Unidirectional CFRP Laminates Research Articles

Experimental Comparative Analysis of the Through-Thickness and In-Plane Compression Moduli of Unidirectional CFRP Laminates

This study explores the experimental characterization of the through-thickness compression properties in unidirectional laminates using cube compression tests. Cubical specimens, each with an edge length of 10 mm, were symmetrically outfitted with biaxial strain gauges and subjected to a compression test. While similar methodologies exist in the literature, this work primarily addresses the potential biases inherent in the testing procedure and their mitigation. The influence of friction-induced non-uniform deformation behavior is compensated through a scaling of the stiffness measurements using finite element (FE) analysis results. This scaling significantly enhances the accuracy of the resulting parameters of the experiments. The ultimate failure of the specimens, originating from stress concentrations at the edges, resulted in fracture angles ranging between 60° and 67°. Such fracture patterns, consistent with findings from other researchers, are attributed to shear stress induced by friction at the load introduction faces. The key findings of this research are the comparisons between the through-thickness modulus (E33c) and strength (X33c) and their in-plane counterparts (E22c and X22c). The results indicate deteriorations of E33c and X33c from E22c and X22c by margins of 5% and 7%, respectively. Furthermore, the results for E22c and X22c were compared with the results obtained through a standard test, revealing a 12% enhancement in strength X22c and 4% underestimated stiffness E22c in the cube compression test.

Automated Quantification of Interlaminar Delaminations in Carbon-Fiber-Reinforced Polymers via High-Resolution Ultrasonic Testing.

This article presents a method of ultrasonic testing (UT) that detects and quantifies interlaminar delaminations in CFRP composites with high resolution in terms of both spatial resolution in the planar dimension and depth into the laminate. Unidirectional and woven CFRP laminates were fabricated for this study, with a PTFE film inserted at various depths throughout the laminate to act as intentional crack initiation sites. All samples were mechanically tested via a three-point, end-notched flexure (ENF) test, followed by a quantification of the extent of the induced interlaminar delaminations using UT and X-ray computed tomography (CT). UT analysis for unidirectional CFRP samples was able to show a clear contrast between the delaminated area and the non-delaminated area. UT analysis of the woven CFRP samples yielded comparable results but required finer tuning of analysis parameters due to the interlocking woven fabric. CT results revealed a significant contrast between the crack and composite; thus, fine geometrical features of the crack front could be observed. UT and CT measurements were then compared, revealing an average difference of 1.09% in the delamination area, with UT overestimating as compared to CT. A UT depth study was also performed to automatically locate the interlaminar delamination at different depths throughout the components, with the delamination being predicted within one lamina interface for all samples. These results demonstrate UT's ability to accurately detect and quantify the extent and location of interlaminar delaminations due to bending.

Comprehensive prediction model of drilling temperature of UD-CFRP laminates considering the combined action of main cutting edge and chisel edge

The accuracy of predicting drilling temperature in CFRP drilling is helpful to process optimization and defect suppression. The previous models for predicting CFRP drilling temperature are few and need to be more systematic. Combined with theoretical modeling and experimental verification, a comprehensive theoretical model is developed to predict the temperature distribution during the drilling process of unidirectional CFRP (UD-CFRP), which considers the synergy of the chisel edge and main cutting edge of the drill bit, as well as the influence of fiber orientation angle (FOA). Results show that the predicted peak temperatures parallel to and vertical to the fiber direction present tiny errors of 2.26–5.27% and 4.44–7.55% respectively, indicating that the model is reliable. At 1 mm from the hole wall of UD-CFRP, the peak temperature decreases with the increase of FOA. The drilling temperature decreases with the increase of feed rate and increases with the increase of spindle speed. At the outlet layer of UD-CFRP, the contribution of spindle speed to temperature rise is more significant than the feed rate.

