Mechanical behavior of composite materials

Composite materials have many mechanical behavior characteristics that are different from those of more conventional engineering materials. Some characteristics are merely modifications of conventional behavior others are totally new and require new analytical and experimental procedures. 

Most common engineering materials are both homogeneous and isotropic  

A homogeneous body has uniform properties throughout, i.e., the properties are independent of position in the body. 

Bodies with temperature-dependent isotropic material properties are not homogeneous when subjected to a temperature gradient, but still are isotropic. 

---

The foregoing characteristics of the mechanical behavior of composite materials have been presented in a qualitative manner without proof. In subsequent chapters, these characteristics will be demonstrated to exist, and further quantitative observations will be made. 

The surface characteristics of materials dominate the performance of the final products in many applications. For instance the mechanical behavior of composite materials are strongly dependent on the nature of the fiber/matrix interface, the lubricant properties are influenced by surface tribological effects, the success of bio-implants is mostly determined by biocompatibility, etc. 

Nanoparticles on the carbon fibers (this can represent a fundamental step for chemical bonding between the fibers and the polymeric, or other typology, matrix and for the subsequent mechanical behavior of composite materials) (Fig 6.20). 

The titanium-based composites with discontinuous reinforcement are attractive materials for a wide range of applications because of their high specific strength and stiffness and good fracture-related properties. Mechanical behavior of these materials depends strongly on both composition and microstructure of matrix and type, size and volume fraction of reinforcing phase. Hot plastic deformation is a powerful tool enhancing mechanical properties of titanium alloys. 

Peter Fratzl s lab studies the relation between (hierarchical) stmcture and mechanical behavior of biological materials, such as mineralized tissues, extracellular matrix, or plant cell walls, as well as bio-inspired composite materials. This is complemented by medically oriented research on osteoporosis and bone r eneration. Fratzl has published more than 350 papers in journals and books, mostly on interdisciplinary materials science topics. He received several international awards for his work including the Max Planck Research Award 2008 from the Humboldt Foundation (together with Robert Langer, Massachusetts Institute of Technology (MIT)) and the Leibniz Award 2010 of the German Science Foundation. In 2010, he was awarded an honorary doctorate from the University of Montpellier, France, and since 2007 he has been foreign member of the Austrian Academy of Sciences. 

For CNTs not well bonded to polymers, Jiang et al. [137] established a cohesive law for CNT/polymer interfaces. The cohesive law and its properties (e g. cohesive strength and cohesive energy) are obtained directly fiom the Lennard-Jones potential from the vdW interactions. Such a cohesive law is incorporated in the micromechanics model to study the mechanical behavior of CNT-reinforced composite materials. CNTs indeed improves the mechanical behavior of composite at the small strain. However, such improvement disappears at relatively large strain because the completely debonded nanotubes behave like voids in the matrix and may even weaken the composite. The increase of interface adhesion between CNTs and polymer matrix may significantly improve the composite behavior at the large strain [138]. 

It is well established that the supermolecular structure of semiciystalline matrix is of pivotal importance to the mechanical and physical behavior of composite materials. We analyzed the formation of the polymorphic forms in polypropylene composites with natural fibers. Moreover, analysis of the effect of processing conditions, during preparation of the composites, on the structure of polymer matrix is a focal issue because polymorphic changes in polypropylene can be stimulated by the temperature of processing and the shearing forces applied. 

By analyzing the fractions collected after hydrolysis, one may obtain the total amount of cellulose, hemicellulose and lignin present in the fiber. Thus, the chemical composition (by weight) for coconut fiber was 28.0% cellulose, 19.8% hemicellulose and 41.1% Hgnin. According to Luz et al. [21], the cellulose, hemicellulose and the hgnin components are responsible for the thermal and mechanical behavior of the material. Thus, it is extremely important to determine the quantity of each of these components in the fiber. 

The use of FRP is convenient also in older metal constmctions, since the mechanical properties of composite materials are well suited to the stmctural features of buildings employing cast iron. The exceptional tensile strength of the fibers makes up for the low resistance of the cast iron and the corrosion resistance of both materials makes the intervention durable. The traditional techniques that rely on welding are unfavorable because fliey require flie complete disassembly of the work, along with an inevitable increase of costs and time consumption. It demonstrated, moreover, the effectiveness of flie application of FRP in improving the brittle behavior of cast iron. 

