Two-dimensional strain-based interactive failure theory for multidirectional composite laminates

A 2-D strain-based interactive failure theory is developed to predict the final failure of composite laminates subjected to multi-axial in-plane loading. The stiffness degradation of a laminate during loading is examined based on the individual failure modes of the maximum strain failure theory, and a piecewise linear incremental approach is employed to describe the nonlinear mechanical behavior of the laminate. In addition, an out-of-plane failure mode normal to the laminate is also investigated to more accurately predict the failure of multidirectional laminates. The theoretical results of the failure model presented are compared with the experimental data provided by the World-Wide Failure Exercise, and the accuracy of the model’s predictive capabilities is investigated.     

Compressive behavior of C/SiC composite sandwich structure with stitched lattice core

C/SiC composite sandwich structure with stitched lattice core was fabricated by a technique that involved polymer impregnation and interweaving. The mechanical behaviors of C/SiC composite sandwich structure were investigated at room temperature. The out-of-plane compressive strength was 20.97 MPa while modulus was 1473.55 MPa. The microstructural evolution on compression fracture surfaces of the stitching yarns was investigated by scanning electron microscopy, and the damage pattern of fibers on compression fracture surface was presented and discussed. Under an in-plane compression loading, the C/SiC composite sandwich structure displayed a linear-elastic behavior until failure. The peak strength and average modulus are 165.61 MPa and 19.74 GPa, respectively. The failure of the specimen was dominated by the fracture of the facesheet.     

Investigation of thermal conductivity and dielectric properties of LDPE-matrix composites filled with hybrid filler of hollow glass microspheres and nitride particles

The hybrid filler of hollow glass microspheres (HGM) and nitride particles was filled into low-density polyethylene (LDPE) matrix via powder mixing and then hot pressing technology to obtain the composites with higher thermal conductivity as well as lower dielectric constant (D_k) and loss (D_f). The effects of surface modification of nitride particles and HGMs as well as volume ratio between them on the thermal conductivity and dielectric properties at 1 MHz of the composites were first investigated. The results indicate that the surface modification of the filler has a beneficial effect on thermal conductivity and dielectric properties of the composites due to the good interfacial adhesion between the filler and matrix. An optimal volume ratio of nitride particles to HGMs of 1:1 is determined on the basis of overall performance of the composites. The thermal conductivity as well as dielectric properties at 1 MHz and microwave frequency of the composites made from surface-modified fillers with the optimal nitride to HGM volume ratio were investigated as a function of the total volume fraction of hybrid filler. It is found that the thermal conductivity increases with filler volume fraction, and it is mainly related to the type of nitride particle other than HGM. The D_k values at 1 MHz and microwave frequency show an increasing trend with filler volume fraction and depend largely on the types of both nitride particles and HGMs. The D_f values at 1 MHz or quality factor (Q × f) at microwave frequency show an increasing or decreasing trend with filler volume fraction and also depend on the types of both nitride particle and HGM. Finally, optimal type of HGM and nitride particles as well as corresponding thermal conductivity and dielectric properties is obtained. SEM observations show that the hybrid filler particles are agglomerated around the LDPE matrix particles, and within the agglomerates the smaller-sized nitride particles in the hybrid filler can easily form thermally conductive networks to make the composites with high thermal conductivity. At the same time, the increase of the value D_k of the composites is restricted due to the presence of HGMs.     

Effects of weave architecture on mechanical response of 2D ceramic composites

A meso-scale finite element model is developed to investigate effects of weave architecture on strain and stress evolution in an eight harness-satin SiC/SiCN composite. Fiber tows are modeled explicitly using elastic rebar layers embedded within elastic/plastic effective medium elements. Effects of through-thickness constraint are investigated using several idealized test geometries, ranging from a single (unconstrained) ply to a fully-constrained two-ply lay-up with periodic boundary conditions in the through-thickness direction. A parallel experimental study of surface strain evolution in a representative SiC/SiCN composite is used to assess the model predictions. The results indicate that, because of bending and straightening of wavy tow segments at the locations of tow cross-overs, strain and stress concentrations arise. The effects are exacerbated by reductions in the constraints on bending and straightening caused by matrix damage, especially in surface plies. The implications of the results in the fracture process and on potential mitigation strategies are discussed.     

