Matrix Failure Research Articles

Enhanced fracture toughness of nanostructured carbon‐fiber reinforced poly(urethane‐isocyanurate) composites at low concentrations

Carbon fiber reinforced poly(urethane‐isocyanurate)‐nanosilica composites CF‐(PUI‐NS) were manufactured by means of the vacuum‐assisted resin transfer moulding technique (VARTM) at very low NS concentrations (0–4 wt%). The high strain to failure of the PUI matrix (>7%) affected tensile tests by CF reorientation. Both the tensile strength and strain to failure were highly dependent on its kinematics. CF(PUI‐NS) caused an increase of the static toughness with a maximum improvement of tensile strain to failure and modulus of +28.8% and +39% at 1 wt% and 2 wt% of NS, respectively. The interlaminar shear strength (GIC) of the composites showed both a deterioration of −12.9% and an improvement of +9.9% for NS concentrations of 1 wt% and 4 wt%, respectively. Regardless of the GIC value, all of the composites prepared with NS presented secondary maxima of the force versus displacement plots, indicating a substantial arrest of the crack propagation velocity after delamination started. Fractographic analysis revealed several features, such as fiber pull‐out, bridging as well as river patterns whereas the composites prepared with NS behaved in a more ductile fashion due to the presence of river patterns and a reduced fiber pull‐out. POLYM. ENG. SCI., 58:1241–1250, 2018. © 2017 Society of Plastics Engineers

Bond Behavior of FRCM Carbon Yarns Embedded in a Cementitious Matrix: Experimental and Numerical Results

The use of inorganic cement based composite systems, known as Fiber Reinforced Cementitious Matrix (FRCM), is a very promising technique for retrofitting and strengthening the existing masonry or concrete structures. The effectiveness of FRCM systems is strongly related to the interface bond between inorganic matrix and fabric reinforcement, and, since the major weakness is often located on this interface, the study of stress-transfer mechanisms between fibers and matrix becomes of fundamental importance.FRCM are usually reinforced with uni-directional or bi-directional fabrics consisting of multifilament yarns made of carbon, glass, basalt or PBO fibers, disposed along two orthogonal directions. The difficulty of the mortar to penetrate within the filaments that constitute the fabric yarns and the consequent non-homogeneous stress distribution through the yarn cross section makes difficult to access the characterization of the composite material. The use of polymer coatings on the fibers surface showed to enhance the bond strength of the interface between fibers and mortar and, as a consequence, to improve the mechanical performance of the composite. The coating does not allow the mortar to penetrate within the filaments while is able to improve the bond between the two materials and to increase the shear stress transfer capacity at the interface.An experimental session of several pull out tests on carbon yarns embedded in a cementitious matrix was carried out. Different embedded lengths have been analyzed, equal to 20, 30 and 50 mm. The carbon yarns object of this study were pre-impregnated with a flexible epoxy resin enhanced with a thin layer of quartz sand applied on the surface.A variational model was proposed to evaluate the pull-out behaviour and failure mechanisms of the system and to compare numerical results to the experimental outcomes. Evolution of fracture in the yarn-matrix system is determined by solving an incremental energy minimization problem, acting on an energy functional which account for brittle failure of matrix and yarn, and for debonding at the yarn-matrix interface. The model was able to accurately describe the three phases of the pull-out mechanism, depending on the embedded length.

Amino-terminated nitrogen-rich layer to improve the interlaminar shear strength between carbon fiber and a thermoplastic matrix

An amino-terminated nitrogen-rich layer of poly(cyclotriphosphazene-co-melamine) (PPM) coating was used to functionalize carbon fiber (CF) to strengthen the interfacial adhesion of CF reinforced copoly(phthalazinone ether sulfone)s (PPBES) through a facile in-situ polymerization. FTIR, Raman and XPS confirm the chemical bonds between CF and PPM. SEM and dynamic contact angle tests demonstrate that PPM coating can enhance surface wettability, roughnes of CF, which can improve interlaminar shear strength and flexural strength of CF/PPBES by 23.2% and 29.3% with no discernable decrease on tensile strength of CFs. According to DMA test, storage modulus and service temperature increase by 15GPa and 6°C. SEM observations certify that the failure mechanism transforms from interface failure for CF/PPBES to matrix failure and fiber broken for CF-PPM/PPBES. Moreover, the successful preparation of PPM coatings on CF surface provides new insights for the fabrication of advanced thermoplastic composites from readily available components via a facile route.

