Key Engineering Materials Vol. 812

Abstract: Post impact microstructural characteristics of 7.62 armour pearcing incendiary bullet was studied on AISI 4340 multilayerd welded joints. The potential application of AISI 4340 steel is found in the construction of combat vehicles using welding process. The welded joints are expected to offer better ballistic resistance like unwelded parent metals in combat vehicles. The traditionally used austenitic stainless steel welding consumables and the transformation effects of the welding process result in inferior ballistic performance of AISI 4340 steel welded joints. Published information revealed that a few attempts were made to successfully resist the bullets at multi layered weld metals by depositing a hard-facing interlayer between traditionally used austenitic stainless steel filler metals and a austenitic stainless steel buttering layer in between the parent metal and the hard – facing interlayer. This paper reports the pre-impact and post-impact microstructural characteristics of multi layered sandwiched joints made of a austenitic stainless steel root, a chromium carbide hard – facing middle layer and a low hydrogen ferritic capping front layer. The effect of the low hydrogen ferritic front layer on the ballistic performance after impact is studied in detail.

Abstract: The fighter planes of the fifth generation, as compared to the fourth, has increased the composite materials use since high mechanical properties enhance the operational performance. However, composite materials present some negative characteristics such as damage progression, leading to delamination or matrix failure. This phenomenon often results in an unknown behaviour. This paper deals with the delamination phenomenon after casual low energy impacts. On different specimens, we performed different investigations through dynamic characterization, followed by modal parameters elaboration. After completing the first phase an opportune simulation model by FEM was developed to compare the obtained results whose values were in agreement with the real case.

Abstract: This research work presents a hierarchical method able to reconstruct the time history of the impact force on a composite wing stringer-skin panel by using the structural responses measured by a set of surface bonded ultrasonic transducers. Time reversal method was used to identify the impact location by the knowledge of structural responses recorded from a set of excitation points arbitrarily chosen on the plane of the structure. Radial basis function interpolation approach was then used to calculate the transfer function at the impact point and reconstruct the impact force history. Experimental results showed the high level of accuracy of the proposed impact force reconstruction method for a number of low-velocity impact sources and energies.

Abstract: The development of active and/or passive systems for acclimatization of soldiers’ uniforms is seen as an important tool to guarantee soldiers’ comfort and, consequently, to improve their performance during military missions. Accordingly, this work aimed at the development of an innovative acclimatization textile system, comprising both active and passive technologies that can act as temperature regulators in response to environmental conditions and soldiers’ body needs. This study presents the main results achieved on the performance assessment of the developed textile system, with emphasis on comfort assessment determined through characterization of the selected materials in terms of physical properties and functionalities in laboratory conditions, characterization of biophysical parameters of the clothing ensemble in a climatic chamber and preliminary field tests in non-controlled environments. This paper summarizes a part of the activities and results of EDA (European Defence Agency)’s ACCLITEXSYS project.

Abstract: This work compares the pure copper (T2 copper)’s stress-strain relationship at different strain rates in the uni-axial tension test and Split Hopkinson Pressure Bar (SHPB) test. Small samples were utilized in the high strain rate SHPB test in which the accuracy was modified by numerical simulation. The experimental results showed that the T2 copper’s yield strength at high strain rates largely outweighed the quasi static yield strength. The flow stress in the stress-strain curves at different strain rates appeared to be divergent and increased with the increase in strain rates, showing great strain strengthening and strain rate hardening effects. Metallographic observation showed that the microstructure of T2 copper changed from equiaxed grains to twins and the interaction between the dislocation slip zone grain boundary and twins promoted the super plasticity distortion in T2 copper.

Abstract: In this paper, the effects of temperature and strain rate on the plastic deformation behavior of 5052 aluminum alloy were investigated by quasi-static tensile test and split Hopkinson pressure bar (SHPB) experiments. Meanwhile, the stress-strain relations obtained through these experiments were employed for calibrating Johnson-Cook (J-C) plastic flow constitutive parameters of 5052 aluminum alloy. The results show that the strain rate sensitivity of 5052 aluminum alloy is insignificant in the range of 0.001s^-1~3000s^-1, while temperature has a great effect on the material plastic behavior. The experimental data are basically consistent with the predicted outcome of J-C constitutive model.

Abstract: This work compares the performance of wet gel electrodes and textile-based electrodes for monitoring the firefighters’ heart rate during on-duty missions. Both types of electrodes are connected to a customized wearable node that acquires the bio-signals and computes the heart rate. Both types of electrodes were evaluated by two subjects and in three different scenarios: sitting, standing and moving the arms, and walking. The tests have demonstrated that in static scenarios the results obtained for both type of electrodes are comparable. However, in dynamic scenarios, the performance of textile electrodes worsens, especially when the wearer is moving the arms. Although the results obtained from textile electrodes during walking are not as good as the wet get electrodes, its information is still useful for monitoring the firefighters’ heart rate.

Abstract: Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) is a material with superior mechanical strength and energy absorption capacity. The orientation of the fiber and the fiber-matrix bond relationship are important factors that affect the performance of UHPFRC. In this study, the pullout performances of hooked end fibers embedded in ultra-high performance concrete (UHPC) matrix under various inclination angles are investigated. It is shown that for the tested fiber and UHPC matrix, the optimum angle for reaching the maximum pullout energy is around 10 degrees; when the inclination angle further increases fiber rupture and matrix spalling occur more frequently. Results from this study can contribute to a better understanding and utilization of fibers effects in UHPFRC.

Abstract: Multifunctional fibrous systems were developed within this research work, giving special importance to the electrical conductivity and antibacterial activity. The functionalization of several natural fibres (jute, sisal, coir, flax and cotton) with silver nanoparticles (Ag NPs) was successfully achieved using a sustainable and eco-friendly method, namely polyethylene glycol (PEG) reduction. FESEM images, GSDR and ATR-FTIR analysis show that the Ag NPs were incorporated onto the fibres surface. The resistivity values obtained by analysing the fabrics without functionalization was about 1.5x10⁷ Ω.m while with the Ag NPs functionalization the resistivity values decreased almost 15000 times, to 1.0x 10³ Ω.m. Jute fibres’ antibacterial efficiency was also studied using the fibres with incorporated nanoparticles. Jute/Ag NPs showed some activity against E. Coli and S. Aureus. Polylactic acid (PLA) was used to develop flexible biodegradable composites with the functionalized jute. The compatibilization of the jute with PLA was successful and the characterization of the final composites was performed by GSDR, ATR-FTIR and TGA.