Toughened Epoxy Matrix Research Articles

Investigation of polyurethane toughened epoxy resins for composite cryotank applications. Part II: Theoretical analysis of toughening mechanisms

Based on the phase separation phenomenon, cryogenic mechanical behaviors and toughening effects for polyurethane toughened epoxy (PU/EP) systems demonstrated in our previous work (Part I), as a followed investigation, this study aims to conduct theoretical analysis of temperature-dependent toughening mechanisms. First, 3D finite element unit cells of PU/EP systems with random particle distributions are established to evaluate the maximum stress concentration factor for the theoretical fracture energy calculation. Then, the temperature-dependent fracture energies of PU/EP systems are investigated quantitatively for the first time, and the theoretical results are shown to be in good agreement with the experimental results. From 10 phr to 40 phr, the particle bridging contribution rises from 47.3% to 65.5%, which elucidates that the particle bridging plays a major role in determining the fracture energy at room temperature. As the temperature decreases from RT to −183°C, taking PU10/EP as an example, the untoughened matrix proportion goes up from 30.9% to 65.2%; the particle bridging contribution decreases from 51.8% to 27.8%; the shear banding contribution drops from 17.3% to 7%. This work explores temperature-dependent toughening mechanisms in depth and offers an accurate quantitative evaluation of fracture energies for toughened epoxy matrix for composites cryotank design.

Mechanical performance and thermal stability of modified epoxy nanocomposite with low loading of graphene platelets

AbstractIn this study, graphene nanoplatelets (GNP) of 0.2%–1.0% by weight were introduced to the rubber toughened epoxy matrix with via a direct method. The effect of GNP loadings on the mechanical (tensile and impact properties) and thermal properties of the rubber toughened epoxy/GNP nanocomposites were evaluated. The results showed that the incorporation of GNP had optimally improved the tensile strength and tensile modulus by 70% and 74%, respectively, at 0.6 wt% filler loading. Thermogravimetric analysis (TGA) and Tg increment result proved the barrier effect via delaying decomposition process which induced by the layered structure of GNP and thus significantly enhanced the thermal stability of epoxy‐based resin, irrespective of GNP loadings (insignificant difference in the decomposition temperature). The fractured surfaces of these nanocomposites showed the good interfacial interaction between the matrix and the GNP nanofiller. X‐ray diffraction (XRD) analysis demonstrated the exfoliation and intercalation states of GNP dispersion in epoxy/LENR.

Strain rate‐dependent deformation behaviors of multi‐layer carbon fiber reinforced polymer laminates

AbstractThis paper focuses on the contributions of diversities of strain rate and orientations for aggravating the diversities of micro failure behaviors on carbon fiber reinforced polymer (CFRP) laminates. A miniature horizontal type tensile tester is employed to conduct experiments with strain rate ranging from 2.6 × 10−6 s−1 to 2.6 × 10−3 s−1. The CFRP laminates are obtained based upon a thermoset toughened epoxy matrix (termed CF/Epoxy) with ply orientations of (0°/0°) and (0°/90°). Significant differences in deformation behaviors of CFRP laminates are determined through tests. The study clearly reveals the strain rate‐dependent deformation modes of CFRP laminates, involving pure fiber fracture, epoxy crack with stepped surface and interface failure with residual voids, determines the “low‐high‐low” variation tendency of Young's modulus and strength as a function of strain rate. Ply orientation‐dependent differences in deformation behaviors are also investigated via severe interfacial shearing effect. A unified model consisted of four deformation modes to is clarified to analyze the complexity of CFRP laminates failure mechanism.

The behaviour of thermoplastic and thermoset carbon fibre composites subjected to low-velocity and high-velocity impact

The present paper describes the results from experimental and theoretical modelling studies on the behaviour of continuous carbon fibre/polymer matrix composites subjected to a relatively low-velocity or high-velocity impact, using a rigid, metallic impactor. Drop-weight and gas-gun tests are employed to conduct the low-velocity and high-velocity impact experiments, respectively. The carbon fibre composites are based upon a thermoplastic poly(ether–ether ketone) matrix (termed CF/PEEK) or a thermoset toughened epoxy matrix (termed CF/Epoxy), which has the same fibre architecture of a cross-ply [03/903]2s lay-up. The studies clearly reveal that the CF/PEEK composites exhibit the better impact performance. Also, at the same impact energy of 10.5 ± 0.3 J, the relatively high-velocity test at 54.4 ± 1.0 m s−1 leads to more damage in both types of composite than observed from the low-velocity test where the impactor struck the composites at 2.56 m s−1. The computationally efficient, two-dimensional, elastic, finite element model that has been developed is generally successful in capturing the essential details of the impact test and the impact damage in the composites, and has been used to predict the loading response of the composites under impact loading.

