Key Engineering Materials Vols. 348-349

Abstract: The Thixomolding® process has a 20-year track record of research with commercial applications beginning in 1992. The apparatus machine is similar to those used for an injection plastic molding: it transforms in a single step magnesium chips heated to a semisolid slurry (inside a barrel and screw) into precision molded parts. Originally employed by Asian manufacturers for consumer electronics and consumer hardware applications (such as notebook computer cases, digital camera bodies and chain saw housings, etc.), the industrial uses of Thixomolding® has been widely extending from last years to other parts suppliers in the Americas, Europe and Asia. Nowadays thixomolded parts there are in automotive and sport equipment as automotive shift cams, bike saddle, fishing reels, snowboard bindings, etc. Thanks to the semisolid and thixotropic state of magnesium, the slurry moves in laminar flow inside the mould. The absence (or very low) of turbulence allows to reduce drastically (and often to eliminate) residual porosity which is typical of die-casting. In this work a thixomolded AZ91D has been characterized by tensile and axial fatigue testing (fatigue ratio R=0.1) both in the as-supplied condition and after a Tx heat treatment.

Abstract: This paper investigates application of slow strain rate tensile (SSRT) testing as a laboratory method to characterize stress corrosion cracking (SCC) susceptibility of type 40Cr steel in the acidic chloride solution. SSRT testing was carried out at room temperature in acidic chloride solution and seawater at different strain rates (3.82×10-7 to 1.41×10-6 s-1)，respectively. The results show that the type 40Cr steel is susceptible to stress corrosion cracking in a certain strain rates (around 1.41×10-6 s-1) in the acidic chloride solution. In tests performed at this strain rate (1.41×10-6 s-1), while the specimens are not susceptible to SCC in seawater. Typical intergranular attack is seen on the fracture surface of the former that had failed due to SCC, and the latter had underwent considerable necking before fracture, also represented by high reduction of area (RoA) and fractographic feature of dimples consistent with ductile failure. Additionally, the results are consistent with the data of electrochemical noise (EN) that is generated of type 40Cr steel exposed to the acidic chloride solution and seawater. This study also supports SSRT and EN technique as an effective testing technique for SCC studies.

Abstract: Description of shear stress-strain relationship for soil-concrete interface during shear fracture process plays an important role in experimental and numerical studies of soil-structure interaction. In this paper, deficiency of traditional hyperbolic model for the shear stress-strain relationship is analyzed, firstly. Then, a new model with 3 parameters for it is established, which can overcome the deficiency of hyperbolic model. Finally, good agreements have been found between the proposed model and laboratory tests.

Abstract: Recycled asphalt mixture is a multiphase system, which consists of sized recycled asphalt pavement (RAP), new asphalt, and/or recycling agents and new aggregates. RAP percentages may have influences on performance of total recycled asphalt mixture. The performance of asphalt mixture is significantly affected by its dynamic response and fatigue performance (crack formation and propagation). Dynamic and fatigue properties of virgin and recycled asphalt mixture are investigated based on Indirect Tensile Test (ITT). It is demonstrated in test results that recycled asphalt mixture is stiffer and more elastic-like than virgin ones with RAP added. The number of load cycles to fatigue failure expected of recycled asphalt mixture is a bit decrease compared with that of virgin ones in low stress level. In addition, high fatigue resistance of recycled asphalt mixture is observed in this test when stress level is high.

Abstract: Damage behaviors of plain concrete (PC), steel fiber reinforced concrete (SFRC), steel fiber reinforced and polymer modified concrete (SFRPMC) are studied in this paper by use of a Split Hopkinson Pressure Bar (SHPB). Three kinds of concrete materials appear obvious strain rate strengthening effects. SFRPMC appears a better resistance and energy absorption ability. A rate-dependent damage model is suggested to depict the impact damage evolution of three kinds of materials under different impact velocities. The simulation results showed the theoretical model could well describe the dynamic behaviors of the three kinds of materials, and steel fibers attribute more to resist crack develop in early stage, “bridge effect” of steel fibers slow up the damage evolution in SFRC, with the addition of polymer, the internal structures of SFRPMC were modified, SFRPMC gains better ductility, and appears a kind of “softening effect”, which makes the damage in SFRPMC develop more slowly than that in PC and SFRC under impact loading.

Abstract: Crack detection of critical beam structures such as bridges and aircraft wings by vibration monitoring is based on understanding how a crack affects the vibration characteristics of a beam structure. Transfer matrix method is a convenient, effective, and hence widely used approach to beam vibration analysis, but a crack in the beam makes this method ineffective. This paper proposes an open crack model that simulates the local stiffness reduction effect of a transverse crack by a rectangular slot to make the transfer matrix method able to analyze vibrations of a cracked beam. The depth of the slot is identical to the depth of the crack, and the equivalent width of the slot is obtained by comparison of stiffness reductions of finite element analysis results and the counterpart transfer matrix method results. Different dimensions of rectangular beams, different crack positions and loading conditions are considered and statistic method is used to improve the generality and accuracy of the model. A calculation example of a cracked cantilever beam is given and the validity of the proposed model is verified with available results of existing models.

Abstract: In the assessment of fatigue life of tubular joints in offshore engineering, hot spot stress range, in conventional method, is frequently used to predict the number of cycles that a tubular joint can sustain before failure from corresponding S-N curves. This method only considers the effect of the peak stress at the weld toe on the fatigue life of a tubular joint, but ignores that of the stress distribution along the weld toe. The effect of the stress distribution along the weld toe on the fatigue life of tubular joints can be evaluated indirectly by analyzing a fracture mechanics parameter, namely stress intensity factor. The stress intensity factor values of the surface cracks in the fatigue failure process for tubular joints subjected to different loading types, thus causing the difference of the stress distributions along the weld toe, are investigated from both numerical analysis and experimental measurement. The results show that the stress distribution along the weld toe has remarkable effect on the fatigue life of tubular joints.

Abstract: The objective of this study is to formulate and evaluate a new training algorithm of Neural Network to predict the inelastic shortening of reinforced concrete members using the column shortening data of high-rise buildings. The new training algorithm of Neural Network for the prediction of column shortening focuses on component of input data and training methods. The validity is examined by training and prediction process based on column shortening measuring data of high-rise buildings. The polynomial fit line of measuring data is used as the training data instead of measuring data. The result shows that the new Neural Network algorithm proposed in this study successfully predicts column shortening of high-rise buildings.

Abstract: Dental implants have been extensively used in prosthetic dentistry over the last two decades. Clinical experience shows that the healing and osseointegration process can heavily influence the success of the implantation. It is critical to understand the damage extent in different time frames. This paper aims at exploring the mechanical damage of dental implantation over the healing process. The 3D finite element analysis (FEA) models were developed based on computerised tomography (CT) scan technology to investigate the load-induced damage of interfacial osseointegration, as well as cortical and cancellous bone tissues. Unlike the existing linear finite element (FE) stress analysis, this study takes into account the damage accumulation and micro-crack nucleation under a framework of bone/interface remodelling. This study reveals the damage in the surrounding bone tissues and bone-implant interfaces at different stages of the healing process, and consequently premature load tolerances are suggested.

Abstract: Detection and identification of structural damage at the earliest possible stage is a vital part of the monitoring and servicing of structural during their lifetime. In this paper, damage monitoring is studied on some kinds of structures use wavelet analysis, such as plate, and framework. The research shows this damage detection technique is made to determine the damage occurring time, positions and degrees and to realize an early discovery and diagnosis on the damage, which offers a basis for realizing lifespan prediction of the structure and ensuring safety of engineering structure.