Elasticity and Yield Strength

The surface stress, or surface tension, and bulk modulus at a specific atomic site share the same dimension of the surface energy density [Eq. (22.1)]  

This expression formulates the correspondence between surface energy density and the mechanical properties. Precisely, the B and r are the derivatives of 

Equation (24.5) indicates that the 7,- and o enhancement depends uniquely on the shortened and the strengthened bonds. The relative values for the local 7,-, t, and y i to those of the bulk measured at To = 0 K follow the same relation 

Numerical reproduction of these quantities derives the quantities of atomic cohesive energy ZibEi(0), binding energy density Eb(0)/Vo, Debye temperature 9d, compressibility jS, thermal expansion coefficient a, etc. 

---

Part AM This part lists permitted individual constnic tion materials, apphcable specifications, special requirements, design stress-intensity vafues, and other property information. Of particular importance are the ultrasonic-test and tou ness requirements. Among the properties for which data are included are thermal conduc tivity and diffusivity, coefficient of theiTnal expansion, modulus of elasticity, and yield strength. The design stress-intensity values include a safety factor of 3 on ultimate strength at temperature or 1.5 on yield strength at temperature. 

Determine the modulus of elasticity and yield strength of a number of household materials. You are to present both measurement results and the models you have used to derive these. 

At compression, the effect of extrusion draw on the strain-stress characteristics of samples is not so pronounced (22). As shown (22), for PP, the increase in the modulus of elasticity and yield strength occurs under compression in a direction normal to that of extrusion. In the case of a load applied along the extmdate axis, there is a decrease in the above characteristics, as compared to the initial material. 

External-pressure failure of shells can result from overstress at one extreme or n om elastic instability at the other or at some intermediate loading. The code provides the solution for most shells by using a number of charts. One chart is used for cylinders where the shell diameter-to-thickness ratio and the length-to-diameter ratio are the variables. The rest of the charts depic t curves relating the geometry of cyhnders and spheres to allowable stress by cui ves which are determined from the modulus of elasticity, tangent modulus, and yield strength at temperatures for various materials or classes of materials. The text of this subsection explains how the allowable stress is determined from the charts for cylinders, spheres, and hemispherical, ellipsoidal, torispherical, and conical heads. 

Autographic load-deformation curves are often drawn during the test. From such a curve, the modulus of elasticity, proportional limit, and yield strength can be determined,... 

Recently Yamaoka and Miyata carried out the reactor irradiation at 20 K on two kinds of aromatic polyimides, Upilex-S and Upilex-R [51]. The stress-strain curves of irradiated Upilex-S films are depicted in Fig. 7. No essential changes in both elastic modulus and yield strength were observed for Upilex-S after irradiation up to 8 x 106 Gy, although a slight decrease of the ultimate elongation was detected. A similar tendency was found in the reactor irradiation of Upilex-R at 20 K, in spite of the fact that its structural unit of the main chain is different from that of Upilex-S. These results indicate that a homologue of aromatic polyimides exhibits the excellent radiation stability even at cryogenic temperatures as well as at ambient temperature. 

Deng et al. (8) investigated the tensile properties of PEEK/MWCNTs, and found increases in the elastic modulus and yield strength at temperatures above and below Tg at 25°C, the tensile modulus increased by -90% for composites including 15 wt% MWCNTs, and the increment reached -160% at 200°C. According to those results, the improvement of MWCNTs in the mechanical behaviour of the matrix is more effective at higher temperatures. Experimental results do confirm that the overall mechanical performance of PEEK/CNT nanocomposites is well above the required for potential aircraft applications. 

Microhardness, therefore, appears to be an elastic-plastic rather elusive parameter (Marsh, 1964). Microhardness as a property is, in fact, a complex combination of other properties elastic modulus, yield strength and strain hardening capacity. One way to differentiate between the reversible and irreversible components of contact deformation is to measure the elastic recovery during unloading of the indenter (Stilwell Tabor, 1961). Extreme cases of depth recovery are best described by soft metals, where it is negligible, and fully elastic rubber, where it is complete. 

Addition of elastomer has been found to increase fracture resistance, but it lowers and reduces elastic modulus, creep resistance and yield strength and increases susceptibility to moisture at elevated temperatures. 

---