Unidirectional Glass Fiber-reinforced Polymer Research Articles

Failure mode maps for glass fibre-reinforced polymer/polyvinyl chloride foam-cored sandwich structures

Purpose This study aims to understand how the facesheet size, orientation and core size influence the analytical failure mechanism mode of glass fibre reinforced polymer (GFRP)/polyvinyl chloride (PVC) sandwich structures subjected to three-point bending. The purpose of this study was to develop failure-mode map of GFRP/PVC sandwich structures. Sandwich structures with different facesheet and core thicknesses were used to develop the failure map. Design/methodology/approach The sandwich structure and facesheet were fabricated using a vacuum-assisted resin infusion method with core sizes of 10, 15 and 20 mm and facesheet thicknesses of 1.5 and 3 mm and were arranged in three different orientations: angle-ply, cross-ply and quasi-isotropic. The key failure modes that occur in sandwich structures were used to predict possible failures in the developed material. Analytical equations were used in MATLAB for each observed failure mode. The probable failure modes, namely, face yielding, core shear and indentation equations, were used to construct the failure maps and were compared with the experimental data. Findings The boundary of the two failure modes shifts with changes in the facesheet and core thicknesses. The theoretical stiffness of sandwich panels was higher than the experimental stiffness. Based on strength-to-weight ratio, specimens E10-4, A15-8 and E20-8 exhibited the best optimum values owing to their shorter distance to the boundary lines. Originality/value In this study, a failure map was used to predict the possible failure modes for different GFRP facesheet orientations and thicknesses and PVC core thickness sandwich structures. Little is known about the prediction of the failure modes of unidirectional GFRP arranged in different orientations and thicknesses and PVC core thicknesses for sandwich structures. Few studies have used failure mode maps with unidirectional GFRP oriented in angle-ply, cross-ply and quasi-isotropic directions as a facesheet for sandwich structures compared to bidirectional mats. This study can serve as a guide for the correct selection of materials during the design process of sandwich structures.

Investigation and calculation of the longitudinal compressive strength of unidirectional glass fiber reinforced polymer considering the fiber orientation distribution

In this study, undulations and their influence on the longitudinal compressive strength of a unidirectional glass fiber reinforced polymer (GFRP) composite are investigated theoretically and experimentally. The objective of this research is to explore the failure mechanisms in FRP and to characterize the mechanical properties of FRP as a function of fiber orientation. For this purpose, a multiscale material model is developed that considers a stochastic fiber orientation distribution (FOD) and models matrix fracture-initiated failure. The relationship between compressive strength and undulation is investigated experimentally on standardized specimens made of unidirectional GFRP. The fiber orientations are measured using X-ray computed tomography and ImageJ image analysis, resulting in a binormal distribution of fiber orientations in the series of samples tested. To examine the failure process in detail, the compression tests are simulated using finite element analysis (FEA). Both the FEA results and the measured compressive strengths confirm the model assumption of matrix fracture-initiated failure under longitudinal compressive loading. The presented analytical model realistically represents the correlation of compressive strength with the FOD.

Mechanical Behaviour of Glass Fibre-Reinforced Polymer/Polyvinyl Chloride Foam Cored Sandwich Structures

This study focuses on the fabrication and analysis of the mechanical behaviour of unidirectional (UD) glass fibre-reinforced polymer (GFRP) facesheet and polyvinyl chloride (PVC) foam core sandwich structures fabricated by a vacuum-assisted resin infusion method (VARIM). These sandwich structures are commonly used in marine and wind turbine blade applications. To date, relatively little knowledge about the functional behaviour of UD GFRP compared to composites reinforced with bidirectional mats is available for day-to-day applications. The effects of the facesheet orientation, facesheet thickness, and core thickness on the mechanical behaviour of the specimens were examined. The UD fibres were oriented in cross-ply (0/90), angle-ply (+45/−45), and quasi-isotropic orientations. Various mechanical properties such as tensile, flexural, flatwise compression, and edgewise compression tests were examined. Characterization of the tensile properties of the facesheet showed that the cross-ply orientation had a higher strength than the angle-ply and quasi-isotropic orientations. The flexural load-carrying capacity of the cross-ply facesheet orientation was superior to the other orientations. The increase in the core thickness changed the flexural failure mode from face yield and core shear to core indentation. Flatwise compression (FWC) was tested to determine the core characteristics of the sandwich structure, and the peak loads of 4.90, 1.81, and 3.90 kN were obtained for 10-, 15-, and 20 mm core thicknesses, respectively. Edgewise compression (EWC) exhibited stable end crushing for thinner facesheet, whereas thicker facesheet showed core crushing and buckling. When the facesheet thickness was increased from 1.5 mm to 3 mm in the EWC, the buckling load increase ranged from 2.53% to 44.83% for core thicknesses 10-, 15-, and 20 mm, respectively.

