Low velocity impact behavior of glass/epoxy cross-ply laminates with different fiber treatments

This paper deals with the influence of the fiber/matrix adhesion quality on the impact behavior of cross-ply glass/epoxy laminates. Glass fibers with two different treatments (one to promote and one to prevent adhesion to the matrix) were embedded in epoxy matrix systems and subjected to low velocity impacts at energies below perforation energy. It will be shown that the laminates with good fiber/matrix adhesion are significantely more damage resistant than the plates with poor adhesion. It will be pointed out that the composites with the more brittle matrix system show the lower damage resistance. For all materials, the absorbed energy correlates well with the amount of impact induced damage. Furthermore, an experimental/mathematical model is introduced that gives the possibility of predicting the maximum deflection and maximum force for any given impact energy. Only one experiment is needed for each material to identify the model parameters. These parameters describe the real material behavior and can be used as characteristic values to classify materials with respect to their impact stiffness and damage resistance.     

Flexural behavior of stepped graphite/epoxy laminates

Cross-ply tapered/stepped laminates with taper angles of 1 and 2° between the top and bottom surfaces were fabricated using T300/943 graphite/epoxy by compression molding. Ply terminations were done internally within the laminate and externally on the surface of the laminate at various cross-sections in order to obtain the taper. Equivalent cross-ply specimens with internal and external ply terminations, respectively, were tested in three point bending. The bending stiffness of the equivalent corss-ply are nearly equal, while the failure modes are significantly different. The tapered laminates with internal ply terminations failed due to a series of delaminations originating at the step corners of the 0° plies. Macroscopically, the specimens with internal ply term nations failed by tensile fracture of the outer plies at relatively higher loads.     

Fabrication and characterization of functionally graded nanoclay/glass fiber/epoxy hybrid nanocomposite laminates

In this work, hardness, tensile, impact, bearing strength and water absorption tests were performed to study the mechanical properties of stepwise graded and non-graded hybrid nanocomposites. Three different stepwise graded nanocomposites and one non-graded (homogeneous) nanocomposite with the same geometry and total nanoclay content of 10 wt% were designed and prepared. Moreover, one neat glass fiber laminate was manufactured. The results of the tests indicated that addition of the graded and non-graded nanoclay improves hardness over neat glass fiber reinforcement. The maximum increase in hardness of about 53% over neat specimen is obtained for specimens that have the highest weight percentage (2 wt%) of the clay nanoparticles on its surface (S-specimen and the side of F-specimen that reinforced with 2 wt% nanoclay). The gradation process results in an increase in hardness of about 11% compared with non-graded (homogeneous) specimen. In addition, an improvement of 11.9% in strain-to-failure is achieved with specimen having greatest amount of nanoclay in the middle over neat glass fiber/epoxy composite. The other nanoclay-filled glass fiber composites have strain-to-failure close to neat glass fiber/epoxy. The addition of nanoclay reinforcement has insignificant effect on ultimate tensile strength, tensile modulus, water absorption, bearing strength and impact strength compared with neat glass fiber/epoxy.     

Effect of patch hybridization on the tensile behavior of patch repaired glass/epoxy composite laminates using acoustic emission monitoring

This paper investigates the tensile response of damaged glass/epoxy composite laminates repaired using hybrid external patches. Hybrid external patches based on glass and Kevlar woven fabrics bonded on both faces of the damaged parent laminate were considered. Five different kinds of plain weave woven fabrics with a different ratio between glass and Kevlar fibers (100/0, 75/25, 50/50, 25/75 and 0/100) were used as the external patches. The intention of using these hybrid patches was to combine the excellent tensile stiffness of Kevlar fiber with the superior resin adhesion property of glass fiber. The virgin and damaged specimens were taken as the reference specimens for comparison of residual mechanical properties and damage mechanisms. Damage evolution and the failure progression of the repaired glass/epoxy specimens were monitored using real-time Acoustic Emission (AE) monitoring technique. The Acoustic Emission (AE) results depict different damage profiles and link them with mechanical test results to reveal the load to a change in failure mechanisms during mechanical loading concerning the influence of each hybrid patches on the performance of repaired glass/epoxy specimens. Good correlation of the acoustic emission results with the photographic images of fractured specimens was obtained. Specimens repaired with the equal volume fraction of glass and Kevlar fibers in the external patches presented the most favorable residual tensile response by effectively releasing the stress concentration in the damaged area.     

