Fiber Kinking Research Articles

Damage progression and failure of single-lap thin-ply laminated composite bolted joints under quasi-static loading

Composite bolted joints are widely used on primary and secondary load-bearing structures of aircrafts. However, investigating the damage progression and failure of composite bolted joints under high bearing loads is challenging due to the geometric, contact and material nonlinearities. In this work, an extensive experimental study has been carried out to investigate and understand the damage evolution and failure of single-lap thin-ply laminated composites bolted joints under quasi-static loading. Quasi-isotropic carbon/epoxy laminates with stacking sequence [45/0/-45/90]4s were selected for fabricating the test specimen. The specimens were observed using X-ray computed tomography (CT) scanning and SEM imaging at different stages of the loading process to evaluate internal damage and deformation characteristics. The results indicate that the bearing failure of composite bolted joints can be interpreted as an accumulated damage process with local compressing, and mainly includes four stages: damage initiation, damage evolution, non-linear softening and catastrophic failure. The major failure modes of the thin-ply laminates are found to be similar to those of traditional thickness composite, including fiber breakage, matrix cracking, delamination, fiber kinking and fiber-matrix splitting. However, the major difference is that the delamination growth in the bearing area in thin-ply composites is suppressed compared with the traditional thickness composites. Therefore, the obtained experimental data provides valuable information for developing mechanism-based failure models of single-lap thin-ply composite bolted joints.

A constitutive model for fiber kinking: Formulation, finite element implementation, and verification

A new mesoscale material model is proposed to analyze structures that fail by the fiber kinking damage mode. The model is formulated following the fiber kinking theory such that the interaction of shear nonlinearity and fiber rotation are tracked throughout the loading history. The model is implemented as a user-defined material in a commercial finite element code. A decomposition algorithm is introduced in the finite element implementation to decouple the mesh size and kink band width. To evaluate the assumptions of the mesoscale model, the results were compared with those of a high-fidelity micromechanical model. A direct comparison between the two models shows remarkable correlation, providing evidence that the key features of the fiber kinking phenomenon are appropriately accounted for in the mesoscale model. The model is demonstrated for a simple unnotched compression configuration. The results show that the model is successful at representing the kinematics of fiber kinking.

Damage characteristics of open-hole laminated composites subjected to longitudinal loads

The damage mechanism of open-hole composite laminates subjected to longitudinal loads was studied by a three-dimensional (3D) progressive damage analysis (PDA) model, which took two essential issues, fiber kinking and shear non-linearity, into consideration. Furthermore, a parameter, damage width, was introduced to indicate the damage extent of the specimen. A good agreement between the simulation and the experiment results demonstrates the effectiveness of the proposed model. Specifically, some interesting characteristics during the damage process were revealed by the finite element analysis. It was found that in open-hole tension (OHT) test, the damage appearing first was matrix tension damage, while fiber micro-buckling emerged first in open-hole compression (OHC) test. From the load- damage width curves of open-hole laminates, it was observed that as long as any fiber damage in 0° ply spreads to the outer edges, the specimen loses its loading capacity. However, this phenomenon, interestingly, cannot be found in 90° or ±45° plies. This characteristic pointed out that the damage of 0° plies in laminated composites can be used as an indicator to reflect the damage extent of composite structures.

Effect of stacking sequence and bundle waviness in quasi-isotropic NCF composites subjected to compression

The current study is focused on the compressive strength of composite materials containing non-crimp fabric (NCF) reinforcement, and how ply stacking sequence and fibre waviness influence onset and growth of damage in such materials. Experiments reveal significant effects from stacking sequence, both on the compressive strength as such, and on the underlying failure mechanisms. The fibre waviness also has a strong influence on the strength. Fibre kinking is seen before ultimate failure for all configurations but some of them also show local delamination prior to kinking. A finite element simulation methodology is developed and used for the studied cases. It handles local variations of fibre orientations by corresponding re-orientation of stiffness matrices at element level. The simulations provide good predictions of intra- and inter-laminar failure considering both in-plane and out-of-plane fibre bundle waviness. The model is further used in a parametric study of the influence from bundle waviness on the compressive strength.

