STUDY ON THE EXPERIMENTAL TECHNIQUES OF DYNAMIC FRACTURE TOUGHNESS OF MATERIALS BASED ON INSTRUMENTED CHARPY IMPACT TEST

A measuring method was proposed to evaluate the dynamic fracture toughness of materials using pre-cracked Charpy impact specimens in this work. Different from the previous methods,a simple way to calculate the dynamic stress intensity factors of three-point bending specimen was developed based on the calculation of fracture specimen’s stiffness using Timoshenko beam theory. The fracture initiation time was detected experimentally and then used to determine the dynamic fracture toughness of materials. The calculated values were compared with the results obtained from finite element simulation,a good agreement was found from the comparison between the theoretical calculation and FEA,indicating that the current method proposed in this study is accurate,reliable and convenient for engineering application

Microstructure and Mechanical Properties of Low-Carbon High-Strength Steel Fabricated by Wire and Arc Additive Manufacturing

Wire and arc additive manufacturing (WAAM) is a novel technique for fabricating large and complex components applied in the manufacturing industry. In this study, a low-carbon high-strength steel component deposited by WAAM for use in ship building was obtained. Its microstructure and mechanical properties as well as fracture mechanisms were investigated. The results showed that the microstructure consisted of an equiaxed zone, columnar zone, and inter-layer zone, while the phases formed in different parts of the deposited component were different due to various thermal cycles and cooling rates. The microhardness of the bottom and top varied from 290 HV to 260 HV, caused by temperature gradients and an inhomogeneous microstructure. Additionally, the tensile properties in transversal and longitudinal orientations show anisotropy characteristics, which was further investigated using a digital image correlation (DIC) method. This experimental fact indicated that the longitudinal tensile property has an inferior performance and tends to cause stress concentrations in the inter-layer areas due to the inclusion of more inter-layer zones. Furthermore, electron backscattered diffraction (EBSD) was applied to analyze the difference in Taylor factor between the inter-layer area and deposited area. The standard deviation of the Taylor factor in the inter-layer area was determined to be 0.907, which was larger than that in the deposited area (0.865), indicating nonuniform deformation and local stress concentration occurred in inter-layer area. Finally, as observed from the fracture morphology on the fractured surface of the sample, anisotropy was also approved by the comparison of the transversal and longitudinal tensile specimens

Investigation into dynamic response of a three-point bend specimen in a Hopkinson bar loaded fracture test using numerical methods

Dynamic fracture toughness of engineering materials at loading rates greater than 10 6 MPa m / s is widely investigated using the modified Hopkinson pressure bar apparatus. For accurate measurement of dynamic fracture toughness, it is essential to thoroughly understand the dynamic effects excited by the stress wave, such as stress wave propagation characteristics in bars/cracked specimen, the contact situation between the specimen and loading point or supports, and the dynamic response of the fracture specimen. In this work, full transient dynamic analysis techniques are used to comprehend “loss of contact” situation of cracked fracture specimen with an incident bar (impactor) and a transmission bar (supports) in a Hopkinson bar loaded two-bar/three-point bend test. A modified Hopkinson bar loaded experimental setup, including striker, incident, and transmission bars and three-point bend fracture specimen, is modeled using the commercial software ANSYS. The dynamic responses of the specimens made of titanium alloy, high-strength steel, and aluminum alloy are analyzed, and the specimen contact states with the incident and transmission bars are investigated using stress state contours of the specimen along with nodal displacement of the specimen and the bars. The dynamic fracture toughness values for the three specimens are also calculated and compared with the experimental results. The simulation results from the current two-bar/three-point bend test indicated that no “loss of contact” occurs during the first load duration as is previously proved experimentally

Research progress in metal foam composites

Metal foam composite is a kind of lightweight composite with low density, high strength, high shielding performance, high damping performance and other characteristics. It has a wide range of application prospects in aerospace, drilling trap floats, artificial bone and other fields, which has attracted people's attention. In this paper, based on the research of the existing literature, the fabrication methods of metal foam composites were introduced, the effect of microstructure on the properties of metal foam composites was analyzed, the progress of mechanical properties, damping properties, shielding properties and heat insulation and their mechanisms of metal foam composites and their applications in relevant fields were reviewed, which provides a theoretical basis for the development of metal foam composites in the future, and the new fabrication technology, modeling research, sandwich structure of metal foam composites and the fabrication of high performance foam hollow sphere composite were also prospected

Construction of a Modified Clip Cage and Its Effects on the Life-History Parameters of <i>Sitobion avenae</i> (Fabricius) and Defense Responses of <i>Triticum aestivum</i>

Clip cages are commonly used to confine aphids or other small insects to a single leaf when conducting plant–small insect interaction studies; however, clip cages are usually heavy or do not efficiently transmit light, which has an impact on leaf physiology, limiting their application. Here, simple, lightweight, and transparent modified clip cages were constructed using punched clear plastic cups, cut transparent polyvinyl chloride sheets, nylon organdy mesh, and bent duck-bill clips. These cages can be clipped directly onto dicot leaves or attached to monocot leaves with bamboo skewers and elastic bands. The weight, production time, and aphid escape rates of the modified clip cages were 3.895 ± 0.004 g, less than 3 min, and 2.154 ± 0.323%, respectively. The effects of the modified clip cage on the growth, development, and reproduction of the English grain aphid (Sitobion avenae Fabricius) in comparison with the whole cage were studied. The biochemical responses of wheat (Triticum aestivum) to the cages were also investigated. No significant differences were observed in the life table parameters, nymph mortality, and adult fecundity in S. avenae confined to clip cages and whole cages, but the clip cages were more time efficient than whole cages when conducting life table studies. Moreover, the hydrogen peroxide accumulation, callose deposition, and cell necrosis in wheat leaves covered by empty clip cages and empty whole cages were similar, and significantly lower than treatments where the aphids were inside the clip cage. The results demonstrate that the modified clip cages had negligible effects on the plant and aphid physiology, suggesting that they are effective for studying plant–small insect interactions

Effect of extraction methods on the preparation of electrospun/electrosprayed microstructures of tilapia skin collagen

Collagen plays a pivotal role in human physiological functions and extracted collagen has multiple potential applications. Tilapia skin can be applied to extract collagen for maximizing the profit of tilapia processing. Electrospinning/electrospraying is novel micro- and nano-techniques to fabricate microfibers and microspheres in a simple and easy way. In this work, we extract collagens from tilapia skin by three types of extraction methods: acetic acid method, hot water method, and sodium hydroxide method. Then, these extracted collagens are characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Fourier transform infrared spectrometry. These extracted collagens have different molecular weights and different protein secondary structures. Finally, these extracted collagens are applied to fabricate electrosprayed microspheres, electrospun microfibers, and mixed microspheres/microfibers with multiple potential applications by adjusting the collagen concentrations. Higher polymer molecular weight only needs lower concentration to produce microfibers. The microfiber diameter increases with the increase of collagen concentration. This work proves that extraction methods have obvious effect on the preparation of electrospun/electrosprayed microstructures of tilapia skin collagen and provide a way to maximize resource utilization of tilapia processing waste