Interlaminar Bond Research Articles

Constructing flexible fiber bridging claws of micro/nano short aramid fiber at interlayer of basalt fiber reinforced polymer for improving compressive strength with and without impact

The high-performance Basalt Fiber Reinforced Polymer (BFRP) composites have been prepared by guiding Micro/Nano Short Aramid Fiber (MNSAF) into the interlayer to improve the resin-rich region and the interfacial transition region, and the flexible fiber bridging claws of MNSAF were constructed to grasp the adjacent layers for stronger interlaminar bond. The low-velocity impact results show that the MNSAF could improve the impact resistance of BFRP composites. The compression test results demonstrate that the compressive strength and the residual compressive strength after impact of MNSAF-reinforced BFRP composites were greater than those of unreinforced one, exhibiting the greatest 56.2% and 73.3% increments respectively for BFRP composites improved by 4 wt% MNSAF. X-ray micro-computed tomography scanning results indicate that the “fiber bridging claws” contributed to better mechanical interlocking to inhibit the crack generation and propagation under impact and compression load, and the original delamination-dominated failure of unreinforced BFRP composites was altered into shear-dominated failure of MNSAF-reinforced BFRP composites. Overall, the MNSAF interleaving might be an effective method in manufacturing high-performance laminated fiber in industrial production.

Outstanding interlaminar strength of carbon fiber reinforced epoxy resin via graphene oxide chemical bridge bonding

Carbon fiber reinforced polymers (CFRPs) are susceptible to interlaminar failure under high shear stress, which reduces their service life. Herein, we report a strategy to address the interlaminar damage problem of CFRPs by utilizing graphene oxide (GO) as a bridge to chemically bond carbon fibers and epoxy resin. Amino groups were grafted on the surface of carbon fibers to react with GO, then active amino functional groups were introduced onto GO, enabling crosslink with the epoxy resin. The chemical bonding force and mechanical interlocking effect of GO tightly integrated the carbon fibers with the epoxy resin, greatly reducing relative slip under high shear stress. This flexible and robust connection resulted in a significant improvement in the interfacial adhesion strength of CFRPs. The GO content and type of amino reactants were optimized to fabricate CFRPs, and the interlaminar shear strength and transverse fiber bundle tensile strength were increased to 76.36 MPa and 34.2 MPa, which were 46.2 % and 77.2 % higher than those of pre-modification materials, respectively. This research demonstrates that the chemical bonding provided by grafting graphene oxide significantly enhances interlaminar bond strength, thereby extending the service life of CFRPs and offering a promising path for manufacturing high-strength composites.

Resin percolation and intimate contact in fast processing of thermoplastic composites

Fusion bonding of thermoplastic composites (TPCs) requires intimate contact before polymer interdiffusion and solidification can establish an interlaminar bond. The lack of such intimate contact results in voids or disbonds. This study shows through cross-sectional imaging and scaling analysis that percolation flow, rather than squeeze flow, of resin through fibers is the dominant mechanism to achieve intimate contact in rapid TPC processes such as automated fiber placement (AFP) and welding. A 1D intimate contact model based on resin percolation, derived from Darcy’s Law, is presented and compared to experiments. While this model proved effective in comparing processing conditions by adeptly capturing trends, it under-predicted resin content in samples processed using AFP with low pressure. The model was extended to high processing rates, suggesting the importance of heated tools and high processing pressures on achieving intimate contact.. In-situ high-speed imaging and ex-situ microscopy revealed void size and morphology based on AFP processing parameters. The results of this study can guide fast processing of thermoplastic composites.

Hybridization Effect on Interlaminar Bond Strength, Flexural Properties, and Hardness of Carbon-Flax Fiber Thermoplastic Bio-Composites.

Hybridizing carbon-fiber-reinforced polymers with natural fibers could be a solution to prevent delamination and improve the out-of-plane properties of laminated composites. Delamination is one of the initial damage modes in composite laminates, attributed to relatively poor interlaminar mechanical properties, e.g., low interlaminar strength and fracture toughness. This study examined the interlaminar bond strength, flexural properties, and hardness of carbon/flax/polyamide hybrid bio-composites using peel adhesion, three-point bending, and macro-hardness tests, respectively. In this regard, interlayer hybrid laminates were produced with a sandwich fiber hybrid mode, using woven carbon fiber plies (C) as the outer layers and woven flax fiber plies (F) as the inner ones (CFFC) in combination with a bio-based thermoplastic polyamide 11 matrix. In addition, non-hybrid carbon and flax fiber composites with the same matrix were produced as reference laminates to investigate the hybridization effects. The results revealed the advantages of hybridization in terms of flexural properties, including a 212% higher modulus and a 265% higher strength compared to pure flax composites and a 34% higher failure strain compared to pure carbon composites. Additionally, the hybrid composites exhibited a positive hybridization effect in terms of peeling strength, demonstrating a 27% improvement compared to the pure carbon composites. These results provide valuable insights into the mechanical performance of woven carbon-flax hybrid bio-composites, suggesting potential applications in the automotive and construction industries.

