Transverse tension fatigue life characterization through flexure testing of composite materials

The transverse tension fatigue life of S2/8552 glass–epoxy and IM7/8552 carbon–epoxy was characterized using flexure tests of 90-degree laminates loaded in 3-point and 4-point bending. The influence of specimen polishing and specimen configuration on transverse tension fatigue life was examined using the glass–epoxy laminates. Results showed that 90-degree flexure specimens with polished machined edges and polished tension-side surfaces had lower fatigue lives than unpolished specimens when cyclically loaded at equal stress levels. The influence of specimen thickness and the utility of a Weibull scaling law were examined using the carbon–epoxy laminates. The influence of test frequency on fatigue results was also documented for the 4-point bending configuration. A Weibull scaling law was used to predict the 4-point bending fatigue lives from the 3-point bending curve fit and vice versa. Scaling was performed based on maximum cyclic stress level as well as fatigue life. The scaling laws based on stress level shifted the curve fit S–N characterizations in the desired direction, however, the magnitude of the shift was not adequate to accurately predict the fatigue lives. Furthermore, the scaling law based on fatigue life shifted the curve fit S–N characterizations in the opposite direction from measured values. Therefore, these scaling laws were not adequate for obtaining accurate predictions of the transverse tension fatigue lives of heterogeneous, fiber reinforced, polymer matrix composites.     

Resin infusion analysis of nanoclay filled glass fiber laminates

This paper focuses on the resin flow characteristics of nanoclay filled glass fiber laminates processed by Vacuum Assisted Resin Infusion Molding (VARIM). Laminates with varying quantities of nanoclays (0–5 wt.%) were prepared and the effect of these nanoclays on the epoxy resin flow characteristics was studied. It was found that the flow rate of resin continuously decreased as nanoclay content continuously increased. The reduction in the flow rate was attributed to the rate of change of curing and the subsequent change in viscosity of the nanoclay filled resin. Analysis of infusion process by Darcy’s law show that the permeability of the fiber decreased in the nanoclay filled resin system. Nanoclay filled laminates show improved static and dynamic mechanical properties than that of unfilled resin composites.     

Improved mechanical properties of an epoxy glass–fiber composite reinforced with surface organomodified nanoclays

An organomodified surface nanoclay reinforced epoxy glass-fiber composite is evaluated for properties of mechanical strength, stiffness, ductility and fatigue life, and compared with the pristine or epoxy glass-fiber composite material not reinforced with nanoclays. The results from monotonic tensile tests of the nanoclay reinforced composite material at 60 °C in air showed an average 11.7% improvement in the ultimate tensile strength, 10.6% improvement in tensile modulus, and 10.5% improvement in tensile ductility vs. these mechanical properties obtained for the pristine material. From tension–tension fatigue tests at a stress-ratio = +0.9 and at 60 °C in air, the nanoclay reinforced composite had a 7.9% greater fatigue strength and a fatigue life over a decade longer or 1000% greater than the pristine composite when extrapolated to 10⁹ cycles or a simulated 10-year cyclic life. Electron microscopy and Raman spectroscopy of the fracture and failure modes of the test specimens were used to support the results and conclusions. This nanocomposite could be used as a new and improved material for repair or rehabilitation of external surface wall corrosion or physical damage on piping and vessels found in petrochemical process plants and facilities to extend their operational life.     

The tensile fatigue behaviour of a silica nanoparticle-modified glass fibre reinforced epoxy composite

An anhydride-cured thermosetting epoxy polymer was modified by incorporating 10 wt.% of well-dispersed silica nanoparticles. The stress-controlled tensile fatigue behaviour at a stress ratio of R = 0.1 was investigated for bulk specimens of the neat and the nanoparticle-modified epoxy. The addition of the silica nanoparticles increased the fatigue life by about three to four times. The neat and the nanoparticle-modified epoxy resins were used to fabricate glass fibre reinforced plastic (GFRP) composite laminates by resin infusion under flexible tooling (RIFT) technique. Tensile fatigue tests were performed on these composites, during which the matrix cracking and stiffness degradation was monitored. The fatigue life of the GFRP composite was increased by about three to four times due to the silica nanoparticles. Suppressed matrix cracking and reduced crack propagation rate in the nanoparticle-modified matrix were observed to contribute towards the enhanced fatigue life of the GFRP composite employing silica nanoparticle-modified epoxy matrix.     

