Experimental Study of Viscoelastic Effects and Aging on Elevated Temperature Damage and Failure in Polymer Composites

An experimental investigation of microcracking andphysical aging effects in polymer matrix composites isdescribed. The goal of the study was to assess the impactof aging on damage accumulation, in terms ofmicrocracking, and the impact of damage on aging andviscoelastic behavior. Results indicate that while theaging times studied have only a limited influence ondamage evolution, elevated temperature and viscoelasticeffects have a profound effect on the damage mode seen forcertain laminates. Some results are counterintuitive,including the lower strain to failure and the catastrophicfailure mode observed at elevated temperatures. Fracturetoughness for transverse cracks is seen to increase withtemperature, however no significant effect of damage onthe aging or viscoelastic properties of the laminates wasobserved.     

Modeling and Prediction of Thermal Cycle Induced Failure in Epoxy-Silica Composites

Epoxy resins filled with dielectric mineral particles are frequently used as insulating materials in power industry applications. Due to their excellent dielectric properties and relatively good thermal performance (resistance, ageing and conductivity) their usability is common and extensive. However, the mechanical performance of the resins is influenced by several factors such as resistance to crack propagation, especially in low temperature applications. This phenomenon is normally linked with appearance of two phase systems where particle filled epoxy material interacts with metallic inserts having significantly different thermal expansion coefficients. This kind of epoxy-metal interface can produce relatively high stresses in the product structure during thermal cycle loading. The paper deals with mechanical problems of power industry products and introduces the methodology for numerical modeling of failure in silica filled epoxy systems subjected to severe temperature gradients. Various aspects of material behavior modeling are covered in this article, including polymerization process, viscoelastic stress relaxation as well as stochastic cracking.     

Thermally Induced Damage Initiation and Growth in Plain and Satin Weave Carbon-Carbon Composites

Carbon-carbon woven composites were analyzed using three-dimensional finite-element analysis to predict the state of the composite after processing. Two types of weaves were examined: plain weave and eight-harness satin weave. Periodic boundary conditions for analyzing eight-harness satin weaves are derived herein. The effect of the stacking sequence of these weaves was investigated. Temperature-dependent material properties and a nonlinear damage model were used in the analysis procedure. Results indicate that there is a significant difference in the as-processed damage states and moduli of these composites for different stacking sequences. Damage initiated as transverse matrix cracks for all weave configurations. This lead to significant damage to the matrix pockets and, in some cases, to tow debonding and shear failure within the tows. In no case was fiber damage predicted.     

Microstructure of YBCO/Semicrystalline Polymer Composites and its Effect on Percolation

Ceramic-polymer composites using conducting YBa₂Cu₃O_7−x (YBCO) powder have been fabricated. Atomic force microscopy (AFM) and Scanning electron microscopy (SEM) images are used to provide new information about the more influent microstructural features. The effect of microstructure on percolation is studied for all the composites. Results indicate that percolation and subsequent positive temperature coefficient (PTC) behavior are related to the powder characteristics, composite microstructure, and ceramic-polymer interface. The reproducibility of the resistivity—temperature behavior for the polyethylene composites upon thermal cycling is illustrated and found that the room temperature resistivity increases by about 1 order of magnitude from the first to the forth cycle.     

Experimental Studies on Damage Growth in Composites under Dynamic Loads

Experimental investigations have been carried out to study the dynamic damage growth in glass/polyester composites. Detonation of two PETN explosive charges on a modified single edge notch (MSEN) specimen provides the dynamic load in the form of a planar tensile wave. High speed photography is used to record the dynamic damage events. The results show that damage grows perpendicular to the loading direction, similar to the static growth; the damage zone splits analogous to the crack branching in unreinforced polyester. The damage propagation velocity in a composite is higher than the crack propagation velocity in polyester resin. The damage area grows at an average rate of 4.3 m²/sec. Static experiments show that about 4 percent of the total energy is spent on the fiber-matrix interface debonding. The damage zone under dynamic loads is much higher than under static loads.     

