TY - JOUR A1 - Fischer, G. A1 - Nellesen, J. A1 - Anar, N. B. A1 - Ehrig, Karsten A1 - Riesemeier, Heinrich A1 - Tillmann, W. T1 - 3D analysis of micro-deformation in VHCF-loaded nodular cast iron by µCT N2 - The impact of very high cycle fatigue (VHCF) load conditions on the microstructure of specimens consisting of nodular cast iron is analyzed by means of micro-computed tomography (µCT) utilizing both monochromatic synchrotron radiation and polychromatic X-ray tube radiation. Using 3D µCT, the microstructure in the region of the smallest cross-sections of shouldered round specimens is imaged in different stages of the VHCF loading. By digital image correlation (DIC) of these tomograms strain fields are analyzed three-dimensionally. Strain levels in the range of a few percent were detected. It is proven that a localization of strain allows to predict the site of the crack which precedes and induces the macroscopic failure of the specimens. KW - VHCF (very high cycle fatigue) KW - µCT KW - Micro-computed tomography KW - Strain analysis KW - Nodular cast iron PY - 2013 U6 - https://doi.org/10.1016/j.msea.2013.04.057 SN - 0921-5093 SN - 1873-4936 VL - 577 SP - 202 EP - 209 PB - Elsevier CY - Amsterdam AN - OPUS4-28533 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Shokouhi, Parisa A1 - Zoëga, Andreas A1 - Wiggenhauser, Herbert A1 - Fischer, G. T1 - Macro-effect of microcracks on sonic surface wave velocity in concrete under compression T2 - 89th TRB Annual Meeting CY - Washington, D.C., USA DA - 2010-01-10 KW - NDT KW - Microcracking KW - Sonic wave velocity KW - Concrete KW - Load KW - Stress PY - 2010 SP - 1 EP - 17 AN - OPUS4-20942 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Fischer, G. A1 - Bohse, Jürgen T1 - Observation and analysis of fracture processes in concrete with acoustic emission (AE) and digital image correlation (DIC) N2 - Fracture processes in concrete can be characterized by the formation of a Fracture Process Zone (FPZ), which is a region of the crack extending between the elastic region ahead of the crack tip over the crack bridging zone to the region where the crack opening is sufficiently large to prevent transfer of load across the crack faces. The formation of cracks and the development of the FPZ have typically been documented by Acoustic Emission (AE) methods and important conclusions regarding the nature of the FPZ and the propagation mechanisms of concrete have been drawn to form the basis of current fracture models for concrete. The study presented in this paper focuses on Mode I cracking of concrete using compact tension specimens and is comparing the results of AE measurements to those obtained from documenting the cracking process by Digital Image Correlation (DIC). The findings from this comparison show that distinctly different AE events occur ahead of the crack tip, in the cementitious matrix at the crack tip and in the wake of the crack due to the increasing separation of the crack flanks and further opening of the crack. The DIC measurements indicate that crack initiation occurs with locally corresponding AE signals and furthermore suggest a continuous path of the crack from initiation to eventual transition to the stress-free zone. Based on these comparative measurements the study suggests that crack formation in unreinforced concrete is initiated by an individual, sharp microcrack rather than by a region of diffuse microcracking ahead of the eventual crack tip. Later on sharp crack branches originate from the main macrocrack path. Furthermore, the measurements with AE and DIC result in information on the nature of the deformation mechanisms occurring in distinct regions of the entire cracking process. AE signals detected using wideband sensors show quite different characteristics in time (waveform) and frequency (bandwidth) domain. T2 - 31st Conference of the European working group on acoustic emission (EWGAE) CY - Dresden, Germany DA - 03.09.2014 KW - Concrete KW - Acoustic emission KW - Digital image correlation KW - Signal analysis KW - Fracture process KW - Source mechanisms PY - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-313471 SN - 978-3-940283-63-4 IS - DGZfP-BB 149 SP - Th.3.A.4, 1 EP - 8 PB - BAM / DGZfP AN - OPUS4-31347 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Shokouhi, Parisa A1 - Zoëga, Andreas A1 - Wiggenhauser, Herbert A1 - Fischer, G. T1 - Surface wave velocity-stress relationship in uniaxially loaded concrete N2 - The sonic surface wave (or Rayleigh wave) velocity measured on prismatic concrete specimens under uniaxial compression was found to be highly stress-dependent. At low stress levels, the acoustoelastic effect and the closure of existing microcracks results in a gradual increase in surface wave velocities. At higher stress levels, concrete suffers irrecoverable damage: the existing microcracks widen and coalesce and new microcracks form. This progressive damage process leads first to the flattening and eventually the drop in the velocity-stress curves. Measurements on specimens undergoing several loading cycles revealed that the velocities show a stress-memory effect in good agreement with the Kaiser effect. Comparing the velocities measured during loading and unloading, the effects of stress and damage on the measured velocities could be differentiated. Moreover, the stress dependency of surface wave velocity proved to be direction-dependent. The velocity increases and decreases the most when measured parallel and perpendicular to the loading axis, respectively. KW - Acoustic emission KW - Acoustoelasticity KW - Microcracking KW - Surface waves KW - Velocity measurement PY - 2012 SN - 0889-325x VL - 109 IS - 2 SP - 141 EP - 148 PB - ACI CY - Farmington, Mich., USA AN - OPUS4-27999 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -