Key Engineering Materials Vols. 452-453

Abstract: In this study, the photoelastic experiment hybrid method was introduced and applied to the fracture problems of the isotropic polycarbonate plate with a central crack under the uniaxial and equibiaxial tensile load. The influences of equibiaxial tensile load on the isochromatic fringes and stress fields, stress intensity factors near the mixed mode crack-tip were investigated. As the results, without relation to the inclined angle of crack, the asymmetric isochromatic fringes around the crack propagation line under uniaxial tensile load has become symmetric and the slope of isochromatic fringe loop has inclined toward crack surface when an equal lateral tensile load was added. Furthermore, the distribution of all stress components have changed from asymmetric shape to symmetric shape, and only the range of compressive stress of σχ/σ0 have changed as compared with the mode I condition under unaxial load with β = 0°. When an equal lateral tensile load was added to uniaxial load, the value of stress intensity factors are little changed when β = 0° but the values of KI /K0 are increased and KII /K0 are become zero, that is, mode I situation when β = 15°~45°.

Abstract: Reinforced concrete construction is very common these days and extensively used both in industrial and commercial buildings. With the gradual rise in occurrences of fire accidents in recent years, a more thorough and quantitative understanding of the damage phenomenon in natural aggregate concrete structures is required. However, little research has been done to study natural aggregate concrete behavior under high temperatures. The mechanical behavior of concrete could actually be more complex under high temperature conditions than at room temperature, for instance. Restoration and reinforcement of the structures exposed to fire may have to be based on residual strength analysis and therefore require a correlation between temperature and mechanical properties. Thus, in order to meet the modern challenges of rapid engineering advances and societal development, further research on the concrete material and its structural behavior at high temperatures becomes extremely important. The present paper deals with investigations on the effect of high temperature exposure on the compressive strength of natural aggregate concrete. Experiments were conducted to study the compressive strength variations with increasing temperatures, up to 700 °C, and the subsequent cooling modes such as natural and spray cooling. Results show that the compressive strength gradually decreases with increasing temperatures.

Abstract: The voids in pure Aluminum always exit in the manufacturing process. The Modified Embedded Atom Method (MEAM) potential is employed in the molecular dynamics (MD) simulation at atomic scale to investigate the interaction between voids under the impact loading for pure Aluminum. The distance between the voids distributed along the loading orientation affects the failure mechanism seriously. The results show that there are 3 kinds of mechanisms with the change of the distance between voids: 1) coalescence takes place within a critical distance between voids under extra loading, 2) when the distance between voids reaches a certain value, each void cracks at 4 locations along with the slide direction <110> of face-centered cubic (fcc), respectively, 3) a stress shield zone appears when the ligament between the voids is at the size between the cases mentioned above, which brings out the phenomena that each of the voids cracks only at 2 locations, and no crack appeared at the stress shield zone.

Abstract: A method of fatigue damage analysis for sandwich panels on ship is studied in this paper. When ship is navigating on the sea, the waves that ship encounters can be regarded as a stochastic process. And responses of ship movement can also be regarded as a series of stochastic spectrums. By using of finite element method, loads spectrums of inner ship structures in different sea conditions can be obtained. Then short term or long term distribution of stress range can be determined. Residual stiffness modal is used to describe the damage of sandwich panels under fatigue loads. And fatigue damage variable D is defined based on stiffness degration. An integral equation to calculate fatigue damage under loads spectrum is constructed to describe the damage of ship sandwich structure in long term and short term stress spectrum distributions.

Abstract: Shape memory alloy (SMA) has various application fields due to its shape memory effect and super elasticity. It can also be embedded into other material to produce SMA reinforced composite (SMAC). SMAC is a good candidate to realize structural smart control. In this paper the mechanical properties of SMAC in the plane stress state is investigated based on the theory of mechanics of composite material and the constitutive model expressing the thermo-mechanical behaviors of SMA.

Abstract: In the conventional hydroxyapatite (HAp) coating, the surface of commercially pure titanium (Cp-Ti) is blasted with Al2O3 grid-blasting powders and then plasma-sprayed with HAp. To improve the adhesive strength of HAp coating, the grid-blasting with Al2O3 powders and subsequently wet-blasting by HAp/Ti mixed powders were applied on Cp-Ti substrate at ambient temperature. On the wet-blasted surface of Cp-Ti, two-layers of coating composed of HAp/Ti bond coat and HAp top coat were deposited by plasma spraying. Both types of HAp-coated specimen could survive up to 107 cycles without spallation of HAp coating at the stress amplitude of 120 MPa under four point bending fatigue test. In order to clarify mechanical failure behavior of the coatings and Ti substrate, acoustic emission (AE) signals during the entire fatigue process were observed. Relationship between AE behavior and cracking process of coated specimen was evaluated. HAp top coat with HAp/Ti bond coat strongly improved the adhesive and cohesive strength, where dense AE signals occurred at the early stage of fatigue test corresponded to plastic deformation of Ti substrate and micro-cracks in coated layers. AE signals occurred at the final stage corresponded to crack propagation in coated specimen and spallations of coated layers.

Abstract: In this contribution we present more developments in the modified discrete element approach (MDEM) which was proposed to model fractures propagation in reservoir rock during production and fluid injection. The new development in this paper includes adding plasticity which allows the material to go into plastic deformation before initiating fractures. Several numerical tests are performed which mimic the real lab tests usually perform on rock samples, this includes direct tension test with initial crack to model crack propagation, and several biaxial compression tests to model the development of shear bands.

Abstract: Presented herein is a finite element investigation into the damage mechanism of the adhesive interface of the rubber-steel bimaterial. The cohesive element layer is used at the interface to simulate the initial loading, initiation and propagation of the damage. From the simulation results, it is found that interface strength exert significant effects on the crack formation in the interface. Smaller interface strength could lead to crack initiation more easily.

Abstract: A practice for analyzing some raw data from fatigue crack growth tests, carried out on specimens obtained from a railway component (wheel), is presented. Despite the data sample is small, it allows to identify the material parameters of the “threshold” propagation model that make this latter amazingly realistic for the examined material. The proposed analysis method is based on the Evolutionary Algorithms theory and uses raw data obtained from C(T) specimens instrumented with crack gauges.

Abstract: According to the piezoelectric material damage theory and the fracture criterion based on damage theory, which has been established by Yang etc., reliability analysis is carried through about a damaged piezoelectric truss. Taken the mechanical damage extension force, electric damage extension force and cross-section area of bar element as stochastic variables, the expression of safety margins of bar element has been proposed. Based on those, the SFEM and PNET method are introduced to solve the reliability index of the structural system. The result indicates positive applied and negative applied electric field have great effect on the reliability of the system, which provides the basis to control the reliability of structural system. Finally, a calculation example is carried out under the conditions of positive applied electric field, negative applied electric field and no electric field. Meantime, the quantification results of the reliability index under three conditions are compared with each other.