Low-velocity impact damage in graphite-fiber reinforced epoxy laminates

An experimental investigation was conducted to identify the failure mechanism and to understand damage propagation in compression-loaded composite structures. The tests were conducted on several laminates of different ply orientation with thicknesses that ranged from 0.56 to 0.79 cm. The panels were damaged by 1.27-cm-diameter aluminum spheres propelled normal to the specimen surface at velocities ranging from 30 m/s to 140 m/s. Results indicate that there is significant internal laminate damage due to low-velocity impact with no surface damage. The internal damage consists of delamination and intraply cracking. Three damage propagation modes were identified as causing specimen failure; delamination, axial load-lateral deformation coupling, and local shear failure.     

Effect of impact damage on the curved beam interlaminar strength of carbon/epoxy laminates

The bending interlaminar strength and bending interlaminar strength after impact of carbon/epoxy-laminated curved beams were studied experimentally using four-point bending test and low velocity impact. First, the post- impact damage of the laminated curved beams with different radii was analyzed based on ultrasonic C-scan images. Then, the effect of impact damage on both the interlaminar strength and the maximum interlaminar radial stresses of the laminated curved beams were investigated. Finally, the full-field displacement distributions of the laminated curved beams were obtained using digital speckle correlation method. Four-point bending experimental results play a significant role for interlaminar strength in evaluating the laminated curved beams with and without impact damage.     

Experimental investigation into glass fiber/epoxy composite laminates subjected to single and repeated high-velocity impacts of ice

Many engineering components in aerospace structures which are made from polymer composite materials are often damaged during service life due to hail ice and bird impact. This study examines the damage which may be incurred by a single and repeated high-velocity impact of 11.7 g cylindrical-shaped ice on glass fiber/epoxy laminated composite panels carried out on a 20-mm diameter smooth barrel gas gun. The laminates were made from E-glass fiber/epoxy resin with 0/90, ±45, chopped strand mat (CSM) and unidirectional fiber orientation and in different stacking sequence. The impact velocity was in the range of 130–140 m/s and the resulting damage extension zones from ice projectile impacts were measured. Damage extension was successfully identified in all specimens subjected to high-velocity ice projectile impact. Results showed specimens with ±45 orientation and CSM fiber exhibited the lowest damage extension. The results also revealed that specimens with plain weave 0/90 lay-up of glass woven roving show the highest damage extension. Extended damages were observed in composite panels under repeated ice projectile impacts. Study of the stacking sequence effect indicated significant role played by presence of ±45 reinforcement in reducing the damage extension in the laminated plates. Delamination constituted the major damage mechanism for most specimens tested followed by matrix and fiber fracture.     

Hygrothermal degradation of the composite laminates from woven carbon/SC‐15 epoxy resin and woven glass/SC‐15 epoxy resin

The degradation mechanism for hygrothermal aging of woven carbon‐epoxy and woven glass‐epoxy composite laminates was investigated in the micro‐scale. Interlaminar shear and cross laminar flexural tests were performed on notched and unnotched specimens to know the mechanical performance of the composite laminates. The Interlaminar Shear Stress (ISS) for both the composites was also evaluated and correlated with the number of hygrothermal cycles. Four‐point bending and tensile or compression shear loading configurations were also used. The stress at the onset of delamination (Delamination Damage Tolerance, DDT) was identified from the load‐deflection curve of the flexural specimens and correlated with the number of hygrothermal cycles. It was found that both the ISS and DDT decrease with the exposure time. Dimensional stability was almost unchanged throughout the aging process, although there was a very little moisture absorption (∼1.3%) in glass‐epoxy and carbon‐epoxy composite laminates. SEM photomicrographs of the delaminated surface show that failure occurs suddenly in a macroscopically brittle mode by crack initiation and propagation method. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers.     

