Cross Ply Research Articles

Experimental and numerical studies of the open hole tensile strength of drilled‐ and molded‐hole unidirectional laminates

AbstractIn this paper, we created a new molded‐hole process to manufacture holes instead of drilling in unidirectional (UD) carbon fiber reinforced plastics avoid fiber discontinuity. Several open hole tensile (OHT) tests were carried out on specimens with Cross ply (CP) and Quasi‐isotropic (QI) stacking sequence with drilled holes and molded holes. At the same time, we established finite element analysis (FEA) models of drilled‐hole laminate and molded‐hole laminate to characterize the stress field around the hole. The numerical analysis results reveals that the max principal strain distribution around the molded hole has much higher coincidence with fiber axial direction than that in drilled‐hole laminate. Moreover, the OHT strength results of QI specimens indicate that the plates with molded holes were more 50% higher than those with drilled holes. This process will provide reference for molded UD CFRP structure holes. And the numerical model can help engineering designer make OHT strength prediction.Highlights A new molded‐hole process for unidirectional prepreg was created. Molded‐hole specimens have higher open hole tensile strength than drilled‐hole pieces. The simulation results were close to the experimental results.

Inter-laminar Strength of NITE-SiC/SiC Composites with Various Fiber Reinforcing Architecture

The mechanical performance of SiC/SiC composites is significantly influenced by the architecture of fiber reinforcement. Among the various fabrication methods, the nano-powder infiltration transition/eutectic (NITE) process is a promising technique that is capable of achieving a dense and stoichiometric SiC matrix. The reinforcement architecture, such as cross-ply (CP) or woven prepreg (WP), is determined during the preform stage of the NITE process, which is crucial in determining the mechanical properties of SiC/SiC composites. In this study, the tensile test and double notch shear (DNS) test were conducted using NITE-SiC/SiC composites to investigate the effect of the fiber reinforcing architecture on the fracture mechanism of SiC/SiC composites. The tensile strength and maximum shear strength of both CP and WP specimens were nearly identical. However, other mechanical properties, particularly those of CP specimens, exhibited significant variability. A comparison of fracture surfaces and load-displacement curve analyses from the DNS tests revealed that the cross points of the longitudinal or transverse fibers act as obstacles to both deformation and crack propagation. These obstacles were found to be more densely distributed in WP specimens than in CP specimens. The variability observed in the mechanical properties of CP specimens is likely due to size effects caused by the sparser distribution of these obstacles compared to the WP specimens.

Comparison of Radial Ply and Cross Ply Tire in Terms of Achieved Rolling Resistance and Soil Compaction in a Soil Test Channel

Many literature sources state that radial ply tires achieve lower rolling resistance values than cross ply tires. From a certain point of view, radial ply tires are gentler on the ground than cross ply tires. The effort was therefore to experimentally verify this statement for two radial ply and cross ply tires similar in shape and size. The work deals with the diagnostics of rolling resistance levels achieved by radial ply and cross ply tires on selected forest soil under the laboratory conditions of a soil test channel. BKT 210/95 R16 Agrimax RT 855 and Özka 7.50-16 8PR KNK 50 were chosen as radial ply and cross ply tires, respectively, and had approximately the same dimensions. The soil in the soil test channel can be characterized as a loamy sand with an average moisture content of 30% and an initial bulk density of 1445.07 kg·m−3. Another monitored parameter was the diagnostics of changes in soil density caused by tire movement in order to assess the degree of soil compaction. From the results of the work, it follows that there is no statistically significant difference between radial ply and cross ply tires in terms of the achieved levels of rolling resistance on the soil. The observed tires also caused intense compaction of the soil in the soil test channel, especially at higher tire pressures and higher vertical loads. The analysis of the results also shows that changes in tire pressure in both tires cause more energy loss and soil compaction than changes in the vertical load.

Effect of the level of anisotropy on the macroscopic failure of notched thin-ply laminates

This work presents a detailed experimental study conducted for a range of different lay-ups using thin-ply carbon fiber reinforced polymer (CFRP) laminates. The selection of the laminates was performed relying on their level of anisotropy. The laminates vary from a quasi-isotropic (QI) laminate, which is weakly anisotropic, to a cross-ply (CP) laminate, which is strongly anisotropic. The laminates were tested in on-axis and off-axis open hole tension (OHT). The main objective was to observe the effect of the level of anisotropy of the laminate on the macroscopic failure and observed failure patterns. It is shown that, contrary to most existing observations so far, depending on the lay-up and consequently its level of anisotropy, open-hole, quasi-homogeneous thin-ply laminates do not necessarily exhibit a fiber dominated failure mode, but could develop sub-critical damage mechanisms in a large extent prior to ultimate failure, reminiscent of what is observed for standard-ply CFRP laminates.

