Creep Mechanisms in Materials and their Control

ANSWER 1:
a) 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)
Fig1. Creep strain Vs Stress Vs time dependency
b) 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).
Fig2. Grain size effects on the creep rate
c) 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).
d) 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).
ANSWER 2:
a) Plain Carbon Steel TTT diagramfor the eutectoid composition can be drawn as:
Fig3. TTT diagram of plain carbon steel
b) 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.
c) 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).
d) 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).
e) 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.
Answer 3:
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
Mass m is given by,
m=btL ρ
Here,
b:breadth of spring

L :Lengthof spring
t :thickness of spring
ρ:density of the material.
From the given constrains:
The Equation for Stiffness S is given as:
S=F /δ
Here,
δ : deflection.
F : force
The δmax Maximum deflection equation is given as:
Maximum Displacement, δmax =1
6
σf L2
tE
Where,
E is modulus of elasticity
L -Length
t is thickness
σ f −Stress
Now,
The deflection equation is written as:
Maximum Displacement δ= 1
48
F L3
EI