Effect of low melting temperature polyamide fiber-interlaced carbon fiber braid fabric on the mechanical performance and fracture toughness of CFRP laminates

Fracture toughness and epoxy matrix brittleness have been main issues in carbon/epoxy composites. Various studies have been reported to improve the fracture toughness and brittleness of the carbon/epoxy composites. Although conventional method such as toughened epoxy matrix dissolving method, interleaving with a thermoplastic interlayer method, incorporating with a thermoplastic particles method improve the fracture toughness and brittleness of the carbon/epoxy composites, there have been no significant improvement. Therefore, in this study, due to improve the fracture toughness and brittleness of the carbon/epoxy composites, applied the carbon fiber/low melting temperature polyamide fiber co-braid fabric. In addition, the toughened effect of carbon fiber co-braid fabric with low melting temperature polyamide fiber and high melting temperature polyamide fiber were investigated. The carbon/epoxy composites manufactured by carbon fiber/low melting temperature polyamide fiber co-braid fabric, the fracture toughness significantly improved about 293% compared to the pristine specimens.

Improving energy dissipation and damage resistance of CFRP laminates using alumina nanoparticles

ABSTRACTIn this paper, the effect of a toughened epoxy matrix on the damage evolution, energy dissipation, and permanent indentation of composite laminates under out-of-plane (transverse) loading is presented experimentally. The epoxy matrix was toughened by 3% alumina nanoparticles with sizes less than 200 nm. A quasi-static indentation test was exploited to characterise the damage modes and evaluate the dissipation of energy of the composite laminate. The dissipated energy was evaluated as the enclosed area between the loading and unloading curves, while the damage resistance was expressed as the number of delaminations and their size. The results showed that epoxy toughened by alumina nanoparticles, showed an improvement in the damage threshold load by 27.3% and higher ultimate load under indentation. Regarding the damage resistance, the toughened laminates showed lower number of delaminated interfaces and lower projected delamination area than untoughened laminates. This is due to the localised damage under the indenter, the matrix cracks at low indentation energy and fibre breakages occur at high indentation energy.

Experimental Characterization of CFRP by NOL Ring Test

Abstract Carbon fiber composites are widely used in aerospace applications such as rocket motor casings, electronic packaging etc., to gainfully exploit their higher specific strength properties. Commercial availability of carbon fibers like T 700 with superior strength needs an epoxy matrix with higher ductility. In this paper, elaborate experimental results obtained on composite specimens made with T 700 carbon fibers in a special toughened epoxy matrix are presented. Tests in accordance with the relevant ASTM specifications were conducted on standard unidirectional tensile test specimens as well as National Ordnance laboratories (NOL) ring test. It was observed from the experimental results that the strength measured by NOL-Ring test is more reliable than the results of unidirectional tensile test. It was also found that the maximum frequency of repetition of failure stress is in between 1675-1725 M Pa. These results are very useful as reliable design inputs for aerospace and automobile applications.

Mechanical properties of hybrid SiC/CNT filled toughened epoxy nanocomposite

Mechanical properties of epoxy nanocomposites filled single filler have been extensively studied by various researchers. However, there are not much discovery on the behavior of hybrid nanocomposite. In this study, single and hybrid nanocomposites of toughened epoxy filled CNT/SiC nanoparticles were investigated. The hybrid nanocomposites samples were prepared by combining CNT and SiC nanoparticles in toughened epoxy matrix via mechanical stirring method assisted with ultrasonic cavitations. Epoxy resin and liquid epoxidized natural rubber (LENR) mixture were first blend prior to the addition of nanofillers. Then, the curing process of the nanocomposite samples were conducted by compression molding technique at 130°C for 2 hours. The purpose of this study is to investigate the hybridization effect of CNT and SiC nanoparticles on mechanical properties toughened epoxy matrix. The total loading of single and hybrid nanofillers were fixed to 4% volume are 0, 4C, 4S, 3S1C, 2S2C, and 1S3C. Mechanical properties of hybrid composites show that the highest value of tensile strength achieved by 3S1C sample at about 7% increment and falls between their single composite values. Meanwhile, the stiffness of the same sample is significantly increased at about 31% of the matrix. On the other hand, a highest flexural property is obtained by 1S3C sample at about 20% increment dominated by CNT content. However, the impact strength shows reduction trend with the addition of SiC and CNT into the matrix. The hybridization of SiC and CNT show highest value in sample 1S3C at about 3.37 kJ/m2 of impact energy absorbed. FESEM micrograph have confirmed that better distributions and interaction observed between SiC nanoparticles and matrix compared to CNT, which contributed to higher tensile strength and modulus.

