Structurally stitched NCF CFRP laminates. Part 2: Finite element unit cell based prediction of in-plane strength

Based on experimental investigations on structurally stitched non-crimp fabric (NCF) carbon fiber/epoxy laminates under in-plane tension, compression and shear loading [1], a finite element based unit cell model was developed to estimate the in-plane strength of NCF laminates taking into consideration the yarn diameter, the stitching pattern and direction as well as the load type. Depending on these parameters, regions with undisturbed and disturbed fiber orientations leading to resin pockets as well as local changes of the fiber volume fraction are taken into account in the model.The comparison of experimental and numerical results showed that the strength of structurally stitched NCF laminates under in-plane tension, compression or shear loading can be predicted with an acceptable accuracy. The overall mean deviation between simulation and experiment observed was between 8% and 13%.     

Mechanical characterization of hierarchical carbon fiber/nanofiber composite laminates

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber–matrix adhesion and indications of CNF-induced matrix toughening.     

Structurally stitched NCF preforms: Quasi-static response

Experimental data are presented for a typical structurally stitched preform, composed of carbon fibre non-crimp fabrics (NCFs) and impregnated with an epoxy resin. The term ‘structural’ presumes here that the stitching yarn does not only consolidate the layers (as the non-structural one does for NCF plies) but forms also a through-the-thickness reinforcement. One stitching technique—tufting—is studied, with 67 tex carbon yarn and several stitching lengths. The test results (in-plane tension, out-plane compression, and 3-point bending) are compared and discussed revealing an influence of stitching and specifics of damage development. The stitching, on the one hand, decreases delaminations and increases the ultimate load. On the other hand, the stitching creates stress–strain concentrators which lead to earlier damage initiation.     

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.     

Evaluation of the interfacial properties of polypropylene composite laminates,reinforced with paper sheets

Laminates, composed of different papers and polypropylene (PP), were fabricated by a manual stacking and hot pressing. The laminates were characterized by mechanical testing and the results were compared to glass fiber reinforced PP. Furthermore, a detailed evaluation of the interfacial properties and the paper structures was carried out by means of data modeling via rule of mixtures (ROM), as well as electron microscope (SEM) analysis. For investigating the influence of the laminate’s composition on the water adsorption behavior, water diffusion coefficients were determined. As a result, laminates with a tensile modulus up to 6 GPa and a tensile strength of 80 MPa were obtained. The property changes of the papers upon processing were successfully modeled, revealing a significant increase of the paper’s mechanical properties after fiber embedding. In general, the obtained results indicate a high potential of paper as a suitable reinforcement material for low to middle strained applications.     

Detection of in-plane fiber waviness in cross-ply CFRP laminates using layer selectable eddy current method

This paper presents an eddy current testing method to detect in-plane fiber waviness in cross-ply carbon fiber reinforced plastic (CFRP) laminates. We propose a method which has high sensitivity to presence of in-plane waviness and can select layers to be inspected. The probe was used to detect artificially induced in-plane waviness in cross-ply CFRP laminates. It was observed that obtained signal has extreme value at the edges and vertex of the waviness, which implies the possibility of precise identification of waviness location. Detectability of subsurface waviness was investigated using 20 layer cross-ply laminate with in-plane waviness at different depths. Experimental results showed that in-plane waviness 18 layers away from the surface could be detected. The minimum misalignment angle of the detected waviness was 7.4°. The effectiveness of the probe and physical background of the obtained signals were verified by finite element method analyses.     

Click coupled stitched graphene sheets and their polymer nanocomposites with enhanced photothermal and mechanical properties

The chemically stitched graphene oxide (GO) sheets were obtained using a click chemistry reaction between azide-functionalized GO and alkyne-functionalized GO. The click coupled GO (GO-click-GO) sheets showed the largely increased electrical conductivity and near infrared laser-induced photothermal properties compared to the GO sheets, which result from formation of triazole ring as a bridging linker between the GO sheets. The polyurethane (PU) nanocomposites incorporating the GO-click-GO sheets exhibited enhanced mechanical properties of breaking stress and modulus than the GO/PU nanocomposites. The modulus of GO-click-GO/PU nanocomposites was higher than that of the GO/PU nanocomposites at the same filler loading of 0.1 and 0.5 wt%. The GO-click-GO/PU nanocomposites also showed a significantly improved photothermal properties compared to the GO/PU nanocomposites at the same filler loading. The click coupled stitched GO sheets in this study can be used as the superior reinforcing fillers for mechanically and photothermally high performance polymer nanocomposites.     

