ESTIMATION OF STRESS CONCETRATION FACTOR - FESB

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Fig.2- Determining of local stress and strain For the supposed exponential relation between stress and number of cycles of the notched and unnotched specimens in the region of quasistatic fracture, it was easy to derive the expression for the approximate estimation of stress concentration factor: * β = β ( β / β ) q s q s log4 N log4 For materials that fail in brittle manner, β s factor depends on α t and notch sensitivity N q (11) at static fracture related to the level of the plastic deformation at static fracture, and there is inside limits 1≤β ≤β . For fully sensitive materials (such as titanium, beryllium and most of its alloys) is β s = β q = α t . s q For ductile materials, including mild steels with σ M < 700N/ mm 2 , the plastic strain is so high at static fracture (N=1/4) that stress concentration effect is relieved by yielding, i.e., it might be taken that β s =1, and consequently * β q = β log4 N log N q q 4 . (11a)
N q 3 5 is an exsclusive characteristic of a material and vary from 10 to 10 for the metals, just like particularly for the steels. 3. Concluding remarks Stress concentration factor calculated in the way described above, is as close to its true value, as suggested computing model is close to the real one. It also depends on basic data reliability, particularly on stress concentration factor boundary values β q, β k and β s . The special attention has to be paid to β q , the suggested calculation of which is good approximation for only the steels. For other materials the additional investigations have to be done to obtain the applicable formula for determination of β q . For that reason the author proposes to apply the known procedure after Fig.2 using obtained formula (10), because this method becomes now much more precise. In such a way the usage of β q is unnecessary, and results are not worse then using it. NOMENCLATURE a- material constant E- modulus of elasticity, MPa k pl - coefficient of cyclic strengthening, MPa m - Wöhler-curve exponent of the unnotched specimen m'- Wöhler-curve exponent of the notched specimen N - number of endurable cycles N gr - boundary number of cycles corresponding to endurance limit R −1 N q - boundary number of cycles corresponding to quasistatical fracture limit n - cyclic strengthening exponent R M - ultimate strength of a material, Mpa R q - unnotched specimen strength at quasistatic fracture boundary, MPa R q ' - notched specimen strength at quasistatic fracture boundary, MPa R −1 - endurance limit of material at -1 stress cycle asymmetry number, MPa α t - theoretical stress concentration factor β k - fatigue stress concentration factor in the zone of elastic strains β k * - fatigue stress concentration factor in the zone of elastic-plastic strains β q - stress concentration factor at quasistatic fracture limit β q * - stress concentration factor at quasistatic fracture zone
Page 1 and 2: -ESTIMATION OF FATIGUE STRESS CONCE
Page 3: 3 Whereas the ratio N / N for the m

ESTIMATION OF STRESS CONCETRATION FACTOR - FESB

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