The opened-up configuration of the artery wall has long been assumed to be stress-free. This is not true in a microscopic level: The aortic media has a layered structure called lamellar unit which is a pair of elastic lamina (EL) and a smooth muscle-rich layer (SML), and the EL has been reported to be stiffer than the SML. If the circumferential stress in the in vivo condition is the same between the two layers, residual stress of each layer should be different because the stress-strain relationships differ. Such residual stress is not released fully by radial cutting, but is released in the area close to the cut surface, causing hills and valleys on the surface due to residual stresses. To check this hypothesis, we have developed a scanning micro indentation tester, and measured the topography and the stiffness distribution of the cut surface. The surface of the porcine thoracic aortas has stiff hills and soft valleys as speculated. Residual stress and strain estimated from the topography and stiffness distribution were almost comparable to those estimated in the ring-like segments in no load condition. Fairly large stress may still reside in the opened-up aortic wall.

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The effect of Ta content on Young's modulus and microstructures of binary Ti-Ta alloys with different mass percents of Ta from 10% to 80% was investigated in order to find a Ta content that gives the suitable Young's modulus for biomedical applications. The experimental results show that both the crystal structure and Ta content have the strong effect on Young's modulus of binary Ti-Ta alloys. The α'' Ti-Ta alloy with mass percents of 30% Ta and metastable β Ti-Ta alloy with 70 % Ta have the nearly same lowest Young's modulus under the given solution treatment. The microstructures of Ti-Ta quenched alloys are very sensitive to Ta content. The quenched alloys exhibit the plate-like HCP martensite (α') structure at a Ta content between 0 and 20 mass%, the acicular orthorhombic martensite (α'') structure at a Ta content between 30 and 50 mass%, metastable β + α'' structure at a Ta content of 60 mass%, and only metastable β structure at a Ta content more than 60 mass%.

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A material test system combined with X-ray computed tomography (CT) is newly developed for material characterization of biological tissue in non-invasive manner. An identification method of in vivo displacement field is investigated through the CT images obtained by the system. Two voxel models are constituted by laying up thicken pixels of the cross-sectional tomograms taken under initial and loaded conditions. Representing the displacement field in terms of mapping function from initial voxel model to deformed voxel model, we pose an inverse problem to identify unknown parameters in the mapping function set by power series of N-th order. A virtual voxel model is embodied through virtual trans-formation of the initial voxel model via tentative mapping function. An error function defined by the voxel values, which is related to brightness of voxels, is minimized with respect to unknown coefficients by means of the SCE-UA method (Shuffled Complex Evolution Method Developed at the University of Arizona). The proposed method is demonstrated through an experimental example using specimen harvested from chicken.

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This study is concerned with the mechanical properties of knee joint ligaments, which are frequently injured in car-to-pedestrian collisions. Such injury makes a lot of disabilities in actions in daily life and it is difficult to recover the original mechanical functions of ligaments. Because of the lack of knowledge of the dynamic properties of these ligaments, there are some difficulties in finite element simulations of a car-to-pedestrian accident. Thus, we performed tensile tests on the rabbit medial collateral ligament (MCL) and anterior cruciate ligament (ACL) to evaluate the strain-rate dependence of their mechanical properties. The failure stress of the MCL increased with the strain rate. The avulsion load of the MCL tibial insertion was lower than the failure load of the MCL in all conditions tested. Three types of failure patterns were observed in the femur-ACL-tibia complex. The tangent modulus and the maximum stress of the ACL complex increased with the strain rate. The strain-rate dependence of the mechanical properties of knee ligaments was evaluated quantitatively. The results are expected to be useful for discussion of the injury criteria for the knee joint and its protection.

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The mechanical properties of biological soft tissue depend on its strain rate. Many experimental studies have been done on the static properties of skeletal muscle in active states. However, there are few data on the dynamic mechanical properties of skeletal muscle in an active state at such a high strain rate as that seen in traffic and sports accidents. With this in mind, we examined the dynamic mechanical properties of skeletal muscle by in situ animal experiments. First, we performed isometric contraction tests to develop an experimental method of controlling muscle contraction. Next we conducted tensile tests of the tibialis anterior muscle under two strain-rate conditions in both the passive and active state to evaluate the strain-rate and muscle contraction dependence of the mechanical properties of skeletal muscle. Based on our experimental results, we found an extensionrate dependence on the failure load was independent of muscle tone. The failure site depended on the extension rate and muscle tone. These findings are expected to provide basic data for muscle injury prevention in traffic and sports accidents.

