Axial behavior of reinforced concrete short columns strengthened with wire rope and T-shaped steel plate units.

yesThis paper presents a relatively simple column strengthening procedure using unbonded wire rope and T-shaped steel plate units. Twelve strengthened columns and an unstrengthened control column were tested to failure under concentric axial load to explore the significance and shortcomings of the proposed strengthening technique. The main variables investigated were the volume ratio of wire ropes as well as geometrical size and configuration of T-shaped steel plates. Axial load capacity and ductility ratio of columns tested were compared with predictions obtained from the equation specified in ACI 318-05 and models developed for conventionally tied columns, respectively. The measured axial load capacities of all strengthened columns were higher than predictions obtained from ACI 318-05, indicating that the ratio of the measured and predicted values increased with the increase of volume ratio of wire ropes and flange width of T-shaped steel plates. In addition, at the same lateral reinforcement index, a much higher ductility ratio was exhibited by strengthened columns having a volume ratio of wire ropes above 0·0039 than tied columns. The ductility ratio of strengthened columns tested increased with the increase of flange width, thickness, and web height of T-shaped steel plates. A mathematical model for the prediction of stress–strain characteristics of confined concrete using the proposed strengthening technique is developed, that was in good agreement with test results

Strain rate dependence of dynamic flow stress of 7075 aluminium alloy at very high strain rates

In order to evaluate the strain rate dependence of the dynamic flow stress of aluminium alloys, 7075-O and -T6, high strain rate tests are performed at strain rates ranging from about 1000/s to 30000/s, and strain rate reduction tests are also conducted in the strain rate range from about 10000/s to 28000/s. A steep increase in the flow stress is observed for both 7075-O and -T6. There is a difference in the strain rate at which the flow stress increases steeply for 7075-O and -T6. A simplified model for dislocation kinetics under a dynamic plastic deformation is proposed to explain the difference, which represents the rate dependence of the viscous drag on the dislocation motion. The model predicts that a mean velocity of a moving dislocation of 7075-T6 is lower than that of 7075-O at a given strain rate and further, 7075-T6 is higher in the moving dislocation density than 7075-O. The increase in the moving dislocation density causes the transition region, or the strain rates at which the steep increase in the flow stress becomes to appear, to shift to the higher strain rate side

Strain rate dependence of dynamic flow stress considering viscous drag for 6061 aluminium alloy at high strain rates

In order to evaluate the strain rate dependence of the dynamic flow stress of aluminium alloys, 6061-O and -T6, high strain rate tests are performed at strain rates ranging from about 1000/s to 30000/s, and strain rate reduction tests are also conducted in the strain rate range from about 10000/s to 20000/s. A steep increase in the flow stress is observed for 6061-O at the strain rate of about 5000/s. The above phenomenon, however, is not observed for 6061-T6 in the strain rate range where the strain rate reduction tests is conducted. A simplified model for a dislocation kinetics under a dynamic plastic deformation is used to consider the deformation mechanism in the above strain rate ranges, which reveals that the steep increase in the flow stress of 6061-O is attributed to the rate dependence of the viscous drag on the dislocation motion. It is estimated that the velocity of moving dislocation of 6061-T6 is lower than that of 6061-O at a given strain rate, and that 6061-T6 is higher in a mobile dislocation density than 6061-O. The model predicts also that the increase in the mobile dislocation density shifts the transition region, or the strain rates in which the steep increase in the flow stress becomes to appear, to the higher strain rate side

Strain rate dependency of dynamic flow stress of iron in wide strain rate range

In order to clarify a rate controlling mechanism of iron, compressive tests for 0.01wt.%C iron are conducted in a strain rate range from about $1\times 10^{-3}$ to $2\times 10^4$/s and at temperatures ranging from 79 K to 473 K. Furthermore, to evaluate the effect of a strain rate history on the dynamic flow stress, strain rate change tests are performed in the strain rate range from about $1.0\times10^4\sim -1.6\times10^4$/s at temperature of 293 K. The results of measurement are treated on the basis of a theory of thermally activated process. In a high strain rate and low temperature ranges, a dominant rate controlling mechanism is a dislocation motion which surmounts the Peierls potential barriers with a aid of the thermally activated formation of kink pairs. Below the strain rate of 1/sec and at temperature of 293 K, the mechanism may be controlled by the thermally assisted cutting of point obstacles with moving dislocations

Transition in rate controlling mechanism of FFC metals at very high strain rates and high temperatures

