Cyclic fatigue crack propagation of nanoparticle modified epoxy

An experimental study on the fatigue performance of nanoparticle modified epoxy was conducted. Seven material systems were examined which were: neat epoxy (E), 6 and 12 weight percent (wt.%) silica nanoparticle modified epoxy (S6, S12), 6 and 12 wt.% rubber nanoparticle modified epoxy (R6, R12), 3 wt.% each of silica and rubber nanoparticle modified epoxy (S3R3) and 6 wt.% each of silica and rubber nanoparticle modified epoxy (S6R6). Effects of those nanoparticles on the fatigue threshold (ΔG_th and ΔK_th) and fatigue crack propagation rates (da/dN) were studied. It was found that, compared to neat epoxy (E), nanosilica (S6, S12) increased ΔG_th (and ΔK_th) but nanorubber (R6 and R12) did not. However, a synergistic effect was observed on the fatigue threshold when both silica and rubber nanoparticles were added into epoxy. All these nanoparticles, individually or conjointly, decreased da/dN with silica the most effective. Morphology of the fracture surface was examined to understand the role of nanoparticles on toughening mechanisms under cyclic loading, which depended on the applied ΔG levels.     

Enhanced fatigue behavior of a glass fiber reinforced hybrid particles modified epoxy nanocomposite under WISPERX spectrum load sequence

Two types of glass fiber reinforced plastic (GFRP) composites were fabricated viz., GFRP with neat epoxy matrix (GFRP-neat) and GFRP with hybrid modified epoxy matrix (GFRP-hybrid) containing 9 wt.% of rubber microparticles and 10 wt.% of silica nanoparticles. Fatigue tests were conducted on both the composites under WISPERX load sequence. The fatigue life of the GFRP-hybrid composite was about 4–5 times higher than that of GFRP-neat composite. The underlying mechanisms for improved fatigue performance are discussed. A reasonably good correlation was observed between the experimental fatigue life and the fatigue life predicted under spectrum loads.     

Fatigue crack growth behavior of silica particulate reinforced epoxy resin composite

In the present work, fatigue crack growth tests of epoxy resin composite reinforced with silica particle under various R-ratios were carried out to investigate the effect of R-ratio on crack growth behavior and to discuss fatigue crack growth mechanism. Crack growth curves arranged by ΔK showed clear R-ratio dependence even under no crack closure, where the values of ΔK_th were 0.82 and 0.33 MPa √m for R = 0.1 and 0.7 respectively. However, crack growth curves arranged by K_max merged into almost one curve regardless of R-ratio, which indicated that crack growth behavior of the present composite was time-dependent. The value of K_max,th were in the range from 0.78 to 1.12 MPa √m. In situ crack growth observation revealed the crack growth mechanism: micro-cracking near the interface between silica particle and resin matrix occurs ahead of a main crack and then micro-cracks coalesce with a main crack to grow. The crack path was in the epoxy matrix, which was consistent with the time-dependent crack growth.     

Carbon nanofibers enhance the fracture toughness and fatigue performance of a structural epoxy system

This study investigates the monotonic and dynamic fracture characteristics of a discontinuous fiber reinforced polymer matrix. Specifically, small amounts (0-1 wt.%) of a helical-ribbon carbon nanofiber (CNF) were added to an amine cured epoxy system. The resulting nanocomposites were tested to failure in two modes of testing; Mode I fracture toughness and constant amplitude of stress tension-tension fatigue. Fracture toughness testing revealed that adding 0.5 and 1.0 wt.% CNFs to the epoxy matrix enhanced the resistance to fracture by 66% and 78%, respectively. Fatigue testing at 20 MPa peak stress showed a median increase in fatigue life of 180% and 365% over the control by the addition of 0.5 and 1.0 wt.% CNF, respectively. These results clearly demonstrate the addition of small weight fractions of CNFs to significantly enhance the monotonic fracture behavior and long-term fatigue performance of this polymer. A discussion is presented linking the two behaviors indicating their interdependence and reliance upon the stress intensity factor, K.     

