Interlaminar Crack Propagation Research Articles

Sandwich-structured electrospun polyvinylidene difluoride sensor for structural health monitoring of glass fiber reinforced polymer composites

Stress monitoring and interlaminar failure detection attract much attention in glass fiber reinforced polymer (GFRP) structural health monitoring area. However, due to limitations of sensing or electrode materials, existing embedding sensors cannot be designed to have both of the abilities without sacrificing mechanical properties. This work fabricated a sandwich-structured sensor, which is composed of three layers of electrospun polyvinylidene difluoride membranes. The upper and lower electrodes are flexible membranes that ensure stable signal collection in the rigid GFRP material system. The sensor can quantitatively monitor the stress based on piezoelectric effect, and interlaminar crack propagation based on parallel plate capacitors. The voltage and capacitance values have a linear relationship with the stress level and crack length (R-square of the functions is 0.999 and 0.933), respectively. Due to the porous microstructure of electrospun membranes, polymer matric can well infiltrate the polyvinylidene difluoride nanofibers while preparing the sensor embedded GFRP. Thus, the perfect bonding of the sensor within GFRP ensures the effective sensing abilities until sample failure and negligible effect (< -7%) on the mechanical properties of GFRP.

A new interpretation of mode I interlaminar fracture in layered materials

Interlaminar crack propagation in layered materials, such as composites, is still a not fully understood phenomenon in fracture mechanics. Experimental observations reveal a broad spectrum of crack propagation velocities during the interlaminar fracture of layered materials under varying loading conditions and rates. While there have been numerous studies examining interlaminar crack propagation under quasi-static, fatigue, and more recently, high-speed loading, there has been a notable lack of endeavors to present and interpret crack growth data within a unified framework. To this direction, we present a new interpretation of interlaminar crack propagation data acquired from a broad test campaign under various mode-I loading conditions (quasi-static, fatigue and high-speed). We connect the crack tip velocity to the rate of change of the SERR to propose a new equation/model that shows potential in presenting and explaining the variety of experimental crack growth data. This correlation arises from a simple mathematical analysis of the rate of change of the SERR using the chain differentiation rule. Subsequently, it is further substantiated by experimental evidence, in an effort to accommodate results from various experiments under a unified description. A robust correlation of the test data is established through a range of slopes which are proven -experimentally- to be characteristic constants of the mode-I interlaminar fracture of a specific material system.

Inspired by nacre: α-ZrP three-dimensional network structures for interlaminar toughening of laminates

This work enhancing the mechanical properties of composite laminates by using a three-dimensional network structure that mimics the nacre-like structure. The optimal interlaminar structure was developed by utilizing α-ZrP with ultrasonic-assisted exfoliation for 6 min. The mode-I critical energy release rate (GIC) and mode-II critical energy release rate (GIIC) of the toughened composite are 104.44 % and 74.20 % higher than those of the unmodified sample. Observation of the region around the crack by SEM and OM reveals that the three-dimensional network structure across the fiber fabric undergoes energy dissipation behaviors, such as fracture, slip and pull-out when cracks penetrate thin walls. In addition, the effect of the thin-walled structure of the α-ZrP aerogel on interlaminar crack propagation is discussed, taking into account the inhomogeneous and multi-surface of the composite interfaces, and the toughening mechanism is investigated.

Mitigating Crack Propagation in Hybrid Composites: An Experimental and Computational Study

The exceptional properties of carbon nanotubes (CNTs) make them ideal nanofillers for various composite materials. In carbon fiber-reinforced polymer (CFRP) composites. CNTs can be grown on the carbon fiber surface to act as a third interface between the fiber and the matrix. However, it was established that the uncontrolled random growth of CNTs could exacerbate delamination in composite structures. Thick nanofiller films could hinder the epoxy from seeping into the carbon fiber, resulting in insufficient interlaminar strength. Hence, the density and distribution of nanofillers play a crucial role in determining the hybrid composite fracture mechanisms. In this investigation, CNTs were grown using the low-temperature technique into specific patterns over carbon fibers to discern their derived composites’ fracture properties. The composite fracture energy release was probed using a double cantilever beam (DCB) test setup and digital image correlation (DIC) to monitor interlaminar crack propagation. A standard finite element simulation model based on the cohesive zone method (CZM) was also utilized to delineate fracture behaviors of the various composite configurations. Results conclude that a coarser pattern of CNT growth enhances resistance to crack propagation, thus improving the interlaminar fracture toughness of a composite structure.

