Question

What are the causes of stress concentration and the methods of reducing stress concentrations?

Answer 1

A stress concentration (often called stress raisers or stress risers) is a location in an object where stress is concentrated. An object is stronger when force is evenly distributed over its area, so a reduction in area, e.g., caused by a crack, results in a localized increase in stress. A material can fail, via a propagating crack, when a concentrated stress exceeds the material's theoretical cohesive strength. The real fracture strength of a material is always lower than the theoretical value because most materials contain small cracks or contaminants (especially foreign particles) that concentrate stress. Fatigue cracks always start at stress raisers, so removing such defects increases the fatigue strength.
Causes
1) Geometric discontinuities cause an object to experience a local increase in the intensity of a stress field. Examples of shapes that cause these concentrations are cracks, sharp corners, holes, and changes in the cross-sectional area of the object. High local stresses can cause objects to fail more quickly, so engineers must design the geometry to minimize stress concentrations.

2) Due to discontinuities in applied loads.

3) Material discontinuities which may occur while manufacturing.

Prevention

A counter-intuitive method of reducing one of the worst types of stress concentrations, a crack, is to drill a large hole at the end of the crack. The drilled hole, with its relatively large diameter, causes a smaller stress concentration than the sharp end of a crack. This is however, a temporary solution that must be corrected at the first opportune time.

It is important to systematically check for possible stress concentrations caused by cracks—there is a critical crack length of 2a for which, when this value is exceeded, the crack proceeds to definite catastrophic failure. This ultimate failure is definite since the crack will propagate on its own once the length is greater than 2a. (There is no additional energy required to increase the crack length so the crack will continue to enlarge until the material fails.) The origins of the value 2a can be understood through Griffith's theory of brittle fracture.

Another method used to decrease the stress concentration is by creating the fillet at the sharp edges. It gives smooth flow of stress streamlines. In a threaded component force flow line is bent as it passes from shank portion to threaded portion as a result stress concentration takes place. To reduce this a small undercut is taken between shank and threaded portion.

Answer 2

The area or the region of a design or material where the stress is significantly higher then the other regions is called as Stress Concentration.

The different steps of stress concentration are

• change in the section of material if done abruptly .
• if the tension material have additional notches, holes and bent structures .
• if after the design is complete additional holes are drilled to fit some shafts.
• if the machinery experiences any type of scratches or dents.
• Large amount of force or pressure is applied on the structure.
• variation in properties of material.
• fillet radius, undercutting and drilling unnecessary or excessive holes.
• if there are stamp marks , surface irregularities etc.

Method to reduce stress concentration

• we must provide gradual transition to the machinery cross sections.
• we can design specifically such as proper notches bents and holes at proper spaces.
• another major thing is to provide material reduction near the notches and put additional grooves.
• We use more then one type of smaller relief joints than using one single long joint.
• joining of common machine joints using welding.
• avoiding sharp corners.

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