Review of low-velocity impact properties of composite materials

This paper is a review of low-velocity impact responses of composite materials. First the term ‘low-velocity impact’ is defined and major impact-induced damage modes are described from onset of damage through to final failure. Then, the effects of the composite's constituents on impact properties are discussed and post-impact performance is assessed in terms of residual strength.     

复合材料蜂窝夹芯结构低速冲击位置识别研究

进行了碳纤维增强复合材料蜂窝夹芯结构的低速冲击实验,采用一种基于应力波和免疫遗传算法的冲击载荷定位方法对蜂窝夹芯结构上的低速冲击载荷进行分析和定位。首先,通过一组事先确定冲击位置的低速冲击载荷产生的冲击应力波实验数据,使用小波变换方法对其在时频域进行分析,获得多个频率上冲击应力波在蜂窝夹芯结构中的传播速度;然后在此基础上,考虑蜂窝夹芯结构中应力波的各向异性特性,采用免疫遗传算法对未知的低速冲击载荷进行位置识别。实验研究结果表明了该方法的可行性和有效性     

热残余应力对内埋光纤光栅传感器性能的影响

将布拉格光纤光栅(FBG)埋植于复合材料T型加筋板结构非干涉区—三角填充区作为应变传感器对复合材料加筋板在固化过程及冲击后压缩过程中的应变变化进行监测。对比了光纤刻栅区采用UV光固化树脂涂层保护和未保护的两种FBG传感器的波谱信号变化; 分析了复合材料在固化成型过程中产生的非轴对称热残余应力对FBG传感性能的影响。结果表明, 刻栅区采用聚合物涂层保护的FBG传感器的半峰宽(FWHM)在固化过程中未发生变化, 并且聚合物涂层可以有效地消除非轴对称热残余应力对光纤光栅反射波谱的影响。在冲击后压缩过程中, 采用聚合物涂层保护的FBG传感器测得的应变与贴于试样表面的应变片测得的应变数据一致性较好。本文对埋植于复合材料加筋板三角填充区的FBG传感器在复合材料固化过程及冲击后压缩过程中应变监测的有效性及可靠性进行了有益的探索。     

Residual torsional strength after impact of CFRP tubes

This paper investigates the residual torsional strength of cylindrical T300-carbon/epoxy tubular specimens damaged by low-velocity impacts. A total of 24 specimens were subjected to a 7 J transverse impact under various torsional preloads. First, the torsional strength of four different lamination sequences is studied. Later it is compared with the residual torsional strength (RTS) of tubes impacted under different torsional preloads. FEM models were developed to investigate the effect of the impact-induced delaminations on the torsion-after-impact strength. The Acoustic Emission (AE) technique was used to study the damage propagation during the torsional loading. Results show that, even if the absorbed energy during impacts is the same, the residual torsional strength of the laminates is highly affected by the torsional preload.     

Drop-weight impact of plain-woven hybrid glass–graphite/toughened epoxy composites

The progressive damage behaviors of hybrid woven composite panels (101.6 mm × 101.6 mm) impacted by drop-weights at four different velocities were studied by a combined experimental and 3-D dynamic nonlinear finite element approach. The specimens tested were made of plain-weave hybrid S2 glass-IM7 graphite fibers/toughened epoxy (cured at 177 °C). The composite panels were damaged using a pressure-assisted Instron-Dynatup 8520 instrumented drop-weight impact tester. During these low-velocity simpact tests, the time-histories of impact-induced dynamic strains and impact forces were recorded. The damaged specimens were inspected visually and using ultrasonic C-Scan methods. The commercially available 3-D dynamic nonlinear finite element (FE) software, LS-DYNA, incorporated with a proposed user-defined damage-induced nonlinear orthotropic model, was then used to simulate the experimental results of drop-weight tests. Good agreement between experimental and FE results has been achieved when comparing dynamic force, strain histories and damage patterns from experimental measurements and FE simulations.     