Improvement of fatigue life by incorporation of nanoparticles in glass fibre reinforced epoxy

The fatigue properties of glass fibre reinforced epoxy laminates modified with small amounts (0.3 wt.%) of nanoparticles (fumed silica SiO₂ and multi-wall carbon nanotubes (MWCNT)) were evaluated by means of static (90°-tensile and stepped tensile) and dynamic fatigue tests. For the MWCNT-modified matrix, the electrical conductivity was measured in situ. The addition of nanoparticles lead to increases in inter fibre fracture strength of up to 16%. More significantly, the high cycle fatigue life is increased by several orders of magnitude in number of load cycles. The increased inter fibre fracture strength could be correlated to the improved fatigue behaviour, as final failure in high cycle fatigue is strongly correlated to matrix cracks. For the MWCNT-modified composites, the state of load and damage state was monitored by conductivity measurements. A correlation between the onset of matrix cracking and increase in electrical resistivity could be drawn enabling self sensing capabilities.     

Electron irradiation effects on interlaminar shear strength of glass or carbon cloth reinforced epoxy composites

Interlaminar tensile shear tests are conducted to study the degradation mechanisms of electron irradiated glass or carbon cloth reinforced epoxy laminates. Interlaminar shear strength decreases significantly after the dose exceeds 3000 Mrad for glass/epoxy, but remains constant up to 12 000 Mrad for carbon/epoxy. SEM photos reveal that debonding of glass fibres and epoxy matrix (or degradation of silane coupling agents) plays an important role in the dose-dependent strength reduction of glass/epoxy laminates. The decrease in the interlaminar shear strength corresponds to that in the three-point bending strength. On the other hand, the SEM fracture appearance is almost dose-independent for carbon/epoxy laminates. In addition, some preliminary irradiation tests are conducted at –120° C to observe the effects of irradiation temperatures.     

Impact and after-impact properties of carbon fibre reinforced composites enhanced with multi-wall carbon nanotubes

The goal of the present study was to investigate the influence of multi-wall carbon nanotubes (MWCNTs) on the impact and after impact behaviour of carbon fiber reinforced polymer (CFRP) laminates. About 0.5% per weight MWCNTs were dispersed via a high shear device in the epoxy matrix (Bisphenol A) of carbon reinforced quasi-isotropic laminates. Subsequently, the modified CFRPs were subjected to low-energy impact and directly compared with unmodified laminates. In previous studies, the beneficial effect of the MWCNT inclusion to the fracture properties of CFRPs has been demonstrated. In terms of the CFRP impact performance, enhanced performance for the CNT doped specimens was observed for higher energy levels. However, the after-impact properties and more specifically compression after impact were improved for both the effective compression modulus and the compression strength. In addition, compression–compression fatigue after impact performance of the CNT modified laminates was also improved, by extending the fatigue life.     

Pristine and amino functionalized carbon nanotubes reinforced glass fiber epoxy composites

Multi-phase composites have been studied by incorporating carbon nanotubes (CNTs) as a secondary reinforcement in an epoxy matrix which was then reinforced with glass fiber mat. Different types of CNTs e.g. amino functionalized carbon nanotubes (ACNT) and pristine carbon nanotubes (PCNT) were homogeneously dispersed in the epoxy matrix and two-ply laminates were fabricated using vacuum-assisted resin infusion molding technique. The issues related to CNT dispersion and interfacial bonding and its affect on the mechanical properties have been studied. An important finding of this study is that PCNT scores over ACNT in composites prepared under certain conditions. This is a very significant finding since PCNT is available at a much lower cost than ACNT.     

