Interlaminar Surface Research Articles

Different-structured nanoclays incorporated composites: Computational and experimental analysis on mechanical properties

The objective of this study is to investigate the mechanical properties of basalt fiber reinforced polymers (BFRPs) with different structured nanoclays incorporation through experimental and computational approaches. To solve the inherent weak interfacial adhesion problem of inorganic basalt fiber is also one purpose of this study. Halloysite nanotube (HNT) and nano Kaolin, which both derived from Kaolin's sub group, were employed to conduct the study due to their distinctive structures. The HNTs were conducted to heat and silane treatments change the structure and enhance the interfacial bonding. A modified Eshelby-Mori-Tanaka micromechanic model was implemented to predict the effective modulus of BFRPs regarding the structures of incorporated nanoclays. The computational and experimental results demonstrate the identical tendency that a flattened nanoclay structure, or larger sectional aspect ratio, is more effective for the modulus improvement. The lower experimental results comparing to computational results are due to the imperfect nanoclay-resin interfacial bonding. Incorporation of different structured nanoclays also improved the interlaminar strength and toughness of BFRPs by experimental approach. The relative dense and uniform distribution of nanoclays was observed on the interlaminar surface of fractured double cantilever beam (DCB) specimens. The bridging effect of tubular structured HNTs which impeded the crack propagation was determined as the mechanism of the improvement of interlaminar properties.

Influence of supplementary cementitious materials on rheological properties of 3D printed fly ash based geopolymer

3D printed fly ash based geopolymer was prepared with fly ash and quartz sand as main raw materials while slag powder and silica fume as supplementary cementitious materials. In this paper, anhydrous sodium silicate powder was used as activator and ATTAGEL-50 thixotropic thickener was used as 3D printing additive. The effect of slag powder and silica fume on the rheological properties of 3D printed fly ash based geopolymer was studied. The results show that the apparent viscosity, thixotropic property, plastic viscosity and yield stress of geopolymer first increased and then decreased with the increase of slag powder and silica fume respectively. Compared with the Bingham (BH) model, the Herschel- Bulkley (H–B) model is more accurate and more suitable for characterizing the rheological model of fly ash based 3D printed geopolymer. The suitable content for slag powder is 10%. On the basis of adding slag powder, silica fume was also used to replace the fly ash, the suitable content for silica fume is 10% in this fly ash-slag powder based 3D printing geopolymer. According to the SEM observations, it is found that the interior of the printing layer of the 3D printing block is more conducive to the production of the geopolymer gel than the interlaminar surface of the printing layers. It is found that the printing layers are connected with each other through bridges.

Thermosetting CFRP interlaminar toughening with multi-layers graphene and MWCNTs under mode I fracture

Unidirectional carbon fiber-reinforced composites are widely applied in aeronautical and astronautical structures. However, the weak interlaminar toughness makes composite structures easily being damaged under the low energy impact. Improving composite interlaminar toughness without increasing thickness becomes possible as the carbon nanotubes(CNTs) and graphene possess superstrong-mechanical property and nanoscale. This paper focuses on enhancing the mode I fracture toughness by adding two nano materials of multi-walled carbon nanotubes(MWCNTs) and multi-layers graphene(mG) into interlaminar surface. MWCNTs and mG are sprayed on the central two plies interlaminar surface at the same density 1 g/m2. Double cantilever beam(DCB) standard test is used to investigate the mode I energy release rate with different toughening strategies. Increasing proportion of the mean fracture toughness of crack propagation are 12.3% (MWCNTs) and 101.4% (mG) respectively. According to SEM analysis, the horizontal multi-layers graphene surface peeling and the vertical pull out mechanisms improve the crack growing resistance and result in large amount of fiber breakage and bridging. It is also revealed that the multi-layers graphene relatively-larger surface area compared with MWCNTs makes the interlaminar fracture toughness increasing notably. Based on finite element method(FEM) of continuum damage mechanics, the original and mG toughened specimen under DCB mode I fracture are modeled and analyzed. The simulation and experiment are in good agreement.

What every spine surgeon should know about transforaminal lumbar interbody fusion surgery for herniated discs.

