Effect Of Ply Thickness Research Articles

Effects of ply thickness and interference-fit on the bearing strength of single-lap countersunk composite joints

The Countersunk composite joints are widely utilized in the load-bearing components of aerospace structures. However, hole-making causes local stress concentration, affecting joints’ bearing capacity. Therefore, a detailed experimental investigation was conducted on the strength under varying parameters including ply thickness, stacking sequence, and interference size in the single-lap countersunk composite joint. The installation force of the thin-ply joint was smaller than that of the thick-ply at the same interference. The initial damage of the thin-ply hole wall caused by the bolt-inserting process was less. The strengths of the two thick-ply joints improved by 10.12% and 9.09% (1.00% interference), respectively, compared with those of thick-ply neat-fit joints. The strengths of the two thin-ply joints increased by 18.18% and 21.78%, respectively, when the interference was increased to 1.44%. SEM imaging revealed that the damage extent of the thin-ply area is smaller with the same interference. Moreover, the fibre kinking angle of the 0° plies at the thick-ply section was more extensive, leading to fibre fracture. Using interference-fit technology to thin-ply laminates could improve the damage resistance of bolted joints and provide a novel idea for applying thin-ply laminates in thin-walled structural components.

Micro-mechanical analysis of the effect of ply thickness on curing micro-residual stresses in a carbon/epoxy composite laminate

During the manufacturing of thermoset-based carbon fiber reinforced polymers (CFRPs), micro-residual stresses are developed in the material, due a mismatch of the chemical-thermal-mechanical properties of the fibers and the polymer matrix. This ultimately leads to a reduction in the mechanical performance of the composite material. In this work, a representative volume element (RVE) of a composite sub-laminate with a 90° ply discretized at micro-scale level, stacked within homogenized plies, is proposed to study the micro-residual stresses and the ply mechanical response considering the in-situ effect caused by the constraining adjacent layers. A model framework is developed using finite elements in Abaqus®/Standard with user-subroutines. The retrieved micro-residual stresses in the 90° layer are generated by the dissimilar chemical shrinkage and thermal expansions, plus the additional residual meso-scale stresses caused by the constraining effect of adjacent plies. The residual stresses in the constrained condition can achieve almost 80 % of the ply failure load for the 0/90/0 sub-laminate, meaning lower sub-laminate strain to failure. Thick laminates retrieve more residual stresses in the transverse 90° layer and exhibit lower constrained strength and more unstable post-failure response, demonstrating that thin-ply laminates exhibit beneficial constraining effects during the curing process.

Effect of ply thickness on tensile and bending performances of carbon fiber reinforced thermoplastic unidirectional laminate

AbstractIt has been explored that thin ply would improve the mechanical performances of carbon fiber reinforced epoxy composites laminate. It has not been known whether there are similar effects on thermoplastic composites. In this article, carbon fiber reinforced polyamide 6 (CF/PA6) unidirectional fabrics with two thicknesses of 0.045 and 0.105 mm were used to fabricate the unidirectional laminate (UDL). Tensile and bending behaviors of the UDL were recorded with a digital image correlation (DIC) system, their damage macro‐morphologies were observed with high‐speed camera during loading. The fracture microstructures of the samples were observed with SEM and optic microscope. The results showed that the tensile and bending strengths of the samples with 0.045 mm thick are 79.8% and 24.8% higher than that with 0.105 mm thick. Thinning ply can inhibit the crack expanding, and the time from crack originating to fracture of the sample is larger than that of the thick ply one. Morphologies of the fractures revealed that the crack mainly expanded in the matrix along the loading direction for thin‐ply laminate, while the crack developed in the fiber cluster for thick‐ply laminate. The results also revealed that the mechanical properties of the laminate can be improved obviously by using thin‐ply.

Fatigue behaviour of thermoplastic glass/polypropylene composite cross-ply laminates: An experimental study with in-situ damage observations and numerical validation

In-situ fatigue damage quantification in non-transparent thermoplastic composites is cumbersome, however, it provides vital information for developing reliable fatigue damage models. This study investigates the tension-tension fatigue behaviour of Glass/Polypropylene laminates. The mechanical response and damage progression in [02/902]s, [902/02]s and [0/90]2s lay-ups are characterized over 500 000 cycles. Microscopic analyses and through-the-width sectioning are considered to compare detected edge cracks to the internal damage state. Effects of ply thickness and location as well as fabrication-induced morphology on fatigue response are investigated. Normalized stiffness reduction and Poisson's ratio evolution in function of measured crack density are compared with numerical results showing good agreements.

