Materials Engineering

Paper instructions:
A report has to be rewritten. It is divided into three sections:
1. Explain the concept of fatigue
2. Explain the concept of creep
3. Explain the properties of aluminium alloy
The report need to be rewritten as to avoid plagiarism.
The format must not be changed.
The pre-written report is attached.

Materials engineering
Explain the concept of fatigue. (How does it affect a component and how can it be prevented during the

design stage?)
In materials science, fatigue is the weakening of a material caused by repeatedly applied loads. It is the

progressive and localized structural damage that occurs when a material is subjected to cyclic loading.

The nominal maximum stress values that cause such damage may be much less than the strength of the

material typically quoted as the ultimate tensile stress limit, or the yield stress limit.
Fatigue occurs when a material is subjected to repeat loading and unloading. If the loads are above a

certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface,

persistent slip bands (PSBs), and grain interfaces. Eventually a crack will reach a critical size, the crack

will propagate suddenly, and the structure will fracture. The shape of the structure will significantly

affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue

cracks can initiate. Round holes and smooth transitions or fillets will therefore increase the fatigue

strength of the structure.
There are two types of fatigue:
• high cycle fatigue
• low cycle fatigue
Design against fatigue
Dependable design against fatigue-failure requires thorough education and supervised experience in

structural engineering, mechanical engineering, or materials science. There are four principal approaches

to life assurance for mechanical parts that display increasing degrees of sophistication.
Design to keep stress below threshold of fatigue limit (infinite lifetime concept)
Fail-safe, graceful degradation, and fault-tolerant design: Instruct the user to replace parts when they

fail. Design in such a way that there is no single point of failure, and so that when any one part

completely fails, it does not lead to catastrophic failure of the entire system.
Safe-life design: Design (conservatively) for a fixed life after which the user is instructed to replace the

part with a new one (a so-called life part, finite lifetime concept, or “safe-life” design practice); planned

obsolescence and disposable product are variants that design for a fixed life after which the user is

instructed to replace the entire device;
Damage tolerant design: Instruct the user to inspect the part periodically for cracks and to replace the

part once a crack exceeds a critical length. This approach usually uses the technologies of non-

destructive testing and requires an accurate prediction of the rate of crack-growth between

inspections. The designer sets some aircraft maintenance checks schedule frequent enough that parts

are replaced while the crack is still in the “slow growth” phase. This is often referred to as damage

tolerant designer “retirement-for-cause”.
Explain the concept of creep. (How does it affect a component and what is the causes?)
In materials science, creep (sometimes called cold flow) is the tendency of a solid material to move

slowly or deform permanently under the influence of mechanical stresses. It can occur as a result of

long-term exposure to high levels of stress that are still below the yield strength of the material. Creep

is more severe in materials that are subjected to heat for long periods, and generally increases as they

near their melting point.
The rate of deformation is a function of the material properties, exposure time, exposure temperature

and the applied structural load. Depending on the magnitude of the applied stress and its duration, the

deformation may become so large that a component can no longer perform its function — for example

creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade.

Creep is usually of concern to engineers and metallurgists when evaluating components that operate

under high stresses or high temperatures. Creep is a deformation mechanism that may or may not

constitute a failure mode.
Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress.

Instead, strain accumulates as a result of long-term stress. Therefore, creep is a “time-dependent”

deformation.
The temperature range in which creep deformation may occur differs in various materials. For example,

tungsten requires a temperature in the thousands of degrees before creep deformation can occur, while

ice will creep at temperatures near 0 °C (32 °F).[1] As a general guideline, the effects of creep

deformation generally become noticeable at approximately 30% of the melting point (as measured on a

thermodynamic temperature scale such as kelvin or rankine) for metals, and at 40–50% of melting point

for ceramics. Virtually any material will creep upon approaching its melting temperature. Since the

creep minimum temperature is related to the melting point, creep can be seen at relatively low

temperatures for some materials. Plastics and low-melting-temperature metals, including many solders,

can begin to creep at room temperature, as can be seen markedly in old lead hot-water pipes. Glacier

flow is an example of creep processes in ice.