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Bone tissue engineering utilizes biomaterial scaffolds seeded with biofactors (cells, genes, and/or proteins) to reconstruct bone defects. The scaffolds must be able to temporarily bear load, shape regenerate tissue, and enhance tissue regeneration through biofactor delivery. Thus, scaffold evaluation requires traditional engineering experiments like non-destructive radiographic evaluation and mechanical testing, as well as biological experiments performed in vitro and in vivo. In this paper we present a paradigm for integrating mechanical and biological scaffold evaluation to determine optimal scaffold design parameters. Specific examples of scaffold material evaluation include micro-computed tomography (micro-CT) scanning, voxel finite element modeling, and mechanical testing. In vitro tests evaluate cell growth on scaffold in static culture. Finally, in vivo evaluation includes gene/cell delivery from scaffolds in immunocompromised mice and mandibular defect filling regeneration with engineered scaffolds.

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Elastic moduli of single trabeculae were determined using micromechanical testing by three-point bending in order to investigate relationships between the mechanical environment of trabecula and its mechanical properties. Single trabeculae harvested from bovine coccygeal vertebrae were machined into beams with a rectangular cross-section by using a custom-made miniature-milling machine. From the load-displacement curve obtained by micromechanical testing, elastic modulus of each trabecular specimen was determined based on the Timoshenko's beam theory. Assuming the trabecular mechanical environment was affected by the trabecular orientation, each five rectangular beam specimens of trabeculae were prepared for axial and transverse directions. As a result of micro-three-point bending test, it was shown that the elastic moduli of the trabecular specimens were 3.03±0.31GPa (mean ± s.d.), and that there was no significant difference between axial and transverse directions. The results indicated that mechanical environment did not affect the mechanical properties of trabeculae, and that mechanical adaptation in the cancellous bone was mainly accomplished by the morphological changes of trabeculae.

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To understand the correlation between the microstructure and the macroscopic properties and behaviors, novel digital image-based multi-scale analysis techniques are presented. They include the mathematical homogenization method, enhanced finite element mesh superposition method and statistical evaluation and visualization of the microscopic morphology of porous ceramics.

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Synergy ceramics exhibits the nonlinear stress-strain behavior and is expected to have the damage tolerance capability. To assess the enlarged reliability of components of such materials, it is indispensable to evaluate the damage tolerance of the material quantitatively. For this purpose, we develop here a constitutive model of the distributed-micro-crack model where inherent defects involved in the material are modeled by micro cracks with a penny shape and when stress is applied they grow up according to the resistance curve for crack propagation (R-curve). Several calculations are conducted with this constitutive model under various conditions, and referring these results, a standard procedure is proposed to evaluate the damage tolerance of the material experimentally.

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In Synergy Ceramics as a Japan government project, the structural design and evaluation technologies for ceramic materials and components have been developed through investigating the relationships between micro- and macroscopic mechanical properties, including stress and strain distribution and fracture behavior. This paper introduces the R&D concept and the results on the experiment and simulation technologies of stresses, and the fracture behavior prediction.

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Three-dimensional elastic analysis for porous ceramic materials was studied by means of combining a image-based modeling and homogenization method. Image-based modeling for porous alumina ceramics was obtained with X-ray CT. The prepared alumina samples had small spherical pores of 20 and 60μm in diameters with the porosities of about 1.0 and 7.0%. After reconstruction of the three-dimensional images, representative volume elements (RVE) were obtained. Homogenization method was applied to the RVE images for calculation of homogenized elastic constants. Using the elastic constants, FEM macroscopic elastic calculation for a global 4-point bending test models was carried out, which gives global stress distributions for the specimens. Under the macroscopic stress conditions, the local stress distributions in the RVE were calculated. High local stresses were shown near the pores, which will be very useful for future fracture analyses of the porous ceramics.

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The major problem with the use of ceramics as structural materials is their brittleness. Recently, the authors developed the silicon nitride, mullite and alumina with very high self-crack-healing ability. This study focuses on the crack-healing behavior of commercial SiC ceramics. The crack-healing behaviors of SiC ceramics were investigated systematically, as a function of crack-healing temperature, time, crack size and temperature dependence of bending strength. The surface of specimens was made the semi-elliptical crack by Vickers indenter. And pre-cracked specimens were healed at 1500℃, 1 hr in air. All fracture tests were performed on a three point loading system with 16 mm bending span. The main conclusions obtained were following; (1) Optimized crack-healing condition are; temperature; 1500℃, time 40〜60 min in air. (2) Maximum crack size which can be healed completely was semi-elliptical surface crack of 450μm in diameter. (3) Heat-proof temperature of crack-healed zone for bending strength was about 600℃. (4) Base material is not sensitive to static fatigue, but crack-healed zone is considerably sensitive to static fatigue at room temperature.

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In this paper the effect of pores on cracking is studied to establish how porosity influences the thermal shock resistance. Experiments have been carried out in which the growth of cracks through simple, well-defined pore arrays has been studied, using poly (methyl methacrylate) as a model system. It is shown that the crack front bows as it passes between the pores causing an increase in the length of the crack front, which is associated with an increase in the applied force required for continued crack growth. Comparison of predictions with experiments shown that the estimated change in the length of the crack front with porosity is very similar to the experimentally observed variation in the fracture energy in different ceramic systems. Using these ideas it is found that the temperature change required to initiate cracking can be increased by adding a pore volume fraction of approximately 0.1, but diminishes at higher pore volume fractions. However to obtain these improvements it is essential to ensure that there is no concomitant increase in the flaw size. The extent of cracking is only marginally affected by porosity.

