Material Science Failures and Crystal Structure Concepts

Material Failure Mechanisms and Concepts

Creep Phenomenon

Creep is the slow and permanent deformation of a material when it is subjected to a constant load or stress for a long period of time at high temperature. Creep becomes significant when the temperature is above 0.4 times the melting temperature of the material (in Kelvin).

Stages of Creep:

Creep occurs in three stages, shown by a creep curve:

1. Primary Creep (Transient Stage)
   • Creep rate decreases with time.
   • Material becomes strain-hardened.
2. Secondary Creep (Steady-State Stage)
   • Creep rate becomes constant.
   • This is the longest and most important stage.
3. Tertiary Creep
   • Rapid increase in creep rate.
   • Formation of cracks and necking.
   • Ends with fracture.

Factors Affecting Creep:

• Temperature
• Applied stress
• Time of exposure
• Material structure and grain size

Creep is important in components working at high temperatures such as:

• Turbine blades
• Boilers
• Jet engines
• Nuclear reactors
• Steam pipes

Prevention / Control of Creep:

• Use creep-resistant alloys.
• Increase grain size.
• Reduce operating temperature.
• Apply protective coatings.

Stress Corrosion Cracking (SCC)

Stress Corrosion Cracking (SCC) is a sudden brittle failure of a material caused by the combined action of tensile stress and a corrosive environment, even when the applied stress is below the yield strength of the material.

Essential Conditions for SCC:

For stress corrosion cracking to occur, all three conditions must be present:

1. Tensile stress (applied or residual).
2. Corrosive environment.
3. Susceptible material.

If any one condition is absent, SCC will not occur.

Characteristics of SCC:

• Cracks are fine and brittle.
• Failure occurs suddenly.
• Occurs at specific environments.
• Difficult to detect by visual inspection.

Examples:

• Stainless steel → cracks in chloride solutions.
• Brass → cracking in ammonia (season cracking).
• Aluminium alloys → cracking in moist or salty environments.

Prevention of Stress Corrosion Cracking:

1. Reduce or remove tensile stresses (stress relieving).
2. Use corrosion-resistant materials.
3. Control the environment.
4. Apply protective coatings.
5. Use inhibitors.

Applications / Importance:

SCC is important in:

• Pipelines
• Boilers
• Nuclear reactors
• Aircraft components
• Chemical processing equipment

Miller Indices for Crystal Planes

Miller indices are a set of three integers (h k l) used to represent the orientation of crystal planes in a crystal lattice. They are obtained from the reciprocals of the intercepts made by the plane on the crystallographic axes and then reduced to the smallest whole numbers. Miller indices help in identifying planes, studying crystal structure, and understanding material properties such as slip, fracture, and atomic packing.

Steps to Find Miller Indices of a Plane:

1. Find the intercepts of the plane on x-, y- and z-axes in terms of lattice constants a, b, c.
2. Take the reciprocals of these intercepts.
3. Clear the fractions by multiplying with the LCM.
4. Reduce to the smallest integers to obtain (h k l).

Important Points about Miller Indices:

• If a plane is parallel to an axis, its intercept is infinite, and the Miller index is zero.
• Negative intercepts are shown by a bar over the index (e.g. $\bar{1}$).
• Miller indices are always integers, not fractions.
• Planes with similar orientation belong to a family of planes, written as {hkl}.

Examples:

• Plane cutting axes at a, infinity, infinity → Miller indices (100).
• Plane cutting axes at a, a, a → Miller indices (111).
• Plane cutting axes at a, 2a, infinity → Miller indices (1 $\frac{1}{2}$ 0).

Importance of Miller Indices:

• Used to describe crystal planes.
• Helps in X-ray diffraction analysis.
• Important in slip systems and deformation.
• Useful in material identification.

Criteria For Selection Of Material

Material selection is the process of choosing a suitable material for a component or product so that it performs safely, economically, and efficiently under given working conditions.

Important Criteria for Selection of Materials:

1. Mechanical Properties

   The material should have required:

   • Strength
   • Hardness
   • Toughness
   • Ductility

   These properties ensure the material can withstand loads and stresses during service.