Experimental and numerical damage characterization of glass/polypropylene multidirectional laminates under quasi-static loading condition

Polypropylene (PP) as a commodity thermoplastic, when reinforced with continuous glass fibers provides cost-effective composites. However, the low glass transition temperature and fabrication induced semi-crystalline morphology of PP set challenges for the experimental characterization especially when the quantification of damage mechanisms in multidirectional laminates is concerned. This paper is aimed at performing in-situ experimental observation, quantification and theoretical modeling of damage mechanisms in glass fiber reinforced polypropylene multidirectional laminates subjected to uniaxial quasi-static loading conditions. Two Stereo Digital Image Correlation systems (3D-DIC) are applied to measure the full-field strains and quantify the extent of damage mechanisms from the specimen's edge. The effects of ply thickness, off-axis ply orientation and location on damage initiation and growth are studied by testing different lay-ups. To validate the experimental measurements, a recent physics-based modeling technique is implemented that can predict the evolution of damage modes as well as their effects on the laminate properties. Good agreements are observed between the experimental measurements and simulation results which verify the accuracy of both analyses.

Effect of ply-thickness on the impact response of interleaved thin-ply non-crimp-fabric laminates: X-ray tomography investigation

Composite laminates subjected to out-of-plane loads develop damage modes that compromise the load-carrying capacity, as in the fuselage skin of an aircraft. Therefore, we aimed to improve the damage resistance and tolerance of carbon/epoxy thin non-crimp fabric (NCF) laminates by (a) reducing the ‘conventional’ ply-thickness (i.e. comparing thick- and thin-plies) (b) interleaving co-polyamide (CoPA) non-woven veils at the interlaminar regions of thin-ply systems. We devised an impact, compression after impact (CAI) and quasi-static indentation (QSI) experimental campaign, where computerized X-ray tomography provided detailed 3D insight on how matrix cracking, delamination and fibre failure interacted during impact and QSI. Thin-ply laminates reduce the amount of matrix cracking and delamination at the cost of premature fibre breakage, therefore decreasing the damage tolerance. Interleaving CoPA veils arrests delamination onset and improves the residual strength with respect to the thin-ply baseline.   [1] S.Sihn, R.Y. Kim, K.Kawabe and S.W. Tsai, Compos. Sci. Technol., vol. 67, no. 6, pp. 996–1008 (2007). https://doi.org/10.1016/j.compscitech.2006.06.008[2] V.A.Ramirez, P.J.Hogg and W.W.Sampson, Compos. Sci. Technol., vol. 110, pp. 103–110 (2015). https://doi.org/10.1016/j.compscitech.2015.01.016

Effects of ply thickness and 0°-layer ratio on failure mechanism of open-hole and filled-hole tensile tests of thin-ply composite laminates

Thin-ply technology provides increasing possibilities for composite structures, expanding the design space for ply thickness and laminate layup. This study examines the effects of ply thickness and the 0°-layer ratio on the strength and damage behavior of thin-ply composite laminates. We conducted open-hole tensile (OHT) and filled-hole tensile (FHT) tests on laminates with various ply thicknesses and 0°-layer ratios, observing the internal damage via X-ray radiography. It was confirmed that the damage retardation effect of the thin-ply reduced both the OHT and FHT strengths. Furthermore, the results revealed that laminates with a higher 0°-layer ratio have higher strengths and the failure of laminates with a higher 0°-layer ratio transitioned to splitting-dominated failure. Finite element analysis of the fastener restraint indicated that the fastener restraint alleviated the stress concentration in the 0°-layer around the hole, potentially leading to a difference in the OHT and FHT strengths.

