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Local Strain Models for Variable Loads

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Part of the book series: Springer Series in Solid and Structural Mechanics ((SSSSM,volume 4))

Abstract

In many practical applications of fatigue loadings with low amplitudes a number of higher load cycles are applied that overcome the threshold of the material yield strength, even if only locally. This circumstance is not taken into account by the classic theory of fatigue at a high number of cycles that assumes elastic states of stress at any point of a loaded body and a limited micro-plasticity at critical points. In the field of high load cycles, damage due to local plasticity is instead the governing phenomenon. Simulation models are developed based on a strain-controlled approach, especially in the cases of complex (i.e. not constant but time-varying) loads amplitudes. This local nonlinear cyclic behavior leads to a damage that occurs for a limited number of cycles \(< 5\times 10 ^ 4\). This numerical limit is conventional, based on experience and therefore subjected to different estimations by different authors. A similar convention emerges in establishing the test conclusion when the first visible crack appears (an event subjected to interpretation).

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Notes

  1. 1.

    Details in A.S.T.M. standardized procedure to determine the fatigue curve in deformation control for lives at low number of cycles, in accordance with \(\textit{ASTM}\)\(\textit{Standard} E 606{-}12\).

  2. 2.

    Using the same specimen could produce results invalidated by the material training by the previous cycles. The best practice is the use of a new specimen for each test.

  3. 3.

    A difficulty could occur in the identification of the stabilized hysteresis loop. Often, depending on the material and strain level, dynamic stability is not reached, i.e. stress amplitude corresponding to an imposed strain amplitude tends to vary continuously during the test and, in some cases, until the rupture of the specimen. In this case a conventional definition of stabilized cycle is given, as the hysteresis cycle that corresponds to the mid-life of the specimen, i.e. to the half number of cycles to failure.

  4. 4.

    In the case of components with states of non-proportional biaxial or triaxial stress (with loads agents not in phase with each other), in order to establish some reasonable estimation of the component strength, the reference specimen for analyzing the cyclic properties of the material must be arranged in such a way as to reproduce the same phase relationships that occur in reality [2, 19].

  5. 5.

    Reversals number (equal to the double number of cycles) distinguishes the semi-cycle with positive strains from the semi-cycle with negative strains and it is more suitable to describe the load history.

  6. 6.

    The concept of equivalence is related to an equivalent damage produced by the sequence of the hysteresis loops generated by real and by the virtual loads.

  7. 7.

    Properly only if the load cycles are followed in descending order.

  8. 8.

    Strain gages provided for large deformations have a very limited fatigue resistance and cannot be used for long load sequences.

  9. 9.

    Otherwise, criteria of multiaxial fatigue [2, 26, 27], must be used.

  10. 10.

    In order to overcome this difficulty a variable expression of \( \nu \) is introduced that must be determined by the cyclic stress-strain curve [35, 36]:

    figure a

    where:

    $$ {\left\{ \begin{array}{ll} {\buildrel \_ \over \nu } &{}= \text {variable Poisson coefficient} \\ E &{}= \text {elasticity modulus} \\ E_s &{}= \text {secant modulus on the cyclic curve} \\ \nu _e &{}= \text {Poisson's coefficient in the elastic field} \end{array}\right. } $$

    Secant modulus \(E_s\) is obtained from the cyclic curve utilizing the effective strain \({\buildrel \_ \over \varepsilon }\):

    $$\gamma _{\textit{oct}} = {2 \over 3} \; \sqrt{2} \; (1 + {\buildrel \_ \over \nu }) \cdot {\buildrel \_ \over \varepsilon }$$

    that for Eq. 8.11 gives:

    figure b

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Freddi, A., Olmi, G., Cristofolini, L. (2015). Local Strain Models for Variable Loads. In: Experimental Stress Analysis for Materials and Structures. Springer Series in Solid and Structural Mechanics, vol 4. Springer, Cham. https://doi.org/10.1007/978-3-319-06086-6_8

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