Interlaminar Fracture Toughness Tests Research Articles

Influence of Interfacial Interaction and Composition on Fracture Toughness and Impact Properties of Carbon Fiber-Reinforced Polyethersulfone.

In this study, the interlaminar fracture toughness and impact strength of polyethersulfone reinforced with continuous carbon fibers were studied. Interlaminar fracture toughness tests were performed using the double cantilever beam method. It was shown that surface modification using the thermal oxidation method of the carbon fibers can strongly increase the interlaminar fracture toughness of the obtained composites. Thus, the maximum value reached 1.72 kJ/m2, which was 40% higher than the fracture toughness of the composites reinforced with initial carbon fibers. Moreover, fractographic analysis using a scanning electron microscope allowed us to highlight the main reasons for the dependence of fracture toughness on fiber content and surface modification conditions of the carbon fibers. It was shown that the main factor that allowed for an increase in fracture toughness was the enhanced interfacial interaction between the fibers and polymer matrix. Additionally, it was found that expectedly, there was a good correlation between interlaminar fracture toughness and interlaminar shear strength results. However, a negative influence of surface modification on the impact properties of composites was found. Such behavior occurred because of higher structural stability and lower exposure to delamination in multiple layers of the composites reinforced with the modified carbon fibers. It was found that impact energy reached ~150 kJ/m2 for the polyethersulfone-based composites reinforced with initial fibers, while the composites reinforced with modified carbon fibers showed impact energy values of only ~80 kJ/m2. Nevertheless, surface modification of carbon fibers using the thermal oxidation method can be an effective method for improving the performance properties of polyethersulfone-based composite materials.

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.

Hierarchical aerospace epoxy composites of carbon fiber and hyperbranched filler: toughening behavior from nanocomposites to composites

In this work the effect of a glassy hyperbranched polymer (G_HBP), characterized by a Tg higher than room temperature (∼77 °C), on the mechanical properties of a patented epoxy resin was investigated. To define an optimized filler concentration, three different G_HBP contents were prepared by a solvent-free technique and improvements of fracture toughness compared to the neat matrix (ΔKC ∼ 16 % and ΔGC ∼ 39 %) and to the corresponding laboratory solvent-based procedure. The solvent-free procedure and the found optimal filler content were considered to manufactured long carbon fiber composite samples of both neat and G_HBP loaded matrix later tested and compared in terms of tensile and flexural properties, as well as fracture toughness performance. Mode I and II interlaminar fracture toughness tests were carried out, revealing remarkable increase of both onset and propagation (ΔGIConset ∼ 29 % and ΔGICprop ∼ 53 %) and preserving the GIIC value (ΔGIIC∼+ 6 %). Validation tests in compression after impact were also performed and analyzed.

Fracture toughness response of glass/epoxy composite in symmetric and asymmetric design layup

Composites are used in transportation industries for their exceptional strength, reduced weight, and cost. Their performance depends upon many factors, such as stacking sequence, fiber volume fraction (FVF), etc. In the present work, glass/epoxy composites are prepared in symmetric (ɸn/θ2n/ɸn//ɸn/θ2n/ɸn) and asymmetric (θn/ɸ2n/θn//ɸn/θ2n/ɸn) configuration with FVF of 48.13 and 61.34%. The laminate performance was evaluated using interlaminar shear strength, flexural, and mode-II interlaminar fracture toughness tests. The results indicate that symmetric layup has better mechanical strength than asymmetric layup. The interlaminar shear strength, flexural, and fracture toughness of asymmetric layup were lower by 14.72–16.39, 12.54–16.72, and 11.92–17.98%, respectively. Further, low FVF specimens had better mechanical strength, and high FVF specimens showed lower strength by 3.67–5.56, 10.75–15.17, and 23.53–28.79%, respectively,

Temperature‐dependent inter‐laminar fracture behavior of waste short carbon fiber embedded glass fiber/epoxy composites

