Fractographic reconstitution of fatigue crack history – Part II

Abstract

A B S T R A C T The most valuable quantitative information contained in fatigue fracture surfaces is related to striations. An alternative source of information, in particular if striation identification is impossible, can be the general characteristics of fracture micromorphology evaluated by digital image analysis. For study of micrographs as image textures, new methods called textural fractography have been developed. In this paper, some of these methods and examples of their application are described. A generalization for the case of a variable cycle loading is proposed, based on a new concept of counting loading cycles. The presented work foreshadows the potential contribution of textural fractography for fractographic reconstitution of fatigue crack history.Keywords fatigue crack growth; image texture; quantitative fractography; variable cycle loading. N O M E N C L A T U R ES a = crack length a = crack increment in one cycle a cc = crack increment under constant cycle loading a real = crack increment under general loading B = harmonic wave C = vector of regression constants F = loading force F = matrix of image features f ij = j-th feature of i-th image h = effectivity of a loading cycle L = vector of logs of crack rates assigned to images N = number of loading cycles N ref = reference number of loading cycles p = period (wavelength) of a harmonic wave v = macroscopic fatigue crack growth rate v ref = RCGR, reference fatigue crack growth rate X = image of fracture surface x ij = local image brightness θ = orientation of a harmonic wave I N T R O D U C T I O NThe knowledge of the real history of a fatigue process is very valuable for design, development and reliable operation of structures exposed to time-variable loading service conditions. Information of this type can be obtained with

Fatigue life prediction of structural steel using acoustic birefringence and characteristics of persistent slip bands

“…7 Therefore, many researchers use various types of fractal dimensions to assess fatigue damage. 7,[9][10][11][12][13][14][15][16] A brief description of digital processing images and calculating the fractal dimension is given below. More details related to software and algorithms for processing and computing can be found in Kabaldin et al 15,16 To eliminate the influence of noise and uneven illumination, a two-dimensional wavelet function was used for image processing.…”

“…7,10,13 Textural fractography has been developed for study of fracture micromorphology. 9 Fractal dimensions of the deformation relief can be used for quantitative assessment of accumulated fatigue damage and remaining life both at the crack initiation stage and the fatigue crack propagation stage. 11,14 3D fractal dimension of corroded surface can be used to predict residual fatigue live of precorroded specimens.…”

Fractographic reconstitution of fatigue crack growth in integrally stiffened panels

“…For constant cycle loading, B = 1, and b = 0. (Another definition of RCGR called physical solution [8] is based on a cycle-by-cycle crack growth model).…”

“…When the linear equation is not suitable, features may be replaced or supplemented by transformed ones, for example f a , logf, etc. Non-significant features from the set f k , k = 1,...u, may be excluded, e.g., by the t-test.For the cases of variable cycle loading, the concept of Reference crack growth rate (RCGR, v ref )[7,8] was proposed with the aim to unify quantitative fractography of all fatigue fractures. The simplest version of this approach may be formulated in following statements:For all cases of fatigue under constant cycle loading, RCGR is equal to the conventional CGR defined as v = a/ N. RCGR is unambiguously related to certain features which are present in all fatigue fractures.…”

Estimation of Fatigue Crack Growth Rate in Heat-Resistant Steel by Processing of Digital Images of Fracture Surfaces

“…Approaches to the quantitative analysis of FS are described in a series of monographs [27][28][29]. According to established notions, the FS pattern on the fracture surfaces is associated with crack increments in each of the load cycles of the element [30][31][32]. Fractographic approaches are most often used to estimate the FCG rate in aluminum and titanium alloys, which are important for aircraft construction.…”