Dr. S & S.S.GHANDHY GOVERNMENT ENGINEERING COLLEGE 3rd year Mechanical Engineering

Topic:- Design Against Fluctuating Loads. Prepared By:- Name:- Enrl.No Gamit Mayur 130230119012 Panchal Karan 130230119036 Variya Ankit 130230119061 Vasava Urvashi 130230119063 Vansekar Vishal 130230119064 Bhalgamiya Nilesh 140233119001 Patel Devansi 140233119009

Fatigue Introduction : In several applications, components have to withstand different kinds of load at different times . Materials subjected to these fluctuating or repeated load tends to show a behavior which is different from what they show under steady loads.

Fatigue It has long been known that a component subjected to fluctuating stresses may fail at stress levels much lower than its monotonic fracture strength, due to a process called Fatigue. Fatigue is an insidious time-dependent type of failure which can occur without any obvious warning. It is believed that more than 95 % of all mechanical failures can be attributed to fatigue.

Fatigue occurs at stress well within the ordinary elastic range as measured in the static tension test. Fracture resulting from fatigue is very difficult to predict and hence a good understanding of fatgue behavior is very important.

Types of fatigue loading: 1.Completely reversed cycle of stress: 2. repeated stress cycles 3. irregular or random stress cycle:

Type of Fluctuating Stresses max min 2 Alternating stress Mean stress m = max min 2 + Ken Youssefi MAE dept., SJSU

Fatigue properties : Fatigue life (N): it is total number of cycles are required to bring about final fracture in a specimen at a given stress. Fatigue life for a given condition is a property of the individual specimen and is arrived at after testing a number of specimens at the same stress.

Fatigue life for P % survival (Np) It is fatigue life for which P percent of samples tested have a longer life than the rest. For example, N90 is the fatigue life for which 90% of the samples would be expected to survive and 10% to fail at a particular stress.

Median fatigue life: it is fatigue life for which 50 % of the population of samples fail and the other 50 % survive at a particular stress.

OR it is the strength of a material for a particular fatigue life. Fatigue strength (σn) It is stress at which a material can withstand repeatedly N number of cycles before failure. OR it is the strength of a material for a particular fatigue life.

Fatigue limit or Endurance limit (σE): it is stress below which a material will not fail for any number of cycles. For ferrous materials it is approximately half of the ultimate tensile strength. For non-ferrous metal since there is no fatigue limit.

Endurance limit is taken to be the stress at which it endures, N number of cycles without failure .N is usually taken as 5 x 108 cycles for non-ferrous metals.

Effect of stress concentration 2) Size effect: 3) Surface Roughness: Factors affecting fatigue: Effect of stress concentration 2) Size effect: 3) Surface Roughness: 4) Surface Residual Stress: 5) Effect of temperature: 6) Effect of metallurgical variables;

Design for Finite Life log .9Sut = log a + b log 103 Sn = a (N)b log Sn = log a + b log N Apply conditions for point A and B to find the two constants “a” and “b” log .9Sut = log a + b log 103 log Se = log a + b log 106 a = (.9Sut)2 Se b .9Sut 1 3 log Sn Kf a = n Design equation Calculate Sn and replace Se in the design equation

The Effect of Mean Stress on Fatigue Life Mean stress exist if the loading is of a repeating or fluctuating type. Mean stress Alternating stress m a Sy Yield line Gerber curve Se Sy Soderberg line Sut Goodman line

The Effect of Mean Stress on Fatigue Life Modified Goodman Diagram Alternating stress m a Sy Yield line Se Safe zone C Goodman line Sy Sut Mean stress

The Effect of Mean Stress on Fatigue Life Modified Goodman Diagram Sy Yield line - Syc Se Safe zone Sut Goodman line C Safe zone - m Sy +m

The Effect of Mean Stress on Fatigue Life Modified Goodman Diagram Se 1 = Sut a m + Infinite life a = Se nf Finite life Sn 1 = Sut a m + a + m = Sy ny Yield Se a + m = Sy ny Yield C Safe zone Safe zone - m - Syc Sy Sut +m

Thank you…