Significance of initiation, propagation and closure of microcracks in high cycle fatigue of ductile metals

Fatigue limit of carbon and Cr Mo steels as a small fatigue crack threshold in high-pressure hydrogen gas

In order to investigate the fatigue limit micro-mechanism of a precipitation-hardened Al–Mg–Si alloy (6061-T6), the alloy was subjected to very-high-cycle fatigue (VHCF) of over 109 cycles by an ultrasonic fatigue method. Two kinds of specimens, one with smooth surface and the other with a small artificial hole on the surface, were compared. The smooth specimens showed no distinct fatigue limit. Conversely, the holed specimens showed clear fatigue limit which had been generally deemed to be absent in non-ferrous alloys. In addition to the conventional fatigue crack growth (FCG) observation by replica technique, metallographically critical analyses by electron backscattered diffraction (EBSD) and cross-sectional focused ion beam (FIB) were conducted to reveal the micro-plasticity associated with FCG. It was found that the fatigue life of smooth specimens at low stress amplitude was controlled by an unstoppable FCG mechanism mediated by persistent slip bands (PSBs). On the other hand, the emergence of distinct fatigue limit in holed specimens was attributed to a non-propagating crack having mode I characteristics in essence. No coaxing effect was, however, confirmed for such non-propagating cracks. The above results, which were somewhat different from previous ones obtained by rotating bending under normal frequency, were discussed in terms of both metallurgical and mechanical points of view.

Fatigue limit investigation of 6061-T6 aluminum alloy in giga-cycle regime

Numerous structural health monitoring systems have been investigated extensively in order to enhance safety level and reduce direct operational costs. This work demonstrates the feasibility study of a new concept, the effective structural health monitoring system. The effective structural health monitoring system detects cracks using a system of capillaries incorporated into a structure. The structure with the integrated capillaries is produced by additive manufacturing, a process of adding material layer by layer. The first objective of this study is to prove that the developed system has reached technological readiness level 3. In order to prove that, four-point bending specimens with the integrated effective structural health monitoring system were tested after being produced by additive manufacturing, more specifically by laser metal deposition. The second objective of the study is to indicate that during four-point bending fatigue tests, the integrated structural health monitoring system has no influ...

Feasibility study on integrated structural health monitoring system produced by metal three-dimensional printing

In this paper, the result from rotating bending fatigue tests of notched specimens are carried out on aluminum cast alloys and high strength steels with H
B≃600 are reported. The threshold stress intensity factor range under a stress controlled condition, ΔK
w, is introduced for the purpose of predicting the fatigue limit of a metal with an arbitrary crack. The ΔK
w value of a long crack, ΔK
wUL, is obtained from the fatigue crack propagation limit σw2 of specimens with a sharp and deep notch; ΔK
w increases with crack length, and ΔK
wUL is an upper limit of ΔK
w. Since there are few σw2 data of steels with H
B≥400, the σw2 values are evaluated by the fatigue crack initiation limit predicted using Linear Notch Mechanics and the relation between σw1 and σw2 at the branch point. Using ΔK
wUL values of many metals with a long crack, the ΔK
wUL values are approximated with a simple formula. Moreover the lower limit value of ΔK
w versus the crack length, ΔK
wLL, is proposed. Then using the ΔK
w and ΔK
wUL formulae and the ΔK
wLL value, the smallness of a fatigue crack is clarified.

Quantitative evaluation of the fatigue limit of a metal with an arbitrary crack under a stress controlled condition – Stress Ratio R =−1

The purpose of this paper is to make clear the reason why the fatigue limit and the coaxing effect exist in steel, using 0.13%C steel and copper specimens. The main results obtained are as follows. In 0.13%C steel specimens ; (1) The incipient fatigue cracks, revealed by light electro-polishing (1∼2μ) after 107 cycles under the stress repetitions of the fatigue limit of the specimen, exist in the ferrite not only at the surface but also in the inside and the tip parts of those cracks become 24∼34% harder than in the virgin state. (2) This incipient fatigue crack appearing by the stress of the fatigue limit is the starting point of fracture, when the specimen fails by a stress greater than the fatigue limit. In copper specimens ; (3) The tip parts of the incipient fatigue crack in copper, appearing after 107 cycles of the stress which corresponds to the fatigue strength for 2×107 cycles, do not become so hard than the ones in 0.13%C steel. These results show that the existence of the fatigue limit in steel is due to the strengthening of the tip parts of the non-propagating micro-crack.

Correlation between the Existence of Fatigue Limit and Non-Propagating Micro-Crack

Using electro-polished low carbon steel specimens, the change in surface states and the incipient fatigue cracks of plain and notched specimens subjected to rotating bending stress were investigated. The main results obtained are as follows. (1) When the limit stress for macro-crack initiation σw1 is repeated, the amount of slip bands appearing at the root of notch is nearly equal, not only after 107 cycles but also at every cycle ratio, even if the maximum stress ασw1 at the bottom of notch is different in various radii of the notches. (2) In the specimens with various radii of the notches, the limit stress for slip bands initiation σws is almost proportional to the stress σw1, that is, σwsb(0.85∼0.9)σw1. (3) The incipient fatigue cracks, which appear on plain specimens after 107 cycles of the stress σw1, can hardly sustain the tensile stress statically and dynamically.

Sin-ichi Nisida

Papers

Correlation between the Existence of Fatigue Limit and Non-Propagating Micro-Crack

Change in Surface States and Incipient Fatigue Cracks in Electro-Polished Low Carbon Steel (Plain and Notched Specimens) under Rotating Bending Stress