Crack Nucleation in Persistent Slipbands

Abstract

The development of persistent slipband topography, crack nucleation, and crack growth in fatigued copper single crystals was studied by an new section technique, which reveals surface topographies with a resolution of 20 nm. Experiments were performed in air, oxygen, hydrogen, water vapor, high vacuum, and ultrahigh vacuum in order to study the influence of the environment on these processes. The development of persistent slipband topography, including intrusions, extrusions, and the protrusion of the whole persistent slipband, does not depend on environment. Crack nucleation and early propagation, however, do strongly depend on the environment. Intrusions can also be distinguished from crack nuclei by their finite vertex angle of approximately 30 deg. Quantitative data of the average protrusion growth as well as crack growth are presented. Chains of large voids were found in high vacuum and ultrahigh vacuum along the trace of the primary slip plane and are interpreted as remnants of rewelded Stage I cracks.

Mechanisms of strain localization, crack initiation and fracture of polycrystalline copper in the VHCF regime

“…Voids are formed in the interior as well as in grain boundaries and close to the specimen surface and agglomerate preferentially in the direction of maximum shear stress and form small stage I shear cracks as detected in [11]. Similar elongated voids have been observed by Hunsche and Neumann, who used the taper sectioning technique and found "chains of large voids" along the trace of primary slip planes in copper single crystals [13]. Deep intrusions and, respectively, small stage I cracks especially at slip band borders, are formed in rather early stages of fatigue loading at each specified cyclic stress, as observed in SEM and AFM studies [7,9] and are assumed to be identical with those observed with FIB and ECC/SEM [11].…”

“…But they may also result from the different ramp start procedure (with ten times higher numbers of cycles) and the ramping of each pulse in the ultrasound experiment, leading to a reduction of the localized strain in the PSBs [6,14], though no influence of a variation of the ramp load length on 'H pl /2 could be detected in the calibration experiments [7,9]. The value of 63 MPa corresponds well with the shear plateau stress of 32 MPa of Neumann for copper single crystals, which were also strongly ramp-loaded [13,14] and results of Yan and Laird [6] and Wang and Laird [15] on enhanced strain localization by ramp loading.…”

“…The "VHCF" as well as "conventional"-PSB thresholds may be considered as thresholds for the formation of PSBs and small cracks at specified numbers of cycles, but higher stresses are necessary to form a "long" crack. (For the difference of PSB and small crack formation see [13]). Measurement of the S-N and Coffin-Manson diagram revealed approximately 100% higher stress/strain amplitudes needed for failure [8,10,12] i.e.…”

Low-strain fatigue in 316L steel surface grains: a three dimension discrete dislocation dynamics modelling of the early cycles. Part 2: Persistent slip markings and micro-crack nucleation

“…The inert environment is reported to have little influence on the kinetics of surface relief formation though it significantly affects crack initiation and propagation [30][31][32][33]. In this study, environmental effects are not accounted for.…”

Extrusions and intrusions in fatigued metals. Part 1. State of the art and history†

“…For obtaining quantitative data on the height of extrusions and/or the depth of intrusions using SEM several special methods were devised (see table 1). Among them two sophisticated techniques provide the most impressive experimental results -section micromilling technique by Hunsche and Neumann [41,42,75] and sharp-corner polishing technique by Basinski and Basinski [43,44,73]. Both methods allow the preparation of cross sections perpendicular to the specimen surface without measurable distortion of the resulting edge and thus provide the sharp true surface profiles of PSMs and initiated fatigue cracks.…”