Understanding Mode I interlaminar toughening of unidirectional CFRP laminates interleaved with aligned ultrathin CNT fiber veils: Thickness and orientation effects

Ultrathin CNT fiber veils (100–300 nm thick) were used as interleaves to improve the interlaminar fracture toughness of unidirectional carbon fiber reinforced polymer composites. These veils, drawn from vertically aligned CNT forests, were directly deposited at the mid-plane of CFRPs either longitudinally or transversely to the carbon fiber. The results revealed that triggering multiple toughening mechanisms is the key to achieve high toughness FRP composites. When the veils were oriented parallelly to the carbon fiber, the crack propagates in a “zig-zag” manner, not only interacting with more CNTs, but also triggering more carbon fiber bridgings, leading to a steeply “rising” R-curve. The superiority and effectiveness of this technique is exemplified by the extremely thin toughening layers (about 300 nm thick) needed to remarkably improve the propagation value of Mode I interlaminar fracture toughness of the CFRP by approximately 107%. The Mode I toughening factor (ηI), defined as the change in Mode I interlaminar fracture toughness per interleaf-to-ply thickness ratio, comes out as high as 713, which is far outweigh the state-of-the-art. However, transversely-placed CNT veils will seriously block the formation of carbon fiber bridgings, remarkably degrading the Mode I interlaminar toughness of the laminates.

Bonded and additively manufactured crack retarders: A comparative study of damage tolerance properties

Fatigue crack grows faster in the integral structure than in the riveted structure. This study compares the bonded, cold-sprayed and ultrasonically consolidated crack retarders to increase the fatigue life of integral airframe structure. The 2024-T351 aluminum M(T) specimen reinforced with crack retarders made of AISI 301 and 316L steels, Ti–6Al–4V titanium alloy and unidirectional CFRP laminate are evaluated in terms of fatigue life, delamination resistance and weight efficiency. Specimens were subjected to constant amplitude loading up to the final failure. A substantial decrease in fatigue crack growth rate was achieved using the ultrasonically consolidated AISI 301 crack retarder.

Very high-cycle fatigue properties of 90° unidirectional CFRP laminates and evaluation of fatigue limits by free volume measurement using positron microscopy

Very high-cycle fatigue properties of 90° unidirectional CFRP laminates and evaluation of fatigue limits by free volume measurement using positron microscopy

Mechanical properties and damage behavior of tapered unidirectional CFRP laminates

An understanding of damage mechanisms induced by drop-off plies in tapered composite laminates is crucial. This study investigated the mechanical properties and damage behaviors of asymmetric tapered unidirectional carbon fiber-reinforced plastic laminates. Damage observations of laminates with simultaneous tapered and staggered tapered structures under monotonic and cyclic tensile loads were done by an optical microscope and X-ray radiography. Based on the results, matrix cracks in the resin pocket and delamination occurred earliest in the simultaneous tapered specimen due to a greater stress concentration in a single large resin pocket in the structure. Intralaminar damages in the staggered tapered specimen with a shorter step spacing occurred mainly in the lower dropped ply as there was a more significant interaction between the neighboring steps in the specimen. The drastic decrease in the stress concentration after the occurrence of damages in the neighboring step(s) then suppressed the occurrence of intralaminar damages in the upper dropped ply. This is in contrast to the specimen with a longer step spacing where the intralaminar damages occurred in both dropped plies due to the distributed stress distribution. Delamination propagated between the dropped plies and continuous (belt and core) plies for both staggered laminates regardless of the length of the step spacing at higher applied stress levels. The results showed that internal ply-drop configuration and step spacing in staggered tapered structures contribute to significant differences in the mechanical properties and damage behavior in the laminates.

Lightning strike damage resistance of carbon‐fiber composites with nanocarbon‐modified epoxy matrices

AbstractCarbon‐fiber reinforced polymer (CFRP) composites are replacing metal alloys in aerospace structures, but they can be vulnerable to lightning strike damage if not adequately protected due to the poor electrical conductivity of the polymeric matrix. In the present work, to improve the conductivity of the CFRP, two electrically conductive epoxy formulations were developed via the addition of 0.5 wt% of graphene nanoplatelets (GNPs) and a hybrid of 0.5 wt% of GNPs/carbon nanotubes (CNTs) at an 8:2 mass ratio. Unidirectional CFRP laminates were manufactured using resin‐infusion under flexible tooling (RIFT) and wet lay‐up (WL) processes, and subjected to simulated lightning strike tests. The electrical performance of the RIFT plates was far superior to that of the WL plates, independent of matrix modification, due to their greater carbon‐fiber volume fraction. The GNP‐modified panel made using RIFT demonstrated an electrical conductivity value of 8 S/cm. After the lightning strike test, the CFRP panel remains largely unaffected as no perforation occurs. Damage is limited to matrix degradation within the top ply at the point of impact and localized charring of the surface. The GNP‐modified panel showed a comparable level of resistance against lightning damage with the existing copper mesh technology, offering at the same time a 20% reduction in the structural weight. This indicates a feasible route to improve the lightning strike damage resistance of carbon‐fiber composites without the addition of extra weight, hence reducing fuel consumption but not safety.

