METALS DESIGN REPORT

METALS DESIGN REPORT

Table of Contents

3Introduction

3Question 1: Laboratory equipment needed and techniques used to measure each of the metal properties given in the spreadsheet

3Ductility

4Yield Strength

4Ultimate Tensile Strength

4Question 2: Describe how the material properties given in the spreadsheet may be used to determine the desired performance characteristics

4Ability to deform inelastically when subjected to severe earthquake loads

5Ability to resist repeated application of cyclic loads

5Ability to resist highest amount of deflection

5Question 3: Enhancing mechanical properties of a ductile steel to meet the desired performance characteristics

7Question 5: Selecting the metal to meet the desired performance characteristics and justification of the choice

8Conclusion

9Bibliography

Introduction

One aspect of engineering design involves selecting materials with chemical, physical, and mechanical properties best suitable for the project under consideration. Mechanical properties determine how a material behaves when loaded. The role of the engineer is to select metals whose mechanical properties will enable them to carry the intended load without failure. Common mechanical properties of metals include toughness, stiffness, brittleness, ductility, malleability, fatigue strength, plasticity, elasticity, resilience, yield strength, and ultimate tensile strength. These properties are determined through special tests. A common engineering test is the tensile test carried using the universal tensile testing machine. It produces load-extension data that is then used to plot a stress-strain curve. The curve provides a lot of information about the metal concerning its mechanical properties.

It is possible to improve the mechanical properties of a metal to make it suitable for a given engineering application through heat and chemical treatments. Common treatments include hardening, annealing, case hardening, tempering, and normalising (Sharma 2003). Heating the metal to specific temperatures results in change of the metal structure for many alloys. Hardening involves heating the metal to elevated temperatures and the cooling it rapidly by dipping it into a quenching medium such as oil. The process is used to increase metal hardness, strength, and brittleness (Sharma 2003). Tempering is done to reduce metal brittles especially after the hardening process. In this process, the metal is heated to temperatures below those used in the hardening process and then allowed to cool in air (Sharma 2003). Annealing is also done to increase ductility. In this case, the metal is heated to the desired temperature and then held at that temperature for a given time before being allowed to cool (Sharma 2003). Normalising is done to increase hardness and strength of a metal. It involves heating the metal to high temperatures, usually above hardening and annealing temperatures, soaking it and then holding it at that temperature for a specified amount of time. The metal is then allowed to cool in still air (Sharma 2003). Addition of carbon, nitride, magnesium, chromium, lead, and manganese at high temperature also alters mechanical properties including hardness and toughness.

This paper selects between four metals whose mechanical properties have been determined through tensile test. Through analysis of provided load-extension data, metal C has been selected as fulfilling the required needs, which include ability to deform inelastically, ability to resist repeated application of cyclic loads, and ability to resist the highest amount of deflection.

Question 1: Laboratory equipment needed and techniques used to measure each of the metal properties given in the spreadsheet

The metal properties being tested are ductility, yield strength, and ultimate tensile strength.

Ductility

Ductility is defined as the ability of a material to undergo inelastic deformation under tensile loading without undergoing considerable degradation of its stiffness or strength (Danyeur 2008). A uniaxial tensile test is usually performed to determine the ductility property of a material. The laboratory equipment used to perform this test is the universal tensile testing machine that applies tensile force on a standard specimen. The equipment records force and extension data that is then used to determine the material’s ductility, which is calculated as the ratio of material elongation at fracture to the original length of the specimen (Vuherer et al. 2007). Alternatively, one can calculate ductility of the material as the ratio of reduction in the cross-section area of the specimen after fracture to the initial cross-section area. When a material is subjected to uniaxial tensile force, it elongates (changes in length). In addition, necking occurs at the region of fracture.

