FRACTURE FATIGUE AND CREEP

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FRACTURE, FATIGUE, CREEP

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FRACTURE, FATIGUE FAILURE AND CREEP:

FRACTURE, FATIGUE FAILURE AND CREEP By ANSARI ZAKIR SAJID Asstt. Professor Department of Mechanical Engg. School of Engg. And Technology Anjuman-I-Islam’s Kalsekar Technical Campus

INTRODUCTION:

INTRODUCTION Failure of material into two or more parts Occurs if metal is stressed beyond its Failure Strength. May also occur if the stress is below yield stress Basic types are Ductile fracture Brittle fracture .1. Ductile fracture: Fractured surface shows cup and cone type of failure Sufficient Plastic deformation before failure . Fig. 2.1 and 2.2 STEPS IN DUCTILE FAILURE: Necking : starts after yield stress 2. Small cavities formation: After necking small cavities or voids are formed either at discontinuities or external particles Less such cavities more is the ductility

INTRODUCTION:

INTRODUCTION 3. Formation of cracks: theses cavities are then linked to form an external crack Spreads in a direction at right angles to the applied load 4. Cup and cone fracture: finally results in cup and cone fracture 2. BRITTLE FRACTURE: Results without any prior indication No change in dia of material No plastic region in the material

FACTORS AFFECTING FRACTURE:

FACTORS AFFECTING FRACTURE STRESS CONCENTRATION (Notch sensitivity): Localized increased in stress due to presence of sharp corners or sudden change in cross sections is called as stress concentration. Theses sharp edges or cross sections are called as stress raisers Stress concentration factor: ratio of stress at notched section to the nominal stress More such sharp corners more will be possibilities of failure For ductile material it is more dominant as the notch is sharp and notch radius is smaller. Can be reduced by proper design like use of fillets, gradual reduction of cross section etc (fig. 3.56 pp134) 2. SPEED WITH WHICH LOAD IS APPLIED More the speed of application of load more will be chances of brittle fracture. 3. THERMAL SHOCK: In presence of thermal shock fracture may appear Eg cracking of glass by pouring hot water

DUCTILE TO BRITTLE TRANSITION IN STEEL:

DUCTILE TO BRITTLE TRANSITION IN STEEL Many materials which are ductile at elevated temperature behaves as brittle at lower temperature This temperature range or temperature where the mode of failure changes is called as ductile to brittle transition temperature As temp decreases dislocation movement becomes difficult and so the plastic deformation At transition temperature σ f = σ y Temperature below transition, σ f < σ y Temperature above transition, σ f > σ y Where σ f =stress to propagate a crack σ y = yield stress Fig. 2.6

FACTORS AFFECTING TRANSITION TEMPERATURE:

FACTORS AFFECTING TRANSITION TEMPERATURE Transition temp increases – When grain size of material increases When alloying elements are added in the material When impurities in metal increases When % of carbon in steel increases Fig. 2.7

GRIFFITH’S THEORY OF BRITTLE FRACTURE:

GRIFFITH’S THEORY OF BRITTLE FRACTURE According to griffith there are microcracks in the bulk of material which causes local concentration of stress to a value high enough to propagate and crack and eventually to fracture Crack will propagate if the decrease in elastic strain energy is at least equal to the crack required to create the new crack surface ASSUMPTIONS: Crack is elliptical shaped with length equal to 2C Thickness of material is unity Crack runs from front to back face Crack tends to increase its length in transverse direction Fig. 2.8 and 2.9

FRACTURE TOUGHNESS:

FRACTURE TOUGHNESS The fracture resistance of a material in presence of crack is called as fracture toughness It is important as most of the engineering materials have some impurities as well as crack These comes during solidification of the metals One of the measures of fracture toughness is the elastic strain release rate G When the rate of release of elastic strain at the crack tip reaches a high value then crack propagates (Gc) Gc = 2( γ + P) γ = energy required to produce unit area of new surface P = energy for plastic deformation of surface of crack per unit area

FACTORS AFFECTING ON FRACTURE TOUGHNESS:

FACTORS AFFECTING ON FRACTURE TOUGHNESS Composition of material: Steel has more fracture toughness than Al Heat treatment: Increase in temperature reduces fracture toughness Service condition: like cyclic load, corrosive environment

FATIGUE FAILURE:

FATIGUE FAILURE Material fails at a stress level far below its fracture stress due to cyclic loading This is called as fatigue failure 90% of failures of mechanical components are because of fatigue Can be recognised from the appearance of the fracture surface . Appears in smooth region as well as rough region Smooth region is because of the rubbing action before failure Fig. 2.11