Effect of interlaminar basalt fiber veil reinforcement on mode I fracture toughness of basalt fiber composites

AbstractComposite structures are vulnerable to delamination under out‐of‐plane loading due to the absence of reinforcing fibers in the thickness direction. This paper introduced the interlaminar basalt fiber veil technique to improve the interlaminar fracture toughness of basalt fiber‐reinforced thermoplastic composite laminates. The reinforcement effect of the basalt fiber veil on GIC of composite laminates was investigated by a double cantilever beam (DCB) test. The toughening effect, damage morphology, and the basalt fiber veil toughening mechanism were analyzed by mechanical response, optical photos, and scanning electron microscopy (SEM). It was found that the basalt fiber veil could significantly inhibit the propagation of interlaminar cracks. Compared to the unreinforced specimen, the maximum load of basalt fiber veil‐reinforced laminate was increased by 73%, the average platform load was increased by 72.7% and the GIC was increased by 157%.Highlights Improvement of GIC of basalt fiber‐reinforced thermoplastic laminates by basalt fiber veil. The toughening mechanism for the basalt fiber veil. The GIC of the fiber veil‐reinforced laminate was 157% higher than the unreinforced laminate.

Mechanical behavior of the polyamide 6.6 nanofiber and MWCNT‐reinforced hybrid nanocomposites

AbstractThe present study examines the influence of polyamide 6.6 (PA 6.6) nanofiber and MWCNTs reinforcements on the mechanical behavior of nanocomposite plates formed by hybridization of carbon fabric (CF) and kevlar fabric (KF). First, PA 6.6 nanofibers were manufactured by electrospinning. The produced nanofibers were placed in the interlaminar of nanocomposite plates. PA 6.6 nanofiber and MWCNT‐reinforced nanocomposite plates consisting of 12 layers were produced by the vacuum bagging method. Tensile, Mode‐II fracture and bending tests were applied to the produced composite plates in accordance with the standards. After the mechanical tests, the nanocomposite plates' macro and micro damage analysis was performed. As a result of the reinforcement of PA 6.6, MWCNTs and PA 6.6‐MWCNTs, the tensile strength, interlaminar fracture toughness and bending strength of the nanocomposite plates significantly enhanced compared to unreinforced samples. In addition, interlaminar crack propagation in reinforced samples became more stable than in the unreinforced sample.Highlights Investigated the impact of PA 6.6 nanofiber and MWCNT reinforcements on nanocomposite plates formed through CF and KF hybridization. Reinforcement with PA 6.6, MWCNTs, and PA 6.6‐MWCNTs significantly improved tensile strength, interlaminar fracture toughness, and bending strength. Mode‐II interlaminar fracture toughness tests demonstrated enhanced crack propagation stability with PA 6.6 nanofiber and MWCNT reinforcements.

Composite laminates reinforced by vertically aligned carbon nanotubes: Detailed manufacturing process, from nanotubes transfer to composite consolidation

Nano-constituent incorporation into composites has been extensively studied in the literature due to its improvement in static and dynamic mechanical properties, as well as its prevention from interlaminar crack initiation and propagation. This work introduces a detailed manufacturing process of nano-engineered composite laminates, from impregnation to consolidation, without damaging the initial morphology of carbon nanotubes transferred on prepreg interfaces. Based on prepreg and vertically aligned carbon nanotubes (VACNTs) synthesized on an aluminium alloy (Al) substrate, the impregnation step allows for the transfer of VACNTs onto the prepreg surface through partial resin-rising capillarity. The Al alloy substrate is then removed from the VACNTs through a separation step, ensuring highly effective and repeatable transfer with more than 80% VACNTs transferred onto the prepreg surface. This paper provides insight into the impregnation and transfer processes and guides the choice of process parameters to ensure minimal VACNTs buckling during the consolidation of the hybrid composites at high pressure in an autoclave.

