Modelling of dimensional stability of fiber reinforced composite materials

Various methods of predicting the expansion and diffusion properties of composite laminates are reviewed. The prediction equations for continuous fiber composites can be applied to SMC composites as the effective fiber aspect ratio in the latter is large enough. The effect of hygrothermal expansion on the dimensional stability of composite laminates was demonstrated through the warping of unsymmetric graphite/epoxy laminates. The warping is very sensitive to the size of the panel, and to the moisture content which is in turn sensitive to the relative humidity in the environment. Thus, any long term creep test must be carried out in a humidity-controlled environment. Environmental effects in SMC composites and bulk polyester were studied under seven different environments. The SMC composites chosen are SMC-R25, SMC-R40, and SMC-R65

Statistical Physics of Rupture in Heterogeneous Media

The damage and fracture of materials are technologically of enormous interest due to their economic and human cost. They cover a wide range of phenomena like e.g. cracking of glass, aging of concrete, the failure of fiber networks in the formation of paper and the breaking of a metal bar subject to an external load. Failure of composite systems is of utmost importance in naval, aeronautics and space industry. By the term composite, we refer to materials with heterogeneous microscopic structures and also to assemblages of macroscopic elements forming a super-structure. Chemical and nuclear plants suffer from cracking due to corrosion either of chemical or radioactive origin, aided by thermal and/or mechanical stress. Despite the large amount of experimental data and the considerable effort that has been undertaken by material scientists, many questions about fracture have not been answered yet. There is no comprehensive understanding of rupture phenomena but only a partial classification in restricted and relatively simple situations. This lack of fundamental understanding is indeed reflected in the absence of reliable prediction methods for rupture, based on a suitable monitoring of the stressed system. Not only is there a lack of non-empirical understanding of the reliability of a system, but also the empirical laws themselves have often limited value. The difficulties stem from the complex interplay between heterogeneities and modes of damage and the possible existence of a hierarchy of characteristic scales (static and dynamic). The paper presents a review of recent efforts from the statistical physics community to address these points.Comment: Enlarged review and updated references, 21 pages with 2 figure

A mass reduction effort of the electric and hybrid vehicle

Weight reduction, cost competitiveness, and elimination of the intrusion beam resulted from the redesign and fabrication using composite materials of the door outer panel and intrusion beam from a Chevrolet Impala. The basis of the redesign involved replacing these two steel parts with a single compression molding using the unique approach of simultaneously curing a sheet molding compound outside panel with a continuous glass fiber intrusion strap. A weight reduction of nearly 11 pounds per door was achieved. Additional weight savings are possible by taking advantage of the elimination of the intrusion beam to design thinner door structures. The parts consolidation approach allows the composite structure to be cost competitive with the original steel design for both the lower production car models and for the near to midterm production vehicles using current state of the art composite production techniques. The design, prototype fabrication, costing, material, properties and compression molding production requirements are discussed

A stable and accurate control-volume technique based on integrated radial basis function networks for fluid-flow problems

Radial basis function networks (RBFNs) have been widely used in solving partial differential equations as they are able to provide fast convergence. Integrated RBFNs have the ability to avoid the problem of reduced convergence-rate caused by differentiation. This paper is concerned with the use of integrated RBFNs in the context of control-volume discretisations for the simulation of fluid-flow problems. Special attention is given to (i) the development of a stable high-order upwind scheme for the convection term and (ii) the development of a local high-order approximation scheme for the diffusion term. Benchmark problems including the lid-driven triangular-cavity flow are employed to validate the present technique. Accurate results at high values of the Reynolds number are obtained using relatively-coarse grids

Microstructure generation and micromechanical modeling of sheet molding compound composites

We introduce an algorithm that allows for a fast generation of SMC composite microstructures. An exact closure approximation and a quasi-random orientation sampling ensure high fidelity. Furthermore, we present a modular framework for anisotropic damage evolution. Our concept of extraction tensors and damage-hardening functions enables the description of complex damage-degradation. In addition, we propose a holistic multiscale approach for constructing anisotropic failure criteria

