Experimental and numerical investigation on damage behavior of honeycomb sandwich panel subjected to low-velocity impact

Abstract

This paper presents the low-velocity impact behavior of sandwich panel with carbon fiber reinforced plastic (CFRP) composite facesheet and Nomex honeycomb core through experimental and numerical methods. Experiments were carried out on two thickness of honeycomb core at various impact energy levels. The dynamic response including contact force history and energy absorption as well as contact duration was recorded. The damage modes were obtained through non-destruction inspection (NDI) C-scan and microscopic observation. A refined three-dimensional finite element model combined with continuum damage mechanics (CDM) was developed with composite plies and detailed honeycomb core. Physically-based Puck’s composite failure criteria and energy based progressive damage model were used to capture the intralaminar damage initiation and evolution, respectively. The interlaminar damage of facesheet and debonding of facesheet/core interface were predicted using cohesive element. The hexagonal honeycomb cells were characterized in FE model with an elasto-plastic constitutive model and damage criterion in detail during impact. The simulation results show good agreements with experiments and the model can be used to predict the low-velocity impact response and impact damage effectively. More detailed responses, such as internal damage details, damage modes and evolution, are observed and discussed with the numerical model proposed.

Introduction

Composite sandwich structures are increasingly used in various applications ranging from energy and aerospace applications due to their high stiffness-to-weight ratio, energy absorption properties and corrosion resistance [1], [2], [3], [4], [5]. However, sandwich structures are also susceptible to low-velocity impact events from foreign objects. Such impact events can be caused by tool drop, hail and debris impact during manufacture, maintenance and service life. The impact damage can result in the reduction of properties especially compressive strength which can lead to catastrophic failure during whole life. Therefore, it is necessary to investigate the complicated impact resistance and damage mechanism of sandwich structures under low-velocity impact [6], [7].

Sandwich structures focused in this paper consist of two thin composite facesheet and a relatively soft Nomex honeycomb core [8]. Recently, composite structure are becoming more attractive to metals because of advantageous properties such as high strength-to-weight ratio [9], [10], [11], [12]. And Nomex honeycomb can also be a suitable choice thanks to good flammability, environment resistance and low dielectric properties [13], [14], [15], [16], [17], [18]. Low-velocity impact can induce different damage modes on facesheet, core material and facesheet/core interface. These damage behaviors depend on various factors including impact energy, material properties, geometric parameters and boundary conditions. Typical failure modes on laminate facesheet mainly contain intralaminar(fiber breakage, matrix cracking) and interlaminar(delamination) damage [19], [20], [21], [22], [23]. Localized core crushing with irreversible deformation always appear in the impact region due to indentation. Meanwhile, global deformation of sandwich structure can cause core shear. The damage mechanism of composite sandwich structures is significantly complicated than conventional laminates [24]. It is mainly due to the interaction of facesheet damage and core deformations.

Experimental, numerical and theoretical methods have been employed to investigate the mechanical behavior of sandwich structures under low-velocity impact [25], [26], [27], [28]. Many researchers obtained the dynamic response and material damage directly through impact experiments and inspection methods. Theoretical investigation is convenient to get the dynamic response with reductions in the cost and time. But some details can hardly be obtained due to assumptions and simplifications. Numerical finite element method (FEM) combined with fracture models has become a common method to analyze and predict the impact procedure. Simulation can obtain extra important information such as internal damage details with appropriate time-consuming and cost-intensive. In practice, researchers tend to use a combination of these methods to investigate impact behavior to improve the designable and predictable capability.

