Xiannian Sun

Effect of Long Multi-walled Carbon Nanotubes on Delamination Toughness of Laminated Composites

Two continuum-mechanics-based mechanistic models are proposed to characterize the pull-out behavior of a long multi-walled carbon nanotube (MWCNT) from its surrounding matrix based on available experimental observations. One model is based on the mechanism of debonding and its propagation along the MWCNT—matrix interface due to weak interfacial shear strength; and the other is based on the sword-in-sheath mechanism after the breakage of the outermost layer in a MWCNT. Both models are then employed to describe the bridging tractions between the two delaminated laminates in a double cantilever beam (DCB) test specimen, and then to study numerically the effect of the long MWCNTs or nano pins on the delamination toughness of laminated composites. The present numerical results reveal that the MWCNTs length, density, and maximum pull-out displacement as well as the interfacial friction shear stress are important parameters affecting the delamination toughness.Two continuum-mechanics-based mechanistic models are proposed to characterize the pull-out behavior of a long multi-walled carbon nanotube (MWCNT) from its surrounding matrix based on available experimental observations. One model is based on the mechanism of debonding and its propagation along the MWCNT—matrix interface due to weak interfacial shear strength; and the other is based on the sword-in-sheath mechanism after the breakage of the outermost layer in a MWCNT. Both models are then employed to describe the bridging tractions between the two delaminated laminates in a double cantilever beam (DCB) test specimen, and then to study numerically the effect of the long MWCNTs or nano pins on the delamination toughness of laminated composites. The present numerical results reveal that the MWCNTs length, density, and maximum pull-out displacement as well as the interfacial friction shear stress are important parameters affecting the delamination toughness.

Nonlinear stress analysis for bonded patch to curved thin-walled structures

Adhesive bonding has been widely used to join or repair metallic and composite structural components to achieve or restore their designated structural stiffness and strengths. However, current analysis methods and empirical databases for composite bonded joint design and composite bonded patch repair are limited to flat plate and/or flat laminate geometries, and the effects of curvature on the performance and durability of composite bonded joints and repairs is not known. This paper presents a novel finite element formulation for developing adhesive elements and conducting 2.5-D simplified stress analysis of bonded repairs to curved structures. Both large deflections of the parent structures and nonlinear adhesive behavior are incorporated in the formulation. An in-house software called BPATCH has also been developed. A variety of numerical examples are presented to illustrate the effects of curvature, moderately large deflection and adhesive nonlinear behavior on stresses in the adhesive layer. Illustrative examples also indicate the effect of patch location, i.e., internal and external patches, patch size and patch thickness.

A New ENF Test Specimen for the Mode II Delamination Toughness Testing of Stitched Woven CFRP Laminates

This paper presents an experimental and numerical investigation on the effects of stitching distribution on the interlaminar fracture toughness of carbon fiber reinforced polymers (CFRPs) using an end notch flexure (ENF) specimen. To avoid premature failure in bending, reinforcing tabs were bonded to either side of the ENF specimens to create a tabbed ENF (TENF) specimen. The effect of stitch distribution on mode II delamination toughness is investigated by considering several stitch distribution patterns. The experimental results indicate that the mode II delamination toughness of stitched TENF specimens can be effectively measured and that stitch distribution does not play a significant role in improving the steady-state mode II delamination toughness of stitched CFRPs. Numerical results are also obtained using both MSC/NASTRAN and an in-house software for the tested specimens. A reasonable correlation exists between the numerical and experimental results.

Progressive Failure Analysis of Laminated Plates with Delamination

A numerical methodology on simulating progressive compressive failure of delaminated plates is presented. A finite element code based on the Reissner-Mindlin plate theory and the Von Karman’s nonlinear plate theory is developed to simulate initial buckling, postbuckling, contact effect of delamination front, delamination growth, fiber-breakage and matrix cracking. Delamination growth is taken into account by applying a fracture mechanics criterion which checks the strain energy release rate (SERR) along the delamination front. Meanwhile, fiber-breakage and matrix cracking are analyzed by a stiffness degradation scheme. Some numerical examples are presented to illustrate different failure mode of delaminated plates. It is found that the delamination growth is significantly affected by the boundary condition, and the stiffness degradation plays an important role in the strength analysis of delaminated plate.

Failure of stitched composite L-joints under tensile loading: experiment and simulation

This article presents an experimental and numerical investigation into the influence of transverse stitching on failure of composite L-joints under tensile loading. Six unstitched and six stitched L-joint specimens were manufactured and tested under quasi static tensile loading. It was observed that the average measured failure load and the associated crosshead displacement for the stitched L-joint specimens are increased significantly compared to those for the unstitched specimens. Full 3D and 2D plane strain finite element (FE) models were developed to simulate both stitched and unstitched L-joints with an implemented stitch element. The load—displacement curves and results predicted via FE models compare favorably with the experimental results. For the stitched L-joints, it is shown that the observed delamination in the elbow region of the flange can be modeled by using a softening model for epoxy layer.