AbstractDelamination is one of the life‐limiting failure modes for fiber reinforced polymer (FRP) composite composites resulting in poor damage tolerance, as this failure mode occurs at considerably lower loads. Owing to the enormous demand and mass production of FRP composites, a large amount of fiber waste is produced worldwide. This triggers concern about utilizing these fiber wastes for some valuable applications. The present study aims to improve the overall damage tolerance of laminated glass/epoxy composites via the introduction of waste short carbon fibers (SCFs) reinforcement in the epoxy matrix. The effect of various SCF contents (0.1, 0.3, and 0.5 wt%) in glass fiber/epoxy composites were evaluated experimentally for their interlaminar fracture toughness in terms of both crack development and propagation under mode I and II interlaminar fracture toughness tests, at elevated temperatures (30, 50, and 70°C). Based on the experimental results, glass/epoxy composite with 0.1 wt% of SCF revealed 13.49% and 20.45% increment in mode I and II interlaminar fracture toughness, respectively, at ambient temperature. A positive reinforcement effect could be noticed for GIC and GIIC values up to 50°C. However, due to residual interfacial stresses, interfacial debonding, and epoxy softening, a negative reinforcement effect could be noticed at 70°C. Delamination initiation and crack propagation mechanisms have been explored at elevated temperatures. Fractographic studies have been carried out using a scanning electron microscope to identify the failure mechanisms.

Composite Interlaminar Fracture Toughness Enhancement Using Electrospun PPO Fiber Veils Regulated by Functionalized CNTs.

In this study, carbon nanotubes (CNTs) are functionalized through diazonium salt reaction to introduce polar groups onto their surfaces. These functionalized CNTs (FCNTs) are added into PPO solutions at different loadings (0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%) and used for electrospinning. The results show that the addition of FCNTs facilitate the production of PPO veils having small fiber diameters. The veils are used as interleaves in CF/EP composite laminates. The Mode I and Mode II interlaminar fracture toughness tests reveal that PPO veils containing 0.5 wt% FCNTs exhibit the optimal toughening. GICini and GIIC have an improvement of approximately 120% and 180% over the untoughened samples, respectively, which is 15% and 26% higher than that of PPO veils containing no CNTs, respectively. The toughening mechanism is also analyzed using scanning electron microscopy (SEM).

Design of damage-resistant hybrid lay-up structures for fiber-reinforced composites based on interface properties

In this work, the poor impact damage resistance of carbon fiber-reinforced plastic (CFRP) composites was improved by mixing carbon fibers with ductile fibers (glass fibers/Kevlar fibers). The damage behavior of five interfaces of the prepared hybrid fiber-reinforced plastic (HFRP) composites was investigated through mode-I and mode-II interlaminar fracture toughness tests and micro-droplet debonding tests. On this basis, a numerical model for predicting the impact damage of HFRP composites was established. Through a comprehensive evaluation of the impact damage resistance indexes obtained from the numerical simulations, the hybrid fiber type, hybrid ratio between different fibers, and hybrid lay-up sequence could be optimized. The specimens with optimized lay-ups were prepared by 3D printing, and the effectiveness of the numerical model was verified through low-velocity impact and compression after impact (CAI) tests. The results show that the introduction of glass fiber layers in CFRP composites can significantly improve the damage resistance of the composites. Compared with CFRP composites, when the volume fraction of the glass fibers is 25%, HFRP composites have a smaller delamination damage area (reduced by 74.08%), furthermore, the pit depth is reduced by 43.01%, the CAI is increased by 17.29%, and the strength retention rate is increased by 31.94%.

Mode I and II interlaminar fracture toughness of glass/carbon inter‐ply hybrid FRP composites: Effects of stacking sequence and testing temperature

AbstractEven though laminated FRP composites show exceptional performance in various modes of loading, they are susceptible to delamination. The ambient temperature and elevated temperature delamination resistance of glass/carbon fiber interply hybrid composites were assessed via mode I and mode II interlaminar fracture toughness tests performed at 30, 50 and 70°C. Interply hybrid composites performed better than composites with single type of fiber. The fracture toughness of inter‐ply hybrid composites varied with the stacking position. At 30°C, mode I fracture toughness value of alternative glass and carbon fiber stacking sequence (G1C1)5 of hybrid composite showed 40% improvement over 10 layered carbon/epoxy (C10) composite. Further, Mode II fracture toughness of C5G5 composite showed 22% enhancement, and C10 exhibited 42% improvement over 10‐layered glass/epoxy composite. Increasing the test temperature of all composites improved their GIC and GIIC values. The increment in the mode I fracture toughness properties of all composite was in the range of 5%–23% at 70°C from its room temperature value. The fractographic images of fractured samples were studied under a scanning electron microscope and failure mechanisms of composites were identified.