Effect of graphene nano-additives on the fatigue limit of fiber-reinforced polymer hybrid composites—A micromechanical modeling procedure

In this article, the effect of graphene nanoadditives on the fatigue limit of unidirectional glass fiber-reinforced polymer (GFRP) composites has been investigated. A micromechanical model based on the crack growth has been used to analyze the fatigue limit response of unidirectional GFRP hybrid composite containing nanoadditives. The influence of different percentages of graphene nanosheets (GNS) and different types of nanofiller dispersion has been investigated by the micromechanical model. The fatigue limit of unidirectional GFRP hybrid composites has been plotted in terms of glass fiber volume fraction, interfacial friction shear stress, interfacial fracture energy and reference length. The fatigue limit of unidirectional composites can be improved by addition of GNS into the polymer matrix of such composite systems. The obtained results on the GNS dispersion type demonstrate that the uniform dispersion of GNS into the polymer matrix increases the fatigue limit of unidirectional GFRP hybrid composites. As the agglomeration density of GNS is low, the fatigue limit of GFRP hybrid composites will be improved more than when the density of agglomeration is high.

Probabilistic Bayesian Approach for Delamination Localization in GFRP Composites Using Nonlinear Guided Waves

Abstract Nondestructive evaluation (NDE) techniques that use nonlinear wave–damage interactions have gained significant attention recently due to their improved sensitivity in detecting incipient damage. This study presents the use of finite element (FE) simulation with the experimental investigation to quantify the effects of guided waves’ propagation through multiple delaminations in unidirectional glass fiber-reinforced polymer (GFRP) composites. Further, it utilizes the outcomes of nonlinear interactions between guided waves and delaminations to locate the latter. This is achieved through probabilistic Bayesian updating with a structural reliability approach. Guided waves interacting with delaminations induce nonlinear acoustic signatures that can be quantified by the nonlinearity index (NLI). The study found that the NLI changes with the interrogation frequency, as confirmed by numerical and experimental observations. By using the numerical outcomes obtained from the nonlinear responses, a Bayesian model-based approach with subset simulation is proposed and subsequently used to locate multiple delaminations. The results indicate that both the log-likelihood and log-evidence are key factors in determining the localization phenomenon. The proposed method successfully localizes multiple delaminations and evaluates their number, interlaminar position, width, and type.

A Micromechanical Modeling Approach for the Estimation of the Weathering-Induced Degradation of Wind Turbine Blades

Glass fiber reinforced polymers (GFRPs) are widely used as composite material for a variety of applications such as wind turbine blades (WTBs). During their operating time, these GFRP structures are exposed to natural weathering conditions, such as low and elevated temperatures, ultraviolet radiation, and moisture. These weathering phenomena influence the material’s mechanical properties due to material aging and the degradation of the composite’s mechanical properties. For a reliable lifetime assessment and the design of a repurposed application of WTBs, the quantification of GFRP’s degradation is required. For this reason, the aim of the current study is to numerically estimate the combined effects of weathering on the mechanical properties of GFRP. Therefore, the effective elastic properties of a unidirectional GFRP composite were determined considering representative volume elements. The required numerical modeling was performed using finite element analysis. The mechanical properties of glass fibers, epoxy resin and their relationship with individual natural aging phenomena were used based on the existing literature values. As a result of the micromechanical modeling, the change of temperature and moisture absorption have the highest effect on the elastic properties on the epoxy resin and thus also on the GFRP composite. The used numerical approach enables a preliminary estimation of environmental-based degradation phenomena of GFRP which can be used at an early stage of developments of composite structures, the reuse of composites or for planning experimental studies considering degradation of these composite materials.