2. Physical Properties

   Includes:

   • Density
   • Melting point
   • Thermal and electrical conductivity

   These properties are important where weight, heat transfer, or electrical use is involved.

3. Chemical Properties
   • Corrosion resistance
   • Oxidation resistance
   • Chemical stability

   Materials used in chemical plants, marine, or humid environments must resist chemical attack.

4. Manufacturing Properties
   • Castability
   • Machinability
   • Weldability
   • Formability

   A material should be easy to manufacture into the required shape without defects.

5. Service Conditions

   Selection depends on:

   • Working temperature
   • Pressure
   • Wear and friction
   • Environment

   The material must perform reliably under actual operating conditions.

6. Cost and Availability
   • Initial material cost
   • Processing cost
   • Maintenance cost
   • Availability in market

   The selected material should be economical and easily available.

7. Aesthetic and Special Requirements
   • Appearance
   • Surface finish
   • Special properties (magnetic, electrical, biocompatible)

Classification of Engineering Materials

Main Classification of Engineering Materials:

1. Metallic Materials

   These materials are good conductors of heat and electricity and have high strength and toughness.

   • Types: Ferrous metals – contain iron (Examples: Steel, Cast iron); Non-ferrous metals – do not contain iron (Examples: Aluminium, Copper, Nickel).
   • Uses: Machine parts, automobiles, structures, tools.
2. Ceramic Materials

   Ceramics are hard, brittle, and resistant to high temperature and corrosion.

   • Examples: Alumina, Silicon carbide, Glass, Refractories.
   • Uses: Furnace linings, insulators, tiles, cutting tools.
3. Polymer Materials

   Polymers are lightweight, corrosion-resistant, and easy to mould.

   • Types: Thermoplastics (PVC, Nylon); Thermosetting plastics (Bakelite, Epoxy); Elastomers (Rubber).
   • Uses: Pipes, insulation, packaging, household items.
4. Composite Materials

   Composites are formed by combining two or more materials to improve properties.

   • Examples: Fibre-reinforced plastic (FRP), Concrete, Carbon fibre composites.
   • Uses: Aircraft, automobiles, bridges, sports equipment.
5. Advanced / Smart Materials

   These materials show special behaviour under external conditions.

   • Examples: Nanomaterials, Shape memory alloys, Biomaterials.
   • Uses: Medical devices, sensors, aerospace applications.

Engineering Requirements of Materials

Engineering requirements of materials are the essential properties and characteristics that a material must have to perform satisfactorily under service and working conditions.

Important Engineering Requirements:

• Mechanical Requirements
  • High strength
  • Adequate hardness
  • Good toughness
  • Required ductility

  These ensure the material can withstand loads and stresses.

• Physical Requirements
  • Suitable density (light or heavy as required)
  • Proper melting point
  • Good thermal and electrical conductivity
• Chemical Requirements
  • Resistance to corrosion
  • Oxidation resistance
  • Chemical stability

  Important for chemical, marine, and outdoor applications.

• Thermal Requirements
  • Resistance to high temperature
  • Low thermal expansion
  • Good thermal shock resistance
• Manufacturing Requirements
  • Good castability
  • Easy machinability
  • Good weldability
  • Formability

  Ensures ease of production.

• Economic Requirements
  • Low cost
  • Easy availability
  • Low maintenance cost

Fibre Reinforced Plastic (FRP)

Fiber Reinforced Plastic (FRP) is a composite material made by combining strong fibres embedded in a plastic (polymer) matrix. The fibres provide strength and stiffness, while the plastic matrix binds the fibres together and protects them from damage.

Constituents of FRP:

FRP mainly consists of two parts:

1. Fibres
   • Glass fibres
   • Carbon fibres
   • Aramid fibres (Kevlar)

   These fibres carry the load and provide high strength.

2. Matrix (Resin)
   • Epoxy
   • Polyester
   • Phenolic resin

   The matrix holds fibres together and transfers stress between them.

Important Properties of FRP:

• High strength-to-weight ratio.
• Lightweight.
• Good corrosion resistance.
• Good fatigue resistance.
• Electrical and thermal insulation.

