Single and double plate impact welding: Experimental and numerical simulation

This paper presents an experimental study and a numerical simulation of impact welding of single and double aluminium plates using a gas gun. The study shows the differences in the bonding quality of the two impacting flyer plates. In general the bonded area between the two flyer plates is less than that between the leading flyer and the target. The simulations show that the back surface of the leading plate flyer exhibits an increase in roughness after impacting onto the target. The numerical modelling in agreement with the experiment shows that the contact surface between two flyers at the time of the contact is not flat.     

Effect of superelastic shape memory alloy wires on the impact behavior of carbon fiber reinforced in situ polymerized poly(butylene terephthalate) composites

Low-velocity impact properties of shape memory alloy (SMA) wires and carbon fiber reinforced poly(butylene terephthalate) obtained by resin transfer molding were characterized. At the subcritical regime the dissipated energy is not affected by the presence of the wires. However SMA has a positive effect on the maximum absorbed energy, since the maximum allowable load is higher. The contribution of the SMA wires to the higher impact performance of the hybrid composite is suggested to be due to their energy absorbing capability, and also to the high reversible force that acts as a healing force.     

Experimental and numerical investigations of low velocity impact behavior of high-performance fiber-reinforced cement based composite

Fiber-reinforced concrete (FRC) has been used in structural applications in order to enhance the structural performance under dynamic loading and reduce cracking and spalling phenomena by increasing toughness, ductility, and tensile strength of the concrete. High-performance fiber-reinforced cement based composite (HPFRC) is a high-strength FRC with enhanced high-performance characteristics. Recent studies have shown that HPFRC has higher impact resistance than other types of concrete. Therefore, it has been widely considered as a promising material for the construction of important and strategic structures. HPFRC panels are tested by drop projectiles up to an impact at which failure occurs. Mechanical properties of HPFRC are obtained to define material parameters in the MAT_SOIL_CONCRETE model in LS-DYNA, which is used to simulate the behavior of HPFRC panel under impact loading and perform parametric studies. Predicted crack and failure patterns on both sides of the HPFRC panel based on finite element simulation are in good agreement with their corresponding experimental results.     

Computer modelling of impact on curved fibre composite panels

This paper presents results of a finite element (FE) analysis study into low energy impact on curved composite panels. The aim of this study was to determine the accuracy and efficiency of a non-linear explicit FE code, MSC.Dytran, and compare results to published experimental data. The study also looked at impact response as a function of composite panel curvature, composite mesh density, impactor weight, velocity and size, and various suggestions are made for improving the accuracy and efficiency of FE analysis procedures in composite low energy impact studies. The paper presents 265 explicit computer simulation results, which show that non-linear FE analysis does provide accurate, efficient and conservative solutions provided various guidelines are followed.     

Low velocity flexural impact behavior of AR glass fabric reinforced cement composites

Fabric–cement composites developed using the pultrusion production process have demonstrated impressive tensile and flexural properties. For instance fabric reinforced composites with bonded Alkali Resistant (AR) glass fabrics exhibit strain-hardening behavior, tensile strength in the range of 20–25 MPa, and strain capacity of the order of 2–5% under static conditions. Properties of these composite systems were investigated under three point bending conditions using an instrumented drop weight impact system. Samples were studied from the viewpoint of the variations of impact load, deflection response, acceleration and absorbed energy. Development of the testing system in terms of components and acceleration response are discussed in detail. Methods of the impact load measurement using three different ways of acceleration response, piezoelectric load washer and conventional strain gage based load cell are discussed. Cement composites with two different fabric contents and four different drop heights of hammer (dropping mass) were tested. Experimental results indicate that for the same drop height, the stiffer beam type specimens have a lower ultimate deflection but a higher load carrying capacity than the plate type specimens. The maximum flexural stress and absorbed energy of composites increase with drop height. In beam specimens, complete fracture does not take place as cracks form and close due to rebound and significant microcracking in the form of radial fan cracking is observed, whereas interlaminar shear is the dominant failure mode in the plate specimens.     