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Lattice defect and local strain, which were introduced by systematically machining the basal plane of sapphire, ware characterized by cross-sectional transmission electron microscopy (XTEM) and convergent beam electron diffraction (CBED) It was found that dislocations and strain were introduced from the surfaces for all specimens examined in this study. Depths of the introduced dislocations were measured by weak beam dark field (WBDF) method, and residual strain in the vicinity of the surfaces were precisely measured by CBED method using the probe size less than 10nm. The experimental CBED patterns were compared with the calculated patterns to quantitatively evaluate the lattice strain around the surfaces. Maximum penetration depths of the dislocations in the specimens ground with #500 diamond wheel, polished with 4〜8μm and under 1μm diamond slurry were 1.7μm, 700nm and 250nm, respectively. In addition, twin defects were introduced in the specimen ground by #500 diamond to accommodated the large lattice strain during the machining. Residual local strain was found to remain at the depths of 1.0μm, 1.2μm and 300nm for the specimens ground with #500 diamond, polished with 4〜8μm and under 1μm diamond slurry, respectively.

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A monolithic SiC ceramic and two SiC-C composite ceramics containing 10vol% and 20vol% of graphite were prepared by hot pressing. The friction and wear behaviors of these materials were evaluated by sliding against sintered silicon carbide under dry conditions using two tribometers: block-on-ring and pin-on-disk. For all the three materials, worn surface was smooth and wear was mild in block-on-ring tests, whilst worn surface was rough and wear was severe in pin-on-disk tests. In the block-on-ring tests, addition of graphite into SiC concurrently achieved a reduction in friction and an increase in wear resistance; however in the pin-on-disk tests, the addition of graphite led to sharply enhanced wear rates despite the lowered friction coefficients.

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We study the application of the millimeter wave heating method to the silicon nitride, which includes Ytterbium oxide (Yb_2O_3) and alumina (Al_2O_3) as additives. Millimeter wave heating method has capability in enhancing densification of sintered bodies. As a result, we successfully obtained high-density sintered bodies with 5-wt% Yb_2O_3 and 3-wt% Al_2O_3. The sintering temperature was 1650℃, which was 200℃ lower compared with the conventional external heating method. Moreover, the annealing at 1220℃ with millimeter wave heating method after the sintering was proved to be effective. The sintered bodies showed excellent mechanical properties at room and high temperatures. The Vickers hardness was 7.5 GPa at 1400℃ in a vacuum, 17.4 GPa at room temperature in the air, and fracture toughness K_<IC> was 9.3 MPa m^<1/2> at room temperature in the air. These experimental results showed that the application of the millimeter wave heating method to the silicon nitride is very effective.

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The mechanical properties of hot pressed Y-stabilized α/β SiAlON ceramics have been studied through assessment of hardness, fracture toughness and bending strength measurements. Wear properties were studied by non-lubricated block on ring and ball on disk type wear tests under a range of loads. The effect of the stabilizing cation was also studied by producing identical compositions with the Y cation being replaced by Yb. In the Y stabilized materials there was a slight decrease in hardness with increasing β content but a 2-fold increase in both strength and toughness due to the development of elongated grains in the microstructure. The same trend in fracture toughness and strength was seen for the Yb materials, although lower density and larger grain size resulted in a lower hardness compared to the Y materials. Under low load wear tests the single phase α materials showed better wear properties, possibly as a result of a lower amount of intergranular phase. However under more severe conditions the high β content materials exhibited better wear properties due to resistance to crack propagation. In the single phase materials under low load, despite lower hardness the Yb samples showed an order of magnitude better wear resistance than comparable Y stabilized ones.

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The simulation method was developed to predict homogenized elastic moduli of ceramic materials from elastic stiffness of each grain within its microstructure. Polycrystalline Al_2O_3, ZrO_2 and Al_2O_3-ZrO_2 ceramics were produced as model samples. Elastic stiffness, c_<ij>, of single crystal cubic-ZrO_2 was measured by four-point bending method. Microstructures of model samples were modeled as two and three-dimensional heterogeneous bodies composed of Al_2O_3 and ZrO_2 grains. The crystallographic 3D-directions of each grain were assumed to be randomly distributed. Some square plates with a unit thickness that cut from the initial two-dimensional grain models and some cubes composed of geometeric cube grains were used to calculate apparent macroscopic elasitc moduli such as Young's modulus, Poisson's ratio and modulus of rigidity. An influence of the model size on apparent elastic moduli was examined by a finite element method. And then, the optimum model size was determined by comparing calculated elastic moduli to experimental data measured by a pulse-echo method. As a result, the scattering of apparent Young's modulus varies narrowly with increasing the number of anisotropic Al_2O_3 and ZrO_2 grains. The number of anisotropic grains within the optimum model was found to be about 400. The calculated elastic moduli for the optimum model coincided with the experimental data.

