New development of self-damping MWCNT composites

Epoxy resin modified with nanofillers cannot be used alone for high performance structural applications due to their low-mechanical properties. Therefore, the objective of this work is to hybridize unidirectional and quasi-isotropic glass fiber composite laminates with 1.0 wt% multi-walled carbon nanotubes (MWCNTs). Results from flexural and damping characterizations showed that the flexural strength and modulus, storage modulus, and damping ratio of MWCNT/E nanocomposite are improved by about 7% ± 1.5% compared to neat epoxy. The enhancement in the flexural strength of quasi-isotropic laminate (20.7%) is about ten times higher than that for unidirectional laminate (2.1%). The flexural moduli of the nano-hybridized laminates are reduced by about 7.5–10.8%. Accordingly, the ultimate failure strain and damping properties are evidently improved. The improvement in damping ratio in some cases is about 100%. The high correlation coefficient (0.9995) between flexural and storage moduli suggests using the dynamic nondestructive tests for evaluation the elastic properties of composites.     

Damage Assessment of CFRP [90/±45/0] Composite Laminates over Fatigue Cycles

The present paper develops a stiffness-based model to characterize the progressive fatigue damage in quasi-isotropic carbon fiber reinforced polymer (CFRP) [90/±45/0] composite laminates with various stacking sequences. The damage model is constructed based on (i) cracking mechanism and damage progress in matrix (Region I), matrix-fiber interface (Region II) and fiber (Region III) and (ii) corresponding stiffness reduction of unidirectional plies of 90°, 0° and angle-ply laminates of ±45° as the number of cycles progresses. The proposed model accumulates damages of constituent plies constructing [90/±45/0] laminates by means of weighting factor η ₉₀, η ₀ and η ₄₅. These weighting factors were defined based on the damage progress over fatigue cycles within the plies 90°, 0° and ±45° of the composite laminates. Damage model has been verified using CFRP [90/±45/0] laminates samples made of graphite/epoxy 3501-6/AS4. Experimental fatigue damage data of [90/±45/0] composite laminates have fell between the predicted damage curves of 0°, 90° plies and ±45°, 0/±45° laminates over life cycles at various stress levels. Predicted damage results for CFRP [90/±45/0] laminates showed good agreement with experimental data. Effect of stacking sequence on the model of stiffness reduction has been assessed and it showed that proposed fatigue damage model successfully recognizes the changes in mechanism of fatigue damage development in quasi-isotropic composite laminates.     

Damage mechanics characterization of transverse cracking behavior in quasi-isotropic CFRP laminates with interlaminar-toughened layers

Microscopic damage behavior in quasi-isotropic CFRP laminates with interlaminar-toughened layers under tensile fatigue loading is investigated. Damage observation is conducted using an optical microscope and soft X-ray radiography. The material used is CFRP with interlaminar-toughened layers, T800H/3900-2. The laminate configurations are quasi-isotropic [45/0/−45/90]_s, [0/45/−45/90]_s and [45/−45/0/90]_s to discuss the effect of stacking sequence on microscopic fatigue damages. A damage mechanics analysis is used to obtain the energy release rate for transverse cracking which is correlated to the transverse crack density growth rate. The modified Paris-law analysis proves to be valid for characterization of transverse crack multiplication when the effect of other damage is small.     

Mechanical Properties and Interlaminar Fracture Toughness of Glass‐Fiber‐Reinforced Epoxy Composites Embedded with Shape Memory Alloy Wires