Shear behavior of concrete beams reinforced with CFRP sheet strip stirrups using wet-layup technique

Fiber-reinforced polymers (FRP) are being used in reinforced concrete structures featuring their lightweight, high tensile strength, nonmagnetic, and corrosion resistance properties, contrasting conventional steel reinforcement. However, low bend strength, anisotropic and non-plastic nature of FRP composites are the major concerns for their assertive applications in construction industry. Pultruded FRP bar stirrups are characterized by premature failure either due to slippage at overlapping region or rupture at bend portion due to kinking of fibers. In this experimental study, close-type direct wound rectangular cross-section CFRP sheet strip stirrups (CSS) were proposed to mitigate the shortcomings of pultruded CFRP bar stirrups (CP). Total of seven full-scale beam specimens reinforced with steel longitudinal bars and CFRP stirrups were cast and tested to investigate the performance of proposed stirrups. The main study parameters include shear reinforcement material type, the cross-sectional geometry of CFRP stirrups, and the inclination angle of CSS stirrups with the beam axis. Specimens reinforced with CSS stirrups provided better shear capacity and crack control ability compared to steel and CP stirrups for same shear reinforcement ratios. Shear capacity of beam corresponding to serviceability limit state was improved up to 13% by changing the inclination of CSS stirrups from 90° to 45°.

Compressive failure of fiber composites containing stress concentrations: Homogenization with fiber-matrix interfacial decohesion based on a total Lagrangian formulation

Compression failure by fiber kinking limits the structural applications of fiber composites. Fiber kinking is especially prevalent in laminates with holes and cutouts. The latter behavior is characterized by strain localization in the matrix material and fiber rotations. To study fiber kinking on the level of the individual constituents, a homogenization of fiber composites is presented. It is based on a total Lagrangian formulation, making it independent of fiber rotations. It accounts for the microstructure of the composite, including fiber-matrix interfacial decohesion, and enables all types of material behavior of the constituents. The response of each constituent of the composite is modeled separately and the global response is obtained by an assembly of all contributions. The model is implemented as a user-defined material model (UMAT) in ABAQUS and used for multiscale modeling of notched unidirectional plies subjected to compression. The model performs well in agreement with a finite element model of an explicit discretization of the microstructure and literature results. The simulations predict the formation of a kink band in near 0-degree plies and show that the open-hole compression strength is sensitive to fiber-matrix interfacial decohesion. The present work suggests a convenient and computationally efficient tool for simulating the elastic-plastic behavior of fiber composites on the fiber-matrix level and predicting the compressive strength of laminates.

Effects of Xylanase Pretreatment on the Quality of Refiner Mechanical Mulberry Branch Fibers

We performed xylanase pretreatment prior to mechanical refining in the production of mulberry branch fibers, with the objective of saving energy and studying the effects of such pretreatment on the quality of the fibers. To determine the effects of the enzyme action, we analyzed the energy required for refining, related yield, and the dimension, deformation, and morphology of the fibers. We found that, with the xylanase pretreatment, the refining energy was reduced by 4%, with the yield of fibers being maintained at >85%. In addition, the fiber bundles were defibered further, resulting in reduced average length of the fiber. Furthermore, the fiber widths increased because of the improved swelling effect of the xylanase pretreatment. However, in some instances, the fine elements were reduced. With a low enzyme dosage, the fiber coarseness decreased remarkably and, because of the swelling and softening effects of the xylanase pretreatment on the mulberry branches, the fiber kink ratios and curl were reduced. Additionally, the mulberry branch tissue was loosened, facilitating fiber separation. In view of these findings, the biomechanical process could be a potentially green and efficient process for the manufacturing of mulberry branch fibers.

The Effect of Single and Double-wefted Styles on the In-plane Impact Behavior of Fabric Carbon-aramid/Epoxy Intralayer Hybrid Composite Laminates

The characterization of in-plane impact properties of epoxy laminates reinforced with intralayer hybrid carbon/aramid fabrics was investigated. The effects of different fabrics, which had architectures of 2D and 3D form, on the impact performance was analyzed by drop-weight impact test. It revealed that different braiding styles had a significant influence on the impact properties. The anti-impact properties were enhanced due to aramid fibers into fabrics, in particular, the fabrics with 3D styles that aramid fibers as the Z-yarns. Laminates reinforced by 3D fabrics containing 25 % aramid fiber presented optimal anti-impact performance, and impact peak force and absorbed energy reached 4217 N and 26.71 J, increased by 135.5 % and 137.2 % relative to pure carbon fabric system, respectively. The results of damage morphologies showed that main damage modes, of fiber kinking, necking and fracture, improved the cushioning effect of laminates, which contributed to the enhancement of impact resistance capacity of composite laminates.