Elastic and electromagnetic monitoring of TRC sandwich panels in fracture under four-point bending

Textile-reinforced cementitious (TRC) sandwich panels with insulation core are mechanical- and material-efficient composites, however, under service loads might develop complex fracture that is still inadequately detected. Among damage mechanisms, the weak core-facing interfacial bond can substantially reduce its load-bearing capacity and lead to premature failure; therefore, early detection of damage is crucial to guarantee optimal long-term structural performance. Periodic inspection is necessary during the service life, however, there is neither a standardized methodology nor monitoring protocol that detects and characterizes premature interfacial debonding. In this study, a novel multi-modal monitoring is developed for the first time, based on acoustic emission (AE), digital image correlation (DIC), and millimeter wave (MMW) spectrometry, to characterize the response to bending of TRC sandwich beams. The core-facing interlaminar bond is artificially weakened during casting to simulate manufacturing defects. The proposed methodology showed high sensitivity to damage and complementarity of the techniques, where, AE allowed to characterize and locate damage, MMW showed high sensitivity to progressive failure, and DIC provided the strain data to validate and interpret AE, and MMW results.

Influence of post-consolidation on continuous carbon-fibre reinforced additively manufactured specimens in bending

Continuous carbon-fibre (CCF) additive manufacturing of composites has significant potential for production of optimised structures. However, stiffness and strength in parts are often significantly lower than manufacturer material data. For thermoset-impregnated CCF, post-consolidation of specimens has greatly increased strength. In this contribution, the effect of post-consolidation for thermoplastic CCF material is presented. Specimens were manufactured on a commercial AM CCF printer and post-consolidation was performed by rapid compression moulding. Porosity was evaluated by X-ray Computed Tomography (CT) and a significant reduction from 5.3vol.% to 1.1vol.% was yielded.Experimental testing is performed in four-point bending with as-printed and post-consolidated samples. Bending stiffness and strength data was obtained. The results show that the bending strength was below expectations in as-printed specimens. Finite Element Analysis indicates that snap-buckling of a small number of outer layers can lead to such a reduced apparent bending strength, and CT scans of fracture surfaces of as-printed specimens reveal the presence of interlaminar debonding. It is shown that post-consolidation highly improves the interlaminar bond, yielding experimentally obtained strength values close to nominal material strength, and the expected kink-band compressive failure is achieved.

Multimodal NDT monitoring of TRC sandwich under bending and early detection of interlaminar debonding

Textile Reinforced Cementitious (TRC) sandwich composites provide a load-bearing, noncorrosive, lightweight, and durable alternative for steel-reinforced sandwich elements, and/or traditional steel-reinforced concrete. However, the composite nature of the material and slender nature of the facings render the fracture behavior complex. Insufficient interlaminar bond can cause premature debonding, substantially reducing the loadbearing capacity of the composite. Non-Destructive Testing (NDT) techniques seem the obvious choice to monitor the damage progression of the material without affecting, nor compromising the behavior of the composite, and predict their service life. In this study, TRC sandwich composites, subjected to quasistatic four-point bending, are monitored with three NDTs. Digital Image Correlation (DIC) allows to measure the surface strains and displacements. MMW Spectrometry, used for the first time in bending damage monitoring, allowed to detect damage such as cracking, or debonding, while Acoustic Emission (AE) allowed to localize and characterize internal cracking. In order to simulate premature debonding, the bond between the tensile TRC facing and the insulation, in the central zone, where the bending moment is maximum, was artificially destroyed. Results show that a weak interlaminar bond reduced the ultimate load of the composite by more than 50%. Additionally, DIC, AE and MMW Spectrometry proved useful to monitor and characterize damage. Multimodal data gathered from the multimodal NDTs showed to be complementary. DIC allowed to interpret AE and MMW Spectrometry data from a surface viewpoint, while AE parameters permit predictions at low load conditions, and detect internal cracking. For instance, the AE behavior at early load stage (less than 15% of the maximum load) of the TRC sandwich with destroyed bond showed significant differences than for the reference TRC sandwich. Specifically, it showed higher RA values, and lower AF than the reference TRC sandwich, suggesting more shear related early activity for, promoted by the damaged bond. Results were corroborated with DIC, while MMW Spectrometry seemed to follow closely the level of damage of TRC sandwich composites under quasi-static four-point bending. EWGAE 35, Ljubljana, Slovenia, 13th – 16th Sep. www.ewg

Predicting and Improving Interlaminar Bonding Uniformity during the Robotic Fiber Steering Process.

