Numerical simulation of incremental strain-rate test on pure copper has been executed to examine whether shear stress and shear strain can be evaluated precisely in the split-Hopkinson torsional bars (SHTB). The shear stress calculated from transmitted waves as a nominal value over the gage length of the specimen shows good agreement with the value evaluated directly from the element of thin walled part of the specimen. On the other hand, the nominal shear strain and shear strain rate calculated on the basis of the relative motion at the both ends of the specimen tend to be larger than the directly evaluated values. This overestimation of shear strain and shear strain rate is due to the strain concentration and the spread of the plastic deformation near the reentrant corners of the specimen. The experimental results of shear strain measured directly by the strain gages cemented on the specimen agree well with the analytical results.
Strain rate sensitivity in FCC metals is known to increase dramatically when the strain rate exceeds about 5×103/s. The phenomenon has been interpreted by transition in rate controlling mechanism of dislocation motion from thermal activation to viscous phonon drag. It is generally known that the phonon drag increases with temperature. Usually, however, the experimental flow stress in the viscous flow range shows opposite temperature dependency. In order to clarify the above contradiction and the mechanism, high strain rate tests are performed for high-purity polycrystalline aluminum and copper in the strain rate range from about 1×103-2×104/s and at temperatures ranging up to 600K. A simplified model for the dislocation kinetics under dynamic plastic deformation is used to consider the deformation mechanism in the above strain rate and temperature ranges. The flow stress calculated in consideration of the temperature dependency of the mobile dislocation density shows fairly good agreement with the flow stress directly measured. The increase in mobil dislocation density with increasing temperature lowers the flow stress and shifts the transition region to the higher strain rate side.
Growth or curtail of spall damage in aluminum plates under repeated impacts has been monitored with a C-scan acoustic microscope (C-SAM) and a PVDF focused transducer. The spall damage or ductile voids were formed by plate impact tests using a gas gun which has a special recovery unit for target plates. The target plate has been impacted up to a maximum of three times with velocities ranging from 100m/s to 200m/s. This experiment has revealed the following: (1) From the C-scan and B-scan images of the spall damage, we can observe the change in three dimensional distribution of voids in aluminum plates under repeated impact. (2) When the second impact stress is higher than the first, which is greater than the spall threshold stress, the voids grow in size and number. (3) On the contrary, a part of voids disappear when the first impact stress is higher than the second, only if the latter is greater than some critical value. (4) The B- and C-scan images overestimate the void size when the voids are less than the ultrasonic beam diameter in size at the focus and are aligned closely in a plane. (5) The changes in B- and C-scan images under repeated impact are well correlated with the change in attenuation (amplitude change of B2 to B1 echo).
To characterize damage and deformation in an impacted area of structure caused by flying foreign objects, it is fundamentally needed to measure the acting force and its distribution over the area. In the present experiments, it was attempted to identify visually the impact force using the pressure sensitive paper ‘Prescale’. A split Hopkinson pressure bar method was used to evaluate the sensitivity of the paper under impact loading. It is found that the pressure sensitive paper instantaneously responds even for the impact loading of which duration is several hundred microseconds, while the sensitivity is significantly reduced when compared to the quasi-static loading. Hertzian impact tests were tried to visually evaluate pressure distributions using the pressure sensitive paper, and the experimental results were discussed.
This paper systematically studies the effect of nonproportional strain history on the flow behavior of FCC metals at high strain rates. Impact compression tests with pre-torsion strain ranging from 0% to 10% are performed at several rates up to 103s-1 by using a split Hopkinson pressure bar method. Two kinds of materials with different stacking fault energy, i.e., pure aluminum and pure copper, are employed to examine also the material dependency. The experimental results demonstrate that there exsts significant interaction between strain history and strain rate effect, which is strongly material dependent. The strain history effect tends to disappear at high strain rates for pure Al, whereas it becomes large under impact loading for Cu. The controlling micromechanism which describes both the above positive and negative interactions is extensively, discussed based on dislocation interaction. The effect of prestraining path is also discussed, where reversed torsion is introduced following 10% pre-torsion. The reversed pre-torsion straining tentatively reduces the strain history effect for Al, while no reduction in flow stress is observed in Cu. These trends are found to become strong for both the materials as the strain rate increases.