Carbon nanotube-grafted carbon fiber polymer composites: Damage characterization on the micro-scale

Abstract Multiwall carbon nanotubes (CNTs) – carbon fibers (CFs) hybrid materials were produced by directly growing CNTs on CFs by means of chemical vapor deposition. For the latter, the oxidative dehydrogenation reaction of C2H2 and CO2 was applied, which allows growing CNTs without damaging the CF surface. Uni-directional nano-engineered carbon fiber reinforced composites (nFRCs) were fabricated by impregnating these hybrid materials with epoxy. The nFRCs subjected to single fiber push-out tests revealed a decrease of the interfacial shear strength (IFSS) of about 36% compared to the carbon fiber composites without CNTs. By means of transverse three-point bending tests performed on pre-notched composite beams inside a scanning electron microscope, the fracturing behavior parallel to the fibers was studied in-situ. The nFRCs showed significantly reduced fiber/matrix debonding while CNTs pull-out, CNTs bridging as well as matrix failure occurred. These results demonstrate that the presence of CNTs in nFRCs affects the stress distribution and consequently the damage initiation as well as the damage propagation. The presence of CNTs suppresses the stress concentration at the fiber/matrix interface and reduces the debonding of CFs from the matrix. However, our results indicate that the stress concentration shifts towards the CNTs' ends/matrix interface and causes promoted matrix failure leading to lower IFSS.

Failure modes and strength prediction of thin ply CFRP angle-ply laminates

In this paper, the tensile strength and failure modes of thin ply carbon fiber-reinforced polymer (CFRP) angle-ply laminates are investigated by experiments and predicted by Finite element method (FEM) and theoretical model. First, experiments are performed to evaluate the tensile behavior of thin ply CFRP angle-ply laminates fabricated with different fiber areal weights of prepreg (20,60,120g/m2) and ply angle (15°, 30°). Experimental results show that thin ply angle-ply laminates present different failure modes, and also the tensile strengths do not increase monotonically with the decrease in fiber areal weight. Second, both theoretical and FEM models are established to predict the strength and failure modes of thin ply CFRP angle-ply laminates with different ply thickness and fiber volume fraction, also two competing mechanisms finally cause the non-monotonic change of strengths of thin-ply laminates which has good agreement with experimental results. Finally, a failure diagram of thin ply angle-ply laminates under tensile load is given, which shows different failure modes (fiber breakage, delamination and transverse matrix failure) for fiber areal weight (0–240g/m2) and ply angle (0–90°). Also the delamination is suppressed and the fiber breakage is extended with the decrease of fiber areal weight in the ply angle range of 10°–25°.

Novel geopolymer based composites reinforced with stainless steel mesh and chromium powder

In this study, two novel geopolymer composites, namely, stainless steel mesh/geopolymer and stainless steel mesh/Crp/geopolymer, were designed and prepared, in which the layers of stainless steel mesh were 4, 7 and 10, respectively, and particle size of chromium powder (Grp) were 74, 38 and 13μm, respectively. The Grp content was 40wt% of metakaolin used to fabricate the resulting geopolymer composites. Phase composition, microstructure, mechanical properties and fracture behavior of the resulting composites were studied. The results suggested the incorporation of stainless steel mesh could significantly improve the mechanical properties of the composites. The flexural strength, elasticity modulus and work of fracture reached the maximum values, 97MPa, 11GPa and 4.2kJ/m2, respectively, for the composite with seven-layers of stainless steel mesh. The presence of Grp could further enhance the mechanical properties of geopolymer composites, especially for the work of fracture, and the mechanical properties gradually improved with the decreasing of particle size of Grp. The reason for the performance improvement after the introduction of Grp was because that the presence of Grp can effectively reduce the debonding between matrix and stainless steel mesh. All composites showed typical pseudo-plastic behavior that offers potential to prevent catastrophic failure of the geopolymer matrix.