Evaluation of fracture toughness of hybrid CNT/CFRP composites

The article is focused on the investigation of interlaminar fracture toughness (IFT) of CFRP modified by SWCNT with 0.1–0.5 wt% concentrations. Mode I IFT is negatively affected by the addition of SWCNTs demonstrating the decrease by 16–46% depending on content. Mode II IFT testing showed a positive influence of SWCNTs resulting in the improvement of IFT up to 35%. The results of these mechanical tests and SEM data along with data of previously conducted research are jointly treated and possible reasons for this behavior are proposed and discussed.

Toughening and Healing of CFRPs by Diels-Alder-Based Nano-Modified Resin through Melt Electro-Writing Process Technique.

In the current study, a novel approach in terms of the incorporation of self-healing agent (SHA) into unidirectional (UD) carbon fiber reinforced plastics (CFRPs) has been demonstrated. More precisely, Diels–Alder (DA) mechanism-based resin (Bis-maleimide type) containing or not four layered graphene nanoplatelets (GNPs) at the amount of 1 wt% was integrated locally in the mid-thickness area of CFRPs by melt electro-writing process (MEP). Based on that, CFRPs containing or not SHA were fabricated and further tested under Mode I interlaminar fracture toughness experiments. According to experimental results, modified CFRPs exhibited a considerable enhancement in the interlaminar fracture toughness properties (peak load (Pmax) and fracture toughness energy I (GIC) values). After Mode I interlaminar fracture toughness testing, the damaged samples followed the healing process and then were tested again under identical experimental conditions. The repeating of the tests revealed moderate healing efficiency (H.E.) since part of the interlaminar fracture toughness properties were restored. Furthermore, three-point bending (3PB) experiments were conducted, with the aim of assessing the effect of the incorporated SHA on the in-plane mechanical properties of the final CFRPs. Finally, optical microscopy (OM) examinations were performed to investigate the activated/involved damage mechanisms.

Study into the Mechanical Properties of a New Aeronautic-Grade Epoxy-Based Carbon-Fiber-Reinforced Vitrimer

The current drive for sustainability demands recyclable matrices for composite materials. Vitrimers combine thermoset properties with reprocessability, but their mechanical performance in highly loaded applications, for instance, composites for aeronautics, is still to be demonstrated. This work presents the complete mechanical characterization of a new vitrimer reinforced with carbon fiber. This vitrimer formulation consists of functional epoxy groups and a new dynamic disulfide crosslinks-based hardener. The testing campaign for the vitrimer composites encompassed tension, compression, interlaminar shear strength (ILSS), in-plane shear (IPS), open-hole tension (OHT) and compression (OHC), filled-hole compression (FHC) and interlaminar fracture toughness tests under mode I and II. Test conditions included room temperature and high temperature of 70 °C and 120 °C, respectively, after moisture saturation. Tension and flexural tests also were applied on the neat vitrimer resin. The results compared well with those obtained for current aeronautic materials manufactured by Resin Transfer Molding (RTM). The lower values observed in compression and ILSS derived from the thermoplastic veils included as a toughening material. This work demonstrates that the vitrimer formulation presented meets the requirements of current matrices for aeronautic-grade carbon-reinforced composites.

Experimental evaluation of the quality of a composite-composite adhesive joint by peeling tests

The evaluation of the quality of composite-composite bonded joints is a major problem for the aeronautical sector given by the fact that a defective joint not only could paralyze the productive process, but it could also involve very high restoration costs. Currently, the quality of the bond is quantified by an interlaminar fracture toughness test. This test measures GIC in a test piece that is bonded theoretically under the same conditions as the actual piece. The measurement of GIC is performed by the DCB test or by the Drum Peel test. Both tests cannot be carried out on the actual part or "in situ". The present article shows a device that carries out the peeling test on the real part allowing to obtain results comparable to those that could be obtained with the traditional procedures of DCB and Drum Peel. In order to verify the validity of the new device that has been designed, an experimental campaign has been carried out to test non-symmetrical composite-composite laminates with different techniques, by comparing the different GIC values obtained. A numerical study carried out in another paper confirms that the evaluation of GIC obtained with the experimental results is adequate.