Creep in Wood Under Variable Climate Conditions: Numerical Modeling and Experimental Validation

This paper deals with the modeling of linear viscoelastic behavior and strain accumulation (accelerated creep) during moisture content changes in timber. A generalized Kelvin–Voigt model is used and associated in series with a shrinkage-swelling element depending on the mechanical and moisture content states of materials. The hygrothermal aging due to climatic variations implies an evolution of rheological parameters depending upon moisture content and temperature. Two distinct viscoelastic laws, one for drying and the other for moistening, are coupled according to the thermodynamic principles when wood is subjected to nonmonotonous moisture variations. An incremental formulation of behavior is established in the finite element program CAST3M (Software developed by C.E.A. (Commissariat á l'Energi Atomique) and an experimental validation from tension creep-recovery tests is presented.     

On Cutter Deflection Profile Errors in End Milling: Modeling and Experimental Validation

In the present paper three dimensional cutter deflections and the corresponding profile errors during end milling are predicted by finite element and bond graph modeling approach. The deflections have been modeled considering the cutter as a cantilever beam fixed at the collet end. Rayleigh beam model is used for modeling to consider the shear force effect, and Castigliano's theorem of strain energy is used to predict the deflections due to radial, tangential, and axial cutting forces. The tool-workpiece contact region is sliced into small elements to apply the forces on the entire contact region and predict deflections more accurately. The predicted deflections at different parametric settings are compared with experimental measurements by measuring the geometric accuracy of the cut profiles. The depth of cut has the most significant influence on profile deviations, while feed and speed have marginal effects. The results reveal that the predictions by finite elements and the bond graph closely matches with the experimental results, and errors of the machined profile are significantly influenced by the radial and tangential deflections. The axial deflection is negligible and leads to insignificant deviation in depth of the cut profile. The proposed model shows that the bond graph simulation takes significantly less computational time and space as compared to the finite element technique.     

A Joint Numerical-Experimental Study on Impact Induced Intra-laminar and Inter-laminar Damage in Laminated Composites

The investigation of the mechanical response of fibre-reinforced composite laminates under impact loads can be very difficult due to the occurrence of simultaneous failure phenomena. Indeed, as a consequence of low velocity impacts, intra-laminar damages, like fibre and matrix cracking, and inter-laminar damages, such as delaminations, can take place simultaneously. These damage mechanisms can lead to significant reductions in strength and stability of the composite structure. In this paper a joint numerical-experimental study is proposed which, by means of non-destructive testing techniques (Ultra-sound and thermography) and non-linear explicit FEM analyses, aims to completely characterise the impact induced damage in composite laminates under low velocity impacts. Indeed the proposed numerical tool has been used to improve the understanding of the experimental data obtained by Non-Destructive Techniques. Applications on samples tested according to the AECMA (European Association of Aerospace Manufacturers) prEn6038 standard at three different impact energies are presented. The interaction between numerical and experimental investigation allowed to obtain an exhaustive insight on the different phases of the impact event considering the inter-laminar damage formation and evolution.     

Experimental and Numerical Analysis of Damage in Woven GFRP Composites Under Large-deflection Bending

Textile-reinforced composites such as glass fibre-reinforced polymer (GFRP) used in sports products can be exposed to different in-service conditions such as large bending deformation and multiple impacts. Such loading conditions cause high local stresses and strains, which result in multiple modes of damage and fracture in composite laminates due to their inherent heterogeneity and non-trivial microstructure. In this paper, various damage modes in GFRP laminates are studied using experimental material characterisation, non-destructive micro-structural damage evaluation and numerical simulations. Experimental tests are carried out to characterise the behaviour of these materials under large-deflection bending. To obtain in-plane shear properties of laminates, tensile tests are performed using a full-field strain-measurement digital image correlation technique. X-ray micro computed tomography (Micro CT) is used to investigate internal material damage modes – delamination and cracking. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus to study the deformation behaviour and damage in GFRP. In these models, multiple layers of bilinear cohesive-zone elements are employed to study the onset and progression of inter-ply delamination and intra-ply fabric fracture of composite laminate, based on the X-ray Micro CT study. The developed numerical models are capable to simulate these features with their mechanisms as well as subsequent mode coupling observed in tests and Micro CT scanning. The obtained results of simulations are in agreement with experimental data.     

