DISCONTINUITY STRESS CLASS NOTES FOR MECHANICAL ENGINEERING

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DISCONTINUITY STRESS

CLASS NOTES FOR MECHANICAL

ENGINEERING

There are various types of discontinuities.

A designer first establishes  the type of

discontinuity. He calculates the discontinuity

stress. These stresses are

considered in the design of pressure vessels

/pipes and structures.

Two types of discontinuities

(i)  Gross structural discontinuity

(ii)  Local  structural discontinuity

Gross Structural Discontinuity

A discontinuity increases the stress or strain. It affects a relatively large portion of the structure.

Gross discontinuity affects the overall stress & strain distribution.

Examples of Gross Structural Discontinuity

(i) Dished head Connection to the cylindrical shell

(ii) Flange connection to the cylindrical shell

(iii) Nozzle connection to the cylindrical shell

(iv) Junction between the shells of different diameters

(v)  Junction between the shells of different thicknesses

Local Structural Discontinuity

A discontinuity which increases the stress or strain. This will affect a relatively small (local)

portion of the structure. Overall stress & strain distribution is not much affected by local discontinuity.

Examples of Local Structural Discontinuity

(i)  Oil hole

(ii) Merger of knuckle and crown radius

(iii) Groove

(iv)  Small dent

(v) Small scratch

Type of stresses

(i)  Primary stress

(ii) Secondary stress

(iii) Peak stress

Tensile, compressive, shear loads cause primary stresses. Pressure, bending moments and torque cause these stresses. Any combination of all these cause these primary stresses.  Equations of equilibrium govern the primary stresses. The primary stresses are not self adjusting. If primary stress exceeds yield stress, it will cause fracture. The maximum value of primary stress is 1.5 the design stress.

Design stress is 2/3 of the minimum specified yield strength for standard materials.  This value is only ½ of the ultimate tensile strength for non-standard materials.

Maximum allowable stress is 90% of the yield strength for all materials.

Secondary Stress

Adjacent parts cause Secondary stress. It  cannot result in fracture or gross distortion of the component. Gross structural discontinuities cause these stresses. Thermal stresses & pre-stresses during fabrication cause these stresses. Any combinations of these sources produce these.

Peak Stress

Peak stress is the increment of stress in thermal stresses. Different coefficient of thermal expansion of connected parts is the reason to produce it.. It is an added stress to the primary and secondary stresses. Only drawback of peak stress is that it can cause a fatigue fracture or brittle fracture.