Damage evolution and energy absorption of E-glass/polypropylene laminates subjected to ballistic impact

High-velocity transverse impact of laminated fiber reinforced composites is of interest in military, marine and structural applications. The overall objective of this work was to investigate the behavior of laminated thermoplastic composites of varying thicknesses under high-velocity impact from an experimental and modeling viewpoint. In order to analyze this problem, a series of ballistic impact tests have been performed on plain weave E-glass/polypropylene (E-glass/PP) composites of different thicknesses using 0.30 and 0.50 caliber right-cylinder shaped projectiles. A gas gun with a sabot stripper mechanism was employed to impact the panels. In order to analyze the perforation mechanisms, ballistic limit and damage evaluation, an explicit three-dimensional finite element code LS-DYNA was used. Material model 162, a progressive failure model based on modified Hashin’s criteria, has been assigned to analyze failure of the laminate. The projectile was modeled using Material model 3 (MAT_PLASTIC_KINEMATIC). The laminates and the projectile were meshed using brick elements with single integration points. The impact velocity ranged from 187 to 332 m s⁻¹. Good agreement between the numerical and experimental results was attained in terms of predicting ballistic limit, delamination and energy absorption of E-glass/PP laminate.     

Oblique Low-Velocity Impact on Fiber-Metal Laminates

In the present study, oblique low-velocity impact (OLVI) on GLARE fiber-metal laminates (FMLs) has been modeled using finite element analysis (FEA) for the first time. Two types of boundary conditions (BCs) can be considered for impactor in the low-velocity impact: constrained (if the impact angle between the impactor and target remains constant during and after the contact) and free (if rotation of the impactor and change in angle with respect to the target is likely) BCs. The more details of these BCs are described in the paper. The OLVI is numerically modeled for four different impact angles. Effect of BC types and impact angle on energy absorption, as well as maximum contact force, is investigated. Due to lack of experimental results for the OLVI on FMLs in the open literature, the authors had to validate the present modelling via the experimental data of the perpendicular low-velocity impact. An excellent agreement was obtained between the numerical results and the experimental data. The results of present study reveal that at the same impact angle, the maximum contact force and energy absorption are greater for the constraint oblique impact (Constrained OLVI). In addition, in the Constrained OLVI, the maximum energy absorption occurs at larger impact angles, while this occurs at smaller impact angles for free oblique impact (Free OLVI). The conclusion to be drawn from all of the cases studied in this paper is that the maximum contact force occurs in the Constrained OLVI with smaller impact angles. Moreover, the maximum energy absorption takes place in the Constrained OLVI at higher impact angles.     

Response of composite sandwich panels with transversely flexible core to low-velocity transverse impact: A new dynamic model

A new computational procedure based on improved higher order sandwich plate theory (IHSAPT) and two models representing contact behavior between the impactor and the panel are adopted to study the low velocity impact phenomenon of sandwich panels comprising of a transversely flexible core and laminated composite face-sheets. The interaction between the impactor and the panel is modeled with the help of a new system having three-degrees-of-freedom, consisting of spring–mass–damper–dashpot (SMDD) or spring–mass–damper (SMD). The effects of transverse flexibility of the core, and structural damping are considered. The present analysis yields analytic functions describing the history of contact force as well as the deflections of the impactor and the panel in the transverse direction. In order to determine all components of the displacements, stresses and strains in the face-sheets and the core, a numerical procedure based on improved higher order sandwich plate theory (IHSAPT) and Galerkin's method is employed for modeling the layered sandwich panel (without the impactor), while the analytic force function developed on the basis of SMDD or SMD model, can be used for the contact force between the impactor and the panel. The contact force is considered to be distributed uniformly over a contact patch whose size depends on the magnitude of the impact load as well as the elastic properties and geometry of the impactor. Various boundary conditions for the sandwich panel have also been considered. Finally, the numerical results of the analysis have been compared either with the available experimental results or with some theoretical results.     