Numerical investigation on behaviors of composite laminates with initial delamination defects under impact and compression after impact

AbstractThe initial delamination defects may arise during the preparation of carbon fiber‐reinforced polymer (CFRP) composite laminates, so its effects need to be considered in the evolution of mechanical properties. In this article, the behaviors of composite laminates with initial delamination defects under impact and compression after impact (CAI) are investigated by numerical method. The simulation framework composed of the micromechanics of failure (MMF) theory and the mixed mode exponential cohesive zone model (ECZM) is constructed. The numerical analysis compares the effects of the interaction between four different initial delamination defects and impact loading on impact response and CAI strength at two impact velocity levels (1.336 and 2.314 m/s). Furthermore, the failure mechanisms of laminates with initial delamination defect are disclosed under CAI loading. The findings indicate that the difference in impact response is minor. The CAI strength is reduced from 250.5/173.5 to 191.5 MPa/139.1 MPa. The sufficient condition for the further significant reduction of CAI strength is that the initial delamination defects merge with the impact‐induced delamination cracks in the width direction of laminates. Moreover, the failure mechanism for CAI condition is mostly matrix damage and interlaminar crack propagation. This work provides a valuable reference for engineering failure analysis of composite laminates.

Effect of Moisture Absorption on Mode II Fracture Behavior of Fumed Silica Reinforced Hybrid Fiber Composite

ABSTRACT This work aims to investigate the influence of water ingression on Mode II interlaminar fracture behavior of a fumed silica-reinforced jute-kevlar hybrid nanocomposite. Shear mode of failure is characterized by using End Notched Flexure (ENF) specimens. Thirteen different composites are prepared based on four layering sequences (i.e., Jute-Jute-Jute-Jute [JJJJ], Jute-Kevlar-Kevlar-Jute [JKKJ], Kevlar-Jute-Jute-Kevlar [KJJK], and Kevlar-Kevlar-Kevlar-Kevlar [KKKK]) and four nanofiller weight proportions (0, 1.5, 3, and 4.5%), with the aim of studying the impact of the layering sequence and nanofiller proportion. Initiation fracture toughness (GIIC init.) is reported for dry and wet conditions of the composites using the Simple Beam Theory (SBT) data reduction method. Among the reported stacking sequences, the stacking sequence JKKJ possesses the highest fracture-resistant material in both dry and wet conditions because of the active involvement of the fiber and matrices at the interlaminar layer resulting from interlaminar crack propagation. The water-aged composite JKKJ with a 3% nanofiller weight proportion shows the highest initiation fracture toughness (GIIC init.) value, 1811.4 J/m2. The composite with the stacking sequence JKKJ and 3% nanofiller weight proportion is recommended, which is proved to be the best Mode II fracture-resistant material under wet conditions among the reported composites.