Compressive behavior of unidirectional FRP with spacial fibre waviness and non-uniform fibre packing

Longitudinal compressive strength of common engineering FRP is known to possess large stochastic variation which highlights the importance of accurate prediction on the compressive failure behaviour. This study employs non-linear 3D FE modeling to investigate the influence of spacial fibre waviness and non-uniform fibre packing on the compressive strength of UD FRP. Cuboid shaped FE model is constructed, and a region of sinusoidal fibre waviness is introduced in the model. Fibre is considered as materially linear-elastic but geometrically non-linear, while matrix is considered as elastic perfectly plastic. The results highlight that in the fibre kinking process fibre rotation may not necessarily occur in the plane with largest preliminary fibre misalignment angle (or fibre waviness plane). The compressive strength of FRP not only depends on fibre misalignment angle but also on fibre waviness direction, while non-uniform fibre packing seems to have insignificant influence on the compressive strength.

Failure of fiber-reinforced composite laminates under longitudinal compression

A micro-mechanics approach is put forward to predict the fiber kink inclination angle and the strength of a unidirectional composite under longitudinal compression. Internal stresses of constituent materials in the kink-band are calculated through Bridging Model; thus, only the constituent fiber and matrix properties of the composite are needed. Considering the non-uniform stress distribution caused by the embedded fiber, the homogenized stresses of the matrix are converted into true values with stress concentration factors before being used for failure analysis. The definition and application of stress concentration factors are introduced. A failure criterion based on Mohr’s theory is established to determine the orientation of potential failure surface and the failure situation of matrix, whereas a maximum normal stress failure criterion is used to detect the fiber breakage. The longitudinal compressive strength of a laminate is defined as the applied load at the moment when a failure of whichever constituent materials occurs first. The practicability and accuracy of the theory have been verified with a comparison between the predictions and the measurements obtained from the worldwide failure exercises and some other literatures.

Dynamic Mechanical Properties of Russian Pinus sylvestris var. mongolica Litv: Experimental and Numerical Simulation

With the Split Hopkinson Pressure Bar (SHPB) experiment and numerical simulation on Russian Pinus sylvestris var. mongolica Litv carried out, the dynamic mechanical properties of the pine wood at high strain rates were researched, and the relevant constitutive models was analyzed. The experiment results suggested that the dynamic mechanical properties of the pine wood were affected by strain rates and wood directions. The stress-strain relationship was mainly influenced by wood directions, but the initial yield stress was related to strain rates and wood directions. When the load was along the straight grain direction, fracture behaviors were caused by shear damage and fiber tensile fracture. The specimens were broken into ‘crushed’ pieces because of fiber and lignin separation, and fiber kink. However, it was chief role for the fracture in transverse grain direction that failure cracks grew down fiber. The dynamic responses described with model based on improved Hashin failure criterion were consistent with actual results, compared with model based on the Hill yield criterion. Related research work provides a certain reference for the related problems research and engineering practice application of Russian Pinus sylvestris var. mongolica Litv.

Failure mechanisms of cross-ply carbon fiber reinforced polymer laminates under longitudinal compression with experimental and computational analyses

This study investigates the failure mechanisms of cross-ply laminates subjected to longitudinal compressive loading. A sequence of failure initiation and propagation process is observed based on optical microscopy images of failed specimens. Specifically, the failure process in the cross-ply laminates involves a combination of four failure mechanisms: fiber kinking, delamination, matrix cracking, and fiber/matrix splitting. We find that the kink-bands in the middle 0° plies of the cross-ply laminates most often show a wedge shape and the angle of matrix cracks in the 90° plies is slightly larger than that of pure 90° plies. The reason has been attributed to the constraining effects by the adjacent plies. The next focus of this study is to propose hybrid micro-macro models to more systematically study the failure mechanisms of the cross-ply laminates. We show that these models accurately predict the combined failure modes of the cross-ply laminates and enable us to closely investigate the interactions of different failure mechanisms. More importantly, the hybrid models achieve great computational efficiency compared to full-scale microstructural models. The combined experimental and computational analyses presented here provide a new level of understanding of the kink-band morphology and damage mechanisms in the cross-ply laminates.