多轴向玻璃纤维增强树脂基复合材料的破坏特性和损伤机制

通过对玻纤增强环氧乙烯基酯树脂(GF/EVE)和玻璃纤维增强不饱和聚酯树脂(GF/UP)复合材料的多轴向铺层设计试件进行低速冲击、弯曲和剪切破坏性力学试验,分析了不同铺层方式的GF/EVE和GF/UP复合材料冲击、弯曲和剪切载荷作用下产生的损伤及失效模式。结果表明:在铺层设计与工艺相同的情况下,CF/EVE的弯曲强度、冲击韧性均优于CF/UP;[0,90]6试件冲击能量吸收性能优于其他五种铺层方式;铺设角设计、树脂基体类型、铺层厚度对层合板剪切力学性能的影响较小。并基于SEM与超声C扫描成像检测(C-SAM)对复合材料的微观界面脱粘机制及损伤演化行为进行阐释。     

Impact behavior and energy absorption of paper honeycomb sandwich panels

Dynamic cushioning tests were conducted by free drop and shock absorption principle. The effect of paper honeycomb structure factors on the impact behavior was analyzed. Results of many experiments show that the dynamic impact curve of paper honeycomb sandwich panel is concave and upward; the thickness and length of honeycomb cell-wall have a great effect on its cushioning properties; increasing the relative density of paper honeycomb can improve the energy absorption ability of the sandwich panels; the thickness of paper honeycomb core has an up and down fluctuant effect on the cushioning properties; with the increase of the thickness of paper honeycomb core, the effect dies down; flexible corrugated paperboard as liners can improve the compression resistance and cushioning properties of paper honeycombs. The research results can be used to optimize the structure design of paper honeycomb sandwich panel and material selection for packaging design.     

Ultrasonic broadband signals monitoring of glass-fiber reinforced cement (GRC) bending tests

In this study, complete ultrasonic monitoring of Glass-fiber Reinforced Cement plates under bending tests was addressed. In this kind of experiment, the mechanical properties of the specimen continuously change during the test, thus, the acquisition time of the ultrasonic signals is a critical variable. In order to overcome this drawback, a new ultrasonic procedure based on broadband signals (chirp) has been applied. Following this line of thought, the analysed ultrasonic parameters have been split into the parameters that only depend on time, and those that depend on both time and frequency. In particular, the frequency dependent attenuation parameter allows characterizing the evolution of the plate being damaged over a wide frequency range and significantly detecting the main two events happening during the experiment: the first crack and the maximum stress point. In short, this paper demonstrates the suitability of ultrasonic broadband signals for characterizing fiber-reinforced cementitious composites under bending stress.     

Experimental and numerical investigation of fabric impact behavior

This paper presents experimental and numerical research regarding blunt trauma resistance of ten fabrics made of high strength fibers. Fabrics of various architecture were examined, including plain woven fabrics, unidirectional laminates and multiaxial fabrics. The fabrics were compared with respect to the depth of the depression formed and the amount of energy transferred to the backing during projectile impact. Absolute values of mentioned parameters were compared, as well as their values after normalization with respect to thickness and areal density of the fabrics. A numerical method for estimating the amount of energy transferred to the backing was proposed.Normalized results, obtained experimentally and numerically, proved that most of the analyzed fabrics provide a similar level of protection, but the best blunt trauma resistance is given by multiaxial fabrics and the least by plain woven fabrics. This study has also shown that the depth of the depression in the backing material is an insufficient parameter in describing protective properties of fabric against blunt trauma. It is possible that impacts into ballistic packages composed of different fabrics with the same depth of depression may cause completely dissimilar injuries because of the amount of energy transferred to the backing material.     

Flexural and impact response of woven glass fiber fabric/polypropylene composites

Impact tests with a falling dart and flexural measurements were carried out on polypropylene based laminates reinforced with glass fibers fabrics. Research has shown that the strong fiber/matrix interface obtained through the use of a compatibilizer increased the mechanical performance of such composite systems. The improved adhesion between fibers and matrix weakly affects the flexural modulus but strongly influences the ultimate properties of the investigated woven fabric composites. In fact, bending tests have shown a clear improvement in the flexural strength for the compatibilized systems, in particular when a high viscosity/high crystallinity polypropylene was used. On the contrary, the low velocity impact tests indicated an opposite dependence on the interface strength, and higher energy absorption in not compatibilized composites was detected. This result has been explained in terms of failure mechanisms at the fiber/matrix interface, which are able to dissipate large amounts of energy through friction phenomena. Pull-out of fibers from the polypropylene matrices have been evidenced by the morphological analysis of fracture surfaces after failure and takes place before the fibers breakage, as confirmed by the evaluation of the ductility index.     