The effects of the content and position of shape memory alloy (SMA) wires on the mechanical properties and interlaminar fracture toughness of glass‐fiber‐reinforced epoxy (GF/epoxy) composite laminates are investigated. For this purpose, varying numbers of SMA wires are embedded in GF/epoxy composite laminates in different stacking sequences. The specimens are prepared by vacuum‐assisted resin infusion (VARI) processing and are subjected to static tensile and three‐point‐bending tests. The results show that specimens with two SMA wires in the stacking sequence of [GF₂/SMA/GF₁/SMA/GF₂] and four SMA wires in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] exhibit optimal performance. The flexural strength of the optimal four‐SMA‐wire composite is lower than that of the pure GF/epoxy composite by 5.76% on average, and the flexural modulus is improved by 5.19%. Mode‐I and II interlaminar fracture toughness tests using the SMA/GF/epoxy composite laminates in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] are conducted to evaluate the mechanism responsible for decreasing the mechanical properties. Scanning electron microscopy (SEM) observations reveal that the main damage modes are matrix delamination, interfacial debonding, and fiber pullout.

     

Unification of strength scaling between unidirectional,quasi-isotropic,and notched carbon/epoxy laminates

An investigation into size effects and notch sensitivity in quasi-isotropic carbon/epoxy laminates was carried out. The purpose is to draw a complete picture of the strength scaling in unidirectional, quasi-isotropic, and notched carbon/epoxy laminates. A link was established between the strength scaling of the unidirectional and quasi-isotropic laminates. Efforts were made to understand the relationship between unnotched and open-hole strengths. For very small holes, the notched strengths approach the unnotched strength limit. A scaling law based on Weibull statistics was used to predict the unnotched laminate strengths. For very large holes, the same scaling law in conjunction with a detailed 3D ply-by-ply FE analysis with matrix cracks in the 90° plies and delamination cohesive interface elements was used to predict the large notched strengths. A good agreement between the modelling and experimental results was achieved. The effects of 90° matrix cracks on unnotched and notched strengths were also studied.     

Mechanical properties of cross-ply and quasi-isotropic composite laminates processed using aligned multi-walled carbon nanotube/epoxy prepreg

This study examined the processing and mechanical properties of cross-ply and quasi-isotropic composite laminates processed using aligned multi-walled carbon nanotube/epoxy prepreg sheets. Three kinds of CNT/epoxy laminates, ([0°/90°]s, [60°/0°/?60°]s, [0°/45°/90°/?45°]s) were successfully fabricated using aligned CNT/epoxy prepreg sheets. The CNT volume fraction was approximately 10%. No visible void or delamination was observed in composite laminates, and the thickness of each layer was almost equal to that of the prepreg. To evaluate the elastic moduli, E₁₁, E₂₂, and G₁₂, of each ply in the laminates, on-axis and off-axis tensile tests (0°, 45°, 90°) were conducted of aligned CNT/epoxy lamina specimens. The Young’s modulus of CNT/epoxy cross-ply and quasi-isotropic laminates agreed with the theoretical values, which were calculated using classical laminate theory and elastic moduli of CNT/epoxy lamina. The respective failure strains of [0°/90°]s, [60°/0°/?60°]s, and [0°/45°/90°/?45°]s laminates are 0.65, 0.92, 0.63%, which are higher than that of 0° composite lamina (0.5%). Results suggest that the failure strain of 0° layer in composite laminates is improved because of the other layers.     

A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites

Experimental investigations on flax and glass fabrics reinforced epoxy specimens, i.e. FFRE and GFRE, submitted to fatigue tests are presented in this paper. Samples having [0/90]_3S and [±45]_3S stacking sequences, with similar fibre volume fractions have been tested under tension–tension fatigue loading. The specific stress-number of cycles to failure (S–N) curves, show that for the [0/90]_3S specimens, FFRE have lower fatigue endurance than GFRE, but the [±45]_3S FFRE specimens offer better specific fatigue endurance than similar GFRE, in the studied life range (<2 × 10⁶). Overall, the three-stage stiffness degradation is observed in all cases except for [0/90]_3S FFRE specimens, which present a stiffening phenomenon of around 2–3% which could be related to the straightening of the microfibrils.     