Failure simulations of open-hole IM7/977-3 coupons subjected to fatigue loading using Autodesk Helius PFA

Finite element simulations of three laminates in open-hole configuration subjected to constant amplitude tension–tension fatigue loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. All coupons were made from unidirectional IM7/977-3 plies, which are composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Government furnished experimental data from an assortment of fatigue loaded unnotched coupons were used to characterize the behavior of the composite material in the simulations. The commercial software Autodesk Helius PFA was used to model the non-linear response of the material. Blind simulations of coupon stiffness and damage at several cycle numbers and residual coupon tensile and compressive strengths are benchmarked against experimental measurements and X-rays. Upon review of the experimental results, a second round of simulations was performed where the modeling strategy was updated to improve correlation to experiment.

Failure simulations of open-hole and unnotched IM7/977-3 coupons subjected to quasi-static loading using Autodesk Helius PFA

Finite element simulations of three laminates in open-hole and unnotched configurations subjected to tension and compression quasi-static loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. The coupons are made from unidirectional IM7/977-3 plies, which are a composite material composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Blind simulations of coupon stiffness, nominal coupon stress at failure and damage evolution are benchmarked against experimental measurements and X-rays. The blind simulations are followed by a second round of simulations where the modeling strategy is modified to improve agreement between the simulations and experiments. In the present article, the commercial software Autodesk Helius PFA is used to model the non-linear response of the composite material. Within Helius PFA, failure is evaluated at the constituent level by extracting the fiber and matrix volume average stress state from the homogenized composite stress state. The relationships between the composite and constituents are developed using multicontinuum theory and a high-fidelity micromechanics model.

Prescribed morphology and interface correlation of MWNTs-EP/PSF hybrid nanofibers reinforced and toughened epoxy matrix

MWNTs-EP were successfully prepared by functionalization of MWNTs with epoxy-based groups, and MWNTs-EP/polysulfone (PSF) hybrid nanofibers were fabricated to obtain ex-situ dispersion and alignment of MWNTs-EP by electrospinning. The prescribed morphology and interface correlation of hybrid nanofibers reinforced and toughened epoxy matrix (RTEP) were investigated. The alignment degree of hybrid nanofibers was enhanced with increasing MWNTs-EP loadings, and MWNTs-EP were found to be well dispersed and aligned along the nanofiber axis. The dispersion and alignment states of MWNTs-EP during inhomogeneous phase separation of RTEP were proposed and verified. MWNTs-EP dispersed and aligned along the orginal nanofiber axis were enveloped, bridged or pinned by PSF spheres arranged in the nanofiber direction. The interface chemical correlation between MWNTs-EP and resin matrix was generated due to the further reaction of epoxide rings on the surface of MWNTs-EP, which resulted in simultaneous improvement of mechanical and thermal properties of RTEP.

Studies on the Synthesis and Characterization of Diglycidylether-terminated Polydimethyl Siloxane-modified Epoxy-Bismaleimide Matrices

Diglycidylether-terminated polydimethylsiloxane (DGETPDMS) toughened epoxy matrix systems were developed and further modified with bismaleimide (BMI), namely N, N‘-bismaleimido-4,4‘-diphenylsulfone. The matrix systems were cured using 4,4‘-diaminodiphenylmethane (DDM) as curing agent. The mechanical, thermal, and electrical properties were studied by standard test methods. The introduction of DGETPDMS into epoxy resin improved the impact strength with reduction of tensile strength, flexural strength and glass transition temperature in comparison with the unmodified epoxy, whereas the incorporation of BMI into epoxy resin improved tensile strength, flexural strength, glass transition temperature and heat distortion temperature in comparison with the unmodified epoxy resin. The incorporation of BMI into DGETPDMS-modified epoxy resin improved thermal stability, dielectric strength, surface resistivity, and arc resistance with increasing loading level. SEM micrographs of both BMI-modified epoxy and BMI-modified DGETPDMS-epoxy systems exhibited homogeneous morphology.