Yield Strength

Yield strength is the stress corresponding to initiation of plastic deformation of the material or the end of elastic deformation. Before reaching yield point, a material will deform elastically, which means that it will resume original shape and size once the load is removed. Beyond this point, plastic deformation will occur such that removal of the load will not be associated with resumption of the original dimensions. A tensile test using a standard tensile testing equipment is used to determine the yield strength of a material. The resultant force extension data is used to plot a stress-strain curve (as shown in figure 1) from which tensile strength is obtained.

Ultimate Tensile Strength

Further increase in load above the yield point increases plastic deformation until a point is reached when the material fractures as shown in figure 1. The stress at this point is known as ultimate tensile strength, which is the maximum amount of stress that the material can withstand without fracture although yielding will have occurred. Beyond this point, fracture occurs.

METALS DESIGN REPORT

Question 2: Describe how the material properties given in the spreadsheet may be used to determine the desired performance characteristics

The desired performance characteristics are fatigue strength (ability to resist repeated application of cyclic loads), ductility (ability to deform inelastically when subjected to severe earthquake loads), and stiffness (ability to resist highest amount of deflection). The material properties presented in the spreadsheet can be used to determine these three material properties.

Ability to deform inelastically when subjected to severe earthquake loads

As earlier mentioned, it is possible to calculate ductility by dividing reduction in cross-sectional area of the specimen with the original cross-sectional area. Ductile materials will undergo considerable necking or elongation before fraction as opposed to brittle materials that will fracture almost immediately after reaching yield point. Therefore, the higher the ratio (reduction in specimen’s cross-sectional area: original cross-sectional area), the more the material is able to deform inelastically (plastically) when subjected to severe earthquake loads. In other words, the higher the ratio, the more ductile the material and hence the more suitable it is for the desired job.

Ability to resist repeated application of cyclic loads

Fatigue strength is the ability of the material to resist repeated application of cyclic loads. There are two approaches to determining this material property, crack initiation and crack propagation. The most common test is crack initiation test whereby the specimen may be subjected to compressive or tensile stresses that are cycled between minimum and maximum stress levels (Vuherer et al. 2007). The stress ratio (A), which is the ration of two stress figures within the stress cycle, is determined as,

METALS DESIGN REPORT 1

Test procedure produces data on number of cycles to fracture (N) and stress amplitude (S), which is used to plot S-N curve. The horizontal axis of the curve represents fatigue limit, which is the maximum strength that the material can withstand when an infinitely large number of load cycles is applied. There is a 50% chance that the metal will fail. However, most non-ferrous metals report fatigue strength instead of fatigue limit because their curves exhibit a gradual drop at high N values (Vuherer et al. 2007).

Therefore, it means that materials that have high values of ultimate tensile strength will posses high values of fatigue strength. Accordingly, when selecting a material for this engineering application, which should satisfy this requirement, it is crucial to choose those with high ultimate tensile strengths. Such materials will withstand high number of load cycles before fracture as opposed to those with lower ultimate tensile strength, which will fail when only a few load fluctuations are applied.

Ability to resist highest amount of deflection

The section before the yield strength is a straight line representing elastic deformation of the material under load. The slope of the curve at this section is known as Young’s modulus or elastic modulus. This material property represents the material’s resistance to deflection. The formula for Young’s modulus is,

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Therefore, if a small change in stress yields a high change in strain, it means that the material is unable to resist high amount of deflection. On the other hand, if a high change in stress (load) yields a small change in strain (extension), it means that the material is stiff and thus resists high amount of deflection. Such a material, which has a high value of Young’s modulus, would be highly applicable for the engineering work under consideration.

Question 3: Enhancing mechanical properties of a ductile steel to meet the desired performance characteristics

The chemical composition and treatment of a material fundamentally determines its mechanical properties. Tests have shown that the fatigue strength of steel can be increased by adding 1% carbon, 1.5% chromium and then heating the material to 8310F. Any heat treatment that increases the tensile strength of a material increases fatigue strength. Heat treatment has been shown to be more effective in improving a material’s fatigue strength than altering the chemical and microstructure. Low temperature tempering of quenched steel increases the fatigue strength of the metal without any negative impact on tensile strength (Sharma 2003). Conclusively, it is possible to increase the ability of the material to resist repeated application of cyclic loads through carbon (adding 1% carbon) and heat treatment (tempering quenched metal at 8310F).

