SEM study of overload effects during fatigue crack growth using an image analysing technique and potential drop measures

Abstract

A B S T R A C TTo study the mechanisms affecting the crack propagation rate for fatigue cracks exposed to an overload, an in situ scanning electron microscope technique was used, together with potential drop measurements. High-resolution images were analysed with an image analysis program to measure the displacements along the crack, and the potential drop technique was employed to measure the electrical contact between the fatigue crack surfaces. The crack closure level could, by image analysis, be determined as close as 1 μm from the crack tip. The indications from the image analysis pointed towards a somewhat lower closure load as compared to the potential drop technique. The effect of an overload on the crack propagation rate was found to depend on the magnitude of the overload in combination with the steady-state conditions. Both overload induced crack retardation and crack acceleration was noticed to occur. a = crack length (m) f = geometry function (-) F = applied tensile load (N) F cl,cc = crack closure load level according to compliance measurements (N) F cl,PD = crack closure load level according to potential drop (N) F cl.before,cc = crack closure level before the overload cycle according to cross correlation (N) F cl,after,cc = crack closure level after the overload cycle according to cross correlation (N) F cl.before,PD = crack closure level before the overload cycle according to potential drop (N) F cl.after,PD = crack closure level after the overload cycle according to potential drop (N) F max = maximum tensile load (N) F ol = maximum load for the overload cycle (N) I = direct current (A) K cl = stress intensity factor level at crack closure (Pam 1 2 ) K max = stress intensity factor level at maximum tensile load (Pam 1 2 ) K min = stress intensity factor level at minimum tensile load (Pam 1 2 ) PD-signal = potential drop over the crack tip (V) r m = distance behind the crack tip where the displacements are measured (m) R = applied steady-state load ratio U = portion of the load cycle that the crack is open

Statistical analysis of damage evolution with a new image tool

“…The information of images at earlier time points is not used. By correlating images from different time points as in Refs [15–17], a more precise detection of micro cracks would be possible. However, the analysed images do not always contain the same surface.…”

“…Some tools are only able to detect and treat one crack or very few cracks 11–18 . In particular, investigations of the crack tip are provided by such approaches 13,15–17 . Other tools can only highlight cracks and damage areas 19,20 .…”

“…[11][12][13][14][15][16][17][18] In particular, investigations of the crack tip are provided by such approaches. 13,[15][16][17] Other tools can only highlight cracks and damage areas. 19,20 Damage areas can be easily highlighted by a threshold value, i.e.…”

Online fatigue crack growth monitoring with clip gauge and direct current potential drop

“…On the other side clip gauge output is compared with direct current potential drop (DCPD) measurements and a visual confirmation. DCPD is a method that is more and more used in fatigue applications, because of its flexibility for geometries and environments [4,5]. It has therefore a wide range of possible applications.…”

Overload effect on the fatigue crack propagation in large‐scale tubular joints

“…The increased volume of plastically deformed materials near the crack tip, caused by a tensile overloading, elevates the compressive residual stresses near the tip and subsequently the plasticity‐induced crack closure, which leads to a decreased crack propagation rate 12,13 . Recent research efforts focus on the understanding of the mechanism of a single overloading applied during a constant‐amplitude cyclic test, through the scanned electronic microscopy (SEM) examination, 14 the X‐ray technique, 15 or the neutron diffraction method 16 . Pommier and Freitas 17 have proved, through a combined experimental and numerical investigation, that the retardation effect caused by the overload depends on both the number of the overloads and the frequency of the overloads applied.…”