5 research outputs found
Characterisation of fatigue crack tip field in the presence of significant plasticity
Characterisation of a fatigue crack tip in the presence of significant plasticity has been challenging due to the lack of suitable tools and lack of knowledge of material constitutive information under cyclic loading. In this paper, Digital Image Correlation (DIC) and integrated finite element (FE) analyses have been used to characterise the crack-tip field beyond the small-scale yielding (SSY) regime in a stainless steel 316L of a compact-tension (CT) specimen under mode I loading conditions. The non-linear characteristics of the near-tip deformation field were verified by the poor fit to the Williamās regression and the overestimation of the stress intensity factor K. The extent of the crack tip plasticity was estimated using a detailed constitutive material model and compared with the estimated by Irwin. The displacement fields local to a stationary fatigue crack were mapped using DIC, and inputted into the FE model as boundary conditions so that an integrated FE analysis was carried out. Fatigue pre-cracking was simulated in the FE analysis prior to the full-field analysis of the fatigue crack tip, including stress/strain distributions ahead of the crack tip and the crack opening displacement (COD) under selected loading conditions. Although a distinct ākneeā was captured as an indication of crack opening from the compliance curves in both the DIC measurements and the FE analyses, consistent with the existing knowledge on the phenomenon of crack closure, it does not appear to correlate with the crack driving force measured by the J-integral
Fatigue crack closure: a myth or a misconception?
In this paper, we have extended our previous study on fatigue crack closure to examine the phenomenon of crack opening displacement (COD) and its impact on the crack tip fields in both 2D and 3D specimen geometries using fullāfield experimental measurements and integrated finite element modelling. Digital image correlation (DIC) and digital volume correlation (DVC) were used to measure the nearātip material responses on the surfaces (DIC) and the interior (DVC) of the specimens. Materials with elasticāplastic and large plastic characteristics were chosen for the study, where plasticityāinduced premature contact between the crack flanks is known to occur. Displacement maps around the cracks were obtained using DIC and DVC at selected load increments and were introduced as boundary conditions into the finite element (FE) models to obtain the āeffectiveā crack driving force in terms of Jāintegral, and the results were compared with those ānominalā from the standard FE analysis. Both visual observation and compliance curves were used to determine the ācrack openingā levels; whilst the impacts of the crack opening on the crack driving force J and the normal strains ahead of the crack tip were evaluated in 2D and 3D. The results from the study indicate that, crack closure, although clearly identifiable in the compliance curves, does not appear to impact on global crack driving force, such as Jāintegral, or strains ahead of the crack tip; hence, it may well be a misconception
In situ mapping of normal strains in the field of a growing fatigue crack in a steel weld using digital image correlation and energy dispersive synchrotron X-ray diffraction
Fatigue crack growth in the Heat Affected Zone (HAZ) of a welded CrNiMoV steel has been
investigated, in situ, using Digital Image Correlation (DIC) and Energy Dispersive X-Ray Diffraction
(EDXD). Compact tension specimens of welded joint were loaded under tension-tension cyclic
loading. The crack tip position and the evolution of the near-tip strains normal to the crack plane
were tracked at selected positions for a growing fatigue crack both on the specimen surface using DIC
and in the bulk of the specimen using EDXD of synchrotron X-rays. The uncertainty and quality of
the DIC measurements were examined with regard to some of the key data processing parameters,
including subset size and the size of measurement window; whilst the measurement errors in the EDXD measurements were also estimated. A ācharacteristicā strain was estimated, in the bulk of the
specimen and on the specimen surface, from the average strains captured at selected positions when
the propagating fatigue crack tip reached these positions