Material Engineering - Lecture - Failure Mechanism, Lecture notes of Material Engineering

Download Material Engineering - Lecture - Failure Mechanism and more Lecture notes Material Engineering in PDF only on Docsity!

Mechanical

Failure

Overview

• (^) Failure Modes
  • (^) Fracture, Fatigue, Creep
• (^) Fracture Modes
  • (^) Ductile, Brittle, Intergranular,

Transgranular

• (^) Fracture Toughness
• (^) Stress Concentrators (Flaws)
• (^) Crack Propagation

Fracture Mechanism

Imposed stress Crack Formation Propagation

• (^) Ductile failure has extensive plastic deformation in

the vicinity of the advancing crack. The process

proceeds relatively slow (stable). The crack

resists any further extension unless there is an

increase in the applied stress.

• (^) In brittle failure, cracks may spread very rapidly,

with little deformation. These cracks are more

unstable and crack propagation will continue

without an increase in the applied stress.

Crack Propagation

Cracks propagate due to sharpness of crack tip

• (^) A plastic material deforms at the tip, “blunting” the crack. deformed region brittle Energy balance on the crack
• (^) Elastic strain energy-
  • (^) energy stored in material as it is elastically deformed
  • (^) this energy is released when the crack propagates
  • (^) creation of new surfaces requires energy plastic

• Evolution to failure:

Moderately Ductile Failure

necking void nucleation

• Resulting fracture surfaces (steel) 50 mm particles serve as void nucleation sites. 50 mm From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton, J. Mater. Sci ., Vol. 6, 1971, pp. 347-56.) 100 mm Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission. Crack fracture propagation Coalescence of cavities

• Ductile failure: -- one piece -- large deformation Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission.

Example: Pipe Failures

• Brittle failure: -- many pieces -- small deformations

(a) SEM image showing spherical dimples resulting from a uniaxial tensile load. (b) SEM image of parabolic dimples from shear loading.

Ductile Failure

1111

Brittle Fracture

Arrows indicate point at failure origination

Distinctive pattern on the fracture surface: V-

shaped “chevron” markings point to the failure

origin.

Intergranular Fracture

• (^) Intergranular failure is typically due to elemental

depletion (chromium) at the grain boundaries or

some type of weakening of the grain boundary due

to chemical attack, oxidation, embrittlement.

Fracture Mechanics

Studies the relationships between:

material properties

stress level

crack producing flaws

crack propagation mechanisms

Fracture Toughness

• (^) Fracture toughness measures a material’s resistance to brittle fracture when a crack is present.
• (^) It is an indication of the amount of stress required to propagate a preexisting flaw.
• (^) Flaws may appear as cracks, voids, metallurgical inclusions, weld defects, design discontinuities, or some combination thereof.
• (^) It is common practice to assume that flaws are present and use the linear elastic fracture mechanics (LEFM) approach to design critical components.
• (^) This approach uses the flaw size and features, component geometry, loading conditions and the fracture toughness to evaluate the ability of a component containing a flaw to resist fracture.

Ductile vs Brittle

• (^) The effect of a stress raiser is more significant in

brittle than in ductile materials.

• (^) For a ductile material, plastic deformation results

when the maximum stress exceeds the yield

strength.

• (^) This leads to a more uniform distribution of

stress in the vicinity of the stress raiser; the

maximum stress concentration factor will be less

than the theoretical value.

• (^) In brittle materials, there is no redistribution or

yielding.

stress-intensity factor (K)

• (^) The stress-intensity factor (K) is used to determine the fracture toughness of most materials.
• (^) A Roman numeral subscript indicates the mode of fracture and the three modes of fracture are illustrated in the image to the right.
• (^) Mode I fracture is the condition where the crack plane is normal to the direction of largest tensile loading. This is the most commonly encountered mode.
• (^) The stress intensity factor is a function of loading, crack size, and structural geometry. The stress intensity factor may be represented by the following equation: KI is the fracture toughness in σ is the applied stress in MPa or psi a is the crack length in meters or inches is a crack length and component geometry factor that is different for each specimen, dimensionless.

Critical Stress

• (^) All brittle materials contain a population of small

cracks and flaws that have a variety of sizes,

geometries and orientations.

• (^) When the magnitude of a tensile stress at the tip of

one of these flaws exceeds the value of this critical

stress, a crack forms and then propagates, leading

to failure.

• (^) Condition for crack propagation: Fracture toughness good diagrams http://www.ndted.org/EducationResources/CommunityCollege/Materials/Mechanical/FractureToughness.htm K ≥ Kc Stress Intensity Factor: --Depends on load & geometry. Fracture Toughness: --Depends on the material, temperature, environment & rate of loading.