Keizo Kishida

Direct observation of glide dislocations in a KCl crystal by the light scattering method

The edge‐like dislocations in a KCl single crystal produced by plastic deformation have been observed successfully by a 90° angle light scattering method using a scanning type microscope constructed with a high‐power Ar‐ion laser (2 W for a wavelength of 514 nm) and a conventional optical microscope. The image of glide dislocations was very sharp and uniform compared with that of grown‐in dislocations. The orientation dependence of light scattering agreed with Moriya–Ogawa’s theory [Philos. Mag. A 41, 191 (1980)].

Measurement of dynamic fracture toughness by longitudinal impact of precracked round bar

Abstract A new procedure to estimate the dynamic stress intensity factor and a simple experimental method were developed for the impact fracture test. The circumferentially precracked round bar specimen was loaded by a tensile stress wave which was generated by the reflection from the free end of the specimen. The dynamic stress intensity factor K(t) was calculated by superposing the elastic response for the crack loaded by the step wave. The transmitted stress σ t through the precracked section was also calculated under the condition of no crack extension. The initiation of fracture was determined as the time of separation of the predicted σ t versus time curve from the measured one. The dynamic fracture toughness K Id of 7075-T651 aluminum alloy was obtained at very high loading rates above 106 MPa √m/s by the present method.

Dynamic Measurement of Elastic Moduli for Composite Materials Using Disk Specimens

We propose a new method to measure orthotropic elastic moduli for composite materials. In this method, the strain histories of a disk specimen subjected to impact loading in various directions are measured in the experiments. These strains are quantitatively compared in the frequency domain with the numerical results of the dynamic finite element analyses and orthotropic elastic moduli are estimated. Using this method, all elastic moduli may be obtained with only one specimen and one test apparatus. We tried to measure elastic moduli for a carbon fiber/epoxy composite material. As a result, it was found that the orthotropic elastic moduli E 1, E 2, G 12 could be measured easily within acceptable accuracy. but this method was suitable for measuring the Poisson’s ratio v 12.

Damage of Glass Subjected to Ultra-Short Pulsed Laser Shock Wave

Damage of a material from the microprocess to macro loss of strength was generally analyzed by the damage variables. The statistic average in space had to be taken to use these variables. This does not complied with the fact that fracture is finally caused by only one fatal crack. Based on the combination of NGA(Nucleation, Growth and Accumulation) model and percolation theory, we propose a novel method to study microcrack evolution that does not need any space average. The application in the study of spallation in glass is presented in this paper. The strong pulsed laser has been used to generate a very short shock wave that propagate radically from center in a disk shape specimen. Damage of spallation crack clusters of different stages were observed in the samples. The microcrack nucleation and growth were included in the computer code to simulate the damage field. The final fracture occurs when the maximum span of microcrack cluster extends to the specimen dimension. The experimental pattern of cracks can be illustrated from two types of simulations using same material parameters in our cylindrical shock wave experiments and uniaxial laser spallation tests for thin plates by other researches. The revealed microcrack percolation system is very complicated. We expect that an elegant fracture criterion be obtained by the analysis of this meso scale microcrack system.

VISCOELASTIC PROPERTIES OF CFRP SUBJECTED TO IMPACT LOADS

ABSTRACT Longitudinal and torsional impact tests are individually performed on two types of CFRP rods, which were made from the same prepreg but in different fiber orientation. The wave histories resulting from longitudinal impact tests on each CFRP rod are resolved into Fourier components in order to determine the complex tensile compliance. In a similar manner, the complex shear compliance is also obtained from the analysis of torsional impact tests. Furthermore, the conformity of viscoelastic properties between two types of CFRP rods is discussed.

Temperature Dependence of Viscoelastic Properties of CFRP Subjected to Impact Loads.

The effect of temperature on viscoelastic properties of CFRP (carbon fiber reinforced plastic) was examined by means of a wave propagation testing. Specimens used in the present study were two types of CFRP rods, which were made from the same prepreg but in different fiber orientation. Longitudinal impact tests were performed in the temperature range -40-80°C, and strain wave histories were resolved into Fourier components in order to determine the complex compliance as well as the viscoelastic model at each temperature. The experimental results revealed that the dynamic compliance (real part of complex compliance) increased with temperature, but change of the loss compliance (imaginary part of complex compliance) was not monotonous as it showed a maximum value at about -20°C and showed a minimum value at about 40°C. Furthermore, shear properties were evaluated from the longitudinal data on the two types of specimens based on the coordinate transformation law of viscoelastic functions (e.g. complex modulus, complex compliance) which were derived in the previous paper.

DETERMINATION OF VISCOELASTIC MODEL FOR CFRP BY WAVE PROPAGATION TESTING

Longitudinal and torsional impact tests were individually performed on two types of CFRP (carbon fibre reinforced plastic) rods with a view to assessing the dynamic properties of the materials. The wave history resulting from the longitudinal impact tests on each CFRP rod was resolved into Fourier components in order to determine the complex tensile compliance. In a similar manner, the complex shear compliance was also obtained from the analysis of torsional impact tests. The viscoelastic models and parameters both for tension and for shear were subsequently determined based on the variations of the complex compliances with frequency. It was found for each material that the 4-element viscoelastic model should be applied to the dynamic behaviour of the materials both in tension and in shear over a wide frequency range.

Behaviour of Materials at High Strain Rates and Cryogenic Temperature

An experimental technique has been developed for investigating behaviour of materials at high strain rates and cryogenic temperature using a split Hopkinson bar system and a liquid helium cryostat. Yield stress and fracture toughness for a mild steel (JIS-S25C) and an aluminum alloy (Al 7075-T651) were measured at strain rates (loading rates) up to 3×103 s−1 (3×106MPa√m/s) and temperatures down to 4.6K. For the mild steel, the dynamic and static yield stresses increased with decreasing temperature and the dynamic yield stress was fairly larger than the static one. The variation of the dynamic yield stress due to temperature was not seen below 77K. The fracture toughness was dropped with decreasing temperature and increasing loading rate. The brittle-to-ductile transition was shifted to higher temperature range by the increase of loading rate. For the aluminum alloy, the yield stress scarcely depended on the temperature. The influences of temperature and loading rate on the fracture toughness were not clearly seen for Al 7075-T651.

A Study of the Isoparametric Timoshenko Beam Element with Reduced Integration

A study of the isoparametric Timoshenko beam element with reduced integration is presented which has four nodes and two degrees of freedom per node. The element stiffness and mass matrices are derived from the same cubic polynomials for the total lateral displacement and the bending slope. The performance of the reduced integration Timoshenko beam element is studied by calculating the natural frequencies of three cantilever beams with different slenderness ratios. It is shown that the rate of convergence of the isoparametric Timoshenko beam element is significantly improved by the use of reduced integration especially for a thin beam.