Morada et al [7] investigated the damage resistance of sandwich panel with ATH/epoxy core under low-velocity impact. The viscoplastic-damage model was used to present the mechanical behavior of ATH/epoxy core. It was found that the primary failure mode is indentation instead of fiber breakage, delamination and facesheet/core debonding. Chen et al [29] investigated the low-velocity impact response of composite sandwich panel through finite element modelling and experiment. The model included facesheet damage, core crushing as well as facesheet/ core debonding. Klaus et al [30] investigated the residual strength after impact of sandwich panels experimentally and numerically. They obtained the residual strength through 4-point bending after impact with different energy. It is observed that the damage and deformations caused by impact test had great influenced on the bending strength of the damaged specimen. And the numerical has also been established to present the impact and bending test with a good agreement. Besant et al [31] established a finite element model to study the low-velocity impact behavior of composite sandwich panels. The metal honeycomb is regarded as an anisotropic elasto-plastic material and the yield criteria are based on combined shear and compression experiment. They observed that the honeycomb absorb energy well through a combination of local crush and shear yielding. Menna et al [32] studied the impact behavior of composite honeycomb sandwich structures numerically. They concerned the damage model, the strain-rate effect and energy absorption capacity. The material parameters were calibrated based on fundamental experiments in order to obtain more reliable simulation results. Feng and Aymerich [33] concerned the development of a FE tool for predicting the damage response of composite foam sandwich structures under low-velocity impact. They proposed the detailed simulation of intralaminar and interlaminar damage. Experiments were performed to observe material damage and calibrate simulations. Qualitative agreement was obtained between FE results and experiment in terms of dynamic response and damage modes. Ivañez et al [34] developed a 3D finite element model to predict the dynamic flexural behavior of composite sandwich beams with foam core. In the FE model, they present the failure damage of woven composite facesheet by using Hou criteria. The results indicated that the compressive properties of foam core greatly influenced failure of sandwich beams directly even though composite facesheet has high strength. Wang et al [35] studied the impact behavior of foam-core sandwich panels subjected to low-velocity impact using experimental and numerical methods. They investigated the influence of the facesheet thickness, core thickness, impact energy and impactor size on the dynamic response and material damage of sandwich plates. When the facesheet thickness increase, the percent of absorbed energy and impact duration decrease, while the peak load increase. Lacy and Hwang [36] established a three-dimensional FE model to analyze the residual compressive strength after impact of sandwich composite structure. They determined several parameters of impacted plate including dent depth, damage area and core crushing zone throughout impact experiment. The damaged facesheet is presented by degraded stiffness meanwhile the constitutive law of intact and damaged core, which is considered as springs, is also different. Leijten et al [37] experimentally investigated the primary sandwich structures under low-velocity impact by considering the influence of different impact energy, materials and geometric sizes on the behavior of impact and compression after impact. They found that damage is more local for thicker core whereas global damage tended to appear for thinner core. The planar damage is influenced obviously by core thickness and density (but not by facesheet). The residual strength mainly depended on the facesheet damage instead of core damage. McQuigg et al [38], [39] studied the low-velocity impact and compression after impact behavior of honeycomb core sandwich panels with thin composite facesheet and had new understanding of damage tolerance for these materials.

Refined numerical models can provide sufficient detailed information of failure mechanism and damage evolution. Many literatures have focused simulation based on different methods containing several categories: failure criteria, fracture mechanics, damage mechanics and plastic theories [40], [41]. FEM simulations based on continuum damage mechanics(CDM) are often utilized with composite failure criteria as damage initiation and followed by stiffness degradation as damage evolution. Despite of complication and interaction of sandwich damage, the material model of composite facesheet and honeycomb core can be considered in FE model individually. On the one hand, the precision of simulation results depends on the failure criteria of composite laminate. Many researchers have investigated low-velocity impact of conventional composite laminates numerically [42], [43], [44], [45], [46].

On the other hand, honeycomb core can also influence the facesheet damage apparently. Early researches regard honeycomb cell as equivalent continuum material using solid element to improve calculation efficiency [47], [48]. However, these approaches might neglect the real behavior of cell walls especially at post-buckling stage. Three-dimensional FE models with detailed cell walls revealing accurate deformation damage progression are becoming more attractive [49], [50], [51], [52]. Honeycomb cell walls are considered as isotropic, orthotropic or further multilayer shells with plasticity.

As outlined above, this paper focuses on the description of low-velocity impact damage resistance of sandwich panels with CFRP facesheet and Nomex honeycomb core. In order to investigate the damage modes and sequence under different impact levels for two core thickness, a series of low-velocity impact tests on two different core thickness were performed through drop weight tower. The impact dynamic response of impacted sandwich panels was characterized in terms of peak force history, energy absorption and contact duration. Failure modes of specimens were observed subsequently through non-destructive inspection (NDI) ultrasonic C-scan and destructive methods. In addition, a refined three-dimensional finite element model was established with intralaminar and interlaminar damage of composite facesheet. Geometrical and material conditions of cell walls of honeycomb core were also considered by using the detailed meso-scale model. The intralaminar damage initiation and evolution of facesheet were predicted with physically-based composite failure criteria and energy-based progressive damage, respectively. While the interlaminar damage including facesheet delamination and facesheet/core debonding was simulated by means of cohesive element. This refined FE model is shown to be able to reproduce the damage of sandwich structures as well as the failure mechanism under low-velocity impact. An objective of the paper is, therefore, to figure out the impact damage with two core thickness under different impact energy levels. This research also aims to develop an available virtual testing model to provide thorough understanding of the damage characterization. Consequently, it can be used on new sandwich designs with wide range of possible configurations before manufacturing for reducing costs and time-consumption. Furthermore, the present FE model can be used to predict residual properties of pre-impact composite sandwich structures with different core thickness.