Skip to Main Content
 

Global Search Box

 
 
 
 

ETD Abstract Container

Abstract Header

PREDICTION OF FATIGUE CRACK NEAR-THRESHOLD CENSORED REGRESSIONS WITH RUN-OUT DATA

Alansary, Yasser Abdo

Abstract Details

2014, Doctor of Philosophy, University of Akron, Civil Engineering.
The discussion is divided into two types of prediction. In the first type statistical models for fatigue life prediction for welded joints are discussed and fitted to experimental data for fillet-welded steel joints where cracks emanate from the weld toe. The models are based on an S-N approach where the number of cycles N to failure is assumed to be directly correlated to the applied nominal stress range. The models assume the existence of a fatigue limit given as a stress range below which no failure will take place. Emphasis is laid on the modeling of the fatigue life close to this limit where the service stresses for welded details often occur. Experimental data in this stress regime are sparse and do not fit the knee point of the conventional bi-linear S-N curve found in design rules and specifications. Therefore, an alternative model where both the fatigue life and the fatigue limit are simultaneously treated as random variables is investigated. The model parameters for this random fatigue-limit model (RFLM) are determined by the maximum likelihood method, and confidence intervals are obtained by the profile likelihood method. The advantage of the model is that it takes into consideration the variation in fatigue limit found from specimen to specimen and that run-out results are easily included. The median S-N curve obtained from the model coincides with the conventional bi-linear curves in the high stress regime (stress ranges higher than 110 MPa), but predicts longer lives as the stress range decreases below 100 MPa. The model gives a nonlinear S-N curve for a log-log scale in the fatigue-limit area; the fatigue life is gradually increasing and is approaching a horizontal line asymptotically instead of the abrupt knee point of the bilinear curve. The nonlinear curve is more in accordance with experimental data. At stress ranges below 100 MPa, the predicted fatigue lives are between 2 to 10 times longer than predictions made by the bilinear Category E curve in AISC Steel Construction Manual. The conclusion is that the rule-based S-N curves may be un-conservative in the stress regime where service stresses frequently occur. A more correct statistical model based on a random fatigue-limit model results in S-N curves that give decreased dimensions for a given fatigue design factor under constant amplitude loading. The second type of prediction is required if inspection is to be carried out in a damage tolerance approach. In this case fatigue process in fillet welded joints is discussed and modeled. As a first approximation, a pure fracture mechanics model was employed to describe the entire fatigue process. The model is calibrated to fit the crack growth measurements obtained from extensive testing on fillet weld joints where cracks emanate from the weld toes. Emphasis is laid on the choice of growth parameters in conjunction with a fictitious initial crack size distribution in order to obtain both reliable crack growth histories and predictions of the entire fatigue life. The model has its shortcomings in describing the damage evolution at low stress ranges due to the presence of a significant crack development period in this stress regime. As an alternative to the fracture mechanics model, a two-phase model (TPM) for the fatigue process was developed and calibrated.
Craig C Menzemer, Dr. (Advisor)
Nao Mimoto, Dr. (Committee Member)
Anil K Patnaik, Dr. (Committee Member)
Tirumalai S Srivatsan, Dr. (Committee Member)
David Roke, Dr. (Committee Member)
320 p.

Recommended Citations

Citations

  • Alansary, Y. A. (2014). PREDICTION OF FATIGUE CRACK NEAR-THRESHOLD CENSORED REGRESSIONS WITH RUN-OUT DATA [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1414502102

    APA Style (7th edition)

  • Alansary, Yasser. PREDICTION OF FATIGUE CRACK NEAR-THRESHOLD CENSORED REGRESSIONS WITH RUN-OUT DATA. 2014. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1414502102.

    MLA Style (8th edition)

  • Alansary, Yasser. "PREDICTION OF FATIGUE CRACK NEAR-THRESHOLD CENSORED REGRESSIONS WITH RUN-OUT DATA." Doctoral dissertation, University of Akron, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=akron1414502102

    Chicago Manual of Style (17th edition)