Continuous Filament Mat Layers Research Articles

Two-dimensional fatigue debonding in GFRP/balsa sandwich panels

An experimental investigation of the two-dimensional (2D) debonding behavior of GFRP/balsa sandwich panels with embedded circular disbonds at the face sheet/core interface subjected to out-of-plane fatigue loads was conducted. The constant amplitude fatigue experiments were performed under load control and different R-ratios. Two face sheet configurations were investigated: a pure woven ply layup (SPA) and a combination of continuous filament mat (CFM) and woven plies (SPB). For the latter, the CFM layers (which are prone to develop fiber-bridging) were placed above and below the woven plies. In contrast to one-dimensional (1D) beam-like fatigue fracture experiments, decreasing crack growth rates and stable crack propagation were achieved under load control as a result of the 2D growth of the disbonds. The fatigue fracture performance obtained under R = 0.1 for the SPB configuration was less efficient than for the SPA configuration owing to the fiber-bridging crushing associated with high fatigue amplitudes. Under higher R-ratios, a reduced amount of fiber-bridging crushing was observed, leading to a corresponding crack arrest and much longer life. The load-displacement hysteresis loops exhibited an increase in the cyclic stiffness within each fatigue cycle throughout the experiments. This stiffening was mainly caused by in-plane stretching stresses that developed in the debonded part of the face sheets whose magnitude and evolution were evaluated based on the in-plane strains monitored during the experiments.

Two-dimensional quasi-static debonding in GFRP/balsa sandwich panels

The two-dimensional (2D) debonding behavior of GFRP/balsa sandwich panels presenting embedded circular disbonds at the face sheet/core interface was experimentally investigated. The bottom of the panels was fixed and out-of-plane quasi-static tensile loads were applied to open the crack. Two different face sheet configurations were studied. The first consisted of a pure woven ply layup while in the second, layers of continuous filament mat (CFM) were placed above and below the woven plies. The inclusion of CFM layers at the face sheet/core interface resulted in an enhanced fracture behavior exhibiting improved load-bearing performance and significant crack arrest. In-plane stretching strains appeared in the deformed face sheets due to the 2D embedded nature of the crack. The magnitude of the stretching was quantified based on the in-plane strains monitored throughout the experiments. As a result of the stretching, shear mode contributions were activated, causing the initiation of secondary cracks in the pure woven configuration and crack migration in the woven/CFM configuration. The migration in the latter occurred from the face sheet/core interface to the woven/CFM interface, generating dense fiber-bridging. This intensified fiber-bridging was found to be the main cause of the already-mentioned improvement in the fracture performance of the woven/CFM configuration.

Evaluation of continuous filament mat influence on the bending behaviour of GFRP pultruded material via Electronic Speckle Pattern Interferometry

Pultrusion is a process allowing the production of unidirectional (roving) fibre-reinforced polymer (FRP) structural elements with constant cross section. Recently, also civil engineers focused their attention on pultruded composite materials as alternative to traditional ones (e.g., concrete, steel). Furthermore, to improve the transverse strength and stiffness with respect to the fibres direction, continuous filament mat (CFM) is often placed within the stacking sequence. The CFM influence on the global mechanical behaviour is not considered by appropriate actual international standards. In this paper, the influence of the CFM layers on the mechanical behaviour of glass fibres pultruded composite material is investigated. In particular, the bending behaviour is analyzed by performing a four-point bending test on specimens extracted from an H-shape member. The experimental analysis was carried out via Electronic Speckle-Pattern Interferometry (ESPI) (handled by phase-stepping technique) to obtain a full-field displacement map and to numerically achieve the longitudinal strains. By imposing the equilibrium conditions and assuming the compression and tensile roving Young's moduli as constant, the CFM Young's moduli are determined. Finally, the mean stress acting on the material is obtained showing that CFM layers have to be considered to correctly evaluate the maximum stress and to optimize the design phase.

Modelling the pultrusion process of an industrial L-shaped composite profile

A numerical process simulation tool is developed for the pultrusion of an industrial L-shaped profile. The composite contains the combination of uni-directional (UD) roving and continuous filament mat (CFM) layers impregnated by a polyester resin system specifically prepared for the process. The chemo-rheology and elastic behavior of the resin are obtained by applying a differential scanning calorimetry (DSC) and a dynamic mechanical analyser (DMA), respectively. The process induced stresses and shape distortions are predicted in a 2D quasi-static mechanical analysis. The numerical process model predicts the residual spring-in angle which is found to be close to the one measured from the real pultruded L-shaped products. The residual spring-in angle is further analyzed using the developed simulation tool for different pulling rates. Through-thickness stress variations are found to prevail inside the part such that the UD and CFM layers have different stress levels at the end of the process. The predicted stress pattern is verified by performing a stress calculation using the classical laminate theory (CLT).