Fatigue behaviour of nanoclay reinforced epoxy resin composites

Nanoparticle filling is a feasible way to increase the mechanical properties of polymer matrices. Abundant research work has been published in the last number of years concerning the enhancement of the mechanical properties of nanoparticle filled polymers, but only a reduced number of studies have been done focusing on the fatigue behaviour. This work analyses the influence of nanoclay reinforcement and water presence on the fatigue behaviour of epoxy matrices. The nanoparticles were dispersed into the epoxy resin using a direct mixing method. The dispersion and exfoliation of nanoparticles was characterised by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fatigue strength decreased with the nanoclay incorporation into the matrix. Fatigue life of nanoclay filled composites was significantly reduced by the notch effect and by the immersion in water.     

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.     

Damping characteristics of nanoclay filled hybrid laminates during medium velocity impact

The objective of this paper is to study the vibrational damping characteristics during medium velocity impact of nanoclay filled glass fiber reinforced epoxy hybrid laminates. A series of laminates with varying degree of nanoclay concentration (0–5 wt.%) and fiber weight fraction (25–75 wt.%) were prepared by vacuum assisted resin infusion molding (VARIM) method. The laminates were subjected to medium velocity projectile impact using in-house built gas gun set-up and the ballistic limit of laminates series was determined. The result indicated that during impact, the laminate undergoes vibrational damping. This damping property is a function of fiber weight fraction and orientation, nanoclay concentration and nanocomposite structure. A 42% increase of ballistic limit was observed for 5 wt.% nanoclay filled hybrid (50 wt.% fiber) when compared with unfilled composite. Structural and modal analysis of hybrids showed that the increased ballistic limit of nanoclay filled hybrids is due to the nanocomposite structure and improved damping and fracture properties.     

Impact response of Kevlar composites with filled epoxy matrix

Kevlar fibres have been widely used as impact-resistant reinforcement in composite materials. The paper studies the impact behaviour as well as damage tolerance of Kevlar/filled epoxy matrix. Two different fillers, cork powder and nanoclays Cloisite 30B, were used in order to improve the impact response of these laminates. For better dispersion and interface adhesion matrix/clay nanoclays were previously subjected to a silane treatment appropriate to the epoxy resin. The fillers adding increases the maximum impact load but the opposite tendency was observed for the displacement. Nanoclays promote higher maximum impact loads, lower displacements, the best performance in terms of elastic recuperation and the maximum residual tensile strength.     

Synergistic effect of cyclic mechanical loading and moisture absorption on the bending fatigue performance of carbon/epoxy composites

This study investigates the effects of hygrothermal condition on the static bending strength, the bending fatigue and the residual bending strength of carbon/epoxy composite laminates. Displacement-controlled three-point bending fatigue tests were conducted on carbon/epoxy composite laminates of immersion for 0, 7 and 14 days, respectively. After 40000 cycles the fatigue test was stopped and the residual properties were measured on the tested specimens. The effects of hygrothermal condition and fatigue on the micrographs of the specimens have been studied by the metallurgical microscope and scanning electron microscope. Experimental results reveal that moisture absorption can accelerate damage propagation of the composite; the accumulation of irreversible structural damage under the cyclic loading leads to a change in the macroscopic mechanical properties of the composites; the bending strength and the residual strength retention decreased with increased immersion time; hygrothermal aging lowered the threshold level for the onset of fatigue.     

Fatigue behaviour in hybrid hollow microspheres/fibre reinforced composites

This article presents the results of a current study concerning the influence of the addition of short fibres on the fatigue behaviour of syntactic foams. The material was obtained by vacuum-assisted resin transfer moulding adding hollow glass microspheres to an epoxy resin acting as binding matrix. Specimens with microsphere contents up to 50% and fibre reinforcement up to 1.2% in volume were tested at three-point bending at room temperature. Foams show significantly lower static and fatigue strength than an epoxy matrix. A significant decrease in the absolute strength with filler increase was observed, and even specific strength decreases for low filler contents and is nearly constant for the higher filler contents. Fatigue strength also decreases with the increase in filler content. The addition of glass fibre reinforcement produces only a slight improvement in flexure strength, while the addition of carbon fibres promotes an important improvement; a hybrid composite containing 0.9% carbon fibre is about 30% stronger than unreinforced foams. An improvement in fatigue strength more than 30% was obtained by the addition of small percentages of glass or carbon fibre.     

