TOPIC 4 | RAW MATERIAL TO FINAL PRODUCT

4.1 PROPERTIES OF MATERIALS

Young's Modulus

The Young's Modulus (or Elastic Modulus) is in essence the stiffness of a material. In other words, it is how easily it is bended or stretched.

The Young's Modulus describes the elastic properties of a solid undergoing tension or compression in only one direction, as in the case of a metal rod that after being stretched or compressed lengthwise returns to its original length. Young’s modulus is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression. Sometimes referred to as the modulus of elasticity, Young’s modulus is equal to the longitudinal stress divided by the strain. Stress and strain may be described as follows in the case of a metal bar under tension.

If a metal bar of cross-sectional area A is pulled by a force F at each end, the bar stretches from its original length L0 to a new length Ln. (Simultaneously the cross section decreases.) The stress is the quotient of the tensile force divided by the cross-sectional area, or F/A. The strain or relative deformation is the change in length, Ln − L0, divided by the original length, or (Ln − L0)/L0. (Strain is dimensionless.) Thus Young’s modulus may be expressed mathematically as Young’s modulus = stress/strain = (FL0)/A(Ln − L0).

This is a specific form of Hooke’s law of elasticity. The units of Young’s modulus in the English system are pounds per square inch (psi), and in the metric system newtons per square metre (N/m2). The value of Young’s modulus for aluminium is about 1.0 × 107 psi, or 7.0 × 1010 N/m2. The value for steel is about three times greater, which means that it takes three times as much force to stretch a steel bar the same amount as a similarly shaped aluminium bar.

Young’s modulus is meaningful only in the range in which the stress is proportional to the strain, and the material returns to its original dimensions when the external force is removed. As stresses increase, the material may either flow, undergoing permanent deformation, or finally break.

When a metal bar under tension is elongated, its width is slightly diminished. This lateral shrinkage constitutes a transverse strain that is equal to the change in the width divided by the original width. The ratio of the transverse strain to the longitudinal strain is called Poisson’s ratio. The average value of Poisson’s ratio for steels is 0.28, and for aluminum alloys, 0.33. The volume of materials that have Poisson’s ratios less than 0.50 increase under longitudinal tension and decrease under longitudinal compression.

Yield Strength

Yield stress or yield strength is the value most often used in engineering calculations. It gives a material a stress value in MPa it can take before plastic deformation. This place is called the yield point. Before it, a material regains its former shape when lifting the load. After exceeding the yield point, the deformation is permanent.

Stress-strain curve

There is a good reason for using yield stress as the most important factor in mechanical engineering. As can be seen from the stress-strain curve, when stress goes beyond the yield point, the damage is not yet catastrophic. That leaves a “cushion” before a construction fails completely to the point of breaking.

Tensile Strength

The ability to withstand pulling or stretching forces (tension).

Ultimate tensile strength, or just tensile strength, is the next step from yield strength. Also measured in MPa’s, this value indicates the maximum stress a material can withstand before fracturing. When choosing a suitable material to tolerate known forces, two materials with a similar yield strength may have different tensile strengths. Having higher tensile strength may help to avoid accidents, if unforeseen forces are applied.


Plasticity, Ductility & Malleability

Plasticity is a mechanical property of materials that shows the ability to deform under stress without breaking, while retaining the deformed shape after the load is lifted. Metals with higher plasticity are better for forming. This is evident in metal bending. Two related mechanical properties of materials are ductility and malleability. 

Ductility has a similar description to plasticity – it is a material’s ability to undergo plastic deformation before breaking. It is expressed as a percent elongation or percent area reduction. Basically, ductility is a property you need when drawing thin metal wires, for example. A good example of such a ductile material is copper. This makes the fabrication of wires possible.

Malleability is, by definition, also similar. But it actually characterises a material’s suitability for compressive deformation. In essence, a metal with good malleability is fitting for producing metal plates or sheets by rolling or hammering.

Gold jewellery is an example of a malleable material

Wire is an example of a ductile material

Toughness

Toughness is a combination of strength and plasticity. A tough material can take hard blows without rupturing. Toughness is often defined as a material’s ability to absorb energy without cracking. 

An example of required toughness is quarry loaders. Throwing huge rocks into the bins results in deformations, not cracks, if the material is tough.

Hardness

High hardness values show that a material resists localised pressures. In simple terms, a hard material is not easy to scrape or punctuate with lasting marks (plastic deformation). It is especially important when heavy wear processes take place. In such circumstances, hard materials like Hardox are suitable. Hardness and toughness are two qualities that account for durability.

Hardness is measured by scratching, bouncing or indentation. The most common way to describe hardness is through indentation hardness. There are different ways to carry out these tests, depending on the material. Each results in a different hardness unit – Brinell, Vickers or Rockwell. If you want to compare 2 materials that have hardness values in different systems, you have to convert them to the same type (e.g Vickers) first.

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Sources of Information:

The following sources were used in some part to help piece the above information together; Fractory, Britannica,