With their high specific stiffness, corrosion resistance and other characteristics, especially their outstanding performance in product weight loss, fiber-reinforced resin matrix composites are widely used in the aviation, shipbuilding and automotive fields. The difficulties in minimizing defects are an important factor in the high cost of composite material component fabrication. Fiber steering is one of the typical means of producing composite parts with increased strength or stiffness. However, fiber waviness is an important defect induced by fiber steering during the fiber placement process. Meanwhile, the laying speeds of the inner and outer tows along the path width direction are different during the fiber steering process, resulting in different interlaminar bond strengths. Therefore, the fiber waviness and uneven interlaminar bonding strength during fiber steering not only affect the dimensions of a composite product, but also influence the mechanical properties of the part. This study aims to reduce fiber waviness and improve interlaminar bonding uniformity along the path width direction using a multi-piece compaction roller. By analyzing the mechanism of the generation of fiber waviness, the interlaminar bonding strength for each tow during fiber steering is investigated. Through analyzing and optimizing the compaction force, laying temperature and laying velocity during fiber steering experiments, the optimization approach is verified.

Effect of filament winding process parameters on interlaminar performance for CF/PP thermoplastic composite

This paper focuses on the in-situ consolidation process of thermoplastic composites by winding, aiming to explore the influence of key process parameters on samples’ quality. During the consolidation of thermoplastic composites, they need to go through resin melting-bonding-consolidation process, in which each parameter affects the interlaminar bond strength. Through preliminary simulation and experiments, the process parameters were selected. In this paper, wound NOL ring samples made of carbon fiber reinforced polypropylene thermoplastic composite prepregs were obtained with a hot gas assisted filament winding equipment. The influence rule of winding temperature, winding tension and winding speed on the interlaminar shear strength were analyzed based on response surface methodology. Through the analysis, in the range of the experiments winding tension and temperature had more effect on the shear strength while speed’s influence was small. With the increase of tension and temperature, the interlaminar shear strength increased, but after reaching a certain value, the laminar shear strength decreased instead.

Intimate contact development for automated fiber placement of thermoplastic composites

The consolidation step in the automated fiber Placement (AFP) of thermoplastic composites is crucial for development of interlaminar bonding strength. Two mechanisms contribute to the interlaminar bond strength development in the interface between the incoming tow and the substrate: (a) the development of intimate contact between the two surfaces and (b) the formation of a fusion bond across those contacting surfaces known as healing. So far, a number of different theoretical intimate contact models have been proposed in the literature to study intimate contact development for different manufacturing processes. However, very limited experimental methods are available to measure degree of intimate contact. To fill this gap, a new experimental approach based on topology of the tape surface is introduced to quantify the degree of intimate contact for the AFP process. The new approach is based on the Bearing Area Curve (BAC) of the surface profile of the tape. To implement the new procedure, an unidirectional strips of carbon fiber/PEEK were laid down on the polished steel tool with different placement rates, compaction forces, hot gas torch (HGT) temperatures, and tool temperatures. The degree of intimate contact is calculated based on the proposed experimental approach and compared with prediction models from the literature. Furthermore, the effect of AFP process parameters on the degree of intimate contact is studied using BAC method and the effective intimate contact model. The results showed that the development of intimate contact is significantly governed by the torch and tool temperature.

Effect of porosity and crystallinity on mechanical properties of laser in-situ consolidation thermoplastic composites

In this paper, the interlaminar bond strength of laser in-situ consolidation (ISC) CF/PEEK thermoplastic composites was investigated under different process conditions. Short beam shear (SBS) tests were conducted to characterize the load-displacement response, the trend of interlaminar shear strength (ILSS), and the mechanism of porosity and crystallinity on ILSS. The experimental results show that porosity has a significant effect on ILSS. Although increasing the tool temperature results in a more stable and homogeneous crystal structure, no correlation is observed between crystallinity and ILSS. The highest measured ILSS value in the heated tool in-situ consolidation (HTISC) group is 59.59 MPa. The aforementioned represents an increase of 61.23% compared to the constant pressure in-situ consolidation (CPISC) group. The proposed method can be used to provide a reference value for acceptance criteria in engineering applications of thermoplastic composites.

