An experimental investigation on the low velocity impact response of composite plates repaired by VARIM and hand lay-up processes

This paper presents results of an experimental investigation on the impact response of repaired and unrepaired glass/epoxy composite plates. Repaired samples were prepared by two different manufacturing methods; vacuum assisted resin infusion process and hand lay-up technique. In order to compare impact response of the repaired and unrepaired samples a number of single impact tests were performed under various impact energies. Damage process of the samples is analyzed from cross-examining load–deflection curves and damaged specimens. From the visual inspection, for the impacted side of the samples, it is noted that the main damage modes for repaired samples are matrix and fiber cracks around point of impact and delaminations while severe matrix cracks expanded through fiber directions are the dominant damage mode for unrepaired samples. At the back surfaces, delaminations and fiber–matrix debonding oriented in the fiber directions are observed for unrepaired samples. However, for repaired samples the fiber fractures through repair line as well as the delaminations become dominant modes. For a reasoning justification in discussing impact test results, interlaminar fracture toughness (Mode I and Mode II) and flexural tests for repaired and non-repaired samples were also conducted.     

Strength evaluations of sinusoidal core for FRP sandwich bridge deck panels

Fiber-reinforced polymer (FRP) sandwich deck panels with sinusoidal core geometry have shown to be successful both in new construction and the rehabilitation of existing bridge decks. This paper is focused on an experimental study of the strength evaluations of a honeycomb sandwich core under out-of-plane compression and transverse shear. The sinusoidal core is made of E-glass Chopped Strand Mat (ChSM) and Polyester resin. The compressive, tensile and shear strengths were first obtained from coupon tests. The out-of-plane compression tests were performed on representative single-cell volume elements of sandwich panels, and the tests included “stabilized” samples to induce compression failure, and “bare” samples to induce local buckling of the core. Finally, four-point bending tests were conducted to study the structural strength behavior under transverse shear. Two types of beam samples were manufactured by orienting the sinusoidal wave either along the length (longitudinal) or along the width (transverse). Both typical shear failure mode of the core material and delamination at the core–facesheet bonding interface were observed for longitudinal samples. The failure for transverse samples was caused by core panel separation. For both single-cell and beam-type specimen tests, the number of bonding layers, i.e., the amount of ChSM contact layer and resin used to embed the core into the facesheet, and the core thickness are varied to study their influence. The experimental results described herein can be subsequently used to develop design guidelines.     

Dynamic models for low-velocity impact damage of composite sandwich panels – Part B: Damage initiation

Equivalent single and multi degree-of-freedom systems are used to predict low-velocity impact damage of composite sandwich panels by rigid projectiles. The composite sandwich panels are symmetric and consist of orthotropic laminate facesheets and a core with constant crushing resistance. The transient deformation response of the sandwich panels subjected to impact were predicted in a previous paper, and analytical solutions for the impact force and velocity at damage initiation in sandwich panels are presented in this second paper. Several damage initiation modes are considered, including tensile and shear fracture of the top facesheet, core shear failure, and tensile failure of back facesheet. The impact failure modes are similar to static indentation failure modes, but inertial resistance and high strain rate material properties of the facesheets and core influence impact damage loads. Predicted damage initiation loads and impact velocities compare well with experimental results.     

Effects of the core density on the quasi-static flexural and ballistic performance of fibre-composite skin/foam-core sandwich structures

Polymeric foams are extensively used as the core materials in sandwich structures and the core material is typically bonded between relatively thin fibre-composite skins. Such sandwich structures are widely used in the aerospace, marine and wind-energy industries. In the present work, various sandwich structures have been manufactured using glass-fibre-reinforced polymer (GFRP) skins with three layers of poly(vinyl chloride) foam to form the core, with the densities of the foam layers ranging from 60 to 100 kg/m³. This study has investigated the effects on the quasi-static flexural and high-velocity impact properties of the sandwich structures of: (a) the density of the polymeric-foam core used and (b) grading the density of the foam core through its thickness. The digital image correlation technique has been employed to quantitatively measure the values of the deformation, strain and onset of damage. Under quasi-static three-point and four-point bend flexural loading, the use of a low-density layer in a graded-density configuration reduced the likelihood of failure of the sandwich structure by a sudden force drop, when compared with the core configuration using a uniform (i.e. homogenous) density layer. The high-velocity impact tests were performed on the sandwich structures using a gas-gun facility with a compliant, high-density polyethylene projectile. From these impact experiments, the graded-density foam core with the relatively low-density layer located immediately behind the front (i.e. impacted) GFRP skin was found to absorb more impact energy and possess an increased penetration resistance than a homogeneous core structure.     