Strain hardening behavior of lightweight hybrid polyvinyl alcohol (PVA) fiber reinforced cement composites

Experimental results on the strain hardening and multiple cracking behaviors of polyvinyl alcohol (PVA) fiber reinforced cementitious composites under bending are reported in this paper. Different hybrid combinations of PVA fibers with different lengths and volume fractions are considered to reinforce the mortar matrix. Among different hybrid combinations, the composite containing 2% thicker PVA fibers of 12 mm length and 1% thinner PVA fibers of 6 mm length and the composite containing 2% thicker PVA fibers of 24 mm length and 1% thinner PVA fibers of 6 mm length showed the best performance in terms of highest ultimate load, largest CMOD (crack mouth opening displacement) at peak load and multiple cracking behavior. The effects of four types of light weight sands on the strain hardening and multiple cracking behavior of hybrid fiber composites are also evaluated in this study. It has been observed that the ultimate load and CMOD at peak load for all light weight hybrid fiber composites are almost the same irrespective of volume fractions of light weight sand. The composites containing finer light weight sands exhibited higher ultimate load than those containing coarser light weight sands. It is also observed that the hybrid fiber composite containing normal silica sand exhibited higher ultimate load than the composites with light weight sands.     

The effect of polyacrylate microstructure on the impact response of PMMA/PC multi-laminates

Polymer armor is widely implemented in many military and commercial applications, particularly where optical clarity is required. The impact mechanics and energy absorption mechanisms of these multi-layered structures are not well understood. This experimental study focuses on the impact response of three-layered structures consisting of poly(methyl methacrylate) (PMMA) and polycarbonate (PC) outlerlayers with various polyacrylate adhesives. Specifically, the effect of varying the microstructure of a soft interlayer incorporated into all-polymer multi-laminates is investigated. Four polyacrylates (VHB 4905, VHB 5925, VHB 4930 and VHB 4936) having a common acrylic base matrix with microstructural variations including combinations of compressible air gaps and rigid microsphere inclusions. To examine how interlayer microstructure affects impact resistance, an instrumented, compressed air driven, experimental setup is utilized to conduct intermediate velocity (9-30 m/s) normal impact testing. The setup is unique in that both force and displacement during impact are recorded independently using a shock accelerometer embedded impactor and optical displacement sensors measuring contact force and out-of-plane deflection, respectively. Quantitative metrics from the data are used to characterize and assess impact response between various configurations. Two impact velocities are tested: 12 and 22 m/s. Several correlations between adhesive microstructure and impact response are observed, including a secondary contact force decrease in multi-laminates having interlayers with microspheres and delamination in multi-laminates having interlayers without air gaps. The multi-laminates with VHB 5925 adhesives (air gaps only) showed the longest cracks and largest fracture area, which directly relates to the greatest displacements and pulsewidths, and smallest second force peaks. Increased fracture compromises structural integrity resulting in more deflection, prolonged impactor contact with the multi-laminate, and decreased elastically stored energy lessening impactor reload. It is demonstrated that this experimental methodology is capable of consistently probing and assessing the local impact response and modulation of an impact load when a multi-layered polymer includes a soft middle layer. Quantitative and qualitative effects of the interlayers’ microstructure on overall impact performance are presented.     

玻璃纤维-碳纤维混杂增强PCBT复合材料层合板的制备及低速冲击性能

采用环状对苯二甲酸丁二醇酯(CBT)预浸料,利用真空袋辅助热压工艺制备了玻璃纤维机织布-碳纤维机织布/聚环状对苯二甲酸丁二醇酯(GF-CF/PCBT)混杂复合材料层合板。利用双悬臂梁(DCB)和三点端部开口弯曲(3ENF)试验对连续纤维增强PCBT复合材料层合板的层间强度做出评估。同时,利用低速冲击试验结合Abaqus/Explicit有限元仿真重点考察了混杂纤维增强PCBT复合材料层合板的低速冲击性能。试验结果表明:尽管CF/PCBT复合材料层合板具有优异的层间性能,当冲击能量为114.3J时,由于CF自身的脆性,CF/PCBT复合材料层合板被完全穿透,而GF-CF/PCBT混杂复合材料层合板只在表面形成凹痕。与纯CF增强PCBT复合材料层合板相比,铺层形式为[CF/GF/CF]25的GF-CF/PCBT混杂复合材料层合板的抗冲击损伤能力提高2倍。仿真得到的云图显示,冲击引起的应力在CF中的分布区域要明显大于在GF中的分布区域。     

Experimental and numerical study of the impact behavior of SMC plates

Steel components absorb impact energy by plastic deformation whilst composite materials absorbing it by damage mechanisms such as fiber debonding, fiber fracture, and matrix cracking. Therefore, in order to properly substitute metal components with composite ones in industrial applications, the impact property of composite materials must be well known. In this study, the impact behavior of sheet molding compounds (SMC), which is widely used in automobile industry due to its relatively low cost and high productivity, was examined both experimentally and numerically. In order to investigate the impact behavior of SMC, an experimental study was carried out by setting up a drop weight impact test system. Using this system, the dissipated impact energies of SMC flat plates were measured to investigate the influence of the mass and shape of impactor, initial velocity, and specimen thickness on the impact behavior.

