Issue 39

S. Seitl et alii, Frattura ed Integrità Strutturale, 39 (2017) 110-117; DOI: 10.3221/IGF-ESIS.39.12 111 concrete, and the differences between the results from the three-point bend tests and wedge splitting tests. K EYWORDS . Fatigue crack behaviour; Three-point bending test; Wedge splitting test; Self-compacting concrete; Paris-Erdogan law. I NTRODUCTION hree-point bending tests (3PBT) as well as wedge splitting tests (WST) are often used to determine the fracture properties of structural materials such as cement based composites [8]. The material properties of normal, vibrated concrete (VC) and its behaviour under static loading are thoroughly researched and well known [9]. However, in order to reliably predict the behaviour of concrete in applications which involve millions of load cycles (e.g. bridges, beam cranes, offshore constructions), more research is required. For example, worldwide there are numerous concrete bridges that suffered excessive multi-decade deflections, some of which have already collapsed [2]. Fatigue behaviour in concrete is a complex process, and even though a tremendous effort has been made by the international scientific community, no universally accepted strategy suitable to efficiently perform the fatigue assessment of concrete has been agreed yet [4, 25, 27, 28]. This article aims to evaluate and compare the fatigue crack propagation rate in VC for different stress ratios using the Paris- Erdogan law [22]. The comparison is based on 3PBT and WST data for four different stress ratios, and was obtained during the research of Korte et al [11-14] . The test data was obtained from static tests (strength of material, fracture toughness, Young’s modulus and Poisson ratio), and by performing cyclic tests on notched specimens, while measuring the crack mouth opening displacement (CMOD) for each load cycle. In this research, finite element analysis software ANSYS [1] was then used to correlate the measured CMOD data with the Paris-Erdogan crack propagation law [22, 27]. Herein, the crack propagation rate d a /d N is plotted against the corresponding stress intensity range ∆ K in a log-log graph. In a final step, the Paris law parameters C and m were obtained through linear curve fitting the data points from these obtained graphs. These parameters are then used to compare and evaluate the fatigue crack behaviour under the four stress ratios, as well as to compare the results from the three-point bend and the wedge splitting tests and data later could be used as input parameters for simulation e.g. ATENA [23, 24] or DOProC [15-16]. T HEORETICAL BACKGROUND atigue may be defined as a process of progressive, permanent internal structural changes in a material subjected to repeated loading. In concrete, these changes are mainly associated with the progressive growth of internal micro cracks, which results in a significant increase of irrecoverable damage [19]. Each load cycle induces microscopic cracks in the cement matrix, which gradually propagate during the loading process until an extended crack pattern is formed, leading to a significant change of the material properties [2]. In this article, the results of cyclic test on VC specimens, subjected to four different stress ratios are evaluated and discussed. In each test, the specimen was subjected to sinusoidal load function until failure, while measuring the CMOD at the crack mouth for each cycle, using a clip gauge. The 3PBT on single edge notched beams is a useful configuration for fracture toughness testing since it can be easily shaped and tested. For the test specimens, a value of S/W=3 was used in which S is the span between the supports, and W the depth of the specimen. Its geometry is included in all international standards for fracture toughness testing [7]. The WST on the other hand was first introduced by Linsbauer and Tschegg [20] and further developed by Brühwiler and Wittmann [5]. It is an interesting test setup, and just like the 3PBT, it can be performed using an ordinary electromechanical testing machine with a constant actuator displacement [26]. The geometry and measurements of the tested 3PBT and WST specimens are given in Fig.. 1. In 1963, Paris and Erdogan proposed a very simple, yet highly useable relationship between the rate of crack propagation d a /d N and the stress intensity range ∆ K , expressed as [22]: m da C K dN   (1) T F