Impact response of integrated hollow core sandwich composite panels

This paper deals with an innovative integrated hollow (space) E-glass/epoxy core sandwich composite construction that possesses several multi-functional benefits in addition to the providing lightweight and bending stiffness advantages. In comparison with traditional foam and honeycomb cores, the integrated space core provides a means to route wires/rods, embed electronic assemblies, and store fuel and fire-retardant foam, among other conceivable benefits. In the current work, the low-velocity impact (LVI) response of innovative integrated sandwich core composites was investigated. Three thicknesses of integrated and functionality-embedded E-glass/epoxy sandwich cores were considered in this study—including 6, 9 and 17 mm. The low-velocity impact results indicated that the hollow and functionality-embedded integrated core suffered a localized damage state limited to a system of core members in the vicinity of the impact. The peak forces attained under static compression and LVI were in accordance with Euler's column buckling equation. Stacking of the core was an effective way of improving functionality and limiting the LVI damage in the sandwich plate. The functionality-embedded cores provided enhanced LVI resistance due to energy additional energy absorption mechanisms.     

Impact response of three-dimensional stitched sandwich composite

The paper aims at evaluating the damage resistance of sandwich structures composed of stitched foam core and glass facesheets subjected to low-velocity impact. To obtain a suitable baseline comparison, the equivalent set of properties was measured for an equivalent unstitched sandwich.Based on the force and energy histories, parameters have been introduced as following: load at incipient damage, maximum load, penetration depth at maximum load, total energy absorbed during impact and impact damage area. The impact resistance of the sandwich structure is greatly improved by the presence of the stitches. Skin/core delamination is limited and initial energy is used to degrade core’s stitches. Moreover the global behavior under impact is influenced by the stitching geometrical parameters.     

泡沫铝夹层结构复合材料低速冲击性能

以泡沫铝为夹芯材料,玄武岩纤维(BF)和超高分子量聚乙烯纤维(UHMWPE)复合材料为面板,制备夹层结构复合材料。研究纤维类型、铺层结构和芯材厚度对泡沫铝夹层结构复合材料冲击性能和损伤模式的影响规律,并与铝蜂窝夹层结构复合材料性能进行对比分析。结果表明:BF/泡沫铝夹层结构比UHMWPE/泡沫铝夹层结构具有更大的冲击破坏载荷,但冲击位移和吸收能量较小。BF和UHMWPE两种纤维的分层混杂设计比叠加混杂具有更高的冲击破坏载荷和吸收能量。随着泡沫铝厚度的增加,夹层结构复合材料的冲击破坏载荷降低,破坏吸收能量增大。泡沫铝夹层结构比铝蜂窝夹层结构具有更高的冲击破坏载荷,但冲击破坏吸收能量较小;泡沫铝芯材以冲击部位的碎裂为主要失效形式,铝蜂窝芯材整体压缩破坏明显。     

十字嵌锁型格栅夹芯结构设计及低速冲击性能

针对传统复合材料格栅夹芯结构极限承载能力较低、单胞封闭易造成水汽凝结的问题,在分析管胞微观结构和功能性的基础上,提出一种新型十字嵌锁型格栅夹芯结构。首先选取最小体积(最小质量)和最小变形(最大刚度)为优化目标,利用第二代非支配遗传算法(NSGA-Ⅱ)完成多目标优化,采用三维Hashin失效准则和改进的刚度退化方法建立格栅夹芯板的冲击渐进损伤有限元分析模型,研究多种低速冲击载荷对不同相对密度夹芯结构的不同位置的破坏机制及力学响应。结果表明：新型格栅夹芯结构表现出良好的低速冲击阻抗,其随芯子的空间分布存在差异,格栅间隙处的抗冲击性能较弱,芯子密度的提高不能有效增强该位置处的冲击强度,夹芯结构所受到的破坏远远大于冲击器撞击格栅交点处的情况;受不同冲击位置和冲击速度的影响,载荷-时间和位移-时间曲线呈现出不同的典型模式,芯子出现屈曲、分层、粘接剥离、折弯变形等失效形式,复合材料上面板发生混合损伤,随着冲击速度的增加,芯子和面板的损伤程度也愈严重。     

