Key Engineering Materials Vols. 417-418

Abstract: Interactions between adjacent, insufficiently separated buildings have been repeatedly observed during major earthquakes. This phenomenon, known as the earthquake-induced structural pounding, may be the reason of local damage at the contact points as well as may lead to the extensive damage at the base of the colliding structure or even initiate its total collapse. In this paper, we examine the importance of inelastic modelling of structural behaviour as the result of damage due to earthquake excitation and structural pounding. The study concerns two adjacent four-storey buildings with different dynamic properties. In the numerical simulations, the nonlinear viscoelastic model is used to model the pounding force during collisions at different storey levels of the structures. The model allows us to take into consideration the dissipation of energy due to damage taking place at the time of collision. Three different ground motion records with different peak ground acceleration levels are used in the study. The comparison between elastic and inelastic damage-involved structural behaviour is investigated. The results of the study show significant changes in the dynamic responses of the inelastic systems as compared to those of elastic ones. The results clearly indicate that modelling the colliding buildings to behave inelastically is really essential in order to obtain accurate damage-involved structural response under earthquake excitation.

Abstract: High speed MODE I crack growth in elastic-plastic materials, involving large scale plasticity and dynamic effects connected to rapid propagation, is faced through a cohesive model to tune force nodal release. The stress resisting to the opening of the edges in the cohesive zone should account of effective stress field ahead crack tip. In this paper a reference value is accounted: it represents the maximum closing stress measured at the crack tip, where the cohesive effects begin. A bi-parametric analytical formulation of stress distribution ahead the crack tip is suggested. The bi-parametric formulation is able to extrapolate the stress at the tip whatever is the T-stress (i.e. the stress acting in the direction of fracture propagation), thus completely defining the cohesive loads.

Abstract: From the emission of dislocations till short crack propagation fatigue is a local process determined by the microstructure. In this paper we present experiments based on refined applications of the scanning electron microscope and focused ion beam technique, which give detailed information about crack initiation and the interaction of short fatigue cracks with microstructural elements.

Abstract: A crack problem in a micropolar piezoelectric solid is considered. By using simplified constitutive relations, the problem can be reduced to the solution of a set of Cauchy singular integral equations with the help of Fourier integral transform technique. Numerical results for stress intensity factors, couple stress intensity factors and electric displacement intensity factors show that micropolar theory can be expected to explain certain size effects in piezoelectric solids.

Abstract: In the field of cast metals, fatigue strength assessment necessarily deals with the strength reduction caused by randomly distributed small-sized defects. When components are taken into account, stress raisers due to component geometry are possible crack initiation sites as well as defects. For the investigation of the overall performance, an appropriate model considering notch sensitivity of parent material and defect behavior shall be used. Unfortunately, several mechanical models of material sensitivity to notches and defects require the knowledge of the un-defected material strength, which is not obtainable in this case. In this contribution, experimental fatigue tests have been carried out on plane and notched specimens, considering small size notches as well. An appropriate analysis of the generated results allowed the evaluation of the material behavior and its notch sensitivity according to critical distances theories. The obtained results have been confirmed also direct inspection of broken samples.

Abstract: Joining timber structural elements using mechanical fasteners goes against the anisotropic and fibrous nature of the material. Adhesive bonding is by far better adapted, since it permits a smoother load transfer. However, the strength prediction of adhesively bonded wooden joints is difficult brittle nature of the adherends, the complex stress distribution as well as the uncertainties regarding the associated material resistance. As a contribution to help close this research gap, the authors have carried out experimental and analytical investigations on adhesively bonded double lap joints composed of timber. This paper describes the experimental and numerical results and suggests a probabilistic method for the strength prediction of joints composed of brittle adherends and adhesives. The method considers the scale sensitivity of material strength modelled using a Weibull statistical function, and considers both the statistical variation and the size effect in the strength of the material. The probabilistic method presents a mechanical explanation for the increased resistance of local zones subjected to high strain or stress peaks.