A stiffness-based phenomenological model to predict the fatigue life of unidirectional CFRP laminates under tensile fatigue loadings

Fibre-reinforced composite materials can withstand large number of loading cycles compared to traditional materials. However, predicting the damage progress and strength loss in a composite structure due to fatigue is difficult because of the complex interaction of different failure mechanisms. This work investigates the decrease in the stiffness and strength in a high-performance undirectional CFRP laminate subjected to on-axis fatigue loads. A stiffness-based phenomenological model is proposed in order to evalute the damage growth due to fatigue through the longitudinal stiffness degradation. Assuming that the laminate ultimate strain is independent of the distributed damage caused by fatigue, the residual strength model can be deduced, or its integral form, the S-N curve. Thanks to this assumption, the residual stiffness model is related to the residual strength model, and thus it has reduced the experimental characterization of phenomenlogical-based approaches. The accuracy of the model is verified by comparing the experimental data against the derived S-N curves.

2D and 3D simulations of dynamic delamination in curved unidirectional CFRP laminates subjected to moment/axial combined loading

Dynamic failure of 30-ply curved unidirectional CFRP laminate under quasi-static moment/axial combined loading used in the experiments of Tasdemir (2018) is investigated numerically. 2D and 3D explicit finite element analyses are carried out using cohesive zone elements and contact interactions to simulate progressive delamination. In both analyses, dynamic delamination initiates at the maximum radial stress location. The resultant crack propagation in both 2D and 3D simulations is observed to be intersonic in the arm regions, with lower crack tip speeds observed for the 3D case. In 3D simulations, a non-uniform crack front is observed, with the crack nucleating at the middle of the width, propagating to the edges and both arms at sub-Rayleigh speed before continuing to propagate in the arms at intersonic speeds. Intermittent local contact regions are observed in 2D simulations behind the crack tip, which are also observed in the 3D simulations at the free edge with a continuous contact line reaching the middle of the crack front, emphasizing the three-dimensional nature of the delamination growth. Load-displacement curves of both 2D and 3D simulations show good correlations with experimental results.

Understanding interlaminar toughening of unidirectional CFRP laminates with carbon nanotube veils

The introduction of nanostructured interlayers is one of the most promising strategies for interlaminar reinforcement in structural composites. In this work, we study the failure mechanism and interlayer microstructure of aerospace-grade structural composites reinforced with thin veils of carbon nanotube produced using an industrialised spinning process. Samples of unidirectional carbon fiber/epoxy matrix composites interleaved with different composition CNT veils were prepared using hot press method and tested for interlaminar fracture toughness (IFT), measured in Mode-I (opening) and Mode-II (in-plane shear), and for interlaminar shear strength (ILSS), evaluated by the short beam shear (SBS) test. The crack propagation mode could be directly determined through fractography analysis by electron microscopy and resin/CNT spatial discrimination by Raman spectroscopy, showing a clear correlation between interlaminar reinforcement and the balance between cohesive/adhesive failure mode at the interlayer region. Composites with full resin infiltration of the CNT veils give a large increase of Mode II IFT (88%) to 1500 J/m2 and a slight enhancement of apparent interlaminar shear strength (6.5%), but a decrease of Mode I IFT (−21%). A toughening factor, defined as the relative increase in toughness per interleaf-to-ply thickness ratio, give a record-high value of 15.8 for Mode II IFT. The results help establish the role of interlayer infiltration, interlaminar crossings and formation of a carbon fiber bridgings, for interlaminar reinforcement with interleaves.

