SCIENCE & MATERIALS IN CONSTRUCTION

Science & Materials in Construction



Science & Materials in Construction

Question 1)

(A)

This section will highlight the values of Young's modules, tensile and comprehensive strength and various other properties in choosing the material for the building such as the outer surface of the building, the main supporting structure and the foundations. It also gives a review of certain fundamental aspects of mechanics of materials, using the material's response to unidirectional stress to provide an overview of mechanical properties without addressing the complexities of multidirectional stress states.

Tensile Strength and Tensile Stress

Perhaps the most natural test of a material's mechanical properties is the tension test, in which a strip or cylinder of the material, having length L and cross-sectional area A, is anchored at one end and subjected to an axial load P - a load acting along the specimen's long axis - at the other. (See Fig. 1.1). As the load is increased gradually, the axial deflection d of the loaded end will increase also. Eventually the test specimen breaks or does something else catastrophic, often fracturing suddenly into two or more pieces. (Materials can fail mechanically in many different ways; for instance, recall how blackboard chalk, a piece of fresh wood, and Silly Putty break.) As engineers, we naturally want to understand such matters as how d is related to P, and what ultimate fracture load we might expect in a specimen of different size than the original one. As materials technologists, we wish to understand how these relationships are influenced by the constitution and microstructure of the material (Geoffrey, 2000, pp. 11).

One of the pivotal historical developments in our understanding of material mechanical properties was the realization that the strength of a uniaxially loaded specimen is related to the magnitude of its cross-sectional area (Gardiner, 1995, pp. 90). This notion is reasonable when one considers the strength to arise from the number of chemical bonds connecting one cross section with the one adjacent to it as depicted in Fig. 1.2, where each bond is visualized as a spring with certain stiffness and strength. Obviously, the number of such bonds will increase proportionally with the section's area. The axial strength of a piece of blackboard chalk will therefore increase as the square of its diameter. In contrast, increasing the length of the chalk will not make it stronger (in fact it will likely become weaker, since the longer specimen will be statistically more likely to contain a strength-reducing flaw.)

Figure 2: Interplanar Bonds (sureface density approximately 1019 m-2)

Stiffness in Tension - Young's Modulus

It is important to distinguish stiffness, which is a measure of the load needed to induce a given deformation in the material, from the strength, which usually refers to the material's resistance to failure by fracture or excessive deformation. The stiffness is usually measured by applying relatively small loads, well short of fracture, and measuring the resulting deformation. Since the deformations in most materials are very small for these loading conditions, the experimental problem is largely one ...