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## Question 1

Question 1

a)Is the creep mechanism Bulk Diffusion highly dependent with stress? Explain why help yourself with drawings.

b)Is the creep mechanism Bulk Diffusion highly dependent on grain size? Explain why help yourself with drawings.

c)Which mechanism of creep can be prevented using a single crystal part? Why?

d)Which mechanism of creep can be accentuated with alloying elements? Why?

Question 2

a.Draw the T-T-T diagram for plain carbon steel of eutectoid composition and show on the diagram the critical cooling curve, the transformation lines, the phases, the axis.

b.Use your diagram to explain the processes of quenching and tempering of the steel.

c.Explain the change of structure with martensitic transformation.

d.Discuss the implications of hardenability with the dimensions of the part to be manufactured.

e.In case the hardenability of the material is not good enough, discuss some strategies to increase the transformation without changing the geometry of the part.

Question 3

Springs for trucks. In vehicle suspension design it is desirable to minimize the mass of all components. You have been asked to select a material and dimensions for a light spring to replace the steel leaf-spring of an existing truck suspension. The existing leaf-spring is a beam, shown schematically in the figure. The new spring must have the same length L and stiffness S as the existing one, and must deflect through a maximum safe displacement δmax without failure. The width b and thickness t are free variables.

In order to design a high performance connecting rod for a racing car, you need to select the suitable material for this application. The engine is a modified engine with the stroke and bore diameter fixed.  Previous experience has demonstrated that in case of failure of high performance connecting rods, they are due to either fatigue or buckling. The engineer leader wants the engine as light as possible. The connecting rod length is L=0.1 m and has to resist F=20000 N in compression.

In order to translate the problem you need to answer these questions:

a)What is the function of the part?

b)What are the constraints of the part?

c)What is the objective?

d)What are the free variables? Discuss the implication of the free variables for the optimal choice.

e)By neglecting the bearing houses calculate the mass of the as a function of material properties to minimise the likelihood of fatigue failure.

f)What is the performance material index for fatigue?

g)By neglecting the bearing houses calculate the mass of the rod as a function of material properties to minimise the likelihood of buckling.

h)What is the performance material index for buckling?

i)Discuss the implication of two performance index for the selection of one material.

Question 1

No, the creep mechanism Bulk Diffusion does not exhibit high-stress dependency.  The creep is generally time dependent. The creep is the deformation in a material which occurs when a particular system is used in similar loading conditions for a prolonged period of time (Schmitt, J et.al 2018)

Creep strain Vs Stress Vs time dependency

The creep rate exhibits a high dependency on the grain size in case of Bulk deformation creep mechanism. In this bulk deformation mechanism, the creep rate is inversely proportional to the size of the grain structure (Vaidya, M et.al 2018).

Grain size effects on the creep rate

TheCoble creepalso known as Grain boundary diffusion creep and the Bulk deformation creep mechanisms can be controlled with the help of single crystal part. As both the bulk deformation and the grain boundary diffusion are depended upon the grain size of the material this single crystal part can be used to control the creep rate (Hirth, G. and Kohlstedt, D.L., 2015).

The creep mechanisms such as the Bulk deformation (Nebarro heriing) Creepand the Grain boundary diffusion (Coble creep) are the most exhibited type of creep in the Alloying elements. As both the mechanisms are grain size dependent mechanism the percentage of creep decreases with the increase in the percentage of grain size (Sun, S.H 2015).

1. Plain Carbon Steel TTT diagramfor the eutectoid composition can be drawn as:

TTT diagram of plain carbon steel

1. Accelerated cooling of ahotspecimen is called as quenching process. The cooling is achieved by the exposure of hot specimen with the quench medium such asFluids like air, water, hydrogen and solids like dry ice, ice etc. The quenching process also influences the strength of the final material.

The different types of quench mediums are oil, gas, caustic sodacold water and salt water.

The material properties such as brittleness, ductility and toughness are greatly changed due to the hardening process. The heat treatment process is done to improve the discussed properties. The heat treatment process adopted for this purpose is known as the tempering. The tempering process is carried out after completing the hardening process. The tempering process involves a slow and steady cooling of the specimen after heating the material to lower martensitic temperature. The process occurs in room temperature.

1. Shape Memory Effect (SME) is the result of the Martensitic transformation of a specimen. The shaipe memory effect is the ability of a specimen to remember its original shape which when heat treated after deformation will return back to its original position (Zhao, Q et.al 2015).
2. The hardness of the material is the measurement of the capacity of the material to withstanding scratches. Also, a hard material will withstand indentation subjected on it. Hard materials have low ductility and also poor machinability. The subtractive machining of a hardened material is really tough so that it needs to be simple in shape (Youssef, K.M et.al 2015).
3. The Hardness property of a material is increased by alloying specimen with carbon. The material hardnessis directly proportional to the amount of carbon alloyed with it. The main process which is adapted to improve hardness is nitriding, Carburizing, flame hardening, carbonitriding, etc.

 Function a) To provide Suspension Constraints a) L = Fixed b) The equation for Max displacement provided c) Stiffness Equation provided Objective a) Minimize mass of all components Free variables a) Width W b) Material choice c) Thickness t

L :Lengthof spring

t :thickness of spring

:density of the material.

From the given constrains:

The Equation for Stiffness S is given as:

Here,

δ : deflection.

F : force

The δmax Maximum deflection equation is given as:

Maximum Displacement,

The deflection equation is written as:

Maximum Displacement

Where,

I : I

L: length

E : modulus of elasticity

Substitute &L2in the Stiffness equation then we will get:

The maximum displacement equation can be written for the thickness t as

## Question 2

Calculation of b from the Stiffness equation:

b = SL3 / (4Ebt3)

Substitute  t and b in the mass equation

With the index maximization, the spring mass is minimized.