Tensile and creep performance of a novel mullite fiber at high temperatures

The novel fiber CeraFib75 with a composition near to pure mullite was analyzed with respect to its potential for high-temperature applications. This mullite fiber free of glass phase was aimed to overcome the strength of commercial oxide fibers at high-temperatures. Tensile tests at room and high temperatures ranging from 900 to 1400 °C and creep tests were performed. Nextel™720, another crystalline mullite-alumina fiber, was tested as a reference. Microstructure and crystal phase analysis of the new fiber revealed mullite grains with traces of γ- and α-alumina in-between; it contains occasionally defects causing a reduced strength at room-temperature. Remarkably, at temperatures beyond 1200 °C, CeraFib75 presented a higher tensile strength than Nextel™720. During tensile tests at 1400 °C, an extended region of inelastic deformation was observed for CeraFib fibers only, which was related to a grain boundary sliding mechanism. Creep rates were of the same order of magnitude for both fibers.     

Thermomechanical behavior of PA6.6 composites subjected to low cycle fatigue

In this study, manifold experiments were conducted to investigate the thermomechanical behavior of short E-glass fiber-reinforced polyamide 6.6 composites subjected to low cycle fatigue loadings. Different hygrometric states, fiber configurations and loading rates were considered. Mechanical, thermal and energy responses of composite specimens were recorded using photomechanic techniques. The influence of water content, fiber orientation and loading rate on these thermomechanical responses was systematically analysed.The mechanical findings indicated that the ratcheting phenomenon was more pronounced for humid composites reinforced with fibers oriented transversely and subjected to a low loading rate. Moreover, the order of magnitude in self-heating was greater for transversal fiber composites conditioned at high relative humidity and subjected to a 10 Hz loading rate. From a thermodynamic standpoint, we also noticed that high proportions of the mean stored energy rate were obtained at a high loading rate, with values exceeded 64%. These values were noticeably altered by the water content and fiber angles, i.e. lower as the relative humidity increased and higher as the fiber angles increased.     

Improving the interlaminar properties of polymer composites using a situ accumulation method to construct the multi-scale reinforcement of carbon nanofibers/carbon fibers

A novel method was developed to realize the situ accumulation of carbon nanofibers (CNFs) in the carbon fiber reinforced polymer composites (CFRPs) to construct the multi-scale reinforcement for improving the interlaminar properties. In this method, the prepreg was sealed by the nanomicroporous nylon membrane, and the excess resin was extracted from the prepreg by the vacuum-assisted method. It was found that the use of nylon membrane resulted in effective CNFs accumulation, especially in the interlayer by scanning electron microscopy. Short-beam strength tests and the end-notched flexure tests were conducted respectively to evaluate the interlaminar properties of CFRPs under shear loading. The results indicated that the interlaminar shear strength (ILSS) and the Mode II interlaminar fracture toughness (G_IIC) of CFRPs made by the filtering membrane-assisted method remarkably increased compared with those prepared without using filtering membrane.     

The voids formation mechanisms and their effects on the mechanical properties of flax fiber reinforced epoxy composites

Unidirectional flax fiber reinforced composites (FFRC) were made by hot press. Effects of processing parameters, including curing pressure, time and temperature on the distribution, shape and content of the voids formed during the manufacturing process of FFRC were investigated. The voids were characterized with the aid of ultrasonic C-scan and optical microscopy. Tensile and interlaminar shear properties of FFRC containing different content and shape of the voids were tested. The results showed that the voids were easily trapped in both the intralaminar and inside the flax yarns of FFRC due to the distinct structural characteristics of flax fibers. The relationships between voids and mechanical properties of the composites were established.     

Damage and permeability in tape-laid thermoplastic composite cryogenic tanks

This work presents a combined experimental and numerical approach to the design and analysis of tape-laid thermoplastic composite cryogenic tanks. A detailed material and defect characterisation of automated tape-laid CF/PEEK is undertaken using optical micrography and 3D X-ray CT (computed tomography) as well as cryogenic testing to investigate damage formation. Resulting material data is used as input for a novel XFEM (extended finite element method)–cohesive zone methodology which is used to predict intra- and inter-ply damage in an internally pressurised cryogenic tank. An optimised tank lay-up is presented and analysed using the numerical method to ensure resistance to microcrack formation and fuel leakage through the tank walls under operating loads.