High Cycle Fatigue: A Mechanics of Materials Perspective part 62. The nomenclature used in this book may differ somewhat from what is considered standard or common usage. In such instances, this has been noted in a footnote. Additionally, units of measurement are not standard in many cases. While technical publications typically adhere to SI units these days, much of the work published by the engine manufacturers in the United States is presented using English units (pounds, inches, for example), because these are the units used as standard practice in that industry. The graphs and calculations came in those units and no attempt was made to convert. | 596 Appendix G 0 0 80 70- 60- n 50- o 40- a 20 10- Notched tension specimens GEAE machined notch FOD simulation specimens P W ballistic impact P W solenoid impact GEAE solenoid impact GEAE machined notch os 30- Ti-6-4 10 30 80 20 40 50 60 70 0 residual stresses assumed present in ballistic and solenoid simulated FOD with compressive zone of inches ASth measured ksi Figure . Prediction of experimental versus predicted threshold stress using the WCN model. model applied to various sources of specimen data involving both FOD and notched specimens is shown in Figure . This figure shows the good correlation of experiment and prediction. WCN Example In order to apply the WCN method an example of its use should prove beneficial. First assume an FOD event has occurred on the leading edge of a blade resulting in a notch with a depth of mm in . The notch has a root radius of mm in. and does not have a crack emanating from it. General formulas for kt can be looked up in handbooks for example 19 . The kt for this particular notch is given in Equation kt p b 1 2b p f p b where f p b is given by Equation . 1 v2 f p b 31 Appendix G 597 For the notch in this example kt is approximately . This is a very sharp notch but kts of this size have been seen in field damage. If this blade material has an endurance limit at 107 cycles of 650 MPa for R then the use of kt would predict that the maximum allowable stress should be MPa. Using the WCN method it is necessary to compute the threshold stress intensity factor for small cracks and correlate that to the threshold applied stress that can be applied to the blade and still remain in the limit of safe operation. The first step is to calculate the small crack parameter ao. Equation defines ao as a function of applied stress ratio 1 r A v. R T a R n _FX re R _ where F for a crack that goes through the thickness in a straight line through crack and for a .
