A study of overload effect on fatigue crack propagation using EBSD, FIB–DIC and FEM methods

Abstract

Abrupt increase in the maximum load during fatigue cycling modifies the deformation conditions at the crack tip, causing plastic flow that may lead to crack closure, introducing residual stress and hardening. The net consequence of these effects is notable crack growth retardation. Despite decades of research in the field, controversy persists regarding the role of each specific mechanism and their interaction. Resolving these issues with the help of experimental observation is related to the difficulty of obtaining local residual stress information at appropriate resolution. The present study examines the effect of overload on fatigue crack grown in a Compact Tension (CT) specimen of aluminium alloy AA6082 (BS HE30). Fatigue crack was grown in the sample under cyclic tension (R=0.1). After the application of a single overload cycle, fatigue loading was recommenced under previous cycling conditions. The crack morphology was investigated using Scanning Electron Microscopy (SEM). Electron Backscattered Diffraction (EBSD) was used to map grain orientation and crystal lattice distortion (pattern quality) in the vicinity of the crack. EBSD analysis of intra-granular misorientation allowed the qualitative analysis of the region around the crack tip location at the time of the overload application. Observations are discussed with a view to identify the roles of crack closure and residual stress effects. Residual stress was evaluated at salient locations around the crack retardation site using the FIB-DIC method which combines the use of Focused Ion Beam (FIB) and Digital Image Correlation (DIC) for residual stress measurement at the (sub)micron-scale. The residual stress field due to overload occurrence was also simulated using Finite Element Method (FEM), and the results compared with experimental observations.

Fatigue crack growth of composite patch repaired Al-alloy plates under variable amplitude loading

“…These delays cycles increases for single overload "q=1" and band overload "q=10" and are respectively, 2.2210 5 and 2.1910 5 cycles. Also, these delays is attributed to the crack closure effect [44][45][46] under application of multiple variable spectrums with band overload from "q=1" to "q=100" cycles. The relationship between the retard ratio ND/NCLA and band overload "q" is plotted in Fig.…”

Elucidating the Mechanism of Fatigue Crack Acceleration Following the Occurrence of an Underload

“…A 2 mm‐thick miniature Compact Tension (CT) specimen (35 mm) was used for fatigue testing. The same sample geometry was successfully adopted in a previous publication . Cyclic loading was applied using a servo‐hydraulic fatigue rig capable of applying cyclic loading up to 20 kN.…”

“…In recent years, publications devoted to this topic largely concerned cases of OL . It is well known that the application of an anomalous OL during constant amplitude fatigue test induces crack growth retardation during the propagation through the affected region.…”

Untersuchung der Mikroschädigung in Baustahl mit ESPI

“…Dies zeigt sich im bisherigen Forschungsstand, dessen Schwerpunkt auf der Untersuchung des Rissfortschritts liegt. Dabei wurde überwiegend die Digital Image Correlation (DIC) für Untersuchungen des Verformungsverhaltens an der Rissspitze sowie der Auswirkungen von Rissschließen und Überlasten und zur Gewinnung bruchmechanischer Parameter eingesetzt . Vergleichsweise wenige Projekte befassten sich mit der Messung von der Risseinleitung und dem Mikrorisswachstum.…”