1 / 18

Chapter 7

Chapter 7. Fatigue Failure Resulting from Variable Loading . Dr. A. Aziz Bazoune King Fahd University of Petroleum & Minerals Mechanical Engineering Department. LECTURE 25. 7-11 Characterizing Fluctuating Stresses.

minda
Download Presentation

Chapter 7

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 7 Fatigue Failure Resulting from Variable Loading Dr. A. Aziz Bazoune King Fahd University of Petroleum & Minerals Mechanical Engineering Department

  2. LECTURE 25

  3. 7-11 Characterizing Fluctuating Stresses • Fluctuating stresses in machinery often take the form of sinusoidalpattern because of the nature of the nature of some rotating machinery. • Other patterns some quite irregular do occur.

  4. 7-11 Characterizing Fluctuating Stresses • In periodic patterns exhibiting a single maximum and single minimum of force, the shape of the wave is not important. • The peaks on both sides (maximum, minimum) are important. • Fmax and Fmin in a cycle can be used to characterize the force pattern. • A steady component and an alternating component can be constructed as follows:

  5. Stress Range • Mean (Midrange Stress) • Stress Amplitude (Alternating Stress) • Stress Ratio • Stress Amplitude (R>0 ) (R =0) (R =-1)

  6. Any varying stress with a nonzero mean is considered a fluctuating stress.

  7. The steady, or static, stress is not the same as the midrange stress. • The steady stress may have any value betweenσminand σmin. • The steady stress exists because of a fixed load or preload applied to a part. • The steady load is independent of the varying portion of the load.

  8. A helical compression spring is always loaded into a space shorter than the free length of the spring. • The stress created by this initial compression is called the steady, or static, component of the stress.

  9. Equations (7-39) use symbols σa and σm as the stress components at the location of scrutiny. • In the absence of a notch, σaandσmare equal to the nominal stressesσaoandσmoinduced by loads Fa and Fm , respectively. • With a notch they are σa = Kfσaoand σm = Kfσmo, respectively.

  10. When the steady stress component is high enough to induce localized notch yielding, the designer has problem. • The first-cycle local yielding produces plastic strain and strain-strengthening. • This is occurring at the location where fatigue crack nucleation and growth are most likely.

  11. Possible ways of quantifying the problem: • Residual Stress Method • All stresses (both mean and alternating) are multiplied by the fatigue stress concentration factor Kf, and correction is made for yielding and resultant residual stresses if the calculated peak stress exceeds the material yield strength. • Nominal Mean Stress Method • In this method, stress concentration factor is applied only to alternating stress. • Reduction in mean stress from not multiplying it by Kf, might be about the same as the reduction in mean stress achieved with the residual stress method by taking yielding and residual stress into account.

  12. 7-12 Fatigue Failure Criteria for Fluctuating Stress • After having defined the various components of stress associated with a part subjected to fluctuating stresses, we want to vary both the midrange stress and the stress amplitude or alternating component, to learn about the FATIGUE RESISTANCE of parts when subjected to such situations. • Many machine elements involve fluctuating stresses about a non-zero mean. • The influence of non-zero mean stress is estimated by using one of several empirical relationships that determine failure at a given life when both alternating and mean stresses are nonzero.

  13. Modified Goodman Diagram • It has midrange stress plotted along the abscissa and all other components of stress plotted on the ordinate, with tension in the positive direction. • The endurance limit, fatigue strength, or finite-life strength whichever applies, is plotted on the ordinate above and below the origin. • The midrange line is a 45o line from the origin to the tensile strength of the part. Figure 7-24 Modified Goodman diagram showing all the strengths and the limiting values of all the stress components for a particular midrange stress

  14. Plot of Fatigue Failures for Midrange Stresses in both Tensile and Compressive Regions. Figure 7-25 Plot of fatigue failures for midrange stresses in both tensile and compressive regions. Normalizing the data by using the ratio of steady strength components to tensile strength Sm/Sut, steady strength component to compressive strength Sm/Suc, and strength amplitude component to endurance limit Sa/S’e enables a plot of experimental results for a variety of steels.

  15. Master Fatigue Diagram. Figure 7-26 Master fatigue diagram for AISI 4340 steel with Sut = 158 Sy = 147 kpsi. The stress component at A are σmin = 20, σ max = 120, σ m = 70, σ o = 50 all in kpsi

More Related