Process-induced strains in RTM processing of polyurethane/carbon composites

The development of residual strains and stresses is critical to manufacture composite structures with the required dimensional stability and mechanical performance. This work uses Fiber Bragg Grating (FBG) sensors to monitor strain build-up in carbon fiber composites with a polyurethane (PU) matrix designed for high production volume applications. The PU matrix presents an initially low viscosity combined with a fast cure reaction, which makes it adequate to very short processing cycles. FBG sensors were incorporated into PU-matrix composites manufactured by vacuum assisted resin transfer molding (VARTM). The measured strains were compared with those obtained with different benchmark epoxy-matrix composites and with those obtained through micromechanical finite element simulations. Results showed that most of the residual strains were built-up during cool-down from the post-curing temperature and that stresses in the PU-matrix composites were comparable to those obtained for epoxies with similar T_g.     

Damage monitoring of CFRP stiffened panels under compressive load using FBG sensors

FBG sensors were embedded in each of two CFRP stiffened panels fabricated by VaRTM. Low-velocity impacts were applied to one of the panels in order to compare the methods of monitoring impact events using FBG sensors. The main impact damage was an interlaminar delamination inside the skin, which could be observed by an ultrasonic C-scan. A monitoring method using the full spectral signals was more effective in evaluating the impact damages in detail than that using the center wavelength. Following the impact tests, buckling behaviors were investigated under compressive loading using FBG sensors and surface-attached strain gauges. The FBG sensors could evaluate strain changes resulting from buckling behaviors under relatively low compressive loading. They could also evaluate damage growth until the final failure and difference of buckling behaviors between panels with and without impact damages.     

Study on the curing process for carbon/epoxy composites to reduce thermal residual stress

In this work, a cure monitoring system using dielectrometry and a fiber Bragg grating (FBG) sensor, was devised to measure the dissipation factor and thermal residual stress of carbon fiber-reinforced epoxy composite materials. Three rapid-cooling points, which were based on the cure initiation point, were chosen as test variables to investigate the effect of cure cycle on process-induced internal strain. The internal strains generated in the composite specimens were measured using embedded FBG sensors. Three-point bending tests were conducted to investigate the effect of thermal residual stress on the flexural strength of the composite specimens.     

Numerical modeling of impact on wood-based sandwich structures

Abstract

The impact behavior of innovative wood based sandwich structures with plywood core and skins made either of aluminum or of fiber reinforced polymer (carbon, glass, or flax composite skins) was investigated numerically. The wood based sandwich structures were subjected to low-velocity/low-energy impacts. An explicit nonlinear numerical model based on volume elements with a cohesive layer was developed. A plastic wood law already implemented in LS-DYNA was used in association with composite type damage criteria. Comparisons with experiments in terms of layer deformations and overall contact laws during impact showed satisfactory results.     

Impact and compression after impact experimental study of a composite laminate with a cork thermal shield

The aim of this paper is to present an experimental study of impact and compression after impact (CAI) tests performed on composite laminate covered with a cork thermal shield (TS) intended for launchers fairing. Drop weight impact tests have been performed on composite laminate sheets with and without TS in order to study its effect on the impact damage. The results show the TS is a good mechanical protection towards impact as well as a good impact revealing material. Nevertheless, totally different damage morphology is obtained during the impact test with or without TS, and in particular at high impact energy, the delaminated area is larger with TS. Afterwards, CAI tests have been performed in order to evaluate the TS effect on the residual strength. The TS appears to increase the residual strength for a same impact energy, but at the same time, it presents a decrease in residual strength before observing delamination. In fact, during the impact tests with TS, invisible fibres’ breakages appear before delamination damage contrary to the impacts on the unshielded sheets.     

Prediction of Impact Contact Forces of Composite Plates Using Fiber Optic Sensors and Neural Networks

Real-time determination of contact forces due to impact on composite plates is necessary for on-line impact damage detection and identification. We demonstrate the use of fiber optic strain sensor data as inputs to a neural network to obtain contact force history. An experimental and theoretical study is conducted to determine the in-plane strains of a clamped graphite/epoxy composite plate upon low-velocity impacts using surface-mounted extrinsic Fabry-Perot interferometric (EFPI) strain sensors. The plate is impacted with a semispherical impactor with various impact energies using the drop-weight technique. The significant features of the strain and contact force response are contact duration, peak strain, and strain rise time. We have designed and built an instrumented drop-weight impact tower to facilitate the measurement of contact force during an impact event. The impact head assembly incorporates a load cell to measure the contact forces experimentally. An in-house finite-element program is used to establish the validity of the EFPI fiber optic sensor contact force response. The finite-element model is based on a higher-order shear deformation theory and accounts for geometric nonlinearity. Experimental load cell data and finite-element impact-induced contact force responses are in close agreement. The load cell data is used to train a three-layer feed-forward neural network which utilizes the Delta Bar Delta back-propagation algorithm. The output of the neural network simulation is the contact force history and the inputs are fiber optic sensor data in two different locations and time in 10-ms intervals. The efficiency and accuracy of the neural network method is discussed. The neural network scheme recovers the impact contact forces without using any complex signal processing techniques.     