Fatigue tensile behavior of carbon/epoxy composite reinforced with non-crimp 3D orthogonal woven fabric

An experimental study of the in-plane tension-tension fatigue behavior of the carbon fiber/epoxy matrix composite reinforced with non-crimp 3D orthogonal woven fabric is presented. The results include pre-fatigue quasi-static test data, fatigue life diagrams, fatigue damage progression, and post-fatigue quasi-static test data for the warp- and fill-directional loading cases. It is revealed that the maximum cycle stress corresponding to at least 3 million cycles of fatigue life without failure, is in the range of 412-450 MPa for both loading directions. This stress range is well above the static damage initiation threshold and significantly above the first static damage threshold (determined by the onset of low energy acoustic emission). The second static damage threshold, determined by the onset of high energy acoustic emission and related to the appearance of local debonds and intensive transverse matrix cracking falls within this range. The established correlation between a 3000,000 cycle fatigue stress limit on one side and the second static damage threshold stress on the other is of a high practical importance, because it will significantly reduce the amount of future fatigue tests required for this class of composites. Surprisingly, for equal maximum cycle stress level, the fatigue life under fill-directional loading appears about three times shorter than that under warp-directional loading. The 100,000 cycle, 500,000 cycle and 1000,000 cycle fatigue loading with 450 MPa maximum cycle stress has resulted in so high variations of post-fatigue static modulus, strength and ultimate strain, that no consistent and statistically meaningful trends could have been established; further extensive experimental studies are required to reliably quantify this effect.     

Nanostructured systems based on SBS epoxidized triblock copolymers and well-dispersed alumina/epoxy matrix composites

In the present research, the effect of addition of (1 wt.% and 3 wt.%) alumina nanoparticles (Al₂O₃) to epoxy modified by poly(styrene-b-butadiene-b-styrene) (SBS) epoxidized triblock copolymer was studied. The microstructure of final hybrid composites was studied with atomic force microscopy (AFM). Composites showed homogeneously dispersed Al₂O₃ nanoparticles in the epoxy matrix containing polystyrene (PS) microphase separated nanodomains. Dynamic mechanical analyses (DMA), flexural and fracture toughness investigations were carried out. The glass transition temperature of epoxy matrix has been retained unchanged by the addition of Al₂O₃ nanoparticles. The nanostructured epoxy systems based on SBS epoxidized triblock copolymer and well-dispersed Al₂O₃ nanoparticles allowed an increase in fracture toughness maintaining the transparency and stiffness of neat epoxy.     

A strategy for improving mechanical properties of a fiber reinforced epoxy composite using functionalized carbon nanotubes

Carbon fiber reinforced epoxy composite laminates are studied for improvements in quasi static strength and stiffness and tension-tension fatigue cycling at stress-ratio (R-ratio) = +0.1 through strategically incorporating amine functionalized single wall carbon nanotubes (a-SWCNTs) at the fiber/fabric-matrix interfaces over the laminate cross-section. In a comparison to composite laminate material without carbon nanotube reinforcements there are modest improvements in the mechanical properties of strength and stiffness; but, a potentially significant increase is demonstrated for the long-term fatigue life of these functionalized nanotube reinforced composite materials. These results are compared with previous research on the cyclic life of this carbon fiber epoxy composite laminate system reinforced similarly with side wall fluorine functionalized industrial grade carbon nanotubes. Optical and scanning electron microscopy and Raman spectrometry are used to confirm the effectiveness of this strategy for the improvements in strength, stiffness and fatigue life of composite laminate materials using functionalized carbon nanotubes.     

