STRUCTURES Young’s Modulus

Tests There are 4 tests that you can do to a material There are 4 tests that you can do to a material 1 tensile This is where a material is stretched to its limits 2 compressive This is where a material is squashed to its limits 3 shear This is where a material is cut along its diameter 4 torsion This is where the material is twisted to its limit

Young's Modulus Thomas Young born June 13, 1773, Milverton, Somerset, England died May 10, 1829, London. Thomas Young born June 13, 1773, Milverton, Somerset, England died May 10, 1829, London.

Mechanical Properties The mechanical properties of a material can be divided into static and dynamic properties. The mechanical properties of a material can be divided into static and dynamic properties. Static properties These include Strength and Hardness Strength is the ability to resist a force without breaking. There are four fundamental types of loading as described in the last slide. Elasticity is the materials ability to return to its original shape after being deformed it will depend on the material and the load carried rubber is elastic up to its breaking point. Plasticity Is the readiness to deform to a stretched state when a load is applied. The plastic deformation is permanent even after the load is removed Plasticine exhibits plastic deformation. Ductility Is the ability to be drawn out longitudally to reduced cross section a ductile material must therefore have high plasticity Hardness Is the resistance to wear or indentation of a material. It is necessary for engine parts or cutting tools where constant friction causes abrasion or wear. Malleability Is the ability of a material to be stretched in all directions without fracture. A malleable material can be hammered into shape.

Dynamic Properties Dynamic Properties These include fatigue, creep and impact resistance where the time or the rate of loading is significant. Creep Is a slow plastic deformation that can occur when a load is applied for prolonged periods of time. It can be observed at room temperature effect and can cause failure at lower stress than static test value. It is an important consideration where a component is subjected to a high temperature for long periods, as in the case of gas turbine blades. Fatigue Is the phenomenon where by which a material can fail at a lower than normal stress if the load is applied many times it will generally result in a small crack in the surface of the material which will gradually extend into the material progressively reducing its ability to carry loads Toughness Is the ability to withstand sudden loads. It is measured by the total energy that the material can absorb. Resilience Is the ability of a material to absorb energy. It depends on the amount the material extends under load as well as the stress it can bear. For this reason materials such as Nylon, that can be stretched considerably, is suitable for tow ropes which have to withstand snatch loads. Brittleness Implies the lack of ductility and toughness. A material that shows no significant plastic deformation before fracture is said to be brittle..

Tensile testing Tensile testing, in which a specimen is loaded to destruction, is one of the most important materials property tests. The results depend on the size and shape of the test piece and specimen sizes should therefore confirm to British standard BS 18:1987 Tensile testing of metals. Specimens should have a central gauge length of uniform cross- section, thereby avoiding regions of high stress concentration. Nonuniformity mat result in premature failure. Tensile specimens can be rectangular or circular in cross-section Tensile testing, in which a specimen is loaded to destruction, is one of the most important materials property tests. The results depend on the size and shape of the test piece and specimen sizes should therefore confirm to British standard BS 18:1987 Tensile testing of metals. Specimens should have a central gauge length of uniform cross- section, thereby avoiding regions of high stress concentration. Nonuniformity mat result in premature failure. Tensile specimens can be rectangular or circular in cross-section

Stress strain curve for low carbon steel A B CD U F Strain e Stress actual stress as the cross- sectional area is reduced 2 Calculated stress based on Original cross-sectional area Resilience: absorbed energy in the elastic region

Tensile properties of low carbon steel Tensile properties of low carbon steel The graph on the board shows the tensile strength extension curve for mild steel with about 0.4% carbon We can see that the specimen extended gradually for a time then went through a transition phase after which it extended more rapidly until fracture occurred. 0-A indicates the values for which the stress is directly proportional to the strain, a result known as hooks law. In this region the ratio of the stress to strain is termed the modulus of elasticity it is given symbol e and is often referred to as the Young's Modulus A is the limit of proportionality 0-B indicates the range in which the specimen extends elastically, and will return to its original length if the load is removed. B is the elastic limit. Band may be coincident with A for some materials. C-D marks the region in which the material undergoes internal structural changes before plastic deformation and permanent set occur. C is the upper Yield point at which the specimen begins to extend. This value of stress is known as the Yield strength. D the lower yield strength D-F shows the region of continued plastic deformation. It is the region where work hardening takes place U marks the ultimate tensile stress (or tensile strength) and represents the maximum nominal stress that a specimen can endure. Beyond this point the specimen narrows or necks and the nominal stress falls F indicates the fracture stress and is the point which the material fails.