Improvement in interfacial fracture toughness of multi-material additively manufactured composites through thermal annealing

An experimental investigation is performed to investigate the effect of post-processing heat treatment on the interfacial fracture toughness of bi-material additively manufactured semi-crystalline polymer composite. An asymmetric double cantilever beam (ADCB) and single-leg bending (SLB) specimens made of polylactic acid (PLA) and Nylon are considered for the mode-I and mixed-mode fracture characterization, respectively. Specimens are isothermally heated in a forced convection oven for a wide range of temperatures and durations. Fracture toughness decreases significantly for both mode-I and mixed-mode conditions when specimens are annealed below the melting temperature of PLA (150 °C). An increase of the crystallinity at the high-temperature annealing prevents the polymer chain mobility, hinders the neck growth, and provides poor intermolecular diffusion resulting in decrease of fracture toughness by 88% as compared to the unannealed specimen. Annealing at 160 °C improves the bi-material interfacial fracture toughness by a maximum of 1225% via sufficient interfacial wetting, higher molecular diffusion, and a longer polymer chain entanglement process. Material transfer, void collapse, and filament impression on the fracture surface of high temperature annealed specimen indicate a better molecular diffusion and strong interlaminar bond at the interface.

Co-consolidation of CF/PEEK tape-preforms and CF/PEEK organo sheets to manufacture reinforcements in stamp-forming process

Co-consolidation is considered one effective joining method to allow novel types of integral structures to be manufactured. In this study, carbon fiber reinforced Polyether-Ether-Ketone partially consolidated tape preforms were co-consolidated with carbon fiber reinforced Polyether-Ether-Ketone organo sheets in stamp-forming process. Interlaminar bond quality of both joining partners is validated in double cantilever beam test. Results exhibit average interlaminar fracture toughness of 2.54 kJ/m2 for stamp-forming specimen, which exceeds interlaminar fracture toughness of reference samples manufactured in autoclave being 1.79 kJ/m2. Further examinations on specimen morphology and mechanical properties indicate distinct assignments to process characteristic cooling rates, which coincides with studies from literature. Accordingly, high cooling rates—as evident in stamp-forming process—are allocated to high toughness, low crystallinity and low bending modulus, causing high interlaminar fracture toughness. Investigations on laminate quality reveal maximum void content of 1.58%.

Parameter Selection for Peel Strength Optimization of Thermoplastic CF-PA6 for <i>Humm</i>3<sup>TM</sup>

For Automated Tape Placement process, degree of bond varies with variation in process parameters and material. Interlaminar bond strength characterization is one of the most important criteria in determining the quality of bond between two layers of thermoplastic tapes. Depending on the bond strength achieved using different process parameters, a process window is defined. Based on the process window an iterative procedure is adopted to find optimum parameters to realize maximum bond strength. This paper aims to investigate the interlaminar bond strength of thermoplastic CF-PA6, during Automated Tape Placement process. A fairly new heating source, a pulsed light solution, i.e. a humm3TM system, which delivers uniform, highly controllable heat to the nip point is used. Experiments were conducted for different process parameters and results obtained using wedge peel test were analyzed. Results acquired help in assessing the material and the heating source in terms of capabilities and efficiency.

The effect of processing temperature on wedge peel strength of CF/PA 6 laminates manufactured in a laser tape placement process

High-performance thermoplastics such as PEEK have been most commonly studied for thermoplastic ATP. Engineering thermoplastics such as PA6 have comparatively low melt viscosities and different processing behaviour. The effect of processing temperature on the quality of CF/PA6 laminates manufactured with a near-infrared laser ATP system is investigated. The inter-laminar bond strength is characterised by wedge peel tests on samples manufactured at different process temperatures from 200 °C to 460 °C at a placement rate of 100 mm/s. Thermal degradation of the polymer was investigated by DSC and DMA. Optical microscopy of cross-sections revealed significant squeeze-out of the matrix for process temperatures exceeding 320 °C, corresponding with a rapid decrease of wedge peel strength. No significant thermophysical changes were detected for temperatures up to 380 °C. Reduction of wedge peel strength at high temperatures was attributed to matrix squeeze-out caused by the low melt viscosity of the polymer and not thermal degradation of the PA6.

