Applied Mechanics and Materials Vol. 907

Abstract: Ultra-high-strength steel (UHSS) structures are exposed to corrosive environments during service, and hydrogen-assisted cracking (HAC) may occur owing to stress corrosion cracking and hydrogen embrittlement. In this study, the HAC threshold stress intensity factor and fatigue life of UHSS steel were evaluated by applying stress in a corrosive environment to prevent structural fracture. For specimen with semicircular slits by electric discharge machining, fatigue limit was obtained by static fatigue test under corrosive environment. The fatigue limit of the crack specimen was evaluated by the fatigue limit of the experiment and HAC threshold stress intensity factor, and comparative evaluation was performed. On the surface of cracks, grain boundaries were embrittled by corrosion, and grains were clearly observed. Meanwhile, cracks in the surface direction propagated slightly, unlike cracks in the depth direction. The static fatigue limit of UHSS (SKD11:HV670) was determined to be 400 MPa, and the fatigue limit of the crack specimen could be evaluated. The experimental results agreed well with the evaluation results.

Abstract: The forged superalloy Inconel 718 and cast superalloy Mar-M247 were joined via friction welding. The welded zone was recrystallized and formed a thin representative layer on the forged superalloy side adjacent to the interface. The effect of the creep property of the welded layer on the behavior of the joint under creep loading was investigated via numerical simulations. Despite the welded layer being relatively narrow (approximately 200 mm), it significantly effects the stress state and the creep damage parameters in and around the welded zone. When the creep rate of the welded layer is lower than that of the base alloy, the stress concentration is dramatically increases adjacent to the interface. However, the creep rate of the welded layer creep rate is higher than that of the base alloy, the interface stress concentration is reduced. The creep strain and stress exhibited an out-phase relation.

Abstract: To clarify the effect of using high-pressure coolant supply in combination with the intermittent cutting of hardened steel ASTM D2, intermittent cutting of hardened steel was performed with various sintered cubic boron nitride compact (cBN) tools. The result revealed that it was effective to use a high-pressure coolant supply method in combination for intermittent cutting of hardened steel. An effective tool material had a bonding phase of WC-Co-Al and a high content of cBN particles. Furthermore, the more suitable cutting edge shape was "a negative-land width of 0.13mm, a negative-land angle of 45°, and R honing of 0.04 mm".

Abstract: The effects of gravel’s geometry and cement content are one of the most important parameters in the construction industry. Two types of aggregates, sharp corners and rounded corners with different sizes, and several samples of various cement contents were prepared. Then the workability, strength, and elastic modulus tests were performed. Experimental results indicate that workability and compressive strength increase with increasing cement content as expected (12% and 22% for workability and 64% and 69% for compressive strength with regard to sharp and rounded aggregate, respectively). Also, in most cases, seismic parameters improve too (maximum of 120% and 107% for energy absorption regarding sharp and rounded aggregate, respectively). The results demonstrate that concrete samples with the largest grain size of 12.5 mm have the most compressive strength, elastic modulus, energy absorption, and ductility in comparison with other samples (maximum 24% and 18% for elastic modulus with reference to sharp and rounded aggregate, respectively). Finally, the results show that the sharp corner aggregates have greater strength and better seismic parameters in comparison with rounded corner (maximum 16% for energy absorption). In contradiction to the above statement, the rounded corners aggregates have more workability than sharp corner ones (up to 18%), and also the workability increases by reducing the gravel size (26% and 19% for sharp and rounded aggregate, respectively). A statistical approach was used to determine the optimal shape and size of aggregate and organize the results scattering.

Abstract: Many building members such as walls, columns, floor contains various components and materials that should be designed to meet their legal obligations outlined in the building regulations especially for fire safety requirements. The performance of those materials under fire-exposure conditions is of major importance in securing safe constructions. To evaluate the conformity of these structural elements, it is necessary to determine the fire-resistance rating as specified according to a wide range of European, American and other test standards. A comparative study was conducted between NFPA 251 and EN 13381-8 to investigate the contribution of the difference in standard specification conditions in affecting the measured fire rating time. For this purpose, six steel columns (of IPE 360 designation) were coated with intumescent paint at the same dry film thickness (DFT) to evaluate the fire resistance rating considering the conditions that listed in both NFPA 251 and EN13381-8. The results showed that the furnace time temperature curves for both standards were functionally equivalent since the standard deviations don’t exceed 5%. Variations in the fire ratings (representing 9% up to 20% of the test time) attributed to the increased temperature sensing locations which provide comparable readings with lesser deviations/ scattering.