Study of Tensile Properties and Deflection Temperature of Polypropylene/Subang Pineapple Leaf Fiber Composites

The development of eco-friendly composites has been increasing in the past four decades because the requirement of eco-friendly materials has been increasing. Indonesia has a lot of natural fiber resources and, pineapple leaf fiber is one of those fibers. This study aimed to determine the influence of weight fraction of pineapple leaf fibers, that were grown at Subang, to the tensile properties and the deflection temperature of polypropylene/Subang pineapple leaf fiber composites. Pineapple leaf fibers were pretreated by alkalization, while polypropylene pellets, as the matrix, were extruded into sheets. Hot press method was used to fabricate the composites. The results of the tensile test and Heat Deflection Temperature (HDT) test showed that the composites that contained of 30 wt.% pineapple leaf fiber was the best composite. The values of tensile strength, modulus of elasticity and deflection temperature were (64.04 ± 3.91) MPa; (3.98 ± 0.55) GPa and (156.05 ± 1.77) °C respectively, in which increased 187.36%, 198.60%, 264.72% respectively from the pristine polypropylene. The results of the observation on the fracture surfaces showed that the failure modes were fiber breakage and matrix failure.

Tensile Properties and Deflection Temperature of Polypropylene/Sumberejo Kenaf Fiber Composites with Fiber Content Variation

The use of synthetic fibers as reinforcement in composites has disadvantage which are unsustainable and an adverse impact on the environment. An alternative reinforcement for composites is natural fiber. Polypropylene and Sumberejo kenaf fibers were used respectively as the matrix and reinforcement. The aim of this research was to obtain the optimum tensile properties and deflection temperature with the variation of kenaf fiber fractions. Polypropylene/kenaf fiber composites were fabricated by hot press method. The kenaf fiber was soaked in NaOH solution before being used as the reinforcement and polypropylene was extruded before being used as the matrix. The weight fractions were varied to produce composites and pristine polypropylene samples were also prepared for comparison. The optimum tensile strength, modulus and deflection temperature were found in the composites with the 40 wt% kenaf fiber fraction with an increase up to 80% and 170% compared to the pristine polypropylene with the values of (60.3 ± 4,3) MPa and (159.1 ± 1,8) °C respectively. The Scanning Electron Microscope observation results in the fracture surface of the composites with the 40 wt% fiber fraction showed a relatively good bonding interface between fibers and the matrix and the failure modes were fiber breakage and matrix failures.

Constitutive Modelling for Anisotropic Damage in Woven E-Glass Reinforcements

It is easy for composite laminates to be damaged by relative lower velocity impact which could give rise to internal delamination that will strongly weaken the compressive strength of laminates. In order to predict the occurrence of matrix failure, the elastic-brittle behaviors of fiber-reinforced composites were modeled constitutively by an anisotropic damage model. The dynamic tensile testing was performed at a constant velocity of 2 mm/min until the sample broke to achieve the mechanical parameters of E-glass reinforcements. The elastic constitutive equation and the constitutive damage model were obtained on basis of the fundamental theory of mechanics about the orthotropic constitutive of reinforcements. The methodology for this constitutive model which is developed by Hashin considered both the effect of fiber and matrix failure. Then, the developed constitutive equations were incorporated into the FE (finite element) codes, ABAQUS, through the user subroutine module to simulate the process of projectile impacting GFRP composite laminates. The results show that the material deformation reaches a maximum at 24 μs, then occurs rebound with the increase of the time. The stress of reinforcements traverse section linearly increases outward from 0 MPa to 509.8 MPa. Material damage area increases with the prolonging of time, and for a fixed time, material damage gradually increases from the edges to the center and reaches a constant value of 1, which means the rupture of the damage process.

