Experimental study on the high-velocity impact behavior of sandwich structures with an emphasis on the layering effects of foam core

In this study, the effects of the core layering of sandwich structures, as well as arrangements of these layers on the ballistic resistance of the structures under high-velocity impact, were investigated. Sandwich structures consist of aluminum face-sheets (AL-1050) and polyurethane foam core with different densities. Three sandwich structures with a single-layer core of different core densities and four sandwich structures with a four-layer core of different layers arrangements were constructed. Cylindrical steel projectiles with hemispherical nose, 8 mm diameter and 20 mm length were used. The projectile impact velocity range was chosen from 180 to 320 m/s. Considering constant mass and total thickness for the core, the results of the study showed that the core layering increases the ballistic limit velocity of the sandwich structures. The ballistic limit velocity of the panels with a four-layer core of different arrangements, compared to the panel with the single-layer core, is higher from 5% to 8%. Also, for the single-layer core structure, by increasing the core density, the ballistic limit velocity was increased. Different failure mechanisms such as plugging, petaling and dishing occurred for the back face-sheet. The dishing area diameter of back face-sheets was proportional to the ballistic resistance of each sandwich structure

High-velocity impact behavior of sandwich structures with AL faces and foam cores—Experimental and numerical study

In this research, the effect of layering of the foam core of sandwich structures with aluminum face-sheets (AL-1050) and also arrangements of these layers on the ballistic resistance of the structures under high-velocity impact were investigated experimentally and numerically. Three single-layer core sandwich structures and four sandwich structures with four-layer core were considered with a total fixed volume (90 ⁎ 90 ⁎ 63 mm3). These structures were impacted by a hemispherical nose cylindrical steel projectile of 20 mm length and 8 mm diameter. The impact velocity range was chosen from 188.7 to 322.6 m/s. The results of this study revealed that, considering constant core mass and total thickness, the core layering increases the ballistic limit velocity of the sandwich structures, so that the ballistic limit velocity of the sandwich structures with four-layer core, with different arrangements of layers, compared to the single-layer core structure is 5 to 8 percent higher, on average. Also, experimental and numerical results are in good agreement. In this research, the effect of parameters such as the sandwich structure core, the thickness of the face-sheets, the projectile nose geometry, the projectile diameter and mass on the ballistic limit velocity were investigated. The study showed that the removal of the core from the sandwich structure led to a 32% reduction in ballistic limit velocity. Increasing the thickness of the back face-sheet (with the constant total thickness of the two face-sheets) increases the ballistic limit velocity by more than 6%. Compared to flat nose projectile, the ballistic limit velocity of a hemispherical nose and conical nose projectiles are respectively 9.5 and 15.6% less. Considering a constant projectile mass, with an increase of 12.5 and 25% in its diameter, the ballistic limit velocity was increased by 6.5 and 14.4%, respectively, and by decreasing the diameter by 5 and 10%, the ballistic limit velocity dropped 7.9% and 13.5%, respectively. Assuming a fixed initial kinetic energy, the increase in the mass of the projectile also reduced the ballistic limit velocity, so that by increasing the 14 and 46.1% of the projectile mass, the ballistic limit velocity was reduced by 8.5 and 18.3%, respectively

Micromechanical Behavior of Potato Tissue by Scanning Electron Microscopy: Effect of Storage Time, Impact Energy and Curvature Radius of Impact Location

Introduction The mechanical impacts occur mainly during harvesting and post-harvesting operations, lead to the breaking of cell membranes in cellular structure that dependS on impact intensity. Furthermore, turgor pressure of potato tissue is influenced by the micromechanical and the physiological changes in the storage duration. Micromechanical changes of potato tissue due to the mechanical impact need to be monitored by microscopic images during storage. Scanning electron microscopy (SEM) is a high-resolution technique used to investigate the micromechanical behavior of potato tissue. Materials and Methods Potato samples (‘Sante’ cultivar), were stored at 5 ± 0.5°C and 85% relative humidity for 16 weeks. By 2-week intervals, potatoes were removed from the storage and then the impact test was done. Experimental factors were impacted energy at three levels of control (no impact was done), impact energy 1 (0.031 ± 0.002 J) and impact energy 2 (0.320± 0.020 J) and the radius of curvature at two levels of (35 and 45 mm). Water content was measured by drying thin slab samples in an oven at 70°C to a constant weight. The cell turgor pressure of potato tissue at 2-week intervals was estimated from the linear regression between turgor values of each mannitol solution (0–0.6 M) and relative volume change. The microstructural changes of impact location on the potato tubers were analyzed by SEM images at 2-week intervals during storage period. The surface and depth sections cutting from the impact location were immediately immersed in 2.5% glutaraldehyde in 0.1 M sodium phosphate buffer (2 h) at 4± 0.5°C. The specimens were then rinsed 3 × 10 min in 0.1 M sodium phosphate buffer (pH 7.2), and dehydrated through an ethanol series, 25, 50, 70, 90 and 100% dry, 15 min each step, 2 × 100%. In this study the HMDS as a high-quality chemical drying was investigated. The sample preparation for SEM observation then followed by chemical drying via HMDS, under a laboratory hood overnight. Analysis of variance test based on completely randomized design (CRD) was considered for all of the data using SPSS 23. Results and Discussion Superior preservation of potato microstructure was obtained by hexamethyldisilazane (HMDS) drying during sample preparation for SEM observation. The MIP software was used for quantitative analysis of SEM images and the microstructural features of potato tissue at the impact location were determined. So that each cell outline was manually separated by drawing the lines along the visible contours of cell walls. Measurement of the impact damage dimensions was done by MIP software for the surface section (major and minor width, w1 and w2) and the depth section (depth, d and major width, w1). The results indicated the significant differences between water content, cell turgor pressure, cell area and cell perimeter over 16 weeks storage. Generally, by increasing impact intensity the water content, cell turgor pressure, cell area and cell perimeter significantly decreased. Also interaction effect of storage time, impact level and radius of curvature for impact damage of potato tissue was significant. Conclusions The cell turgor pressure at the impact location on the potato tubers had the similar trend with the water content. SEM investigation showed that potato parenchyma, which was high preserved by HMDS drying, had consisted of the pentagon and hexagonal thin-wall cells with the average cell area of 23.14 × 103 ± 0.178 μm2, the average cell perimeter of 564.98 ± 2.008 μm at week 0. The higher impact damage was at week 16 of storage, impact level 2 and the radius of curvature of 35 mm compared to the other treatment