Damage-mechanics mesoscale modeling of composite laminates considering diffuse and discrete ply damages: Effects of ply thickness

Thin-ply composites are gaining significant attention owing to their high damage resistance and their ability to enhance design flexibility in a composite structure. Herein we proposed a damage mechanics model considering diffuse and discrete ply damages in thin-ply composite laminates for mesoscale simulation. To investigate the effects of ply thickness on damage evolution, loading and unloading tests were conducted using cross-ply, angle-ply, and quasi-isotropic laminates with several ply thicknesses. The damage evolution was quantitatively evaluated using continuum damage mechanics. The evolution laws of damage were formulated considering the ply thickness dependency. A damage simulation using the proposed model demonstrated that the retardation and suppression effects of thin-ply were captured by the proposed model. The simulation results also validated that the stiffness degradation and failure strength of laminates with three different ply thicknesses were predicted with good accuracy by considering both diffuse and discrete damages.

Modelling damage in multidirectional laminates subjected to multi-axial loading: Ply thickness effects and model assessment

A meso-scale damage modelling framework for composite laminates is assessed against multi-axial experimental data obtained from specimens subjected to combined tension/compression and shear loading using a modified Arcan fixture (MAF). Two laminates made from unidirectional carbon fibre-reinforced epoxy plies of different thickness are considered to investigate the ply thickness effect on the damage behaviour of modified open-hole specimens. Intra-laminar damage is predicted using a continuum damage model combined with a fibre-aligned mesh, while cohesive zones are used to account for inter-laminar damage. The multi-axial open-hole specimen strength is predicted with a mean relative error of 15% across all the investigated lay-up and loading configurations, while the predicted damage maps show encouraging agreement with full-field measurements obtained using digital image correlation (DIC). However, the assessment reveals limitations of the assumption of small strains and rotations in the formulation of the model that lead to spurious model behaviour for shear-dominated damage modes.

Effect of ply thickness and orientation on fatigue delamination of laminated composites using cohesive zone model

Fibre reinforced polymer (FRP) laminate composites display anisotropic behaviour on account of alternating phases of fibre and matrix in suitable weight fractions. Recent research has focussed on structural analysis of these composites pertaining to various strengths like tensile, compressive, flexural etc. This paper attempts to estimate delamination failure of these laminated composites on basis of “cohesive zone modelling”. Modelling of laminated composites with an open interface area is done in Ansys® Workbench (ACP) as per required weight/volume fraction. The effect of fibre orientation and thickness of lamina is analyzed on force reaction and deformation due to delamination failure in epoxy carbon laminated composite. The longitudinal plies with fibre orientation of 0° are seen to have a very good structural response against delaminating loads as compared to cross ply laminates with a ply fibre orientation of 90°. Also, the deformation corresponding to maximum force reaction in delamination of composite laminate is not affected by increasing or decreasing number of plies.

Experimental and numerical investigation of ply size effects of steel foil reinforced composites

The effect of ply thickness on the notch sensitivity and bearing properties on carbon fibre reinforced polymer composites and their hybrid laminates with steel foils were studied. Laminates with ply thicknesses of 0.3 mm and 0.03 mm comprising of CFRP and hybrid laminates were manufactured and characterized using tension, open hole tension and double lap bearing tests. A 25% ply substitution was found to double the bearing load with extensive plastic deformation in the joint while maintaining high stress and maintaining constant cross-sectional thickness in the laminate. With a good agreement between the finite element predicted values and failure behaviour, the damage initiation and progression behaviour could be observed experimentally. We numerically captured (i) rapid failure of 0° plies in the thin ply CFRP hybrid and (ii) continuous delamination with significant plastic deformation for the thick ply CFRP hybrid. The numerical results significantly reduce future experimental work when designing hybrid laminates and could allow the laminate lay-up to be tailored for load cases. Both the experiments and numerical models underline the distinct size effects occurring with respect to the ply thicknesses when hybridising a very ductile metal with a brittle yet strong composite material.

