Temperature and loading rate effects in the mode II interlaminar fracture behavior of carbon fiber reinforced PEEK

The combined effect of varying loading rate and test temperature on the mode II interlaminar fracture properties of AS4/carbon fiber reinforced PEEK has been investigated. End notch flexure tests have shown that this thermoplastic‐based composite system offers a very high value of interlaminar fracture toughness at room temperature. Increasing the test temperature leads to a reduction in the mode II interlaminar fracture toughness of the composite, with the value at 150°C being approximately one half of the room temperature value. In contrast, increasing the crosshead displacement rate has been shown to increase the value of G_IIc by up to 25%. A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry. Here, extensive plastic flow within the crack tip region was observed in all specimens. It is believed that the rate sensitivity of G_IIc reflects the rate‐dependent characteristics of the thermoplastic resin.     

Temperature and Loading Rate Effects on the Fracture Behaviour of Adhesively Bonded GFRP Nylon-6,6

The combined effect of varying test temperature and loading rate on the Mode II fracture toughness of plasma-treated GFRP Nylon-6,6 composites bonded using a silica-reinforced epoxy adhesive has been studied. End notch flexure tests have shown that the adhesive system used in this study offers a wide range of fracture energies that are extremely sensitive to changes in temperature and loading rate. Increasing the test temperature resulted in a substantial reduction in the Mode II fracture toughness of the adhesive, with the value of G_IIc at 60°C being approximately one-half of the room temperature value. In contrast, increasing the crosshead displacement rate at a given temperature has been shown to increase the value of G_IIc by up to 250%. Compression tests performed on bulk adhesive specimens revealed similar trends in the value of [sgrave]_y with temperature and loading rate. In addition, it was found that the plasma treatment employed in this study resulted in stable crack propagation through the adhesive layer under all testing conditions.

A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip was achieved using the double end notch flexure (DENF) geometry, which was considered in tandem with the fracture surface morphologies. Here, changes in the degree of matrix shear yielding and particle-matrix debonding were used to explain the trends in [sgrave]_y and G_IIc.     

Mode I and Mode II delamination resistance and mechanical properties of woven glass/epoxy–PU IPN composites

The effect of polyurethane on the mechanical properties and Mode I and Mode II interlaminar fracture toughness of glass/epoxy composites were studied. Polyurethanes (PU) synthesized using polyols and toluene diisocyanate were employed as modifier for epoxy resin by forming interpenetrating polymer network. The PU/Epoxy IPN was used as matrix material for GFRP. PU modified epoxy composite laminates having varying PU contents were prepared. The effect of PU content on the mechanical properties like interlaminar fracture toughness (Mode I, G_1c and Mode II, G_IIc), tensile strength, flexural strength, and Izod impact strength were studied. The morphological studies were conducted on the fractured surface of the composite specimen by scanning electron microscopy (SEM). Tensile strength, flexural strength, and impact strength of PU‐modified epoxy composite laminates were found to increase inline with interlaminar fracture toughness (G_1c and G_IIc) with increasing PU content to a certain limit and then it was found to decrease with increase in PU content. It was observed that toughening of epoxy with PU increases the Mode I and Mode II delamination toughness up to 17 and 120% higher than that of untoughened composite specimen, respectively. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

The influence of cooling rate on the fracture properties of a glass reiforced/nylon fiber-metal laminate

The effect of varying cooling rate on the microstructure and resulting mechanical properties of a novel fiber-metal laminate (FML) based on a glass fiber-reinforced nylon composite has been investigated. Polished thin sections removed from plain glass fiber/nylon composites and their corresponding fiber-metal laminates indicated that the prevailing microstructure was strongly dependent on the rate of cooling from the melt. Mode I and Mode II interlaminar fracture tests on the plain glass fiber reinforced nylon laminates indicated that the values of G_Ic and G_IIc averaged approximately 1100 J/m² and 3700 J/m² respectively at all cooling rates. The degree of adhesion between the aluminum alloy and composite substrates was investigated using the single cantilever beam geometry. Here, the measured values of G_c were similar in magnitude to the Mode I interlaminar fracture energy of the composite, tending to increase slightly with increasing cooling rate. The tensile and flexural fracture properties of the plain composites and the fiber metal laminates were found to increase by between 10% and 20% as the cooling rate was increased by two orders of magnitude. This effect was attributed to over-aging of the aluminum alloy plies at elevated temperature during cooling. Finally, fiber metal laminates based on glass fiber/nylon composites were shown to exhibit an excellent resistance to low velocity impact loading. Damage, in the form of delamination, fiber fracture, matrix cracking in the composite plies, and plastic deformation and fracture in the aluminum layer, was observed under localized impact loading. Here, the fast-cooled fiber metal laminates offered superior post-impact mechanical properties at low and intermediate impact energies, yet very similar results under high impact energies.     