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A strain evaluation technique using a micro focus X-ray beam was developed. An electron gun with two electron lenses was used in order to steadily make a fine focus of an electron beam on a copper target. The target current was 84μA, and an X-ray brightness 52×10^9W/m^2 was obtained. X-ray was condensed to a converging angle 0.09 deg with a convergent unit, and the minimum focus diameter was 60μm. A direct beam intensity was 8.5×10^5 cps. The principal strain in a sapphire (1-12) plane was measured. The maximum principal strain ε_1 was in extremely good agreement with the estimated value. In the case of the second principal strain ε_2 and the minimum principal strain ε_3, the error of the sample loaded more than 300με was within 70με. The diffraction patterns were able to be detected for a single grain in a translucent alumina.

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The fracture tests of porous ceramics were carried out under tensile and bending conditions, and a formula model of fracture process was investigated in order to clarify the fracture properties. As the formula model, the assemblies of pore-element and matrix-element were proposed, and the experimental results were estimated using an inverse analysis method. As the results, the stress-strain behaviors of porous ceramics under tensile load were successfully simulated using our devisal formula model.

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The domain switching near the fatigue crack tip was observed by synchrotron X-ray diffraction and AFM for non-poled and poled PZT specimens. The diffraction patterns taken from a local area of 100μm in diameter were processed to obtain the ratio, I_<004>/I_<400> , of the integrated intensity of 400 to 004 diffractions as a measure of domain switching. The intensity ratio, I_<004>/I_<400> was decreased both at the notch root and the tip of fatigue cracks, suggesting the 004 direction aligned on the specimen surface and perpendicular the crack line due to fatigue loading. The amount of domain switching near the crack tip is larger for NP specimen than for PL specimen, which corresponds to the larger resistance of NP specimens to crack growth. The domain switching near the crack tip occurring under load was detected as the depression zone by AFM. The size of the depression zone observed at the maximum load was about 6μm which was equal to the grain size, and the zone disappeared when unloaded. The depth of the depression zone changes as a function of the applied load, and the relation between the depth and the applied load showed a hysteresis loop. The amount of loop expansion of hysteresis was larger in NP specimen, and the load at the onset of depression was also larger.

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Notched specimens of porous silicon carbide with porosity 37% were fatigued under four-point bending at frequencies of 30 and 0.3Hz. The fatigue life expressed in terms of time was rather insensitive to the test frequency, while that expressed in terms of cycles was much shorter for the case of 0.3Hz than for 30Hz. The relation between the load and the compressive strain was recorded at prescribed cycles during fatigue tests. At the beginning of fatigue cycling, the relation between load and strain was linear. As the number of cycles increases, the hysteresis loop moved toward the right side and also expanded. The crack extension from the notch was determined from the compliance change by FEM method. The crack propagation curve was divided into Stages I, II and III. In Stage I, the crack propagation rate decreased even though the applied stress intensity factor got larger with crack extension. The rate was rather constant in Stage II, and then increased in Stage III. This anomalous behavior is caused by crack-tip shielding due to asperity contact and bridging by SiC particles. Fractographic observations showed that the fracture path was along the binder phase between SiC particles, more precisely along the interface between particles and binders.

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Numerical simulation method for analyzing the fracture behavior of brittle materials was developed on the basis of the initiation of micro-cracks. Deformation and the fracture behavior of porous ceramics with initial cracks were numerically simulated. The mean stress model was adopted as the condition of the initiation and propagation of micro-cracks. The micro-cracks were located in a regular manner as the bonds of a square lattice. The stress distribution and stress intensity factors at micro-crack tips were calculated numerically by the body force method(BFM). As the variance of the critical strength increased, and as the initial crack length became long, the stress-strain relationship became nonlinear. When the initial crack length was small, the fracture occurred not from initial crack tip. The size of the non-damaging defects could be evaluated by the proposed method. The resistance curve calculated was independent of the initial crack length. The calculated results agreed very well with the experimental results.

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This paper presents a feasibility study on locating through-width delaminations in fiber-reinforced plastic beams using Lamb waves. An active diagnostic system, consisting of a surface-mounted piezoelectric patch and a vibration sensor, is proposed. The fundamental anti-symmetric Lamb wave mode is chosen as the diagnostic wave. It is generated by applying to the patch an excitation voltage in the form of sinusoidal bursts. Experiments are performed to reveal the wave propagation in clampedfree Kevlar/epoxy beams. The results are used to verify the accuracy of a finite element (FE) model. The FE model is then applied to locate delaminations in some artificial beams. By performing Wavelet Transform (WT) on the predicted acceleration traces, the propagating Lamb waves can be extracted. Based on the arrival times of the extracted waves, delaminations are located with satisfaction. Improve-ment on the damage localization process is possible by eliminating the boundary reflections as well as a priori knowledge on both the size and depth of the delamination.