For numerical predictions, a modified damage model for SMC was developed and adopted in the user defined material subroutine of the commercial simulation program LS-DYNA3D. For the sake of improving efficiency of impact simulations, the SMC material property was determined in consideration of the local differences of the fiber volume fractions. The dissipated impact energies under various conditions and the reliability of the developed impact simulation process were examined through comparisons of the predicted data with the experimental results.

From this comparison, it was found that, in the scope of current study, the specimen thickness is the most important parameter that should be considered in the design of SMC components for the aspect of impact behavior.     

Evolution and calibration of a numerical model for modelling of hybrid-fibre ECC panels under high-velocity impact

A material model for hybrid-fibre engineered cementitious composites (ECC) under impact loading is developed and calibrated in this paper, and size effect, appropriate erosion criteria and strain rate effect are investigated and accounted for in the model. Employing the new material model, a numerical model and modelling technique are developed to model the impact behaviour and impact process of hybrid-fibre ECC panels using LS-DYNA commercial software. The material model and the numerical model developed in this paper are validated against the experimental results.     

Modelling of aluminium foam core sandwich panels under impact perforation

Aluminium foam core sandwich panels are good energy absorbers for impact protection applications, such as light-weight structural panels, packing materials and energy absorbing devices. In this study, the high-velocity impact perforation of aluminium foam core sandwich structures was analysed. Sandwich panels with 1100 aluminium face-sheets and closed-cell A356 aluminium alloy foam core were modelled by three-dimensional finite element models. The models were validated with experimental tests by comparing numerical and experimental damage modes, output velocity, ballistic limit and absorbed energy. By this model the influence of foam core and face-sheet thicknesses on the behaviour of the sandwich panel under impact perforation was evaluated.     

Flexural behavior of reinforced concrete (RC) beams retrofitted with hybrid fiber reinforced polymers (FRPs) under sustaining loads

This paper reports experimental studies of reinforced concrete (RC) beams retrofitted with new hybrid fiber reinforced polymer (FRP) system consisting carbon FRP (CFRP) and glass FRP (GFRP). The objective of this study is to examine effect of hybrid FRPs on structural behavior of retrofitted RC beams and to investigate if different sequences of CFRP and GFRP sheets of the hybrid FRPs have influences on improvement of strengthening RC beams. Toward that goal, 14 RC beams are fabricated and retrofitted with hybrid FRPs having different combinations of CFRP and GFRP sheets. The beams are loaded with different magnitudes prior to retrofitting in order to investigate the effect of initial loading on the flexural behavior of the retrofitted beam. The main test variables are sequences of attaching hybrid FRP layers and magnitudes of preloads. Under loaded condition, beams are retrofitted with two or three layers of hybrid FRPs, then the load increases until the beams reach failure. Test results conclude that strengthening effects of hybrid FRPs on ductility and stiffness of RC beams depend on orders of FRP layers.     

玄武岩纤维增韧混凝土冲击性能

采用三点弯曲冲击试验装置, 结合超声波测试技术, 研究了玄武岩纤维质量分数为0%～0.60%时, 玄武岩纤维增韧混凝土（Basalt Fiber Reinforced Concrete, BFRC）的冲击性能及其损伤演化规律, 研究了混凝土冲击破坏过程中基于超声波波速的损伤演化过程, 并应用体视显微镜观测了冲击过程中试件表面裂纹的发展, 分析了玄武岩纤维提高混凝土冲击韧性的机制。结果表明: 玄武岩纤维对混凝土的抗压强度无明显改善, 但可以显著提高混凝土的冲击韧性, 当纤维质量比为0.36%时冲击韧性提高了2.2倍。各玄武岩纤维掺量下混凝土的冲击破坏均表现出脆性特征, 但玄武岩纤维的加入有效提高了混凝土对冲击能量的吸收, 其临近破坏时损伤变量较素混凝土提高了40%～83%; 玄武岩纤维混凝土冲击破坏过程表现出多缝开裂的特征, 在最终破坏时主裂缝附近有明显的副裂缝出现。