Advanced Materials Research Vols. 891-892

Abstract: This study investigates mechanical and microstructural properties of P460N steel after Shielded Metal Arc Welding (SMAW). Three specimens with groove angles of 45 °, 60 ° and 75° were prepared and welded for this purpose. Tensile strength test, impact test, grain size test, macro and micro hardness test and metallographic test were performed on weld metal (WM), Heat Affected Zone (HAZ) and Base Metal (BM) and the results were compared. In the specimen with groove angle of 60°, yield strength was more than the base metal. Impact energy results in HAZ were different, so that HAZ areas in the specimens with 45° and 60° groove angles have less impact energy compared to the base metal but in the specimen with 75° groove angle, impact energy was more than the base metal. This increase can be due to formation of intermetallic compounds. After checking mechanical and microstructural properties of all the specimens, the groove angle of 60° was confirmed to be used at designing and construction of a reservoir with thickness of 14mm and working pressure of 40 bars.

Abstract: In this study, three - dimensional porous carbonate apatite (CO₃Ap) materials with the chemical compositions and structures similar to cancellous bone were produced via phosphorization of porous calcite precursor in hydrothermal condition. In order to make porous calcite precursor, negative replication of polyurethane foam that named as inverse ceramic foam method was conducted. When the polyurethane template occupied within the ceramic solid walls disappeared due to burning at high temperature, interconnected hollow pathways were produced. Polyurethane foam was used as a porogen - template firstly was coated layer by layer with synthetic resin to modify morphology and enlarge thickness of struts so as to expand porous area for satisficing cellular bioactivities. Calcium hydroxide (Ca(OH)₂) slurry was then infiltrated into resin coated-polyurethane foam. Heat treatment in atmosphere of oxygen and carbon dioxide gases was carried out to eliminate polyurethane template and induce carbonation process. Ca(OH)₂ was converted to calcite with the internal porous channel architecture simulating polyurethane foam struts network. That interconnected porous calcite was subsequently transformed to CO₃Ap with remaining the same macroporous structure through hydrothermal treatment in phosphate solution. The porous CO₃Ap materials were implanted in the tibia of Japanese male rabbits and removed after a period of 3 months. The bone formation response of the three - dimensional porous carbonate apatite in vivo has been preliminary studied using micro-computed tomography (µ-CT) scanner. The results showed that the porous implant materials have sufficient mechanical strength to provide structural support during bone remodeling and successfully bond with host bone.

Abstract: A new approach to reliability investigations of multistate complex systems with dependent components at variable operation conditions called critical infrastructures is proposed. The multi-state reliability function of the critical infrastructure system is defined and determined for an exemplary m out of l critical infrastructure. In the developed model, it is assumed that the system components have the multistate exponential reliability functions with dependent departures rates from the subsets of the reliability states. The model is adopted to reliability prediction of an oil piping transportation system operating at maritime port.

Abstract: A new approach to reliability optimization of multistate complex systems with dependent components at variable operation conditions called critical infrastructures is proposed. The reliability function of the critical infrastructure system is introduced and optimized. Further, the procedure of the critical infrastructure reliability optimization using the linear programming method is proposed and adopted to reliability optimization of an oil piping transportation system operating at maritime port.

Abstract: Recently a novel strategy to improve the fatigue resistance of precipitation hardened aluminium alloys has been proposed, which is based on dynamic precipitation in partially aged material. In this paper, the effect of mean stress and alternative temperature treatments that can enhance the high cycle fatigue resistance through the mentioned mechanism were investigated. The material used is an under-aged 2024 aluminium alloy, which showed superior fatigue life compared to the peak aged condition. The recorded behaviour was observed to be more effective at lower stress amplitudes and lower R values. As the dynamic precipitation may not be able to keep up with the damage evolution, dedicated experiments were conducted to insert periods of controlled healing in a stress free state.

Abstract: Determination of fatigue crack growth characteristics under shear-mode loading is a rather complicated problem. To increase an efficiency and precision of such testing, special specimens enabling simultaneous propagation of shear cracks under II, III and II+III loading modes started to be used rather recently. K-calibration of these specimens was performed and, after unique pre-crack and heat-treatment procedures, effective thresholds in several metallic materials could be measured. However, a description of crack growth rate in terms of appropriate fracture mechanics quantities demands a precise assessment of plastic zone size under various shear-mode loading levels. This contribution is focused on the numerical elasto-plastic analysis of stress-strain field at the crack tip in specimens made of a pure polycrystalline (ARMCO) iron. The results reveal that the small scale yielding conditions are fulfilled in the near-threshold region. Starting from ΔK values approximately two times higher than the threshold, however, the ΔK_J or ΔJ approach should already be utilized. Probably the most interesting result of the analysis lies in a simple procedure that enables us to separate individual loading components ΔK_J,II and ΔK_J,III, applied in the mixed-mode II+III part of the specimen, by comparing elasto-plastic and elastic solutions.