Strength Optimisation of Hybrid Bolted/Bonded Composite Joints Based on Finite Element Analysis.

A finite element analysis (FEA) was conducted to examine the behaviour of single-lap quasi-isotropic (QI) and cross-ply (CP) hybrid bolted/bonded (HBB) configurations subjected to tensile shear loading. Several critical design factors influencing the composite joint strength, failure conditions, and load-sharing mechanisms that would optimise the joining performance were assessed. The study of the stress concentration around the holes and along the adhesive layer highlights the fact that the HBB joints benefit from significantly lower stresses compared to only bolted joints, especially for CP configurations. The simulation results confirmed the redundancy of the middle bolt in a three-bolt HBB joint. The stiffness and plastic behaviour of the adhesive were found to be important factors that define the transition of the behaviour of the joint from a bolted type, where load sharing is predominant, to a bonded joint. The load-sharing potential, known as an indicator of the joint's performance, is improved by reducing the overlap length, using a low-stiffness, high-plasticity adhesive, and using thicker laminates in the QI layup configuration. Enhancing both the ratio of the edge distance to the hole diameter and washer size proves advantageous in reducing stresses within the adhesive layer, thereby improving the joint strength.

Effects of Carbon Fiber Reinforced Plastic Laminate on the Strength of Adhesive Joints

Carbon fiber composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. The use of carbon fiber reinforced plastics (CFRP) for the repair of steel structure is also advantageous, for example to avoid welding or bolting which can weaken structure and add fire risk. This study is based on adhesive bonded double lap shear (DLS) joints where the inner adherend is steel and the outer adherend is cross plies unidirectional (UD) CFRP. Overlap lengths of 25-125 mm with CFRP thickness of 3 and 6 mm were considered and two different orientation angles [0o,90o] and [90o,0o] laminates have been considered. The overall objectives of the paper are to investigate the delamination of the CFRP laminates within the DLS joints under quasi-static loading and both experimental and numerical techniques were used. The numerical study was based on 2-D model using strength-limit method, taking into consideration the UD properties of the plies and the resin layers separating these. As a result, it was concluded that the data obtained from 2-D finite element analysis were coherent with experimental results and additional to that fiber orientation angles of the laminate markedly affected the failure load of joints, failure mode and stress distributions appeared in adhesive and composite

Bolt-Hole Elongation of Woven Carbon-Epoxy Composite Plates and Joints Using the Digital Image Correlation Technique

The elongation of the bolt hole is an important parameter for assessing the failure of bolted joints. However, direct experimental measurement using strain gauges and extensometers is difficult. This article shows that digital image correlation (DIC) can overcome the difficulties and provide important indications of the failure mechanisms of bolted joints. Hole elongation was measured using DIC in the following carbon/epoxy composite configurations: standard open-hole tensile (OHT) and filled-hole tensile (FHT), single-lap shear only-bolted (OB), and single-lap shear hybrid-bolted/bonded (HBB) joints. For each configuration, the hole-elongation changes were tracked for cross-ply (CP) and quasi-isotropic (QI) stacking sequences with two thicknesses. In the tensile load direction for OHT and FHT cases, CP showed a greater hole elongation than QI. However, the opposite trend was observed in the transverse direction. In OB joints, bypass loads contributed more to the hole elongation than bearing action. In HBB joints, it has been observed that the adhesive significantly reduces hole elongation, particularly for CP configurations. Moreover, it was found that in HBB joints, hole elongation was independent of laminate lay-up, while it was very determinative in OB joints.

The Tensile Behavior of Hybrid Bonded Bolted Composite Joints: 3D-Digital Image Correlation versus Finite Element Analysis.

