Determination of initiation value of mode I interlaminar fracture toughness of unidirectional polymer composites

The use of interlaminar fracture tests to measure the delamination resistance of unidirectional composite laminates is now widespread. However, because of the frequent occurrence of fiber bridging and multiple cracking during the tests, it leads to artificially high values of delamination resistance, which will not represent the behavior of the laminates. Initiation fracture from the crack starter, on the other hand, does not involve bridging, and should be more representative of the delamination resistance of the composite laminates. Since there is some uncertainty involved in determining the initiation value of delamination resistance in mode I tests in the literature, a power law of the form G_IC= A · Δ a^b (where G_IC is mode I interlaminar fracture toughness and Δ a is delamination growth) is presented in this paper to determine initiation value of mode I interlaminar fracture toughness. It is found that initiation values of the mode I interlaminar fracture toughness. G_ICini, can be defined as the G_IC value at which 1 mm of delamination from the crack starter has occurred. Examples of initiation values determined by this method are given for both carbon fiber reinforced thermoplastic and thermosetting polymers.     

Effect of graphene oxide on the interlaminar fracture toughness of carbon fiber/epoxy composites

Graphene oxide (GO) nanoparticles were introduced in the interlaminar region of carbon fiber–epoxy composites by dispersing it in a thermoplastic polymer carrier such as polyvinylpyrrolidone (PVP). Mode‐I fracture toughness (G_IC) was investigated using double cantilever beam testing to evaluate the effect of the GO on the delamination behavior of the composite. The GO content was varied from 0% to 7% by weight as a function of the PVP content. Improvement of ～100% in the Mode I fracture toughness (G_IC) was observed compared to composites with no GO. The optimum amount of nanoparticles for improving the interlaminar fracture toughness was found to be ～0.007% by weight of the composite. The increase in the value of flexural strength value was also observed. Scanning electron microscopy of fracture surfaces, X‐ray diffraction, and transmission electron microscopy, and reflectance Fourier transform infrared spectra, as well as Raman spectroscopy results, are presented to support the conclusions. POLYM. ENG. SCI., 59:1199–1208 2019. © 2019 Society of Plastics Engineers     

Effects of temperature on interlaminar fracture toughness of fibre reinforced plastic composites made by newly proposed rubber pressure moulding technique

Abstract

The present paper has investigated the effect of temperatures (i.e.?70–100°C) on the interlaminar fracture toughness (ILFT) of fibre reinforced plastic (FRP) composite panels made by a recently developed process known as the rubber pressure moulding (RPM) technique. The RPM technique is based on the matching die set, where the die is made of hard metal like steel and the punch from flexible rubber like materials. The use of flexible rubber punch helps to intensify and uniformly redistribute pressure (both operating pressure and developed hydrostatic pressure due to the flexible rubber punch) on the surface of the product. Natural rubber was used to prepare rubber punch in this investigation. For performance evaluation of FRP composites made by the RPM technique, FRP composites were also made by conventional method and tested at the same temperatures. It is observed that Mode I ILFT of FRP composites decreases towards higher and lower extremes of the temperature range selected. FRP composites made by the RPM technique show a higher Mode I ILFT over the 25–100°C temperature range than those made by the conventional process.     

The effect of loading rate on the fracture toughness of fiber reinforced polymer composites

This article is a detailed review of the strain rate dependence of fracture toughness properties in polymer composite materials. An attempt is made to draw together all the strain rate studies done in the past and to elucidate the reasons given by the authors of the reviewed papers for the trends resulting from their studies to better understand the strain rate effects on the fracture toughness of fiber reinforced polymer composite materials. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 899–904, 2005     

Mechanical behavior and fracture toughness characterization of high strength fiber reinforced polymer textile composites

The mechanical and fracture behavior of polymer composites are the subject of great interest from many years and still interesting among the researchers. Composites are extremely used for their superior mechanical, thermal and fracture toughness properties in various sectors such as automobile, aerospace and defense applications. In this article, unidirectional and woven high strength glass, carbon and Kevlar fiber reinforced polymer textile composites are taken into consideration for the comprehensive review of mechanical behavior and fracture toughness characterization. Current review work began with the introduction to polymer textile composites with its manufacturing stages, processing techniques and factors affecting the performance under mechanical loading. The mechanical behavior of high strength fiber reinforced polymer (HSFRP) textile composites was discussed in tension, compression, flexural, low velocity and high velocity impact loading with the recent numerical and experimental characterization studies. Textile geometrical modeling and CAE tools are also described for numerical characterization. Under the influence of mechanical loading on composites, failure occurs actually due to the crack initiation and propagation, so it is also required to characterize. Significant elements of fracture mechanics are well described for the better understanding of fracture toughness characterization. Mode-I, Mode-II, Mode-III interlaminar and Mode-I intralaminar fracture toughness characterization are widely explained by considering the effect of filler content, fiber orientation and fiber volume fraction. Fracture toughness characterization techniques and research summery are uniquely presented by considering various factors under one umbrella for better understanding of fracture behavior. Statistical Weibull distribution is also presented for the failure prediction of composites.