Pull-out behavior of straight steel fibers from asphalt binder

The interaction between a straight steel fiber and the surrounding asphalt matrix is investigated through single fiber pull-out tests and numerical simulations. Based on experimental observations from 240 pull-out tests for various temperatures, displacement rates, and fiber dimensions, pull-out failure modes are classified into three types: matrix, interface, and mixed failure modes. The experimental data suggests that there is a critical shear stress beyond which fiber-binder interface debonding will occur. Detailed finite element analyses employing a nonlinear viscoelastic constitutive model for asphalt are carried out to explain the observed test results. The numerical simulations show that the stress distribution along the fiber surface varies substantially with temperature and fiber dimensions. At −20°C, the concentration of interfacial shear stress becomes significant as the embedded aspect ratio of fibers becomes higher, and causes a decrease in the value of average interfacial shear stress at the onset of the interface debonding. The simulations also showed that this stress concentration decreases at 0°C as the contribution of viscous behavior becomes more significant. The simulation results confirm that the shear bond strength between steel fiber and asphalt binder is 6.9MPa, and it is independent of temperature, loading rate, and fiber dimensions within the ranges used in the study.

Experimental analysis and micromechanical models of high performance renewable agave reinforced biocomposites

Abstract The present work deals with the experimental study of high performance biocomposites reinforced with optimized agave fibers, as well as the successive implementation of reliable micromechanical models that can be used at the design stage. In detail, systematical experimental analyses performed on biocomposites with epoxy or PLA matrix, have permitted to highlight that for short fibers biocomposites the reinforcing leads to a significant improvement of the matrix stiffness, whereas the particular damage mechanism based essentially on the matrix failure with consequent tensile failure of the fibers aligned with the applied load, does not allow to obtain an actual reinforcing of the matrix. Such a result is strictly related to the limited fiber stiffness, that lead to a low reinforcing of the matrixes. Consequently, as confirmed by the experimental evidence, the mechanical performance of such biocomposites is not affected significantly by the improvement of the matrix-fiber adhesion provided by the mercerization. Also, the best results are obtained by using the stiffer agave marginata fibers, extracted by simple mechanical pressing of the leafs, without any successive treatment. For long fiber biocomposites, instead, the experimental analysis has shown that for a given fiber treatment, the use of the more deformable PLA allows a better exploiting of the fiber properties, i.e. it leads always to more eco-friendly biocomposites with higher mechanical strength. Moreover, the use of the agave marginata permits to obtain biocomposites with strength higher than the biocomposites reinforced with the common agave sisalana (sisal), with improvements until to about 50%; also, the use of fibers extracted by simple leafs pressing, allows the user to obtain high performance renewable biocomposites, characterized by high stiffness and strength comparable with that obtained by using mercerized fibers. Finally, the detailed analysis of the damage mechanisms, performed also by proper 2D and 3D micrographs, has permitted to implement accurate theoretical models that allows the user accurate predictions of the mechanical performance of biocomposites reinforced with short fibers or long fibers.

A novel aircraft energy absorption strut system with corrugated composite plate to improve crashworthiness

ABSTRACTA novel crashworthy aircraft strut system with corrugated composite plate and hinge joints is proposed and investigated. The hinge joints could transfer axial impact load to the corrugated composite plate (CCP). The best crashworthy CCP is guaranteed to enhance the energy absorption ability. To verify the impact dynamic performance, the progressive failure model is adopted. Both of the fibre and matrix failure modes are considered in the failure criterion. To give a better failure behaviour, the single shell and stacked shell approaches are researched and compared. Based on the validated model, the sensitivities of layer number, layer angle and cross-sectional size are given. Numerical results show that the proposed crashworthy fuselage structure could enhance the energy absorption ability. The intra-laminar and delinamination could be simulated by the stacked shell model. A similar energy absorption ability is demonstrated for the different CCPs with progressive failure behaviour. An unstable failure behaviour may be exhibited if the lateral stiffness and strength is not enough to support the impact process.

Research of the micromechanics of composite materials with polymer matrix failure under static loading using the acoustic emission method

Micromechanics of composite materials’ failure has been investigated under static loading by means of acoustic emission (AE) method. The results showed that for samples made of fiberglass with transverse fibers with respect to the applied load, the process of destruction both in the deformation parameters and in the parameters of total AE has two stages of damage accumulation. At the same time the parameter of total AE shows that the process of destruction begins 5–6% earlier than it is shown by the deformation parameter. Also the nature of the total AE change was analysed.