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.     

Mode II interlaminar delamination of composite laminates incorporating with polymer ultrathin fibers

This study makes use of electrospinning to produce epoxy polymer ultrathin fibers. Those fibers were collected as nonwoven sheets with various thicknesses. The collected ultrathin fibrous sheets (UFS) were incorporated into ply interfaces of a glass/epoxy composite laminate. End‐notch flexure (ENF) specimens were employed to measure the mode II interlaminar fracture toughness (G_IIC) of the resulting laminate. Scanning electron microscopy (SEM) was carried out to investigate the fracture surface and fracture mechanism of tested samples. Results indicate that there were some reinforcing effects of the UFS on the G_IIC when the thickness of the sheets was not more than 0.13 mm for a single sheet and 0.06 mm for multi‐sheets incorporated. The mechanical properties were quickly decreased and became poorer when a higher thickness of the sheets was used. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Load‐deflection behavior of fracture toughness testing of ceramics by SEVNB method

This paper aims to explore the load‐deflection behavior of fracture toughness testing of ceramics by single‐edge V‐notched beam (SEVNB) method. The fracture toughness of Si₃N₄, 3Y‐TZP, SiC, and 8Y‐FSZ ceramics were measured by SEVNB method and single‐edge notched beam (SENB) method, respectively. The load‐deflection behavior varies with R‐curve behavior of the ceramics, the test methods and the loading conditions. Through comparative analysis, the results show that the actual fracture toughness of ceramics by SEVNB method can be determined by maximum flexure load and the notch length at loading rate of 0.05 mm/min in air. The obtained actual fracture toughness values of Si₃N₄, 3Y‐TZP, SiC, and 8Y‐FSZ ceramics are 5.2 ± 0.21, 4.5 ± 0.12, 3.2 ± 0.15, and 1.6 ± 0.07 MPa ^. m^1/2, respectively.     

Effect of deposited carbon nanotubes on interlaminar properties of carbon fiber‐reinforced epoxy composites using a developed spraying processing

Carbon fiber‐reinforced epoxy composites, with incorporated carboxylic multiwall carbon nanotubes (CNTs), were prepared using vacuum‐assisted resin infusion (VARI) molding, and the in‐plane and out‐of‐plane properties, including mode‐I (G_Ic) and mode‐II (G_IIc) interlaminar fracture toughness, interlaminar shear strength (ILSS), tensile, and flexural properties were measured. A novel spraying technique, which sprays a kind of epoxy resin E20 with high viscosity after spraying the CNTs, was adopted to deposit the CNTs on the surface of carbon fiber fabric. The E20 was used to anchor CNTs on the fabric surface, avoiding that the deposited CNTs were removed by the infusing resin during VARI process. The spraying processing, including spraying amount and spraying sequence, was optimized based on the distribution of CNTs on the fibers. After that, three composite specimen groups were fabricated using different carbon fiber fabrics, including as‐received, CNT‐deposited with E20, and CNT‐deposited without E20. The effects of CNTs on the processing quality and mechanical properties of carbon fiber‐reinforced polymer composites were studied. The experimental results show that all studied laminates have uniform thickness with designed values and no obvious defects form inside the laminates. Compared with the composite without CNTs, depositing CNTs with E20 increases by 24% in the average propagation G_Ic, by 11% in the propagation G_IIc and by 12% in the ILSS, while it preserves the in‐plane mechanical properties, However, depositing CNTs without E20 reduces interlaminar fracture toughness. These phenomena are attributed to the differences in the distribution of CNTs and the fiber/matrix interfacial bonding for different spraying processing. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers     

Synthesis and characterization of fluorinated poly(etherimide)s toughening for carbon fiber‐reinforced epoxy composites