Open-hole fatigue testing of UD-GFRP composite laminates containing aligned CNTs using infrared thermography

The present study aims to characterize the open-hole fatigue (OHF) behavior of unidirectional glass fiber-reinforced polymer matrix (UD-GFRP) epoxy composites containing aligned multi-walled carbon nanotubes (MW-CNTs) locally reinforcing the composite in the vicinity of the hole. The performance of these multi-scale composites has been compared with that of composites containing randomly oriented MW-CNTs around the hole. UD-GFRP containing neat epoxy (without MW-CNTs) as a matrix were the control samples in the study. Infrared thermography has been used to investigate the damage evolution in composites during the OHF tests. MW-CNTs, aligned perpendicular to the loading direction, improved the fatigue strength of the UD-GFRP composites by over 12% compared to the control samples. Fractography results are also presented in support of the findings from fatigue testing. Infrared thermography is presented as a method to rapidly determine the high cycle fatigue strength of the OHF specimens.

3D Finite Element investigation in high speed drilling of unidirectional Glass Fiber Reinforced Polymer composite

The unidirectional Glass Fiber Reinforced Polymers (UD-GFRP) composites have wide spread usage in the current modern industries, because of their significant mechanical behavior over conventional materials and metals. Drilling operation is a vital machining process of making holes with high precision during the pre-assembly process of the laminates. Hole with higher precision is needed to guarantee the strength of the joint made during the assembly process. However, the effect of key drilling parameter settings like cutting speed, tool diameter, tool geometry, drill material and feed rate is more significant in achieving acceptable dimensional tolerance and quality holes. The effect of the thrust forces in the quality of the hole and chip morphology is very high especially during the high speed drilling process. Hence, the prediction and investigation of the critical thrust force is very much needed to reduce the machining induced defects and thereby improving the effectiveness of the drilling process. In this research work, the prevalidated 3D Finite Element Model (3D FEM) developed in ABAQUS/Explicit software is utilized to analyze and explore the complex cutting mechanics behind high speed drilling of GFRP composite. The effect of machining parameters settings on thrust forces are predicted and reported using 3D FE analysis.

Quantitative Visualization of Buried Defects in GFRP via Microwave Reflectometry.

Glass fiber-reinforced polymer (GFRP) is widely used in engineering fields involving aerospace, energy, transportation, etc. If internal buried defects occur due to hostile environments during fabrication and practical service, the structural integrity and safety of GFRP structures would be severely undermined. Therefore, it is indispensable to carry out effective quantitative nondestructive testing (NDT) of internal defects buried within GFRP structures. Along with the development of composite materials, microwave NDT is promising in non-intrusive inspection of defects in GFRPs. In this paper, quantitative screening of the subsurface impact damage and air void in a unidirectional GFRP via microwave reflectometry was intensively investigated. The influence of the microwave polarization direction with respect to the GFRP fiber direction on the reflection coefficient was investigated by using the equivalent relative permittivity calculated with theoretical analysis. Following this, a microwave NDT system was built up for further investigation regarding the imaging and quantitative evaluation of buried defects in GFRPs. A direct-wave suppression method based on singular-value decomposition was proposed to obtain high-quality defect images. The defect in-plane area was subsequently assessed via a proposed defect-edge identification method. The simulation and experimental results revealed that (1) the testing sensitivity to buried defects was the highest when the electric-field polarization direction is parallel to the GFRP fiber direction; and (2) the averaged evaluation accuracy regarding the in-plane area of the buried defect reached approximately 90% by applying the microwave reflectometry together with the proposed processing methods.

Defect Size Characterization in Unidirectional Curved GFRP Composite by TSR Processed Pulse and Lock in Thermography: A Comparison Study

Glass fiber reinforced polymer (GFRP) curved composite is widely used in industries due to its high corrosive resistance nature. GFRP curved composites are involved in many industries like petrochemical industries for handling oil and gas at offshore platforms, chemical processes such as chemical storage tanks, desalination, and water treatment. Since alloy steel materials get corroded by environmental factors such as excess salinity in the surrounding environment, mud deposition, and sulfur crude accumulation makes alloy steel pipes are expensive to maintain. Every year billions of dollars can be saved by corrosion prevention using Glass Fiber Reinforced Polymer (GFRP) pipes instead of alloy steel pipes. In-service stage of the GFRP pipe or tank, different types of defects are forming such as void, delamination, and wall loss (pits). Among all these defects, pits or wall loss is one of the severe defects which may lead to leakage accidents. The objective of the study is the quantification of defect size by using TSR processed pulsed and lock-in thermography and analyses their capabilities in defect size quantification. TSR processed the PT image and the signal to noise ratio was used to estimate the defect size quantification. For defect size measurement, the TSR-processed PT thermal results are recommended and the near-surface defects can be measured with high accuracy in LT.

