Key Engineering Materials Vols. 480-481

Abstract: The integration of network education technology and mathematics curriculum has become the hot issue of mathematics teaching reform in higher vocational education institutions. The paper has emphatically introduced the construction situation and implementation process of network education in advanced mathematics. Through investigation and statistical analysis on network education teaching effect, it verified that network education has the superiority that the conventional education cannot match. However, each teaching model has its own advantages and disadvantages. Finally, based on the teaching practice, the paper made some suggestions and countermeasures on further development of network education, and provided the valuable teaching experimental basis for the mathematics educators.

Abstract: In a variety of materials, the ultrasonic propagation velocity is different. Using ultrasonic reflection principle in the material, the measuring instrument detected the thickness of the measured object. AT89C51 microcontroller as the system core, with ultrasonic probe, transmitter circuit, receiver circuit, counter circuit, LCD display circuit together, completed the measurement of materials thickness. The measurement range was up to a 1.2 ~ 200mm. Practice shows that the instrument is low cost, high speed, high precision, reliability, adaptability, etc. So it has broad application prospects.

Abstract: In this paper, we use acoustic emission (AE) system to collect the AE signals and analyze the damage evolution during the monotonic compression test. Based on the experimental correlation diagram of the load and characters of the acoustic emission, the reference load of failure was found. The experimental results also revealed the characters of the source of the acoustic emission signals after the wavelet packet decomposition and frequency spectrum analysis. The frequency range of the matrix cracking is on the range of 125~187.5 kHz, while the frequency range of layer debonding is wide, it is not just on the low-frequency range but on the high-frequency range. The frequency of fiber breakage is on the high frequency range, nearly on the range of 375~437.5 kHz.

Abstract: The purpose of this study was to investigate the effect of operating conditions on nugget diameter and joint strength of KOVAR alloy and SPCC steel in projection welding of dissimilar metals. All projection welds were produced with a capacitor charging power supply. Harris and Riley type projection was embossed on SPCC steel sheet. The nugget diameter was estimated using a peel test. The lap-joint strength of projection welds was evaluated using a tensile-shear test. The results indicated that the nugget diameter and joint strength increase as the weld current increase. Increasing the electrode force will produce a small nugget diameter. The nugget diameter of projection welds is a good indicator of the lap-joint strength.

Abstract: Aluminum alloy 7A04 compressed at high deformation temperature and large deformation is applied in two different heat treatment (T5 and T6), then microstructure and properties of the alloy after heat treatment are investigated. The mechanical properties are studied by means of the tensile testing and the hardness testing. The microstructure characteristics and the fractorgraphy analysis are respectively investigated with optical microscopy and SEM. The experimental results indicate that after T6 heat treatment, the second-phase particles dispersed in the matrix, which can efficiently improving the strength of the alloy, but reduced the toughness. After T5 heat treatment, the coarse and discontinuous second-phase is distributed along the grain boundary, which can lead to the low strength and high toughness because of the lack of the strengthening phase in the grains. The fracture appearance is intercrystalline fracture after T6 heat treatment, and dimple transgranular fracture after T5 heat treatment.

Abstract: With scanning Kelvin probe and microscopy technique, the degradation process of the micro-arc oxidation (MAO) treated magnesium alloy was investigated under a 3% NaCl droplet. Because of the unique two-layer structure of the MAO specimen, the droplet penetrated in and spread through the external loose layer during the initial several minutes. Subsequently, with the increase of time, only a thin layer of electrolyte was left on the surface of the MAO specimen. During the entire corrosion process, the potential exhibited a valley type distribution, only the width of potential valley increased with time. Based on above observations, a model was proposed for describing the contamination mechanism of MAO treated magnesium alloy under droplet.

Abstract: The distributions of corrosion potential and galvanic current of 304 stainless steel under a NaCl droplet were studied by using the wire beam electrode (WBE). It was found that the distributions of the electrochemical parameters were heterogeneous with isolated anodic and cathodic zones appeared randomly. During the corrosion process, the polarity of some anodes changed with the evolution of time. The localized corrosion rate and heterogeneity increased firstly, and then decreased afterward with the increase of time, which can be attributed to the cooperative effects of the aggressive ions and the corrosion products.

Abstract: Flexible composites with high performance are widely used in geotextiles and some other fields. In this research work, the author studied the tensile behaviors of four neat woven fabrics and two coated woven fabrics. The comparison of the tensile strength between the uncoated and coated woven fabric revealed the effect of coating on the tensile strength of flexible composites which will be beneficial for the design and manufacture of flexible composite with high quality.

Abstract: Yarns with high performance are more and more widely used in geotextiles. In this paper, the author researched the effect of twist on the tensile behaviors of high performance yarns. Due to the superior tensile strength of these yarns, the tensile tests can’t be operated on the normal textile yarns testing machine and all yarns were tested on the fabric strength tester. The results reveal that the tensile strength of yarns will firstly increase, and then decrease with the increasing twist. The simple analyses were also proposed to demonstrate this phenomenon.

Abstract: This work conducted a non-linear finite element model associated with arc efficiency to simulate the temperature distribution and residual stress. A three-dimensional finite element analysis of temperature and stress in butt-joint TIG welds was performed using commercial software ANSYS. This model includes adjusting Gaussian distribution heat flux, alternating temperature dependent material properties, and managing thermal elasto-plastic material behavior. Computational results for both the temperature distribution and the residual stress are compared with available experimental data to confirm the accuracy of this technique. The simulated results of temperature distribution and residual stress are in good agreement with corresponding experimental data. The greatest value of this work does not lie in its ability to predict the magnitude and distribution of weld temperature and residual stress. Rather, this work proposed that prediction errors in a finite element model can be eliminated by modifying the arc power distribution function.