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ASPECTS OF MATERIALS FAILURE
SKMM 4133 Failure of Engineering Components and Structures MODULE 3 ASPECTS OF MATERIALS FAILURE
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SKMM 4133 Failure of Engineering Components and Structures
FAILURE OF MATERIALS Modes of failure Mechanical properties and behavior Failure (yield) theories Factor of safety Overview of fracture mechanics Stress states at crack tip Stress intensity factor Fracture toughness Ductile-to-brittle transition behavior
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Failure Versus Fracture
SKMM 4133 Failure of Engineering Components and Structures Failure Versus Fracture Failure Inability of a component to perform according to its intended function. Fracture Separation of a component into two or more parts.
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SKMM 4133 Failure of Engineering Components and Structures
Modes of Failure Gross Yielding Fatigue Fracture Creep Rupture Buckling Static Delayed Fracture
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Fracture of a Specimen in a Tension Test
SKMM 4133 Failure of Engineering Components and Structures Fracture of a Specimen in a Tension Test Load cell Extensometer Crosshead Specimen grips Specimen Data acquisition system
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Mechanical Properties and Behavior
SKMM 4133 Failure of Engineering Components and Structures Mechanical Properties and Behavior Properties Values y0.2% (MPa) 429 u (MPa) 604 E (GPa ) 208 n 0.0935 K ( MPa ) 682.65 r2 0.9857 Stress-strain curves for Type 316 SS
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Mechanical Properties of Some Materials
SKMM 4133 Failure of Engineering Components and Structures Mechanical Properties of Some Materials MATERIALS E GPa (106psi) So MPa(ksi) SU AISI 1040 steel 207 (30) 413 (60) 620 (90) Stainless Steel 314 193 (28) 205 515 (75) 7075 Al alloy 72 (10.5) 105 (15) 230 (33) Gray cast iron - 152 (22)
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SKMM 4133 Failure of Engineering Components and Structures
Theory of Failure Yield Criterion The material of a component subjected to complex loading will start yielding when the (parametric stress) reaches the (characteristic stress) in an identical material during a tensile test. f() = Other parameters: Strain Energy Specific stress component (shear stress, maximum principal stress)
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SKMM 4133 Failure of Engineering Components and Structures
Theory of Failure Maximum-distortion-energy theory
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SKMM 4133 Failure of Engineering Components and Structures
Theory of Failure Maximum-normal-stress theory
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SKMM 4133 Failure of Engineering Components and Structures
Factor of Safety Ffail is determined from experimental testing of the material Fallow is the allowable or working load F.S. ≥ to avoid failure
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Fracture Mechanics - Overview
SKMM 4133 Failure of Engineering Components and Structures Fracture Mechanics - Overview WWII Liberty Ships Photo by Neil Boenzi, The New York Times.
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SKMM 4133 Failure of Engineering Components and Structures
Mechanics of Materials A branch of mechanics that studies the relationships between external loads applied to a deformable body and the intensity of internal forces acting within the body. Fracture Mechanics The mechanics that describes the response of materials to loading in the presence of crack or crack-like defects.
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Scope of Fracture Mechanics
SKMM 4133 Failure of Engineering Components and Structures Scope of Fracture Mechanics
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Linear Elastic Fracture Mechanics (LEFM)
SKMM 4133 Failure of Engineering Components and Structures Linear Elastic Fracture Mechanics (LEFM) Fracture mechanics within the confines of the theory of linear elasticity. Analytical procedure that relates the stress magnitude and distribution in the neighborhood of a crack to: the nominal applied stress crack geometry (size, shape) and orientation material properties An underlying principle is that unstable fracture occurs when the stress-intensity factor at the crack tip reaches a critical value.
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Basic Loading of Cracked Bodies
SKMM 4133 Failure of Engineering Components and Structures Basic Loading of Cracked Bodies
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Stress Field Ahead of Crack Tip
SKMM 4133 Failure of Engineering Components and Structures Stress Field Ahead of Crack Tip Complex state of stress exists in the vicinity of a crack tip
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SKMM 4133 Failure of Engineering Components and Structures
Crack-tip Stress Stress magnitude at the crack tip approaches (mathematically) an infinite value
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SKMM 4133 Failure of Engineering Components and Structures
Crack-tip Plasticity There is always a plastic zone at the crack tip
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Stress Intensity Factor
SKMM 4133 Failure of Engineering Components and Structures Stress Intensity Factor 2a Crack τxy σyy σxy θ r σ The stress intensity factor, KI describes the crack tip stresses. (or Y) – geometry factor
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SIF – Finite Width Correction
SKMM 4133 Failure of Engineering Components and Structures SIF – Finite Width Correction For 2a<<W,
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Values of KI for different loading conditions and geometries
SKMM 4133 Failure of Engineering Components and Structures Values of KI for different loading conditions and geometries Adapted from Fig. 8.8, Callister 6e.
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Stress Intensity Factor
SKMM 4133 Failure of Engineering Components and Structures Stress Intensity Factor
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Stress Intensity Factor
SKMM 4133 Failure of Engineering Components and Structures Stress Intensity Factor
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Fracture Toughness of Some Materials
SKMM 4133 Failure of Engineering Components and Structures Fracture Toughness of Some Materials Fracture toughness represents the resistance of materials to resist cracking. Fracture toughness values are determined from fracture toughness tests. Based on data in Table B5, Callister 6e.
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Fracture Toughness of Some Materials
SKMM 4133 Failure of Engineering Components and Structures Fracture Toughness of Some Materials MATERIALS KIC MPa√m Aluminum 2024 7075 7178 26 24 35 Steel BS 816M40 BS816 M40 BS 535 A99 99 60 14 Titanium IMI 318 115 55 SY (MPa) 455 495 490 860 1515 2070 910 1035
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Condition for Fracture
SKMM 4133 Failure of Engineering Components and Structures Condition for Fracture Fracture occurs when the applied stress intensity factor, KI reaches the value of the fracture toughness, KIC of the material
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Impact Test SKMM 4133 Failure of Engineering Components and Structures
(Charpy)
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Impact Energies of Some Materials
SKMM 4133 Failure of Engineering Components and Structures Impact Energies of Some Materials MATERIALS Impact energy J (ft-lb) AISI 1040 steel 48.8 (36) Gray cast iron (class 20) 20 (15) Ti-6Al-4V 22 (16)
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Fracture Surfaces of Impact Test Specimens
SKMM 4133 Failure of Engineering Components and Structures Fracture Surfaces of Impact Test Specimens
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Ductile-to-Brittle Tansition (DBT) Temperature, TDBT
SKMM 4133 Failure of Engineering Components and Structures Ductile-to-Brittle Tansition (DBT) Temperature, TDBT Transition Temperature (ASTM specification) The temperature at which specimens show a fracture of 50 pct. shear and 50 pct. cleavage. TDBT Energy absorbed BCC Temperature FCC Brittle ductile Nil-ductility temperature (NDT) The ref. point in the transition range giving the limiting condition of temperature-stress combination under which catastrophic fracture can occur.
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