Initiation Of First Crack Research Articles

Fatigue characterization of embedded layers in CFR Composites

The stress-based approach with experimental S/N curves (stress amplitude vs. number of cycles) is a well-known approach for lifetime prediction of components, also in the composite industry. The problem with this so called "Wöhler" approach is that only final fatal failure of specimen can be described. But for a real definition of end-of-lifetime the identification of failure mechanisms and the corresponding sequence of effects are essential. Besides that for embedded layers, the initiation of first cracks does not lead to failure of the whole specimen.Within this work two different layups with carbon fibre reinforced polymers in UD 90° respectively embedded UD 90° within UD 0° direction was used. Starting from these two layups, a simulation was done for different end-tab-materials and several taper angles to get an overview on the lowest risk factor for breakage at the tab/specimen transition region of the specimen. Regarding the simulation results, specimens were produced and different end tabs were added. Quasi-static as well as fatigue tests were performed and compared based on the calculation with classical laminate theory. Also a comparison between simulation and testing of the crack region for the UD 90° specimen was done. With this work, a base for further variation of specimen geometry was established to monitor the initiation and propagation of (micro-)cracks in the embedded UD 90° layers with different techniques.

Behavior of concrete beams reinforced with hybrid steel and FRP composites

This paper presents a nonlinear finite element model to investigate the behavior of hybrid Fiber Reinforced Polymers and steel reinforcement. Different types of Fiber Reinforced Polymers; CFRP and GFRP; were used along with steel rebars in the studied concrete beams. The study was conducted using the nonlinear finite element program “ANSYS”. Nonlinear material models for the components of the concrete beam were used in the three dimensional finite element models. The outcomes got from finite element analysis were confirmed against experimental results. A broad parametric study was conducted to explore the effect of replacing steel reinforcement by different types of FRP bars. The study showed that the contribution of steel rebars to FRP rebars in concrete beams improved beam ductility and eliminated the unfavorable brittle failure of the concrete beam. In addition, it is better to use steel rebars as top reinforcement in concrete beams with hybrid reinforcement. In hybrid GFRP/steel reinforced concrete beams, a significant reduction in stiffness and a noticeable increase in the beams' deflection after the initiation of first crack and yielding of steel reinforcement were observed. On the other hand, in hybrid CFRP/steel reinforced concrete beams showed a better performance during cracking initiation and propagation.

Structural health monitoring of slab-column connections using FBG sensors

Health monitoring of reinforced concrete structures is a very important technique to maintain structural integrity. Early detection of damage can save lives and costly repairs. The information obtained from monitoring is generally used to plan and design maintenance activities, increase the safety, verify hypotheses, and reduce uncertainty. The current research is focused on the development and optimization of a structural health monitoring concept to detect damage initiation for slab-column connections. The research objective is to detect, localize and quantify flexural (service) cracks and shear cracks in slab-column connections. The experimental program is divided into three phases. The first phase is focused on detection of flexural cracks using embedded fibre Bragg grating (FBG) sensors. Detection of flexural cracks includes the initiation of first crack as well as the initial crack width. The second phase is focused on mapping of crack patterns using FBG sensors arrays. The last phase of the experimental work includes detection and localization of shear cracks as well as quantification of high compressive (bearing) zones using three-dimensional configuration of FBG sensor arrays. The current research presents the first phase of the experimental work. Test results revealed that embedded FBG sensors exhibited excellent performance and it was able to monitor the width and location of the first crack.