Engineering Materials 1

Question 1

What are the No. of material selection parameters? State e.g. of each.

Answer 1

Bulk Mechanical, Surface, Economical, Aesthetic, Manufacturing, Non-mechanical properties

Question 2

2 bonds primary/secondary , What are they describe their bondings? give properties of each. Give examples.

Answer 2

Primary bonds are Covalent and Ionic Bonds.
Covalent bonds occur when the electrons are shared between atoms usually between groups 4, 5 and 6.
They usually form liquids and gases.
Properties are: High Strength, High elastic Modulus, Brittle, high melting pt, low electrical and heat conductivity. Diamond, polymers, gases O2 N2
Ionic bonds occur when e transferred from one atom to the other. Creates electrstatic attraction between them. Occurs usually between group 1,2 and 7, 6, 5. The transfer of the electrons will result in the valence shells being complete. They are same properties are covalent. E.g. are Alumina, MgO

Question 3

What are ceramics? What bonds? deformation? Why? Mechanical properties?

Answer 3

Ceramics are inorganic and non-metallic materials that are commonly electrical and thermal insulators, brittle and composed of more than one element (e.g., two in Al2O3). Made from compounds of a metal and a non metal.
Ceramic bonds are mixed, ionic and covalent, with a proportion that depends on the particular ceramics.
The brittle fracture of ceramics limits applications. Flaws Lead to crack formation, and crack propagation (perpendicular to the applied stress) is usually transgranular, along cleavage planes. The flaws cannot be closely controlled in manufacturing; this leads to a large variability (scatter) in the fracture strength of ceramic materials.

Question 4

What is an intramolecular force ? And what are intermolecular forces? give examples. and why intermolecular forces occur. give examples of compounds with intermolecular forces.

Answer 4

Any force that holds together the atoms making up a molecule or compound. Contains all types of chemical bonds. They are stronger than Intermolecular forces which are present between atoms and molecules that are not bonded. e.g. are dipole-dipole, Vanderwaals forces, Ion-dipole forces. They occur because of non-uniform charge distribution within an atom.
Polymers for e.g. have secondary bonds between the chains of their atoms.

Question 5

What is an ideal solid?

Answer 5

Have zero Volume, Point mass. Perfectly elastic collisions. Have zero kinetic energy.

Question 6

Combine Young’s Modulus and the Forces between atoms and derive E/Young’s modulus (abit hard), given
the diagram

Answer 6

E = S0/r0

Question 7

Define elastic Modulus, Elastic deformation, poisson’s ratio.

Answer 7

Also known as young’s modulus is a merasure of the material to resist elastic deformation. Elastic deformation is instantaneous and fully reversible. When the load is removed, the material returns back to its original shape. Poisson’s ratio is a lateral strain (change in width) over transverse strain ( changes in length)

Question 8

Difference between Eng stress/strain and true stress/strain. What is shear stress? Define/give formula of Bulk Modulus, Shear modulus.

Answer 8

Engineering stress is F/original Area regardless of how the area of the material is changing under the load. True stress takes into account the change in Area as well. Engineering strain is change in lenght over original length. True strain is in increments of true stress. Shear stress is the component of stress coplanar to the cross section of the material. shear strain is the tan(y). Bulk Modulus(K) is the relative change in volume of a material due to unit stress or pressure. Shear Modulus(G) is the torsional stiffness. Shear stress/Shear strain.

Question 9

What are viscous materials? Viscoelasticity? Define compressibility.

Answer 9

Viscosity is a measure of a fluid’s resistance to flow. Large viscosity resists flow. Low viscosity flows easily. A property of materials that exhibits both viscous and elasticity under deformation. Compressibility is the change in volume of a shape per unit stress.

Question 10

What is yield point? how do we find it? What UTS? Why is it higher than stress at failure?

Answer 10

The point on the stress/strain graph where elastic behavior ends and plastic deformation starts. Where material starts to yield. Not easily defined so we take .1% strain offset - which is called proof stress. Ultimate Tensile stress is maximum stress on the engineering stress strain graph. Due to necking, the area right before failure decreases, which reduces stress.

