Tham khảo tài liệu 'crc press - mechanical engineering handbook- mechanics of solids part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 1-120 Section 1 Multiaxial Fatigue Complex states of stress are common in engineering components and in fatigue analysis they may cause serious difficulties. There are many methods available but none of them are adequate for all cases. The simplest situations that might be handled reasonably well involve fully reversed loading by in-phase or 180 out-of-phase proportional stresses at the same frequency. Multiaxial fatigue testing is difficult and expensive so it is often desired to use uniaxial test data for predicting the multiaxial behavior. A typical approach for this is based on computing an effective stress amplitude ơe from the amplitudes of the principal stresses ơ 1a ơ2a ơ3a. With the concept of the octahedral shear yield criterion a ơ -c .f ơ -ơ 2 ơ -5 2 e 12 1a 2a 2a 3a 3a 1a where in-phase stresses are positive and 180 out-of-phase stresses are negative. The life is estimated by entering ơe on the appropriate S-N curve. Note that mean stresses localized or general yielding creep and random frequencies of loading further complicate the problem and require more sophisticated methods than outlined here. Strain vs. Life e-N Curves A strain-based approach is necessary in fatigue when measurable inelastic strains occur. In general total strain consists of elastic plastic and creep strains with the latter two being in the category of inelastic strains t e p c When ep or and c are dominant the life is relatively short and the situation is called low-cycle fatigue LCF as opposed to high-cycle fatigue HCF where e is dominant. The mechanics of LCF can be understood by first considering hysteresis loops of elastic and plastic strains as defined in Figure . FIGURE Hysteresis loop. Simplistically HCF means a thin loop a straight line at very long life and LCF means a fat loop. Strain-life plots are especially useful in the LCF regime where material properties f ơf obtained in monotonic tension tests are directly useful in fatigue life .