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This A failure criterion with the existence of coupling terms is employed to investigate the progressive failure in anisotropic laminated composite plates. The criterion allows interaction between fiber and matrix properties. The paper is aimed to investigate the contribution of the coupling terms. A mathematical model and computational model are presented for the analysis. The deformation of the plates is predicted based on higher order shear deformation theory. Variation of material properties through thickness is used and accommodated by a discrete layer approach. Stresses calculated are used in the present failure model to determine the first ply failure and last ply failure, by progressively reducing the stiffness of the laminas. Finally, the first ply failure and last ply failure results are used to determine the lower and upper bounds within which the true load carrying capacity lies. The numerical results obtained shows some improvement compared to other failure criteria.

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Polymer-matrix composites containing continuous carbon fibers are the dominant lightweight structural materials. The monitoring of the damage is important for structural health monitoring and hazard mitigation. The monitoring of the strain is relevant to structural vibration control and security. This paper reviews the use of the composites themselves to sense their own damage and strain, thereby removing or reducing the need for embedded or attached sensors, which suffer from high cost and poor durability. Both damage and strain affect the volume electrical resistivity of a composite, thereby allowing the monitoring of damage and strain in real time by volume resistivity measurement. The contact resistivity of the interlaminar interface (i.e., the interface between the laminae) provides another attribute for monitoring. A configuration involving two crossply laminae provides a two-dimensional array of sensors and an x-y grid of electrical interconnections, thereby allowing spatial distribution sensing.

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For a cryogenic tank of a next generation RLV, a laminated composite tank is one of the key technologies. For the fuel tank made from laminated composites, matrix cracks are significant problems that cause leak of fuel. In the present paper, electric resistance change method is adopted to detect matrix cracking of the laminated composites. Tension load in fiber direction causes electric resistance increase, and fiber breakages also cause the electric resistance increase of the CFRP laminates. In order to distinguish electric resistance changes due to the piezoresistivity and the fiber breakages, gage factor of reloading condition is employed in the present study. Experimental investigations were performed using cross-ply laminates, and as a result, it can be revealed that the gage factor is a useful indicator for matrix crack creation of cross-ply laminates.

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The information of applied external forces is very important and useful in the assessment of integrity of structures. An identification method of impact force is presented in this paper. First, the relation between force histories and strain responses is formulated based on the finite element method (FEM). By applying this formulation, an error vector that indicates the force location is defined with strain responses measured at multiple points in a structure. The inverse problem can be solved with the conventional least-square method, but in an actual system, the identified force history has a large oscillation because of the influence of measurement noise and FEM modeling error. To overcome this difficulty, second, the present paper proposes a reliable force identification method that imposes the penalty on the derivative of the force history. The present method has been applied to carbon fiber reinforced plastic (CFRP) laminated plates with a SMART Layer embedded lead zirconate titanate (PZT) piezoelectric sensors. The experimental results for two kinds of plates are reported in this paper. According to the results of force location and history identification, the validity of the present method is verified, and the influence of the modeling error involved in the finite element model is discussed.

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Measurement of strain of tires in-service is effective to improve reliability of tires and ABS system. Since conventional strain gages have high stiffness and require lead wires, the conventional strain gages are cumbersome for the strain measurement of the tires. Sensors of Micro-electro-mechanical systems are also usually high stiffness themselves, and those are not applied to tires. These backgrounds require a new low cost wireless sensor for tires. In the present study, a new strain measurement system utilize electric capacitance change of steel wire reinforced tires is proposed and experimentally investigated. A small oscillating circuit is embedded in the tire; deformation of the tire brings capacitance change of the tire comprises steel wire and rubber; the change of the capacitance makes the change of oscillating frequency of the oscillating circuit. Measurement of the frequency of the oscillating circuit enables us to measure the strain of the tire wirelessly. A rectangular specimen cut from a commercially available tire is adopted as a specimen. Tension test is performed and the frequency of the oscillating circuit is measured during the test. As a result, the method is experimentally proved to be effective for the wireless strain monitoring of tires.

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Carbon Fiber Reinforced Plastic (CFRP) is composed of electric conductive carbon fibers and electric insulator resin. Many researchers have reported electric resistance change of CFRP due to fiber breakages and electric resistance change due to applied strain for unidirectional CFRP laminates. Piezoresistivity is electric resistance change with applied strain, and many researchers have reported the piezoresistivity of CFRP. There are, however, large scatter in the measured piezoresistivity in the fiber direction even during tensile loading: both positive piezoresistivity and negative piezoresistivity are reported. In the present study, therefore, basic piezoresistivity measurements are performed using specimens made from single-ply and multi-ply laminates using a precise four-prove method. As a result, positive piezoresistivity was obtained for both single-ply and multi-ply specimens and negative piezoresistivity was obtained from a multi-ply specimen without polishing surface

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The piezoelectric thin film (PVDF: polyvinylidene fluoride) sensors with good dynamic sensing characteristics can be used to monitor low velocity impacts on composite structures. The relation between the impact forces and the signals of the PVDF sensor for a composite sandwich beam was derived. Impact tests at low energy without inducing damage were performed on an instrumented drop weight impact tester. The measured signals of the PVDF sensors attached on the surface of the beam agreed well with the simulated signals in the forward problem. In the inverse problem, the iterated Tikhonov regularization method was used to reconstruct the impact forces from the PVDF sensor signals. Most of the reconstructed impact forces showed good agreement with the measured forces. The comparison results showed that the piezoelectric thin film sensor could be used to monitor low velocity impacts on composite sandwich structures.