Question 11

Describe differences in fracture between brittle and ductile materials. Are materials compressible in plastic flow?

Answer 11

Brittle fractures have very little plastic deformation whereas…. Not compressible. Tensile deformation due to change in shape only and not volume, so v = poisson’s ratio remains 0.5, prior to necking.

Question 12

How can engineers use this info? In what region of the stress/strain graph should we use materials.

Answer 12

Far below the yield point. Use safety factors depending on the application. Typically between 2 - 4.

Question 13

What is static load and Dynamic load? And what does each type do to the structure of the material?

Answer 13

Static load is when the load is constant. Dynamic has cycles of tension and compression which encourage crack propagation and also leads formation of cracksat areas of stress concentration. Static is less likely to form cracks but grow them over time.

Question 14

What is strain hardening. Give another name. Define ductility. Rs between strength and ductility?

Answer 14

Also known as cold working, In ductile materials loading past yield stress can increase yield strength in materials. also reduces ductility. Induced during manufacture. Ductility is the ability of the material to deform plastically before fracture. Strength and ductility are often inversely proportional.

Question 15

define toughness

Answer 15

Toughness is the ability of the material to deform plastically without fracture. Also defined as the area of stress-strain. It’s also the product of strength and ductility.

Question 16

Define cystralline, polycrystalline, and amorphous.

Answer 16

Crystalline, In a crystal, the atoms or molecules are arranged in a regular periodic manner. The degree of crystallinity has a big influence on mechanical properties and transparency, diffusion. polycrystalline structures have several different types of crystallites of varying size and orientation. Amorphous solid has no regular periodic manner in the ordering of molecules.

Question 17

What dictates mechanical properties in crystals? And what denotes the way of packing of atoms. ?

Answer 17

way of Packing of atoms. Thermodynamics, all crystals will tend to a state of minimum energy however, the structures can change with the addition of heat.

Question 18

What is unit cell and space lattice.

Answer 18

Unit cells in a crystal are the smallest repeating units. Space lattice describes the overall of the 3 dimensional structure of the repeating unit in a crystal.

Question 19

What are Miller indices. And what do they describe.

Answer 19

The reciprocals of the intercepts of x,y,z rounded to 1 they describe the planes within the unit cycle.

Question 20

What are the crystal structures? and how many atoms in each unit cell of each structure? what are the coordination numbers

Answer 20

BCC - Body centered Cubic 2 atoms/unit cell, Coord no. = 8 FCC, Face centered cubic 4 atoms/unit cell, Coord no. 12 HCP, hexagonal closest packed - 6 atoms. Coord No. 12

Question 21

How do you find density of atoms in the crystal. give equations of finding the length of the edge of each unit cell for each crystalline structure type.

Answer 21

No. of atoms/unit cell by Volume/unit cell. Volume/unit cell is a3. whereas a for BCC is 4r/√3. Fcc is 4r/√2, HCP is 2r

Question 22

What are the effects of imperfections and dislocations. Give some examples.

Answer 22

The experimental values of yield strength for e.g. is much higher than the practical values because imperfections act as a source of slip/deformation.
Grain boundaries are interference between two grains. The larger the grains, the lower these defects - so more ductile. They are 2D defects. Surface defects. Thermal effects due to elevated temperatures. missing or foreign particles in the lattice structures. MISALLIGNMENT of rows in the crystal structure also known as dislocations.

Question 23

Explain edge dislocations. What are screw dislocations?

Answer 23

They occur when there is an extra row of particles/atoms. They allow for the material to yield at lower than expected values. Dislocations also move parallel to the plane. Screw dislocations occur when the atoms are dislocated along the line of cut.

Question 24

Describe the process of ductile fracture

Difference between the deformation of ductile fracture and brittle fracture.

Answer 24

Necking, Microvoids occur, then microvoids coalescences which forms a crack, which then propagates to finally fracture. Note : all solids contain cracks, this increases localized stresses which causes fracture. Ductile materials deform through shear. Movement or gliding of dislocations. This takes place at the elastic/plastic stage if the stress/strain graph. Brittle fracture have little movement at the dislocations, and no evidence of necking. fracture surface is perpendicular to the stress.