The plate impact experiment was made to find out a way of controlling the stress history of target plate. In the experiment, a flat target plate of aluminum alloy was impacted by an impactor plate laminated by two kinds of metal plates, traveling at high velocity. The waveform of stress pulse was measured by the embedded manganin gauges and were compared with the waveform which was calculated on the basis of impedance-matching method. The calculated plateau values of stress agreed with the experimental results within 4per cent error in average. It is shown that the method of using a laminated impactor plate is useful in controlling the history of stress in a target plate.
An aluminum projectile was impact-welded on a stainless steel target using a nitrogen gas gun at impact velocity over 250m/s. The effect of surface roughness at the impact face of the target on the bonding area was examined using scanning acoustic tomography, and the strength of the bonding area was evaluated by a tension test. The microstructure and element distribution in aluminum/stainless steel joint were analyzed by means of a scanning electron microscope and energy dispersive X-ray spectroscopy (EDX). The experimental results of the concentrations of elements in the compound layer at the interface were compared with those obtained by a simple theoretical analysis. The following results were obtained. The bonding area of the joints increased with the decrease of surface roughness. However, the increment of the bonding area due to the decrease of surface roughness hardly influenced the bonding strength. The thickness of the compound layer increased with impact velocity. In addition, the area of uniformly distributed elements in the layer extended with the increase in thickness of the layer. The concentrations of elements measured in the layer using EDX hardly varied with the impact velocity and were close to those of the theoretical result in which an equivalent heat was generated at the impact faces of a projectile and a target.
As a series of the studies on the mechanical properties of spider threads, the effects of ultra-violet rays (UV) and water was investigated using drag lines of Nephila clavata (Jorougumo) and capture threads of Argiope amoena (Koganegumo). It was shown for the effect of water that 1) the drag line was contracted as soon as immersed in water (super contraction), while the capture thread was not so contracted and the viscid droplets attached to it were dissolved in water, and 2) the super contracted drag line behaved like a capture thread with a J shaped load-draw ratio curve. For the effect of UV, it was found that 1) the degradation due to UV irradiation with short wave length was accelerated more than that with long one and 2) the degree of degradation for each wave length was governed by the integrated irradiation energy of UV.
The plane problem of a crack terminating at an interface in dissimilar materials subjected to thermal loading is considered. The thermal stresses near the crack tip are theoretically derived using the Airy stress function through the Mellin transform, and those are represented by a linear combination of the singular solutions of type rp-1 and rp-1 logr, the no singularity solutions and the particular solutions which are independent of distance r from the crack tip. Then, the solutions are determined from roots (a real root, a complex root and a double root) of an eigen equation and unknown functions which depend on the wedge angles of the materials, their mechanical properties and the root. When the root is a double root, the singular solution of type rp-1 logr, together with the singular one of type rp-1, appears in the thermal stresses. The appeared order and variation of the root are shown on the k12 (stiffness ratio)-φ1 (wedge angle of material 1) plane.
Damage mechanics is applied to the prediction of transverse cracking in composite laminates. The analysis is performed by assuming that the average crack opening displacement of a transverse crack can be approximated by that in an infinite homogeneous transversely isotropic medium. The analysis elucidates the thermomechanical properties of the laminates with transverse cracks and gives the energy release rate associated with transverse cracking. By assuming that a transverse crack occurs when the energy release rate reaches a critical value and by incorporating the thermal residual stresses arising from the mismatch of thermal expansion coefficients between plies, the transverse crack density is predicted as a function of the laminate stress. The prediction is compared with the experimental results of CFRP cross-ply (0/90m/0) (m=4, 8 and 12) laminates. An advantage of the present analysis is that it can be applied to an arbitrary laminate.