Numerical Analysis of Unidirectional GFRP Composite Mechanical Response Subjected to Tension Load using Finite Element Method

Composite material is a well-known structural material which is increasingly adopted as an engineering structure material. Glass fiber reinforced polymer offers the lightweight and high strength characteristics that is required for the modern industry, such as aviation, automotive, wind power, and marine technology. One of the important mechanical characteristics of the composite materials are the tensile properties, because it is well known as the material strength. Therefore, the investigation of mechanical response on the glass fiber reinforced polymer (GFRP) tensile test using numerical analysis is important for the estimation of structural response of the GFRP complex structure, such as boat construction. The objective of this research is to assess and estimate the mechanical response of the GFRP composite material subjected to tension load using finite element method. The linear transversely isotropic model is developed to estimate the unidirectional glass fiber GFRP with the configuration of fiber orientation angles of 0°, 30°, 45°, 60° and 90°. The results show that FE simulation are capable to detect the specimen response during the tensile test. The maximum discrepancy of the estimated stress strain diagram is about 16.5% to 32% compared to experimental data. The larger orientation angle has shown the larger discrepancy value. It is found that the increment of discrepancy value is generated by the nonlinearity behavior of the material due to the domination of polymer material behavior on the large orientation angle. Otherwise, the FE models have estimated accurately the ultimate strength, maximum displacement and fracture load. It can be concluded that the linear transversely isotropic model is adequately accepted as the estimation method of the GFRP composite structure response.

Erosion Mapping of Through-Thickness Toughened Powder Epoxy Gradient Glass-Fiber-Reinforced Polymer (GFRP) Plates for Tidal Turbine Blades

Erosion of tidal turbine blades in the marine environment is a major material challenge due to the high thrust and torsional loading at the rotating surfaces, which limits the ability to harness energy from tidal sources. Polymer–matrix composites can exhibit leading-blade edge erosion due to marine flows containing salt and solid particles of sand. Anti-erosion coatings can be used for more ductility at the blade surface, but the discontinuity between the coating and the stiffer composite can be a site of failure. Therefore, it is desirable to have a polymer matrix with a gradient of toughness, with a tougher, more ductile polymer matrix at the blade surface, transitioning gradually to the high stiffness matrix needed to provide high composite mechanical properties. In this study, multiple powder epoxy systems were investigated, and two were selected to manufacture unidirectional glass-fiber-reinforced polymer (UD-GFRP) plates with different epoxy ratios at the surface and interior plies, leading to a toughening gradient within the plate. The gradient plates were then mechanically compared to their standard counterparts. Solid particle erosion testing was carried out at various test conditions and parameters on UD-GFRP specimens in a slurry environment. The experiments performed were based on a model of the UK marine environment for a typical tidal energy farm with respect to the concentration of saltwater and the size of solid particle erodent. The morphologies of the surfaces were examined by SEM. Erosion maps were generated based on the result showing significant differences for materials of different stiffness in such conditions.

Bi-axial behaviours of abraded and rehabilitated FRP decks as anisotropic plates under concentrated wheel loading

For in-service GFRP (glass fibre-reinforced polymer) cellular bridge decks, wheel loads can induce damage of the anti-skid surfacing-to-deck bond, thereby loosening fragments of surfacing that then cause abrasion of the deck’s top flange under further wheel loads. Indeed, over the last 20 years, wheel load-induced damage to the top flanges of FRP decks on the road network has been observed in different countries, after periods in service ranging from a few days to a few years, due to inadequate design. Bonding of uni-directional GFRP panels to the abraded multi-directional flange constitutes a potentially effective rehabilitation strategy, but it also influences the deck’s local wheel-load response by altering the anisotropy of the top flange. The present paper uses an experimental-numerical study to explore this role of anisotropy for both abraded and rehabilitated decks. The experiments captured the top flanges’ peak biaxial strains, using gauges installed on the flange soffits to avoid damage by the wheels. The numerical model captures the morphology of the deck, the actual spread of the tyre load over the contact zone and spatial variations in anisotropy of the abraded or built-up flanges and of the webs. The results show that the uni-axially dominated rehabilitation strategy strongly improves bi-axial response to static wheel loads. The numerical model encouragingly predicts the bi-axial strains for each of the abraded and rehabilitated decks, and reliably predicts the strain reductions due to rehabilitation. Refined meshes along the tyre-loaded local flange span, with coarser meshes in adjacent spans, enable these reasonable predictions of response. It is concluded that top flange soffit gauges installed via ad-hoc tooling inserted through holes in the bottom flange, in tandem with rigorous numerical modelling, constitutes an effective two-pronged approach for elucidating wheel load effects.