Advantages of FRP:

• High strength with low weight.
• Resistant to corrosion and chemicals.
• Long service life.
• Easy to mould into complex shapes.

Applications of FRP:

• Aircraft and aerospace components.
• Automobile body parts.
• Boat hulls and marine structures.
• Pipes, tanks, and storage vessels.
• Sports equipment (helmets, rackets).

Fatigue and Fatigue Limit

Fatigue:

Fatigue is the failure of a material due to repeated or fluctuating stresses, even when the maximum stress is below the yield strength of the material. Fatigue failure occurs suddenly after a large number of stress cycles and usually shows little or no plastic deformation. Fatigue is common in components subjected to cyclic loading such as shafts, gears, springs, aircraft wings, and bridges.

Stages of Fatigue Failure:

Fatigue failure occurs in three stages:

1. Crack initiation – small cracks start at stress concentration points.
2. Crack propagation – cracks grow gradually with each load cycle.
3. Final fracture – sudden failure of the component.

Fatigue Limit (Endurance Limit):

The fatigue limit is the maximum stress level below which a material can withstand an infinite number of stress cycles without failure.

• Materials like steel and iron show a clear fatigue limit.
• Materials like aluminium and copper do not have a definite fatigue limit.

S–N Curve:

The fatigue behaviour of materials is represented by an S–N curve, which is a plot of Stress (S) versus Number of cycles to failure (N). The curve helps determine the fatigue life and fatigue limit of materials.

Factors Affecting Fatigue:

• Surface finish.
• Stress concentration.
• Type of loading.
• Material composition.
• Operating environment.

Caustic Embrittlement

Caustic embrittlement is a type of localized corrosion in which boiler steel becomes brittle and develops cracks due to the action of concentrated caustic alkalis, mainly sodium hydroxide (NaOH), at regions under high tensile stress. It is also called intergranular corrosion, because the attack mainly occurs along the grain boundaries of the metal.

Where and Why it Occurs:

Caustic embrittlement mainly occurs in steam boilers, especially at:

• Riveted joints.
• Bends.
• Crevices.
• Welded regions.

These areas experience high stress, which makes them more susceptible to cracking.

Causes of Caustic Embrittlement:

Boiler water often contains sodium carbonate ($\text{Na}_2\text{CO}_3$) used for softening. At high temperature and pressure, sodium carbonate decomposes as: $\text{Na}_2\text{CO}_3 + \text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{CO}_2$. The sodium hydroxide (NaOH) formed enters small cracks and crevices by capillary action. As water evaporates, NaOH becomes highly concentrated and attacks the steel.

Mechanism:

1. Formation of NaOH in boiler water.
2. Entry of NaOH into stressed regions.
3. Concentration of NaOH in crevices.
4. Attack along grain boundaries.
5. Loss of ductility and strength.
6. Development of brittle cracks.

Characteristics:

• Occurs only in alkaline conditions.
• Localized, not uniform corrosion.
• Cracks are intergranular.
• Failure may be sudden and dangerous.
• Difficult to detect at early stage.

Prevention of Caustic Embrittlement:

1. Use phosphate conditioning instead of carbonate.
2. Add tannins, lignin, or sodium sulphate.
3. Maintain proper alkalinity control.
4. Design boilers to reduce stress concentration.
5. Regular inspection and water treatment.

Importance / Effect:

If not prevented, caustic embrittlement can lead to:

• Boiler leakage.
• Sudden failure.
• Accidents and economic loss.

Hydrogen Embrittlement

Hydrogen embrittlement is a phenomenon in which a metal, especially steel, becomes brittle and weak due to the absorption of hydrogen. As a result, the material may crack or fail suddenly under stress, even at stresses below its normal strength.

How Hydrogen Enters the Metal:

Hydrogen can enter the metal during:

• Acid pickling.
• Electroplating.
• Welding.
• Corrosion reactions.
• Exposure to hydrogen-rich environments.