Damage evolution of angle-ply SCS-6/Ti composites under static and fatigue loading

The effect of fibre orientation and laminate stacking sequence on the tensile and fatigue behaviour of SCS-6/Ti 15-3 composites were investigated. The laminates used in this study were: (90)₆, (0/ ± 45)_s, (0/90)_s, and (90/ +-45)_s. The initiation and progression of microstructural damage at various stress levels was thoroughly characterized. It was found that fatigue life at high applied stresses were controlled by fibre fracture; progressive damage involving fibre fracture, interfacial debonding and matrix cracking became dominant at low applied stresses. Observation of the damage mechanisms in the angle-ply laminates under cyclic loading suggests that increasing the fibre-matrix bonding strength may improve the load carrying capability and fatigue life of laminates containing off-axis plies.     

Characterization of transverse tensile, interlaminar shear and interlaminate fracture in CF/EP laminates with 10 wt% and 20 wt% silica nanoparticles in matrix resins

The transverse tensile properties, interlaminar shear strength (ILSS) and mode I and mode II interlaminar fracture toughness of carbon fibre/epoxy (CF/EP) laminates with 10 wt% and 20 wt% silica nanoparticles in matrix were investigated, and the influences of silica nanoparticle on those properties of CF/EP laminates were characterized. The transverse tensile properties and mode I interlaminar fracture toughness (G_IC) increased with an increase in nanosilica concentration in the matrix resins. However, ILSS and the mode II interlaminar fracture toughness (G_IIC) decreased with increasing nanosilica concentration, especially for the higher nanosilica concentration (20 wt%). The reduced G_IIC value is attributed to two main competing mechanisms; one is the formation of zipper-like pattern associated with matrix microcracks aligned 45° ahead of the crack tip, while the other is the shear failure of matrix. The ratio of G_IIC/G_IC decreased with the concentration of silica nanoparticles, comparable with similar CF/EP laminates with dispersed CNTs in matrix. Fractographic studies showed that interfacial failure between carbon fibre and epoxy resin occurred in the neat epoxy laminate, whereas a combination of interfacial failure and matrix failure occurred in the nanosilica-modified epoxy laminates, especially those with a higher nanosilica concentration (20 wt%).     

Open hole fatigue characteristics and damage growth of stitched plain weave carbon/epoxy laminates

Fatigue response of stitched plain weave carbon/epoxy laminates containing circular holes is experimentally investigated. Two carbon/epoxy laminates of cross-ply [(0/90)]₂₀ and quasi-isotropic [(±45)(0/90)₂(±45)₂(0/90)₂(±45)₂(0/90)]_s are reinforced using Kevlar-29® yarns in through-thickness direction. The laminates are drilled to produce a circular hole with diameter of 5.7 mm. Stitch configuration for cross-ply laminates is round stitch and parallel stitch, while that for quasi-isotropic laminates is parallel stitch only. For round stitch configuration, the hole is surrounded by circular stitch line of 7-mm diameter. For parallel stitch, the distance between two stitch lines (spacing) is 15 mm. In all, three independent cases are presented in this paper: Case 1 (cross-ply laminates, round stitch, tension–tension fatigue); Case 2 (cross-ply laminates, parallel stitch, tension–tension fatigue); Case 3 (quasi-isotropic laminates, parallel stitch, compression–compression fatigue). In each case, comparison with unstitched laminates is made. Case 1 shows that round stitch reduces tension fatigue curve of carbon/epoxy laminates. Round stitch seems to aggravate the damage, which is emanating from the hole rim of laminates. It gradually diverts the damage towards the edge of the specimen and causes premature fatigue failure. Case 2 shows that although parallel stitch generally does not influence the fatigue life of laminates, the damage growth due to parallel stitch is apparently unstable after 8 million cycles. As a result, laminates with parallel stitch eventually fail before reaching 10 million cycles. In contrast, unstitched laminates are able to sustain fatigue load for more than 10 million cycles. Case 3 shows that under compression fatigue load, fatigue limit of stitched plain weave laminates is better than that of the unstitched ones due to damage redistribution along the stitch lines.     

