Important Answers: Steady Stresses and Variable Stresses In Machine Members

1. Define factor of safety.

Factor of safety (FOS) is defined as the ratio between the maximum stress and working stress.

       2 What are the various phases of design process?

               Recognition of need

               Definition of problem

               Synthesis

               Analysis and Optimization

               Evaluation

               Presentation

        3 List out the factors involved in arriving at factor of safety.

               Material properties

               Nature of load

               Presence of localized stress

               Presence of initial stress

               Mode of failure

        4 Define Endurance limit.

Endurance limit is the maximum value of completing reversed stress that can sustain an infinite number (10⁶) of cycles without failure.

       5 What are the factors affecting endurance strength of a material?

            Load

            Surface finish

            Size

            Temperature

            Impact

            Reliability

        6  What is eccentric load and eccentricity?

An external load, whose line of action is parallel but does not coincide with the centroidal axis of the machine component, is known as an eccentric load. The distance between the centroidal axis of the machine component and the eccentric load is called eccentricity. (Ex.) C-clamps, Punching machines, brackets, offset connecting links etc.

7. What is stress concentration and stress concentration factor?

The irregularity in stress distribution caused by abrupt changes of form is called stress concentration. Stress concentration factor = maximum stress at the change of cross section / nominal stress.

     8 What are the factors affecting selection of material for machine element?

            Load applied

              Purpose and operating conditions of the part.

              Suitability for manufacture.

              Minimum weight and optimal size

            Availability and cost.

9        Define Resilience.

Resilience is the property of the material to absorb energy and to resist shock and impact loads. This property is essential for spring materials.

10. Define the Morphology of Design.

Morphology of design consisting of problem formulation analysis search for alternative an evaluation decision taking and specification of the solution.

     11   What are the various theories of failure? a. Maximum principal stress

theory b. Maximum shear stress theory

c. Maximum principal strain theory d. Maximum strain energy theory

12. Why normal stress theory is not suitable for ductile materials?

Ductile materials mostly fail by shearing. But this theory considers only tensile or compressive stresses. So this is not suitable for ductile materials.

13. What is an impact load? Give examples

If the time load application is less than one third of the lowest national period of vibration of the part, the load is called an impact load.

Example: Punching presses, hammers, loads exerted on cams during the motion due to eccentricity, loads imposed on gear teeth due to irregular tooth profile

       14  What are the types of fracture?

       Ductile

fracture b. Brittle fracture

15. Explain size factor in endurance strength.

Size factor is used to consider the effect of the size on endurance strength. A large size object will have more defects compared to a small one. So, endurance strength is reduced. If K is the size factor,

Actual endurance strength = Theoretical endurance limit x K