     

Tensile properties and fracture toughness of carbon-fiber felt reinforced carbon composites at high temperature

Two kinds of carbon felt reinforced carbon composites, C/C-A and C/C-B, containing respectively pitch carbon fibers and PAN carbon fibers, have been developed to enhance the fracture mechanics properties. The fracture toughness values of these new composites were measured as a function of temperature up to 2400°C. These results are compared with those of a carbon fiber cloth reinforced carbon composite containing rayon fibers (C/C-C) and a fine grain isostatic graphite (IG-11). Major differences among these materials and their distinctive features are discussed.     

Interlaminar fracture toughness of selectively stitched thick carbon fibre reinforced polymer fabric composite laminates

Abstract

Carbon fibre reinforced polymer fabric specimens prepared from selectively stitched thick laminates have been tested under mode I (tension) and mode II (shear) loading, similar to already established tests used for thin unidirectional specimens. The respective interlaminar fracture toughness characteristics were derived for laminates of different stitching configurations. Results indicated significant interlaminar fracture toughness increase for all stitched samples compared with non-stitched samples, especially under mode I loading. It was concluded from parametric investigations that carbon thread stitching is more effective than its aramid counterpart in improving interlaminar fracture toughness. This is attributable to its higher stiffness and better bonding to the carbon fibre reinforced polymer system compared with the aramid thread.     

The influence of various kinds of PA12 interlayer on the interlaminar toughness of carbon fiber‐reinforced epoxy composites

Modifying the impact toughness of carbon fiber‐reinforced epoxy composites by introducing thermoplastic inserts in the interlaminar layer is state‐of‐the‐art. This article compares the introduction of thermoplastics in continuous and discontinuous form. Test plate samples were produced using unidirectional noncrimp carbon fabrics with two different aircraft resin systems: HEXFLOW RTM6 (Hexcel) and Cycom 890 RTM (Cytec). In addition, Polyamide 12 (PA12) was laid in the interlaminar layer in the forms of two different laid scrims, as powder or as nonwoven fabric (NWF). The performance of the resulting combinations was assessed by testing the samples in Mode I and II interlaminar fracture toughness (GIc and GIIc), interlaminar shear strength (ILSS), and compression strength after impact (CAI). The results show that in nearly all the tests a fine‐mesh laid scrim performs similarly to a NWF with twice the weight per surface area. They show furthermore that the curing dynamics of the resin systems together with the melting characteristics of the thermoplastic during processing have an important effect on the performance of the test samples. Hardening of the resin before the PA12 reaches its melting point hinders the compacting of the thermoplastic. This limits the reduction in the original thickness of the insert, leading to an increase in the sample thickness and, thus, reducing the fiber volume content. Otherwise, the discrete arrangement of the laid scrim has positive effects on the material properties of the composite at elevated temperatures, considerably reducing the falloff in ILSS resulting from the temperature‐dependent Young's modulus of PA12. POLYM. COMPOS., 36:1249–1257, 2015. © 2014 Society of Plastics Engineers     

Enhancement of interlaminar fracture toughness of carbon fiber–epoxy composites using polyamide‐6,6 electrospun nanofibers

In this study, carbon fiber–epoxy composites are interleaved with electrospun polyamide‐6,6 (PA 66) nanofibers to improve their Mode‐I fracture toughness. These nanofibers are directly deposited onto carbon fabrics before composite manufacturing via vacuum infusion. Three‐point bending, tensile, compression, interlaminar shear strength, Charpy impact, and double cantilever beam tests are performed on the reference and PA 66 interleaved specimens to evaluate the effects of PA 66 nanofibers on the mechanical properties of composites. To investigate the effect of nanofiber areal weight density (AWD), nanointerlayers with various AWD are prepared by changing the electrospinning duration. It is found that the electrospun PA 66 nanofibers are very effective in improving Mode‐I toughness and impact resistance, compressive strength, flexural modulus, and strength of the composites. However, these nanofibers cause a decrease in the tensile strength of the composites. The glass‐transition temperature of the composites is not affected by the addition of PA 66 nanofibers. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45244.     