Model development for work of fracture of hybrid composites

ABSTRACTThe theoretical modeling for work of fracture of hybrid composites was investigated. Current available models for the work of fracture mostly consider single fiber composites and are not supported by experimental evidence. This is due to the complex nature of work of fracture, and presence of far too many variables for the experimental validation such as interaction of fibers with matrix and with each other. In this work, a model was developed for hybrid composites based on the modification of rule of mixtures, considering pull‐out, fracture, debonding, and stress redistribution for fibers along fracture for matrix. Later, the model was experimentally evaluated by GF‐hemp‐PP hybrid composites, and showed a better agreement in comparison to a prevalent model for work of fracture. Aside from matrix failure, the dominant failure mechanisms were fiber stress redistribution for long fiber composites and fiber pull out for short fiber composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44711.

A damage mechanics model for low-velocity impact damage analysis of composite laminates

ABSTRACTA method was developed to predict numerically the damage of composite laminates with multiple plies under low-velocity impact loading. The Puck criterion for 3D stress states was adopted to model the intralaminar damage including matrix cracking and fibre breakage, and to obtain the orientation of the fracture plane due to matrix failure. According to interlaminar delamination mechanism, a new delamination criterion was proposed. The influence of transverse and through-thickness normal stress, interlaminar shear stress and damage conditions of adjacent plies on delamination was considered. In order to predict the impact-induced damage of composite laminates with more plies quickly and efficiently, an approach, which can predict the specific damage of several plies in a single solid element, was proposed by interpolation on the strains of element integration points. Moreover, the proposed model can predict specific failure modes. A good agreement between the predicted delamination shapes and sizes and the experimental results shows correctness of the developed numerical method for predicting low-velocity impact damage on composite laminates.

Dynamic failure of basalt/epoxy laminates under blast—Experimental observation

The dynamic failures of flat and single-curved woven basalt/epoxy laminates subject to blast loading were investigated experimentally. Number of fiber layers ((0/90)9, (0/90)18 and (0/90)28) and radii of curvature (infinity, R= 500mm and R= 250mm) were considered for the laminates in the experiment. A four-cable ballistic pendulum system was utilized to measure the impulse from the blast loading applied to the specimens. The tests were primarily conducted to characterize the deformation and failure modes of the laminates subject to blast loading, followed by a series of postmortem macroscopic and microscopic examinations to analyze the failure mechanism of laminates. The results showed that the blast-resistance of basalt/epoxy laminates was greatly enhanced through increasing the thickness. By decreasing the radius of curvature, the deformation mechanism of laminates under blast impact changed from flexural modes to indentation modes. Optical and scanning-electron (SEM) micrographs showed that the macroscopic delamination was mainly caused by matrix failure and debonding between fiber and matrix. Meanwhile, extensive fiber breakage led to the further macroscopic fracture of the laminates.

Progressive failure simulation in laminated composites under fatigue loading by using discrete damage modeling

The discrete damage modeling method is extended for progressive failure analysis in laminated composites under fatigue loading. Discrete damage modeling uses the regularized extended finite element method for the simulation of matrix cracking at initially unknown locations and directions independent of the mesh orientation. A material history variable in each integration point is introduced and updated after each loading increment, corresponding to certain load amplitude and number of cycles. The accumulation of the material history variable is governed by Palmgren-Miner’s rule. Cohesive zones associated with mesh-independent cracks are inserted when the material history parameter reaches the value of 1. Cohesive zone model consistently describing crack initiation and propagation under fatigue loading without any assumption of initial crack size is proposed. The fatigue properties required for matrix failure prediction include shear and tensile S-N curves as well as Mode I and II Paris law parameters. Tensile fiber failure is assumed unaffected by fatigue. All input data required for model application are directly measured by ASTM tests except tensile fiber scaling parameter and compression fiber failure fracture toughness, which were taken from literature sources. The model contains no internal calibration parameters. Fatigue damage extent, stiffness degradation and residual tensile and compressive strength of IM7/977-3 laminates have been evaluated. Three different layups, [0/45/90/-45]2S, [30/60/90/-60/-30]2S and [60/0/-60]3S, were modeled and tested. The predictions captured most experimental trends and showed good agreement with X-ray CT damage assessment; however, significant further work is required to develop reliable methodology for quantitative composite durability prediction.

