope. The device is manufactured by conventional surface micro machining process using 15µm-thickness epitaxially grown polysilicon. The total size of the device is 4.8mm x 4.8mm in area including 1mm x 1mm core gyro structure and pads for wire bonding. It is designed as two-gimbaled structure which separates inner and outer gimbals. This leads two nearly identical modes of gyroscope, i.e. driving and sensing mode, to mechanically decoupled ones, resulting in enhanced performance. Experiments are accomplished through frequency analysis and dynamic tests using a detection circuit, a vacuum chamber and a rate table. Through experiments, it is confirmed that the designed gyroscope has 0.003 deg/sec resolution, 0.00095 deg/sec/ Hz noise equivalent density, 0.57 % FSO nonlinearity error and 0.02 deg/sec bias stability.

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