The objective of this study is to exmine the mechanical properties of construction joints between existing and newly placed concrete under combined tensile and shear stresses. Loading tests are conducted by using push off type specimens. The joint surface of existing concrete is roughened by shot blast and a half of the specimen is reconstructed by new concrete using ultra rapid hardening cement. The insufficient treatment of joint surface of the old concrete causes the lowering of tensile rigidity, while shearing rigidity is almost the same as that of the other specimen. The shearing and tensile rigidities of non jointed concrete and concrete shot blasted properly are not dependent on the combination of shearing and tensile forces. For the jointed concrete shot blasted insufficiently, the shearing rigidity decreases with the increase of tensile force and the tensile digidity also becomes lower by the action of shearing force. Both the tensile strength and shearing strength of jointed concrete become small compared to those of non jointed concrete. The ratio of reduction in tensile strength is larger than that in shearing strength. The strength of jointed concrete under combined tensile and shear stresses can be evaluated by Mohr's failure envelope expressed by parabora tangent to both tensile strength circle and compressive strength circle.
The static strength and delayed fracture strength characteristics of the glass used for CRTs were examined. The influences of surface roughness, thickness and test conditions on them were examined. The main results obtained are summarized as follows. (1) The influence of surface roughness on the static strength was especially recognized. (2) The delayed fracture strength of the scratched specimen, which was normalized by the static strength, was lower than those of the polished and abraded specimens.
Polyetherimide (PEI) resin has excellent characteristics in mechanical, thermal, and electrical properties, particularly in crack resistance. Thus, its applications are being extended to precision components for electric and electronic devices or medical equipment. In the area of medical care, the repetition of water wash or UV (ultraviolet) ray irradiation on tools must be applied after medical treatment in order to prevent bacterial infection. From these view points, this study was made to evaluate the degradation behavior of PEI resin caused by water absorption and UV ray irradiation. And for the purpose of higher functionalization of PEI resin, the surface coating of titanium (Ti) thin film was tried by vacuum vapor deposition or assisted vapor deposition using the ion beam mixing method. The effect of thin film formation upon water absorption characteristics was examined.
The experimental results of fatigue at low stress levels around the fatigue limit have large scatter. For the design of structures and machines, it is important to evaluate the scatter and to decide the design margin. In this paper, the statistical method to decide the fatigue limit is proposed. In the proposed method, the fatigue limit is decided with considering the distribution of fatigue strength. Because each fatigue data is transformed into normalized fatigue strength, this method has the advantage point that the distribution of fatigue strength and the fatigue limit can be found by relatively small samples. Using the proposed method, the computer system to decide the design margin for fatigue is developed. In this system, the S-N curve with fatigue limit and the distribution of fatigue strength are obtained at first. Then the design margin with a given probability of failure and a confidence level is calculated. The developed system is applied to some experimental results for fatigue test. As a result, the S-N curve with the design margin has obtained easily. It is recognized that the obtained S-N curve is decided after consideration of scatter.
The volume resistivity of conductive filler filled epoxy resin was measured under shear flow. A rheometer with double-cylinder type sample cell was employed in order to measure the volume resistivity and shear stress under shear flow. The outer and inner cylinders of the rheometer were used as the electrodes. Nickel particles and two kinds of carbon-fibers with different aspect ratios were used as the conductive fillers for the samples. It was found that the volume resistivity was affected by shear rate and the volume content and shape of fillers. The shear rate was varied from 10-2 to 102sec-1. The minimum volume resistivity was observed for the fiber filled samples in a range of shear rate from 1 to 10sec-1. The minimum volume resistivity indicated by shear rate for these samples changed when the aspect ratio and volume content of fibers were varied. The volume resistivity of the particle filled samples monotoneously decreased with increasing shear rate. The almost same volume resistivity for the fiber filled samples was observed at the same shear stress, even though the aspect ratio and content of fibers were varied. At the same time, the particle filled samples showed the same tendency in relationship between shear stress and volume resistivity.