An assessment of mechanical behavior of glass fiber/epoxy composites with secondary short carbon fiber reinforcements

AbstractUtilizing recycled or short fibers for making useful engineering composite component is a present research trend and requirement. Current research work explicates the effect of short carbon fibers (SCF) used as secondary reinforcement in woven glass/epoxy hybrid composites and evaluates their mechanical properties. Different concentrations (0.1, 0.3, and 0.5 wt%) of SCF were incorporated to assess the tensile, flexural, and interlaminar fracture toughness (Mode I and II) of the resulted composites. Composites with 0.1 wt% SCF showed 29.02% and 16.08% increment in the tensile and flexural strength, respectively. For Mode I and Mode II interlaminar fracture toughness tests, 0.1 wt% SCF‐GE samples showed the highest strain energy release rate. Double cantilever beam specimens were used for Mode I testing and an improvement of 13.49% invalue was observed. End‐notched flexure method was used for the Mode II tests, and 0.1 wt% SCF‐GE samples showed 20.45% increase invalues. This paper explains the various reasons and mechanisms for obtaining such results. Scanning electron microscopy of the fractured surfaces was carried out to perceive the diverse failure micro‐mechanisms that could have been involved.

Influence of hygrothermal ageing on the mechanical properties of unidirectional carbon fibre reinforced powder epoxy composites

A systematic investigation was carried out in this work to evaluate the effect of seawater ageing on the mechanical properties of different composites manufactured with three types of unidirectional carbon fibres, based on a novel powder epoxy system. The flexural behaviour of the samples with and without accelerated seawater ageing was evaluated. A significant strength reduction due to seawater ageing was observed in the composites. Changes within the materials are also evaluated by interlaminar fracture toughness testing and dynamic mechanical thermal analysis . Interlaminar fracture toughness mode-I testing using the Double Cantilever Beam (DCB) test, revealed that the interfacial adhesion differences in combination with the moisture uptake could alter the fracture resistance of the composites, hence emphasising the importance of the interfacial bonding strength. A Finite Element analysis was carried out to gain insight on the important modelling parameters, for mode I dominated failure , and compared with the experimental results. Fracture surface examination by scanning electron microscopy revealed delamination , fibre debonding/bridging and plasticising of resin due to seawater effect.

Electrical Conductivity of Glass Fiber-Reinforced Plastic with Nanomodified Matrix for Damage Diagnostic.

The electrical conductivity of glass fiber-reinforced plastic (GFRP) with epoxy matrix modified by multiwall carbon nanotubes (MWCNT) was studied. The electrical conductivity of nanomodified lamina and multi-layered GFRP was investigated on several levels using a structural approach. Components of the electrical conductivity tensor for unidirectional-reinforced monolayer were calculated similarly as in micromechanics using the conductivity of the nanomodified matrix. The electrical conductivity of multilayer composite was calculated using laminate theory and compared with values measured experimentally for various fiber orientation angles. Calculated and experimental data were in good agreement. The voltage distribution measured throughout the laminate allowed detecting the damage in its volume. The electrode network located on the laminate surface could determine the location, quantification, and geometry of the damage in the GFRP lamina modified with MWCNT. Experimental and calculated electrical resistance data for GFRP double-cantilever beam specimens were investigated in Mode I interlaminar fracture toughness test. Results demonstrate that electrical resistance could be successfully used for the diagnostic of the crack propagation during interlaminar fracture of the MWCNT-modified GFRP.

Effect of material nonlinearity on the toughness evaluation in quasi-static mode II interlaminar fracture toughness tests of composite laminates

In mode II interlaminar fracture toughness tests of composite laminates, the fracture toughness evaluated based on linear elastic fracture mechanics deviates from the true toughness due to the material nonlinearity of the matrix. In this study, an elastoplastic finite-element analysis was performed on a new quasi-static mode II interlaminar fracture toughness test, i.e., the doubly end-notched tension (DENT) test, to investigate the difference between the perceived energy release rate and the J-integral. The effect of material nonlinearity was compared with other quasi-static mode II interlaminar fracture toughness tests: three- and four-point bend end-notched flexure tests. Analysis of the DENT showed that for a given J-integral loading, the size of the plastic zone at the delamination crack tip was the same regardless of the crack length. The difference between the perceived energy release rate and J-integral in the DENT test with respect to the plastic zone size was independent of the crack length, and was smaller compared with that of flexure-type tests. Furthermore, the compliance calibration method was applied to the elastoplastic analysis results of the DENT test to reduce the evaluation difference of the energy release rate.