An Empirical Study on Ballistic Resistance of Sandwich Targets with Aluminum Facesheets and Composite Core

Abstract This study attempted to examine the ballistic resistance of sandwich structures with aluminum facesheets, polyurethane core and polyurethane foam reinforced with aluminum pins. The main focus was on the effect of variations in density of core foam and the effect of adding different percentages of aluminum pins on energy absorption and ballistic resistance of sandwich structures under the impact of blunt and conical nose projectiles at high velocities (170 to 260 m/s). The results firstly demonstrated that any increase in the density of foam led to greater energy absorption. Secondly, the ballistic limit of sandwich structures with composite core was more than the foam core by 18 percent. Thirdly, the use of aluminum pins not only enhanced core resistance, but also altered the shape of damage and energy absorption in the rear facesheet. Finally, the ballistic limit of blunt projectile was greater than that of conical nose projectile

An experimental investigation on crack effect on the mechanical behavior and energy absorption of thin-walled tubes

Energy absorption capacity and collapse of cylindrical and square thin-walled aluminum tubes with a crack shaped trigger under axial compression are studied in this paper. Furthermore, the effects of length, angle, location and situation of cracks on the mechanical behavior of tubes are investigated. The results of this research show that the cracks change the collapse processes and folding modes; this effects are greater for the cylindrical tubes; the maximum load is reduced between 4.92% and 31.33% for cylindrical and between 2.55% and 18.52% for square tubes; the cracks increase the crush force efficiency up to 67.03% and 31.06%, and absorbed energy up to 30.45% and 30.16% for cylindrical and square tubes, respectively. The maximum load for all of the cracked tubes is less than that of intact tubes and increasing the crack angle from 0 degrees to 45 degrees decreases the maximum load and from 45 degrees to 60 degrees increases it. Finally, parallel cracks are more effective than perpendicular cracks

Experimental and numerical investigation of the effect of the combined mechanism of circumferential expansion and folding on energy absorption parameters

In this research, in order to increase energy absorption of thin-walled tubes, a combined deformation mechanism is proposed which involves the simultaneous combination of circumferential expansion and folding. Such a combined mechanism was not concerned in the literature. The study is carried out both experimentally and numerically. A special device was designed and made to conduct experimental tests on tubes. The samples were made of aluminum, and quasi-static loading was applied at two different speeds of 10 and 200 mm/min. Energy absorption parameters including specific energy absorption (SEA), crushing mean force, initial peak force, the deformation mode and crush force efficiency (CFE) were studied. Experimental results showed that combined mechanism (without lubrication) could increase absorbed energy up to 123% compared to the folding mechanism. If the lubricant is used, the increase will be up to 97%. The combined deformation mechanism (without lubrication) increases absorbed energy up to 94% compared to the circumstantial expansion. This value will be 107% with lubrication. In addition, the initial peak force in the combined mechanism decreases between 8% and 36% relative to the folding mechanism. The circumstantial expansion in the proposed mechanism is complete and the expansion stroke length is 100%, while this stroke was less in the previous researches due to design restrictions. Numerical simulations were conducted using LS-Dyna software and there is good agreement between the numerical results and experimental data

Effects of buckling initiators on mechanical behavior of thin-walled square tubes subjected to oblique loading

Thin-walled structures usually collapse in Eulerian buckling mode under oblique loads. Energy absorption capacity and crush force efficiency of the structure in this type of collapse are low. Collapse initiators are used to improve these properties. In this research, effect of collapse initiators on energy absorption characteristics of square tubes under oblique quasi-static loads is investigated both experimentally and numerically. Initiators are in the form of cuttings on the tube corners. Results show that collapse initiators in most of the specimens change deformation mode from general buckling to progressive buckling and decrease considerably the peak load; therefore increase crush force efficiency. Furthermore, effect of location and number of initiators is studied. There is good agreement between the numerical results and data from experiments. (C) 2012 Elsevier Ltd. All rights reserved