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

Abstract

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 "characteristic" strain was estimated, in the bulk of the 2 specimen and on the specimen surface, from the average strains captured at selected positions when the propagating fatigue crack tip reached these positions.

J-integral analysis: An EDXD and DIC comparative study for a fatigue crack

“…The mean value across all the detectors was used to map FWHM at each ( , ) coordinate measurement point. The maximum value of FWHM was quite localised (Figure 5b), and a cross-correlation algorithm could be used for efficient subpixel image registration between the FMHM maps [16]. This allowed changes in the position of the FWHM peak (or maximum spatial value) to be tracked with subpixel accuracy (1 pixel = 100 µm).…”

“…Full details of the experiment that provided the raw data analysed in this work have been published elsewhere [16], and are summarised briefly here. The test specimen was fabricated from a HSLA ('high strength low alloy', yield strength ~800 MPa) steel weld joint (Cr2Ni4MoV), which had been annealed at 550°C for 40 hours to relieve the welding residual stresses.…”

Full‐field characterisation of crack tip deformation and fatigue crack growth using digital image correlation—a review

“…Near‐tip strain accumulation has been monitored in situ during cyclic loading, and quantitative experimental evidence of normal strain ratchetting has been found ahead of the crack tip, in support of a hypothesis of strain ratchetting as a mechanism for fatigue crack growth . In addition, strains have been monitored at selected, fixed observation points in the field of a growing fatigue crack, and a critical or onset strain was identified from instantaneous strains at the crack tip as the crack approached the observation points (Figure ). Such information is significant towards discovering a local crack driving force, albeit within a continuum mechanics domain.…”

“…The normal strains recorded, in situ, as a function of cycles, tracked at selected positions on the crack path (A) at a stress intensity of Δ K = 30 MPa√m. The black circles indicate the onset strains when the crack tip reached the measurement positions [Colour figure can be viewed at wileyonlinelibrary.com]…”

Analysis of the correlation between micro-mechanical fields and fatigue crack propagation path in nodular cast iron

“…On the other hand, experimental characterization of the full 3D deformation field at the level of the microstructure is very challenging. The reason is that the limited number of matrix grains between neighboring nodules precludes the application of energy dispersive X-ray diffraction [21], whereas measurements using X-ray Laue micro-diffraction [22,23] require long time to probe a volume sufficiently large to be statistically relevant and this may lead to stress relaxation during the measurement. In addition, absorption tomography combined with digital volume correlation, which have proved to be very effective for the determination of the local stress intensity factor in SGI [24][25][26], lacks spatial resolution if the nodules alone are used as markers.…”