Utilization of Waste Silk Fabric as Reinforcement for Acrylonitrile Butadiene Styrene Toughened Epoxy Matrix

The blend matrix epoxy-acrylonitrile butadiene styrene (ABS) is prepared with varying weight percentage (2, 4, 6, 8, and 10) of ABS content in epoxy and the tensile, flexural, and impact properties of this blend matrix is determined. The blend matrix, which contains 4% ABS content in epoxy shows maximum strength and this is used as the matrix to prepare composites. Composites are prepared with varying weight fraction of silk fabric as the reinforcement and pure epoxy and ABS-epoxy as matrix. The developed composites are tested for their mechanical properties and it is found that the strength of the composites increases after toughening epoxy resin with ABS polymer.

Effect of Fly Ash Content on Impact, Compression, and Water Absorption Properties of Epoxy Toughened with Epoxy Phenol Cashew Nut Shell Liquid–Fly Ash Composites

Fly ash, a waste product generated in a thermal power plant in huge quantities has been posing problems of disposal. The purpose of this study is to make a meaningful utilization of fly ash as the filler in pure epoxy resin matrix and 20% epoxy phenol cashew nut shell liquid (EPCNSL)toughened epoxy matrix. The properties under consideration are impact, compression, and water absorption. Composites are made with varying proportions of fly ash in epoxy resin and 20% EPCNSL-toughened epoxy resin. The compression properties are measured on a computerized universal testing machine according to ASTM procedure. The impact strength is determined using an izod impact tester for unnotched specimens. The toughened matrix filled with fly ash composites shows improved compression modulus over pure epoxy–fly ash composites. The toughened matrix composites show better water absorption property over pure epoxy–fly ash composites.

Utilization of Granite Powder as Filler in Epoxy Phenol Cashew Nut Shell Liquid-toughened Epoxy Resin for Impact and Compression Strength

Granite powder, a waste product generated while sizing granite slabs has been posing problems of disposal. The purpose of this study is to make a meaningful utilization of granite powder as the filler in epoxy phenol cashew nut shell liquid (EPCNSL)-toughened epoxy matrix. The properties under consideration are impact and compression. Composites are made with varying proportions of granite powder in pure epoxy and EPCNSL-toughened epoxy matrix system. The impact strength is determined by using an Izod impact tester. The compression properties are measured on a computerized universal testing machine according to ASTM standards. A comparison of the properties of the composites filled with untreated and treated granite powder establish that the treatment of granite powder imparts better reinforcing properties.

Damage tolerance of a stitched carbon/epoxy laminate

Polymer composite laminates are prone to delamination when impacted. This behaviour generally results in a low damage tolerance, which is of great concern for load carrying applications. The possibility of improving the low velocity and ballistic impact damage tolerances of a carbon fibre brittle epoxy laminate by stitching has been investigated by determination of damage width, compression strength and other properties after falling weight and ballistic impact. The damage tolerance of the stitched laminate was found to be much better than that of a prepreg laminate with a brittle epoxy matrix (914C), better than that of a laminate with a toughened epoxy matrix (6376C) and about equal to that of a laminate with a very tough thermoplastic matrix (APC-2). The compression strength after a 45 J low velocity impact of a quasi-isotropic laminate 3 mm thick was 150 MPa for the 914C laminate and 270 MPa for the stitched laminate. In the ballistic case compression strength was increased by about 50% for near quasi-isotropic laminates of thickness 5 mm.

Characterization of the Fiber-Matrix Interphase and its Influence on Mechanical Properties of Unidirectional Composites

