Stress Controlled Fatigue

• Stress-controlled fatigue is generally related to high cycle (low stress) fatigue, in which a major part of the material behaves elastically.

• Even though the material immediately adjacent to the notch may become plastic, both the extent of plastic zone, and the stress in it are limited.

• Since stress is directly proportional to strains, conventionally, the fatigue strength is expressed in terms of stress.

• One of the earliest investigations of stress-controlled cyclic loading effects on fatigue life was performed by a german engineer named August Wohler in 1893 who studied railroad wheel axle failure.

• His work on fatigue marks the first systematic investigation of S-N Curves, also known as Wöhler curves, to characterise the fatigue behaviour of materials.

• Several important facts were revealed from his investigation

• First, the number of cycles to failure increases with decreasing stress range. Below a certain stress range, which is referred to as fatigue endurance limit, the fatigue life is infinite.

• Second, the fatigue life is reduced dramatically by the presence of a notch.

• These observations indicate that fatigue is a three-stage process involving initiation, propagation, and a final failure stage

N : Periyodik yükleme sonucunda kırılmanın oluştuğu tekrar sayısı

Süresiz yorulma dayanımı

• The scatter in the S-N curves is due to these factors :• Wall thickness• C orrosion• Type and condition of the material including a

number of metallurgical variables.• Test environment, specimen surface, alignment of

the test machine etc• Residual stress, mean stress or stress ratio • Local stress peaks (notch effects)

• Dayanımı 900 N/mm olan çeliğin, çeşitli yüzey durumuna göre yorulma dayanımları:

• Ayna gibi parlamış yüzeyde ise çekme dayanımının % 50 si yorulma dayanımıdır: 450 N/mm

• Taşlanmış yüzeyde bu değer % 12 daha azdır. 450x0.88=396 N/mm

• Haddelenmiş ise bu değer % 53 daha azdır. 450x0.47=212 N/mm

• Korozif bir ortamda çalışılıyorsa % 73 daha azdır. 450x0.27= 122 N/mm