ABSTRACT:This paper deals with the determination of crack widths of concrete beams during load tests from monocular image sequences. The procedure starts in a reference image of the probe with suitable surface texture under zero load, where a large number of points is defined by an interest operator. Then a triangulated irregular network is established to connect the points. Image sequences are recorded during load tests with the load increasing continuously or stepwise, or at intermittently changing load. The vertices of the triangles are tracked through the consecutive images of the sequence with sub-pixel accuracy by least squares matching. All triangles are then analyzed for changes by principal strain calculation. For each triangle showing significant strain, a crack width is computed by a thorough geometric analysis of the relative movement of the vertices.

The paper at hand presents an investigation of the tensile behavior of high-strength, strain-hardening cement-based composites (HS-SHCC), reinforced with a single layer of continuous, two-dimensional textile made of ultra-high molecular weight polyethylene (UHMWPE). Uniaxial tension tests were performed on the bare UHMWPE textiles, on plain HS-SHCC, and on the hybrid fiber-reinforced composites. The bond properties between the textile yarns and the surrounding composite were investigated in single-yarn pullout experiments. In order to assess the influence of bond strength between the yarn and HS-SHCC on the tensile behavior of the composites with hybrid fiber reinforcement, the textile samples were analyzed both with, and without, an additional coating of epoxy resin and sand. Compared to the composites reinforced with carbon yarns in previous studies by the authors, the high elongation capacity of the UHMWPE textile established the higher strain capacity of the hybrid fiber-reinforced composites, and showed superior energy absorption capacity up to failure. The UHMWPE textile limited the average crack width in comparison with that of plain HS-SHCC, but led to slightly larger crack widths when compared to equivalent composites reinforced with carbon textile, the reason for which was traced back to the lower Young’s modulus and the higher elongation capacity of the polymer textile.

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An image sequence analysis procedure is developed to quantitatively analyze complex multiple crack patterns in tension tests of fiber-reinforced composite specimens. Planar textured surfaces of such specimens can be observed with a monocular image sequence using a camera of suitable spatial and temporal resolution. Due to the narrow crack paths, a dense high-precision displacement vector field is computed applying least-squares image matching techniques. Some uniformly distributed matching points are triangulated into a mesh. To measure deformations, principal strains and crack widths are computed for each face. Stretched triangles presumably containing one or multiple cracks are subdivided into three new triangles in order to densify the mesh in critical regions. The subdivision is repeated for some iterations. The crack width computation of the triangles requires at least three vertices and its displacements. Due to the dense displacement vector field, there are more points available. In this paper, an algorithm for the crack width computation in a least-squares fit is presented. Keywords Crack width • Multiple crack pattern • Deformation measurement • Triangle mesh Zusammenfassung Strategie zur Rissbreitenmessung multipler Rissstrukturen in monokularen Bildsequenzen bei der Materialprüfung im Bauwesen Zur quantitativen Untersuchung komplexer multipler Rissmuster in Dehnungsversuchen mit Probekörpern bestehend aus faserbewährten Kompositen wird eine auf Bildsequenzanalyse basierende Methode vorgestellt. Planare, texturierte Oberflächen solcher Probekörper können mit monokularen Bildsequenzen unter Nutzung einer Kamera mit geeigneter räumlicher und zeitlicher Auflösung beobachtet werden. Aufgrund der engen Risspfade wird ein dichtes, hochgenaues Verschiebungsvektorfeld mit der Punktverfolgungstechnik der Kleinsten-Quadrate-Anpassung berechnet. Eine gleichverteilte Auswahl der verfolgten Punkte bildet eine ausgedünnte Punktmenge, mit Hilfe derer ein Dreiecksnetz bestimmt wird. Um Deformationen zu messen, werden Hauptdehnungen und Rissbreiten für jede Facette berechnet. Gedehnte Dreiecke, die möglicherweise von einem oder mehreren Risse durchlaufen werden, werden in 3 neue Dreiecke unterteilt, um das Netz in kritischen Regionen zu verdichten. Die Unterteilung wird mehrfach wiederholt. Für die Rissbreitenberechnung von Dreiecken werden mindestens 3 Punkte mit ihren Verschiebungen benötigt. Wegen des dichten Verschiebungsfeldes stehen mehr Punkte zur Verfügung. In dieser Publikation wird ein Algorithmus zur Rissbreitenberechnung in Form einer Ausgleichung vorgestellt.

Knowledge about the interior and exterior camera orientation parameters is required to establish the relationship between 2D image content and 3D object data. Camera calibration is used to determine the interior orientation parameters, which are valid as long as the camera remains stable. However, information about the temporal stability of low-cost cameras due to the physical impact of temperature changes, such as those in smartphones, is still missing. This study investigates on the one hand the influence of heat dissipating smartphone components at the geometric integrity of implemented cameras and on the other hand the impact of ambient temperature changes at the geometry of uncoupled low-cost cameras considering a Raspberry Pi camera module that is exposed to controlled thermal radiation changes. If these impacts are neglected, transferring image measurements into object space will lead to wrong measurements due to high correlations between temperature and camera’s geometric stability. Monte-Carlo simulation is used to simulate temperature-related variations of the interior orientation parameters to assess the extent of potential errors in the 3D data ranging from a few millimetres up to five centimetres on a target in X- and Y-direction. The target is positioned at a distance of 10 m to the camera and the Z-axis is aligned with camera’s depth direction.