Mechanical Properties Research of Carbon Fiber Composite Laminates

This paper expounds the basic theory of composite mechanics, and discusses the damage forms, damage analysis and failure criteria of composite materials. Then, the basic mechanical properties of unidirectional CFRP laminates with different layers, including modulus of elasticity, strength and so on, were obtained through a large number of experiments. Based on the experimental data, the relationship between the number of layers and the properties of materials was studied. The brittle fracture process of CFRP laminates was simulated by finite element analysis based on progressive damage model and compared with the force-displacement curves obtained by experiments. The validity of progressive damage model was proved.

Experimental study of orthogonal cutting of unidirectional CFRP laminates

This article relates to the study of orthogonal cutting of unidirectional carbon epoxy composite laminates T800S/M21 with tool angle influences. Many positive and negative values of the cutting angle (γ) and of the angle χ2 defined by the direction of the cutting speed and the fibres direction of the workpiece are used. Then, this papers focus on the study of the cutting mechanisms, the chip morphology (its mode of formation and its shape), and the surface quality obtained according to the cutting forces evolution. Finally, study explains angle influence on surface roughness.

Experimental study of orthogonal cutting of unidirectional CFRP laminates

Experimental study of orthogonal cutting of unidirectional CFRP laminates

Thermal defect characterization and strain distribution of CFRP laminate with open hole following fiber laser cutting process

High power fiber laser cutting of CFRP composites has been proven to be feasible with high efficiency, while there are still several challenges due to different carbon fiber-epoxy resin properties and sensitivity to thermal heat. This paper studied the influence of processing parameters on machined hole quality and quasi-static tensile behavior of unidirectional CFRP laminate with open hole (6.0 mm in diameter) following fiber laser cutting process. Hole surface morphology was analyzed using optical microscope and scanning electron microscopy. The area of heat affected zone and defect factor were quantified to understand the effect of machining parameters on thermal defect. It was found that both cutting speed and laser power were statistically significant with respect to HAZ recorded at hole entry, while cutting speed was the only significant factor for HAZ observed at hole exit. The full-field strain distribution during tensile loading was characterized using 3D digital image correlation (DIC) technique. Surprisingly, laser processing parameters showed limited effect on surface strain distribution (εxx, εxy, εyy) during tensile loading. Some highly localized strain zones were observed on the tangent of transverse edge of the hole, corresponding to final failure mode of splitting due to inevitably slight shear force during tensile loading and weak interface strength between carbon fiber and epoxy resin. Numerical simulation results of strain distribution were in good agreement with the DIC analysis.

An efficient numerical approach to the prediction of laminate tolerance to Barely Visible Impact Damage

An efficient numerical approach for the prediction of the Compression After Impact (CAI) strength of aerospace-grade CFRP laminates when exposed to Barely Visible Impact Damage (BVID) is proposed. The approach is based on mapping relevant BVID features, i.e. delaminations, onto an efficient CAI finite element model based on continuum shell discretization, and can be used on Low-Velocity Impact (LVI) results obtained experimentally or by means of high-fidelity virtual tests. It is proposed that delaminations may be represented by simplified shapes, and only the ones at critical through-thickness locations need to be mapped, allowing the clustering of several plies in a single shell layer. General guidelines, that are potentially valid for a wide range of unidirectional CFRP laminates, are proposed to identify relevant and critical BVID features to be mapped onto the efficient CAI modelling. The approach was validated for five laminates of AS4/8552 material, covering a range of different thicknesses, overall achieving CAI strength predictions within 5% of the experimental results. In comparison with the alternative high-fidelity CAI virtual testing approach, this method leads to computational efficiency gains of an order of magnitude. Moreover, the full simulation of the sequence LVI plus CAI steps can be accelerated by a factor of four.