Because of their dual crosslinked nature, both networks exert a unique control over the size, shape, and composition of the phase domains in an IPN. The morphological detail strongly influences, in turn, the physical and mechanical behavior of the material. While Chapter 5 detailed several ways of synthesizing IPNs, little mention was made of how crosslink density, order of polymerization, overall composition, etc. affect the final product. The objective of this chapter will be to explore the interrelationships among synthesis, morphology, and glass transition behavior. Mechanical and engineering properties will be treated in Chapter 7. 

Xu H S, Li Z M, Yang S Y, Pan J L, Yang W and Yang M B (2005) Rheological behavior comparison between PET/HDPE and PC/HDPE microfibrillar blends, Polym Eng Sci 45 1231-1238. Nuriel H, Klein N and Marom G (1999) The effect of the transcrystalline layer on the mechanical properties of composite materials in the fiber direction. Compos Sci Technol 59 1685-1690. Sapoundjieva D, Denchev Z, Evstatiev M, Fakirov S, Stribeck N and Stamm M (1999) Transcrystallization with reorientation in drawn PET-PA12 blend as revealed by WAXS from synchrotron radiation, J Mater Sci 34 3063-3066. 

The particle size distribution resulting from a milling operation is primarily determined by both the method of particle size reduction as well as the mechanical properties of the material such as fracture toughness, elastic modulus, and hardness. Thus, two extrudate samples with different mechanical properties milled under the same conditions will yield different particle size distributions. Beyond the intrinsic properties of the system, the mechanical behavior of extruded material is also affected by features of the bulk extrudate itself such as air bubbles, particle inclusions, or other defects that can increase the apparent brittleness of the material. Foamed extrudate, for example, could have different milling behavior as compared to a nonfoamed extmdate of the same composition. 

In particular carbonate sands are often used as a fill for land reclamation. Origin and composition, typical properties and mechanical behavior of these materials are described in section 9.2. 

Buyanov, A., Gofman, I., Revel skaya, L., Khripunov, A., Tkachenko, A., 2010. Anisotropic swelling and mechanical behavior of composite bacterial cellulose-poly (acrylamide or acrylamide-sodium acrylate) hydrogels. Journal of the Mechanical Behavior of Biomedical Materials 3 (1), 102-111. 

For academic applications. Engineering Mechanics of Composite Materials, by Isaac M. Daniel and Ori Ishai, is a good textbook that discusses the behavior of composite materials from micromechanical and macromechanical perspectives. 

The inherent anisotropy (most often only orthotropy) of composite materials leads to mechanical behavior characteristics that are quite different from those of conventional isotropic materials. The behavior of isotropic, orthotropic, and anisotropic materials under loadings of normal stress and shear stress is shown in Figure 1-4 and discussed in the following paragraphs. 

James Martin Whitney, A Study of the Effects of Coupling Between Bending and Stretching on the Mechanical Behavior of Layered Anisotropic Composite Materials, Ph.D thesis. Department of Engineering Mechanics, The Ohio State University, Columbus, Ohio, 1968. (Available from University Microfilms, Inc., Ann Arbor, Michigan, as no. 69-5000.)... 

The objective of this chapter is to address introductory sketches of some fundamental behavior issues that affect the performance of composite materials and structures. The basic questions are, given the mechanics of the problem (primarily the state of stress) and the materials basis of the problem (essentially the state of the material) (1) what are the stiffnesses, (2) what are the strengths, and (3) what is the life of the composite material or structure as influenced by the behavioral or environmental issues in Figure 6-1 ... 

The basic nature of composite materials was introduced in Chapter 1. An overall classification scheme was presented, and the mechanical behavior aspects of composite materials that differ from those of conventional materials were described in a qualitative fashion. The book was then restricted to laminated fiber-reinforced composite mafeffals. The basic definitions and how such materials are made were then treated. Finally, the current and potential advantages of composite materials were discussed along with some case histories that clearly reveal how composite materials are used in structures. 

Obviously, the foregoing description of problems in the mechanics of composite materials is incomplete. Some topics do not fit well within the logical framework just described. Other topics are too advanced for an introductory book, even at the graduate level. Thus, the rest of this chapter is devoted to a brief discussion of some basic lamina and laminate analysis and behavior characteristics that are not included in preceding chapters. 

---