Influence of embedded gap and overlap fiber placement defects on the microstructure and shear and compression properties of carbon–epoxy laminates

This paper presents results from an experimental study of the influence of embedded defects created during automated fiber tape placement, on the mechanical properties of carbon/epoxy composites. Two stacking sequences have been examined, [(−45°/+45°)₃/−45°] and [90°₄/0°₃/90°₄], in which gaps and overlaps have been introduced during fiber placement. These materials have been cured in an autoclave either with or without a caul plate, then analyzed by ultrasonic C-scan. The microstructures were characterized by scanning electron microscopy. In-plane shear tests were performed on the ±45° laminates and showed that the use of a caul plate does not affect mechanical behavior of plies in the embedded defect region. Compression tests were performed on 0°/90° laminates and in this case the presence of a caul plate is critical during polymerization as it prevents thickness variations and allows defects to heal.     

Transverse crack growth behavior considering free-edge effect in quasi-isotropic CFRP laminates under high-cycle fatigue loading

The high-cycle fatigue characteristics focused on the behavior of the transverse crack growth up to 10⁸ cycles were investigated using quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates whose stacking sequence was [−45/0/45/90]_s. To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz in addition to 5 Hz. In this study, to evaluate quantitative characteristics of the transverse crack growth in the high-cycle region, the energy release rate considering the free-edge effect was calculated. Transverse crack growth behavior was evaluated based on a modified Paris law approach. The results revealed that transverse crack growth was delayed under the test conditions of the applied stress level of σ_max/σ_b = 0.2.     

Evaluation of three in-plane shear test methods for advanced composite materials

This paper is concerned with the evaluation of three in-plane shear test methods for advanced carbon fibre composites for aerospace applications. To accomplish this goal, the losipescu, ± 45° tensile and 10° off-axis tensile shear test methods were evaluated for three advanced epoxy matrix materials (Narmco 5245C, Hexcel F584 and American Cyanamid Cycom 1806) reinforced with Hercules IM6 carbon fibres. The values of in-plane shear moduli obtained from the three test methods and three materials were then used with other previously determined elastic constants to predict the tensile moduli of (+45°/0°/−45°/90°)_6s laminates. Comparison of the predicted and experimental laminate tensile moduli showed that any one of the three shear test methods was appropriate for determining the in-plane shear modulus to predict tensile moduli of symmetric laminates which consist of equal numbers of 0°, +45°, −45° and 90° oriented laminae.     

Fatigue life estimation of graphite/epoxy laminates under consideration of delamination growth

A closed form approach to the assessment of the fatigue life of graphite/epoxy laminates under cyclic tension–compression loading has been developed. The model is mechanistic and uses cyclic energy release rates for prediction of delamination growth and of critical delamination sizes which induce buckling and the final failure of the laminates. Tests performed with graphite/epoxy specimens of stacking order [0_n, ?_m]_s with severed central plies [?], and of stacking order [0₂, +45, 0₂, ?45, 0, 90]_s with a central unloaded hole, indicate good correlation between estimated values and observed delamination growth, critical buckling strength of separated plies and load cycles to failure.     

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/−45₂/0₂]_S, [45/90/−45/0]_2S and [45/90/−45/0]_4S were examined. These were selected on the basis that under quasi-static loading the [45₂/90₂/−45₂/0₂]_S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]_2S and [45/90/−45/0]_4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1 × 10⁶ cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.     

Quantification of flexural fatigue life and 3D damage in carbon fibre reinforced polymer laminates

Carbon fibre reinforced polymer (CFRP) laminated composites have become attractive in the application of wind turbine blade structures. The cyclic load in the blades necessitates the investigation on the flexural fatigue behaviour of CFRP laminates. In this study, the flexural fatigue life of the [+45/−45/0]_2s CFRP laminates was determined and then analysed statistically. X-ray microtomography was conducted to quantitatively characterise the 3D fatigue damage. It was found that the fatigue life data can be well represented by the two-parameter Weibull distribution; the life can be reliably predicted as a function of applied deflections by the combined Weibull and Sigmodal models. The delamination at the interfaces in the 1st ply group is the major failure mode for the flexural fatigue damage in the CFRP laminate. The calculated delamination area is larger at the interfaces adjacent to the 0 ply. The delamination propagation mechanism is primarily matrix/fibre debonding and secondarily matrix cracking.     