Effect of autofrettage on durability of CFRP composite cylinders subjected to out-of-plane loading

The objective of this study is to evaluate effects of autofrettage on durability of composite cylinders subjected to impact loading. Impact tests and damage observation were performed on the composite cylinders with and without autofrettage. Internal pressure tests were also conducted on the cylinders after impact tests. Simulated cycle tests were conducted on the rectangular plate specimens simulated as composite cylinders after impact tests. For the composite cylinder with autofrettage, absorbed energy became smaller, which was due to suppression of the damages, such as delamination. As a result, residual strength became larger and fatigue life became longer. It was clarified that CFRP composite cylinders with proper autofrettage were superior in the impact resistance.     

Low-velocity impact and residual tensile strength analysis to carbon fiber composite laminates

In this paper, low-velocity impact characteristics and residual tensile strength of carbon fiber composite laminates are investigated by experimentally and numerically. Low-velocity impact tests and residual tensile strength tests are performed using an instrumented drop-weight machine (Instron 9250HV) and static test machine (Instron 5569), respectively. The finite element (FE) software, ABAQUS/Explicit is employed to simulate low-velocity impact characteristics and predict residual tensile strength of carbon fiber composites laminates. These numerical investigations create a user-defined material subroutine (VUMAT) to enhance the damage simulation which includes Hashin and Yeh failure criteria. The impact contact force and the tensile strength are accurately estimated using the present method. Two different tensile damage modes after different impact energies are observed. The degradation of residual tensile strengths can be divided to three stages for different impact energies, and amplitudes of degradation are affected by stacking sequences.     

Predicting low velocity impact damage and Compression-After-Impact (CAI) behaviour of composite laminates

Low-velocity impact damage can drastically reduce the residual strength of a composite structure even when the damage is barely visible. The ability to computationally predict the extent of damage and compression-after-impact (CAI) strength of a composite structure can potentially lead to the exploration of a larger design space without incurring significant time and cost penalties. A high-fidelity three-dimensional composite damage model, to predict both low-velocity impact damage and CAI strength of composite laminates, has been developed and implemented as a user material subroutine in the commercial finite element package, ABAQUS/Explicit. The intralaminar damage model component accounts for physically-based tensile and compressive failure mechanisms, of the fibres and matrix, when subjected to a three-dimensional stress state. Cohesive behaviour was employed to model the interlaminar failure between plies with a bi-linear traction–separation law for capturing damage onset and subsequent damage evolution. The virtual tests, set up in ABAQUS/Explicit, were executed in three steps, one to capture the impact damage, the second to stabilize the specimen by imposing new boundary conditions required for compression testing, and the third to predict the CAI strength. The observed intralaminar damage features, delamination damage area as well as residual strength are discussed. It is shown that the predicted results for impact damage and CAI strength correlated well with experimental testing without the need of model calibration which is often required with other damage models.     

基于FBG传感技术的复合材料T型加筋板低速冲击损伤监测

针对碳纤维增强树脂复合材料低速冲击损伤的实时监测,设计将布拉格光纤光栅（FBG）传感器埋植在复合材料T型加筋板结构的三角填充区,在线监测复合材料T型加筋板冲击损伤过程。分别将FBG传感器埋植于复合材料层合板内部和复合材料T型加筋板的三角填充区,对比FBG传感器的埋入对复合材料层合板和复合材料T型加筋板力学性能的影响。结果表明,内埋FBG传感器的复合材料层合板试样的拉伸强度比未埋植传感器的层合板试样降低了约5%,但在FBG传感器的破坏应变范围内,FBG传感器可以准确、实时地监测复合材料的应变信号。将FBG传感器埋入复合材料T型加筋板的三角填充区,内埋FBG传感器的T型加筋板样件压缩破坏载荷与未埋植的样件基本一致。通过对比T型加筋板蒙皮上冲击位置、冲击能量对FBG传感器测得的冲击过程持续时间和最大应变值的影响,表明冲击过程持续时间随着冲击能量增大而延长,最大应变值随着冲击距离的增加呈下降趋势,而最大应变值随着冲击能量的增大呈上升趋势。利用FBG传感器测得的应变信号可初步实现对复合材料T型加筋板蒙皮冲击损伤位置及冲击能量的实时监测。      