Experimental study on the tensile behavior of unidirectional and plain weave CFRP laminates under different strain rates

In the present work, static and dynamic tensile tests of unidirectional and plain weave CFRP laminates used for retrofitting building structures were carried out using a universal testing machine and a high-speed servo-hydraulic testing machine (INSTRON VHS 160/100-20), respectively. The strain rate that the CFRP laminate experienced in the tests varies from 4E-5s−1 to 160s−1. Test results show that the tensile behavior of both unidirectional CFRP and plain weave CFRP are strain rate dependent, while the latter is more obvious than the former. Failure processes of the CFRP laminates were also observed with the aid of a high-speed camera. It is found that the failure mode of the CFRP laminate at high strain rate is quite different from that at quasi-static loading and low strain rate, which is believed to contribute to the strain rate sensitivity of both the unidirectional and plain weave CFRP laminate. Empirical formulas of dynamic increase factor (DIF) of tensile strength, Young's modulus and failure strain of the CFRP laminates were derived as a function of strain rate within the strain rate range tested. Available testing data were also collected from other papers and analyzed together with the testing data from the present study.

A new model for fatigue life prediction based on infrared thermography and degradation process for CFRP composite laminates

In this paper, a new fatigue life prediction methodology is proposed by combining stiffness degradation and temperature variation measured by InfraRed Thermographic (IRT) camera. Firstly, the improved thermographic method is used to determine the fatigue limit by using the data of stabilized temperature rising. Following this, a two-parameter model is proposed to characterize the stiffness degradation of CFRP laminates with the increase of cycle numbers. After the calibration parameters and the calculation of the normalized failure threshold stiffness, the whole S-N curve can be obtained in a very short time. The proposed model is applied to both the experimental data of triaxially braided CFRP laminates from literature and those of unidirectional CFRP laminates obtained from our fatigue tests. Results show that predicted S-N curves have a good agreement with traditional tests. The principal interests of this model could be listed as follows: (i) it is a more general criterion applicable to different materials; (ii) it has more physical senses; (iii) it allows the determination of the total S-N curve for composite materials in a short time.

Effect of specimen size on longitudinal strength of unidirectional carbon/epoxy composite laminates (part 2, splitting strength)

The effects of specimen size as well as notch size on longitudinal splitting failure in notched unidirectional carbon/epoxy laminates are examined. For this purpose, static tension tests are performed on thickness dimension (1D), in-plane dimensions (2D) and all the dimensions (3D) scaled notched specimens, respectively. The experimental results show that splitting strength has a tendency to increase by about 5–10% with increasing thickness dimension of a notched specimen. In contrast, splitting strength tends to decrease by about 20% not only with increasing in-plane dimensions but also with increasing all the dimensions, regardless of notch size. It is suggested that the 3D scaling effect in splitting strength of notched specimens is governed by the 2D scaling effect involved, since the 1D scaling effect is smaller. On the basis of these observations, a scale-sensitive notch size effect law is established. It is demonstrated to adequately describe the specimen-size effect as well as the notch-size effect in the longitudinal splitting strength of the unidirectional carbon/epoxy composite laminates. It is also shown that the effective normalized mode I fracture toughness for splitting failure in the unidirectional CFRP laminate can indirectly be evaluated by means of the scale-sensitive notch size effect law and it is close to the fracture mechanics test results in literature.

An experimental investigation into the high velocity penetration resistance of CFRP and CFRP/aluminium laminates

Carbon fibre-reinforced composite materials are of high potential as protective casing in the aerospace area, acting as an effective solution to lighten components against the collision. The high velocity penetration resistance abilities of unidirectional CFRP laminates and two carbon fibre reinforced aluminium laminates CRALL2/1 and CRALL3/2 (fabricated from T700S CFRP layers combined with aluminium alloy 2024-T3 layers) were evaluated by the ballistic tests with a flat, hemispherical or conical nosed projectile. Revealed from ballistic tests that fracture modes, ballistic limits and specific energy absorptions of CRALLs and CFRP were sensitive to nose shapes. Higher ballistic limits and specific energy absorption ability were performed by CRALLs than monolithic CFRP impacted by all shapes due to the strain rate hardening effect and failure conversion effect. In particular situation of flat nose projectiles penetrating, the specific energy absorption of the CRALL3/2 was 8% higher than that of monolithic aluminium alloy 2024-T3 at same thickness. The CRALLs may then be designed as effective lightweight structures to protect frames against collision in the aerospace area and outperform the traditional single CFRP laminates.