Experimental and numerical investigation of short GFRP- reinforced steel tubular short columns

Abstract This study presents the experimental and analytical modelling of externally bonded glass fibre-reinforced polymer (GFRP) sheets for strengthening circular steel tubular short columns. The GFRP sheets are easy to handle due to the lightweight property and the high-strength can provide a degree of restraint to delay buckling of the thin steel wall. Four short columns of cold-formed steel circular hollow section (CHS) were experimentally tested. One column is conventional (without GFRP wrap) and other 3 columns are externally bonded unidirectional (hoop) GFRP sheets which are varying with numbers of layers (1, 2 and 3 layers). In addition to the experimental studies, finite element analysis was also carried out using ABAQUS software. The experimental result shows that the steel tube is strengthened by GFRP sheets and enhances the axial section capacity. The load carrying capacity of externally bonded GFRP column increases about 17%−49% on comparison with the column without confinement. The study also concluded that the column externally warped with 3 layers of GFRP sheet is found to be optimum and it attains maximum load caring capacity. The prediction of analytical results has been found to be in good correlation with the experimented results.

Behavior of GFRP strengthening masonry walls using glass fiber composite anchors

This paper presents experimental and theoretical investigations on the short-term behavior of stone masonry walls constructed with lime mortar. The walls were strengthened using unidirectional GFRP (Glass fiber reinforced polymers) sheets to resist out-of-plane loads. Two methods of anchorage were used to enhance the behavior of strengthened stone masonry walls by preventing premature peeling-off of the GFRP sheets caused by GFRP debonding. Nine GFRP strengthened limestone masonry wall specimens including two control specimens were tested under combined constant low axial load levels and out-of-plane bending caused by two point monotonic loading. All the specimens have height-to-thickness (h/t) ratio of 7.1. The main variables investigated were GFRP reinforcement ratio, arrangement of GFRP, axial load level, and anchorage method. The strengthened walls showed an improvement in the out-of-plane performance which was significantly enhanced by anchorage. The increase in wall ultimate load carrying capacity reached 100%.

Lathe Parameters Optimization for UD-GFRP Composite Part Turning with PCD Tool by Taguchi Method

The aerospace and automobile sectors are widely utilized the polymer composites. The composite materials, like unidirectional glass fiber reinforced polymer (UD-GFRP), is difficult to machine due to its anisotropic that is non-homogeneous character and such material requires special cutting tools. The proposed work is going to examine the tool wear, quality of the surface and forces generated in the various stages of inputs given to the machining of unidirectional glass fiber reinforced polymer (UD-GFRP) composites. The assessment of the machining incorporates tool wear investigations, surface roughness investigations and quality of material by varying input parameters. The Taguchi optimization technique with experimental design of L9 orthogonal array employed. The parameters range identified by trail runs and observations of conducted machining utilized for optimization. The Turning process parameters of cutting velocity or speed, rate of tool movement or feed rate and cutting depth on composite part or depth of cut were considered. The other factors, like tool material i.e., Poly-Crystalline Diamond (PCD) tool, its cutting regime (dry), profile of cutting tool are considered as constant parameters. The responses, like tool wear, surface finish, and cutting force, were measured against various input parameters, while machining the composite (UD-GFRP) composite part. The objective of this research is to establish relationship among various operating parameters to achieve desired results. That is major focus of the work on the economic condition for getting better values based on setting of input parameters.