Effects of T-tabs and large deflections in DCB specimen tests

A simple strength of materials analysis was developed for a double-cantilever beam (DCB) specimen to account for geometric nonlinearity effects due to large deflections and T-tabs. A new DCB data analysis procedure was developed to include the effects of these nonlinearities. The results of the analysis were evaluated by DCB tests performed for materials having a wide range of toughnesses. The materials used in the present study were T300/5208, IM7/8551-7, and AS4/PEEK. Based on the present analysis, for a typical deflection/crack length ratio of 0.3 (for AS4/PEEK), T-tabs and large deflections cause a 15 percent and 3 percent error, respectively, in the computer Mode 1 strain energy release rate. Design guidelines for DCB specimen thickness and T-tab height were also developed in order to keep errors due to these nonlinearities within 2 percent. Based on the test results, for both hinged and tabbed specimens, the effects of large deflection on the Mode 1 fracture toughness (G sub Ic) were almost negligible (less than 1 percent) in the case of T300/5208 and IM7/8551-7; however, AS4/PEEK showed a 2 to 3 percent effect. The effects of T-tabs G sub Ic were more significant for all the materials with T300/5208 showing a 5 percent error, IM7/8551-7 a 15 percent error, and, AS4/PEEK a 20 percent error

Efficient Multiscale Homogeni-zation Model for Sheet Molding Compound (SMC) with Random Wavy Fibers and Overlapping Structures

학위논문(박사) -- 서울대학교대학원 : 공과대학 항공우주공학과, 2023. 8. 윤군진.멀티스케일 유한요소 해석(FEA)은 복합재료의 거동을 모델링하는데 효과적인 방법으로 널리 사용되고 있다. 그러나 멀티스케일 모델은 높은 계산 비용으로 인해 실제 적용 에는 한계가 있다. 본 논문은 정확도를 유지하면서 계산 시간을 줄일 수 있는멀티스케일 클러스터 기반 자기상관 해석SCA(Self-consistent clustering Analysis)방법을 제안하여 복합재료 해석에 적용하는 것을 목표로 한다. 제안된 모델링 방법을 SMC (Sheet Molding Compound) 복합재료에 적용하여 해석의 효율성과 정확성을 타 해석방법과 비교 검증하였다.. 먼저 SMC 복합재료의 효율적인 모델링을 위해 개선된 해석적 균질화 모델과 제안된 클러스터 기반 SCA 모델을 제안하고 비교검증하였다. SMC복합재료의 복잡한 구조로 인해 전통적인 균질화 모델은 그 거동을 정확하게 모사하는데 한계가 있다. 따라서 개선된 해석적 균질화 모델은 wavy 형상의 섬유, 연성 손상, SMC 칩 방향 및 SMC 칩의 겹침과 같은 요소를 고려하여 Mori-Tanaka 방법을 통합하였다. 반면에 클러스터 기반 SCA 모델은 계산 효율성을 유지하면서 무작위로 wavy 모양의 섬유와 무작위로 방향이 지정된 SMC 칩을 고려하는 수치적 멀티스케일 모델이다. 두 가지 균질화 모델은 엄밀하게 검증되었으며 SMC 복합재료의 거동을 모사하는 데 효과적이고 정확함을 입증하였다. 제시된 SCA 모델과 전통적인 멀티스케일 유한요소해석 모델의 비교를 통해 정확성과 타당성을 유지하면서 계산 시간을 크게 줄인 것을 확인하였다. 섬유와 SMC 칩의 다양한 기하학적 특성이 복합재료의 기계적 특성 예측에 미치는 영향도 분석하였다.Multiscale finite element analysis (FEA) has proven to be an ef-fective method in modeling the behavior of composite materials. However, their practical applicability is limited due to the high com-putational cost associated with multiscale models. This thesis pro-poses a multiscale clustering-based self-consistent analysis (SCA) for composite materials, which aims to reduce computation time with-out sacrificing accuracy. The proposed approach addresses the need for efficient modeling of SMC composites and the challenges of ap-plying a multiscale homogenization model to real-world applications. An improved analytical homogenization model and a clustering-based SCA model are presented and compared for the efficient modeling of SMC composites. Due to the complex structure of SMC, traditional homogenization models face challenges in accurately capturing its behavior. The improved analytical homogenization mod-el incorporates the Mori-Tanaka method, considering factors such as wavy fiber, ductile damage, SMC chip orientation, and overlap of SMC chips. On the other hand, the clustering-based SCA model is a numerical multiscale model that considers randomly wavy fibers and randomly oriented SMC chips while maintaining computational effi-ciency. Both homogenization models are thoroughly validated and demonstrate their effectiveness and accuracy in capturing the be-havior of SMC composites. The comparison between the presented SCA model and traditional multiscale FEA models highlights the sig-nificant reduction in computation time achieved while maintaining ac-curacy and validity. The influences of various geometric features of fibers and SMC chips are demonstrated in predicting the mechanical properties of composites.Chapter 1. Introduction 1 1.1. Study Background 1 1.2. Purpose of Research & Thesis Overview ５ Chapter 2. Homogenization Methodology ９ 2.1. Lippmann Schwinger Homogenization 10 2.2. Cluster-based LS Homogenization 13 2.2.1 Domain Decomposition 15 2.2.2 Interaction tensor 18 2.2.3 Cluster-based Iterative Self-consistent Analysis 18 2.3. Mori-Tanaka Homogenization 22 2.4. Improved Mori-Tanaka Homogenization 25 2.4.1 Mori-Tanaka Homogenization with Interface Damage 25 2.4.2 Mori-Tanaka Homogenization with random wavy fiber 29 2.4.3 Mori-Tanaka Homogenization with plasticity and ductile damage 33 2.5. Comparison of LS homogenization and Mori-Tanaka Homogenization 42 Chapter 3. Multiscale Homogenization for SCA composites 49 3.1. Multistep Analytical Homogenization 49 3.1.1 1st Homogenization 50 3.1.2 2nd Homogenization 52 3.1.3 3rd Homogenization 55 3.2. Multiscale cluster based SCA homogenization 57 Chapter 4. Results 64 4.1. Results of analytical homogenization 64 4.1.1 Effects of interfacial damage 64 4.1.2 Effects of plasticity and ductile damage 65 4.2. Results of multistep homogenization for SMC 70 4.2.1 Model Validation with literatures 70 4.2.2 Effect of overlapping 73 4.2.3 Effect of waviness and orientation 74 4.2.4 The SMC micromechanics model under cyclic loading 76 4.3. Results of multiscale cluster based SCA simulation 77 4.3.1 Validation of multiscale cluster based SCA simulation 77 4.3.2 The effect of waviness and orientation 80 4.3.3 The comparison of multiscale SCA simulation and multistep homogenization of SMC 82 Chapter 5. Conclusions 84 Bibliography 85 Abstract in Korean 94박