Effects of ply thickness and architecture on the strength of composite sub-structures

This work presents an experimental study on the effect of: ply thickness; ply-level hybridization; and type of ply architecture on the damage mechanisms that dominate failure and strength of multidirectional laminates. Nine effectively equivalent 0°dominated multidirectional laminates made from unidirectional carbon fibre tapes (UD), non-crimp fabrics (NCF) and spread-tow fabrics (STF) with thickness ranging from 67 μm to 268 μm are defined and tested under plain strength tension and compression, open-hole tension and compression, filled-hole compression and tension-bearing. An overall improvement of the strengths with the decrease of ply thickness is observed for the UD and NCF laminates, but nearly no effect of ply thickness is detected for STF laminates. Ply-level hybridization, where thin off-axis plies are combined with thicker grade 0°plies contributes to the improvement of the laminate response, especially when thicker 0°plies are used. However, hybrid-ply laminates are more sensitive to the loading direction and are, therefore, only suitable in structures where highly oriented loadings are expected. Fabric-based laminates do not show especial susceptibility to early failure compared to UD laminates. Their use can, therefore, be an economical solution that does not compromise the structural response, ensuring the weight benefits of composite materials at lower processing costs.

Evaluation of the in-situ damage and strength properties of thin-ply CFRP laminates by micro-scale finite element analysis

A micro-scale simulation scheme is developed in this study to evaluate in-situ damage and strength properties of CFRP laminates with various ply thicknesses. To capture both the initiation and propagation of transverse cracks, the microscopic random fiber configuration and the constraint effect from neighboring plies should be carefully considered. This study considers the representative volume element (RVE), consisting of the ‘inhomogeneous’ ply in which the solid elements individually modeled the fiber and matrix, and the ‘homogenized’ plies, homogeneously modeled by the shell elements. Matrix damage and debonding between fiber and matrix were modeled in the inhomogeneous ply to reproduce transverse crack propagation. Furthermore, in the RVE simulation, the key degree of freedom method was incorporated to evaluate the in-situ properties of each ply effectively. The validity of the proposed tool was examined by comparing the predicted cracking behavior with the results of unidirectional tensile tests on cross-ply laminates having different 90º ply thicknesses. The effects of ply thickness on the in-situ damage and strength properties of cross-ply laminates were numerically investigated, and the continuum damage mechanics model for thin-ply laminate was proposed based on the numerical results.

Closed-form formulae for prediction of homogenized ply-properties and laminate thermo-elastic constants in symmetric laminates containing ply cracks in multiple orientations

The aim of this study is to develop a novel model for predicting homogenized properties of a cracked ply in a symmetric laminate using the crack density and thickness of the ply. In doing so, the periodic boundary conditions are applied to the RVEs in a three-dimensional finite element model to calculate the thermo-elastic constants of symmetric laminates containing uniformly and non-uniformly spaced cracks under in-plane and out-of-plane loading conditions. On the other hand, homogenized material properties of the cracked ply are explicitly determined by averaging procedures for both stress and strain components. Using computed results and nonlinear least square fitting, an analytical formula is proposed to describe the mechanical properties of the cracked ply, which takes into account the effect of ply thickness, material properties and crack density. Different cracked laminates are tested to indicate the potential of the developed method in analyzing complex crack geometries in multiple orientations. Moreover, the applicability and accuracy of the suggested model are verified by comparing the results with those of experiments, variational approach and finite element method.

Effect of Ply Thickness on Impact Damage Modes of Thin-ply CFRP Laminates

Effect of Ply Thickness on Impact Damage Modes of Thin-ply CFRP Laminates

Modelling the size and strength benefits of optimised step/scarf joints and repairs in composite structures

Adhesive bonding offers a better load transfer between adherends, for the assembly and repair of composite structures, compared with mechanical fastening methods. One drawback of adhesive bonded repairs is the considerable amount of material which must be removed, around the damaged region, to ensure adequate load transfer. Optimised geometries that account for the highly-orthotropic properties of individual composite plies are investigated, including a novel ‘fibre-oriented’ scarf approach that is inspired by an existing fibre-oriented step design. These methods aim to reduce the length of joint and repair bonding regions by at least 36%, compared with conventional step and scarf geometries of a similar strength.Size-reduction benefits are predicted using a MATLAB tool that is applicable for any composite laminate, and parametric analysis used to assess the effect of ply thickness, the number of plies, stacking sequence and taper angle.Cohesive Zone Models of joints and repairs with conventional and fibre-oriented designs are used to predict and compare the ultimate strength of each configuration. The existing fibre-oriented step design appears to show no benefit over a conventional step design. However, the novel fibre-oriented scarf approach results in a 33–40% reduction in the size of the bonding region compared to a conventional scarf design with similar strength. Analysis further indicates a 17–22% increase in ultimate strength for joints and repairs with the same bonding region size that employ the optimised fibre-oriented scarf design.