Abstract: This paper presents the development of a three degrees-of-freedom (DOF) compliant parallel mechanism (CPM) with spatial motions, i.e., two rotations about the X and Y axes and one translation along the Z axis (θX-θY-Z). Such CPM is synthesized by the improved beam-based structural optimization method. The obtained results suggest that the proposed CPM is able to produce totally decoupled motions illustrated by a diagonal stiffness/compliance matrix, a large workspace of ±22,5 degrees × ±22,5 degrees × ±9,6 mm, fast dynamic response with the first natural frequency of ~100 Hz and high stiffness ratios between actuating and non-actuating directions (with stiffness ratios of 6210 and 2706 for translations and rotations respectively). Finite element analysis (FEA) is employed to evaluate the actual performance of the synthesized CPM with Ti6Al4V material in order to verify the correctness of the synthesis method. The effectiveness of the improved beam-based structural method is demonstrated by the good agreement between the simulation and predicted data with the highest deviations are 8.8% and 6.7% for the stiffness and dynamic properties respectively. In addition, some comparisons are carried out to investigate the advantages as well as disadvantages of the proposed CPM and existing designs. The comparison results show that the 3-DOF CPM presented in this paper has many merits compared to its counterparts. It can be used in various applications, e.g., the micro/nano positioners and alignment systems in precision engineering field due to its good mechanical properties (high stiffness ratios and fast dynamic behavior), large work range and fully-decoupled motion characteristic.

Abstract: Multi-pass solar air heater is attributed to the increase in efficiency due to reduce top heat loss. In this paper, a quadruple-pass solar air heater equipped with longitudinal fins on both sides of an absorber plate was investigated for efficiencies and axial temperature distribution. A mathematical model is formulated in form of ordinary differential equation (ODE) from eight heat transfer equations to solve for four local temperatures of airflow and four local temperatures of surfaces (two glass covers and two absorber plates). ODEs are solved by numerical integration and validated by the comparison with the published data. The current approach is conducted since an analytical model and parametric study on quadruple-pass solar air heater has not been found in the open literature. Among the fin parameters including thickness, quantity, and height, the fin height has a great influence on thermal efficiency. Thermal efficiency can reach 65.7% at maximum fin height. Reynolds number of 5500 achieves maximum effective efficiency of 63.5%. When the Reynolds number is large, heat transfer in the fourth pass is poor due to the sharp drop in surface temperature at high airflow rate.

Abstract: In the field of food cooling and freezing computation, the accurate determination of thermo-physical properties such as specific heat, thermal conductivity and density is necessary and specially important to optimize the energy consumption of equipment. These parameters depend on the food composition and temperature usually determined by theoretical or experimental method or the combination of both. Experimental method using differential scanning calorimetry (DSC) is the best approach to yield correct results. However, DSC method requires expensive equipment and it might be not suitable for limited funding in some Vietnamese research centers. Therefore, the line heat source was chosen in this study.This paper introduces a new approach to determine thermal properties of the Vietnamese Pangasius in the temperature range from-40 °C to +40 °C using the line heat source. The results indicated that there is good correllation between the obtained measured data and the results calculated using theoretical models as well as other published experimental results.

Abstract: Currently, the difficulty of delivering packages, goods in transportation is a very concerning issue in both urban and suburban areas when traffic congestion is increasing significantly. Therefore, the purpose of this study is to design an unmanned aerial vehicle using quadrotor to transport goods such as food, home appliances or drugs, and blood to customers. This paper presents the sizing algorithm applying blade element momentum theory to determine necessary specifications for the electric propulsion system of multicopters. These results are then used to design the appropriate configuration that can handle the mission profile. Finally, analyzing and evaluating structural strength and failure of the designed delivery drone by applying the Tsai-Wu criterion.