No abstract

The conditions for non-propagating LEFM type fatigue cracks were investigated on an Inconel, 617 Alloy in the range of K, , , between 10 and 50 MPa mliz under four different types of fatigue loading conditions. In all tests, K,,, was held constant during the fatigue cycling prior to determining the non-propagation condition. It was found that with decreasing range of applied K the fatigue tolerance range A&,, increases. Furthermore, there is slight increase of AK,,,, with decreasing &ax of approximately 2&30% when K,,, is decreased from 50 to 10 MPa m'". The results of the four types of tests are considered in respect to the damage in the near-region of the crack front, i.e. increasing &,, increases the damage zone and therefore decreases the fatigue tolerance range A K , , , , . NOMENCLATURE urn,". umar = Minimum and maximum stress, in a cycle K,,,, K,,, = Minimum and maximum stress intensity factor, of a cycle R = umm/u,,, = Stress ratio AK = K,,, -Kmn = Stress intensity range K,,, = Threshold stress intensity range determined at a growth rate of 10-'orn/cycle Kop = Stress intensity factor at which a fatigue crack opens KeH,,, = Stress intensity range which does not produce fatigue crack growth

Rapid Solidification Processing (RSP), of metals and alloys, is establish by increasing of the undercooling applying high cooling rates (102 - 106 K/s) or by reduce nucleation sites using low cooling rates (1 K/s). Melt undercooling opens new solidification pathways for new nonequilibrium phases and unusual microstructures. Several techniques have been developed to reduce nucleation sites and produce increased undercooling in metals and alloys including the fluxing technique. The aim of this paper is to study the influence of the undercooling level on microstructures of eutectic Sn-57wt%Bi alloy by using the fluxing technique. A morphological change from eutectic to eutectic plus primary dendrites bSn was observed when the undercooling increase from 10 K to 19 K and a refinement of the primary dendrites bSn was observed when the undercooling increase from 19 K to 29 K. Increasing the undercooling led to a higher growth rate, hence morphological refinement occur.

Service-type fatigue loading conditions are critically analyzed with respect to their relevance for a fatigue crack growth threshold. It was found that a threshold should be experimentally determined as a function of the monotonic-plastic zone size (i.e., as a function of Kmax). For each Kmax the cyclic-plastic zone size depends on the applied ΔK and threshold testing should cover the range ΔK ≈ ΔKth and ΔK (R = 0) for tension fatigue. For a given plastic zone, Kmax and ΔK fixed, the threshold ΔKth could occur anywhere between Kmax and Kmin. This study presents experimental procedures and furnishes results for Inconel 617 and a Ti-6A1-4V alloy which allow one to analyze the influence of the three parameters cited above. It demonstrates the following: • The threshold decreases “linearly” with increasing monotonic-plastic zone size; this confirms existing data. • The cyclic-plastic zone has minimal influence on the fatigue threshold with Ti-6A1-4V, while this influence is substantial in the case of Inconel 617. • A shift of threshold between Kmax and Kmin does not alter the magnitude of the threshold.

Fatigue thresholds have been determined for Inconel 617 and Ti-6Al-4V that are unaffected by any closure phenomenon. These results show that a nonpropagation condition exists between Kop and Kmax of unique amplitude ΔKT which is unaffected by the position between Kop and Kmax. This experimental fact opens up the possibility of measuring the dividing point Kop (fixing the ΔKeff) based on a propagation/nonpropagation criterion. Three such test methods are proposed. Results of these measurements are compared with Kop determinations via load-displacement curves made in parallel on Inconel 617. Comparison of both results show that they contradict each other. It is concluded that the two different experimental procedures determine and characterize two independent phenomena affecting the fatigue crack growth process differently. Characterizing the two different phenomena by Kop is incorrect.