21-09-2013, 02:12 PM
CRACK AND ITS PROPAGATION
CRACK AND ITS PROPAGATION.ppt (Size: 1.04 MB / Downloads: 35)
WHY DOES A CRACK OCCUR
A mechanical part is exposed to a complex, often random, sequence of loads, large and small. These eventual loads tends the object/part to crack.
WHEN DOES A CRACK OCCUR
Fracture strength:
Fracture strength, also known as breaking strength, is the stress at which a specimen fails via fracture.
Ultimate strength:
Ultimate strength may be defined as the stress at which that a material can withstand while being stretched or pulled before necking.
Yield strength:
Yield strength may be defined as the stress at which a material begins to deform plastically.
The S-N curve:
A very useful way to visualize time to failure for a specific material is with the S-N curve. The "S-N" means stress verse cycles to failure.
If the material is loaded below fatigue stress, then it will not fail, regardless of the number of times it is loaded.
Material such as aluminum, copper and magnesium do not show a fatigue limit.
Crack initiation:
Stage-1:
The crack may be caused by surface scratches caused by handling, or tooling of the material; threads (as in a screw or bolt); slip bands or dislocations intersecting the surface as a result of previous cyclic loading.
NON DESTRUCTIVE TESTINGNDT)
Non destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage.
Magnetic Particle Inspection
The part is magnetized.
Finely milled iron particles coated with a dye pigment are then applied to the specimen.
These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity.
This indication can be visually detected under proper lighting conditions.
Rail inspection
The first rail inspections were done visually.
Rail inspection cars are the answer to today’s high mileage inspection needs.
The probes and transducers are mounted on carriages located underneath the inspection car.
Modern day inspection cars now use multiple NDT methods.
Induction and ultrasound methods can be used in rail inspection cars and operate at testing speeds of more than 30 mph.