Rapid simulation of impacts on composite sandwich panels inducing barely visible damage

The finite element based design tool, CODAC, has been developed for efficiently simulating the impact behavior of sandwich structures consisting of two composite face sheets and a compliant core. To achieve a rapid and accurate stress analysis, three-layered finite shell elements are used. A number of macromechanical damage models are implemented to model damage onset and damage growth.

The transient impact analysis is assessed via an experimental impact test program on honeycomb sandwich panels. Force–time histories and damage sizes are examined. The influence of distinct damage and degradation models on the impact response is analyzed. Results show that the presented time-efficient methodology is capable of accurately modeling core failure behavior and rapidly simulating low-velocity impacts which induce barely visible damage.     

明胶鸟弹撞击复合材料蜂窝夹芯板试验

开展明胶鸟弹撞击复合材料蜂窝夹芯板试验,研究夹芯结构在软体高速冲击下的损伤形式,分析相关因素对结构动态响应结果的影响。通过CT扫描对复合材料蜂窝夹芯板内部进行检测可知,面板出现分层、基体开裂、纤维断裂、凹陷、向胞内屈曲等损伤形式,蜂窝芯出现芯材压溃、与面板脱粘的损伤形式;分析复合材料蜂窝夹芯板后面板的动态变形过程及撞击中心处位移-时间数据可知,复合材料蜂窝夹芯板在撞击过程中出现由全局弯曲变形主导和局部变形主导的两种变形模式;通过对比不同工况下的复合材料蜂窝夹芯板损伤程度可知,复合材料蜂窝夹芯板损伤程度随鸟弹撞击速度的增加而增大;蜂窝芯高度为10 mm的复合材料蜂窝夹芯板较蜂窝芯高度为5 mm的复合材料蜂窝夹芯板的损伤程度大;初始动能较大的球形鸟弹较圆柱形鸟弹对复合材料蜂窝夹芯板造成的冲击损伤程度更大。      

The Fatigue Behavior of an Aluminium Foam Sandwich Beam Under Alternating Bending

Aluminium foam‐sandwiches which are applied in car bodies, e.g., as side impact protection structures, are loaded not only by quasistatic, but also by cyclic forces. If these fatigue loadings induce damage of the foam‐sandwich structure, the stiffness, strength and impact behaviour may be adversely affected. The present study shows results of bending fatigue tests on an Aluminium foam‐sandwich material that was manufactured by a powder metallurgy process with succesive rolling and foaming. The sandwich had a foam core with 10 mm and outer sheet layers with thickness 1 mm. Alternating bending tests with normal stresses parallel to the sheet plane were realised using a servoelectric fatigue testing machine. The sinusoidal loading was momentum controlled with a load ratio of R = –1 and frequencies up to 50 Hz. The fatigue limit was calculated from 18 tests using the stair case method with an ultimate number of cycles of 10⁷. The cyclic deformation behaviour within the HCF‐ and the LCF‐regime was determined from hysteresis loops of the bending moment versus the bending angle which were measured at selected load cycles during each test.The material shows pronounced cyclic hardening at the beginning of the fatigue loading due to work hardening processes mainly within the sheet layers. Afterwards, a cyclic neutral behaviour occurs until the end of the test. Damage by fatigue crack initiation generally starts within the sheet layers, mostly near large and deep pores within the gauge length. Subsequently, the cracks propagate firstly within the sheet layers and after that through the foamed core of the sandwich perpendicular to the bending axis.     