Specific influence of polyethersulfone functionalization on the delamination toughness of modified carbon fiber reinforced polymer processed by resin transfer molding

The specific influence of polyethersulfone (PES) end‐functionalization with chlorine or hydroxyl end groups at same molar mass on PES‐epoxy composites based on a high‐performance tetra‐epoxide with di‐amine hardener resin (RTM6) is investigated in terms of morphology, thermal behavior, and toughness. A model study on PES filaments embedded in epoxy precursor is first performed to compare the interdiffusion and resulting morphology upon curing. PES‐OH shows a larger interdiffusion distance compared to PES‐Cl in the model systems and the laminates. This effect is more pronounced at high heating rate. Cross sections and fracture surfaces of composite panels are analyzed by scanning electron microscopy (SEM) coupled with energy dispersive X‐ray (EDX) spectroscopy to establish the link between the microstructures and fracture mechanisms. The toughness of PES‐OH‐modified epoxy composites is doubled compared to unmodified reference panels, whereas the PES‐Cl shows no improvement. The favorable influence of PES‐OH is ascribed to enhanced miscibility, interfacial adhesion and morphology, resulting from the better affinity between hydroxyl‐terminated PES and the epoxy‐resin. POLYM. ENG. SCI., 59:996–1009, 2019. © 2019 Society of Plastics Engineers     

Crack growth and fracture behavior of fabric reinforced polymer composites

The objective of this study is to understand and characterize crack deflection and sub-crack growth in fabric laminate composites. The theory of crack digression based on the Cook-Gordon mechanism of crack blunting and the criterion developed by Kendall were used to study the crack propagation phenomenon. A simple approach has been developed to evaluate the cohesive and adhesive fracture energies, which play a vital role in the study of strength and toughness of the fabric laminate composites. The effects of strain rate and quasi-static crack velocity on these energy values were identified. This study explored the possibility of selftoughening in an otherwise brittle composite system. Two competing mechanisms have been identified that control crack propagation in fabric laminate composites.     

Tearing mode interlaminar fracture toughness of composite materials

The Simplified Split Cantilever Beam (SSCB) is proposed in this work and compared with the Split Cantilever Beam (SCB) to obtain the tearing mode interlaminar fracture toughness. The materials considered are single‐fiber system composites and interply hybrid composites. For interply hybrid composites, three different types of stacking sequence for SSCB specimens, which are [0/0//0],[0/0//0]. and [0/0//0], are tested to compare their suitability. Finite element analysis combined with a modified crack closure integral has been applied to separate the different components of the strain‐energy release rate. In addition, the method of compliance calibration was used to calculate Gc values. The effects of crack growth, initial crack length, specimen width, and number of glass fiber plies were also studied. The results show that SSCB testing has a more dominant Mode III component and more stable Gc values than SCB testing. For SSCB testing, the crack growth and the specimen width for the range considered have no clear effects on the interlaminar fracture toughness, but the initial crack length should be carefully selected to obtain corrected values. The tearing mode interlaminar fracture toughness of interply hybrid composites is higher than that of carbon/epoxy composites, and the three different types of stacking sequence considered are all suitable to approximate the Mode III interlaminar fracture toughness for interply hybrid composites.     

Mode I interlaminar fracture toughness of Nylon 66 nanofibrilmat interleaved carbon/epoxy laminates

Carbon/epoxy laminates interleaved with laboratory scale electrospun Nylon 66 nanofibrilmat and spunbonded nonwoven mats were investigated. The effect of the nanoscale fibers on the fracture toughness of the composite under pure Mode I loading was evaluated. It was shown that the nanofibrilmat is responsible for a major interlaminar fracture toughness improvement, as high as 255–322%, compared to a noninterleaved carbon/epoxy reference laminate. We further studied the improvement mechanism of the electrospun nanofibrilmat compared to a commercial spunbonded nonwoven Nylon 66 mat. A combination of two interlayer fracture mechanisms responsible for the toughness improvement is suggested: the first is related to the high energy dissipated by bridged thermoplastic nanofibers and the second is attributed to the generation of a plastic zone near the crack tip. The interlaminar fracture mechanisms of both electrospun nanofibrilmat and the nonwoven mat interleaving was analyzed and discussed. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Mechanical properties of natural carbon black reinforced polymer composites

This article describes the development of new carbon black material from agricultural waste (wood apple shells) by using pyrolysis method at various carbonization temperatures (400, 600, and 800°C) and used as reinforcement in polymer composites. The wood apple shell carbon black (WAS‐CB) particulates are characterized by proximate analysis, energy dispersive spectroscopy (EDS), and scanning electron microscope (SEM). Results showed that due to increases in carbonization temperature the percentage of carbon improved in the carbon black particles. Furthermore, various tests were performed to determine the effect of new carbon black material on the mechanical properties of composite at different filler loading. The results indicated that mechanical properties like tensile strength, tensile modulus, flexural strength, and flexural modulus are improved as the increase in the carbonization temperature and filler loading. The filler‐matrix bonding was analyzed by SEM. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41211.     