Energy absorption efficiency of carbon fiber reinforced polymer laminates under high velocity impact

In this paper, response of carbon fiber reinforced polymer (CFRP) laminates subjected to high velocity impact has been investigated by experimental and numerical methods. Experiments using a two-stage light gas gun were conducted to investigate the impact process and to validate the finite element model. The energy absorption efficiency (EAE) of CFRP laminates with different thickness was investigated. According to the results of experiments and numerical calculations, thin CFRP laminates have a good EAE under relative higher velocity impact; by contraries, a superior EAE is displayed in thick laminates under relative lower velocity impact. Subsequently, EAE of CFRP laminates was compared with that of 304 stainless-steel plates. In a specific impact velocity range, EAE of CFRP laminates is higher than that of 304 stainless-steel. Thus, CFRP laminates have a potential advantage to substitute the metal plates to be used in high velocity impact resistance structures under a specific impact velocity range.     

Compression after impact test (CAI) on NOMEX™ honeycomb sandwich panels with thin aluminum skins

Sandwich panels are used in industrial fields where lightness and energy absorption capabilities are required. In order to increase their exploitation, a wide knowledge of their mechanical behavior also in severe loading conditions is crucial. Light structures such as the one studied in the present work, sandwich panels with aluminum skins and Nomex^™ honeycomb core, are exposed to a possible decrease of their structural integrity, resulting from a low velocity impact. In order to quantitatively describe the decrease of the sandwich mechanical performance after an impact, an experimental program of compression after impact tests (CAI) has been performed. Sandwich panel specimens have been damaged during a low velocity impact test phase, using an experimental apparatus based on a free fall mass tower. Different experimental impact energies have been tested. Damaged and undamaged specimens have been consequently tested adopting a compression after impact procedure. The relation between the residual strength of the panel and the possible relevant parameters has been statistically investigated. The results show a clear reduction of the residual strength of the damaged panels compared with undamaged ones. Nevertheless, a reduced dependency between the impact energy and the residual strength is found above a certain impact energy threshold.     

Multi-site impact response of S2-glass/epoxy composite laminates

High velocity transverse impact to laminated fiber reinforced composites is of interest in marine, military and civilian applications. Most studies in literature have addressed single point isolated impact events; while this work draws distinction in that we consider multi-site sequential and simultaneous impacts to composite structures. The overall objectives of this work were to investigate the response of laminated composites subjected to high velocity, multi-site impacts from a modeling and experimental viewpoint. Energy absorption, new surface creation, and failure mechanisms from sequential and simultaneous multi-site high velocity impacts are compared to assess additive and cumulative effects of these scenarios. Finite element modeling (LS-DYNA 3D) was used to gain insight into failure modes, energy absorption, and damage prediction. The modeling results correlated well with experimental data obtained from three layer laminates of vacuum assisted resin transfer molding (VARTM) processed S2-glass/SC-15 epoxy. The impact damage has been characterized using optical nondestructive evaluation (NDE) techniques. Specimens subjected to sequential impact exhibited average of 10% greater energy absorption and 18% increase in damage than specimens impacted simultaneously.     

Effect of physical and geometrical parameters on transverse low-velocity impact response of sandwich panels with a transversely flexible core

The effects of important physical and geometrical parameters on transverse low-velocity impact response of composite sandwich panels have been studied in this paper. Impacts are assumed to occur normally over the top and/or the bottom face sheets, at arbitrary locations and with different impactor masses and initial velocities. For deriving closed-form solutions for the contact force, displacements of the impactor and the panel in the transverse direction, the sandwich panel has been modeled as a discrete three-degrees-of-freedom dynamic system with equivalent masses and springs (SM). The dynamic response of the panel is based on the improved higher-order sandwich plate theory (IHSAPT) and both thick and thin panels have been analyzed. The effects of transverse flexibility of the core, and boundary conditions are considered. Also, the area of the contact patch between the impactor and the panel can be varied as it changes with contact duration. The numerical results of the analysis have been compared either with the available experimental results or with some theoretical results. It is established that the dynamic behavior of the sandwich panel depends on various parameters, such as the aspect ratio and the length-to-thickness ratio of the panel, core thickness, boundary conditions of the panel and impactor parameters like its potential energy, velocity and the location of contact point, etc.     