The aim of this study is to report our experiences on managing large lumbar disc herniations with several symptoms by surgery with transforaminal lumbar interbody fusion (TLIF) and to shed light for spine surgeons about TLIF surgery. We retrospectively evaluated our database of patients with various lumbar spine pathologies who underwent TLIF surgery from 2014 to 2017. We separated 18 patients who had been operated on for extruded disc herniation, which causes severe pain and radicular symptoms. The pain was quantified by visual analog scores (VAS). The disability status were pre-operatively and post-operatively evaluated by the Oswestry Disability Index (ODI). We evaluated the patients for at least two years. Interbody fusion was detected by routine radiographs at six, 12, and 24months after surgery. An ODI outcomes analysis demonstrated a statistically significant improvement in the six and 24-month mean scores compared with the pre-operative scores on the same scales. Patients' mobility improved significantly after surgery, as indicated by the decrease in the Oswestry Disability Index from 72 to 23 over twoyears (p < 0.001). Pain rapidly decreased in all patients and continued to decrease at the time of the latest follow-up. The mean pre-operative VAS scores for pain was 8.8; it had improved to 2.4 after surgery (p < 0.05). Within the follow-up period of twoyears, the ascertained mean VAS declined from 8.8 to 1.4 (p < 0.001). The average disc space height at the herniated levels was fairly well maintained. No patient had evidence of implant failure. Interbody fusion was graded as definitely solid in 100% of cases twoyears post-operatively. One patient displayed a superficial wound infection. Following appropriate debridement and antibiotics, the wound healed without sequelae. No major complications were observed, including permanent neurological deficit, pulmonary embolism, peri-operative cardiac event, or death. The findings of our study and those in the literature showed that primary herniated disc patients with radicular and chronic low back pain, degenerative changes, bi-radicular symptoms, and instability are required to have fusion after a discectomy. Being a heavy-duty worker is also a criterion for fusion surgery. TLIF is performed by a unilateral approach preserving the interlaminar surface on the contralateral side, which can be used as a site for additional fusion. As an effective results TLIF procedure should be chosen for fusion surgery.

Enabling internal electronic circuitry within additively manufactured metal structures – the effect and importance of inter-laminar topography

PurposeThis paper aims to explore the potential of ultrasonic additive manufacturing (UAM) to incorporate the direct printing of electrical materials and arrangements (conductors and insulators) at the interlaminar interface of parts during manufacture to allow the integration of functional and optimal electrical circuitries inside dense metallic objects without detrimental effect on the overall mechanical integrity. This holds promise to release transformative device functionality and applications of smart metallic devices and products.Design/methodology/approachTo ensure the proper electrical insulation between the printed conductors and metal matrices, an insulation layer with sufficient thickness is required to accommodate the rough interlaminar surface which is inherent to the UAM process. This in turn increases the total thickness of printed circuitries and thereby adversely affects the integrity of the UAM part. A specific solution is proposed to optimise the rough interlaminar surface through deforming the UAM substrates via sonotrode rolling or UAM processing.FindingsThe surface roughness (Sa) could be reduced from 4.5 to 4.1 µm by sonotrode rolling and from 4.5 to 0.8 µm by ultrasonic deformation. Peel testing demonstrated that sonotrode-rolled substrates could maintain their mechanical strength, while the performance of UAM-deformed substrates degraded under same welding conditions ( approximately 12 per cent reduction compared with undeformed substrates). This was attributed to the work hardening of deformation process which was identified via dual-beam focussed ion beam–scanning electron microscope investigation.Originality/valueThe sonotrode rolling was identified as a viable methodology in allowing printed electrical circuitries in UAM. It enabled a decrease in the thickness of printed electrical circuitries by ca. 25 per cent.

Characterization of carbon nanotube enhanced interlaminar fracture toughness of woven carbon fiber reinforced polymer composites

In this study, the effect of multi-walled carbon nanotubes (CNTs) on the Mode I Interlaminar fracture toughness or the strain energy release rate of woven carbon fiber reinforced composites has been investigated and characterized by double cantilever beam tests. The CNTs were uniformly dispatched on the surface of woven carbon fiber prepregs with the CNT loading ranging from 0–4.0 g/m2 by an increment of 0.5 g/m2. The prepregs were then layered and cured with an out-of-autoclave processing. SEM examination of the fractured surfaces revealed that the CNTs were uniformly distributed in the interlaminar surface. The experimental results demonstrate that a small CNT loading can increase the interlaminar fracture toughness because extra energy is required to break CNTs for crack propagating through. At the CNT loading of 1.0 g/m2, the fracture toughness of sample increases up to 32%. As the CNT loading is increased beyond 1.0 g/m2, a consistent decrease in the fracture toughness is observed, which can be attributed to the slippage among CNTs leading to initiation of micro-fracture in the matrix.

