Engineering Material Technology: Failure and Prevention

Failure of Engineering Materials

• Failure of engineering materials is an undesirable event due to human lives being put in jeopardy, economic losses, and interference with product availability.
• Causes of failure include improper materials selection and processing, inadequate design, and misuse of components.
• It is the engineer's responsibility to anticipate and plan for possible failure, assess its cause, and take preventive measures.

Fracture

• Simple fracture is the separation of a body into two or more pieces in response to an imposed static stress at low temperatures relative to the material's melting temperature.
• Two fracture modes are possible: ductile and brittle, classified based on the material's ability to experience plastic deformation.

Principles of Fracture Mechanics

• This subject allows quantification of relationships between material properties, stress level, crack-producing flaws, and crack propagation mechanisms.
• Design engineers can anticipate and prevent structural failures using fracture mechanics.

Stress Concentration

• Measured fracture strengths for brittle materials are lower than predicted by theoretical calculations due to microscopic flaws or cracks at the surface and interior of the material.
• Stress concentration occurs around internal cracks, with the magnitude of localized stress decreasing with distance away from the crack tip.

Fracture Toughness

• The critical stress required for crack propagation in a brittle material is described by the expression σc = Kc / √(πa), where Y is a dimensionless parameter.
• The Kc value for a thick-specimen situation is known as the plane strain fracture toughness.

Design Using Fracture Mechanics

• The design (or critical) stress must be less than the material's fracture toughness to prevent failure.
• The maximum allowable flaw size ac is determined by the design stress and fracture toughness.

Impact Fracture Testing

• Impact testing is a technique used to determine the fracture toughness of a material.

Fatigue

• Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses.
• Failure can occur at a stress level lower than the tensile or yield strength for a static load.
• Fatigue failure is brittle-like in nature, with little to no gross plastic deformation.

Cyclic Stresses

• Failure occurs by the initiation and propagation of cracks, with the fracture surface perpendicular to the applied tensile stress.

S-N Curve

• The S-N curve is a graph that shows the relationship between the number of stress cycles and the stress magnitude.

Factors that Affect Fatigue Life

• Mean stress and surface effects can affect fatigue life.
• Environmental effects, such as thermal fatigue and corrosion fatigue, can also contribute to fatigue failure.