Engineering materials and metallurgy - Failure mechanism and testing

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 Two prominent mechanisms of plastic deformation ◦ slip and twinning. Slip  Slip is the mechanism of deformation where one part of the crystal moves glides or slips over another part along certain planes called slip planes. eg Mg Cu Al Ni .  During slip each atom usually moves same integral number of atomic distances along the slip plane producing a step but the orientation of the crystal remains the same. Steps observable under microscope as straight lines are called slip lines.  Slip occurs most readily in specific directions slip directions on certain crystallographic planes.  Generally slip plane is the plane of greatest atomic density and the slip direction is the close packed direction within the slip plane. Dr.K.RaviKumar Dr.N.G.P.Institute of Technology

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The direction of slip planes is indicated in a piece of metal after deformation by slip bands on the surface of the metal. Factors influencing slip  external load and the corresponding value of shear stress produced by it.  The geometry of crystal structure.  The orientation of active slip planes with the direction of shearing stresses generated.

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Schematic presentation of different plastic deformation mechanism

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 It results when a portion of crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite symmetrical way.  The twinned region divides the crystal into two regions in such a way that the twinned portion of the crystal is a mirror image of the parent crystal. The plane of symmetry is called twinning plane.  Each atom in the twinned region moves by a homogeneous shear a distance proportional to its distance from the twin plane  FCC - Ag Au Cu.  BCC - α-Fe Ta  HCP - Zn Cd Mg Ti Twinning may be caused by impact thermal treatment or by plastic deformation.

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 Failure can be defined in general as an event that does not accomplish its intended purpose. Failure of a material component is the loss of ability to function normally. Causes for failure  Improper materials selection  Improper processing  Inadequate design  Misuse of a component  Improper maintenance

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Structural elements and machine elements can fail to perform their intended functions in three general ways  Excessive elastic deformation buckling type of failure- controlled by the modulus of elasticity not by the strength of the material.  Excessive plastic deformation or yielding- controlled by the yield strength of the material.  Fracture ◦ Ductile/brittle fracture ◦ Creep- At elevated temperatures failure occurs in form of time- dependent yielding ◦ Fatigue- occurs in parts which are subjected to alternating or fluctuating stress

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Stages of ductile tensile fracturecup-and-cone fracture

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Exhibits three stages –  1 after necking cavities form usually at inclusions at second-phase particles in the necked region because the geometrical changes induces hydrostatic tensile stresses  2 the cavities grow and further growth leads to their coalesce resulting in formation of crack that grows outward in direction perpendicular to the application of stress  3 final failure involves rapid crack propagation at about 45 ْ to the tensile axis

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 Takes place with little plastic deformation  It occurs often at unpredictable levels of stress by rapid crack propagation.  The direction of crack propagation is very nearly perpendicular to the direction of applied tensile stress. This crack propagation corresponds to successive and repeated breaking to atomic bonds along specific crystallographic planes and hence called cleavage fracture .

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 Takes place in three stages  1 plastic deformation that causes dislocation pile- ups at obstacles  2 micro-crack nucleation as a result of build-up of shear stresses  3 eventual crack propagation under applied stress aided by stored elastic energy

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Schematic presentation of interior and surface cracks

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 Griffith proposed that a brittle material contains number of micro-cracks which causes stress rise in localized regions  When one of the cracks spreads into a brittle fracture it produces an increase in the surface energy of the sides of the crack. Source of the increased surface energy is the elastic strain energy released as crack spreads.  Grif fith’ s criteria for propagation of crack include these terms as: a crack will propagate when the decrease in elastic energy is at least equal to the energy required to create the new crack surface.  According to Griffith such as crack will propagate and produce brittle fracture when an incremental increase in its length does not change the net energy of the system.

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 Fatigue occurs when a material is subjected to repeated loading and unloading.  Failures occurring under conditions of dynamic or alternating loading are called fatigue failures.  If the loads are above a certain threshold microscopic cracks will begin to form at the surface.  Fatigue failure usually occurs at stresses well below those required for yielding or in some cases above the yield strength but below the tensile strength of the material.  These failures are dangerous because they occur without any warning. Typical machine components subjected to fatigue are automobile crank-shaft bridges aircraft landing gear etc.  Fatigue failures occur in both metallic and non-metallic materials

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Reasons for occurrence of fatigue fracture  a a maximum tensile stress of sufficiently high value.  b a large enough variation or fluctuation in the applied stress.  c a sufficiently large number of cycles of applied stress.

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 I. crack initiation at high stress points stress raisers ◦ crack initiation – includes the early development of fatigue damage that can be removed by suitable thermal annealing  II. propagation ◦ slip-band crack growth – involves the deepening of initial crack on planes of high shear stress. This is also known as stage-I crack growth. ◦ crack growth on planes of high tensile stress – involves growth of crack in direction normal to maximum tensile stress called stage-II crack growth  d III. final failure by fracture ◦ final ductile failure – occurs when the crack reaches a size so that the remaining cross-section cannot support the applied load.

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Factors That Affect Fatigue Life  Mean stress lower fatigue life with increasing s mean .  Surface defects scratches sharp transitions and edges. Solution:  polish to remove machining flaws  add residual compressive stress e.g. by shot peening.  case harden by carburizing nitriding exposing to appropriate gas at high temperature Environmental Effects  Thermal cycling causes expansion and contraction hence thermal stress if component is restrained.  Solution:  eliminate restraint by design  use materials with low thermal expansion coefficients. Corrosion fatigue. Chemical reactions induced pits which act as stress raisers. Corrosion also enhances crack propagation. Solutions:  decrease corrosiveness of medium if possible.  add protective surface coating.  add residual compressive stresses.

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 creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses.  Creep is more severe in materials that are subjected to heat for long periods and near melting point  Creep always increases with temperature.  The rate of this deformation is a function of the material properties exposure time exposure temperature and the applied structural load.

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 In primary creep the strain rate is relatively high but slows with increasing strain. This is due to work hardening.  The strain rate eventually reaches a minimum and becomes near constant. This is due to the balance between work hardening and annealing thermal softening. This stage is known as secondary or steady-state creep.  In tertiary creep the strain rate exponentially increases with strain because of necking phenomena.

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 There are several standard types of toughness  Three of the toughness properties that will be discussed in more detail are 1 impact toughness 2 notch toughness and 3 fracture toughness.

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 Nominal dimensions of gauge test section: ◦ 128 mm long and 10 mm2 Izod ◦ 55 mm long and 10 mm2 Charpy

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Shear Stress load/area in shear

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