Low velocity impact damage evaluation in fiber glass composite plates using PZT sensors

In this work PZT sensors are proposed to characterize the impact effects in fiber glass composite plates. To provide effective and reliable results the analysis was accomplished on samples submitted to multiple impacts, then guaranteeing the same testing conditions for different impact levels. The analysis of the impact effect has been made by two parameters; amplitude response and time shift. PZT sensors have been bonded to the samples in a pitch-and-catch configuration and the Lamb wave symmetrical mode (S₀) signal was used. The results demonstrate that the two evaluated parameters are able to characterise the different damage types occurring in these composites, as well as to evaluate their severity. It was also observed that amplitude predict well the defect size, whenever fiber-breakage occurs.     

Effect of repeated impacts on the response of plain-woven hybrid composites

The repeated low-velocity impact responses of hybrid plain-woven composite panels were studied by drop-weight experiments. Non-hybrid S2-glass-fiber/toughened epoxy and IM7 graphite fiber/toughened epoxy as well as hybrid S2-glass–IM7 graphite fiber/toughened epoxy composite panels were impacted repeatedly using a pressure-assisted Instron-Dynatup 8520 instrumented drop-weight impact tester. During the low-velocity impact tests, the time histories of impact forces, absorbed impact energies and panel central deflections were recorded. The relations between the impact force and central deflection, whose slope represented the dynamic contact stiffness, were then constructed. The damaged specimens were inspected visually and using the ultrasonic C-Scan method. The effects of hybridization and lay-up sequence on the repeated drop-weight impact responses of woven composites were investigated. It was observed that damage accumulations could be slowed down using hybridization. It was also witnessed that the lay-up configuration of a hybrid composite had a significant influence on damage accumulation rate. The hybrid specimens with glass–epoxy skins survived the double number of successive impacts compared to hybrid specimens with graphite–epoxy skins.     

Low energy impact damage monitoring of composites using dynamic strain signals from FBG sensors - Part I: Impact detection and localization

Fiber reinforced composite materials risk to suffer from subsurface, barely visible, damage induced by transverse relatively low energy impacts. This two-paper series presents a method for the localization of an impact and identification of an eventual damage using dynamic strain signals from fiber Bragg grating (FBG) sensors. In this paper, the localization method allowing to predict the impact position based on interpolation of a reference data set is developed and validated. The data utilized in the method are the arrival times of the asymmetric zero order Lamb waves at the different sensors. A high rate interrogation method based on intensity modulation of the Bragg wavelength shift is used to acquire the FBG signals. The localization method allows to predict the impact position with a good accuracy and therefore the inspection of the laminate can be limited to this region.     

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.     

Bird strike event monitoring in a composite UAV wing using high speed optical fiber sensing system

In this study, high speed bird strikes on a composite structure were successfully monitored using optical fiber sensors. Four multiplexed optical fiber sensors in a single cable were surface-bonded on the leading edge of a composite UAV wing box. In order to acquire those high frequency signals, a newly developed interrogation system was used to process strain signals from four sensors simultaneously at a sampling frequency of 100 kHz. Before the bird strike tests, pre-impact tests using a rubber hammer were performed to verify the suitability of the FBG signal acquisitions. The pre-test data were used in the neural network training procedures to estimate the bird strike locations. Then, the bird strike tests were accomplished using dummy projectiles and a pneumatic gun. The one-pound dummy birds, made of gelatin, hit the leading edge with a maximum speed of 201 km/h. The impact signals were successfully recorded during the tests and their frequency characteristics were then analyzed. Finally, the strike locations were estimated with the neural network which was trained through the pre-tests. The average error was 33.6 mm.