The fiber-matrix interphase is commonly altered in graphite fiber rein-forced composites by changing the level of surface treatment imparted to the fibers and by using different fiber sizings. In this study, the influence of both surface treatment and fiber sizing on the mechanical properties of unidirectional composites was studied. Three material systems having the same high modulus Apollo carbon fibers (manufactured by Courtoulds Research) and HC 9106-3 toughened epoxy matrix, but with different fiber sizing and surface treatment, were used to study the effects of the interphase. For convenience, these are designated 810 A, 820 A and 8100 systems. The fibers used in the 810 A and 8100 systems received 100% industry standard surface treatment, while the fibers used in the 820 A system received 200% industry standard surface treatment. The "A" and "O" represent unreacted bisphenol-A epoxy and polyvinylpyrrolidone (PVP) sizing respectively. The formation of distinctly different interphase in these three material systems was confirmed using a pernanganic etching technique. The etching study also revealed a highly nonuniform distribution of PVP in the 8100 system. Unidirectional tensile test results indicated significant reduction in the longitudinal stiffness of the PVP sized 8100 laminates. However, the tensile strength and failure strain of this system were vastly greater than those of the epoxy sized 810 A laminates. The systems with 10% and 200% levels of fiber surface treatment had similar longitudinal tensile properties. Transverse tensile test results indicated that the interracial bond strength was greatest in the 820 A system and lowest in the PVP sized 8100 system. The transverse flexural strength of the 8100 and 820 A systems were however found to be significantly greater than that of the 810 A system. It is postulated that the differences in unidirectional mechanical properties in these material systems are essentially due to differences in the interfacial bond strength and efficiency of load transfer from the matrix to the fiber.

高分子系複合材料 くさび挿入試験によるＣＦＲＰ積層材のモードＩ層間破壊じん性評価

A new estimating method for mode I interlaminar fracture toughness of CFRP laminates was proposed with the aim of applying to dynamic fracture test. In this method, a wedge indentor was used to cause mode I interlaminar fracture in a precracked coupon specimen. That is, compressive load, which is convenient to dynamic test, can be used in this method, while tensile load, which is not appropriate to dynamic test, is necessary for the conventional double cantilever beam test. Finite element analyses and quasi-static experiments were carried out to validate an estimating formula for the mode I energy release rate, which is based on the beam theory and the compliance method. As the result, this method is found to be able to estimate the mode I interlaminar fracture toughness of CFRP laminates and to obtain the same stress field as the one formed in the double cantilever beam specimen. But in this method one has to measure accurately the crack length at the fracture initiation, which is difficult under high rate loading. Three types of CFRP laminates were tested in order to investigate the effect of the toughness of matrix resin. They are: T300/2500 (Toray) which has standard epoxy matrix resin, IM600/133 (Q-C133, Toho rayon) which has toughened epoxy matrix resin, and HTA/PEEK (Toho rayon) which has thermoplastic matrix resin of high fracture toughness. The experimental results show that the toughness of the matrix resin scarcely affects the estimation of the mode I interlaminar fracture toughness and that the fracture toughness estimated by the present method is comparable to that estimated by the double cantilever beam test.

Effect of matrix resin on delamination fatigue crack growth in CFRP laminates

The objective of this study is to examine the effects of matrix resin and fibers on the mechanisms and mechanics of delamination fatigue crack growth in unidirectional CFRP laminates. The near-threshold growth of delamination fatigue cracks was investigated with seven kinds of unidirectional laminates made from brittle epoxy matrix, toughened epoxy matrix and thermoplastic PEEK matrix. Tests were carried out under mode I loading at different stress ratios. The fatigue crack growth behavior of laminates tested here was mainly controlled by the toughness of the matrix resin. The crack growth rate under different stress ratios was a unique function of the equivalent stress intensity range proposed by the authors. The parameter, γ, to indicate the relative contribution of the maximum stress to the cyclic stress was determined from the analysis of the stress-ratio dependency. For the toughened matrix system, the crack growth rate was mainly controlled by the stress range; the γ value was small. On the other hand, for the case of brittle epoxy, the contribution of the maximum stress was large; the γ value was large. The resistance against fatigue crack growth increased with the increase of the static fracture toughness. However, the ratio of the fatigue threshold to the static fracture toughness decreased with the decrease of the γ value. Thus, the increase of the fracture toughness does not fully contribute to the fatigue crack growth resistance. The difference in growth behavior due to matrix resin and fibers was discussed on the bases of mechanism consideration and fractographic observation.

Application of modeling to the control and optimization of composites processing

The general features of a master model for the description of the thermochemorheological changes in the matrix during the processing of polymer based composites, developed in recent years, are reviewed and applied here to describe the behavior of a new commercial toughened epoxy matrix, characterized through a complete experimental program. The master model is then combined with an intelligent system developed to monitor and predict the real curing temperatures during autoclave processing, providing the boundary conditions of the mathematical model. The integrated system allows the prediction ‘on line’ of part temperature changes as a function of the programmed air temperature and the optimization of the cure cycle, minimizing the difference between the actual temperature and the value recommended by process specification.