Low-velocity impact-induced damage of continuous fiber-reinforced composite laminates. Part I. An FEM numerical model

A numerical model for simulating the process of low-velocity impact damage in composite laminates using the finite element method is presented in this paper, i.e. Part I of this two part series on the study of impact. In this model, the 9-node Lagrangian element of the Mindlin plate with consideration of large deformation analysis is employed. To analyze the transient response of the laminated plates, a modified Newmark time integration algorithm previously proposed by the authors is adopted here. We also proved that the impact process between a rigid ball and laminated plates is a stiff system, therefore a kind of A(α) stable method has been advocated here to solve the motion equation of the rigid ball. Furthermore, various types of damages including delamination, matrix cracking and fiber breakage, etc. and their mutual influences are modeled and investigated in detail. To overcome the difficulty of numerical oscillation or instability in the analysis of the dynamic contact problem between delaminated layers using the traditional penalty methods, we have employed dynamic spring constraints to simulate the contact effect, which are added to the numerical model by a kind of continuous penalty function. Moreover, an effective technique to calculate the strain energy release rate based on the Mindlin plate model is proposed, which can attain high precision. Finally, some techniques of adaptive analyses have been realized for improving the computational efficiency. Based on this model, a program has been developed for numerically simulating the damage process of cross-ply fiber-reinforced carbon/epoxy composite laminates under low-velocity impact load. In Part II, this numerical model will be verified by comparing with the experimental results. Also the impact damage will be investigated in detail using this numerical approach.     

Influence of fiber orientation and thickness on the response of glass/epoxy composites subjected to impact loading

Composite laminates, made of glass/epoxy using compression molding technique, were subjected to impact loading. The ballistic limit and energy absorption capacity of the laminates were obtained. Experiments were carried out to study the effect of fiber orientation and thicknesses on ballistic limit and energy absorption of the laminates, by using a rigid conical bullet having 9.5 mm diameter and mass of 7.5 g in an air gun. Analytical expressions were obtained to find the ballistic limit, residual velocity and energy absorption capacity of the laminates. The expressions obtained by considering the various damage modes, which were involved in penetration, when laminates subjected to impact loading. The values obtained from analysis were compared with experimental results and good agreement was found. The strain rate sensitivity of the glass/epoxy composites was considered for analysis.     

Experimental and numerical investigations of low velocity impact on functionally graded circular plates

This paper addresses low-velocity impact behaviour of functionally graded clamped circular plates. An experimental work was carried out to investigate the impact behaviour of FG circular plates which is composed of ceramic (SiC) and metal (Al) phases varying through the plate thickness by using a drop-weight impact test system. The influence of the compositional gradient exponent and impactor velocity on the contact forces and absorbed energies was concentrated on the tests. The explicit finite element method, in which a volume fraction based elastic–plastic model (the TTO model) was implemented for the functionally graded materials, was used to simulate their drop-weight impact tests. Effective material properties at any point inside FGM plates were determined using Mori–Tanaka scheme. The experimental and numerical results indicated that the compositional gradient exponent and impactor velocity more effective on the elasto-plastic response of the FG circular plates to a low-velocity impact loading. The comparison at the theoretical and experimental results showed that the use of the TTO model in modelling the elasto-plastic behaviour of FG circular plates results in increasing deviations between the numerical and experimental contact forces for ceramic-rich compositions whereas it becomes more successful for metal-rich compositions.     

