Materials Science and Engineering Properties Notes
Materials Science and Engineering Properties
Course Information
Course Code: FAEN:203
Lecturer: Bismark Mensah, PhD
Department: Materials Science & Engineering, University of Ghana (UG)
Email: bismrkmensh@ug.edu.gh
Introduction
Encouragement for students: "IT IS NOT TOO LATE, YOU CAN DO IT! START NOW!"
Emphasis on achieving a high GPA to facilitate future success.
Case Studies Highlighting Material Failures
Case 1: Melcom Disaster (November 2012)
Overview of the incident: Resulted in the loss of 9 lives.
Conclusion: Failure of materials can lead to disasters.
Case 2: Space Shuttle Disasters
1982 Incident:
Specific cause: Failure of two redundant O-ring seals in the Space Shuttle's right solid rocket booster (SRB).
Resulted in disintegration over the Atlantic Ocean.
1986 Challenger Disaster:
Causes considered: Failures associated with O-rings.
Reference: Activities of Late Prof. Allen Gent, including former students like Prof. Nah and Dr. Bismark Mensah.
Definitions of Material
Material:
Definition: Physical matter that occurs either naturally or is manufactured, characterized by specific physical properties and characteristics.
Types of Materials
Principal Material Families:
Metals:
Properties: Malable (can be bent), strong, conducts electricity, generally non-transparent.
Ceramics:
Properties: Brittle (cannot be bent), electric insulators, can be transparent.
Polymers:
Properties: Lighter, soft, do not retain new shapes when bent, used as electrical insulators, can be transparent.
Materials Selection/Design Principles (S-3Ps)
Factors to consider: Structure, Processes, Properties, Performance.
Properties of Materials
Structural and Bonding Considerations
Nucleus and Bonding:
Sensitive Properties at Electronic and Atomistic Levels:
Density, Modulus, Specific Heat, Expansion Coefficient, Saturation Magnetization.
Microstructure-sensitive Properties:
Strength, Toughness, Elongation, Electrical and Thermal Conductivities.
Bonds in Materials:
Metals: Characterized by metallic bonds (e.g., Iron (Fe)).
Ceramics: Comprised of ionic/covalent bonds (e.g., Silica (SiO2)).
Polymers: Contain covalent and secondary bonds (e.g., Polyethylene).
Causes of Material Failure
Factors Affecting Defects in Polymers
Defects include:
Branching, Crosslinks, Reinforcement-matrix interactions, and Crack Formation.
Molecular Weights (MW):
Variants: LDPE (Long Branched), LLDPE (Short Branched), and HDPE (Linear).
Defects in Metals
Types of Defects
Defects in Crystals:
Point Defects: Vacancies, Interstitial, Schottky, Frenkel.
Line Defects: Edge, Screw dislocation.
Surface Defects: Grain boundary defects, Twin boundary..
Volume Defects: Voids, inclusions, and precipitates.
Physico-Mechanical Properties of Materials Mechanical Strength
Tensile/Ultimate Strength ($ au_{ts}$):
Definition: The maximum stress at which a material separates when subjected to tension.
Formula: au_{ts} = rac{F}{A} where $F$ is force and $A$ is the cross-sectional area.
Elastic Moduli
Elastic (Young’s) Modulus:
Definition: The ratio of tensile stress to axial elastic strain.
Stress and Strain relation: Most materials have a linear relationship.
Toughness and Ductility
Toughness:
Defined via the stress-strain curve, encompassing strength and ductility measures.
Fracture Toughness ($K_{1c}$):
Resistance to crack propagation, measured with introduced crack length and tensile stress.
Formula: K_{1c} = Y au^* ( rac{ ext{length}}{2})^{1/2}, where $Y$ is a geometric factor.
Bending Strength
Flexural Strength or Modulus of Rupture:
Definition: Maximum surface stress in a bent beam at failure.
Bending Modulus ($E$):
Calculated as: au_b = rac{8FL}{ ext{π}d^3}, where $L$ is length, $F$ is force, and $d$ is diameter.
Fatigue Strength
Endurance Limit ($ au_e$):
Resulting from cyclic loading and microcracks formation. Fracture occurs after many cycles ($N_f > 10^7$).
Additional Mechanical Properties
Shear Strength: Stress tolerance under torsion before yielding or fracturing.
Compressive Strength: Reflects max compressive force that material can sustain.
Ductility: Indicates plastically deformable materials.
Toughness: Ability of material to deform plastically before fracturing.
Hardness: Resistance to surface indentation or deformation.
Creep Resistance: Resistance to deformation under prolonged loads at high temperatures.
Impact Strength: The capacity to absorb sudden dynamic impacts.
Coefficient of Friction and Wear Coefficient: Measure of friction and surface wear, respectively.
Electronic and Thermal Properties of Materials
Band Theory
Band Gap:
Definition: Minimum energy distance between the valence and conduction bands.
Elements and their respective band gaps (in eV):
Silicon (Si): 1.12
Silver: 2.51
Polyethylene (PE): 8.28
Graphene: 1–2
Electrical Conductivity
Mechanism for conductive polymers: Delocalization of electrons along the conjugated polymer backbone allows for free movement.
Thermal Properties
Specific Heat (S): Quantity of heat needed to change the temperature of a unit mass by one degree, measured in $J/kg·K$.
Heat Capacity (H): The heat required to change the temperature of an entire substance by one degree. Formula: H = m imes S
Thermal Expansion: Change in material dimensions (linear, area, volume) upon heating.
Thermal Conductivity: Ratio of heat flow through a material; higher in pure solid metals.
Melting Point: The transition temperature from solid to liquid; depends on bonding forces.
Thermal Diffusivity: Ratio of thermal conductivity to heat capacity influences heat distribution speed.
Thermal Shock Resistance: Ability of material to withstand abrupt temperature changes.
Dielectric Properties of Materials
Dielectric Constant
Formula: K = rac{ϵ}{ϵ0}; where $ϵ$ refers to permittivity of the substance and $ϵ0$ is permittivity of free space.
Factors Affecting Dielectric Constant:
Temperature: Increases dipole moment and dielectric constant.
Heating Effects: Can lead to energy dissipation (dielectric loss).
Humidity: Increased humidity decreases dielectric strength.