C. K. Pithawala College of Engineering and Technology Topic:-STRESS CONCENTRATION Group number:-4 Mechanical 3rd Year 5th Sem Guild:-Prof. Mahesh Patel Name:-Patel Gopal(130090119073) Patel Gunjan(130090119074) Gajjar Snehal(130090119024) Borasia Rohan(130090119101)

Table of contain Introduction Causes of Stress Concentration Stress Concentration Factors ……….Theoretical Stress Concentration Factor(kt) ……….Fatigue Stress Concentration Factor(kf) ……….Notch Sensitivity(q) Methods of Reducing Stress Concentration Reference

Introduction A stress concentration (often called stress raisers or stress risers) is a location in an object where stress is concentrated. An object is strongest 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 of Stress Concentration Holes and slots Notches or grooves Ribs,gussets,and posts Sharp wall thickness transitions Surface roughness Bosses Corners

Stress Concentration Factors Theoretical Stress Concentration Factor(kt) Geometric stress concentration factors can be used to estimate the stress amplification in the vicinity of a geometric discontinuity. kt=maximum stress/nominal stress kt=σmax/σnom

Fatigue Stress Concentration Factor(kf) In design under fatigue loading, stress concentration factor is used in modifying the values of endurance limit while in design under static loading it simply acts as stress modifier. This means Actual stress= t k × calculated stress. For ductile materials under static loading effect of stress concentration is not very serious but for brittle materials even for static loading it is important.

Fatigue Stress Concentration Factor(kf) Conti…. It is found that some materials are not very sensitive to the existence of notches or discontinuity. In such cases it is not necessary to use the full value of t k and instead a reduced value is needed. This is given by a factor known as fatigue

Fatigue Stress Concentration Factor(kf) Conti…. strength reduction factor f k and this is defined as k= Endurance limit of notch free specimens Endurance limit of notched specimens

Increase of theoretical expected stress over the nominal stress Notch Sensitivity(q) The notch sensitivity of a material is a measure of how sensitive a material is to notches or geometric discontinuities. q= Increase of actual stress over the nominal _____________ stress ______________ Increase of theoretical expected stress over the nominal stress

Notch Sensitivity(q) conti…. Where Kf=fatigue stress concentration factor

Methods of Reducing Stress Concentration A number of methods are available to reduce stress concentration in machine parts. Some of them are as follows: Provide a fillet radius so that the cross-section may change gradually. Sometimes an elliptical fillet is also used. If a notch is unavoidable it is better to provide a number of small notches rather than a long one. This reduces the stress concentration to a large extent. If a projection is unavoidable from design considerations it is preferable to provide a narrow notch than a wide notch. Stress relieving groove are sometimes provided. These are demonstrated in figure

Methods of Reducing Stress Concentration conti…

Methods of Reducing Stress Concentration conti…

Reference Wikipedia https://en.wikipedia.org/wiki/Stress_concentration Stress Concentration by Version 2 ME,IIT Kharagpur

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