Novel‐fluorinated poly(etherimide)s (FPEIs) with controlled molecular weights were synthesized and characterized, which were used to toughen epoxy resins (EP/FPEI) and carbon fiber‐reinforced epoxy composites (CF/EP/FPEI). Experimental results indicated that the FPEIs possessed outstanding solubility, thermal, and mechanical properties. The thermally cured EP/FPEI resin showed obviously improved toughness with impact strength of 21.1 kJ/m² and elongation at break of 4.6%, respectively. The EP/FPEI resin also showed outstanding mechanical strength with tensile strength of 91.5 MPa and flexural strength of 141.5 MPa, respectively. The mechanical moduli and thermal property of epoxy resins were not affected by blending with FPEIs. Furthermore, CF/EP/FPEI composite exhibited significantly improved toughness with Mode I interlaminar fracture toughness (G_IC) of 899.4 J/m² and Mode II interlaminar fracture toughness (G_IIC) of 1017.8 J/m², respectively. Flexural properties and interlaminar shear strength of the composite were slightly increased after toughening. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Effect of physical aging on the toughness of carbon fiber-reinforced poly(ether ether ketone) and poly(phenylene sulfide) composites. I

The effect of physical aging on the penetration impact toughness and Mode I interlaminar fracture toughness of continuous carbon fiber (C.F.) reinforced poly(ether ether ketone) (PEEK) and poly(phenylene sulfide) (PPS) composites has been investigated by using an instrumented falling weight impact (IFWI) technique and a double cantilever beam (DCB) test. Composite materials studied are aged below their glass transition temperature (T_g) at various periods. Initiation force and energy of damage, failure propagation energy, impact energy and ductility index (D.I.) are reported. The Mode I critical value of strain energy release rate (G_IC) of the unidirectional carbon fiber-reinforced PEEK (APC-2) composites is obtained. Results show that aging has a significant effect on the toughness of both composite materials. Energy absorbed during impact decreases with the increase of aging temperature and period. The PEEK/C.F. composites exhibit a higher retention of impact toughness than that of the PPS/C.F. composites after aging; however, the PPS/C.F. composites show a much higher ductility index. The Mode I fracture mechanism of the APC-2 composite is a combination of stable and unstable failure and shows a “stick-slip” behavior. Owing to the formation of a relative rigid structure, the fracture toughness (G_IC) of APC-2 decreased with the increase of aging temperature and period.     

Mechanical properties of zirconia-based ceramics asfunctions of temperature

Commercially available ceramics, MgO–ZrO₂, CeO₂–ZrO₂, and an in-house fabricated zirconia-toughened mullite were examined in this study for use as a structural component in diesel engines. The fast fracture strengths of these materials were measured by loading ASTM C-1161-B specimens in four-point flexure at 30 MPa/s and at 20, 200, 400, 600, and 850 °C. The dynamic fatigue or slow crack growth susceptibility was assessed at 20 and 850 °C by combining the fast fracture strengths with strength data obtained by testing the same specimens in four-point flexure at 0.30 and 0.003 MPa/s stressing rates, as specified in the ASTM C 1368 standard. Fracture toughness was measured following the ASTM C-1421 standard and using chevron notch specimens in three-point flexure at room and elevated temperatures. The strength of the zirconia-toughened mullite was invariant to increases in the temperature and decreases in the loading rate, while the MgO–ZrO₂ and CeO₂–ZrO₂ materials exhibited strength degradation as temperatures increased and the loading rates decreased. Temperature was observed to have the greatest influence on facture toughness. As temperatures increased, the fracture toughness values dramatically decreased for all the materials examined in this study. Improvements in the fracture toughness are needed most for these ceramic materials in order to meet the structural requirements and to develop a more durable and reliable diesel engine component.     

Evaluation of interlaminar-toughened poly(etherlmide)-modified epoxy/carbon fiber composites

Amorphous poly(ether imide) has been used as interlaminar toughening particulate agent in laminated carbon fiber/epoxy composites. Mode I and Mode II delamination fracture toughness was characterized using the double cantilever beam (DCB) and end-notched flexure (ENF) specimens. The delamination surface was examined using a scanning electron microscopy (SEM) to investigate relationships between the morphology and properties. The results revealed that the PEI-modified composites exhibited a significantly increased fracture toughness, which increased with the PEI content. G_IC was improved from 165 to 540 J/m² (at 1 mm/min crosshead speed). G_IIC was improved more significantly from 290 to 1300 J/m². It is believed that these values could be further improved if the processing cycle were to be optimized.     