3D damage characterisation and the role of voids in the fatigue of wind turbine blade materials

The fatigue mechanisms of Glass Fibre Reinforced Polymer (GFRP) used in wind turbine blades were examined using computed tomography (CT). Prior to mechanical testing, as-manufactured [+45/−45/0]_3,s glass/epoxy specimens were CT scanned to provide 3-dimensional images of their internal microstructure, including voids. Voids were segmented and extracted, and individual characteristics and volumetric distributions were quantified. The coupons were then fatigue tested in uniaxial loading at R = −1% to 40% of the nominal tensile failure stress. Some tests were conducted to failure for correlation with the initial void analysis and to establish failure modes. Other tests were stopped at various life fractions and examined using CT to identify key damage mechanisms. These scans revealed transverse matrix cracking in the surface layer, occurring predominantly at free edges. These free-edge cracks then appeared to facilitate edge delamination at the 45/−45° interface. Propagation from sub-critical, surface ply damage to critical, inner ply damage was identified with either a −45/0° delamination, or a 0° fibre tow failure allowing a crack to propagate into the specimen bulk. Final failure occurred in compression and was characterised by total delamination between all the 45/−45° plies. A quantitative void analysis, taken from the pre-test CT scans, was also performed and compared against the specimens’ fatigue lives. This analysis, to the authors’ knowledge the first of its kind, measured and plotted approximately 10,000 voids within the gauge length of each specimen. The global void measurement parameters and distributions showed no correlation with fatigue life. A local ply-level investigation revealed a significant correlation between the largest void and fatigue life in the region of the laminate associated with the crack propagation from sub-critical to critical damage.     

The influence of subzero temperatures on fatigue behavior of composite sandwich structures

The fatigue lives and failure modes of foam core carbon/epoxy and glass/epoxy composite sandwich beams in 4-point bending were characterized from room temperature (22 °C) down to −60 °C. Similar previous investigations had focused on elevated temperatures only, but the low temperature fatigue behavior must be understood so that these materials may be evaluated for possible use in the hull structures of ships, which operate in cold regions. Core shear was found to be the dominant fatigue failure mode for the test specimens over the entire temperature range from 22 °C down to −60 °C. Significant increases in the useful fatigue life with brittle type core shear failure were observed at low temperatures by comparison with the corresponding room temperature behavior. Fatigue failure at the low temperatures was catastrophic and without any significant early warning, but the corresponding failures at room temperature were preceded by relatively slow but steadily increasing losses of stiffness. Two different approaches were used to investigate stiffness reductions during fatigue tests, and both approaches led to the same conclusions. Experimental observations regarding the location of fatigue crack initiation were confirmed by static finite element analyses for both materials.     

Fatigue behaviour of glass fibre reinforced epoxy composites enhanced with nanoparticles

Nanoparticle reinforcement of the matrix in laminates has been recently explored to improve mechanical properties, particularly the interlaminar strength. This study analyses the fatigue behaviour of nanoclay and multiwalled carbon nanotubes enhanced glass/epoxy laminates. The matrix used was the epoxy resin Biresin® CR120, combined with the hardener CH120-3. Multiwalled carbon nanotubes (MWCNTs) 98% and organo-montmorillonite Nanomer I30 E nanoclay were used. Composites plates were manufactured by moulding in vacuum. Fatigue tests were performed under constant amplitude, both under tension–tension and three points bending loadings. The fatigue results show that composites with small amounts of nanoparticles addition into the matrix have bending fatigue strength similar to the obtained for the neat glass fibre reinforced epoxy matrix composite. On the contrary, for higher percentages of nanoclays or carbon nanotubes addition the fatigue strength tend to decrease caused by poor nanoparticles dispersion and formation of agglomerates. Tensile fatigue strength is only marginally affected by the addition of small amount of particles. The fatigue ratio in tension–tension loading increases with the addition of nanoclays and multi-walled carbon nanotubes, suggesting that both nanoparticles can act as barriers to fatigue crack propagation.     

