Papers by Author: Wei Guo Guo

Abstract: In this paper, dynamic mechanical property tests under different tempreture and strain rate of high strength steel 18NiC250 are conducted by means of Hopkinson pressure bar technique. The results of tests show 18NiC250 steel is not only very sensitive on strain rate, but also sensitive to temperature. The rate relative constitutive model of this steel is obtained and well predicts dynamic mechanical property after yielded.

Abstract: Lightweight foamed concrete is one kind of new and important runway arresting materials for airplanes and other vehicles. To study its crushing and flowing behavior under different loading rates, an electric-driving screw testing machine, and an Instron VHS 8800 higher strain-rate testing machine are used. Crushing mechanical characteristics, and deformation and failure mode with the density of 0.20g/ccm are systematically analysed, while crushing strain-rate range from 0.001/s to about 102/s. Results show this foamed concrete suffers three stages of deformation during loading, namely elastic region, crushing plateau region and densification region; and it presents very low shear strength. Based on testing results, a phenomenological model is established, and comparing model predictions with experimental results, a good agreement is obtained.

Abstract: In this paper, to determine the dynamic strength model for steels, a new approach which does not rely on the Hopkinson bar test has been proposed. As the DH36 steel for example, using the results of Taylor impact test and the quasi-static compression test, the initial parameters of Johnson-Cook plastic strength model have been fitted out, then the initial strength parameters have been optimized using the optimization techniques of the sparse Taylor impact cylinder. It has been shown that the optimized results in numerical simulation are consistent with results of Taylor impact test, and the optimized Johnson-Cook model can also well describe flow stress curve fitted from the Hopkinson bar test.

Abstract: For investigating the effect of temperature on the dynamic properties of concrete material, tests for cylindrical concrete specimens at 23°C ~ 800°C were carried out by using Split Hopkinson Pressure Bar (SHPB) apparatus, and the strain rates ranged from 30/s to 220/s. Effects of temperature and strain-rate on the dynamic behavior of concrete were analyzed. The results show that: above 4000C, the dynamic compressive strength of concrete decreases with increasing temperature, and the enhancements of strain-rates on the compressive strength of concrete depend significantly on temperatures. Moreover, both strain-rate and temperature can enhance the peak strain of concrete.

Abstract: To understand the damage mechanism and obtain the fatigue life of the oriented sheets of polymethyl methacrylate (PMMA) glasses, the quasi-static compression tests and the constant amplitude fatigue testing with two notch stress concentration factors are carried out respectively, using the CSS44100 electromechanical universal materials testing machines and a MTS servohydraulic testing machine. The damage region and fracture section of the deformed and failed sheet samples are examined with the aid of an optical microscopy. The results show that, 1) the oriented PMMA glasses drawn through vertical bi-direction have remarkable thermomechanical anisotropic characteristic, that is, it has the higher hardness and strength along the normal direction of the sheet panel, it will burst out and scatters into bigger pieces under the perforation; 2) the S-N curve of the oriented sheet specimens shows that its fatigue life is not sensitive on the notch at lower amplitude stress levels; and 3) the crack initiation of the oriented PMMA glasses often originates the interbeds area or mid region of the notch edge under fatigue cycle loading.

Abstract: The split Hopkinson bar (SHB) technique has been widely used to determine dynamic plastic/damage properties of materials at high strain rates. But presently, a number of researchers used it to measure the so-called dynamic Young’s moduli. This paper aims at the feasibility to determine Young’s moduli by conventional SHB. The strain-stress curves of two kinds of metals, 45# steel and LY-12 aluminum, were measured by a conventional SHB facility. The corresponding numerical experiments were performed by ABAQUS/Explicit. The curves obtained by the both procedures agree well in elastic stage (strain range 0-0.5% assumed). But the slopes of the linearly fitted curves, i.e. the so-called dynamic Young’s moduli, are much different from the corresponding real values. The significant errors come from the non-uniformity of stress in specimen during the beginning stage of loading process. The results clarify that it is unfeasible to measure Young’s moduli by conventional SHB.

Abstract: In the present paper, in order to better understand the third type “dynamic strain aging” occurring during the plastic flow of metals, the uniaxial compressive experimental data ever obtained in University of California, San Diego using an Instron servo-hydraulic testing machine and the Hopkinson technique are systematically analysed. These experimental data cover the plastic flow stress of several fcc, hcp, bcc polycrystalline materials and several alloys at a broad range of temperatures (77K – 1,100K) and strain rates (0.001/s – 10,000/s). In analysis, the appearing region of the “dynamic strain aging ” under different temperatures and strain rates are respectively plotted by the curves of stress vs temperature, and stress vs strain for fcc, hcp and bcc metals. The results show that: (1) this third type “dynamic strain aging ” occurs in all hcp, bcc and fcc polycrystalline or alloy materials, and there are different profiles of stress-strain curve; (2) the “dynamic strain aging ”occurs in a matching coincidence of the temperature and strain rate, its temperature region will shift to higher region with increasing strain rates; (3) bcc materials do not have an initial pre-straining strain as the onset of work-hardness rate change for the “dynamic strain aging ”; and (4) based on the explanations of dynamic strain aging with serration curves (Portevin-Lechatelier effect) and other explaining mechanisms of references, The mechanism of third DSA is thought as the rapid/continuous formation of the solute atmospheres at the mobile dislocation core by the pipe diffusion along vast collective forest dislocations to result in a continuous rise curve of flow stress. Finally, several conclusions are also presented.