Improving through-thickness conductivity of carbon fiber reinforced polymer using carbon nanotube/polyethylenimine at the interlaminar region

The through-thickness conductivity of carbon fiber reinforced polymer (CFRP) composite was increased by incorporating multiwalled carbon nanotubes in the interlaminar region. Carbon nanotubes (CNTs) were dispersed in a polyethylenimine (PEI) binder, which was then coated onto the carbon fiber fabric. Standard vacuum-assisted resin infusion process was applied to fabricate the composite laminates. This modification technique aims to enhance the electrical conductivity in through-thickness direction for the purpose of nondestructive testing, damage detection, and electromagnetic interference shielding. CNT concentrations ranging from 0 to 0.75 wt% were used and compared to pristine CFRP samples (reference). The through-thickness conductivity of the CFRP exhibited an improvement of up to 781% by adopting this technique. However, the dispersion of CNT in PEI led to a viscosity increase and poor wetting properties which resulted in the formation of voids/defects, poor adhesion (as shown in scanning electron micrographs) and the deterioration of the mechanical properties as manifested by interlaminar shear strength and dynamic mechanical analysis measurements.     

Enhancement of fracture toughness of carbon fiber laminated composites using multi wall carbon nanotubes

In this paper, the effect of Multi Wall Carbon Nanotubes (MWNT) as a toughening agent of laminated composites is experimentally investigated. Carbon fiber laminates were manufactured by resin film infusion technique in which the resin flows in the through-the-thickness direction. The modified polymer systems showed 17% improvement in the stress intensity factor (K_Ic), whereas the laminated composites showed up to 48% improvement in Mode I and 143% improvement in Mode II fracture toughness. Scanning Electron Microscope (SEM) was then used to study the fractured surface and to explain the contrasting behavior of the MWNT-modified polymers when compared to the laminates.     

Sliding mode interlaminar fracture toughness of interply hybrid composite materials

To measure the sliding mode interlaminar fracture toughness of interply hybrid composites, end notched flexure (ENF) specimens with three different types of stacking sequence have been utilized. Finite element analysis is applied to separate the contribution from different modes on the strain energy release rate. In addition, the methods of beam theory, compliance, and compliance calibration to calculate the G_C values are compared. The effects of interface friction, crack length, and specimen width are also discussed. The results show that the crack growth in the three types of specimens is dominated by the sliding mode and the Mode II interlaminar fracture toughness can be approximated. The compliance method is not recommended for hybrid ENF specimens and the effects of interface friction can be neglected. To get rid of the edge effect, the specimen width must be carefully chosen, while the fracture toughness does increase with the initial crack length.     

The effect of silicon carbide whiskers on the Mode I interlaminar fracture of carbon fiber composites

This paper reports on the Mode I interlaminar fracture toughness improvement of carbon fiber-epoxy composites as a result of incorporating SiC whiskers in the epoxy matrix. Five laminates of unidirectional carbon fiber-epoxy composites at different weight fractions of SiC whiskers were manufactured using hand layup vacuum bagging process. Optical and scanning electron microscopic analysis were conducted to give an insight into the fracture morphogoloy, failure mechanisms, and the energy dissipation mechanisms created by the presence of the whiskers in the composite. Experimental results showed that composites containing 5 wt% whiskers exhibited 67% increase in the crack initiation interlaminar fracture toughness G_IC, whereas it exhibited 55% increase in the maximum G_IC compared to pristine composite. The optical and SEM fractographs revealed a strong relation between the microstructure of the fractured surfaces and the energy release rate trend of the composites.     

Interlaminar fracture toughness and fatigue fracture of continuous fiber-reinforced polymer composites with carbon-based nanoreinforcements: a review

ABSTRACT

This review critically examines the recent developments in the use of carbon-based nanofillers as additional reinforcement to enhance the interlaminar properties of FRP composites. The low interlaminar strength of FRP composites results in delamination failure. The various nanoreinforcement strategies and their effect on fracture toughness, interlaminar shear strength (ILSS) and interlaminar fatigue are discussed in detail to prevent this delamination failure. Important findings on various factors that influence the interlaminar properties of multi-scale composites are presented by discussing various intrinsic and extrinsic toughening processes. Moreover, an overview of simulation techniques is provided to predict the delamination onset and propagation.