Size reduction of WO3 nanoparticles by ultrasound irradiation and its applications in structural nanocomposites

The porous WO₃ (pore size 2–5 nm) nanoparticles were synthesized using a high intensity ultrasound irradiation of commercially available WO₃ nanoparticles (80 nm) in ethanol. The high resolution transmission electron microscopic (HRTEM) and X-ray studies indicated that the 2–5 nm uniform pores have been created in commercially available WO₃ nanoparticles without much changing the initial WO₃ nanoparticles (80 nm) sizes. The nanocomposites of WO₃/SC-15 epoxy were prepared by infusion of 1 wt.%, 2 wt.% and 3 wt.% of porous WO₃ nanoparticles into SC-15 epoxy resin by using a non-contact (Thinky) mixing technique. Finally the neat epoxy and nanocomposites were cured at room temperature for about 24 h in a plastic rectangular mold. The cured epoxy samples were removed and precisely cut into required dimensions and tested for their thermal and mechanical properties. The HRTEM and SEM studies indicated that the sonochemically modified porous WO₃ nanoparticles dispersed more uniformly over the entire volume of the epoxy (without any settlement or agglomeration) as compared to the unmodified WO₃/epoxy nanocomposites.     

CF/EP composite laminates with carbon black and copper chloride for improved electrical conductivity and interlaminar fracture toughness

An experimental study was conducted to improve the electrical conductivity of continuous carbon fibre/epoxy (CF/EP) composite laminate, with simultaneous improvement in mechanical performance, by incorporating nano-scale carbon black (CB) particles and copper chloride (CC) electrolyte into the epoxy matrix. CF/EP laminates of 65 vol.% of carbon fibres were manufactured using a vacuum-assisted resin infusion (VARI) technique. The effects of CB and the synergy of CB/CC on electrical resistivity, tensile strength and elastic modulus and fracture toughness (K_IC) of the epoxy matrix were experimentally characterised, as well as the transverse tensile modulus and strength, Mode I and Mode II interlaminar fracture toughness of the CF/EP laminates. The results showed that the addition of up to 3.0 wt.% CB in the epoxy matrix, with the assistance of CC, noticeably improved the electrical conductivity of the epoxy and the CF/EP laminates, with mechanical performance also enhanced to a certain extent.     

Crashworthiness characteristics investigation of silk/epoxy composite square tubes

This research concentrates on the evaluation of crashworthiness characteristics of natural silk/epoxy composite square tubes energy-absorbers. Composite laminate specimens were subjected to static axial compression load and experimental evaluation of the energy absorption capability of silk/epoxy composite. Specimens were in the form of square cross-sections with the dimension of 80 mm × 80 mm and a radius curvature of 5 mm. The variables in the experiment were the length of the tubes built 50 mm, 80 mm and 120 mm. Meanwhile, the thickness of the walls, consisting of laminates of silk/epoxy of 12, 24 and 30 plies, correspond to equivalent wall thickness of 1.7 mm, 3.4 mm and 4.2 mm, respectively. The parameters measured were the total absorbed energy (E_total), and the crash force efficiency (CFE). E_total is the measure of the amount of energy that the structure can withstand without failure and thus is a measure of its strength, while CFE gives a quantitative indication of the mode of failure of the composites. The mode of failure was observed using photography.     

Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress-strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10⁵ cycles at the frequency of 0.1 Hz and as 2 × 10⁵ cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Interfacial load transfer in carbon nanotube/ceramic microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance the composite delamination resistance, and possibly improve its toughness and any matrix-dominated elastic property as well. In the present work aligned CNTs were grown upon ceramic fibers (silica and alumina) by chemical vapor deposition (CVD) at temperatures of 650 °C and 750 °C. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on pristine as well as on CNT-grown fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Significant increases (up to 50%) are observed in the fiber tensile strength and in the interfacial adhesion (which was sometimes doubled) with all fiber types upon which CNTs are CVD-grown at 750 °C. We discuss the likely sources of these improvements as well as their implications.     

