Kenichi Sakaue

Instantaneous phase-stepping photoelasticity for the study of crack growth behaviour in a quenched thin glass plate

A new approach to simultaneous acquisition of phase-stepped photoelastic fringes using a CCD camera equipped with a pixelated micro-retarder array is described for the investigation of time-variant problems. This method is applied to the study of quasi-static crack growth in a quenched thin glass plate. The distributions of the principal direction as well as the principal stress difference around a growing crack tip are obtained by the proposed method. Then, not only the mixed-mode stress intensity factors but also the T-stress are evaluated from the distribution of the principal stress difference, and they are validated using the reconstructed phase maps. The results show that the proposed instantaneous phase-stepping technique is effective for the study of the crack growth behaviour in a thin glass plate.

Simultaneous photographing of rapidly bifurcating cracks on both surfaces of plate specimens with pulsed holographic microscopy

When brittle materials break under external force, fast propagating cracks appear often whose propagation speed is from 200 m/s to 2000 m/s. The fast propagating cracks suddenly bifurcate into two cracks when the propagation speed is high enough. But the mechanism of the rapid crack bifurcation has not been well understood. In the present study, two optical systems of pulsed holographic microscopy are applied to take photographs of rapidly bifurcating cracks in PMMA plate specimens. The cracks are of the opening mode and propagate at a speed more than 600 m/s. The photographs of the cracks are simultaneously taken on the both sides of the plate specimens about 10 μs after bifurcation. The photographs show that, in many cases, the shape of branch cracks on one side of a specimen is apparently different fom that on the other side of the specimen. The fact indicates that the rapid crack bifurcation in PMMA is of three-dimensinal phenomenon. Also shown is that two branched cracks have different crack opening displacements. It means that the bifurcation of the cracks are asymmetric just after bifurcation.

Recent Progress of Experimental and Measuring Technology. Measurement of Opening Displacement of Bifurcated Cracks by High-Speed Holographic Microscopy and Moire Interferometry.

High-speed holographic microscopy is applied to take successive microscopic photographs of a fast propagating crack at bifurcation. The crack speed is about 650 m/s. From the photographs, crack opening displacements (CODs) of the mother crack and two bifurcated branches are measured along the crack. A specimen with a notch is also made, where the notch has the bifurcated notch tip same as the bifurcated fast propagating crack. Opening displacement of the notch is measured by Moire interferometry under static loading condition. The measurement results show that the opening displacements of both the mother crack of the bifurcated fast propagating crack and the mother notch of the bifurcated notch are proportional to square root of the distance r from the crack or the notch tip. The opening displacements of the two bifurcated branches of the notch are also proportional to √(r). But the CODs of the bifurcated branches of the fast propagating crack are not always proportional to √(r).

Instantaneous phase-stepping photoelasticity and hybrid stress analysis for a curving crack under thermal load

Stress fields around an oscillating crack tip in a thin plate are analyzed using a hybrid method of photoelasticity and finite element analysis. Instantaneous phase-stepping photoelasticity using a CCD camera equipped with a pixelated micro-retarder array is used for measuring the stress fields around a propagating crack tip in a quenched thin glass plate. The distributions of the principal direction as well as the principal stress difference around a growing crack are obtained. Then, the values of the principal direction and the principal stress difference are used for determining the boundary condition for a local finite element model. Using the boundary condition that is determined from the measurement results inversely, the stress distributions around a crack are evaluated. It is shown that the stresses around the crack tip can be evaluated using the proposed hybrid method. Results show that the proposed hybrid method is effective for the study of crack growth behavior in the quenched glass plate.

The effect of Ni,Ge elements on microstructure and mechanical properties of Sn-Ag-Cu solders

Sn-Ag-Cu solder alloys have been widely used as lead-free solders following the enactment of the 2007 RoHS Directive. Recently, the content of Ag has been demanded to decrease in the solders for wave soldering process as the cost of Ag has already become very expensive. On the other hand, it was considered that the mechanical strength of the solder tends to be not enough as the amount of Ag decreases. Therefore, the low-Ag solders must be carefully developed to keep the mechanical properties. In this paper, we study the effect of adding small amount of (Ni,Ge) elements on microstructure and mechanical properties of the low-Ag lead-free solders of which the amount of Ag is 0.3wt% and 1.0wt%. Indentation testing method was used to obtain mechanical properties including elastic, plastic and creep deformations in this study.