This study examines the behavior of hybrid bolted/bonded (HBB) joints loaded in tensile shear comprising plain weave carbon/epoxy laminates in quasi-isotropic (QI) and cross-ply (CP) layups. It proposes a combined approach of 3D digital image correlation and finite element analysis (FEA) to assess their behavior. To apply the FEA simulation accurately, a single layer of plain fabric was replaced with [0/90]s lamination. Experimental standard open-hole tension test results, as well as only bolted (OB) and HBB, along with FEA predictions, confirmed the accuracy of the substitution method. The FEA, calibrated by experimental results, provides insight into the distinctive characteristics of HBB joints in comparison with bonded and bolted joints. Critical considerations include material properties, damage modeling, adhesive characteristics, and mass scaling. The FEA results underscored the pivotal role of adhesives in HBB joints, rendering them akin solely to bonded configurations. HBB joints retain their geometry better than OB joints with considerably less out-of-plane displacement, following a sinusoidal trend. Moreover, the overall behavior of the two layups demonstrates that CP benefits from having higher strength than QI, especially at the critical hole located closer to the grip side.

A Machine Learning Model for Flaw Identification in Fibre-Reinforced Composites

A Haar cascade classifier is a machine learning (ML) algorithm used for object detection. In this paper, the Haar algorithm is introduced in the context of a non-destructive evaluation of fibrereinforced composite (FRC) structures. The Haar learning model is used for flaw identification from thermal images. Thermal images are created from cross-ply (CP) carbon fibre-reinforced laminates with flat-bottomed holes (6–10 mm) of different depths from the surface (0.5–1.5 mm). After training is complete, the model successfully detects similar artificial flaws in previously unseen thermal images. In doing so, the feasibility of Haar classifiers for automatic evaluation of FRCs is established.

Mathematical and simulation based investigation towards deformation behaviour of bombyx mori moth silk /vinyl ester reinforced laminated composite

ABSTRACT This paper focuses on the composite design with fibres loaded in the longitudinal direction as well as orientated at some certain angles. The study uses Bombyx Mori Moth Silk (BMM Silk) as fibres and vinyl ester as matrix material. The two sorts of composites having cross ply and angle ply laminate were considered for deformation study using the finite element analysis. According to the ASTM D 3039 testing standard, the tensile test was carried out to design and composite study of their occurred deformation. The rule of mixture was employed in order to evaluate the elastic constants of the prepared orthotropic composite laminate along the fibre direction and the evaluation of orthogonal stresses in transverse direction was carried out using the finite element procedure. The maximum orthogonal stress value of cross ply and angle ply was found to be 44.902 and 47.779 MPa, respectively. Also, the deformation occurred for both laminates was found to be of 0.083603 and 0.076428. This concludes that the deformation occurrence is more in case of angle ply and is less in cross ply laminates.

Effect of MWCNTs on mechanical properties of woven cross ply GFRP laminates

The impact of multiwalled carbon nanotubes (MWCNTs) on the structural integrity of woven cross ply glass fiber reinforced polymer (GFRP) has been examined in this study. Eight-ply laminates of fiber orientation (0˚/90˚) with neat epoxy GFRP, 0.5 wt% and 1.0 wt% MWCNTs modified GFRP were manufactures by using vacuum supported hand layup technique. According to ASTM standards, rectangular samples were cut from the prepared composite laminates for mechanical testing such as three-point bending test, tensile test, interlaminar shear strength test, in-plane shear and compression test. Addition of 0.5 wt% and 1.0 wt% of MWCNTs in cross ply GFRP laminates enhanced the ultimate tensile stress, compressive stress, bending stress, interlaminar shear stress and in-plane shear stress.

Three-dimensional scaled boundary finite element method to study interfacial imperfections in thick laminated composite plates undergoing bi-axial bending

In this work, the scaled boundary finite element method (SBFEM) is employed to model weakly bonded thick laminated composite plates subjected to bi-axial bending. A three-dimensional modelling technique incorporated in the SBFEM framework is used for thick laminated plates, where two-dimensional plate models cease to provide accurate results. Weakly bonded interfaces are modelled using spring layer models with the incorporation of zero-thickness interface elements between the layers. The interface element is subjected to behave in a linearly elastic manner capturing the slippage between the layers. Using SBFEM, it is possible to have fewer discretisation in the through thickness direction of the plate (or individual lamina) without compromising the accuracy of the results. Additionally, the size of interface elements can be kept small without altering the size of the adjoining bulk element. Validation examples covering symmetric, anti-symmetric cross ply and angle ply laminates have been solved using the proposed approach. Results from the proposed method using SBFEM are compared with well-established methods for modelling thick laminated composite under bi-axial bending.

