Applied Composite Materials (v.22, #3)

Effect of Nesting on the Out-of-Plane Permeability of Unidirectional Fabrics in Resin Transfer Molding by Liangchao Fang; Jianjun Jiang; Junbiao Wang; Chao Deng (231-249).
The nesting of layers has great effect on the permeability which is a key parameter in resin transfer molding (RTM). In this paper, two mathematical models were developed to predict the out-of-plane permeability of unidirectional fabrics with minimum and maximum nesting, respectively. For different zones of characteristic yarn arrangement in the unit cell, the local permeability was modeled as a function of geometrical yarn parameters. The global permeability was then modeled as a mixture of permeabilities of different zones with the electrical resistance analogy. The influences of local permeability of each zone on the global value of unit cell were deeply researched. In addition, two different fabrics were tested and a reasonably good agreement was found between the model predictions and experimental results. We also found that the permeability values were two orders of magnitude larger with minimum nesting than with maximum nesting. However, the differences between minimum nesting and maximum nesting decreased with increasing fiber volume fraction.
Keywords: Nesting; Permeability; Resin transfer molding; Unidirectional fabrics

Pull-Through Mechanical Behavior of Composite Fastener Threads by Zhidong Guan; Junwu Mu; Fei Su; Tianya Bian; Yongjie Huang; Zengshan Li (251-267).
A method was proposed to test the pull-through mechanical behavior of fastener threads, which were fabricated from weave carbon/carbon (C/C) composites. The damage morphologies of the C/C fastener threads were observed through an optical microscope and high-resolution micro-CT systems. The acoustic emission (AE) technique was utilized to track the damage progression of threads during loading up to fracture in terms of AE event rate which has an exponential type profile. Finally, A 3D finite element damage evolution model of composite threads was established based on continuum damage mechanics to calculate the thread load distribution and damage progression. The relations between the pitch and the load distribution, as well as between different fabrication directions and ultimate loads, were investigated by using this model. The stress in the first thread was analyzed based on the tapered cantilever assumption. The results show that, the first thread is brittle fracture at the root where is the higher stress level region of the threads and it is the initial damage. The load distribution in C/C threads is not uniform and not improved as the value of pitch decreases. Load capacity of C/C threads is different result from the fabrication direction. Numerical results agree well with experimental results.
Keywords: Composite fastener threads; Pull-through test; Load distribution; Acoustic emission

Nowadays wind energy is widely used as a non-polluting cost-effective renewable energy resource. During the lifetime of a composite wind turbine which is about 20 years, the rotor blades are subjected to different cyclic loads such as aerodynamics, centrifugal and gravitational forces. These loading conditions, cause to fatigue failure of the blade at the adhesively bonded root joint, where the highest bending moments will occur and consequently, is the most critical zone of the blade. So it is important to estimate the fatigue life of the root joint. The cohesive zone model is one of the best methods for prediction of initiation and propagation of debonding at the root joint. The advantage of this method is the possibility of modeling the debonding without any requirement to the remeshing. However in order to use this approach, it is necessary to analyze the cyclic loading condition at the root joint. For this purpose after implementing a cohesive interface element in the Ansys finite element software, one blade of a horizontal axis wind turbine with 46 m rotor diameter was modelled in full scale. Then after applying loads on the blade under different condition of the blade in a full rotation, the critical condition of the blade is obtained based on the delamination index and also the load ratio on the root joint in fatigue cycles is calculated. These data are the inputs for fatigue damage growth analysis of the root joint by using CZM approach that will be investigated in future work.
Keywords: Wind turbine blade; Delamination index; Cohesive zone model; Fatigue debonding; Load ratio

Multi-Scale Modeling and Damage Analysis of Composite with Thermal Residual Stress by Geng Han; Zhidong Guan; Zengshan Li; Mi Zhang; Tianya Bian; Shanyi Du (289-305).
In order to analysis thermal residual stress and its influence on the strength of composite, the hierarchical multi scale simulation method is applied. A microscopic computational model of single fiber composite with thermal residual stress is built to research the stress distribution. Then the damage initiation discipline details of unidirectional composite are researched, and the effects of different fiber arrangements on thermal residual stress distribution, damage initiation and the different final failure behaviors of fiber regular distribution and random distribution under tension and compression are researched in details. It shows that in fiber regular arrangement, damage initiation in interface appears evenly and in matrix it appears at somewhere randomly. But in fiber random arrangement, initial damage focuses at the resin pockets between closely packed fibers with both interface and matrix damage. The maximal thermal residual stress in fiber random arrangement model is larger than that in fiber regular arrangement model. And it reaches the normal strength of the interface and thus causing the initiation of interface damage. Also the failure modes of composites under transverse tension and compression with and without residual stress are quite different from each other. The strength and failure path of different RVE and loading are showing respectively in this paper.
Keywords: Fiber reinforced composites; Interface; Thermal residual stress; Computational mechanics; Multiscale modeling; Damage initiation and evolution

A micromechanical approach is adopted to study the role of viscoelasticity on the fatigue behavior of polymer matrix composites. In particular, the study examines the interaction of fatigue and creep in angle ply carbon/epoxy at 25 and 114 °C. The matrix phase is modeled as a vicoelastic material using Schapery’s single integral constitutive equation. Taking viscoelsticity into account allows the study of creep strain evolution during the fatigue loading. The fatigue failure criterion is expressed in terms of the fatigue failure functions of the constituent materials. The micromechanical model is also used to calculate these fatigue failure functions from the knowledge of the S-N diagrams of the composite material in longitudinal, transverse and shear loadings thus eliminating the need for any further experimentation. Unlike the previous works, the present study can distinguish between the strain evolution due to fatigue and creep. The results can clearly show the contribution made by the effect of viscoelasticity to the total strain evolution during the fatigue life of the specimen. Although the effect of viscoelsticity is found to increase with temperature, its contribution to strain development during fatigue is compromised by the shorter life of the specimen when compared to lower temperatures.
Keywords: Polymer matrix composite; Viscoelasticity; Creep; Fatigue; Micromechanics

Liquid Resin Infusion (LRI) processes including VARI and VARTM have received increasing attention in recent years, particularly for infusion of large parts, or for low volume production. This method avoids the need for costly matched metal tooling as used in Resin Transfer Moulding (RTM) and can provide fast infusion if used in combination with flow media. Full material characterisation for LRI analysis requires models for three dimensional fabric permeability as a function of fibre volume content, fabric through-thickness compliance as a function of resin pressure, flow media permeability and resin viscosity.The characterisation of fabric relaxation during infusion is usually determined from cyclic compaction tests on saturated fabrics. This work presents an alternative method to determine the compressibility by using LRI flow simulation and fitting a model to experimental thickness measurements during LRI. The flow media is usually assumed to have isotropic permeability, but this work shows greater simulation accuracy from combining the flow media with separation plies as a combined orthotropic material. The permeability of this combined media can also be determined by fitting the model with simulation to LRI flow measurements. The constitutive models and the finite element solution were validated by simulation of the infusion of a complex aerospace demonstrator part.
Keywords: Finite Element analysis (FEA); Resin flow; Permeability