Dynamic mechanical study of epoxy,epoxy/glass,and glass/epoxy/wood hybrid composites aged in various media

It was proposed and subsequently established that wrapping of red oak wood crossties with epoxy impregnated glass fiber composites will impart longer service life and better stiffness and strength characteristics to these hybrid ties than conventional ones and will help them better withstand environmental extremes. The objective was to understand the degrading effects of aqueous (distilled water), saline (NaCl), acidic (HCl), and alkaline (NaOH) solutions, as well as accelerated aging and freeze/thaw cycling environments on the dynamic and static mechanical properties of these hybrid materials (i.e., wood, wrapped with fiber reinforced resin) and their components. Also micrographs of composite samples, obtained through scanning electron microscopy (SEM), were studied to determine the failure mechanism of composite specimens aged in different environments. Results showed that immersion in aging media lowered the glass transition temperature (T_g) and enhanced apparent phase separation in the samples because of polymer plasticization. In water immersion, the T_g and the stiffness increased with time owing to continued resin curing. At ambient temperature, sustained load had little effect on the mechanical behavior of the aged samples. The extent of degradation was the least for samples aged in salt solution. Soaking in room‐temperature acid solution was most damaging to pure red oak wood samples. Six‐cycle aging did not damage the neat resin or the hybrid samples, whereas it damaged pure wood specimens. Therefore, the composite wrapping around the wood core of the hybrid sample protected it sufficiently, thereby preventing damage to the hybrid specimen during the aging process. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Low moisture absorptive epoxy formulations for Gr/Ep laminates

The incorporation of 2-undecyl-imidazole (C₁₁Z) and chromium acetylacetonate (Cr(acac)₃) additives into N, N, N′, N′-tetraglycidyl diaminodiphenylmethane (TGDDM)/diaminodiphenylsulfone (DDS) epoxy formulations was found to significantly decrease the moisture absorption of their prepared graphite/epoxy laminates. However, the same additions did not much affect the moisture absorption of the cured TGDDM/DDS neat resins. Thus, the former case was attributed to the processing effects in view of the fact that C₁₁Z pre-reacting with TGDDM during lay-up process produced the ether-linkage polymer chains in the epoxy networks and raised the viscosity. The following compacting stage of laminates was believed to squeeze the small molecules such as unreacted TGDDM, DDS and Cr(acac)₃, toward the surface of carbon fibers and increase the chance of Cr(acac)₃ to block the hydroxyl groups in the epoxy networks produced by the reactions between TGDDM and DDS. Some evidence was provided to support the above hypothesis in this study.     

Flexural behavior of pMWCNTs filled glass fiber/epoxy nanocomposites: Synthesis and interfacial failure

Glass fiber reinforced polymer composite (GFRP) encounter many practical situations during its application, exposed to different temperature fluctuation. Absorption of moisture and the fluctuated thermal environment cause mechanical degradation of GFRP. The current plan is to explore the mechanical and chemical behavior of pristine multiwall carbon nanotubes(pMWCNTs)-glass fiber strengthens epoxy composites during thermal shock (TS). The laminates were prepared by the hand-lay-up process followed by compression molding. The thermal shock was performed at the temperature for the required samples was 135°C for 24 h, followed by −135°C for 24 h. A flexural test has been carried out at 1 mm/min loading speed. 0.2 wt% pMWCNTs filled nanocomposite appeared to have the highest flexural strength and modulus compared to ambient samples. The thermomechanical behavior of nanocomposites has been accomplished by analyzing the dynamic mechanical thermal analysis graph (DMTA). Field emission scanning electron microscope (FESEM) analyzed the fracture surface of in-situ mechanical failure samples to find out the primary failure modes for strengthening and weakening mechanisms. The glass transition temperature (T_g) of the nanocomposite observed decreased due to homogeneous dispersion of pMWCNTs, imparting some effect on crosslink density and reinforcement up to 0.2 wt%. This study reveals that the uniform distribution of pMWCNTs and thermal shock treatment enhances the matrix stiffness and improves the mechanical properties.     

Impact performance of laminates made of syntactic foam and glass fiber reinforced epoxy as protective materials

Impact performance of two‐dimensional quasi‐isotropic laminates subjected to impact loading with flat‐ended impactors has been studied in terms of impact stress, strain rate, and volume fraction of laminae. A simple model was formulated to predict impact stress within an elastic limit as a function of volume fractions of laminae. Individual impact parameters for syntactic foam and fiber‐reinforced epoxy were experimentally obtained at impact energy levels of 0.54 and 0.87 Joule, and used to predict impact stress of the laminates made of the same materials. A reasonable agreement between predictions and experimental results were found. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2306–2310, 2003     

Creep and relaxation behavior of woven glass/epoxy substrates for multilayer circuit board applications

The creep behavior of a common woven glass/epoxy composite substrate for multilayer circuit board applications was characterized using dynamic mechanical analysis (DMA). The creep compliance was measured in both the warp and fill directions of the composite over a temperature range of 30°C to 155°C. The creep compliance of the neat FR-4 epoxy matrix was also characterized for comparison with the composite response. Master creep curves were obtained for the neat resin and the composite in the warp and fill directions assuming thermorheologically simple behavior and applying the time-temperature superposition principle. The creep data was fit to a Prony series and then converted to relaxation data in the Laplace domain. Micromechanical models were developed to predict the relaxation behavior of the woven glass/epoxy composite from the elastic properties and the geometry of the glass fabric and relaxation behavior of the neat resin. Model predictions were compared with experimental data.     