Assessment of the mechanical behaviour of glass fibre composites with a tough polydicyclopentadiene (PDCPD) matrix

The use of a tough thermoset polydicyclopentadiene (PDCPD) as a matrix material for composites was explored. A PDCPD–glass fibre composite and an equivalent epoxy composite were compared. Fibre–matrix adhesion quality was assessed by transverse bending tests. The materials were subjected to compression tests, impact tests, static tensile tests and tensile fatigue tests. The results indicate that the tough behaviour of the PDCPD matrix markedly influences the composite damage resistance. The size of the impact damage in the PDCPD composite was half of that in the epoxy composite. The tensile tests indicated no significant difference in tensile strength, but the damage before failure was found to be much more severe in the epoxy samples. The fatigue results showed a much lower variation in fatigue life for the PDCPD material than for the epoxy material, as well as clear differences in damage development for the two materials.     

Nano-enhanced composite materials under thermal shock and environmental degradation: A durability study

Carbon nanotubes (CNTs) were incorporated at 0.5 wt% in epoxy resin using sonication at two different levels of amplitude (50% and 100% of 400 W nominal sonication power). The CNTs modified epoxy systems were used to manufacture carbon fibre reinforced laminates (CFRPs). All specimens were subjected to thermal shock and hygrothermal exposure. The presence of CNTs did not alter the water absorption profiles for the epoxy resin, but it resulted in a spectacular 40% reduction in the water uptake at equilibrium for the CFRPs. The interlaminar shear strength of the CFRPs was not significantly affected by the thermal shock cycles; however it was reduced by 50–60% after hygrothermal exposure. The addition of CNTs led to slightly lower interlaminar shear strength values in the as-manufactured state. However their presence did not accelerate the deterioration of the strength after the environmental exposure. Although the addition of CNTs did not significantly influence the thermomechanical properties of the resin, they were beneficial in the case of the CFRPs since (i) they enhanced the storage modulus and glass transition temperature and (ii) limited the deterioration of these properties after thermal shock and hygrothermal exposure. The amplitude level during sonication which determined the dispersion state and length of the CNTs had a clear effect on the durability of the studied systems.     

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

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

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.     

Evaluation on the thermal and mechanical properties of HNT-toughened epoxy/carbon fibre composites

Halloysite nanotubes (HNT) were effectively incorporated into epoxy resin and used for infusion of carbon fibre textiles, resulting in epoxy/halloysite nanotube/carbon fibre (EP/HNT/CF) multi-scale composites. The distribution of nanotubes in the composites was examined by SEM. The thermomechanical properties of the composites were characterized by dynamic mechanical analyser (DMA). A 25% enhancement was recorded for the storage modulus of EP/HNT/CF composite in the glassy state. Moreover, the T_g of the laminates increased with the addition of HNT, and the values were even higher than the T_g of their matrix. Additionally, the Izod impact strength of the composites has been improved. These results indicate a synergistic effect between HNT and carbon fibres.     

Effects of stress waveform and water absorption on the fatigue strength of angle-ply aramid fiber/epoxy composites

The influences of stress waveform and water absorption on the tension–tension fatigue fracture behavior were investigated in ±45° angle-ply laminates of aramid fiber reinforced epoxy matrix composite. For dry specimens, the fatigue strength under negative pulse waveform was higher than that under the positive pulse waveform. Rotation of fibers to the longitudinal direction, which resulted from creep deformation caused by the cyclic loading superimposed on the maximum stress hold time, decreased the compliance, thereby increasing the fatigue life under the negative pulse waveform. Water absorption degraded the fiber/matrix interfacial strength and caused the swelling of the matrix, which resulted in decreases in the static tensile strength and fatigue strength. Although the strength under the negative pulse waveform was slightly higher than that under the positive one, the influence of stress waveform on fatigue strength was smaller in wet specimens.