Fatigue behavior of continuous-discontinuous sheet molding compounds

Sheet molding compound (SMC) composites have been well-established as nonstructural and semi-structural components in vehicles due to their high lightweight potential. Over the past decade, increasing need for further vehicle weight reduction has fueled endeavors to further improve mechanical properties of SMC and thereby to expand their suitability as structural components. One promising approach to achieve this goal is the combination of discontinuous glass fiber SMC with local continuous carbon fiber reinforcement, which is currently being investigated by the German-Canadian research training group GRK 2078. Such hybrid continuous-discontinuous composites enable the production of components with excellent mechanical properties while maintaining the advantages of the SMC process including geometric flexibility, short cycle times and cost efficiency. However, several factors prevent the exploitation of the composite’s full potential. Major obstacles for a safe and efficient application of continuous-discontinuous SMC are the lack of knowledge regarding their fatigue behavior and about uncharted effects of hybridization on damage behavior under cyclic loading conditions. This thesis describes a systematical analysis of the fatigue behavior of continuous-discontinuous SMC both under cyclic tensile and bending loads at different temperatures and frequencies. The hybrid composite, which consists of a discontinuous glass fiber SMC core and unidirectional carbon fiber SMC face plies, shows significantly higher fatigue resistance compared to discontinuous SMC without continuous reinforcement. The effect of hybridization is more pronounced under cyclic loading than under monotonic loading, which is a result of distinct damage mechanisms acting at distinct applied stresses. The mechanical behavior of continuous-discontinuous SMC is dominated by the continuous plies at high stresses and by the discontinuous ply at low stresses. The effect of hybridization is particularly distinctive under cyclic bending load on account of the composite’s sandwich-like structure. In addition, relative stiffness degradation of continuous-discontinuous SMC is less pronounced over a larger loading period compared to discontinuous SMC. While damage evolution within the discontinuous SMC ply remains largely unaffected by hybridization, the continuous SMC plies are enabled to withstand significantly higher cyclic loads than continuous SMC specimens that are not a part of a hybrid composite. When using a cross-ply instead of a unidirectional continuous reinforcement, early initiation of cracks in the 90° ply at comparatively low stresses leads to rapidly growing delaminations and large cracks in the discontinuous SMC ply. Consequently, the effect of hybridization under cyclic loading is less pronounced. While continuous-discontinuous SMC is largely insensitive to a change in frequency, enhanced temperature leads to early failure of the unidirectional ply on the compression loaded side, which results in a significant decrease of fatigue life

Microstructure generation and micromechanical modeling of sheet molding compound composites

We introduce an algorithm that allows for a fast generation of SMC composite microstructures. An exact closure approximation and a quasi-random orientation sampling ensure high fidelity. Furthermore, we present a modular framework for anisotropic damage evolution. Our concept of extraction tensors and damage-hardening functions enables the description of complex damage-degradation. In addition, we propose a holistic multiscale approach for constructing anisotropic failure criteria