In order to clarify the rate controlling mechanism of FCC metals at the high strain rates, a test is conducted for high-purity polycrystalline aluminium and copper in the strain rate range from about 103 ≈ 2x10-4/s at temperatures ranging up to 600K. A simplified model for a dislocation kinetics under a dynamic plastic deformation is used to consider the deformation mechanism in the above strain rate and temperature ranges. The increase in the mobile dislocation density with increasing temperature lowers the flow stress and shifts the transition range to the higher strain rate side. The results indicate also that a internal stress decreases slightly with increasing a temperature, which reflects the temperature dependency of cross-slip of a screw dislocation. It is confirmed that a steep increase of the flow stress observed at the high strain rates is attributed to the transition in a rate controlling mechanism of a dislocation motion from the thermal activation to the phonon drag

ULTRASONIC ATTENUATION IN METALS DEFORMING AT HIGH RATES OF STRAIN : DATA ANALYSIS BY USING A KINETIC MODEL OF DISLOCATION LOOPS

In order to obtain experimental knowledge on the behaviour of dislocations at high rates of strain, time-resolved measurements of the ultrasonic attenuation and velocity in specimens undergoing dynamic plastic deformation have been tried. In this paper, some analyses of the ultrasonic data for polycrystalline aluminium are made by using a kinetic model for the motion of dislocations cutting through the forest dislocations. The model given its quantitative basis by the ultrasonic data can describe fairly well the strain rate dependency of the flow stress over a wide range of strain rate

DYNAMIC FLOW STRESS RESPONSE OF ALUMINIUM TO SUDDEN REDUCTION IN STRAIN RATE AT VERY HIGH STRAIN RATES

L'objectif du travail est d'évaluer les effets de la vitesse de déformation instantanée et l'état structural correspondant à l'histoire de la vitesse de déformation pendant un écoulement dynamique. Pour cela ont été conduits des essais dans lesquels la vitesse de déformation est rapidement abaissée pendant la déformation plastique d'un aluminium polycristallin, dans une gamme 8000 - 20000/s. Un nouvel appareil a été conçu afin d'obtenir une brusque réduction de la vitesse de déformation. Les résultats montrent que la vitesse de déformation instantanée joue un rôle plus important que l'état structural dû à l'histoire de la vitesse de déformation.In order to evaluate the effects of the instantaneous strain rate and the structural state reflecting the strain rate history upon the dynamic flow stress, the tests in which the strain rate is suddenly decreased during the dynamic plastic deformation are conducted for high purity polycrystalline aluminium in a strain rate range from about 8000 to 20000 /s. To obtain a sufficiently steep reduction in the strain rate a new apparatus is devised. The results indicate that the instantaneous strain rate plays a more important role than the strain rate history for the dynamic flow stress in such a very high strain rate range in which the steep increase in the strain rate sensitivity of the flow stress is observed

AN ULTRASONIC STUDY OF THE BEHAVIOR OF DISLOCATIONS UNDER HIGH SPEED PLASTIC DEFORMATION IN ALUMINUM

In order to detect the behavior of dislocations at high rates of strain, time-resolved measurement of the ultrasonic attenuation under dynamic plastic deformation was made. The experimental technique, the theoretical background of this method and the results of the measurements made for polycrystalline aluminum are presented together with brief discussion

Two strain rate change tests for derivation of constitutive relationship of metals at very high rates of strain

Deux types de test de changement de la vitesse de déformation ont été effectués comme suit. Un test de réduction de la vitesse de déformation a été conduit sur l'aluminium, le cuivre, le fer et le niobium à des vitesses de déformation allant jusqu'à environ 20000/s. Des mesures d'atténuation de l'onde ultrasonore superposée à la déformation plastique dynamique - test associant le changement de la vitesse de déformation à la perturbation de haute fréquence de celle-ci - ont été faites sur l'aluminium et le cuivre à des vitesses de déformation jusqu'à environ 10000/s. Les résultats de ces deux tests montrent que, l'histoire de la vitesse de déformation a un effet faible sur le stress de déformation.Two types of strain rate change test are conducted. The strain rate reduction test is made for aluminium, copper, iron and niobium at strain rates up to about 20000/s. The attenuation measurement of the ultrasonic pulse superimposed upon the dynamic plastic deformation, a kind of strain rate change test with a high frequency perturbation of the strain rate, is made for aluminium and copper at strain rates up to about 10000/s. Results show that, in the above high strain rate range, the effect of the strain rate history upon the strain rate dependency of the flow stress is small