A new approach to attenuate low velocity impact damages on CFRP structures

Carbon Fibre Reinforced Polymers (CFRP) have been widely used in advanced markets, such as, the aeronautic, aerospace and military ones. Despite their extraordinary mechanical properties and low density, advanced composites, due to their intrinsic brittleness and layer-by-layer nature, present some vulnerability when submitted to impact, dynamic and flexural loading. Low velocity impacts (LVI) are one of the most dangerous events that composite laminate can face in their life time. These solicitations may develop imperceptible internal laminate damages, namely delaminations, that tend of propagate in service, compromising part performance. In order to improve interlaminar crack propagation, some works were already carried out using interleaved thin veils between laminate layers to enhance the interlaminar Mode I and II fracture toughness. In this work, a study was carried out on the distribution of stresses across the thickness of a carbon/epoxide laminate typically used in structural aircraft components, to define between which specific layers would be better to interleave thin veils made from four different materials (glass, carbon, aramid and polyester) to achieve better LVI damage tolerance. Samples made from the selected structural carbon/epoxy laminate were then produced by vacuum bag infusion with those thin veils located between their most appropriated layers to be submitted experimental testing and compare their mechanical and LVI performances with those without using interleaved veils. All studied laminates were observed under scanning electron microscopes (SEM) for assessing their processing quality and submitted to interlaminar shear strength (ILSS) and LVI tests to evaluate their delamination resistance and impact response, respectively.

The effects of back-up on drilling machinability of filament wound GFRP composite pipes: Mechanical characterization and drilling tests

Composite structures used in many engineering fields are produced using different production methods according to the design characteristic. The filament winding (FW) method, one of the most important of these production techniques, is preferred in the manufacturing of high-pressure vessels, pressure pipelines, and construction components. Composite pipes produced by the FW method are assembled using various processing methods depending on their usage. Drilling is one of the most used method among these processing techniques. The drilling-induced damages negatively affect the mechanical properties of the composite pipes during service. Therefore, it is important to determine the optimum cutting parameters and a suitable back-up material so that minimal damage occurs during the drilling process. In this study, firstly, mechanical properties of the glass fiber reinforced polymer (GFRP) produced by the FW method at ± 55° configuration were determined and damage characteristic based on ring tensile test was analyzed. Then, the effects of back-up use on the drilling machinability of FW GFRP pipes were investigated. Results showed that back-up use in drilling composite pipes plays a key role in reducing delamination, borehole damages, and surface roughness although it leads to occur higher thrust forces. Also, it helps prevent the propagation of interlaminar cracks around the hole. • Back-up use slightly increase the thrust force in drilling of FW GFRP pipes. • In drilling without using back-up, more severe delamination damages occur in the area close to the hole edges owing to intralaminar and interlaminar cracks. • Damage modes were intralaminar cracks, fiber breakages, uncut fibers, and debonding. • Back-up use improves surface quality in drilling FW GFRP pipes.

Effects of delamination in drilling glass/polyester composite

Considering failures during machinery processes such as drilling, a precautionary analysis involving delamination and the corresponding dissipated energy is required, especially for composite structures. In this context, because of the complexity of both the analysis procedure and experimental test setup, most studies prefer to represent mode I and III interlaminar crack propagation instead of that involving mode II. Therefore, in this study, the effect of mode II delamination and corresponding interlaminar crack propagation was considered during the drilling process of multilayered glass/polyester composites using both numerical and experimental approaches. In the experimental procedure, the mechanical properties of the glass/polyester specimens were obtained according to ASTM D3039. In addition, the interlaminar mixed-mode (I/II) loadings were determined using an ARCAN test fixture so that the fracture toughness of glass/polyester could then be identified. The mode II critical strain energy release rate (CSERR) was then obtained using an experimental test performed using an ARCAN fixture and the virtual crack closure technique (VCCT). It was determined that the numerical approach was in accordance with the experiments, and more than 95% of crack propagation could be attributed to mode II compared to the two other modes.

Effects of graphene geometrical characteristics to the interlaminar fracture toughness of CFRP laminates

This research paper explores and highlights the effect of aspect ratio (AR) and specific surface area (SSA) of Graphene Nano-Platelets (GNPs) on the interlaminar fracture behavior of carbon fiber-reinforced polymer (CFRP) composites. Various types of GNPs having different aspect ratios were dispersed in an epoxy system by using the three roll milling technique at a fixed content of 0.5 wt.%. The blends resulting from this procedure were used for the impregnation of carbon fabrics aiming in obtaining nano-modified epoxy pre-impregnated fabrics (Prepregs). The prepreg material was used for the fabrication of CFRPs by using the autoclave technique. The resistance of composites to interlaminar crack propagation was measured by performing Mode I and II fracture tests utilizing the DCB and ENF configurations. The fracture surfaces of the tested samples were investigated post-mortem using Scanning Electron Microscopy (SEM). We show that the graphene Nano-Platelets with higher AR and SSA combined lead to enhanced interlaminar fracture toughness under mode I and mode II loading as compared to the unmodified composite system.