Transient Dynamic Response and Failure of Sandwich Composite Structures under Impact Loading with Fluid Structure Interaction

The objective of this study is to examine the Fluid Structure Interaction (FSI) effect on transient dynamic response and failure of sandwich composite structures under impact loading. The primary sandwich composite used in this study consisted of a 6.35?mm balsa core and a multi-ply symmetrical plain weave 6?oz E-glass skin. Both clamped sandwich composite plates and beams were studied using a uniquely designed vertical drop-weight testing machine. There were three impact conditions on which these experiments focused. The first of these conditions was completely dry (or air surrounded) testing. The second condition was completely water submerged. The final condition was also a water submerged test with air support at the backside of the plates. The tests were conducted sequentially, progressing from a low to high drop height to determine the onset and spread of damage to the sandwich composite when impacted with the test machine. The study showed the FSI effect on sandwich composite structures is very critical such that impact force, strain response, and damage size are generally much greater with FSI under the same impact condition. As a result, damage initiates at much lower impact energy conditions with the effect of FSI. Neglecting to account for FSI effects on sandwich composite structures results in very non-conservative analysis and design. Additionally, it was observed that the damage location changed for sandwich composite beams with the effect of FSI.     

齿板-玻璃纤维/聚氨酯泡沫芯夹层梁的低速冲击性能

提出了一种由齿板-玻璃纤维（TP-GF）混合面板和聚氨酯（PU）泡沫芯材组成的新型TP-GF/PU泡沫夹层梁,结构中金属板通过齿钉压入GF与内部芯材连接,该夹层梁采用真空导入模压工艺制作。通过低速冲击试验,研究了不同冲击能量、纤维厚度和泡沫密度下TP-GF/PU泡沫夹层梁的冲击响应和损伤模式,并与普通的夹层梁进行了对比分析;通过双悬臂梁试验研究了混合夹层梁的界面性能,计算了夹层梁的应变能释放率。结果表明:在22 J、33 J、44 J能量冲击下,泡沫芯材密度为150 kg/m3的TP-GF/PU泡沫夹层梁的最大接触力较普通夹层梁分别提高了31.2%、48.6%、33.3%,冲击能量吸收分别增加了17.2%、11.3%、15.5%;随着冲击能量、面板纤维层数及芯材密度的增加,TP-GF/PU泡沫夹层梁最大接触力增大,密度较低的TP-GF/PU泡沫夹层梁损伤形式主要为面板的局部弯曲,而芯材密度较高的TP-GF/PU泡沫夹层梁则以穿透损伤为主;增加泡沫芯材密度和面板纤维厚度能够提高TP-GF/PU泡沫夹层梁的抗冲击性能,随着芯材密度的增大TP-GF/PU泡沫夹层梁的应变能释放率峰值越高,界面性能越好。      

高速冲击载荷作用下钎焊蜂窝夹层板动态响应

目的 以钎焊高温合金蜂窝夹层板为研究对象,分析其在弹丸高速冲击作用下的力学性能。方法 采用轻气炮冲击加载试验结合有限元模拟,对蜂窝夹层板开展不同冲击强度下的动态响应和失效研究。开展含高速冲击损伤的蜂窝夹层板侧压试验,研究损伤模式对剩余强度的影响。结果 冲击强度对夹层板的失效过程和失效模式有着明显的影响,当冲击条件不足以使得迎弹面发生侵彻时,夹层板失效为表面压痕损伤;随着冲击强度的提高,出现不同程度的局部芯层压缩;当冲击强度大于临界值时,迎/背弹面陆续被侵彻,夹层板出现侵入损伤及贯穿损伤。结论 高速冲击损伤使得蜂窝夹层板的侧压失效模式,由理想塑性屈曲转变为局部失稳,侧压极限载荷大幅降低。     

泡沫填充的S型褶皱复合材料夹芯板低速冲击响应特性

为了研究泡沫填充褶皱夹芯结构低速冲击响应特性与损伤机制,采用热压法制备了玻璃纤维增强S型褶皱夹芯板,并使用聚氨酯泡沫进行了填充,通过落锤试验机对夹芯板节点与基座两个位置进行了冲击试验。研究表明,冲击位置对泡沫填充褶皱夹芯板的失效模式存在影响。当冲击位置为节点时,夹芯板芯子以凸侧面曲面壁压溃断裂失效为主,泡沫的填充起到了提供力矩的作用。当冲击位置为基座时,夹芯板芯子以凹侧面曲面壁撕裂和凸侧面曲面壁压溃失效为主,夹芯板损伤沿板厚度方向扩展充分,导致冲击载荷均匀化。在相同冲击能量下,节点与基座冲击相比,夹芯板的最大载荷力提高,并且比较稳定。此外,节点载荷峰值产生的冲击位移较低于基座冲击。      