The influence of different glass fiber/epoxy matrix combinations on the durability under severe moisture impact

The growing success of fiber reinforced polymers (FRP) as material for the construction of high-performance lightweight structures used under maritime environmental conditions, requires foremost the knowledge about their long term durability. As the supplier market is still growing fast, methods which allow manufacturers to distinguish between suitable and less suitable composites are needed. In this study, the effects of moisture on the mechanical properties of glass fiber reinforced polymers (GFRP) using different glass fiber fabrics are investigated under several ageing and testing conditions. Focusing on the fabrics and introducing an ageing method prior to composite manufacturing, allows to describe the proportions of fiber, matrix, sizing and interphase damage to the composites durability in more detail. Absorption quantity related testing after ageing at temperatures between 8°C and 50 °C highlights the resulting effects on the tensile strength of mostly unidirectional GFRPs. The strength of composites based on fabrics with high resistance to moisture degradation decreases steady but moderate during absorption. This effect is mainly associated with changes of matrix properties. However, less durable composites show a two-stage behavior. In this case, severe interphase damage and cracking leads to an additional drastic strength decrease when exceeding a defined amount of water absorption.

Internal GFRP Reinforcement of Low-Grade Maritime Pine Duo Timber Beams.

This paper presented an experimental structural-scale test campaign used to analyze the flexural behavior of low-grade maritime pine (Pinus pinaster Ait.) duo timber beams reinforced with an internal glass fiber-reinforced polymer (GFRP) sheet. For this purpose, thirty (30) unreinforced duo beams and thirty (30) duo beams internally reinforced with a unidirectional GFRP sheet with an areal mass of 1200 g/m2 were produced and tested. The addition of a low GFRP reinforcement ratio (1.07%) in the tension zone of the duo beams provided an average improvement of 8.37% in bending stiffness (EI) and an increase of up to 18.45% in ultimate moment capacity. In addition to this improved bending behavior, the internal GFRP reinforcements seemed to decrease the influence of wood singularities and wood heterogeneity on mechanical properties, which allowed for better characteristic values to be reached and for the achievement of results with less variability.

Novel method of healing the fibre reinforced thermoplastic composite: A potential model for offshore applications

Abstract Continuous fibre reinforced thermoplastic composites are increasingly finding their use as engineering materials in many industries due to the excellent fire, smoke and toxicity performance. However, the composite component produced using automated continuous fibre reinforced thermoplastic tapes laying machine are susceptible to sudden failure emanating from microscale cracks. This study demonstrates the healing potential of a layered Glass Fibre Reinforced Polymer (GFRP) composite consisting of alternative layers of GFRP and a magnetic polyamide-6 (PA-6) nanocomposite (PNC) film. The self-healing process is presented in three steps, viz. (i) polymer nanocomposite synthesis, (ii) preparation of the layered GFRP layered composite sample and (iii) self-healing and testing of GFRP layered composite sample. Firstly, the multilayer dog bone sample consisting of a magnetic polymer nanocomposite (PNC) film sandwiched between thermoplastic unidirectional GFRP tapes are prepared. Healing is triggered by exposing the damaged multilayer sample to microwave causing selective heating of nanocomposite film and its subsequent melting. The healing process completes when liquid polymer fills the micro-crack in the multilayer tape through capillary action and solidifies upon cooling. The healing yields 84% of the undamaged tensile strength recovery. Results demonstrate the potential application of an autonomous self-healing method for thermoplastic composite used in the offshore environment.

Effect of Graphene on Machinability of Glass Fiber Reinforced Polymer (GFRP)

Glass fiber reinforced polymers (GFRPs) are used extensively in many industries because of their low cost and high mechanical properties. Even if composite manufacturing processes are well controlled and allow to fabricate near net shapes, machining operations are still necessary to complete the manufacturing. As a composite material, GFRP machining remains difficult because of its heterogeneous and anisotropic character. This work intends to investigate the effect of graphene addition to the epoxy matrix of GFRP on its machinability. The epoxy was filled with 1 wt% graphene by mixing, sonicating, and then being used to produce unidirectional GFRP laminate by hand layup methods. Thermocouples were bonded on a chemical vapor deposition (CVD) diamond coated tool in order to record cutting temperatures during the trimming process. The cutting forces were recorded and the resulting surface roughness after trimming was measured to qualify properly the machinability of the modified GFRP. Compared to the reference material (GFRP without graphene), the additive improved the machining process by decreasing the cutting temperature and forces as well as the surface roughness without deteriorating the inter-laminar shear strength.