However, these metal treatments negatively affect ductility or the ability of the metal to deform inelastically. In general, processes aimed at improving tensile strength and fatigue strength reduce the material’s ductility because they increase brittleness property. Enhancing ductility is achieved through annealing whereby heat is applied slowly to the metal until an elevated temperature is achieved. The metal is then held in this high temperature for a long time before being cooled slowly (Sharma 2003). This process can be done after hardening processes to relieve internal stresses that develop in the material after they undergo heat treatment processes. In addition, annealing refines grain structure of the material to make it more ductile then before such that it can undergo considerable plastic deformation before fracture.

Question 4: sketching and labeling the expected stress-strain relationships up to the point of failure for the four metals

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METALS DESIGN REPORT 4

Initial cross-sectional area for metal A = 78.744mm2

Initial cross-sectional area for metal B = 81.089mm2

Initial cross-sectional area for metal C = 82.034mm2

Initial cross-sectional area for metal D = 80.531mm2

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Question 5: Selecting the metal to meet the desired performance characteristics and justification of the choice

The elimination method will be used in selecting the desired metal. Metal A and metal B fail to satisfy the property of ability to deform inelastically. From the above curve, these two metals have low values of elongation before failure, which means that they will easily fracture when a load is applied. They have low ductility levels. In fact, metal B is a brittle metal as characterised by lack of plastic (inelastic) deformation. Its stress-strain curve shows that it failed immediately after reaching the yield point, which indicates that it did not undergo any inelastic deformation. As such, it is incapable of deforming inelastically.

In addition, these two metals fail to meet the second condition of ability to resist repeated application of cyclic loads. As earlier mentioned, this property is represented by the ultimate tensile strength of a material. From the figure above, metal A and metal B have low ultimate yield strength, which further rules them out.

Two metals are now remaining, C and D. The former has the highest ultimate tensile strength, which means that it is highly capable of resisting cyclic loads. However, it has a lower ductility level than metal D, which means that metal D will perform better than C on the ability to deform inelastically. The third requirement, ability to resist deflection, will be used to rule out between these two metals. As aforementioned, this property is reflected by the material’s Young’s modulus. The material with a higher Young’s modulus would be appropriate for this job because it will be stiff and would thus resist deflection. Metal C has a higher elastic modulus than metal D, which means that the former is stiffer than the latter. The gradient of the linear (elastic) part of the curve is the Young’s modulus. The higher the gradient, the higher the material property. From the curve shown in figure 2, the gradient for the elastic section of Metal C’s curve is higher than that of metal D’s curve. Therefore, one can conclude that metal C is stiffer and more appropriate than metal D. Conclusively, metal C has been selected for the application because it meets all the three required performance requirements.

Conclusion

Tensile test helps in getting crucial information about a material regarding its mechanical properties and consequently its behaviour when loaded. The task at hand in this assignment was to select the best material for the engineering project. This undertaking is very crucial in engineering design and construction to ensure that materials used are able to withstand applied loads or forces that are expected during project use. Therefore, an understanding of properties of engineering materials is critical to the success in civil and construction engineering. After critical analysis using provided data, metal C has been found best applicable. Its performance can be enhanced through carbon and heat treatment.

Bibliography

Danyeur, A 2008, Insitu expanding foam based carbon/epoxy sandwich jackets for column retrofit. ProQuest LLC.

Sharma, RC 2003, Principles of heat treatment of steels. New Delhi, India: New Age International(P) Limited, Publishers.

Vuherer, T., Godina, A., Burzic, Z., Gliha, V. (2007). Fatigue crack initiation from microstructurally small Vickers indentations. Metalurgija, vol. 46, no. 4, pp. 237-243.