Caustic method is an optical method to measure the stress intensity factor of cracks. The method is widely used by the researchers of fracture mechanics, since it has three advantages. The first advantage is the simple optical system. Usually one doesn't need to use a laser as a light source. The second is that the higher-order terms of the stress field near crack tips don't much affect the stress intensity factors measured by caustic method. The last is that the caustic method can be applied easily to measure the dynamic stress intensity factor of fast propagating cracks, because it is an optical method. However, one often needs to develop new techniques when he or she applies the caustic method to dynamic fracture phenomena. The present article describes the holographic caustic method that can record and reconstruct caustic beams of fast propagating cracks at the instant of dynamic fracture. The holographic caustic method makes it possible to figure out the stress field near the tips of fast propagating cracks.

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Shadow optical method of Caustics has been applied in order to measure thermal stress distribution around a cylindrical heater inserted in a thin disk of acrylic. Caustics pattern has circular shape centered to the heater. Caustics pattern appears at certain stress value and then increases in diameter (Caustics Size). As soon as heating process goes to its steady state, Caustics pattern movement tends to slow and finally stops. A strong relation between Caustics pattern size and position of the highest value of the radial thermal stress has been observed. By this experiment a theoretical Caustics pattern size equation has also been successfully obtained. The comparison between the behavior of Caustics pattern movement and the thermal stress distribution and its behavior during heating process engages us to conclude that Caustics experiment method could be used to measure radial thermal stress gradient.

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Holographic caustic method is applied to measure the dynamic stress intensity factor of fast propagating cracks not only in the region of the singular stress field of plane stress but also in the 3-dimensional stress field in the vicinity of the crack tips. The caustic method is based on the assumption that the initial curves of caustics are in the singular field of plane stress, hence, it doesn't give the correct values of the stress intensity factor if the initial curves are in the 3-dimensional stress field. In order to obtain the correct K-values in the 3-dimensional stress field, two correction curves are examined and are compared with the correction curve obtained from stationary cracks. It is shown that the correction curves obtained from the measured K-values of fast propagating cracks are approximately equal to that obtained from the stationary cracks within the accuracy of measurement.

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Crack bifurcation phenomena depend on loading condition and characteristic of material. Therefore, we try to observe simple crack bifurcation phenomena by the dual loading system. In this study, the method of caustics and a high-speed photography system are used to record the dynamic crack bifurcation phenomena.
At first, the experiments for dynamic crack propagation are carried out under various loads. Occurrence of crack bifurcation depends on load balance in the dual loading system. For the cases of dynamic crack bifurcation and nonbifurcation, the fracture mechanics parameters are evaluated based on the experimental results. For all crack bifurcation cases, the energy flux values at the crack bifurcation moment are almost constant, which indicates an existence of critical material value governing the dynamic crack bifurcation.
Next, the special cases of crack bifurcation are discussed. The asymmetric crack bifurcation is occurred under thesame governing low for dynamic crack bifurcation. Also the fracture mechanics parameters at each crack tip have different values. In one more special case, single crack bifurcates to three propagating cracks. These three cracks start propagating from same material point.

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Brewster et al. have previously shown that the ultrasonic caustics can be applied for sizing the surface and the internal cracks in the circular rods by using the shear wave. However, the application technique by employing the longitudinal wave and the general evaluation procedure for the rods with an arbitrary diameter are not shown. Moreover, the accuracy of this technique has not been discussed in detail. In this study, we investigated about those points to improve the ultrasonic caustics technique. The crack is evaluated by employing the shear wave and the longitudinal wave, and the general relationship between an angle β₀ of the receiver and a ratio a/D of the crack length a to the diameter D of rods is given for determining the crack length. The immersion method is employed to aluminum rods with a surface crack in the radial direction to verify that the minimum crack size of 2mm is evaluated by the ultrasonic caustics technique under present experimental conditions. It is also shown that the length of the surface crack in oblique direction and the length of the internal crack in radial direction can be evaluated by this technique.

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Various implants are employed to recover the function of the femur by providing mechanical fixation. However, these implants can have a very limited life and require replacement. In order to optimize implant life, it is necessary to improve the design of components and implantation techniques, which is clearly also beneficial to both patients and hospitals. The determination of the real displacement patterns across implanted bone is important in developing alternative and improved implants. Holographic interferometry, a displacement analysis technique permitting whole-field inspection of deformation patterns, was used as the experimental method. In this study, a one-shot holographic interferometry is developed for simultaneous measurements of three aspects of femur. The optical system consists of two sets of plane mirror each arranged to face the postcrior and antcrior of fcmur. Three objective images reconstructed from holograms are constructed in the CAD system and analyzed. By the proposed method, experiments become easy and the reliability of the data is improved. The feasibility of proposed method is shown by measuring displacement of the intact model bone.

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In displacement measurement by holographic interferometry, an interferogram is recorded before and after deformation of a specimen using a double exposure method. The fringe pattern obtained from holographic interferometry is the superposition of the complex amplitudes before and after deformation. Then the reconstructed fringe pattern represents contours of out-of-plane displacement. Recently, digital holography was developed. In digital holography, a hologram is recorded on a CCD and the reconstructed image can be obtained from the hologram with a computer. In the case of phase-shifting digital holography, the complex amplitude is obtained from phase-shifted holograms using Fresnel diffraction algorithm. In this study, we propose a method to measure out-of-plane displacement using the phase-shifting digital holography. The displacement distribution of a cantilever is measured by combining holographic interferometry and phase-shifting digital holography. A removal technique of speckle noise is proposed where the speckle noise is suppressed by using the mask made from the reconstructed image.

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Phase analysis methods for projected gratings have been often used for non-contacting shape measurement. We have developed the integrated phase-shifting method for real-time shape and deformation measurement with high resolution. But there is a problem that the measuring error becomes larger when the object is moving speedily because several images are captured while shifting the grating phase. In order to solve this problem, we developed a DMD reflection-type CCD camera that we call a DMD camera. The each DMD pixel works as a high-speed controllable shutter of the corresponding CCD pixel. In this paper, we propose an integrated phase-shifting method using correlation for a real-time shape and deformation measurement. This method can analyze phase distributions of projected gratings from an image taken by the DMD camera. The principle and an experimental result to apply this method to a shape measurement are shown.

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Micro-indentation technique was applied to evaluate the mechanical properties of corroded surface layers of Ni-alloy, 316SS, Ti and Ta, which were exposed into the corrosive gaseous environment consisting of HI+I₂+H₂O+H₂. The corrosion condition was chosen so as to simulate one of the environments in the thermochemical hydrogen production, IS process, where the degradation due to the corrosion attack by HI and the hydrogen embrittlement is a key issue from the viewpoint of material integrity. The micro-indentation tests using a spherical indenter and the inverse analysis with Kalman filter were performed to quantitatively estimate material constants of corroded surface layer. Ni-alloy exhibited good corrosion resistance and Ta was remarkably influenced by the hydrogen embrittlement.

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