Comparison of Analytical Approaches Predicting the Compressive Strength of Fibre Reinforced Polymers.

Common analytical models to predict the unidirectional compressive strength of fibre reinforced polymers are analysed in terms of their accuracy. Several tests were performed to determine parameters for the models and the compressive strength of carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP). The analytical models are validated for composites with glass and carbon fibres by using the same epoxy matrix system in order to examine whether different fibre types are taken into account. The variation in fibre diameter is smaller for CFRP. The experimental results show that CFRP has about 50% higher compressive strength than GFRP. The models exhibit significantly different results. In general, the analytical models are more precise for CFRP. Only one fibre kinking model’s prediction is in good agreement with the experimental results. This is in contrast to previous findings, where a combined modes model achieves the best prediction accuracy. However, in the original form, the combined modes model is not able to predict the compressive strength for GFRP and was adapted to address this issue. The fibre volume fraction is found to determine the dominating failure mechanisms under compression and thus has a high influence on the prediction accuracy of the various models.

Damage growth and strain localization in compressive loaded fiber reinforced composites

To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking which is typically induced in compression. We propose a new set of continuum damage models that are able to predict fiber kinking response under compression. A structure tensor based formulation is established at the unidirectional ply level, where the elastic material response is governed by transverse isotropy. To consider geometrical effects in conjunction with fiber kinking instability, a continuum damage formulation at finite strain is developed. The fracture area progression includes a convective and a local damage production involving a finite progression speed. In this framework, two damage evolution models are considered; one non–local model including the gradient damage effect and a local one, without the gradient enhancement. The models are implemented in a FE–code and validated for a compression loaded specimen. The models are computationally robust and can predict the localized nature of fiber kinking. A thorough sensitivity study is presented to show how the different formulations influence the predicted responses.

A progressive damage model for 3D woven composites under compression

A progressive damage model is proposed to investigate the damage initiation and evolution of 3D woven composites under uniaxial compression at a micromechanical level. The typical compressive experiments were carried out. Based on the observations, the compression failure modes of 3D woven composites mainly include fiber kinking, transverse failure of fiber tow, matrix fracture, and interfacial debonding. The initial damage criteria are according to the physically based failure criteria for the fiber kinking, the Puck criteria for the transverse failure of fiber tow, and the maximum stress criterion for the matrix. The damage of fiber tow–matrix interfacial is simulated through cohesive contact. Particularly, the fiber’s initial misalignment angle is taken into account in the damage model. The simulated compression results agree well with the experimental ones. The compressive stress–strain response of the 3D woven composite is forecasted. The damage evolution of each constituent of the 3D woven composite is obtained. The results show that the influence of the fiber’s initial misalignment angle on the compression strength of the 3D woven composite needs to be considered.

Determination of the longitudinal compressive strength of a CFRP ply through a tensile test on a laminate

In this study, an innovative test is proposed to identify the longitudinal compressive strength of a unidirectional ply. The key idea consists in designing a laminate that, when subjected to a tensile loading, fails by compressive failure in its central 90°-ply, due to the Poisson effect, without any prior damage. Six specimens have been tensile tested to failure. No intra-laminar matrix damage could be detected before the final failure. Fibre kinking in the 90°-ply is observed experimentally in the failed specimens. This damage mechanism, located in the gauge section of the specimens, leads to the final failure. A fast computational identification method is used to determine the longitudinal compressive stress and strain within the 90°-ply at failure, from this specific tensile test. The identified average failure properties are consistent with those obtained through conventional compression tests, but the associated scattering is much lower. Consequently, this innovative method leads to an increase in the design allowable, resulting in higher performance designs.