Buckling analysis of three dimensional braided composite plates under uniaxial loading using Inverse Hyperbolic Shear Deformation Theory

Three dimensional braided composites exhibit several distinct advantages which are not realised in traditional laminates. This is due to the out of plane orientation of some fibers & fully integrated nature of fiber arrangements of three dimensional braided composites that enhance stiffness/strength in the thickness direction and eliminate the inter-laminar surface characteristics. A recently developed Inverse Hyperbolic Shear Deformation Theory (IHSDT) has proved its accuracy and efficiency for static and buckling analysis of laminated composite and sandwich structures. However, its efficacy for the three dimensional braided composites has not examined so far in the literature. In present study, applicability and efficacy of IHSDT has been analysed for the buckling analysis of three dimensional braided composite plates. The effects of various parameters of three dimensional braided composite on the buckling response are examined in details. It is observed that the geometric parameter (aspect ratio), fiber volume fraction and braiding angle have a significant effect on the buckling response of the braided composite plates. It is also observed from the results that the IHSDT accurately predicts the buckling responses of the braided composite plates.

Delamination propagation analyses of spar wingskin joints made with curved laminated FRP composite panels

Initiation and propagation of inter-laminar delamination in adhesive bonded spar wingskin joint (SWJ) made with laminated fibre-reinforced plastic (FRP) composite curved panels have been studied employing three-dimensional finite element analyses. In-plane and out-of-plane normal and shear stress distributions are seen to be highly three-dimensional in nature. Tsai-Wu coupled stress failure criteria have been employed to identify critical locations of onset of delamination-induced damage. This occurs underneath the toe-end of the spar overlap and at the inter-laminar surface between the first and second plies of the curved FRP wingskin panel. Significant edge effects on the joint strength have been observed due to the curvature geometry of the composite wingskin panels. Non-linear finite element analyses have been carried out for study of delamination propagation using contact and multi point constraint (MPC) elements. The use of contact elements prevents inter-penetration of delaminated surfaces. Whereas, sequential release of MPC elements facilitates computation of opening, sliding and cross-sliding modes of delamination-induced strain energy release rates (SERR) by using virtual crack closure technique. Variation in delamination lengths significantly effects the variation of peel and inter-laminar shear stresses and different modes of SERRs. Variations on the two delamination fronts are seen to be quite different indicating dis-similar propagation rates. The Mode I SERR (GI) predominantly governs the delamination propagation in the SWJ.

Design and analysis of a smart graded fiber-reinforced composite laminated plate

The present work deals with the design of a graded fiber-reinforced composite lamina and graded laminates with an objective of reduced inter-laminar stress-discontinuity in composite laminates. First, the micro-structure of composite lamina is designed and its graded elastic properties are predicted. Next, its utility is demonstrated through the proposition of a new lamination scheme in which a conventional laminated composite plate (CLCP) is converted to a graded laminated composite plate (GLCP). The usefulness of the present design in attaining continuous stresses at the inter-laminar surface of composite laminates is verified through a static analysis. For conversion of a CLCP into a GLCP, the changes in the dynamic characteristics are also presented by analyzing the nonlinear frequency responses of CLCP/GLCP integrated with a layer of active constrained layer damping (ACLD). The dynamic analysis clarifies the suitability of proposed lamination scheme for the use of GLCP under transverse harmonic loads.

Compression–After–Impact Strength and Surface Morphology in Toughened Composite Materials

Interleaved toughening is a promising approach of improving fracture toughness in composites. Past observations of toughened particles have been conducted using the Optical Microscopy or Scanning Electron Microscopy for localized microstructures at a specific location; however, these methods may not be able to provide a 3–D, birds–eye view of the global particle distribution on a composite interlaminar surface. In this paper, we discovered an interesting optical phenomenon which can be used to observe surface morphology in toughened composites. This novel optical technique is able to reveal the detailed morphological information of toughened particles and individual fibers, including particle distribution uniformity, particle size variation, and fiber misalignment. It was found that particle distribution uniformity is a critical factor in differentiating the high and low Compression–After–Impact (CAI) laminates. High–CAI laminate had uniform and denser particle distribution, whereas low–CAI laminate had very non–uniform particle distribution.

A novel optical technique for observation of global particle distribution in toughened composites

One promising approach to improving fracture toughness is interleaved toughening. Past observations of toughened particles have typically been conducted using the optical microscope; however, such an approach may not be able to provide a 3-D, birds-eye view of how the toughened particles are distributed on a composite interlaminar surface. In this paper, we developed a simple yet novel optical technique for observing global particle distribution on a composite laminate surface. IM7/8551-7, a material used for the fan blades of the GE90, the world’s most powerful commercial aircraft jet engine, was used to demonstrate the strength of our optical technique, which produces high-quality morphological information. High-GIC laminate had more uniform and denser particle distribution with a smaller particle size, whereas low-GIC laminate had larger, but far less toughened particles. The same optical technique can also be used on other particle-toughened composite systems.