Fracture toughness improvement of CFRP laminates by dispersion of cup-stacked carbon nanotubes

Several techniques are introduced to enhance the interlaminar fracture toughness of CFRP laminates using cup-stacked carbon nanotubes (CSCNTs). Prepared CSCNT-dispersed CFRP laminates are subject to Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests in order to obtain mode-I and mode-II interlaminar fracture toughness. The measured fracture toughnesses are compared to that of CFRP laminates without CSCNT to evaluate the effectiveness of CSCNT dispersion for the improvement of fracture toughness. All CSCNT-dispersed CFRP laminates exhibit higher fracture toughness, and specifically, CSCNT-dispersed CFRP laminates with thin epoxy interlayers containing short CSCNTs have three times higher fracture toughness than CFRP laminates without CSCNT. SEM observation of fracture surfaces is also conducted to investigate the mechanisms of fracture toughness improvement. Crack deflection mechanism is recognized in the CSCNT-dispersed CFRP laminates, which is considered to contribute the enhancement of interlaminar fracture toughness.     

Fabrication and mechanical properties of well-dispersed multiwalled carbon nanotubes/epoxy composites

Multiwalled carbon nanotubes (MWCNTs)/epoxy nanocomposites were fabricated by using ultrasonication and the cast molding method. In this process, MWCNTs modified by mixed acids were well dispersed and highly loaded in an epoxy matrix. The effects of MWCNTs addition and surface modification on the mechanical performances and fracture morphologies of composites were investigated. It was found that the tensile strength improved with the increase of MWCNTs addition, and when the content of MWCNTs loading reached 8 wt.%, the tensile strength reached the highest value of 69.7 MPa. In addition, the fracture strain also enhanced distinctly, implying that MWCNTs loading not only elevated the tensile strength of the epoxy matrix, but also increased the fracture toughness. Nevertheless, the elastic modulus reduced with the increase of MWCNTs loading. The reasons for the mechanical property changes are discussed.     

A survey of test methods for multiaxial and out-of-plane strength of composite laminates

This review paper gives an overview of test methods for multiaxial and out-of-plane strength of composite laminates, with special consideration of non-crimp fabrics (NCF) and other textile systems. Tubular and cruciform specimens can provide arbitrary in-plane loading, while off-axis and angle-ply specimens provide specific biaxial loadings. Tensile and compressive out-of-plane strength may be determined by axial loading of specimens with a waisted gauge section, while bending of curved specimens allow determination of the out-of-plane tensile strength. Tests suited for out-of-plane shear strength include the short beam shear test, the inclined double notch test and the inclined waisted specimen. Testing of arbitrary tri-axial stress states using tubular or cruciform specimens with superimposed through-the-thickness loading is highly complex and significant problems have been reported in achieving the intended stress states and failure modes. Specific tri-axial stress states can be obtained by uniaxial loading of specimens with constrained expansion, as in the die channel test.     

Mechanical and electrical properties of polymer-derived Si-C-N ceramics reinforced by octadecylamine - Modified single-wall carbon nanotubes

Polymer-derived Si-C-N ceramics reinforced by homogeneously distributed octadecylamine-functionalized single-walled carbon nanotubes (SWCNTs) were synthesized using a casting process, successive pressureless cross-linking and thermolysis. We find that the incorporation of even small amounts of modified SWCNTs leads to a remarkable improvement of mechanical and electrical transport properties of our composites. In particular, we find twofold enhancement of fracture toughness. The Youngs modulus and the hardness show increase by ∼30% and 15%, respectively. Furthermore, the electrical conductivity was found to increase more than five orders of magnitude even for a tube content of 0.5 wt.%.     