Mechanical properties of carbon fiber/fullerene-dispersed epoxy composites

This study examined the effect of fullerene dispersion on the mechanical properties of carbon-fiber reinforced epoxy matrix composites (CFRPs). Mechanical properties such as tension, compression, open-hole compression, comparession after impact (CAI), binding, short beam shear, and interlaminar fracture toughness were evaluated for [0]₈, [90]₁₆, [45/0/?45/90]_2S laminates. Tension and compression strengths increased 2–12% by dispersing 0.5% of fullerene into the matrix resin. Furthermore, interlaminar fracture toughness of the composite was improved by about 60%. It was revealed that a small amount of fullerene (0.1–1 wt.%) increased the failure strain of epoxy resin itself, thereby improving the CFRP strength.     

Effect of temperature on low velocity impact damage and post-impact flexural strength of CFRP assessed using ultrasonic C-scan and micro-focus computed tomography

The effect of temperature on the low velocity impact resistance properties and on the post-impact flexural performance of CFRP laminates were studied. With this aim, 150 × 75 mm cross-ply carbon fibre/epoxy laminates with a [0/90/90/0]_2s layup, therefore with a total of sixteen layers, were impacted at ambient temperature (30 °C) and at elevated temperatures (55, 75 and 90 °C) at a velocity of 2 m/s using a drop weight impact tower. This was followed by flexural tests carried out at ambient temperature using a three-point bending rig. Damage assessment of impact and post-impact behaviour were carried out using ultrasonic C-scan and microfocus X-ray computed tomography (μCT). Interrupted flexural tests using μCT allowed delamination propagation to be observed. In general, lower projected damage was observed at elevated temperatures, which resulted also in a possible hindrance to delamination and shear cracks propagation during impact and in a greater amount of retained flexural strength after impact.     

Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates

The reinforcement effects of two nanofillers, i.e., multi-walled carbon nanotube (MWCNT) and vapor grown carbon fiber (VGCF), which are used at the interface of conventional CFRP laminates, and in epoxy bulk composites, have been investigated. When using the two nanofillers at the interface between two conventional CFRP sublaminates, the Mode-I interlaminar tensile strength and fracture toughness of CFRP laminates are improved significantly. The performance of VGCF is better than that of MWCNT in this case. For epoxy bulk composites, the two nanofillers play a similar role of good reinforcement in Young’s modulus and tensile strength. However, the Mode-I fracture toughness of epoxy/MWCNT is much better than that of epoxy/VGCF.     

Bisphenol A based polyester binder as an effective interlaminar toughener

Bisphenol A based thermoplastic polyesters are commonly used in the industry as binders, or tackifiers, to produce cost-saving preforms in Liquid Composite Moulding processes such as Vacuum Assisted Resin Transfer Moulding (VARTM). However, it is often reported that the presence of these polyesters has a detrimental effect on the mechanical properties of the resulting composite laminates. In contrast, this study shows that interlaminar toughness can be increased without negatively affecting other properties by coating the reinforcing plies with a bisphenol A based thermoplastic polyester if some precautions are taken in mind.The polyester was added to an epoxy resin in order to study its effect on the thermophysical properties and fracture toughness of the bulk epoxy. The polyester molecules acted as a plasticizer for the epoxy resin when the polyester was added in low amounts. This increased the bulk fracture toughness of the epoxy resin by 30%. Polyester modified glass/epoxy laminates were produced and tested for Mode I interlaminar fracture toughness and flexural properties. The increased toughness of the epoxy matrix led to a 60% increased Mode I interlaminar fracture toughness of the laminates, without negatively affecting flexural stiffness and strength of the laminates.