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.     

Time–temperature dependency of mode II fracture toughness for bisphenol A type epoxy resin

We investigated the mode II fracture toughness (K_IIc) with time and temperature dependence of the bisphenol A type of epoxy resin. We performed an asymmetric four‐point bending test under various conditions of temperature and displacement rate. We found that K_IIc strongly depended on the displacement rate and the temperature, even at room temperature. Moreover, it was governed by the time–temperature equivalence principle in regard to the fracture time. The time–temperature dependency of K_IIc was similar to that of the loss modulus (E″), and the transition of brittle to ductile fractures occurred nearly simultaneously when E″ peaked. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 51–55, 2005     

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     

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 effects of additives on curing properties,resin contents,and mechanical properties of graphite/epoxy composites

The effects of additives such as boron trifluride-monoethylene amine (BF₃MEA) and fumed silica in the TGDDM/DDS epoxy formulations on the curing properties, resin contents, and mechanical properties of their graphite/epoxy (Gr/Ep) composites were investigated. The addition of BF₃MEA increased the viscosity of resin as well as the resin contents of cured laminates because of its catalytic effect. Although the fumed silica was considered a thickening agent, it also acted like a co-catalyst with BF₃MEA. As the resin content of cured laminates was increased, the excess resin was likely to accumulate in the interlaminar region, which increased the interlaminar shear strength but decreased the flexure strength as well as the interlaminar fracture toughness value, G_IC.     

Mechanical Behavior and Fracture Toughness of Poly(L‐lactic acid)‐Natural Fiber Composites Modified with Hyperbranched Polymers

Summary: The use of hyperbranched polymers (HBP) with hydroxy functionality as modifiers for poly(L ‐lactic acid) (PLLA)‐flax fiber composites is presented. HBP concentrations were varied from 0 to 50% v/v and the static and dynamic tensile properties were investigated along with interlaminar fracture toughness. Upon addition of HBP, the tensile modulus and dynamic storage modulus (E′) both diminished, although a greater decline was noticed in the static modulus. The elongation of the composites with HBP showed a pronounced increase as large as 314% at 50% v/v HBP. The loss factor (tan δ) indicated a lowering of the glass transition temperature (T_g) due to a change in crystal morphology from large, mixed perfection spherulites to finer, smaller spherulites. The change in T_g could have also resulted from some of the HBP being miscible in the amorphous phase, which caused a plasticizing effect of the PLLA. The interlaminar fracture toughness measured as the critical strain energy release rate (G_IC) was significantly influenced by HBP. At 10% v/v HBP, G_IC was at least double that of the unmodified composite and a rise as great as 250% was achieved with 50% v/v. The main factor contributing to high fracture toughness in this study was better wetting of the fibers by the matrix when the HBP was present. With improved ductility of the matrix, it caused ductile tearing along the fiber‐matrix interface during crack propagation.

ESEM photograph of propagation region of the interlaminar fracture toughness specimens with 30% v/v of HBP.     

Fracture of adhesive joints and laminated composites

The mechanisms of resin controlled failure in adhesive joints and composites (delamination and transverse cracking) are examined. An in-situ failure model based on the fracture mechanics principles is applied here to describe the failure processes involved. The model centers on the crack tip plastic zone developed in the thin resin layer between the fibers or the adherends. The plastic zone in the resin layer is heavily influenced by a dominant slow varying stress distribution, approximated to be r^?m/2 dependent with m ? 1 (r is the distance from the crack tip). The adhesive or composite fracture toughness G*_IC can then be expressed as a function of several resin properties of comparable importance: modulus E, yield stress σ_y, resin G_IC and residual stress. The relative significance of the resin properties on the adhesive or composite fracture is discussed. The effects of temperature, loading rate, and resin toughening on such failure as a result of the corresponding variations in resin properties are also addressed.     

The fracture toughness of natural fibre- and glass fibre-reinforced SMC

The fracture toughness properties, in terms of stress intensity factor K_Ic and strain energy release rate G_Ic, of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO₃ filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30?vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20?vol.-%, with K_Ic values of around 6?MPa.m^?1/2.