Mechanism (Explanation):

1. Hydrogen atoms penetrate into the metal surface.
2. Hydrogen diffuses into the crystal lattice.
3. It accumulates at grain boundaries, dislocations, or voids.
4. Internal pressure builds up.
5. Metal loses ductility and becomes brittle.
6. Cracks develop and lead to sudden fracture.

Characteristics of Hydrogen Embrittlement:

• Failure is brittle in nature.
• Occurs without warning.
• More severe in high-strength steels.
• Cracks usually start at stress-concentrated regions.
• Difficult to detect before failure.

Materials Commonly Affected:

• High-strength steels.
• Nickel alloys.
• Titanium alloys.

Prevention of Hydrogen Embrittlement:

1. Use low-hydrogen welding electrodes.
2. Heat treatment (baking) to remove absorbed hydrogen.
3. Avoid excessive acid cleaning.
4. Apply protective coatings.
5. Proper material selection.

Applications / Importance:

Hydrogen embrittlement is critical in:

• Pressure vessels.
• Pipelines.
• Aircraft components.
• Fasteners and bolts.

Surface Imperfection

Surface imperfections are defects or irregularities present on the outer surface of a material. These defects are usually formed during manufacturing processes such as casting, rolling, forging, machining, welding, or heat treatment. Surface imperfections reduce the strength, fatigue life, appearance, and service life of components.

Causes of Surface Imperfections:

Surface imperfections occur due to:

• Improper manufacturing conditions.
• Poor mould or die design.
• Excessive stress or deformation.
• Improper cooling or heat treatment.
• Mechanical damage during handling.

Common Types of Surface Imperfections:

1. Cracks
   • Fine or visible breaks on the surface.
   • Caused by thermal stress, fatigue, or improper cooling.
2. Scratches and Grooves
   • Produced during machining or handling.
   • Act as stress concentration points.
3. Laps and Seams
   • Formed during rolling or forging.
   • Overlapping metal folds on the surface.
4. Blow holes and Pits
   • Small cavities on the surface.
   • Caused by trapped gases during casting.
5. Scale and Oxide Layers

   Formed due to oxidation at high temperature.

Effects of Surface Imperfections:

• Reduce fatigue strength.
• Initiate crack formation.
• Lower corrosion resistance.
• Affect dimensional accuracy and appearance.

Detection of Surface Imperfections:

Surface imperfections are commonly detected by:

• Visual inspection.
• Dye penetrant testing.
• Magnetic particle testing.

Prevention:

• Proper manufacturing control.
• Smooth surface finishing.
• Correct heat treatment.
• Careful handling and storage.

Types and Applications of Glass

Glass is a hard, brittle, transparent, and amorphous material made mainly from silica ($\text{SiO}_2$) along with other oxides. It is widely used in engineering, construction, optics, and daily life because of its useful physical and chemical properties.

Types of Glass:

1. Soda–Lime Glass
   • Composition: Silica + Soda + Lime.
   • Properties: Cheap, transparent, easily moulded.
   • Applications: Window panes, bottles, glassware.
2. Borosilicate Glass
   • Contains boron oxide.
   • Properties: High thermal shock resistance, low expansion.
   • Applications: Laboratory glassware, kitchenware (ovenware), chemical apparatus.
3. Lead Glass (Flint Glass)
   • Contains lead oxide.
   • Properties: High refractive index, brilliant appearance.
   • Applications: Optical lenses, decorative items, radiation shielding.
4. Safety Glass
   • Types: Toughened glass and laminated glass.
   • Properties: Does not break into sharp pieces.
   • Applications: Automobile windshields, doors, windows, buildings.
5. Optical Glass
   • High purity glass.
   • Properties: Precise refractive index, clarity.
   • Applications: Cameras, microscopes, telescopes, spectacles.
6. Fibre Glass
   • Glass in the form of fine fibres.
   • Properties: Lightweight, strong, corrosion resistant.
   • Applications: Insulation, FRP products, boat hulls, roofing sheets.

General Applications of Glass:

• Construction (windows, doors, facades).
• Electrical insulation.
• Medical and laboratory equipment.
• Optical instruments.
• Domestic and decorative items.