Key Engineering Materials Vols. 462-463

Abstract: Fiber Reinforced Polymer (FRP) has been effectively used for strengthening concrete beams. In this study, nonlinear finite element (FE) model of the beam strengthened with CFRP is established to analyze the debonding failure caused by the adhesive hollow defects. The constitutive relationship of the adhesive layer in the FE models is similar to Monti’s bi-linear mode. Three different locations of the hollows are considered in the FE models. The principal stresses in the concrete, the debonding stresses in the adhesive and the stress in CFRP are calculated. The results show that the effect of the locations of the hollow is marginal on the principal stresses in the concrete. When the hollow appears close to the interfacial end, the debonding stresses in the adhesive and the shear stress in the CFRP are significant, which easily causes the debonding failure.

Abstract: Debonding failure mode usually occurs in the concrete structure flexural strengthened with fiber reinforced polymer (FRP) under cyclic loading. This paper presents an experimental investigation into the fatigue behavior of the FRP-concrete interface of reinforced concrete (RC) beams strengthened with prestressed FRP. 8 small-scale beams were tested under three-point bending cyclic loading. The propagation behavior of the fatigue interface cracks is addressed, and curves showing the growth law of interface cracks are presented. Results from these tests show that the propagation process of interface cracks had three stages, including rapid, stable and unstable growth. The stable propagation phase experienced the most part of the whole test, and the failure mode of all failed beams was debonding following the fatigue fracture of the tensile steel bars. In addition, the influence of FRP prestressing level on the fatigue lives of strengthened beams is discussed, and an empirical formula is developed to predict the fatigue lives of such members. The results show that the fatigue life increases with the prestressed level of FRP. This study provides an insight on the potential long-term performance of FRP-strengthened beams submitted to fatigue loading conditions.

Abstract: The early-age shrinkage cracking of concrete plays an important role to the accelerated deterioration and shortening the service life of concrete structures. Modern concretes are more sensitive to cracking immediately after setting, which is due to material characteristics (lower water/binder ratio and higher cement content) and external environmental fluctuations (humidity and temperature change). Determination of concrete free shrinkage is the basis of shrinkage cracking research. Analytical models of the autogenous shrinkage and drying shrinkage are established in this paper. The calculated results agree well with the experimental results.

Abstract: The fatigue tests were conducted for cold-drawn eutectoid steels having different activation energies and microstructures to investigate the mechanism of fatigue strength degradation by irreversible hydrogen. The fatigue strength of a sample with low activation energy was decreased by irreversible hydrogen, whereas the fatigue strength of a sample with high activation energy was not. When the activation energy for irreversible hydrogen becomes small, the desorption of irreversible hydrogen from its trap site is easily induced by cyclic loading, which results in a decrease in fatigue strength. When a sample having high activation energy was annealed at 473 K, the fatigue strength was decreased by irreversible hydrogen although the activation energy barely changed. This is because the precipitation of fine carbides due to annealing causes higher sensitivity to hydrogen embrittlement. High sensitivity to hydrogen embrittlement was concluded to induce the fatigue strength degradation by irreversible hydrogen. Therefore, the degradation of fatigue strength by irreversible hydrogen depends on the activation energy and sensitivity to hydrogen embrittlement.

Abstract: Stress distribution analysis on four types of stress corrosion cracking (SCC) specimen is presented in this paper. This work was performed using commercial finite element based software, ANSYS. Two types of mesh arrangements, fixed mesh with different mesh size and adaptive mesh, were employed in this work to study the effect of mesh size on stress distribution of SCC specimen. Four types of SCC specimen were studied in this work, i.e. C-ring specimen, tensile test specimen, pre-crack cantilever beam specimen and U-bend specimen. Simulation result shows that stress distribution on the SCC specimen much affected by mesh size and arrangement, especially for specimen with notch. By performing the stress analysis, less effort needed in order to determine the location of corrosion potential measurement on the SCC specimen. This will be very helpful for future work of SCC study.

Abstract: This paper deals with two-dimensional plane stress wrinkling model of elastic/plastic annular plate. Based on energy method and nonlinearity of strain-displacement law, a bifurcation functional in polar coordinate is derived analytically. This technique leads to the critical conditions for the onset of the elastic/plastic wrinkling of the flange during the deep-drawing process. Tresca yield criterion along with deformation theory of plasticity are utilized and the material of the plate is assumed to behave perfectly plastic. Moreover, the influence of the blankholder upon wrinkling and on the number of the generated waves is quantitatively predicted by the suggested scheme. The main advantage of the proposed solution is its better agreement with the experimental and analytical results found by the other resarchers.

Abstract: Water absorption behavior and flexural strength properties of carbon fiber reinforced plastics (CFRP) under hot-wet environment were examined. Those of epoxy resin were also examined for reference. Weight gains of CFRP and epoxy resin were measured after immersion in distilled water at temperatures under 90°C. Quasi-static flexural tests of CFRP and epoxy resin were conducted after immersion for 180 days. Weight gains of CFRP and epoxy resin increased with increasing water temperature. After immersion for 180 days at 90°C, weight gain of CFRP became 3.3times higher and that of epoxy resin was 2.3 times higher than that at RT, respectively. When CFRP and epoxy resin were immersed in distilled water at 90°C, weight gains of CFRP and epoxy resin increased and then decreased. Flexural strengths of CFRP and epoxy resin decreased in distilled water at temperatures less than 90°C. Flexural strengths of dried CFRP and epoxy resin after immersion recovered but were lower than that of virgin CFRP and epoxy resin. Debonding of fiber/resin interface and crack initiation in epoxy resin in distilled water resulted in the strength reduction.

Abstract: The paper discusses the compressive behaviour of two materials, the conventional aluminium honeycomb and the new titanium alloy micro lattice blocks. The new titanium alloy micro lattice structure is being developed as core material candidate in sandwich construction for aerospace application. Experimental tests have been done on the blocks in order to compare its property with the aluminium honeycomb. Compression strength as well as compressive behaviour of both materials are compared and observed. The mechanisms that contributed to the differences in their performance are discussed and this will be used to improve the geometrical and structural design of micro lattice structure in order to achieve properties that are superior or at least comparable with that of aluminium honeycomb.

Abstract: From micro perspective, crumb rubber concrete (CRC) is viewed as a composite consisting of mortar matrix, aggregates and rubber particle. In this paper, based on random aggregate model with different aggregate shape in planar, the mechanical properties of CRC using linear Mohr-coulomb constitutive relation are studied by nonlinear finite element method under uniaxial compression on mesoscopic. The number of random aggregates is calculated in two-dimension by Walraven formula. Circular random aggregate model, elliptic random aggregate model and polygonal random aggregate model are established. Stress-strain curves under varieties of conditions are derived and compared with the test results. The results show that the simulative stress-strain curve fit the reality very much. In the numerical analysis, the aggregate shape has little effect on the mechanical properties of CRC.

Abstract: This paper presents the optimisation of real-time performance of the genetic algorithm clustering method. This performance optimisation concerns the population diversity and limitation and is based on actual runtime of the algorithm. A real-time ticker is incorporated into the algorithm for actual runtime measurement. For population diversity and limitation, a controlled k-means analysis is performed on the population of solutions to determine its diversity. Achieving a less diverse population in less amount of time without sacrificing the accuracy of the algorithm will help reduce the time-complexity of the algorithm, thus opening up the potential for the algorithm to cluster data in higher dimensions. Results from this study will be used for improving the method of clustering fatigue damage features of automotive components using genetic algorithm based methods.