Probing deformation of double-walled carbon nanotube (DWNT)/epoxy composites using FTIR and Raman techniques

Two of the limitations of carbon nanotube (CNT) polymer composites have been the low volume fraction of nanotubes and inadequate load transfer from the polymer to the stiff CNT. Here, we have utilized functionalized mats of double-walled nanotubes (DWNT) to obtain 10 wt.% DWNT in an epoxy matrix, with strength approaching those of quasi-isotropic carbon fiber composites. We used the transmission FTIR technique with in situ loaded specimens to monitor spectral shift per unit applied stress for understanding load transfer behavior at the nanotube–epoxy interface. Tests show that in most cases a tensile stress causes negative FTIR peak shift in the neat epoxy, but this behavior is not always observed for the epoxy matrix in the composite. The FTIR data can be used successfully to estimate the average matrix stress in the composite and thereby the average stress in the nanotubes. In situ Raman studies using the G′ peak are also conducted to obtain complementary information on average tensile stress in the DWNT in the loading direction. The shift response is found to be ∼37 cm⁻¹/GPa.     

Mechanical properties of epoxy composites with high contents of nanodiamond

Mechanical properties and thermal conductivity of composites made of nanodiamond with epoxy polymer binder have been studied in a wide range of nanodiamond concentrations (0-25 vol.%). In contrast to composites with a low content of nanodiamond, where only small to moderate improvements in mechanical properties were reported before, the composites with 25 vol.% nanodiamond showed an unprecedented increase in Young’s modulus (up to 470%) and hardness (up to 300%) as compared to neat epoxy. A significant increase in scratch resistance and thermal conductivity of the composites were observed as well. The improved thermal conductivity of the composites with high contents of nanodiamond is explained by direct contacts between single diamond nanoparticles forming an interconnected network held together by a polymer binder.     

Fatigue life evaluation for carbon/epoxy laminate composites under constant and variable block loading

This paper presents the results of current research on the fatigue life prediction of carbon/epoxy laminate composites involving twelve balanced woven bidirectional layers of carbon fibres and epoxy resin manufactured by a vacuum moulding method. The plates were produced with 3 mm thickness and 0.66 fibre weight fraction. The dog bone shape specimens were cut from these plates with the load line aligned with one of the fibre directions. The fatigue tests were performed using load control with a frequency of 10 Hz and at room temperature. The fatigue behaviour was studied for different stress ratios and for variable amplitude block loadings. The damage process was monitored in terms of the stiffness loss. The fatigue life of specimens submitted to block loading tests was modelled using Palmgren–Miner’s law and taking in to account the stress ratio effect. The estimated and experimental fatigue lives were compared and good agreement was observed.     

Cryogenic delamination growth in woven glass/epoxy composite laminates under mixed-mode I/II fatigue loading

This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures.     

Mechanisms and impact of fiber–matrix compatibilization techniques on the material characterization of PHBV/oak wood flour engineered biobased composites

Fully biobased composite materials were fabricated using a natural, lignocellulosic filler, namely oak wood flour (OWF), as particle reinforcement in a biosynthesized microbial polyester matrix derived from poly(β-hydroxybutyrate)-co-poly(β-hydroxyvalerate) (PHBV) via an extrusion injection molding process. The mechanisms and effects of processing, filler volume percent (vol%), a silane coupling agent, and a maleic anhydride (MA) grafting technique on polymer and composite morphologies and tensile mechanical properties were investigated and substantiated through calorimetry testing, scanning electron microscopy, and micromechanical modeling of initial composite stiffness. The addition of 46 vol% silane-treated OWF improved the tensile modulus of neat PHBV by 165%. Similarly, the tensile modulus of MA-grafted PHBV increased 170% over that of neat PHBV with a 28 vol% addition of untreated OWF. Incorporation of OWF reduced the overall degree of crystallinity of the matrix phase and induced embrittlement in the composites, which led to reductions in ultimate tensile stress and strain for both treated and untreated specimens. Deviations from the Halpin–Tsai/Tsai–Pagano micromechanical model for composite stiffness in the silane and MA compatibilized specimens are attributed to the inability of the model both to incorporate improved dispersion and wettability due to fiber–matrix modifications and to account for changes in neat PHBV and MA-grafted PHBV polymer morphology induced by the OWF.