缝合泡沫夹层复合材料低速冲击数值模拟及实验

在ABAQUS分析平台中建立了缝合泡沫夹层复合材料在低速冲击下的动力学有限元模型,采用杆单元模拟缝线树脂柱的作用,基于Hashin破坏准则模拟层板面内损伤,通过各向同性硬化本构模型利用等效塑性变形模拟泡沫夹芯损伤演化。针对相同铺层的缝合和未缝合泡沫夹层结构,模拟了相同冲击能量下的低速冲击响应过程及面板、泡沫的损伤情况,数值结果与实验结果吻合较好,证明了该方法的有效性和准确性。研究结果表明,在低速冲击下,泡沫夹层结构引入缝线后虽然降低了泡沫缓冲吸能的作用,使得面板表面受到较大的冲击破坏,但增强了整体刚度,增大了面板抵抗弯曲变形的能力,减小了内部面板的损伤,使其在改善复合材料面板易分层缺陷的同时还依然拥有优良的面内性能。     

复合材料夹芯板低速冲击后弯曲及横向静压特性

对低速冲击后的复合材料Nomex 蜂窝夹芯板进行了纯弯曲和准静态横向压缩实验, 用X 光技术、热揭层技术和外观检测等对板内的损伤进行测量, 分析了被冲击面在受压情况下蜂窝夹芯板的弯曲破坏特点, 对比了横向静压与低速冲击所造成的板内损伤, 讨论了不同横向压缩速度时接触力P-压入位移$h 的变化规律和损伤情况。结果表明: 低速冲击可使蜂窝夹芯板的弯曲强度大幅度降低; Nomex 蜂窝夹芯板对低速冲击不敏感。      

Impact response of fiber metal laminate sandwich composite structure with polypropylene honeycomb core

Fiber metal laminates (FMLs) were used as skin on polypropylene honeycomb core to form a sandwich structure. Impact response was measured by conducting a series of low-velocity impact test. Impact force and the force time history were recorded and analyzed. It was found that the maximum impact load increased up to a threshold value at which it plateaus while the energy absorption in the structure increased with increasing impact energy. Post-impact optical image showed a change in damage area with increasing impact energy. The impact damage threshold energy for the sandwich structure was clearly shown in the range of impact energy between 7.84 J and 11.76 J where damages including delamination of the skins and global bending of the structure were observed.     

Study on impact properties of through-thickness stitched foam sandwich composites

Through-thickness stitched foam core sandwich composites were fabricated by using RTM process; and impact performance and damage extent were studied at 1–70 J impact energy levels. The results show that two sharp peak loads and a low-loading plateau appear on the load-time plots at 1–30 J impact energy levels; both sharp peak loads can be considered as the course of penetrating top and bottom facings, a low-loading plateau has the characteristics of penetrating foam cores. Compare to the unstitched samples, the average damage angle of stitched samples increase by 48%, the maximal cracking width and penetration depth of the stitched samples decrease by 67% and 4% at 25 J impact energy levels.     

Experimental study of the low-velocity impact behaviour of primary sandwich structures in aircraft

The susceptibility of sandwich structures to localised (impact) damage is one of the main reasons why the sandwich concept is not yet used in large primary aircraft structures of airliners. The objective of this work is to experimentally investigate the damage tolerance of representative composite sandwich panels for primary aircraft structures. Instrumented low-velocity impact tests were performed on sandwich specimens consisting of carbon Non-Crimp Fabric/epoxy facings and a Rohacell (PMI) foam core. Both internal and external damage resulting from these impact events was evaluated.The foam core material has a considerable influence on the amount of damage detected by ultrasonic TTU C-scan. CAI tests however showed that this core damage has no significant influence on the residual compressive strength of the specimens.