Outline of Todays Lecture(a) What is Fracture?(b) Fundamentals of Fracture (Introduction).(c) Common Causes of Failure(d) Modes of Fracture.(e) Stages of Fracture.(f) Ductile Fracture. (g) Process / Stages of Ductile fracture.(h) Surface Characteristic of Ductile Fracture.

What is Fracture? Simple fracture is the separation or

fragmentation of a body into two or more pieces in response to an imposed stress that is static (i.e., constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material.

The applied stress may be tensile, compressive, shear, or torsional.

Fracture occurred due to only defects because no components are produced without any defect.

Fundamentals of Fracture; (Introduction) Failures of structures can occur at various

locations, for instance, at home, on the road, in a machine, in an aircraft, or in a big plant.

The Failure of engineering materials is almost always an undesirable event for several reasons; these include1) Human lives that are put in jeopardy (danger), or

(Injuries to Personal)2) Economic losses, i.e. Damage the Property,

3) Facilities: The interference with the availability of products and services.

4) Loss of Production. 5) Shut Down of an entire Plant.

Even though the causes of failure and the behavior of materials may be known, prevention of failures is difficult to guarantee.

The usual causes are improper materials selection, processing, inadequate design of the component and its misuse.

It is the responsibility of the engineer to anticipate (predict) and plan for possible failure , in the event that failure does occur, to assess its cause and then take appropriate preventive measures against future incidents. Or Learning from Past Failure helps to prevent future mistake.

Extensive study of carburized and hardened gears for heavy duty trucks, machine tools, mining machines, diesel engines etc. showed that 38% of failure resulted from surface problems (like pitting), 24% from fatigue, 15% from impact and 23% from miscellaneous causes.

Common Causes of Failures:

1) Improper design Underestimate of service stress Undesirable geometrya) Stress concentratorsb) Inadequate radii at cornersc) Inaccessibility for inspection

d) Difficult to fabricate Improper choice of materials Improper choice of heat treatment Environmental effects Ad-hoc modifications

2) Defective manufacturing Materials

a. Materials mix-upb. Poor quality of materialc. Nonconformity to specificationsd. Undesirable phases and nonmetallic inclusions

Castingsa. Porosityb. Inclusionsc. Segregation

Forgingsa. Foldsb. Fissuresc. Unfavorable flow lines

Weldsa. Improper fusionb. Cracks in heat affected zonec. Wrong filler material

Machined partsa. Poor surface finishb. Grinding burns

Heat treatmenta. Quench cracksb. Overheatingc. Under-tempering

Surface treatmenta. Improper cleaningb. Improper plating

c. Improper post treatment Assembly

a. Defective joiningb. Misorientation and misalignmentc. Inadequate supportd. Improper tighteninge. Improper balancingf. Damages by toolsg. Assembly stressesh. Distortion

3) Improper inspection Deviations from approved tolerances Deviations from acceptance standards Wrong techniques Non-adherence to schedules of inspection and calibration of inspection tools Shortcuts to meet targets

4) Operation, service abnormalities, and abuses Inadequate lubrication Improper cleaning No adherence/obedience to operating instructions

a. Over speedingb. Over loadingc. No trained operators

Unexpected service conditions Temperature and stress excursions Vibrations Changes in environment

5) Environmental effects Various types of chemical corrosion Stress corrosion Corrosion fatigue Erosion

6) Maintenance Wrong replacement Wrong tools Excessive concessions Inadequate inspection Ad hoc alterations Inadequate condition monitoring and preventive maintenance

7) Combination of the preceding Combined action of some of the preceding causes

8) Sabotage (Disruption/Damage) Deliberate damage by antisocial acts Mechanical damages Explosive sabotage

Modes of Fractures:

Two fracture modes are possible; classification is based on the ability of a material to experience plastic deformation.

i.e., Ductile and BrittleBrittle Ductile

Stress

Strain

Tensile Stress-strain behavior of both fracture type as

shown in figure

(a) Very ductile: soft metals (e.g. Pb, Au) at room T, polymers, glasses at high T

(b) Moderately ductile fracture typical for metals(c) Brittle fracture: ceramics, cold metals, cast iron,

Knowledge of ductility of material is important for at least two reasons.

1) It indicates to a designer that, the degree to which a structure/material will deform plastically before fracture.

2) It specifies the degree of allowable deformation during fabrication operation.

Stages of Fracture:

Any fracture process involves two stages/steps.1) Crack initiation.(start/beginning/commencement)2) Propagation in response to an imposed stress.

Ductile Fracture:

Ductile fracture is characterized by extensive plastic deformation in the vicinity (area/locality) of an advancing crack.

The process proceeds relatively slowly as the crack length is extended.

Such a crack is often said to be stable. That is, it resists any further extension unless there is an increase in the applied stress.

Ductile fracture is almost always preferred for two reasons. 1) The presence of plastic deformation gives warning that

fracture is imminent (pending), allowing preventive measures to be taken.

2) More strain energy is required to induce ductile fracture.

Process / Stages of Ductile fracture:

(a) Initial necking.(b) Small cavity formation.(c) Coalescence of cavities forms a crack.(d) Crack propagation.(e) Final shear fracture at a 45° angle relative to the tensile direction.

Crack grows 90o to applied stress

45O - maximum shear stress

a) inclusions in a ductile matrix, (b) void nucleation, (c) void growth, (d) strain localization between voids, (e) necking between voids, and (f) void coalescence and fracture.

Fig: Mechanism for ductile crack growth: (a) initial state, (b) void

growth at the crack tip, and, (c) coalescence of voids with the

crack tip.

First, (a) and (b) after necking

begins, small cavities, or

micro-voids, form in the

interior of the cross section,

Next, (c) as deformation

continues, these micro-voids

enlarge, come together, and

join to form an elliptical crack,

which has its long axis

perpendicular to the stress direction. The crack continues to grow

in a direction parallel to its major axis by this micro -void

coalescence process.

Finally, (d) and (e) fracture proceeds by the rapid propagation of a

crack around the outer perimeter of the neck by shear deformation

at an angle of about 45° with the tensile axis—this is the angle at

which the shear stress is a maximum.

Cone and Cup

Surface Characteristic of Ductile Fracture:

Sometimes a fracture having this characteristic surface contour is

termed a cup and- cone fracture because one of the mating

surfaces is in the form of a cup, the other like a cone.

Cup Shape

Figure: Scanning electron microscope (SEM) fractograph which shows ductile fracture in a low carbon

steel

Deformation

Basic Concepts for Deformation Millers indices: Miller indices give the crystallographic

information in terms of crystallographic planes and directions.

Number of atoms per unit cell (BCC, FCC, HCP)

Atomic Packing factor.

defects