Fracture and fatigue behavior of Bambusa Vulgaris-Schrad Bamboo

This paper presents the results of an experimental study of the fatigue and fracture behavior of Bambusa Vulgaris-Schrad bamboo. Mechanisms of crack initiation and growth are elucidated under monotonic and cyclic compressive failure or flexural loading. The microscopic studies of fatigue and fracture explore the effects of fiber/crack/ply orientation on crack growth and toughening mechanisms in single-edge notched and double -edge notched fracture mechanics bend specimens, which are studied in the “crack-arrestor” and “crack-divider” orientations. The compressive fatigue behavior of Bambusa Vulgaris-Schrad bamboo is also investigated in smooth un-notched specimens. The resulting stress-life behavior is shown to occur in a regime in which the maximum stresses exceed the critical conditions for the onset of progressive fiber buckling and shear band formation. This results ultimately in the nucleation and propagation of interlaminar cracks and shear cracks across the Bambusa Vulgaris-Schrad structures. The implications of the results are discussed for the design of robust bamboo structures.

Improved mode Ⅰ interlaminar fracture toughness of random polypropylene composite laminate via multiscale reinforcing formed by introducing functional nanofibrillated cellulose

Interface engineering and phase engineering of composites offer a promising route to rationally tune the interlaminar toughness of laminates, as high toughness plays an important role in preventing interlaminar failure and ensuring the safety of components. Herein, unlike the previous melt-mixing process, we coated the second reinforcement TONFC (TEMPO-oxidized nanofibrillated cellulose) on the surface of the main reinforcement (continuous glass fiber) by sol impregnation, and successfully prepared the commingled yarn random polypropylene (PPR) composite laminate with high interlaminar fracture toughness. The surface groups of each raw material of the composite system before and after the reaction were characterized and thereby obtaining the interface strengthening mechanism. Besides, the heterogeneous nucleation effect of TONFC and the mechanism of inducing ꞵ-phase formation were revealed by analysis of crystallization behavior and crystal structure. Further, through a double cantilever beam (DCB) test and morphology analysis, the effect mechanism of the addition of TONFC on the interlaminar crack propagation of PPR composite laminate was explored. This work provides a rapid and efficient avenue for the interlaminar modification of commingled yarn composites.

A comparative study of a quasi 3D woven composite with UD and 2D woven laminates

To develop a composite with high in-plane properties and sufficient delamination resistance, a quasi-three-dimensional (Q3D) woven composite has been designed. In Q3D, some yarns in each layer are woven into those in the adjacent layers and form a 3D interlocking network. This study examines the performance of a carbon fiber/SC-15 epoxy triaxial Q3D [(0°/60°/−60°)4] composite. For comparison, a [0°/60°/−60°]4 composite laminate with unidirectional (UD) plies and a [0°/60°/−60°]4 laminate with two-dimensional woven (2DW) layers were fabricated using the same carbon fiber and epoxy system. The in-plane tensile, interlaminar fracture, and low-velocity impact tests were performed for all three types of composites. The results show that the Q3D composite has the in-plane tensile modulus and strength comparable to that of UD and 2DW composites, but offers a higher resistance to interlaminar crack propagation and impact damage. This work provides insights into the influences of fiber structures on composite properties and performances.

