Composites strength and stiffness

There are numerous test methods that have been used to characterize the fiber-dominated composite strength and stiffness for filament wound structures. A number of these test methods have been standardized by the ASTM D30 Committee [39], These standardized tests methods include ... 

Effect of Interphase Design on Composite Strength and Stiffness... 

In their review article, Coleman and coworkers asserted that, in order to achieve effective reinforcement, CNTs should fulfill four main system requirements. First, a large aspect ratio is required to maximize the load transfer to the nanotubes, which is central for optimizing composite strength and stiffness. Second, the CNT must form a good dispersion the nanotubes must be uniformly dispersed as individually coated nanotubes within the polymer so that efficient load transfer to the nanotube network is accomplished. Moreover, good distribution of CNTs maximizes stress distribution while minimizing stress-concentration... 

Properties of typical thermosetting and thermoplastic polymer matrices and single fibres are listed. The former have relevance to the upper working temperature and off-axis properties of the composite, while the latter can be used to estimate composite strength and stiffness parallel to the fibre direction. 

Today, carbon fibers are still mainly of interest as reinforcement in composite materials [7] where high strength and stiffness, combined with low weight, are required. For example, the world-wide consumption of carbon fibers in 1993 was 7,300 t (compared with a production capacity of 13,000 t) of which 36 % was used in aerospace applications, 43 % in sports materials, with the remaining 21 % being used in other industries. This consumption appears to have increased rapidly (at 15 % per year since the early 1980s), at about the same rate as production, accompanied by a marked decrease in fiber cost (especially for high modulus fibers). 

Reinforcing fibres have diameters varying from 7 /im to 1(X) /im. They may be continuous or in the form of chopped strands (lengths 3 mm-50 mm). When chopped strands are used, the length to diameter ratio is called the Aspect Ratio. The properties of a short-fibre composite are very dependent on the aspect ratio - the greater the aspect ratio the greater will be the strength and stiffness of the composite. 

In this book, attention will first be focused on macromechanics because it is the most readily appreciated of the two and the more important topic in structural design analysis. Subsequently, micromechanics will be investigated in order to gain an appreciation for how the constituents of composite materials can be proportioned and arranged to achieve certain specified strengths and stiffnesses. 

Fiber-reinforced composite materials such as boron-epoxy and graphite-epoxy are usually treated as linear elastic materials because the essentially linear elastic fibers provide the majority of the strength and stiffness. Refinement of that approximation requires consideration of some form of plasticity, viscoelasticity, or both (viscoplasticity). Very little work has been done to implement those models or idealizations of composite material behavior in structural applications. 

Figure 1-24 Strength and Stiffness of Composite Materials and Metals...

Not all of the strength and stiffness advantages of fiber-reinforced composite materials can be transformed directly into structural advantages. Prominent among the reasons for this statement is the fact that the joints for members made of composite materials are typically more bulky than those for metal parts. These relative inefficiencies are being studied because they obviously affect the cost trade-offs for application of composite materials. Other limitations will be discussed subsequently. 

The topic of materials with different strengths and stiffnesses in tension than in compression will not be covered further in much depth (except to report different strengths) because research on such materials is still in its infancy. However, the topic is very important for the general class of composite materials, if not fiber-reinforced laminated composites. Ambartsumyan and his associates first reported research on this topic in 1965 [2-9]. A few Americans have also investigated some aspects of the mechanics of these materials (see Jones [2-10], Bert [2-11], and Bert and Reddy [2-12]). 

One of the key elements in laminated composite structures design is the ability to tailor a laminate to suit the job at hand. Tailoring consists of the following steps. We want to design the constituents of the laminate, and those constituents include the basic building blocks of the individual laminae and as well how they are oriented within the laminate. We design those constituents to just barely meet (with an appropriate factor of safety) the specific requirements for, say, strength and stiffness. 

Fibers are often regarded as the dominant constituents in a fiber-reinforced composite material. However, simple micromechanics analysis described in Section 7.3.5, Importance of Constituents, leads to the conclusion that fibers dominate only the fiber-direction modulus of a unidirectionally reinforced lamina. Of course, lamina properties in that direction have the potential to contribute the most to the strength and stiffness of a laminate. Thus, the fibers do play the dominant role in a properly designed laminate. Such a laminate must have fibers oriented in the various directions necessary to resist all possible loads. 

High stiffnesses and strengths can be attained for composite laminates. However, these characteristics are quite different from those of ordinary materials to which we often need to fasten composite laminates. Often, the full strength and stiffness characteristics of the laminate cannot be transferred through the joint without a significant weight penalty. Thus, the topic of joints or other fastening devices is critical to the successful use of composite materials. 

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