Energy Absorption Characteristics of Web-Core Sandwich Composite Panels Subjected to Drop-Weight Impact

The objective of the study was to characterise the energy absorption of composite panels with tied cores, subjected to a drop weight impact test. Numerical simulations based on explicit finite element analysis have successfully modelled low velocity impact tests carried out on sandwich panels with web-core structure and plastic foam. The numerical model has been validated in terms of the failure behaviour of the panel and the variation of the contact force after the initial peak load corresponding to flexural failure. The numerical model is used for a better interpretation of the test results and of the failure mechanisms within the structure. The contribution to the overall energy absorption of the different parts composing the panels has been studied, with the aim of evaluating the feasibility of using low density foam in combination with web-core reinforcement in structural applications.     

Crushing energy absorption of GFRP sandwich panels and corresponding monolithic laminates

Steady quasi-static compression of GFRP monolithic laminates and sandwich panels made of a randomly oriented continuous filament mat/polyester were undertaken. The effects of facing/laminate thickness, trigger collapse system and aspect ratio on their failure mechanisms, hence their energy absorption capability were examined. A numerical model, using a non-linear finite element explicit code, LS-DYNA, was used for pre-analysis of the effect of aspect ratio. A collapse trigger configuration was also studied numerically. The experimental data showed that high values of energy absorbed per unit mass were a predominant feature of the thickest monolithic laminates and sandwich panels with the thickest facings. The monolithic laminates showed higher specific energy than their sandwich panel counterparts. It seems that this difference was due to instability of the sandwich specimens.     

High Velocity Impact Response of Composite Lattice Core Sandwich Structures

In this research, carbon fiber reinforced polymer (CFRP) composite sandwich structures with pyramidal lattice core subjected to high velocity impact ranging from 180 to 2,000 m/s have been investigated by experimental and numerical methods. Experiments using a two-stage light gas gun are conducted to investigate the impact process and to validate the finite element (FE) model. The energy absorption efficiency (EAE) in carbon fiber composite sandwich panels is compared with that of 304 stainless-steel and aluminum alloy lattice core sandwich structures. In a specific impact energy range, energy absorption efficiency in carbon fiber composite sandwich panels is higher than that of 304 stainless-steel sandwich panels and aluminum alloy sandwich panels owing to the big density of metal materials. Therefore, in addition to the multi-functional applications, carbon fiber composite sandwich panels have a potential advantage to substitute the metal sandwich panels as high velocity impact resistance structures under a specific impact energy range.     

复合材料加筋板低速冲击损伤的数值模拟

建立了复合材料加筋板在横向低速冲击载荷作用下的渐进损伤有限元模型。该模型考虑了复合材料加筋板受低速冲击时的纤维断裂、基体开裂及分层脱粘等五种典型的损伤形式, 在层内采用应变描述的失效判据, 结合相应的材料性能退化方案, 通过编写VUMAT用户自定义子程序以实现相应损伤类型的判断和演化。在层间以及筋条与层板间加入界面元, 模拟层间区域的情况, 结合传统的应力失效判据和断裂力学中的能量释放率准则来判断分层损伤的起始和演化规律。通过对数值模拟结果与实验数据的比较, 验证了模型的合理性和有效性。同时探讨了不同位置、不同冲击能量以及含初始损伤(脱粘)等因素对复合材料加筋板低速冲击性能的影响。     

Contact force history analysis of composite sandwich plates subjected to low-velocity impact

Usually the modified Hertzian contact law or experimental static indentation law has been used to analyze low-velocity impact response of composite laminates. In composite laminated plates subjected to low-velocity impact, usually indentation by impact is very small and also energy absorption by indentation is negligible, so ‘spring element method’, which proposed by author recently, can be well applied to investigate impact response. In the present study ‘lumped mass method’ also had been proposed by author to approximately calculate contact force history of composite laminates will be conceptually described as well as the spring element method. And it will be discussed that how the spring element method can be applied to composite sandwich plates. Finally numerical results easily obtained from finite element analysis based on the spring element method using general-purpose commercial FEM software is compared with experimental results. The comparison shows overall agreement.     