A multi-scale framework for layered composites with thermo-rheologically complex behaviors

A multi-scale modeling framework is proposed for generating the effective nonlinear thermo-viscoelastic responses of multi-layered and thick-section composites. The modeling framework is demonstrated for multi-layered composite systems consisting of two alternating fiber-reinforced polymeric layers: unidirectional fiber (roving) and continuous filament mat (CFM). The viscoelastic behavior of the polymeric matrix is considered as stress-dependent and thermo-rheologically complex. Two simplified 3D micromechanical models with periodic boundary conditions are formulated for the roving and CFM. In addition, a sublaminate model is developed for the 3D effective homogenized through-thickness response of the roving and CFM layers. This framework provides an effective time-dependent material response while simultaneously recognizing the corresponding deformation at the microconstituents under overall thermo-mechanical loadings at the macro-level. Stress correction algorithms are developed at each scale to enhance computational efficiency and accuracy. Short-term (30 min) creep tests on off-axis multi-layered specimens are also conducted under combined stresses and temperatures to calibrate in-situ fiber and matrix properties and verify the predictions of the multi-scale framework. A time-shifting method is applied to create long-term material behaviors from the available short-term creep data. Verification of the multi-scale material framework is also done for the overall long-term responses. Finally, long-term structural analyses under thermo-mechanical loadings are conducted by integrating the multi-scale material framework to finite element (FE) structural analyses.

Multiscale Nonlinear Framework for the Long-Term Behavior of Layered Composite Structures

This paper presents an integrated micromechanical–structural framework for local–global nonlinear and time-dependent analysis of fiber reinforced polymer (FPR)composite materials and structures. The proposed modeling approach involves nested multiscale micromodels for unidirectional and continuous filament mat (CFM) layers. In addition, a sublaminate model is used to provide a 3-D effective anisotropic and continuum response to represent the nonlinear viscoelastic behavior of a through-thickness periodical multilayered material system. The 3-D multiscale material framework is integrated with a displacement-based finite-element code to perform structural analyses. The time-dependent responses in the unidirectional and CFM layers are exclusively attributed to their matrix constituents. The Schapery nonlinear viscoelastic model is used with a newly developed recursive–iterative integration method applied for the polymeric matrix. The fiber medium is linear and transversely isotropic. The in situ long-term response of the matrix constituents is calibrated and verified using long-term creep coupon tests. Good prediction ability is shown by the proposed framework for the overall viscoelastic behavior of the layered material. Material and geometric nonlinearities of I-shape thick composite columns, having vinylester resin reinforced with E-glass unidirectional (roving) and CFM layers, are studied to illustrate the capability of the multiscale material-structural framework. Nonlinear elastic behavior and creep collapse analyses of the I-shaped column are performed. The recursive–iterative and stress correction algorithms, which are implemented and executed simultaneously at each material scale, enhance equilibrium and avoid misleading convergent states.

Mode-I fracture toughness testing of thick section FRP composites using the ESE(T) specimen

Experimental and numerical analyses are performed to determine the translayer mode-I fracture toughness of a thick-section fiber reinforced polymeric composite using the eccentrically loaded, single-edge-notch tension, ESE(T) specimen. Finite element analyses using the virtual crack closure technique were performed to assess the effect of material orthotropy on the mode-I stress intensity factors in the ESE(T) specimen. The stress intensity factors for the proposed ESE(T) geometry, are calculated as a function of the material orthotropic parameters. The formula is validated for a class of thick composite materials. The thick composite tested in this study is a pultruded composite material that consists of roving and continuous filament mat layers with E-glass fiber and polyester matrix materials. Data reduction from the fracture tests was performed using two methods based on existing metallic and composite ASTM [ASTM E 1922, Standard Test Method for Translaminar Fracture Toughness of Laminated Polymer Matrix Composites, Annual Book of ASTM Standards, 1997; ASTM E 399, Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials, Annual Book of ASTM Standards, 1997] fracture testing standards. Criteria for assessing test validity and for determining the critical load used in calculating the fracture toughness were examined. Crack growth measurements were performed to determine the amount of stable crack growth before reaching critical load. The load versus notch mouth opening displacement, for different crack length to width ratios is affected by material orthotropy, nonlinearity, and stable crack propagation. The mode-I translayer fracture toughness and response during crack growth is reported for ESE(T) specimen with roving layers oriented both, transverse and parallel to the loading direction.