Estimation of Damage Thickness in Fiber-Reinforced Composites using Pulsed Thermography

Nondestructive testing (NDT), including active thermography, has become an inevitable part of composite process and product verification, post manufacturing. However, there is no reliable NDT technique available to ensure the interlaminar bond integrity during composite laminate integration, bonding or repair where the presence of thin air gaps in the interface of dissimilar polymer composite materials would be detrimental to structural integrity. This paper introduces a novel approach attempting to quantify the damage thickness of composites (the thickness of air gaps inside composites) through a single-side inspection of pulsed thermography. The potential of this method is demonstrated by testing on three specimens with different types of defect, where the Pearson correlation coefficients of the thickness estimation for block defects and flat-bottom holes are 0.75 and 0.85, respectively. This approach will considerably enhance the degradation assessment performance of active thermography by extending damage measurement from currently two dimensions to three dimensions, and become an enabling technology on quality assurance of structural integrity.

A mesoscale beam-spring combined mechanical model of needle-punched carbon/carbon composite

Needle-punched carbon/carbon composite (NP C/C) has high temperature resistance and reinforced interlaminar strength. Needle-punching technique improves the integrality of composite, nevertheless, it degrades the in-plane properties due to introducing a number of defects. In this study a mesoscale mechanical model was proposed, in which a kind of circular arc beam element (CABE) was established to characterize fiber bundle with punching defeats and extended spring elements (ESEs) were modeled to represent intralaminar and interlaminar bonds of fiber bundles. Uniaxial tension and compression experiments for a NP C/C were conducted to validate the proposed numerical model and good agreement was found between model predictions and test data.

Assessment of Delamination in Tensylon� UHMWPE Composites by Laser-induced Shock

Ultra-High Molecular Weight Polyethylene (UHMWPE) composites are the result of recent developments in material research for ballistic protection due to their ability to absorb the kinetic energy of the bullet by various mechanisms of dissipation, among which an important one is delamination. In order to study this mechanism independently, the laser induced shock wave testing procedure has been used on thin Tensylon� laminate samples. Laser-induced shock represents a modern approach that can be used for assessing the interlaminar bond strength between two plies of a composite material, in dynamic conditions, at high strain rates representative for a ballistic impact. Through this technique, a delamination failure stress threshold can be determined. In the present work, the laser induced shock technique was applied on the commercial UHMWPE material called Tensylon�. The delamination threshold of this material was determined by using the Novikov approach, and, compared to the literature, the results match the values determined by other means of measurement.

Fishnet model with order statistics for tail probability of failure of nacreous biomimetic materials with softening interlaminar links

The staggered (or imbricated) lamellar “brick-and-mortar” nanostructure of nacre endows nacre with strength and fracture toughness values exceeding by an order of magnitude those of the constituents, and inspires the advent of new robust biomimetic materials. While many deterministic studies clarified these advantageous features in the mean sense, a closed-form statistical model is indispensable for determining the tail probability of failure in the range of 1 in a million, which is what is demanded for most engineering applications. In the authors’ preceding study, the so-called ‘fishnet’ statistics, exemplified by a diagonally pulled fishnet, was conceived to describe the probability distribution. The fishnet links, representing interlaminar bonds, were considered to be elastic perfectly-brittle. However, the links may often be quasibrittle or almost ductile, exhibiting gradual postpeak softening in their stress-strain relation. This paper extends the fishnet statistics to links with post-peak softening slope of arbitrary steepness. Probabilistic analysis is enabled by assuming the postpeak softening of a link to occur as a series of finite drops of stress and stiffness. The maximum load of the structure is approximated by the strength of the kth weakest link (k ≥ 1), and the distribution of structure strength is expressed as a weighted sum of the distributions of order statistics. The analytically obtained probabilities are compared and verified by histograms of strength data obtained by millions of Monte Carlo simulations for each of many nacreous bodies with different link softening steepness and with various overall shapes.

Optimization of Process Parameters for Robotic Fibre Placement

Compared with labour-intensive and time-consuming processing of conventional fabrication methods, the robotic fiber placement process greatly improves the flexibility of the fiber placement process and allows for the construction of more complex structures. Based on the study of the placement process, the intimate contact process and healing process were analysed theoretically. The key process parameters for affecting the quality of the composite component were put forward:hot gas torch temperature, compaction force and laying velocity. In order to analyse the fabrication process for laying the cylindrical parts with 0-degree tow direction, the model of the process parameters coupling affected the interlaminar bond strength was established, according to the design of the response surface method. The reliability and validity of the model were verified by the analysis of variance. The optimal process parameters of fiber placement were obtained. The results show that the model is effective and the optimum peeling force of the laying products is 24.1 N under the optimum process parameters.