Abstract: A NFE model is constructed to analyze the heating steady thermal stress in a ceramic/FGM/metal composite EFBF plate considered temperature dependency. From numerical calculation, when T0=Ta=300K and Tb=1 000K, the stress distributions in the plate were obtained. The results are as follows. With the increase of the FGM thickness, the stress distribution is more reasonable, and the largest tensile stress reduces by 45.64%. With the increase of M, the stress change increases obviously, and the compressive stress on the surface of ceramics reduces by 56.0%. Compared with A=0, the compressive stress of A=3.99 on the surface of metal increases by 94.2%, and the stress on the surface of ceramics changes from compressive stress to tensile stress. When T0=300K, Ta=700K, compared with Tb=1 050K, when Tb=1 800K, the compressive stress on the surface of metal increases 13.62 times, and the maximum compressive stress on the surface of ceramics increases 5.22 times. Compared with the two-layered ceramic/metal composite plate, the stress of ceramic/FGM/metal composite EFBF plate is very gentle, and the maximum tensile stress reduces by 44.2%. Compared with constant material properties, the maximum compressive stress on the surface of metal considered temperature dependency reduces by 59.1%. The results provide the foundations of theoretical calculation for the design and application of the composite plate.

Abstract: In view of the randomness in terms of high arch dam load, resistance and failure calamity loss as well as the fuzziness in terms of evaluation conclusion, a high arch dam risk evaluation system is established by means of risk analysis method. Natural factors, structural factors and human factors that lead to high arch dam failure are summed up on the basis of statistics. Through qualitative analysis coupled with quantitative estimation, it is determined that high arch dams generally involve five major failure modes: abutment rock mass destabilization, excess cracking, arch dam & dam foundation entire destabilization, extreme dam-overflow and destabilization of dam body along base plane. The state functions of individual major failure modes are established. An approach is made to the correlativity among the major failure modes and among the random variables within individual failure modes, and it is suggested that risk rate, economic loss risk value and life loss risk value should be used to assess the risk of high arch dams. A certain high arch dam abutment instability risk evaluation has been provided.

Abstract: Hot section parts of combined cycle gas turbines are susceptible to degradation due to high temperature creep, crack formation by thermal stress, and high temperature oxidation, etc. Thus, regularly repairing or replacing the hot section parts such as gas turbine blades is inevitable. For this purpose, revolutionary and advanced repair technologies for gas turbines have been developed to enhance reliability of the repaired parts and reduce the maintenance cost of the gas turbines. The cold spraying process, which has been studied as not only a new coating technology but also as a process for obtaining a thick deposition layer, is proposed as a potential repairing solution. The process results in little or no oxidation of the spray materials, so the surfaces stay clean, which in turn enables superior bonding. Since the operating temperature is relatively low, the particles do not melt and the shrinkage on cooling is very low. In this study, the cold spraying conditions were optimized by taking into account the particle kinetic energy and the rebound energy for application in repairing gas turbine blades. A high quality cold-sprayed layer is that which has lowest porosity; thus the spraying parameters were optimized to achieve low-porosity layer, which was verified by scanning electron microscopy (SEM).

Abstract: Different amounts of fiber added samples were prepared by standard ceramic processing routes and sintered at different temperatures. Although powder packing characteristics of the matrix material were negatively affected with increasing fiber content; certain improvements were observed for the density, MOR and water absorption values both for green and sintered states. Fracture surfaces of the samples after three-point bending test were investigated via detailed SEM observations and phase analyses were performed by XRD measurements. It is found that phase transformation controlled fiber-matrix integration starts with increasing sintering temperature and degree of bonding between fiber/matrix interfaces can be arranged by selecting optimum sintering temperature. Aluminosilicate fiber addition was found efficient for improving mechanical properties of clay-kaolin matrix and the mechanism of the improvement can be grouped into two categories i.e. (1) brittle fiber – brittle matrix interactions via well known pulled-out, crack deflection and bridging mechanisms prior to fiber-matrix integration (2) further densification via phase transformation controlled fiber-matrix integration after high sintering temperatures.