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In this paper, mechanical property and crack closure behavior of shape memory alloy (SMA) fiber reinforced composite subjected to thermal stress are investigated. For this purpose, three types of polycarbonate matrix composites reinforced by TiNi fiber with different super elasticity and shape memory effect at room temperature are fabricated. Thermal test is performed by changing the applied temperature at the range from -40℃ to 80℃ in thermostatic bath. The behaviors of crack closure and mechanical properties under tensile load are investigated by analyzing the characteristic of photoelastic fringe patterns near the crack tip of the specimens. The effects of prestran and heating temperature on stress intensity are discussed are suitable SMA heating conditions for phase transformation under low temperature are found.

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It is difficult to analyze bending properties of Filament Winding Cylinders (FW Cylinders) because fibers are complicatedly wound and FW Cylinders are not composed of laminae. In this study, Finite Element Method (FEM) was proposed to evaluate the flexural rigidities of FW Cylinders with several winding angles, and the results of FEM analyses were compared with results of 3-point bending tests. As a result, both results of FEM analyses and experiments were quantitatively agreed to each other. And it was found that flexural rigidities of FW Cylinders which were made with small winding angles became large values, but large deformation occurred on the cross-section at the central loading point by bending load, especially for thin wall FW Cylinders. Moreover, it became clear that flexural rigidities were largely influenced by the thickness of FW Cylinders.

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Even though standardized fracture mechanics tests for continuous fiber-reinforced polymer-matrix composites do not require sophisticated tools, the development of these methods has profited from the application of advanced technology. A crucial measurement in fracture mechanics testing is the determination of the delamination length (crack length). A number of advanced non-destructive test methods will be compared with respect to their sensitivity, precision and compatibility with the standardized fracture mechanics test set-up. Recently, there has been renewed interest in the analysis of the "damage" or "bridging" zone that is produced in delamination growth in fiber-reinforced composites. Test methods that yield information on these zones will, usually, also be applicable for damage detection in composite structures.

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Acoustic Emission (AE) analysis using passive piezoelectric sensors is a standard tool for damage detection and damage severity assessment in structures made of fiber-reinforced polymer-matrix composites. Recently developed active piezoelectric elements (active fiber composites) can serve both as actuators and sensors. If implemented into composite structures, in principle, they offer a possibility both for damage detection and corrective action with one single device. As a first step towards this goal, model experiments on fiber-reinforced composites under mechanical loading have been performed. The sensors response of the AFC elements is compared with that of standard AE sensors. A particular emphasis is put on damage threshold detection.

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Static tensile tests and their fracture process of the unidirectional Si-Ti-C-O/BMAS composite (V_f=0.5) and Si-Ti-C-O fiber-bonded composite (V_f=0.9) were studied at room temperature in order to clarify the fracture behavior of fiber reinforced ceramics. The observation revealed the following common features of the fracture behavior. (i) First, matrix cracking was initiated in both composites. (ii) Then, the number of matrix cracking increased with the strain increase. In spite of matrix cracking, the associated interfacial debonding was suppressed by the compressive residual stress of the matrix in the fiber axis direction. (iii) Fiber breakage occurred when the stress reached about 90% of the fracture strength of the composites. Once fiber breakage occurred, large scale debonding was enhanced due to the tensile residual stress of fiber. (iv) Finally, overall fracture of the composite occurred, accompanied by a large number of fiber breakage. The difference of fracture process between both composites was the macroscopic behavior. The slope of the stress-strain curve of the Si-Ti-C-O/BMAS composite decreased from the initial one owing to matrix cracking. On the other hand, that of the Si-Ti-C-O fiber-bonded composite was almost the same until the final fracture in spite of the matrix cracking accumulation. The higher fiber volume fraction of the Si-Ti-C-O fiber-bonded composite was responsible for more suppressed interfacial debonding. The observation results were realized in computer by means of the approximate simulation using the modified shear lag analysis combined with the Mote Carlo method which enables to describe the relationship of mesoscopic damage of interfacial debonding, matrix cracking and fiber breakage to the macroscopic fracture behavior of unidirectional fiber reinforced ceramics. The effect of the fiber volume fraction on the fracture behavior of unidirectional fiber reinforced ceramics was discussed on the basis of experiments and simulation.