Interlaminar fracture toughness of a quasi 3D braided composite

Fiber reinforced composite materials are a heavily sought after material for next generation vehicles for light-weighting components due to their high specific strength and stiffness. However, these materials have relatively weak interlaminar strength and are prone to delamination. This is especially the case when a delamination crack already exists. Quasi-3D (Q3D) braided composites seek to solve this issue by weaving the bias tows into the adjacent (above and below) plies. The plies are physically connected through fiber tows as opposed to being bonded simply by the epoxy, and the composite will achieve a higher interlaminar strength due to fiber failure being required for crack propagation as opposed to simply matrix failure. The [Formula: see text] UD and Q3D carbon composites are investigated in this study for their better in-plane isotropy. Mode I and Mode II interlaminar fracture toughness tests were conducted on UD and Q3D samples. In Mode I experiments, the samples were continuously loaded to full beam split using the double cantilever beam method to obtain the fracture toughness throughout the sample. 4ENF is used to measure the Mode II fracture toughness to create a full R-curve for the architectures.

Cyclic Olefin Copolymer Interleaves for Thermally Mendable Carbon/Epoxy Laminates.

Thin cyclic olefin copolymer (COC) foils were used as intrinsic thermoplastic healing agents in carbon fiber (CF)-reinforced epoxy laminates. COC films were produced by hot pressing and were interleaved in the interlaminar regions between each EP/CF lamina, during the hand layup fabrication of the laminates. Three samples were produced, i.e., the neat EP/CF laminate without COC, and two laminates containing COC layers with a thickness of 44 μm and 77 μm, respectively. It was observed that the fiber volume fraction decreased, and the porosity increased with the introduction of COC layers, and this effect was more evident when thick films were used. These two effects, combined with the sub-optimal adhesion between COC and EP, caused a decrease in the mechanical properties (i.e., the elastic modulus, flexural strength, interlaminar shear strength and interlaminar fracture toughness) of the laminates. Specimens subjected to mode I interlaminar fracture toughness test were then thermally mended under pressure by resistive heating, through the Joule effect of conductive CFs. A temperature of approximately 190 °C was reached during the healing treatment. The healing efficiency was evaluated as the ratio of critical strain energy release rate (GIC) of the healed and virgin specimens. Healed specimens containing COC layers of 44 μm and 77 μm exhibited a healing efficiency of 164% and 100%, respectively. As expected, the healing treatment was not beneficial for the neat EP/CF laminate without COC, which experienced a healing efficiency of only 2%. This result proved the efficacy of COC layers as a healing agent for EP/CF laminates, and the effectiveness of resistive heating as a way to activate the intrinsic healing mechanism.

The Effect of Transverse Shear in Symmetric and Asymmetric End Notch Flexure Tests-Analytical and Numerical Modeling.

This paper focuses on the effects of transverse shear and root rotations in both symmetric and asymmetrical end-notched flexure (AENF) interlaminar fracture toughness tests. A theoretical model is developed, whereas the test specimen is subdivided into four regions joined by a rigid interface. The differential equations for the deflection and rotations of each region are solved within both the Euler–Bernoulli simple beam theory (SBT) and the more refined Timoshenko beam theory (TBT). A concise analytical equation is derived for the AENF deflection profile, compliance, and transverse shearing forces as a function of the specimen geometry, stacking sequence, delamination length, and fixture span. Modeling results are compared with numerical finite element analyses, obtaining a very good agreement. Performed analyses suggest that even in the case of symmetrical and unidirectional laminates considered as pure mode II fracture, a complex compression/tension and bending moment state is present, as well as a slight contribution of anti-planar shear at the vicinity of the crack tip.

A study of the effects of NIR laser radiation on interlaminar fracture toughness of CFRP

This research focusses on the analysis of two near-infrared (NIR) lasers’ characteristics with regard to the resulting interlaminar fracture toughness and the identification of a sweet spot for delamination free laser ablation of CFRP.Earlier studies showed that nanosecond-pulsed UV and MIR lasers are able to ablate CFRP without causing an increased risk for delamination as opposed to NIR lasers. However, NIR lasers are more flexible regarding industrial integration. Therefore, the risk of delamination induced by NIR radiation needs to be minimised. It stems from the CFRP’s optical properties and leads to NIR radiation to be mostly absorbed on the carbon fibres’ surface, indirectly removing the matrix and thus risking delamination below the fibres. Mode I interlaminar fracture toughness (GIc) tests evaluate this risk, using two adhesively bonded CFRP surfaces, where one surface received surface treatment.In this work, experiments with two NIR lasers achieved GIc results that matched or surpassed those of mechanically milled references.