In-plane shear testing of thick-section pultruded FRP composites using a modified Arcan fixture

A modified Arcan fixture with butterfly specimen geometry is designed to measure the in-plane shear response of thick-section pultruded FRP composites. The objective of the proposed testing method is to determine both the material shear stiffness and its non-linear stress–strain response up to ultimate stress. The tested pultruded specimens include two alternating layers in the form of a unidirectional glass roving and continuous filament mat layers. Finite element models for the butterfly specimen are generated to examine the effects of the notch radius and material orthotropy on the uniformity and distribution of stresses in the gage area. Butterfly geometry with a blunted notch and roving orientation parallel to the applied load is found to have a uniform shear stress in the gage section. The axial-shear response is measured under different biaxial stress states by varying the angle of the applied load. The tested non-linear shear stress–strain responses compare favorably to results previously obtained from off-axis compression tests used to calibrate a multi-axial constitutive model for this material. Results from strain gage and Infrared thermography measurements provide confirmation for the effectiveness of the fixture and the specimen geometry.

A micromechanical constitutive framework for the nonlinear viscoelastic behavior of pultruded composite materials

This study introduces a new three-dimensional (3D) micromechanical modeling approach for the nonlinear viscoelastic behavior of pultruded composites. The studied pultruded composite system consists of vinylester matrix reinforced with E-glass roving and continuous filament mat (CFM) layers. Micromechanical models are introduced for the roving and CFM layers each having a unit-cell with four fiber and matrix subcells. In addition, a sublaminate model is used to provide for a nonlinear equivalent continuum of a layered medium with alternating roving and CFM layers. The roving layer consists of unidirectional fibers embedded in the matrix; it is idealized as doubly periodic array of fiber with square cross-sections. The CFM layer consists of relatively long, swirl, and randomly oriented filaments. This system is idealized using a weighted-average response of two simplified micromodels with fiber and matrix dominated responses. A new iterative procedure is introduced along with a recursive integration method for the Schapery nonlinear viscoelastic model used for the isotropic matrix subcells of the two micromodels. The fiber medium is considered as transversely isotropic and linear elastic. Incremental micromechanical formulations of the above three micromodels are geared towards the time integration scheme in the matrix phase. New iterative numerical algorithms with predictor–corrector type steps are derived and implemented for the micromodels in order to satisfy the fiber and matrix viscoelastic constitutive relations along with the micromechanical equations in the form of traction continuity and strain compatibility between the subcells. Experimental creep tests are performed with coupons cut from E-glass/vinylester pultruded plate to calibrate and predict the nonlinear viscoelastic response. Three sets of pultruded specimens having off-axis angles: 0°, 45°, and 90° were tested at room temperature under different applied compression stress levels.

Nonlinear behavior of pultruded FRP composites

Abstract Coupon tests are investigated and used to calibrate three-dimensional (3D) micromechanical models and to verify their prediction for the nonlinear elastic behavior of pultruded fiber reinforced plastic composites. The tested composite material system is made from E-glass/vinylester pultruded composite plate with both glass roving and continuous filament mat (CFM) layers. Tension, compression, and shear tests were performed, using off-axis coupons cut with different roving reinforcement orientations. The overall linear elastic properties are identified along with the nonlinear stress–strain behavior under in-plane multi-axial tension and compression loading. The tests were carried out for coupons with off-axis angles: 0, 15, 30,45, 60, and 90°, where each test was repeated three to five times. Finite element analyses are used to simulate the off-axis tests and examine the effects of coupon geometry, end-clamping condition, and off-axis orientation, on the spatial distribution of the axial strains at the center of the coupons. Lower initial elastic modulus and a softer nonlinear stress–strain responses were consistently observed in the tension tests compared to those in compression, for all off-axis (roving) orientations. The nonlinear behavior can be attributed to the relatively low overall fiber volume fractions (FVFs) in pultruded composites and the existence of manufacturing defects, such as voids and microcracks. It is also shown that the end-clamping effects for the tested geometry are relatively small at the center and allow extracting the nonlinear stress–strain response of the anisotropic material. The analytical part of this study includes two (3D) micromechanical models for the roving and CFM layers. Shear tests are used to calibrate the in situ nonlinear elastic properties of the matrix. Good prediction ability is shown by the proposed micromodels in capturing the stress–strain behavior in the off-axis tests.