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Eingeladener Vortrag
- nein (9)
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.
One trend in today’s structural design for automobiles is towards application of stainless Steel in addition to traditional carbon Steel. This is mainly due to the advantages offered by stainless steel to the designer, i.e. its high corrosion resistance, heat resistance, strength and formability. Welding of austenitic stainless steel involves a high risk of solidification cracking due to low melting eutectics containing different kinds of impurities and to accumulated strain behind the weld pooi in the mushy zone. As a result, the quality of stainless steel welded joints will greatly be affected which may give cause for safety concerns. The intention of this study was to investigate the hot cracking sensitivity of laser welded fully austenitic stainless steels and to determine the critical local strain and strain rate from the formation of solidification cracks. C02-laser welding experiments were conducted using the steel X8CrMnNi19-6-3 (Nirosta H400). Solidification cracking susceptibility was examined with the help of the Controlled Tensile Weldability (CTW) Test developed by BAM Federal Institute for Materials Research and Testing in Berlin. The critical strain and strain rate of solidification cracking was measured at the surface of the workpiece directed to the laser beam in the close vicinity of the weld pool by using the Digital Image Correlation (DIC) technique and diode laser as the illuminating source.
The purpose of this article is to demonstrate how the position of a package affects the results in guided and unguided compression tests. Digital image correlation (DIC) shows how localized stresses occur. The positions of the jerricans were moved 1 cm in every direction from the center. The different positions showed how guided and unguided compression applied different loads to thejerrican.
Furthermore, the guided and unguided compressions were compared directly using DIC to show how areas of stress concentrations formed and the localized stress changed. In general, when testing the jerricans with unguided compression, the swivel platen tilted backwards because the rear side of the jerrican was not as stiff as the front side with the lid. Guided and unguided compression tests will give different results because the stress concentrations are different. The position of the jerrican in the stacking test has drastic affects on the outcome of the fest. The stresses between the jerrican bottom and stacking plate were analyzed with a sensor mat and compared the guided and unguided loads.
Guided and unguided compression tests were performed on jerricans and small and large corrugated fiberboard boxes. The different methods produced different results affecting whether the test was passed
or failed.
Surface-applied fibre optic strain sensors were investigated using a unique validation facility equipped with application-independent optical reference systems. First, different adhesives for the sensor's application were analysed regarding their material properties. Measurements resulting from conventional measurement techniques, such as thermo-mechanical analysis and dynamic mechanical analysis, were compared with measurements resulting from digital image correlation, which has the advantage of being a non-contact technique. Second, fibre optic strain sensors were applied to test specimens with the selected adhesives. Their strain-transfer mechanism was analysed in comparison with conventional strain gauges. Relative movements between the applied sensor and the test specimen were visualized easily using optical reference methods, digital image correlation and electronic speckle pattern interferometry. Conventional strain gauges showed limited opportunities for an objective strain-transfer analysis because they are also affected by application conditions.
In-situ analysis of solid state phase transformation in TRIP-aided steels by synchrotron diffraction
(2011)
Energy dispersive synchrotron diffraction (EDXRD) analysis and 3 dimensional digital image correlations were conducted to investigate the stress and strain effected transformation behavior during tensile loading of low alloyed TRansformation Induced Plasticity (TRIP) steel. This technique allowed for phase specific stress measurement during certain tensile load steps in the elastic and also plastic regime. Additionally the simultaneous determination of the load dependent phase content was realized. The results show that the martensite transformation starts only after exceeding the overall yield point and is finished before reaching the uniform elongation, whereas a large portion of the austenite remains unchanged in the structure. Furthermore, the martensite transformation related to the stress in the γ-phase and α-phase was analyzed and quantified.
Direct measurement and quantification of phase transformation in a low-alloyed transformation induced plasticity steels depending on the tensile load as well as determination of the real true stress and true strain values were carried out in-situ using high energy synchrotron radiation. Digital image correlation technique was used to quantify more precisely the true strain values. The aim of the work was to obtain a better understanding of the phase transformation of commercial low-alloyed transformation induced plasticity steel depending on the true strain and true stress values.