Full-field through-the-thickness strain distribution study of hybrid multi-bolted/bonded single-lap composite joints using digital image correlation

This study proposes a digital image correlation (DIC) technique for through-the-thickness deformations of multi-bolted single-lap (SL) shear hybrid bolted/bonded joints made of carbon/epoxy laminates. Through-the-thickness peel strains are obtained throughout the overlap region during the entire tensile test. It is observed that real-time monitoring of the strain in the overlap region informs on the location of the failure before the load reaches 50% of its ultimate value. The peel strain-load curves for both the overlap ends superimpose perfectly up to about 40% − 50% of the applied tensile load and diverge considerably beyond this level. Similar behavior occurs for shear strain and is confirmed for quasi-isotropic (QI) and cross-ply (CP) laminate stacking sequences. It is also found that peel and shear strains, as well as joint rotation in CP, are higher than those of QI. These results can be useful for the design of this type of joint.

An experimentally validated 3D progressive fatigue damage model for fatigue life prediction of Flax-epoxy laminates

A finite element model that incorporates a 3D progressive fatigue damage model (3D PFDM) is developed to simulate the fatigue behavior of Flax-epoxy (FE) composites. Input parameters of the model are determined from testing the material properties in all directions. A double-notched shear test is used to characterize the out-of-plane shear properties of the laminate under both static and fatigue loading conditions. The 3D PFDM, which employs a stochastic distribution of the material properties, is implemented thorough a user-defined subroutine in ABAQUS finite element software. Results of the finite element analysis of voids-containing laminates are in good agreement with experimental results. The predicted fatigue life of all layups is more influenced by voids at higher loading levels compared to laminates without voids. A further verification of the 3D PFDM model was performed using the cross ply and quasi-isotropic configurations and comparing the predicted free-edge delamination at the interface of the 90° ply and adjacent layers to the SEM micrographs of free edges of specimens. From a practical perspective, the proposed 3D PFDM could be applied to a wide range of composite materials to predict edge delamination in laminates with different fiber orientations.

A comprehensive 2 Dimensional and 3 Dimensional FEM study of scarf repair for a variety of common composite laminates under in-plane uniaxial and equibiaxial loadings

Due to their potential to recover strength and stiffness with a minimum impact on aerodynamic performance of a damaged structure, scarf repairs are boosted as a viable repair option for primary aero-structures. So far, most of the experimental and numerical studies have been limited to joint specimens and their equivalent 2 Dimensional models and a few number of studies attempted to examine the results using real scarf repair geometry. Suggestions previously made to justify the difference between scarf joint and scarf repair strength and possible solution to diminish the difference are challenged in current work. Here, it is tried to investigate the strengths and shortcomings of 2D scarf joint modelling and their influence on the associated results by promoting 3 Dimensional Finite Element Modelling of complete geometry of the scarf repair. A method to assign material properties to a planar geometry of composite material has been developed that provides the possibility of plane strain, plane stress, and generalized plane strain modelling options of a scarf repair cross section under uniaxial load. Besides, accuracy of scarf joint specimen as a representative to scarf repair to predict load carrying capacity of a circular scarf repair is investigated. The study mainly focuses on 2D and 3D FEM simulation results discrepancies considering the effect of angle ply, cross ply and various stacking sequences of quasi-isotropic laminate under uniaxial and equi-biaxial loads. Results show that the laminate lay-up angles and stacking sequence substantially affect the 2D and 3D simulations agreement. Also, the observed discrepancies in scarf repair and scarf joint results are not limited to the effects of plastic deformation of the adhesive, but it seems the modelling shortcomings greatly affect the results. In overall, the current work demonstrates that for a particular laminate, joint specimen does not represent reasonably a scarf repair. In fact, the load carrying capacity estimated based on joint specimen data can mislead decision making procedure in a way that results in rejection of an eligible repair. To avoid inaccurate strength estimation, a 3D simulation of a scarf repair especially for a load other than uniaxial tension or compression is strongly recommended.

Low-velocity impact response of MWCNTs toughened CFRP composites: Stacking sequence and temperature effects

Despite impressive specific mechanical properties, carbon fiber reinforced polymer (CFRP) composites are strongly susceptible to impact events due to their low interlaminar fracture toughness which fosters delamination phenomena detrimental to the residual mechanical properties of the laminate. The incorporation of nano-fillers in the neat matrix is a suitable way to improve both Mode I and Mode II CFRP fracture toughness. In view of this, the present work assessed the toughening effect of multi-walled carbon nanotubes (MWCNTs) on the impact and post-impact response of carbon fiber reinforced epoxy composites evaluating the effect of two key design parameters: ply stacking sequence and operating temperature. Two stacking configurations, i.e., cross-ply (CP) and quasi-isotropic (QI), and three testing temperatures, i.e. −40 °C, room temperature and +80 °C, were considered. The results highlighted a higher damage tolerance of QI laminates characterized by a superior residual flexural strength than CP for all impact energies and temperatures. Moreover, the toughening effect played by MWCNTs on QI samples was disclosed for all temperatures with an improvement in the normalized residual flexural strength up to 10%. The enhanced damage tolerance must be ascribed to the bridging effect and the pull-out phenomena provided by CNTs which delay matrix cracks opening and propagation.