The comparison of low‐velocity impact resistance of aluminum/carbon and glass fiber metal laminates

This article presents the low‐velocity impact response of fiber metal laminates, based on aluminum with a polymer composite, reinforced with carbon and glass fibers. The influence of fiber orientations as well as analysis of load‐time history, damage area and damage depth in relation to different energy levels is presented and discussed. The obtained results made it possible to determine characteristic points, which may be responsible for particular stages of the laminate structure degradation process: local microcracks and delaminations, leading to a decrease in the stiffness of the laminate, as well as further damage represented by laminate cracks and its perforation. The damage mechanism of fiber metal laminates is rather complex. In case of carbon fiber laminates, a higher tendency to perforation was observed in comparison to laminates containing glass fibers. Delaminations in composite interlayers and at the metal/composite interface constitute a significant damage form of fiber metal laminates resulting from dynamic loads. Fiber metal laminates with glass fibers absorb energy mainly through plastic deformation as well as through delamination initiation and propagation, whereas laminates containing carbon fibers absorb energy for penetration and perforation of the laminate. POLYM. COMPOS. 37:1056–1063, 2016. © 2014 Society of Plastics Engineers     

Dynamic-mechancial response of graphita/epoxy composite laminates and neat resin

The dynamic mechanical properties of carbon-fiber-reinforced, epoxy-matric-composite laminates subjected to loading perpendicular to the plane of lamination and of neat epoxy resin are reported. The dynamic mechanical measurements were performed in the frequency range from 0.1 to 40 Hz and at temperatures between 20° and 200°C at deformation levels within the linear viscoelastic region by the use of a Dynastat apparatus. It is found that thein-phase and out-of phase stiffnesses superpose to form master curves covering a frequency range of 12 decades. By a suitable scaling procedure of the master curves, it is found that thein-phase stiffiness has tbe same shape and the out-of-phase stiffness has the same dispersion for all laminates irrespective of the stacking sequence and are nearly identical to those for neat epoxy resin, An empirical function for the relaxation modulus is developed that, when converted to dynamic modulus, gives a good overall agreement for both components of the dynamic stiffness as a function of frequency. Absorbed moisture is found to cause a reduction in thhe elevated temperature mechanical properties of a laminate due to a reduction in the glass-transition temperature of the resin. It is also found thatthemositure absorption is a reversible process, in the sense that the initially dry properties of the laminate are recovered afterredrying the wet sample.     

Degradation and enhancement of the fatigue behavior of glass/graphite/epoxy hybrid composites after accelerated aging

The tensile fatigue characteristics of graphite/glass/epoxy hybrid composites were investigated before and after exposure to various high humidity environments. Accelerated aging treatments of 98 percent RH and 120°F were applied both continuously for 500 and 1000 h and coupled with a 350°F thermal shock after ?67°F low temperature exposure. This latter thermohumidity cycle was applied for 500 h total exposure time. Cyclic loading tests showed that degradation due to the moisture and thermohumidity conditioning treatments was in-most cases small. Some bereficial behavior of the humidity exposure was also noted. The hybrid fatigue characteristics both before and after exposure were a blend of the nearly flat advanced fiber composite S-N behavior and the highly curved glass fiber composite S-N behavior. This was attributed to a mixture of failure modes for the hybrid system.     

Polymer/clay aerogel‐based glass fabric laminates

Laminates of polymer/clay aerogels and glass fabric sheets were prepared with varying epoxy adhesion application levels. A poly(amide‐imide) and an epoxy (1,4‐butanediol diglycidyl ether/2,6‐diaminopyridine) were chosen as the two “foam core” polymers; both single‐layered and double‐layered glass fiber laminates were investigated. The adhesion between polymer clay aerogels and glass fibers was quantified using the T‐peel method. The peel strength properties were found to increase as adhesive loading increased up to an optimal value, after which peel strength declines. Flexural and compressive testing of the laminates was also performed as a way of measuring mechanical strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012