Initiation of free-edge delamination in composite laminates

Edge delamination onset on composite laminate was investigated for a carbon/epoxy G947/M18 composite material. On the edge of laminates, out-of-plane stresses arise, even up to material failure. Layer thickness is also known as influencing delamination onset stress. Making use of (i) a conventional model assuming plies homogeneity, damageable elastic behaviour, plane interlaminar surface also infinite interfacial stiffness and (ii) a local stress tensor correction near the edge, allows an efficient calculation of the full stress tensor, thanks to an asymptotic method. Then, a stress criterion was studied. The assessment of criterion parameters from test results was based on conjugate gradient method and experimental thickness effect. Edge delamination tests were performed on several specimens with various lay-ups and loaded under interlaminar shear, tension and compression. Acoustic emission was used to detect delamination initiation. As expected, these tests have exhibited layer thickness influence on onset stress. The assessment of shear and tension parameters shows good agreement between theory and experimental results. But the influence of interlaminar compressive stresses seems insufficiently taken into account in the criterion formulation.

Onset of free-edge delamination in composite laminates under tensile loading

Edge delamination onset on composite laminates has been investigated for a carbon/epoxy T800/914 composite material. On the edge of laminates, out-of-plane stresses arise, even up to material's failure. Layer thickness is also known as well to influence delamination onset stress. Making use of a conventional model (that is to say assuming plies homogeneity, elastic linear behaviour, plane interlaminar surface and interface's infinite stiffness) and of a local stress tensor correction near the edge, allows, thanks to an asymptotic method, an efficient calculation of the full stress tensor. A stress criterion has then been studied. Criterion parameters assessment, from test results, has been focused on, based on conjugate gradient method and experimental thickness effect. Edge Delamination Tests have been performed on several specimens of various layups. Interlaminar shear, tension, as well as shear and tension combination have been investigated. Acoustic emission was used to detect delamination onset. As expected, these tests have exhibited layer thickness influence on onset stress. Shear parameter assessment shows good agreement between theory and experimental results. A single set of parameters is necessary to predict delamination for different layups. But experimental testing for both tensile and mixed mode has shown that failure may not be interlaminar, as expected, but intralaminar.

Influence of microstructure on modal work-of-fracture of carbon fibre/pitch-derived carbon composites

Influence of microstructure on multi-modal work-of-fracture of carbon fibre/pitch-derived carbon composites (C/C composites) was studied. C/C composites in which matrix textures were flow type and mosaic type were prepared. Besides, C/C composites were heat-treated at 1000°C-2800°C. Modal work-of-fractures of tensile fracture and interlaminar shear fractures of the composites were simultaneously estimated. The modal work-of-fracture of tensile fracture was governed by the energy dissipation in fibre pullout process. The energy dissipation in fibre pull-out process was changed by the volume fraction of fibre, texture of matrix carbon and development of graphite crystallite. On the other hand, the modal work-of-fracture of interlaminar shear fracture was explained by debris density on the interlaminar shear fracture surface.

A PARTIAL HYBRID DEGENERATED PLATE/SHELL ELEMENT FOR THE ANALYSIS OF LAMINATED COMPOSITES

A degenerated plate/shell element based on the combined energy variational principle and the equivalent single-layer model is proposed. It is derived from the 3-D continuum equation by imposing one constraint on the 3-D isoparametric solid element: a straight line normal to the mid-surface before deformation remains straight but not normal after deformation. The continuities of interlaminar stresses are satisfied at the interlaminar surface and the number of degrees of freedom per node is independent of the number of layers. In this work, the combined energy variational principle is used to overcome transverse stress continuity limitations of single-layer models. The traction-free conditions are satisfied on the upper and lower surfaces of a laminate by assuming the transverse stress components independently. The transverse normal strain is taken into account in order to consider the full 3-D effect in a laminated composite. The iso-function method and the classification method of the stress modes are used to construct the assumed stress field which contains a minimum number of stress modes and guarantees no zero energy mode in the element stiffness matrix. Three examples are presented to illustrate the efficiency and accuracy of the element.