On the suitability of a Discrete Element Method to simulate cracks initiation and propagation in heterogeneous media

The present paper investigates the suitability of a Discrete Element Method (DEM) to simulate cracks initiation and propagation in heterogeneous media. We focus our studies on the cohesive hybrid-particulate model in which the continuous medium is modeled using a cohesive beam model. Previous works exhibited the ability of such an approach to simulate the mechanical behavior of continuous materials under various solicitations. The DEM is actually well-suited to take into account local heterogeneities and complex fracture phenomena occurring at very fine scales. In a first step, several tests are performed in the context of a 2D homogeneous medium in order to better quantify micro-macro scale transition effects related to the DEM on cracks initiation and propagation. Then, in a second step, unidirectional fibre-reinforced composites are modeled using 2D models taking into account a brittle matrix failure and interfacial debonding effects. Numerical tensile tests are set up for two main configurations: a single-fiber composite constituted of a single fiber embedded in an alumina matrix and the case of multi-fiber composites constituted of parallel fibers also embedded in an alumina matrix. The results exhibit the suitability of the DEM to yield suitable stress fields and crack patterns in the investigated heterogeneous media. Scale transition effects are noticeable in terms of stress fields but turn out to be relatively limited in the mechanism leading to cracks initiation and propagation. Furthermore, the competition between debonding and failure is also well captured whatever the fiber arrangement.

Rock mechanics of shear rupture in shale gas reservoirs

Hydraulic fracturing is an effective technology to improve shale gas wells production. The key problem in shale gas well hydraulic fracturing is opening hydraulic fractures to connect natural fractures and weak planes, in turn, creates the fracture network. The shear failure of matrix, natural fracture and weak plane are the main rock rupture models used in shale gas reservoirs. In this paper, firstly, a bottom hole pressure calculation model is built to show that large hole perforations and big pipeline diameters should be used to decrease perforation friction. Secondly, shale rock, natural fracture and weak plane shear failure models, based on rock mechanics theory, are presented. The calculated results show that horizontal differential principal stress has a significant influence on shear failure and the slippage of natural fracture and weak plane. A higher value of differential principal stress indicates a lower shale rock shear failure pressure, which brings benefits to weak plane slippage. Elastic modulus and Poisson's ratio all have effects on weak plane shear slippage. The influences of elastic modulus on shear slippage have a greater impact than Poisson's ratio. A reasonable dip angle of weak plane is also provided to enhance weak plane slippage. The practical-approach built in the paper does not require more rock mechanics parameters. The results have potential to enrich shale rock shear rupture theory and benefit fracture mesh system research.

Pull-out behavior of different fibers in geopolymer mortars: effects of alkaline solution concentration and curing

Reinforcing geopolymer materials with fibers can enhance tensile and flexural strengths and fracture toughness. The bond between fiber and geopolymer matrix is a critical factor that needs to be investigated to optimize the performance of the fiber reinforced composite. In this study, single fiber pull-out tests are conducted on steel and polypropylene fibers embedded in geopolymer matrices; in addition, OPC mortars are tested as control condition. The following parameters are investigated: fiber type (i.e. steel and polypropylene) and shape, concentration of alkali solution in the geopolymer matrix, and curing conditions. Bond-slip performance, failure modes, and slip resisting mechanisms of different matrices and fibers are compared and discussed. The fiber deformation ratio, a novel parameter, is introduced to quantitatively investigate the effect of fiber shape on the mortar performance. In case of steel fibers, the geopolymer-fiber composite performs better for lower fiber deformation ratios, where the full fiber pull-out mechanism can be exploited. For higher deformation ratios, the strong bearing forces developed, combined with the high adhesion strength of the geopolymer-steel fiber interface, lead to more brittle failure mechanisms, such as fiber breakage or matrix failure, as observed in end-deformed and length-deformed steel fibers, respectively.