Design of Machine Elements: Unit 3 - Design Against Fluctuating Loads, Summaries of Mechanical Systems Design

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DESIGN OF MACHINE ELEMENTS

Unit 3

DESIGN AGAINST FLUCTUATING LOADS

ME – 1, Batch - C

Unit contain Stress concentration - causes & remedies, fluctuating stresses, fatigue failures, endurance limit, design for finite and infinite life, Soderberg and Goodman diagrams, Modified Goodman diagrams

• A plate with a small circular hole, subjected to tensile stress is shown in Fig. The distribution of stresses near the hole can be observed by using the Photo- elasticity technique.
• The localized stresses in the neighbourhood of the hole are far greater than the stresses obtained by elementary equations.
• Stress concentration is defined as the localization of high stresses due to the irregularities present in the component and abrupt changes of the cross section

• stress concentration factor: It is denoted by Kt and defined as,

Causes:

Variation in Properties of Materials Load Application Abrupt Changes in Section Discontinuities in the Component Machining Scratches

• A fl at plate with an elliptical hole and subjected to tensile force, is shown in Fig. the theoretical stress concentration factor at the edge of hole is given by,
• The stress concentration charts are based on either the photo-elastic analysis of the epoxy models using a circular polariscope or theoretical or fi nite element analysis of the mathematical model. That is why the factor is called theoretical stress concentration factor.
• The ductility or brittleness of the material and the type of load— whether static or cyclic—affects the stress concentration.

• Therefore, there is a difference between the stress concentration indicated by the theoretical stress concentration factor and the actual stress concentration in the compon
• The designer should consider the following guidelines 1) Ductile Materials Under Static Load When the stress in the vicinity of the discontinuity reaches the yield point, there is plastic deformation, resulting in a redistribution of stresses. This plastic deformation or yielding is local and restricted to a very small area in the component. There is no perceptible damage to the part as a whole. 2)Ductile Materials Under Fluctuating Load when the load is fluctuating, the stress at the discontinuities may exceed the endurance limit and in that case, the component may fail by fatigue. Therefore, endurance limit of the components made of ductile material is greatly reduced due to stress concentration.

Reduction of stress concentration factor

• Fluid Flow analogy
• When there is sudden change in cross-section, bending of stress lines is very sharp and severe resulting in stress concentration. Therefore, stress concentration can be greatly reduced by reducing the bending by rounding the corners.
• There are different methods to reduce the bending of the stress lines at the junction and reduce the stress concentration Force flow around sharp corner Force flow around Rounded corner

1) Additional Notches and Holes in Tension Member

3) Drilling Additional Holes for Shaft

4) Reduction of Stress Concentration in Threaded Members