Study of trimming damages of CFRP structures in function of the machining processes and their impact on the mechanical behavior

The main focus of this paper is to investigate the defects generated by different machining processes (namely burr tool machining, abrasive water jet machining ‘AWJM’ and abrasive diamond cutter ‘ADS’) and their impact on the mechanical behavior of CFRP in quasi-static (compression and inter-laminar shear) and tensile–tensile fatigue tests. The cutting conditions are selected so that different levels of degradation can be obtained. The machined surface is characterized using roughness measuring devices with and without contact and SEM observations. The results show that the defects generated during the trimming process with a cutting tool are fibers pull-out and resin degradation. These defects are mainly located in the layers with the fibers oriented at −45° and 90°. However, when using abrasive water jet and abrasive diamond processes, the defects generated have the form of streaks and are not dependent on the fiber orientation. Furthermore, the results of quasi-static tests performed on specimens machined by cutting tools show that AWJ specimens offer a better resistance in compression but the ADS samples offer higher inter-laminar-shear strengths. Moreover, the results of fatigue tests show that specimens machined with a burr tool offer higher endurance limit. Finally, it is concluded that the type and the mode of the mechanical loading (quasi-static fatigue) affect the mechanical response of CFRP and favor a given machining process.     

MWNT reinforced polyurethane foam: Processing, characterization and modelling of mechanical properties

The mechanical properties of a foam material changes when the foam is reinforced with nanoparticles. In this paper it is investigated how the addition of multi-walled carbon nanotubes (MWNTs) influences the effective properties of polyurethane foam. Both pure and nano-reinforced foams containing different amounts of MWNT are produced and both pristine and functionalized MWNT are used as reinforcement. The MWNT are dispersed in the polyol using high-shear mixing with various mixing times to examine how that influences the properties of the produced foams. SEM is used to characterize the microstructure of the produced foams and these examinations reveals that the foam changes from a completely closed cell material for the pure PU foam to a partly open celled foam when adding MWNT. Compressive tests are performed in order to determine the strength and stiffness of the produced foams and the increase in these properties are very dependent on both the wt.% of MWNT and the mixing time used to disperse them in the polyol. Furthermore, the effective properties of the reinforced foams are determined using the Mori-Tanaka (MT) method and generally the correlation between the experimentally and numerically determined properties improves when the mixing time used increases for a constant wt.% of MWNT.     

Evaluation of impact damage mechanism of multi-axial stitched CFRP laminate

This study focuses on multi-axial stitched fabric, which is a thick, high performance reinforcement for large-scale composite structures. The effects of impact damage on multi-axial stitched CFRP laminates molded by vacuum-assisted resin transfer molding (VARTM) method were evaluated. Impact damage within material was evaluated by ultrasonic scanning device and optical cross-sectional observations. Probed images obtained by both non-destructive and destructive methods were compared, and internal damage distributions of multi-axial stitched CFRP laminates were clarified. In addition, residual compressive strength and fatigue property of impact-damaged CFRP laminates were evaluated by in situ damage growth monitoring using the thermo-elastic stress analyzer (TESA). Three-dimensional damage distribution of impacted CFRP laminate was obtained from ultrasonic C-scan images and cross-sectional photographs. Damage progress behavior was observed on a destructive and non-destructive basis by post-impact fatigue (PIF) test.     

Effects of stacking thickness on the damage behavior in CFRP composite cylinders subjected to out-of-plane loading

The purpose of this study is to evaluate effects of stacking thickness on the microscopic damage behavior in a filament wound carbon fiber reinforced plastics (FW-CFRPs) composite cylinder subjected to impact or quasi-static out-of-plane loading. From both tests, thicker CFRP improved the stiffness of the cylinder and decreased the resultant plastic deformation due to indentation. From the cross-sectional observation, it is clarified that fiber breakages were localized for the specimens with impact tests more than 10-layers and specimens with quasi-static tests more than 15-layers. In order to discuss the relation between the damage and the absorbed energy, damage depth ratio was defined as fiber damage depth per unit CFRP thickness. To normalize the effect of thickness, absorbed energy ratio was also defined as absorbed energy per unit CFRP thickness. Absorbed energy ratio as a function of absorbed energy ratio was expressed as one master curve regardless of loading conditions.     

