Key Engineering Materials Vols. 348-349

Abstract: It is a prerequisite to formulate material parameters affecting chloride penetration into concrete. This paper is focused on defining intrinsic parameters of concrete, linking a theoretical solution for durability design in submerged concrete under marine environment. Material parameters, such as surface chloride content, chloride diffusivity, critical threshold critical content, chloride binding capacity are formulated.

Abstract: The fatigue testing of the specimen with a central open hole was carried out to investigate the influence of the surface roughness on the fatigue lifetime. Fracture morphology investigations were used by scanning electron microscope. The reliability analysis of the specimen was carried out by detail fatigue rating. The results revealed that the less the roughness is, the longer the fatigue life of open hole and the higher the value of DFR (Detail Fatigue Rating) is. Under the same technological conditions, the effect of corner crack on fatigue life of open holes is greater than the effect of internal crack. So, the effect of a decrease of roughness on the fatigue life of an open hole is similar to a change of crack type from a corner crack to an internal crack.

Abstract: The finite element analysis fully coupled fatigue damage evolution is implemented on the user subroutine UMAT of the finite element software ABAQUS. The fully coupled method developed with damage mechanics and the finite element analysis is performed on calculation of fatigue damage accumulation of the critical welded member in the Tsing Ma Bridge. The calculated result shows that the fatigue damage in the critical welded member is accumulated in the region of toe of welding. The value of faitgue life calculated by the fully coupled method is smaller than that by the uncoupled method, which suggests that there exists interaction between the fatigue damage evolution and the structural response. The linear Miner’s Law is widely used however conservative for the evaluation of fatigue life of bridge on service. The above results provide feasible method for accurate evaluation of fatigue damage in bridge components based on the hot spot stress analysis and the damage mechanics theory.

Abstract: When a suitable pulse current is introduced into a metal die containing a crack, the current concentration effect is immediately apparent, small welded joints are formed at a small sphere near the crack tip inside the metal die by metal melting as a result of the heat concentration effect. An axial symmetric metal die with a half–embedded crack on the top face is the object studied. The high intensity pulse current is switched on and flows from the outer to the inner surface of the hollow metal die. The complex function method is used to solve for the temperature field around the crack tip at the moment when the pulse current is switched on. The expression for the temperature field provides the basis for determining the current density and current direction. The experiment surveys show that the temperature around the crack tip grows up instantly above the melting point of the metal, the round crack tip is melted.

Abstract: This study is focused on examining the effect of critical crack width in combination with crack depth on chloride penetration into concrete. Because concrete structures have to meet a minimum service-life, critical crack width has become an important parameter. Specimens with different crack width / crack length have been subjected to rapid chloride migration testing (RCM). The results of this study show a critical crack width of about 0.012 mm. Cracks smaller than this critical crack width are considered not to have a significant influence on the rate of chloride transport inwards, while chloride penetration does proceed faster above this critical crack width.

Abstract: The purpose of this study is to define the amount of critical corrosion in steel that causes cracks on the cover of concrete. Major variables in the amount of critical corrosion are concrete strength, thickness, and the cover of concrete. In this experiment, stress concentration is a newly added factor. According to the results of this experiment, it is evident that the amount of critical corrosion decreased according to the increase in stress concentration factors. In addition, electric currents were measured using a constant voltage generator. When a constant voltage was connected to the corrosion circuits, it was discovered that the magnitude in the change was similar to the displacement on the cover of concrete and the electric current according to time.1)

Abstract: As concrete is a type of porous materials, water or air freely permeates concrete. Therefore the durability of concrete decreases. However, porous material with a rust inhibitor may allow permeation of water into concrete. In addition, there may be permeation of water through the rust inhibitor at the location of steel frames. The objective of the study is to investigate the penetration depth of concrete under water forced conditions with pressure.

Abstract: The purpose of this study is to propose a stiffness reduction factor for flat plate systems under lateral loads. According to current design provisions, slab stiffness under lateral loads should account for stiffness reduction due to the effects of cracks. Several researchers have conducted for evaluating the stiffness reduction in flat plate slab systems under lateral loads. However, no research is found for establishing strength reduction factor with respect to the level of applied moment. This study attempted to propose equations for calculating stiffness reduction factor with respect to the level of applied moment (Ma) represented by the ratio of Ma to the cracking moment of the slab (Mcr). For this purpose, test results of 20 interior slab-column connections were collected. For each specimen, stiffness reduction was measured with respect to Ma/Mcr. To verify the proposed factor, this study conducted the experimental test of interior connection under quasistatic cyclic loading, from which load-deformation curve was obtained. The curve was compared with that obtained from the effective beam width method with the proposed stiffness reduction factor. It shows that the proposed factor accurately predicts stiffness reduction in flat plate systems.

Abstract: Impacts of meteoroids and space debris on pressure vessels can have detrimental consequences for any mission. Depending on the parameters of the impacting particle and the characteristic of the vessel, the damages can range from relatively uncritical craters in the vessel’s surface to the catastrophic bursting of vessels, which besides the loss of vessel may result in severe secondary damages to surrounding components. In order to investigate failure mechanisms of thin-walled aluminum pressure vessels under hypervelocity impact of space debris, a non-powder two-stage light gas gun was used to launch Al-sphere projectiles impacting on unshielded and shielded vessels. Damage patterns and mechanisms leading to catastrophic rupture are discussed. Experimental results indicate that the impact kinetic energy of the projectile and the gas pressure in the vessel have an important effect on the damage modes of the vessel. On the one hand, high pressure gas can lead to a vessel blast. On the other hand, high pressure gas can mitigate the impact of the debris cloud on the rear wall of the vessel. Catastrophic rupture of unshielded gas-filled vessels can be avoided when the impact energy is less than a certain limit value. When the bumper is perforated, damage of shielded pressure vessel might be fatal for vessels with high gas pressure.

Abstract: In this paper, coherent gradient sensing (CGS) and digital speckle correlation method (DSCM) are introduced to study the K-dominance of static crack tip in functionally gradient materials (FGMs) with a crack oriented along the direction of the elastic gradient. And the numerical simulation is analyzed through finite element method (FEM). Firstly, the CGS and DSCM equations at the mode-I static crack tip of FGMs are developed, which can be used to calculate the stress intensity factors of FGMs. Secondly, three kinds of FGMs specimens with different variation of the modulus are prepared to observe the influences of the property variation on the K-dominance. Then three-point-bending experiments are carried out. The interference fringe pictures of CGS and the speckle patterns for DSCM on the specimens are shot through the camera. Thirdly, based on the results of the experiments, the stress intensity factors of three kinds of FGMs specimens are calculated by CGS and DSCM. Meanwhile, the stress intensity factors are obtained by FEM. Finally, comparing the results from CGS, DSCM and FEM, the K-dominance of mode-I static crack tip in FGMs is discussed in detail. It is found that the K-dominance of FGMs and homogenous material is almost same when the gradient index in FGMs is relatively small.