X-Ray Microtomography of Fatigue Crack Closure as a Function of Applied Load in Al-Li 2090 T8E41 Samples
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ABSTRACT Crack closure is held to be responsible for very low fatigue crack growth rates in many alloys such as Al-Li 2090 T8E41, and early crack face contact during unloading or prolonged contact during loading seems to reduce the driving "force" for crack extension. High resolution x-ray computed tomography (i.e., microtomography) allows one to image the entire volumes of samples and to quantify opening as a function of applied load over the entire crack surface. Crack closure results are reported for a fatigue crack grown under load ratio R=0. 1 in a compact tension sample of Al-Li 2090 T8E41; the crack was free to choose its path unconstrained by side-grooves which are normally used to suppress crack deflection. The inter-relationship between crack path, crack face contact and applied load level are discussed. INTRODUCTION Under ambient conditions, fatigue crack growth rates in L-T oriented compact tension samples of Al-Li 2090 T8E41 (i.e., the load axis along the plate's rolling direction L and crack growth along the plate's long-transverse direction T) are typically an order of magnitude lower than those of other Al alloys [1,2]. In the absence of side-grooves, fatigue cracks in samples of this near peak aged Al-Cu-Li-Zr alloy often grow with such a strong mode II component that ASTM standard test conditions are violated, and comparison with data from samples of different alloys
with "better" behaving cracks is problematic. The rough surfaces of fatigue cracks in Al-Li 2090 T8E41 are dominated by large asperities (peaks on one crack face and matching valleys opposite) and by significant deflections from the nominal crack plane defined by the notch. The center of thick plates of Al-Li 2090 T8E41 have very different textures and much lower fatigue crack growth rates than the outer surfaces [3, 4]. Therefore, the processes slowing crack growth in the plate centers are controlling fatigue crack propagation resistance. The geometry of large asperities in the plate centers correlates with the average texture in this volume [2-4], and the large asperities appear to form when the crack cuts through groups of five to ten adjacent pancake-shaped grains with nearly identical crystallographic orientations [5-7]. One result of the crack face roughness in Al-Li 2090 T8E41 is fatigue crack closure, the phenomenon where crack faces come into contact prematurely during unloading of the sample (i.e., before the minimum stress of the fatigue cycle is reached) or where the crack faces remain in contact much longer than expected during unloading [8-11]. Roughness induced fatigue crack closure in Al-Li 2090 T8E41 samples was quantified using changes in the slope of load-deflection curves [1, 4], and x-ray microtomography has been used to quantify where and at what stresses the crack faces contact [12-15]. Conventionally, the closure's effect on crack propagation is rationalized by using AK•ff = K.
- Kd instead of AK = K.
- K,,,, , where the subscripts denote
the maximum (of a fatigue cycle), closure and minimum stress intensit
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