Residual compressive strength of laminated plates with delamination

A study of residual compressive strength in delaminated laminates is presented. A methodology is proposed for simulating the whole compressive failure responses, such as initial buckling, postbuckling, contact of delamination front region, delamination propagation, fiber breakage, and matrix cracking etc. An finite element analysis (FEA) of the residual compressive strength is conducted on the basis of the Von Karmans nonlinearity assumption and the first-order shear deformation plate theory, combined with a stiffness degradation scheme. The numerical analysis models and methods are briefly introduced in this paper and some numerical examples are presented to illustrate it. From numerical results and discussion, it is clear that the compressive failure response involves complex multi-failure modes during compressive process. The method and numerical conclusions provide in this paper should of great value to engineers dealing with composite structures.

Curvature Effect on Fracture Toughness of Cracked Cylindrical Shells Bonded with Patches

Adhesively bonded patch repair has been widely used as an efficient and economical method to extend the service life of cracked structural components. Most of the currently available analysis methods and empirical databases for composite bonded patch repair to flat structures are computationally efficient and easy to use. However, the current knowledge on composite bonded repair for flat structures cannot be directly applied to curved repairs. A novel adhesive element developed by the authors in conjunction with a shell element is employed to investigate the fracture toughness at the crack tip of a cracked cylindrical shell bonded with a composite patch. To validate the present finite element model for curved composite patch repairs, the stress intensity factors in a flat composite patch repair are first computed by the strain energy release rate analysis method and compared with those available in the literature. For the curved patch repairs, a full three-dimensional finite element analysis is also conducted to validate the proposed numerical model. Some selected numerical examples are given to demonstrate the effect of curvature on the fracture toughness of a cracked cylindrical shell bonded with a composite patch subjected to different types of loading.

Optimization of Ply Drop-offs in Bonded Patch to Cylindrical Shell Structures

Ply drop-off is a technique widely used to achieve gradual thickness change in composite laminate, and it can be utilized to form boundary tapering of a composite patch bonded to a parent structure. This paper firstly presents a new adhesive element formulation with a nonneutral reference plane that takes into account the geometrical features of ply drop-off and can be applied to conduct 2.5-D simplified stress analysis of bonded repairs to curved structures. The new formulation is numerically validated by comparing results calculated using both the present formulation and a full three-dimensional finite element formulation available in commercial software. Sequential Linear Programing (SLP) method is then employed in conjunction with a fully implemented automatic mesh generation algorithm to minimize adhesive stresses, namely, the peak peel, shear or Von Mises stresses, through optimally selecting ply drop-off parameters. The objective of this paper is to ensure effective bonding between a patch and an undamaged parent shell structure via minimizing maximum stress along the boundary of adhesive layer. Several numerical examples are presented to demonstrate that an optimum ply drop-off in a bonded patch can dramatically reduce peak stresses in the adhesive layer, particularly the peak positive peel stress.

Modelling composite reinforcement by stitching and z-pinning

Publisher Summary Fiber-reinforced composite laminates have been widely used in weight-critical applications, especially in aircraft structures, due to their significant advantages over traditional engineering materials. However, a critical drawback to this traditional layered 2D architecture is its relatively low interlaminar fracture toughness, which makes the laminates susceptible to delamination when subjected to interlaminar stress concentrations. For aircraft structures made from Carbon-Fiber Reinforced Plastic [“CFRP”] composites, delamination is inevitable as a result of low energy impact, such as hail impact, accidental impacts from dropped tools during manufacturing, maintenance and servicing, and impacts from stones on the tarmac during take-offs and landings. To retain adequate strength, stiffness, and fatigue performance of the delaminated composite structures, thick laminates or improved interlaminar properties are often employed. Apparently, an increase in laminate thickness leads to higher structural weight and material costs. Consequently, a considerable amount of research has been devoted to improving the damage tolerance of a composite structure, either by selecting better materials or by utilizing stitching, knitting, z-pinning, weaving, or braiding in order to introduce through-thickness reinforcement into the laminate. As only minor improvements are achieved in delamination toughness by using tougher matrices, there have been more recent efforts devoted to through-thickness reinforcement by stitching and z-pinning.

DESIGN CURVES FOR METALLIC Z- PINNED COMPOSITE LAMINATED DOUBLE CANTILEVER BEAMS

Composite laminated double cantilever beams, reinforced by metallic z-pins, are studied in this paper. Generally, a pull-out model only considering the axial frictional traction is used to evaluate the through-the-thickness reinforcements, such as stitches and fibrous z-pins, due to their negligible bending-resistance capability. However, a metallic z-pin can carry a higher transverse load which may be improper to be ignored. Therefore, a novel z-pin model is developed to take into account the transverse bending traction provided by a metallic z-pin, i.e., a metallic z-pin provides both the axial frictional traction and the transverse bending traction. Correspondingly, design curves for composite laminated double cantilever beams (DCB), reinforced by the metallic z-pins, are developed and presented.