Introduction to Material Failure

Introduction to Material Failure

• Material failure can be static (ductile, brittle) or dynamic (creep, stress relaxation, fatigue)
• Applied tensile stress amplifies at notch/incision tips, making it easier to tear materials
• Material failures have caused significant disasters throughout history, highlighting the importance of understanding them.

Case Studies of Material Failure

• 300-meter long oil tank fractured due to crack propagation originating from a small flaw, amplified by stress at sea
• Boeing 737-200 experienced explosive decompression. Metal fatigue, exacerbated by crevice corrosion from a humid and salty coastal environment
• Stress cycling of the fuselage from compression and decompression during short hops
• Proper maintenance would have detected and prevented this accident.

Material Failure Types

• Mechanical failures categorized into deformation-related (time-dependent, time-independent) and fracture (static, dynamic).
• Elastic deformation can be a type of failure, as it may fail to achieve desired goals. Examples include a fork bending on wood. Rubber band stretching beyond ability or plastic deformation.
• Time-dependent failures include creep (increasing deformation under constant stress) and stress relaxation (reducing load under constant deformation). These are sensitive to elevated temperatures.
• Static fracture is categorized into ductile and brittle.
• Dynamic failures include fatigue (repeated loading), fretting corrosion (repetitive mechanical contact). Example: hip joint implant, vibration-related scenarios.

Simple Fracture

• Simple fracture is the separation of a body into pieces. Due to imposed static stress/constant/minimal changing stress/slow varying with low temperature.
• Categorized into ductile (substantial plastic deformation, high energy absorption) and brittle (little or no plastic deformation, low energy absorption).
• Ductile materials have inclined fracture planes; brittle materials have flat fracture surfaces.
• Ductility is relative and depends on temperature, strain rate, stress state.
• Burst pipes: cast iron/brass (brittle) fracture into small pieces with little deformation, whereas ductile pipes deform significantly before fracturing.

Stages of Ductile Fracture

• Flaws are inevitable.
• Five stages from necking to fracture: initiation of microvoids/cavities - enlargement and coalescence - formation of crack - crack propagation to exterior - rupture via shear deformation.
• Shear deformation occurs at 45 degrees to the tensile loading axis(maximum shear stress) yielding a cup-and-cone fracture.
• Interior regions show irregular/fibrous plastic deformation.

Features of Brittle Fracture

• Brittle fracture happens with negligible visible deformation, rapid crack propagation, and flat fracture surfaces.
• Fracture surfaces have distinctive patterns like V-shaped markings (chevron pattern), radiating lines/ridges, which point back towards the origin of initiation of crack
• Patterns diminish with reducing grain size (increasing stiffness) and diminish in amorphous material due to the random microstructure.
• Crack propagation in crystalline materials aligns along specific crystallographic planes (cleavage) through grain (transgranular).
• Sometimes crack propagation is along grain boundaries (intergranular).

In-Class Paper Fracture Demonstration

• Demonstrates how notches/folded papers make it easier to tear paper
• Fracture patterns differ depending on type of treatment to paper.

In-class Quiz

• Revision of previous module/term.