三维编织复合材料的微结构与力学性能研究进展

综述了国内外近年来对三维编织复合材料细观结构力学模型和力学性能的研究进展.细观结构力学模型研究主要是通过分析三维编织体的拓扑结构建立力学分析模型,主要有"米"字枝状模型、纤维倾斜模型和三细胞模型.对力学性能的研究包括刚度性能和强度性能研究,刚度性能的预测分析方法有解析法、基于均匀化理论的方法和有限元数值仿真;而强度性能的预测大多采用有限元方法来进行.最后,总结了目前研究中存在的问题并对以后的工作进行了展望.     

Modeling of Moisture Diffusion in Permeable Fiber-Reinforced Polymer Composites Using Heterogeneous Hybrid Moisture Element Method

This study proposes a two-dimensional heterogeneous hybrid moisture element method (HHMEM) for modeling transient moisture diffusion in permeable fiber-reinforced polymer composites.
The HHMEM scheme is based on a heterogeneous hybrid moisture element(HHME), with properties determined through an equivalent hybrid moisture capacitance/conductance matrix. This matrix was calculated using the conventional finite element formulation in space discretization as well as the θ-method in time discretization with similar mass/stiffness properties and matrix condensing operations. A coupled HHME-FE scheme was developed and implemented in computer code MATLAB in order to analyze the transient moisture diffusion characteristics of composite materials containing multiple permeable fibers. The analysis commenced by comparing the performance of the proposed scheme with that of conventional FEM to model the moisture diffusion process. Both hexagonal and square fiber arrangements were studied. Having validated its performance, the scheme was then employed to investigate the relationship between the volume fraction of the permeable fibers in the resin composite and the rate of moisture diffusion. It was found that the moisture diffusion was significantly retarded as the volume fraction of the fibers increased. The HHMEM approach proposed in this study provides a straightforward and efficient means of modeling transient moisture diffusion in composite materials containing multiple permeable fibers. This is because only one HHME moisture characteristic matrix of fibers requires calculation for all HHMEs sharing the same characteristics. Furthermore, varying volume fractions can be modeled without modifying the original model simply by controlling the size of the inter-phase region within the HHME domain.     

Experimental and Numerical Investigations on the Ballistic Performance of Polymer Matrix Composites Used in Armor Design

Ballistic properties of two different polymer matrix composites used for military and non-military purposes are investigated in this study. Backside deformation and penetration speed are determined experimentally and numerically for Kevlar 29/Polivnyl Butyral and Polyethylene fiber composites because designing armors for only penetration is not enough for protection. After experimental ballistic tests, a model is constructed using finite element program, Abaqus. The backside deformation and penetration speed are determined numerically. It is found that the experimental and numeric results are in agreement and Polyethylene fiber composite has much better ballistic limit, the backside deformation, and penetration speed than those of Kevlar 29/Polivnyl Butyral composite if areal densities are considered.     

Printing Polymer Nanocomposites and Composites in Three Dimensions

Recent advances in materials science and three‐dimensional (3D) printing hold great promises to conceive new classes of multifunctional materials and components for functional devices and products. Various functionalities (e.g., mechanical, electrical, and thermal properties, magnetism) can be offered by the nano‐ and micro‐reinforcements to the non‐functional pure printing materials for the realization of advanced materials and innovative systems. In addition, the ability to print 3D structures in a layer‐by‐layer manner enables manufacturing of highly‐customized complex features and allows an efficient control over the properties of fabricated structures. Here, the authors present a brief overview mainly over the latest progresses in 3D printing of multifunctional polymer nanocomposites and microfiber‐reinforced composites including the benefits, limitations, and potential applications. Only those 3D printing techniques that are compatible with polymer nanocomposites and composites, that is, materials that have already been used as printing materials, are introduced. The very hot topic of 3D printing of thermoplastic composites featuring continuous microfibers is also briefly introduced.