Improving delamination resistance through tailored defects

Delamination is a well-known damage mode exhibited by laminated composites which has been extensively studied in unidirectional laminates (UD). In this work, a novel strategy consisting of the introduction of a small interlaminar defect to enhance the fracture resistance to delamination of UD composites is presented. This toughening concept is based on the simultaneous propagation of multiple interlaminar cracks along different interfaces. A preliminary numerical study showed that the minimum size required by the initial defect to trigger dual propagation was 4 times the ply thickness for intermediate and thin plies for any applied mixed mode, except for pure mode II. An experimental campaign to prove the toughening concept with 1 and 3 defects at adjacent interfaces was carried out and compared against a reference configuration under three different applied mode mixities (ϕglob=0.4,0.2 and 0). The single defect configuration reached an improvement in the fracture resistance of +300% under pure applied mode I, and +80% and +100% for applied mixed modes of 0.2 and 0.4, respectively. The configuration with 3 defects reached even higher fracture toughness values: +430% in pure applied mode I, and +115% and +100% for applied mixed modes of 0.2 and 0.4, respectively. A finite element model employing cohesive zones successfully predicted both, the phenomenon of multiple crack propagation, and the effective fracture resistance of the process. Interestingly, among the dissipation mechanisms reported experimentally, the extensive promotion of fiber bridging under applied pure mode I was observed, which was not reported in the baseline configuration.

Temperature effect on the mechanical properties and damage mechanisms of a glass/thermoplastic laminate

The aim of this study is to evaluate the temperature effect on the mechanical properties and damage mechanisms of a Glass/Elium 150 laminate composite. Quasi-static indentation tests are carried out at different temperatures to highlight the temperature dependency of different parameters of the samples (stiffness, maximum load, stored elastic energy, and applied energy). The different damage mechanisms involved in the collapse of the composite are observed at the macro-, meso-, and micro-scopic scales using optical and scanning electron microscopy. The influence of temperature on these damage mechanisms is discussed based on post-mortem observations. It has been highlighted that the kind and the severity of damage are strongly temperature dependent. Below 20℃, fiber breakage, strand failure, intra- and inter-laminar crack propagation are identified. At 60℃, plastic flow of the polymer matrix is observed, which modifies micro-crack propagation at the fiber/matrix interface. Above 90℃, only intra-laminar micro-cracking occurs. To conclude, based on all these observations the kinetics of damage appearance are discussed.

Investigation of mode II interlaminar fracture toughness of lignocellulosic laminated specimens – an experimental and numerical approach

Mode II interlaminar fracture toughness of lignocellulosic laminate composite materials made of beech veneer with urea formaldehyde resin and rye flour was investigated using the end notched flexure specimens (ENF). In order to determine the critical value of the strain energy release rate GIIc associated with the interlaminar crack onset, the experimental data analysis was conducted by means of a mixed mode bending device in conjunction with a compliance calibration approach for data reduction. The interlaminar crack propagation under pure mode-II loading was also simulated by finite element analysis using general contact in Abaqus/Explicit, with Virtual Crack Closure Technique (VCCT) as fracture criterion. Good agreements between the load-displacement curves obtained through the use of finite element analyses and the experimental testing results were obtained.

Improved interlaminar crack resistance of glass fiber/poly (phenylene sulfide) thermoplastic composites modified with multiwalled carbon nanotubes

ABSTRACTThe influences of carboxyl multiwalled carbon nanotubes (MWNTs) on the interlaminar crack initiation and propagation behaviors of glass fiber reinforced poly(phenylene sulphide) (PPS) composites prepared by aqueous suspension prepregging were studied. Interlaminar shear strength (ILSS) as well as the mode‐I fracture toughness was evaluated as a function of the MWNTs contents in the PPS resin. Failure morphology was also investigated to figure out the toughening mechanism. The laminates modified with 1 wt% MWNTs were found to perform 23% and 25% higher ILSS and fracture toughness, respectively. The significant increase indicated that interlaminar damage resistance and tolerance were raised by the addition of MWNTs.