Materials are often placed in service at ‘relatively high temperatures’ and exposed to static mechanical

stresses. These stresses are less than the yield strength of the material but nevertheless can cause

plastic deformation to take place – particularly over a long period of service time. This phenomenon is

known as creep. Note that the term ‘relatively high temperatures’ means high homologous

temperatures (Tservice/Tmelting) and is a measure of how near the temperature is to the melting point

of the material concerned.
So room temperature (20°C) is a low homologous temperature for steel (melting point around 1600°C),

but is a high homologous temperature for tin-lead solder (melting point around 180°C).
Creep is observed in all material types – in metals it only becomes important at temperatures greater

than about 0.4Tm (where Tm is the melting point in Kelvin). Soft metals such as tin and lead creep at

room temperature while aluminium and its alloys creep around 250°C. Steel creeps at about 450°C while

nickel-based alloys (nimonics) creep at around 650°C. A typical creep curve is shown in Figure 1 below.

Properties of high strength aluminium alloy and processing methods

High strength aluminium alloy
It is commonly available in tempered grades such as 6061-T6 (solutionized and artificially aged). 6061 is a

precipitation hardening* aluminium alloy, containing magnesium and silicon as its major alloying

elements. It has good mechanical properties. It is one of the most common alloys of aluminium.

*Precipitation hardening, also called age hardening, is a heat treatment technique used to increase the

yield strength of malleable materials, including most structural alloys of aluminium.
The alloy composition of 6061 is:
•   Silicon minimum 0.4%, maximum 0.8% by weight
•   Iron no minimum, maximum 0.7%
•   Copper minimum 0.15%, maximum 0.40%
•   Manganese no minimum, maximum 0.15%
•   Magnesium minimum 0.8%, maximum 1.2%
•   Chromium minimum 0.04%, maximum 0.35%
•   Zinc no minimum, maximum 0.25%
•   Titanium no minimum, maximum 0.15%
•   Other elements no more than 0.05% each, 0.15% total
•   Remainder Aluminium (95.85%–98.56%)

T6 temper 6061 has an ultimate tensile strength of at least 42,000 psi (300 MPa) and yield strength of at

least 35,000 psi (241 MPa). More typical values are 45,000 psi (310 MPa) and 40,000 psi (275 MPa),

respectively.[4] In thicknesses of 0.250 inch (6.35 mm) or less, it has elongation of 8% or more; in thicker

sections, it has elongation of 10%. T651 temper has similar mechanical properties. The typical value for

thermal conductivity for 6061-T6 at 77°F is around 152 W/m K. A material data sheet defines the fatigue

limit under cyclic load as 14,000 psi (100 MPa) for 500,000,000 completely reversed cycles using a

standard RR Moore test machine and specimen. Note that aluminium does not exhibit a well-defined

“knee” on its S-n graph, so there is some debate as to how many cycles equates to “infinite life”. Also

note the actual value of fatigue limit for an application can be dramatically affected by the

conventional de-rating factors of loading, gradient, and surface finish.

ALUMINUM DIE CASTING
Aluminium die casting is a casting process that is characterized by forcing molten aluminium under high

pressure into a mould cavity. The mould cavity is created using two hardened tool steel dies which have

been machined into shape and work similarly to an injection during the process. The casting equipment

and the metal dies represent large capital costs and this tends to limit the process to high volume

production. Manufacture of parts using die casting is relatively simple, involving only four main steps,

which keeps the incremental cost per item low. It is especially suited for a large quantity of small to

medium sized castings, which is why die casting produces more castings than any other casting

process. Die castings are characterized by a very good surface finish (by casting standards) and

dimensional consistency.

Forging
6061 is an alloy that is suitable for hot forging. The billet is heated through an induction furnace and

forged using a closed die process. The weight of the products which can be manufactured has a huge

tolerance. They can start from less than a kilogram and it can range up to 580 metric tons. Once this

process has been completed, the product often requires a secondary process. So for this instance, a 5

axis CNC milling machine can be used for mill the pulley gear. Forging tooling is generally cheaper than,

for example, high-pressure die cast tooling, and the production rate is higher. Offsetting this is

generally higher raw material costs, associated with the necessary alloying of raw materials to provide

desirable heat treatable characteristics. Consequently, many aluminium forgings are used in highly

stressed applications.

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