STRESS CYCLES:

STRESS CYCLES Mean stress Range of stress Alternating stress

MECHANISM OF FATIGUE FAILURE:

MECHANISM OF FATIGUE FAILURE Three step mechanism of fatigue failure Crack nucleation: starts with nucleation of crack at some point in the bulk Crack growth: The developed crack grows in a direction perpendicular to the applied load. Fracture: the crack then grows with every cycle of stress, the increment is indicated as a minute ripple. These ripples are visible under microscope Burnished or smooth surface is due to rubbing action Fig. 2.13

THEORIES OF FATIGUE FAILURE:

THEORIES OF FATIGUE FAILURE OROWAN’S THEORY: Metal consists of external inclusions or areas of stress concentration. If the total plastic strain in this region exceeds the critical value then a crack is formed. 2. Wood’s theory: The back and forth fine slip movements of fatigue could build up notches or ridges at the surface. These notches acts as stress raisers and in this way starts fatigue cracks which leads to fracture. Fig 2.14 3. Cottrell and Hull theory: In this theory it is based on the interaction of edge dislocation on slip systems Fatigue crack starts from such interactions fig. 2.15 4. Mott theory: Similar but deals with interactions of screw dislocations

FATIGUE/ENDURANCE LIMIT AND SN CURVES:

FATIGUE/ENDURANCE LIMIT AND SN CURVES

INFLUENCE OF IMPORTANT FACTORS ON FATIGUE:

INFLUENCE OF IMPORTANT FACTORS ON FATIGUE Notch effect: presence of notch or sharp irregularities reduces fatigue life. Surface effect: fatigue is influenced by the surface effect. Surface roughness: fatigue strength decreases with increase in the surface roughness more smooth the surface more will be fatigue strength. Changes in surface properties: decabourization reduces fatigue strength. surface hardening heat treatment increases fatigue strength electroplating reduces fatigue strength Surface residual stress: surface residual compressive stresses are desirable. Corrosion fatigue: simultaneous effect of cyclic load and chemical attack is called as corrosion fatigue The rate of crack propagation increases

INFLUENCE OF IMPORTANT FACTORS ON FATIGUE:

INFLUENCE OF IMPORTANT FACTORS ON FATIGUE Thermal fatigue: fatigue caused because of temperature generated stresses is called as thermal fatigue. in this failure occurs by repeated cycles of thermal load. Prestressing: A process of loading an engineering component under controlled conditions to a cyclic stress to a fixed number of cycles prior to any possibility of fatigue failure. understressing is desirable and improves fatigue life overstressing is undesirable as it reduces fatigue life.

CREEP:

CREEP INTRODUCTION: Slow plastic deformation of metal under constant stresses and at constant temperature for prolonged period. Can occur at temperature below room temperature called low temperature creep The one occuring at elevated temp is called high temp creep Due to this effect material can fail at much lower stress Essential for design of I.C engines, boilers, ships, submarines When temp increases Mobility of atoms increases Also dislocations increases with increase in temp Slip systems change or additional slip systems are introduced

MEASUREMENT AND REPRESENTATION OF CREEP:

MEASUREMENT AND REPRESENTATION OF CREEP Most mechanical properties are time independent at room temperature Creep is a time dependent phenomena There should be some data representing creep Called as creep strength or rupture strength Defined as the stress at a given temperature that produces a steady state creep rate of fixed amount Also defined as the stress to cause strain of 1% at given temperature Rupture strength refers to the stress required to cause fracture of the specimen under creep in a specified time. Normally data represented as shown in fig. on log-log graph

CREEP TEST:

CREEP TEST

MECHANISM OF CREEP:

MECHANISM OF CREEP The following are the mechanisms of creep failure 1. Dislocation climb: In primary creep dislocations move fast but then gets piled up at various barriers At temperature more than 0.5 Tm dislocation climb occurs

CREEP TESTING:

CREEP TESTING

CREEP CURVE:

CREEP CURVE

CREEP RESISTANCE:

CREEP RESISTANCE Resistance to failure under creep Can be increased by restricting the motion of dislocations Can be done by adding alloying elements which do not diffuse rapidly will improve creep resistance Properties of creep resistant materials: High melting point Should be coarse grained structure Should be precipitation hardened Should have high oxidation resistance

CREEP FRACTURE:

CREEP FRACTURE

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