Tensile and compressive damage assessment of a novel sandwich composite structure made of Kevlar/flax/epoxy hybrid laminates

This is a study to quantify damage progression of a new Kevlar/flax/epoxy (KFE) hybrid composite made with 12 layers of pure flax fiber fabric sandwiched between 4 layers of pure Kevlar fiber weave. There were four versions of the composite: 0° unidirectional, 0°/90° cross-ply, ± 45° angle-ply flax fibers, and Kevlar fibers-only as the control. Composite plates were subjected to progressive load-unload levels in tension and compression until failure and were evaluated for stiffness degradation, permanent strain, and crack density. Unidirectional KFE laminates exhibited less damage accumulation compared to cross ply and angle ply configurations. Damage progression and failure of KFE hybrids mainly involved cracks through flax fiber bundles and at flax fiber/epoxy interfaces. The reported data are useful for damage-limited design structures made from a combination of natural and synthetic fibers.

Fabrication and preliminary assessment of aramid reinforced polymer composite material

A knowledge of the material constituents and fabrication process is generally required for easing the composite material properties study and characterization. This paper presents the methodology of aramid reinforced polymer composite material preliminary assessment and fabrication. The demonstration is carried out through the development of mechanical properties characterization specimen. The composite material specimen is fabricated by utilizing the open mould with hand lay-up method, where three types of laminates include of unidirectional, orthotropic, quasi-isotropic cross plies are demonstrated. A template is applied for preparing specific fibre orientation 45°angle. Plies are lay accordingly with guided by a square guide for minimizing the off-orientation angle defect. Preliminary assessment is included of verification of unidirectional aramid fabric quality, cured specimen physical defect, internal defect through micrography analysis and material volume fraction prediction. Average specimen fibre volume fraction of 0.64 is predicted by determined the fibre filament geometry detail through micrography analysis. There are several fabrication defects had been identified. The defect identification findings shall be referred for further improvement of material preparation and fabrication method.

Electric conductivity and surface potential distributions in carbon fiber reinforced composites with different ply orientations

The electric conductivity and surface potential distributions of carbon fiber laminated panels with different ply orientations have been investigated. We found that the unidirectional (UD) lamina has higher conductivity along the carbon fiber direction than the perpendicular direction, and equipotential contours also show different gradients along the two directions. The cross-ply (CP) and quasi-isotropic (QI) laminates have the mixed effects of the UD lamina electric conductivity and ply orientations, while the surface potential distributions mostly depend on the surface lamina direction. The conductivity along the thickness direction depends on each lamina and inter-laminar bonding. A finite element analysis model was also developed to show the effect of ply orientation on potential distribution. The CP and QI laminates with 0° surface ply have uniform potential distributions and isotropic electricity behaviors. The results could be used to monitor damage locations and design electric composite materials.

Ballistic performance of quasi-isotropic CFRP laminates under low velocity impact

The perforation characteristics of fiber reinforced laminates is crucial for the design of protective civil and military structures. This paper investigates the perforation characteristics (ballistic limit velocity, residual velocity, perforation energy) of cross ply and quasi-isotropic (QI) carbon fiber reinforced polymer (CFRP) laminates under the impact of a rigid conical steel bullet. The influence of thickness and ply orientation on these characteristics is also studied for a wide range of velocities. The perforation characteristics of these laminates were determined, numerically as well as experimentally. A numerical model is developed by using Hashin damage model to understand the behavior of laminates under high velocity impact. The accuracy of the model is assessed by comparing its prediction with experimental results of cross ply laminates. Then, impact perforation study of different possible configurations made of quasi-isotropic (QI) CFRP laminates, oriented at 0°, 90°, 45° and −45° directions are carried out with the help of validated numerical model. The perforation characteristics predicted with the help of numerical model is in good agreement with the experimental results. Optimal configuration is achieved in terms of energy absorption and damage resistance for better performance under impact loading.