Recording caustic images by phase-shifting digital holography

Phase-shifting digital holography is applied to record the caustics of a stationary crack at arbitrary distances. The stress intensity factors of the crack are measured from the reconstructed images of the caustics. The measurement results show that the stress intensity factors are almost constant in the region where the initial curve radius, i.e., distance from the crack tip, is greater than half the specimen thickness. In addition, the stress intensity factors decrease in a three-dimensional stress field. These results correspond to those by Rosakis and Ravi-Chandar. This confirms that this method is applicable to measure the stress intensity factor of cracks.

Evaluation of Viscoelastic Characteristics of Polymer by Using Indentation Method

Five-step indentation test is used to evaluate viscoelastic characteristic of acrylonitrile butadiene styrene resin, ABS resin. Five-step indentation test consists of two indentation cycles and the creep compliance is evaluated from the second indentation cycle, because the measured penetration depth in second cycle is unaffected by plastic deformation. The creep compliance is evaluated by the indentation theory for an axisymmetric conical indenter and elastic-viscoelastic correspondence principle. Furthermore, in order to evaluate the creep compliance, FEM is used to analyze the indentation problem, in which the conical indenter is impressed on an impression. The results show that creep compliance evaluated by FEM are comparable to that by tensile test. This result means that the creep compliance of polymer can be evaluated by 5-step indentation test.

Measurement of Crack Tip Displacement Field in Desiccating Paste

Desiccation cracking is a phenomenon in which cracks are caused by shrinkage of drying paste material. In the present study, digital image correlation, DIC, is applied to measure the displacement field at the crack tip in a drying paste. In order to obtain magnified images around the crack tip, the control method for crack nucleation position and crack propagation direction is newly developed. Furthermore, stress intensity factors are evaluated by iterative procedure from the measured displacement field. The results show that the stress intensity factors are varied during crack propagation because of kinked crack path ant the fracture toughness depends on the paste thickness even the paste is composed of the same powder. In addition, it is found that the fracture toughness is independent of the crack propagation speed. The dependence of the fracture toughness on the paste thickness is noticeable results to analyze the fracture of desiccation cracking.

Improvement of elliptically polarized white light technique for photoviscoelasticity

Viscoelastic materials such as rubbers and plastics are widely used not only in industrial fields but in daily life. The mechanical properties of the materials are remarkably dependent on time and temperature. Photoviscoelastic technique has been used for stress field measurement in the kind of material. However, the conventional techniques are not suitable to measure fringe order and the principal direction of birefringence simultaneously. Yoneyama et al proposed an elliptically polarized white light technique which can measure fringe order and the principal direction of birefringence simultaneously with a single exposure. However, the technique involves still three severe problems. First, the accuracy in determination of the principal direction of birefringence is severely influenced not only by noise but by the accuracy of fringe order determination, because the principal direction of birefringence is determined by utilizing the attenuation of light intensity and the fringe order. Second, it is difficult to determine fringe order automatically and accurately. And the third, this technique needs long time to manipulate images. In this paper, the authors describe how to improve the technique and to solve the problems. In this situation, the polarization angle for the largest attenuation in light intensity is calculated and examined to obtain good results. Also, an algorithm for the determination of fringe order is developed. As the results, the angle for the highest attenuation is obtained as 45 degrees, and the fringe order is successfully determined by applying a window field reference technique.

Displacement measurement by phase-shifting digital holography with convolution technique

Phase-shifting digital holography enables the measurement of surface displacements on an object body. In the present study, the convolution integral technique is adopted to reconstruct holographic images. The most important feature of the technique is that spatial resolution is independent on the distance of image reconstruction, and is suitable for displacement measurement of a body with depth. Also, the resolution obtained by the technique is expected to be higher than that obtained by the conventional reconstruction technique. The displacement fields are obtained using phase difference data before and after deformation. In this paper, experimental results of the case of a cantilever beam show good agreement with theory, also the algorithm developed for the convolution integral technique is shown to be effective and useful to the image reconstruction.