Localization in anisotropic elastoplastic micropolar media: Application to fiber reinforced composites

In this paper, an anisotropic elastoplastic constitutive model is formulated for fiber reinforced composites, undergoing large geometric deformation. The composite material is modeled as a micropolar continuum, which unlike a classical or a Cauchy continuum, takes into account the higher order fiber bending and twisting modes of deformation. In micropolar theory, the rotational degree of freedom is independent of the displacement field. This is utilized to express the microrotation tensor that represents the rotation of the fibers. An anisotropic micropolar yield criterion is introduced by extending Hill’s criterion in classical elastoplasticity and including couple-stresses. The geometric and materially nonlinear theoretical model is derived. The corresponding updated Lagrangian (UL) finite element formulation is discussed. In literature, micropolar theory has been applied to problems associated with localized deformation, where classical theories fail to capture this phenomenon due to the loss of ellipticity of the governing equations. These previous analyses have been done on isotropic micropolar media, and in order to initiate the onset of localization, a softening relationship between the equivalent stress and strain has been introduced. Since the introduction of material softening into the constitutive relation has been a topic of contention in the field of continuum mechanics, it is shown to be unnecessary. Instead, localization is induced due to geometric and material nonlinearity, along with misalignment of the principal material directions with respect to the dominant loading directions that introduce axial-shear coupling. In fiber reinforced composites, localization occurring from compressive loading is representative of fiber kink banding, which is associated with a snap-back behavior in the macroscopic stress-strain response.

Analyses of press formability of CFRP sheet considering the fiber kinking and the ductile behavior of resin

Establishment of a press forming method of carbon fiber reinforced plastic (CFRP) is desired. However, fundamental research on a ductility improving mechanism of CFRP is still insufficient. Since unidirectional CFRP shows different characteristics in compression and tensile deformation, accurate expression in both characteristics is necessary to investigate factors that affect formability. To express fiber kinking and ductile behavior of resin, a microscale model that separated fiber and resin was made. The fiber part in FEM model was tilted as initial misalignment and Gurson-Tvergaard-Needleman (GTN) model was applied to the resin part. To investigate the influence of design parameters such as temperature and initial void fraction on formability, this study performed tensile, compression and bending analyses by changing the resin temperature and initial void fraction. Results of compression analysis showed that the higher the temperature and initial void fraction, the earlier fiber kinking occurrence. Bending analysis showed a similar tendency. These results are physically reasonable. Therefore, these numerical experiments confirmed that the model used in this research is valid for studying factors that affect formability.

Experimental and computational analysis of failure mechanisms in unidirectional carbon fiber reinforced polymer laminates under longitudinal compression loading

This study investigates the failure mechanisms of notched unidirectional (UD) carbon fiber reinforced polymer (CFRP) laminates subjected to longitudinal compressive loading. A sequence of failure initiation and propagation is observed based on optical microscopy images of specimens during failure. The micrographs reveal that the main failure mechanism in UD laminates is fiber kinking failure. The influence of manufacturing defects and voids on fiber kinking mechanisms is also analyzed. The results show that the voids in the resin-rich area lead to kink-band splitting, while in some cases the voids cause the kink-band to deflect in a new direction depending on the location of the voids. The next focus of this study is to propose a computational micromechanics model considering local fiber waviness to study the fiber kinking failure mechanism. The computational results on kink-band formation and stress-strain response show good consistency with our experimental analysis. Finally, the failure envelopes of σ11-σ22 and σ11-τ12 subjected to combined loading conditions are obtained from the computational model, which provide essential inputs for future theoretical failure criteria.

Compressive Failure of Fiber Composites: A Homogenized, Mesh-Independent Model

Micromechanics models of fiber kinking provide insight into the compressive failure mechanism of fiber reinforced composites, but are computationally inefficient in capturing the progressive damage and failure of the material. A homogenized model is desirable for this purpose. Yet, if a proper length scale is not incorporated into the continuum, the resulting implementation becomes mesh dependent when a numerical approach is used for computation. In this paper, a micropolar continuum is discussed to characterize the compressive failure of fiber composites dominated by kinking. Kink banding is an instability associated with a snap-back behavior in the load–displacement response, leading to the formation of a finite region of localized deformation. The challenge in modeling this mode of failure is the inherent geometric and matrix material nonlinearity that must be considered. To overcome the mesh dependency of numerical results, a length scale is naturally introduced when modeling the composite as a micropolar continuum. A new approach is presented to approximate the effective transversely isotropic micropolar constitutive relation of a fiber composite. Using an updated Lagrangian, nonlinear finite element code, previously developed for incorporating the additional rotational degrees-of-freedom (DOFs) of micropolar theory, the simulation of localized deformation in a continuum model, corresponding to fiber kinking, is demonstrated and is found to be comparable with the micromechanics simulation results. Most importantly, the elusive kink band width is a natural outcome of the continuum model.