K1c (Fracture toughness index )> 15 Mpa.m0.5 considered for the accidental impacts.

Material Selection:

The list of materials which are suitable.

 Material Property Steel Expensive Heavy Carbon Fiber Reinforced Plastics Low weight Expensive Super Strength Rubbers Light Available abundantly

1. Given data:

Temperature and Temperature 1,  T1: 450K&t1: 30 secs

Temperature and Temperature 1,  T2 : 430K& t2: 81.5s

We know that,

Time for injection can be given as,     ti=

Q is the amount of  heat transfer

C is the Specific heat capacity

Injection timing relation is given as:

Q = =     8.05 E4J/mole

C =  =  = 7.325E7

The overall time :

Toverall   =   = 3.457 Seconds

The Overall time is obtained at 3.457 Seconds.

After curing the thermosetting polymer could not be reverted back to its initial state. It follows an Irreversible path. The thermosets experience creep when the plasticpolymers arebeing exposed to thermal, structural and various kind of loadings for a long time period. Even though they have very high rigidity the creep happens. Tensile creep and buckling creep the commonly occurred type of creep in thermosetting plastic polymers. The mechanism of creep in these polymers are temperature and time-dependent. It is alsostress dependent which is actedon the specimen.In thermoplastic Thermally Activated glide Creep mechanism is the type of Creep Mechanism that acts upon the polymers. All the Plastic polymers such asthermoplastics&thermosetexhibit creep, but the thermoset polymer material are strongerthan that of the thermoplastics because of their polymer structure.

The structure of the plastic helps in the prediction of the creep amount that the specimen is prone to. The thermoplastic consists of a structure known as chain linked polymer structure and they exhibit low opposition to creep when compared to the thermoset plastic polymer.The thermoset plastic consists of a Cross-linked polymer structure.So that from the structure we can predict the creep rate of a specimen (Froidevaux, V et.al 2015).

Thermoplastic and Thermoset plastic Structure

The equation is used to study the creep is the viscoelastic equation:

The injection timing relationship equation is given as:

The common terms between the viscoelastic equation and the injection timing equation are temperature and time. The viscoelastic equation is more concentrated on the geometry of the pipe. The injection timing equation is related to the temperature, heat transfer and time.

The connecting rod is an important part of an IC Engine- Internal Combustion engine. It is one the most major parts which make up the engine. The Connecting rodtranslates the linear motion of the piston into oscillating motion. The Connecting rod acts as a connecting link between the crankshaft and the piston. The connecting rod geometry consists of two mounting holes one for piston another for the crankshaft. The linear motion of the piston due to the expansion stroke is translated into rotary motion of crankshaft via oscillating motion of the connecting rod.

 Constraints a) L = Fixed b) Buckling should be considered c) The material should be strong and light
1. Constraints:
2. The objectives are to make a connecting rod which can Withstand the Force load of 20KN, Lighter & stronger and resistive to fatigue & buckling.
3. The variable which is not bound to any constraints is known as the free variables. The free variables provide us with the free will to choose them. Such as the material choice, width, length and thickness, etc.
4. The mass equationfor the connecting rod:

Mass, m = (Ma)+ (Mb)+(Mc)

Ma Rectangle

Mb Circle 1

Mc Circle 2

] ρ    in kilograms

Here,

ρ-  Material density

Da& Db – Diameter of the upper and lower circles.

b – the breadth of the connecting rod

W- width of the connecting rod

t- the thickness of the connecting rod

1. The Fatigue material indices is a mathematical expression of fatigue load factor governing the system it can be found at:

M = E/ρ

E = FL2 /

M = FL2ρ /

1. The Connecting rod Mass is calculated in subdivision e as:

) ρ    in kg

W2 t +  ]X ρ

1. The Material index for buckling is given as:

Material index,

Buckling force,

1. Applying of two performance indices gives an enormous flexibility and control over the product design. The equations formed are more strong and accurate. The outcome of the design will be more clear and accurate.

References:

Schmitt, J., Lu, P. and Jones, J., NITRIDE SOLUTIONS Inc, (2018). Bulk diffusion crystal growth of nitride crystal. U.S. Patent Application 15/860,463.

Vaidya, M., Pradeep, K.G., Murty, B.S., Wilde, G. and Divinski, S.V., (2018). Bulk tracer diffusion in CoCrFeNi and CoCrFeMnNi high entropy alloys. Acta Materialia, 146, pp.211-224.

Hirth, G. and Kohlstedt, D.L., (2015). The stress dependence of olivine creeps rate: Implications for extrapolation of lab data and interpretation of recrystallized grain size. Earth and Planetary Science Letters, 418, pp.20-26.

Sun, S.H., Koizumi, Y., Kurosu, S., Li, Y.P. and Chiba, A., (2015). Phase and grain size inhomogeneity and their influences on creep behaviour of Co–Cr–Mo alloy additive manufactured by electron beam melting. Acta Materialia, 86, pp.305-318.

Zhao, Q., Qi, H.J. and Xie, T., (2015). Recent progress in shape memory polymer: New behaviour, enabling materials, and mechanistic understanding. Progress in Polymer Science, 49, pp.79-120.

Youssef, K.M., Zaddach, A.J., Niu, C., Irving, D.L. and Koch, C.C., (2015). A novel low-density, high-hardness, high-entropy alloy with close-packed single-phase nanocrystalline structures. Materials Research Letters, 3(2), pp.95-99.

Froidevaux, V., Negrell, C., Laborbe, E., Auvergne, R. and Boutevin, B., (2015). Thermosetting material by a thermoresponsive cross-linking using retroDiels–Alder and, in situ, Thia-Michael reactions. European Polymer Journal, 69, pp.510-522.

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[Accessed 03 August 2024].

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