Ceramics elastic modulus values

Bar chart of room-temperature stiffness (i.e., elastic modulus) values for various metals, ceramics, polymers, and composite materials. 

The stress-strain curves for cortical bones at various strain rates are shown in Figure 5.130. The mechanical behavior is as expected from a composite of linear elastic ceramic reinforcement (HA) and a compliant, ductile polymer matrix (collagen). In fact, the tensile modulus values for bone can be modeled to within a factor of two by a rule-of-mixtures calculation on the basis of a 0.5 volume fraction HA-reinforced... 

There are two possible approaches to the selection of materials from the standpoint of thermal shock resistance. The first is suitable for glass and fine dense ceramics and was discussed in Section II. 5. 2. With these materials, it is necessary to avoid formation of primary cracks which originate at the surface and propagate rapidly into the interior where they are the cause of extensive fracture. In this case, the favourable properties include high strength and high thermal conductivity, and low elasticity modulus and expansion coefficient values. 

The strength or modulus of rupture quoted for glass ceramics is determined by loading test bars in flexure. This value can vary by a factor of ten or more for a given material, depending on the surface finish of the test specimens. Fracture strength, ur, depends on the size of surface flaws c, the elastic modulus E, and the work of fracture yr ... 

The value of the elastic modulus, often called the Reuss bound, is identical to that for transverse loading on a fibre composite, and gives a value for the elastic modulus normal to the layers. In both of these equations, E, E and Ep are the elastic moduli of the ceramic, matrix and particles, respectively, and Vc (equal to 1.0), Vm and Vp are the corresponding volume fractions. 

A single experimentally obtained cooling curve of the elastic modulus, such as the one shown in Figure 2 (circular markers), can be used in conjunction with Equations (1) and (10) can be used to find the appropriate values of an for a particular porous ceramic system. The modulus predicted by Equation (10) is shown by the solid curve in Figure 2. Thus, the agreement between model and experiment is very good for this material. 

However, flexural strength, strain-to-failine, and fracture toughness values of HA-ceramics are significantly less than those of bone, whereas the elastic modulus is much higher [8, 9], These mechanical mismatches influence the reliability of ceramics when implanted into the bone tissue. To improve the mechanical compatibility, a composite approach may be promising. The combination of different materials within a composite structure may lead to a composite material that reveals specific physical, chemical and/or mechanical properties, particularly resulted from a synergy principal. 

The microhardness technique is used when the specimen size is small or when a spatial map of the mechanical properties of the material within the micron range is required. Forces of 0.05-2 N are usually applied, yielding indentation depths in the micron range. While microhardness determined from the residual indentation is associated with the permanent plastic deformation induced in the material (see section on Basic Aspects of Indentation), microindentation testing can also provide information about the elastic properties. Indeed, the hardness to Young s modulus ratio HIE has been shown to be directly proportional to the relative depth recovery of the impression in ceramics and metals (2). Moreover, a correlation between the impression dimensions of a rhombus-based pyramidal indentation and the HIE ratio has been found for a wide variety of isotropic poljuneric materials (3). In oriented polymers, the extent of elastic recovery of the imprint along the fiber axis has been correlated to Young s modulus values (4). 

All ceramic materials are elastic, and hence show very little bending under load. They do not exhibit any creep under load. The modulus of rupture type of test is the routine test most commonly used in the ceramic industry, and gives the figure generally quoted for the strength of the material. It must be remembered that the value obtained for any particular body depends on the cross-sectional area of the test piece thus figures quoted from test results may be higher than those obtained on actual articles, which usually have a thicker section than the test piece. 

Composites provide an atPactive alternative to the various metal-, polymer- and ceramic-based biomaterials, which all have some mismatch with natural bone properties. A comparison of modulus and fracture toughness values for natural bone provide a basis for the approximate mechanical compatibility required for arUficial bone in an exact structural replacement, or to stabilize a bone-implant interface. A precise matching requires a comparison of all the elastic stiffness coefficients (see the generalized Hooke s Law in Section 5.4.3.1). From Table 5.15 it can be seen that a possible approach to the development of a mechanically compatible artificial bone material... 

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