Bird strike damage analysis in aircraft structures using Abaqus/Explicit and coupled Eulerian Lagrangian approach

The subject of this paper is numerical prediction of bird strike induced damage in real aeronautical structures using highly detailed finite element models and modern numerical approaches. Due to the complexity of today’s aeronautical structures, numerical damage prediction methods have to be able to take into account various failure and degradation models of different materials. A continuum damage mechanics approach has been employed to simulate failure initiation and damage evolution in unidirectional composite laminates. Hashin’s failure initiation criteria have been employed in order to be able to distinct between four ply failure modes. The problem of soft body impacts has been tackled by applying the Coupled Eulerian Lagrangian technique, thereby avoiding numerical difficulties associated with extensive mesh distortion. This improvement in impactor deformation modelling resulted in a more realistic behaviour of bird material during impact. Numerical geometrical and material nonlinear transient dynamic analyses have been performed using Abaqus/Explicit. The main focus of the work presented in this paper is the application of the damage prediction procedure in damage assessment of bird impact on a typical large airliner inboard flap structure. Due to the high cost of gas-gun testing of aircraft components, experimental testing on the real flap structure could not have been performed. In order to evaluate the accuracy of the presented method, the bird and composite damage model have been validated against experimental data available in the literature.     

A numerical study of ply orientation on ballistic impact resistance of multi-ply fabric panels

This paper presents a detailed finite element (FE) analysis aiming to investigate numerically the impact deformation of multi-ply fabric panels with angled plies. The purpose of the investigation described in this paper is to study numerically the way in which the multi-ply panels deform and to identify the energy absorption in different panel constructions. The FE model was created using ABAQUS to simulate the transverse impact of a projectile onto various woven fabric panels. Influencing factors such as the impact velocity, panel construction and the number of plies are taken into account in the FE simulations. The numerical predictions show that the orientation of plies significantly affects the energy-absorbing capacity of the multi-ply fabric panels. The angled panels always increase the energy-absorbing capacity, compared with the aligned panel, by as much as 20%, depending on the number of plies in the panel. In addition, the stacking sequence of oriented plies also plays an important role in absorbing the energy. For the multi-ply fabric panel with large numbers of plies, there is an optimised sequence of plies which can maximise the energy-absorbing capacity of the panel. An important aspect of the work is validation of the numerical technique. It is shown that the FE predictions are highly consistent with the experimental study [1].     

S-玻纤编织物增强层板的冲击损伤研究

采用空气炮,渗透剂增强Ｘ射线照相法对受到中心横向冲击的一系列平纹ｇｌａｓｓ／ｅｐｏｘｙ层板以及由此产生的损伤情况进行了研究,并采用有限元程度对层板的应力状态进行了分析。基于以上研究,发现;相同动能下,小质量,高速度的弹体在板中产生的损伤面积大于大质量,低速度弹体,层板铺层角的变化对冲击载荷－时间曲线无显著影响;     

LOW VELOCITY PERFORATION BEHAVIOUR OF POLYMER COMPOSITE SANDWICH PANELS

The paper describes low-velocity impact tests on square panels made from two polymer composite sandwich constructions, namely woven glass vinyl ester skins with Coremat core and woven glass epoxy pre-preg skins with honeycomb core. The impact velocity was up to 8 m s^-1 with an impact mass of up to 30 kg giving a maximum impact energy of 882 J. This maximum energy gives full perforation of the panels. The panels were 0.5 m by 0.5 m with clamped but free to pull in boundary conditions. The impactor geometry considered was a 50 mm diameter hemisphere. Results are expressed in the form of energy and failure mode plots and it is shown that the energy absorbing capabilities of the panels increase with the velocity of impact. The increase in energy absorption is attributed to an increase in the core crush stress and skin failure stress at high strain rates. Some discussion is given on the influence of the energy absorbing capabilities of constituent materials on the overall energy absorption behaviour of the panel. Suggestions have also been made for increasing panel perforation energy.