Effect of ply thickness and ply level hybridization on the compression after impact strength of thin laminates

There is a lack of research available on how thin laminates respond to impact and post impact loads, even though thin structures are used in present-day aircrafts. This experimental paper employs thick, standard and thin uni-directional plies to investigate the effect ply thickness has on thin laminates on their impact and compression after impact (CAI) response. Further, we propose two hybrid laminates where thick or standard plies are mixed with thin plies, respectively, in an effort to improve the CAI strength of thin laminates. Results reveal that, contrary to thick laminates, thin laminates made of only thin plies exhibit extensive fibre failure, leading to a considerably reduced CAI strength. Moreover, the hybrid laminate where thick 0° plies are mixed with thin plies improves the CAI strength by 40% over the thin ply baseline laminate. Thus, hybridization with thin laminates appears to be an economic prospective in terms of improving damage tolerance.

Effect of Ply Thickness and Matrix Resin on Flexural Properties of Thin Ply CFRP Laminates

Effect of Ply Thickness and Matrix Resin on Flexural Properties of Thin Ply CFRP Laminates

Simulation of the Mechanical Response of Thin-Ply Composites: From Computational Micro-Mechanics to Structural Analysis

This paper provides an overview of the current approaches to predict damage and failure of composite laminates at the micro-(constituent), meso-(ply), and macro-(structural) levels, and their application to understand the underlying physical phenomena that govern the mechanical response of thin-ply composites. In this context, computational micro-mechanics is used in the analysis of ply thickness effects, with focus on the prediction of in-situ strengths. At the mesoscale, to account for ply thickness effects, theoretical results are presented related with the implementation of failure criteria that account for the in-situ strengths. Finally, at the structural level, analytical and computational fracture approaches are proposed to predict the strength of composite structures made of thin plies. While computational mechanics models at the lower (micro- and meso-) length-scales already show a sufficient level of maturity, the strength prediction of thin-ply composite structures subjected to complex loading scenarios is still a challenge. The former (micro- and meso-models) provide already interesting bases for in-silico material design and virtual testing procedures, with most of current and future research focused on reducing the computational cost of such strategies. In the latter (structural level), analytical Finite Fracture Mechanics models—when closed-form solutions can be used, or the phase field approach to brittle fracture seem to be the most promising techniques to predict structural failure of thin-ply composite structures.

Towards aerospace grade thin-ply composites: Effect of ply thickness, fibre, matrix and interlayer toughening on strength and damage tolerance

Thin-ply composites represent a promising approach to further improve the performance of carbon fibre composite structures thanks to their ability to delay the onset of matrix cracking and delamination up to the point of fibre dominated failure. However, this increased strength comes with a more brittle failure response which raises concerns on damage tolerance. Thus a careful material optimization is needed to address this trade-off. In this work, eight different formulations of thin-ply composites ranging from low modulus to high modulus carbon fibres are evaluated to understand the effects of the fibre and matrix constituents on the onset of damage and strength in unnotched tensile (UNT) tests of quasi isotropic laminates for ply thicknesses between 300 and 30 microns. The obtained experimental data are combined in master curve diagrams for simplified material selection process. It is observed that certain thin-ply composites with a ply thickness t < 134 μm can reach UNT strength corresponding to or approaching the ultimate strain of the fibres as well as UNT stress at onset of damage as high as 92% of the latter. Based on this knowledge, a novel aerospace grade toughened thin-ply composite system is developed which can reach a quasi-isotropic UNT strength above 1 GPa (>95% of the fibre strain). The newly developed composite is further optimized to improve damage tolerance by toughening the resin and selected interfaces. The effect of those modifications on damage tolerance are evaluated through compression strength after impact (CAI) tests and open hole tensile tests (OHT). It is found that an optimized interlayer toughened thin-ply composite based on 68 microns plies of intermediate modulus fibre can reach both outstanding strength properties with comparable or better CAI and OHT strength compared to current aerospace grade composites.