University of Al-Qadisiyah Al-Qadisiyah Journal for Engineering Sciences 1998-4456 7 4 2014 12 28 THE EFFECT OF STRESS RATIO ON FATIGUE THRESHOLD OF CRAK IN MODE (I) دراسة تأثير تغيير نسبة الإجهاد على بداية تشقق الكلل للطور الأول 187 200 95871 EN Journal Article 2014 12 01 In the present study, the effect of the stress ratio change on the threshold of the fatigue cracks for an aluminum alloy have been researched. This aluminum alloy was taken from a blade of Russian MI25 helicopter. A specimen of MI25 blade has been cut into testing specimens of standard dimensions; the stress intensity factor as a basic variable has been calculate and used by making an initial crack in the test specimens. The test have been conducted for five different stress ratio : ( R = - 1 , 0.0 , 0.1 , 0.3 , 0.5 ) : Several tested have been done to study the effect of these change on the threshold of fatigue crack , The result obtained in this research were compared to some well knows previous scientific theories and research in this field . At each stress ratio ( R ) , several specimens have been used to obtain the threshold stress intensity factor for that value of ( R ) . From the results obtained , it is clear that the value of the threshold stress intensity factor range (ΔK) decreases as the stress ratio increases. Whereas ; at a negative stress ratio: (ΔK) increase for threshold fatigue crack https://qjes.qu.edu.iq/article_95871_2c29c841bf5a9603392bb17494e4e7c3.pdf