Question 25

What is stress intensity factor? What does it depend on? the formula? What is fracture toughness and what is their relationship. An equation perhaps?

Answer 25

The stress intensity factor is used to predict the stress intensity at the end of the cracks caused by remote or residual stress. It depends on the geometrey of the sample, Size and location of crack, size and distribution of the load. Fracture toughness is a measure of the ability of the material to resist fracture fracture, also expresses a material’s resistance to brittle fracture when a crack is present. Fracture toughness is a specific value of intensity factor. The equation connecting them is the griffith equation which is

Question 26

Define Creep. and stress relaxation? and the relationship between Temperature at initiation of of creep and melting point. For both ceramics and metals.

Answer 26

Creep is a time-dependent deformation under constant load which generally occurs at an elevated temperature, known as thermal creep but it can also happen in room temperature, albeit slower. As a result of this, the material undergoes an increase in length. Creep is an increase in strain under constant stress. Stress relaxation is a decrease in stress under constant strain. Tcreep is at about 0.3-0.4 of melting point for metals and 0.4-0.5 of melting point for ceramics.

Question 27

The stages of creep ?

Answer 27

On a creep strain vs time graph the initiation step is called instantaneous creep, then secondary creep, then tertiary creep and finally rupture

Question 28

Describe the two mechanisms of Creep deformation

Answer 28

Diffusion creep and dislocation creep. Diffusion creep occurs via diffusion of atoms within a grain due to a difference in energy gradient which in this case is caused by the applied stress. For e.g. the applied stress creates regions of high stress in extremities of the grain along the loading. So the atoms tend to diffuse towards these directions. This results in the elongation of the grains and ultimately of the material. Diffusion distances are shorter in fine grains hence, they are more susceptible to creep.
Dislocation creep is a mechanism that involves the movement of dislocations within the material. This mechanism tends to dominate at higher stresses and lower temperatures.

Question 29

What is the arrhenius equation?

Answer 29

it shows the relationship between temperature and the reaction rates.

Question 30

What is fatigue failure and how does it occur? How does it differ to fast fracture?

Answer 30

Fatigue failure describes the failure of a material due to crack growth within a material. This can occur due to repeated applied loads or cyclic loading which leads to propagation of cracks which finally results to the whole system unable to be load bearing. It is progressive and localized damage that occurs due to cyclic loading. As a result the nominal value of the load may be much less than the UTS.

Question 31

What is fatigue strength, what is fatigue life. What is endurance or fatigue limit. What affects the behaviour of the S-N graph curve?

Answer 31

Fatigue strength is the number of cyclic stress range where failure will occur after n number of times. Fatigue life is the expected number of cycles bearable for a given stress value in the stress range. Fatigue limit is the max stress range that can be applied without failure. properties that affect the S-N curve are corrosion, material defects, residual stress, Temperature, stress magnitude.

Question 32

State Basquin’s law and Coffin-Manson law. What is considered high cycle and what is considered low cycle.

Answer 32

Basquin law states that for stresses below the yield point, and for High cycle, a linear relationship is seen for failure. Coffin-Manson law however, states that for low cycle fatigue Basquin’s law no longer holds. Above 10,000 cycles in a lifetime is considered as high cycle.

Question 33

What design considerations to implement to prevent fatigue and cracks from occurring.

Answer 33

Cold working, allowing, making finer grains. Avoid the presence of surface cracks/defects as almost all cracks start from the surface. Avoid machining marks, gas voids, during casting or welding. Avoid localized regions of stress. Avoiding sharp edges, local stress concentrations.

Question 34

Describe the fatigue mechanisms

Answer 34

First, crack initiation when cracks start to form at stress concentration regions. Crack propagation - due to cyclic loading the cracks are incrementally and slowly increased in size. which increases stress intensity factor and crack propagation rate. Final failure occurs when the crack size (a) is so large that the Nf > the fracture toughness.