Flat Samples Fatigue limit stress as a function of notch depth for the flat specimens subjected to a static indent with a 2 mm diameter chisel at a stress ratio (R) of 0.1 is shown in Figure 4. Data from this investigation, denoted MLS, are shown alongside those from a prior investigation using the same material and indentor, denoted AFIT [2]. While there are not many data points, it can be seen that the AR specimens (hollow symbols) clearly show a larger debit in fatigue strength. The greatest fatigue debit occurs in those specimens that exhibit a loss of mass through chipping, a finding consistent with prior observations by Kaufman and Meyer [10] and Martinez [11]. In the plot, the expected failure stress from a fatigue notch factor, Kf, is shown, based on the assumption that the radius of the indent is that of the indentor. 0 100 200 300 400 500 600 0 200 400 600 800 1000 R=0.1 Flat MLS AR AFIT AR MLS SR Calc from Kf Fatigue limit stress, MPa Notch depth, Figure 4: Effect of notch depth on the fatigue limit stress for R=0.1 on quasi-static impacted flat samples. Role of Residual Stresses Similar to what happens when compressive residual stresses are applied under surface treatments such as in laser shock peening [12], the beneficial effects of residual stresses seem to be more prominent at low stress ratios than at high ones. It is important to note that a residual stress superimposed on the stresses generated from fatigue loading does not change the stress range (or stress intensity factor range, ∆K, in the case of a cracked body) but, rather, changes the values of the maximum and minimum stresses by the same amount. Thus, compressive residual stresses superimposed on tensile fatigue stresses at a particular value of R will reduce the effective value of R for crack initiation. For low values of applied R, the effective R may be negative. Recent unpublished data from our laboratory indicate that the range of compressive stresses is much less effective than the same range in tension in high cycle fatigue. For the crack propagation phase after initiation, the compression region does not contribute to crack growth. For the situation where the residual stresses are in tension, the effective value of R increases although the stress range remains unchanged. In this case, tests at increasingly positive values of R can be detrimental because the magnitude of the fatigue limit stress decreases with increasing values of R, as evident in a Goodman diagram [5]. Further, any beneficial effects of compression for originally negative R can be removed. SUMMARY AND CONCLUSIONS Observations in this investigation and previous work [3,4] indicate that scatter in fatigue strength data involving FOD from ballistic impact, and even quasi-static indentations, have a tendency to obscure trends in causes of observed behavior. Nonetheless, the data obtained for fatigue limit strength on LE specimens indicate that the damage inflicted, excluding cases
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