A PARTIAL HYBRID DEGENERATED PLATE/SHELL ELEMENT FOR THE ANALYSIS OF LAMINATED COMPOSITES

A degenerated plate/shell element based on the combined energy variational principle and the equivalent single-layer model is proposed. It is derived from the 3-D continuum equation by imposing one constraint on the 3-D isoparametric solid element: a straight line normal to the mid-surface before deformation remains straight but not normal after deformation. The continuities of interlaminar stresses are satisfied at the interlaminar surface and the number of degrees of freedom per node is independent of the number of layers. In this work, the combined energy variational principle is used to overcome transverse stress continuity limitations of single-layer models. The traction-free conditions are satisfied on the upper and lower surfaces of a laminate by assuming the transverse stress components independently. The transverse normal strain is taken into account in order to consider the full 3-D effect in a laminated composite. The iso-function method and the classification method of the stress modes are used to construct the assumed stress field which contains a minimum number of stress modes and guarantees no zero energy mode in the element stiffness matrix. Three examples are presented to illustrate the efficiency and accuracy of the element.

Three dimensional stress analysis of symmetric composite laminates under uniaxial extension and in-plane pure shear

This paper presents the stress and strain distribution obtained numerically in the thickness direction in the central region of symmetric composite laminates under uniaxial extension and in-plane pure shear loading. It is shown that in the symmetric angle-ply laminates, near the interlaminar surface, there is an interlaminar boundary-layer effect. The abrupt change of the fiber orientation at the interlaminar surface causes a high concentration of stresses and a local three dimensional stress state.

Mechanical Properties of Multidirectional Fiber Composites

An investigation was conducted to systematically evaluate the structure- property relationships in both 3-D and the corresponding 2-D composites. Macroscopic failure modes and microfailure mechanisms in these composites were studied as a function of various material parameters. In particular, the influence of the stitched Z-direction fibers on the mechanical behavior of an otherwise 2-D composite was investigated. The 3-D composites have been found to possess a greater damage resistance than the conven tional 2-D composite materials. The Z-direction (or third-direction) fibers effectively reduce the extent of delamination by increasing the interlaminar surface energy. The total energy absorbed by a composite during impact loading, three-point, and four-point bend ing test is therefore raised without creating an excessive amount of delamination. The third directional fibers are found to have a profound effect on the failure mechanisms.

The characterization of uniaxial graphite fiber/epoxy composites by mechanical testing and scanning electron microscopy

AbstractThe effect of the constituent material on the mechanical properties and on the overall crack propagation behavior of uniaxial graphite/epoxy composites was studied using two different types of graphite fibers, i.e., PAN precursor and pitch precursor fibers in combination with an epoxy known for its toughness. The tensile properties and interlaminar shear strength (ILSS) of composites with various fiber volume fraction were studied using an Instron Universal Testing Machine. The fracture surface of composites was studied using scanning electron microscopy (SEM). The surprising result from the tensile tests is that the percent elongation of AS4/epoxy composites was much higher than that of individual components, i.e., the epoxy and the fiber. The stress–strain curves of AS4 fiber/epoxy composites show yielding. The yielding is interpreted to result from shear stress concentration due to the fiber breakage. Two causes of failure mode were observed in this study: (1) ductile plastic deformation by the PAN‐based AS4/epoxy composites; (ii) brittle catastrophic failure shown by the pitch‐based VSB‐16/epoxy composites. The tensile strengths and moduli of the composites increased as the volume fraction of fibers increased. The principal fracture features of AS4/epoxy composites under tension were fiber pullout (bundles and single fibers) and very rough fracture surface due to the pullout of fiber bundles with epoxy, whereas VSB‐16/epoxy composites under tensile and bending loads showed only tensile (or compressive) failure. The composites of epoxy/PAN (polyacrylonitrile)‐based fibers (AS4) showed greater ILSS than those of the pitch‐based fiber (VSB‐16)/epoxy. The most characteristic features of the interlaminar shear failure surface of AS4/epoxy composites are delamination and formation of hackles.

On higher order shear deformation theory of laminated composite panels

In this paper we examine the suitability of higher order shear deformation theory based on cubic inplane displacements and parabolic normal displacements, for stress analysis of laminated composite plates including the interlaminar stresses. An exact solution of a symmetrical four layered infinite strip under static loading has been worked out and the results obtained by the present theory are compared with the exact solution. The present theory provides very good estimates of the deflections, and the inplane stresses and strains. Nevertheless, direct estimates of strains and stresses do not display the required interlaminar stress continuity and strain discontinuity across the interlaminar surface. On the other hand, ‘statically equivalent stresses and strains’ do display the required interlaminar stress continuity and strain discontinuity and agree very closely with the exact solution.