Flexural properties of hemp fibre reinforced polylactide and unsaturated polyester composites

In this work, flexural strength and flexural modulus of chemically treated random short and aligned long hemp fibre reinforced polylactide and unsaturated polyester composites were investigated over a range of fibre content (0-50 wt%). Flexural strength of the composites was found to decrease with increased fibre content; however, flexural modulus increased with increased fibre content. The reason for this decrease in flexural strength was found to be due to fibre defects (i.e. kinks) which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fibre treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fibre/matrix adhesion.     

Indentation of polyethylene laminates by a flat-bottomed cylindrical punch

Cross-ply polymer laminates reinforced by ultra-high molecular weight polyethylene (UHWMPE) fibers and tapes have been subjected to quasi-static indentation by a flat-bottomed, circular cross section punch and their penetration resistance and failure mechanisms investigated. Three fiber- and two tape-reinforced grades progressively failed during indentation via a series of unstable failure events accompanied by substantial load drops. This resulted in a ‘saw-tooth’ load versus indentation depth profile as the load increased with indentation depth after each failure event. The penetration behavior scaled with the ratio of the thickness of the remaining laminate to the diameter of the punch, and the indentation pressure scaled with the through thickness compressive strength. Failure occurred by ply rupture. The results are consistent with penetration governed by an indirect tension failure mechanism, and with experimental reports that tape-reinforced materials have a similar ballistic resistance to the higher tensile strength fiber-reinforced grades in rear-supported test conditions.     

Review of the mechanical properties of carbon nanofiber/polymer composites

In this paper, the mechanical properties of vapor grown carbon nanofiber (VGCNF)/polymer composites are reviewed. The paper starts with the structural and intrinsic mechanical properties of VGCNFs. Then the major factors (filler dispersion and distribution, filler aspect ratio, adhesion and interface between filler and polymer matrix) affecting the mechanical properties of VGCNF/polymer composites are presented. After that, VGCNF/polymer composite mechanical properties are discussed in terms of nanofibers dispersion and alignment, adhesion between the nanofiber and polymer matrix, and other factors. The influence of processing methods and processing conditions on the properties of VGCNF/polymer composite is also considered. At the end, the possible future challenges for VGCNF and VGCNF/polymer composites are highlighted.     

Structure and properties of highly oriented polyoxymethylene/multi-walled carbon nanotube composites produced by hot stretching

Highly-oriented polyoxymethylene (POM)/multi-walled carbon nanotube (MWCNT) composites were fabricated through solid hot stretching technology. With the draw ratio as high as 900%, the oriented composites exhibited much improved thermal conductivity and mechanical properties along the stretching direction compared with that of the isotropic samples before drawing. The thermal conductivity of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawing. Microstructure observation demonstrated that the POM matrix had an ordered fibrillar bundle structure and MWCNTs in the composite tended to align parallel to the stretching direction. Wide-angle X-ray diffraction results showed that the crystal axis of the POM matrix was preferentially oriented perpendicular to the draw direction, while MWCNTs were preferentially oriented parallel to the draw direction. The strong interaction between the POM matrix and the MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs.     

Positive size and scale effects of all-cellulose composite laminates

Negative size effects are commonly reported for advanced composite materials where the strength of the material decreases with increasing volume of the test specimen. In this work, the effect of increasing specimen volume on the mechanical properties of all-cellulose composites is examined by varying the laminate thickness. A positive size effect is observed in all-cellulose composite laminates as demonstrated by a 32.8% increase in tensile strength as the laminate thickness is increased by 7 times. The damage evolution in all-cellulose composite laminates was examined as a function of the tensile strain. Enhanced damage tolerance concomitant with increasing specimen volume is associated with damage accumulation due to transverse cracking and strain delocalisation. A transition from low-strain failure to tough and high-strain failure is observed as the laminate thickness is increased. Simultaneously, scale effects lead to an increase in the void content and cellulose crystallinity at the core, with increasing laminate thickness.