Abstract: The use of composite materials as a replacement for commonly used metals such as aluminium and steel are increasing in the engineering industry, particularly in the aerospace sector. The move towards light weight and high stiffness structures that have good fatigue durability and corrosion resistance has led to the rapid move from metal to composites. This change allows for further flexibility in design and fabrication of various components and joints. There are three main categories of joints used in composite materials – mechanically fastened joints, adhesively bonded joints and the combination of the two called hybrid joints. In order to adequately understand the effectiveness of these joints, substantial testing and validation is required, particularly in the use of hybrid joints for real life applications. Static testing, load distribution and parametric studies of hybrid joints have been investigated by various researchers; however further work is still required in understanding the durability and fatigue of hybrid joints and ensuring that both the adhesive and mechanical fasteners can work together effectively in producing an optimum joint. Mechanical fastening alone in composite laminates is not a preferred joining method as they create high stress concentrations around the fastener holes. Adhesive bonding although has numerous benefits it is difficult to detect the bond defect particularly in cases where weak bonds can occur during applications and it is sensitive towards the environmental conditions. Thus hybrid joints are seen arguably as being more effective in joining composite components together and offer greater residual strength. Hence the performance, strength and long-term durability of these joints need to be further investigated and be applied to practical situations whilst assisting in repair certification.

Abstract: A review of some of the various fatigue models introduced over the years for both metallic materials, in particular aluminium alloys followed by fatigue and durability concerns associated with composite materials. The move towards light weight and high stiffness structures that have good fatigue durability and corrosion resistance has led to the rapid move from metal structures to composite structures. With this brings the added concern of certifying new components as the damage mechanisms and failure modes in metals differ significantly than composite materials such as carbon fiber reinforced polymers (CFRP). The certification philosophy for composites must meet the same structural integrity, safety and durability requirements as that of metals. Hence this is where the challenge now lies. Substantial work has been conducted in the reparability of composite structures through bonding using various adherend thicknesses and joint types and has been shown to have higher durability than mechanically fastened repairs for thin adherends however these are currently unacceptable repair methods as they cannot be certified. Repairs are designed on the basis that the repair efficiency can be predicted and should be designed conservatively with respect to the various failure modes and include the surrounding structure.

Abstract: The effect of different grain sizes on the fatigue performance of high manganese TWIP steel (Twinning-Induced Plasticity) in the low-cycle fatigue regime was investigated. The average grain sizes in the fine grained condition were 2 5 μm and after heat treatment in the coarse grained condition about 80 μm were obtained. Pronounced twin-dislocation interactions especially in small grains strengthen the steel during monotonic deformation. Twin boundaries act as obstacles for dislocation slip, and thus, further reduce the effective grain size, which affects the fatigue response as well. The samples were monotonically and cyclically deformed at room temperature. The results reveal that the grain size has a significant influence on the mechanical as well as on the cyclic performance. Especially under cyclic loading differences in the resulting stress levels and cyclic stability can be observed. To clarify the microstructure evolution before and after fatigue with different constant strain amplitudes the samples were analyzed by means of transmission electron microscopy (TEM).

Abstract: A three-dimensional crystal plasticity (CP) finite element model is developed to reproduce the grain level stress concentration and deformation of polycrystalline aluminium alloy 7075 (AA7075) during fatigue experiments. The grains contained in the model possess the same size and crystallographic orientations obtained from electron back-scatter diffraction experiments. A modified CP constitutive model, which considers the backstress evolution, is employed to describe the mechanical behaviour of AA7075 under cyclic loading. A round-notched specimen from a fatigue test is simulated using the proposed CP model. Convergence studies in terms of mesh density and plastic deformation zone size are carried out to determine the appropriate conditions for the simulation. The simulation results are compared with those obtained using the elasto-plastic model and the CP model without grain morphology. The comparison indicates that with the embedded grain morphology, the proposed model can capture very well the local response induced by the microstructure features, which is vital to the accurate fatigue life prediction of aluminium alloys.