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A CONTRIBUTION ON THE PHOTOGRAMMETRIC DETERMINATION OF
THE CRITICAL CRACK-TIP-OPENING DISPLACEMENT
M. GRUBER and O. KOLEDNIK
Technische Universität Graz, Abteilung für Fernerkundung, Bildverarbeitung und Kartographie,
Graz, Austria
Erich Schmid-Institut für Festkôrperphysik der Osterreichischen Akademie der Wissenschaften,
Leoben, Austria
Abstract: The critical crack tip opening displacement (CODi) which characterizes the fracture
toughness of engineering materials is determined by using stereo images from the scanning electron
microscope (SEM). The surface pair of the stretched zone region is analyzed and documented in
three dimensions. Further the parameters of the photogrammetric task and the accuracy of the
measurement are pr esented.
Zusammenfassung: Die quantitative Untersuchung der Bruchoberflächen und die Bestimmung
geometrischer Größen in diesem Bereich, insbesondere der Rißöffnungsverschiebung, ist für die
Erforschung von Bruchvorgängen sehr wichtig. Im folgenden werden zwei Bruchmechanikproben
anhand der vorliegenden rasterelektronenmikroskopischen Aufnahmen untersucht und die
Oberfläche der jeweiligen Bruchhälften photogrammetrisch ausgewertet. Die mikroskopische
Aufnahme erfolgte durch Kippen der Proben stereoskopisch und erlaubt daher eine räumliche
Vermessung. Durch die Identifikation identer Punkte auf den zugehörigen Probenhälften wird die
geometrische Beziehung zwischen beiden Oberflächen hergestellt. Der Vergleich von Querprofilen
an der Rißfront führt zur quantitativen Bestimmung der Rißöffnungsverschiebung, die als
Kenngröße für die Bruchzähigkeit der Probe dienen kann.
Introduction
The fracture toughness of a metallic body depends
primarily on the amount of the plastic deformation energy
which must be put into the body to produce an increment
of crack extension. For a fracture under small-scale
yielding conditions the plastic deformation is confined to a
region close to the new fracture surfaces. The plastic
deformation can be detected by putting together the
broken pieces of the body and measuring the misfit
between the upper and the lower fracture surfaces. This
can be done by making stereo image pairs of the two
fracture surfaces in the scanning electron microscope
(SEM) and analyzing corresponding regions on the
photograms [2]. In [3] this method was applied to measure
the critical crack tip opening displacement CODi, for a
ductile fracture. CODi (which is the maximum
displacement which appears at the tip of a fatigue pre-
crack before the crack begins to grow) is a measure of the
fracture thoughness of the material. It is the purpose of the
current investigation to detect the misfit between the two
fracture surfaces of a transcrystalline brittle (cleavage)
fracture and to determine CODi.
Material
The material investigated is an annealed structural steel
with 0,1796 C. The microstructure consists of ferrite grains
(with a mean intercept length of 17 pm) with small carbide
particles and great number of elongated MnS-inclusions
inbedded.
At room temperature the yield strength and the ultimate
tensile strength were about G^ ys = 298 MPa and
Gurs = 426 MPa. At -196? C the tensile test gave a
fracture strength of G yrs = 990 MPa without any
macroscopic plastic yielding.
Testing Procedure
From this material two fracture mechanics specimens
(CT1-specimens) were machined with a ST-crack plane
orientation, i.e. the MnS-inclusions are directed parallel to
the crack front. Specimen I was both precracked by fatique
and broken up in liquid nitrogen (-196° C), Specimen II
was precracked at room temperature and broken up in