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The change in the electric resistance in CFRP due to mechanical loading and thermal history was experimentally investigated. First, the surface electric resistivity and piezoresistivity in the linear region were measured for unidirectional laminates. Secondly, quasi-static tensile tests as well as loading/unloading tensile tests were conducted for on-axis and off-axis unidirectional specimens to measure the change in the resistance together with strain up to failure. Finally, the resistance change in the transverse direction of unidirectional laminates during thermal history was measured to monitor the shrinkage due to post-cure. In order to predict the piezoresistance response in the linear region, a two-dimensional model was proposed. It was found that the electric resistance-strain curves during quasi-static tensile loading exhibit nonlinear behavior after linear region, especially in the off-axis specimens. The permanent change in the electric resistance was observed in all the specimens after loading/unloading. During the thermal cycle, both the transverse strain and the resistance varied with temperature and time and they showed permanent change at the end of the cycle. Thus, post-cure behavior can be monitored by measuring the electric resistance in the transverse direction.

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The present work first involved the fabrication of continuous hybrid Carbon Fiber Reinforced Polymer (CFRP) in the forms of sheet and rod. Second, the health monitoring functions of hybrid CFRP composites and CFRP-strengthened beams are also demonstrated by the resistance measurement method. The results reveal that the hybrid CFRP and concrete beams strengthened with hybrid CFRP possess, intrinsically, the function of detecting the damages in terms of the electrical conductivity and piezoresistivity of carbon fibers. As designed, the resistance changes in a stepwise manner as a function of strain or stress. In addition, the hybrid CFRP are also able to provide a useful warning of the impending of catastrophic fractures, and they can sustain further loads even after the initial rupture of high modulus carbon fibers.

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When an electromagnetic wave transmission line is constructed inside a material or structure, or a material or structure itself can be treated as a transmission line, damage or change in material properties may be detected using time-domain response of the line to a step input voltage signal. In this paper, we will present the results of experiments on two applications of this method as a health monitoring tool of composites. One is the detection of fiber breakage of CFRP. A microstrip line was constructed with a copper foil tape and the CFRP plate, and fiber breakage near the surface was successfully detected as a rise of the time-domain response at the corresponding position. The other is to add a damage-sensing function to an FRP repair patch that is being developed for repairing a damaged metal structure, such as aged aircraft structure. In this case, the metal under the patch and a carbon fiber strand in a carbon/glass fiber hybrid layer embedded in the patch could be treated as a microstrip line, and the model damage (slit) under the patch was successfully detected as a rise of the response at the corresponding position.

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Laminated composites consisting of unidirectional plies exhibit the complicated characteristics on deformation and strength because ply-cracking damage, delamination and fiber breakage develop from an early stage of deformation. In this paper, the damage and fracture process of CFRP cross-ply laminates under monotonic and cyclic loading has been investigated. Tensile tests in various directions and fatigue tests in the axial direction were carried out on three kinds of CFRP cross-ply laminates. In the tensile tests, the ply-cracking damage develops under 0? and 90? tension, while the nonlinear deformation due to nonelastic property of the matrix resin is predominant under off-axis tension. On the semi-log S-N curves obtained by the fatigue tests in the 0? direction, the fatigue fracture is described by a straight line in the high cycle fatigue region and the fatigue limit is not recognized clearly. In the fatigue tests, the fiber-peeling damage in 0° plies develops in addition to the ply-cracking damage in 90° plies. Ply-cracking density in 90° plies is characterized as a function of number of stress cycles. On the specimen fractured by fatigue, the most of ply-cracking damage develops at the first cycle, and slightly increases during fatigue. This damage behavior is insensitive to the stress level. Therefore, it is concluded that the ply-cracking damage is not a main factor controlling the fatigue fracture. On the other hand, fiber-peeling damage in 0° plies initiates from the edges of 0° plies and evolves in the whole area of the laminate with an increase in stress cycles. The evolution of the fiber-peeling damage strongly depends on the stress level. The fatigue fracture seems to occur when the 0° plies lose their load carrying capacity by the evolution of the fiber-peeling damage. Therefore, it is concluded that the fiber-peeling damage in 0° plies is the main factor controlling fatigue fracture.

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Experimental tests have been carried out on the mechanical behavior of CFRP 1×7 strand cables with a central core subjected to tensile and torsional loading conditions. As a consequence of helical structure, their tensile and torsional properties are strongly coupled. In addition to AE monitoring, the failure process of each helical string and a straight core has been probed independently by means of electrical resistance method. Unloading-reloading tests have also been done to investigate the effects of progressive cyclic loading. The electrical resistance method, which requires no external sensor, works successfully for the sensitive detection of the fiber breakage in the core and helical strings and hence, the present cable can be regarded as a smart structural element with self-diagnostic ability.

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Process induced thermal residual stresses and matrix failure of unidirectional CFRP has been investigated by finite element methods. Partial discrete model composites consisting of a microscopic area of fibers and matrix surrounded by a homogeneous area were chosen. Four cases have been investigated concerning the formation of residual stresses and initial matrix failure: A free UD-laminate, a constrained UD-laminate, a cross ply laminate and a thick laminate which is subjected to a temperature gradient during cool down. On the basis of experimental results from thermo-mechanical tests of the neat resin, the temperature dependent matrix stress/strain behavior was formulated and implemented into the finite element program. The actual stress state depending on different boundary condition could be described. The parabolic failure criterion was incorporated into the FE-Analysis. The failure criterion is based on the temperature dependent strength of the matrix, in order to take the competition of increase in residual stresses and increase of strength into account. The authors showed that the approach of a partial discrete model is suitable to determine the initial matrix failure of different macroscopic specimens under consideration of micro-mechanical effects. The results showed that high tri-axial stresses occur in the constrained laminate, which lead to initial matrix failure. The interaction of the 0°-layer in a cross-ply laminate lead to a stress state which causes initial matrix failure in the 90°-layer. The consideration of a temperature gradient changes the stress distribution in the matrix but shows a small influence on the maximum residual stress values. In this case initial matrix failure can be excluded.

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Fiber-optic distributed sensors, which can measure physical variables as a function of position along an optical fiber, are regarded as attractive sensors for structural health monitoring (SHM). Brillouin optical time domain reflectometry (BOTDR) can be used for measurements of strain distributions on structures, such as aircrafts, ships and bridges. In the BOTDR system for strain measurements, a pulsed light is launched into an optical fiber and the Brillouin backscattered light is measured. A short pulse is required to improve the spatial resolution. However, the measurement accuracy of BOTDR extremely becomes worse when the pulse width is less than 10 ns. When using 10 ns pulse, the accuracy and the spatial resolution are about ±30με and 1 m, respectively. This fact puts limitations on its applicability to SHM, because it is difficult to detect strain perturbation within the length of the spatial resolution. In this study, we developed a new technique with BOTDR to detect inhomogeneous strain fluctuating sharply within the spatial resolution. This technique is based on the fact that the profile of the Brillouin spectrum changes depending on strain distributions. We confirmed the dependency of the Brillouin spectrum on the strain distributions theoretically and experimentally.

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The validity of a failure criterion based on the idea of severity near the notch root is investigated experimentally. An experimental method is presented which examines the effects of notch geometry and fiber length on the damage and strength for notched plates of short glass fiber reinforced polypropylene (GF/PP) subjected to static tension. We used the GF/PP plate contains 30% E-glass fiber by weight. The mean value of fiber length was 0.4 and 3.5 mm. A diameter of fiber is about 0.01 mm. To evaluate the damage, we measured the luminance distributions by means of the luminance-measuring system with a CCD camera. It seems that the effects of fiber length were remarkable in the growth process of the damage and fracture strength for GF/PP plate. On the basis of the idea of severity near the notch root mentioned above, the experimental results can be explained.

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For an extrinsic Fabry-Perot interferometric sensor, the compensating demodulation technique to maintain the sensitivity at the quadrature is particularly necessary in applying the interferometer to detecting acoustic emission signals because of signal-fading problems. In this paper, we developed the stabilization control sensor system that is composed of an EDFA (erbium-doped fiber amplifier) source, a Fabry-Perot (FP) tunable filter and a control-circuit board. As an application of the system, we detected a simulated acoustic emission signal induced by a lead pencil-break with the stabilization control sensor system. Finally, the acoustic emissions of fracture signals generated by a tensile test of a cross ply composite specimen were successfully detected with this sensor system.

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Health monitoring is a major concern not only in the design and manufacturing but also in service stages for composite laminated structures. Excessive loads or low velocity impact can cause matrix cracks and delaminations that may severely degrade the load carrying capability of the composite laminated structures. To develop the health monitoring techniques providing on-line diagnostics of smart composite structures can be helpful in keeping the composite structures sound during their service. In this presentation, we discuss the signal processing techniques and some applications for health monitoring of composite structures using piezoceramic sensors and fiber optic sensors.

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Electromagnetic acoustic transducers (EMATs) are applied to detection of delamination in glass fiber reinforced plastic (GFRP) laminates. Since the delamination lowers the strength of the FRP, it should be detected nondestructively to assure the structural safety. Lamb wave may be useful in detecting the thickness reduction due to delamination, because its velocity depends on the plate thickness. In this study, while delaminating uniaxial GFRP laminate stepwise, we measure the Lamb wave velocity at each level of delamination. The Lamb wave of the lowest symmetric mode is used to avoid the interference with the other modes. The EMATs are used to transmit and receive the Lamb wave. Since the GFRP is nonconductive, coils are embedded in the laminates. When magnets are put on the coils in order to compose the EMATs, the Lorentz force acting on the current in coil wire launches the Lamb wave. Scan of the measuring point can be achieved by sliding the magnet. Both the group velocity and the phase velocity are measured using broadband pulse and burst wave, respectively. The delamination can be evaluated as the area of elevated velocity. Accuracy of evaluation of delamination front is discussed.

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