Illuminating Histories: Cracking Open the Boundaries of the Church

In order to identify the effect of crack depth and opening on the girder, finite element method (FEM) has been used in this paper. In FE analysis, six nodded two dimensional plane elements (PLANE-2) are considered. Each node has two degree of freedom such as UX and UY. For the plane elements, a plane stress width/thickness option is chosen. For analytical model of crack of the concrete bridge girder, crack opening was increased from 0.2 mm to 1mm at an interval 0.2 mm and crack depth also increased from 30 mm to 150 mm at an interval 30 mm. The models were discreatized by a triangular mesh and convergence test was executed to obtain satisfactory results from the Plane-2 element. From the numerical result, it is seen that the principal stress become a higher with increased the crack depth and also crack opening with respect to load increasing. But the crack depth at 90 mm and crack opening at 0.6 mm, it has more effect on the girder because the stress concentration is higher than other crack depth and opening.

Experimental studies considering the dynamic transient signals of a notched rotor (i.e. a rotor with a transverse open crack) are investigated in order to examine the possibility of detecting the presence of open cracks in rotating machinery for low or high rotor accelerations. Firstly, the orbital patterns, spectrum cascade plots of the Power Spectral Density, evolutions of the 2× amplitudes of rotor with and without an open transverse crack are compared and discussed in the case of low accelerations of the rotor system. Secondly, the non-stationary vibration signals of the rotor with an open transverse crack at high accelerations are investigated. The timefrequency features and a tool based on the Continuous Wavelet Transform (CWT) to detect open cracks in a rotor system are discussed. It is demonstrated that both the Continuous Wavelet Transform (CWT) and changes on the 2× harmonic components are robust indicators for crack detection.

Comparison studies of open and breathing crack detections of a beam-like bridge subjected to a moving vehicle

On the finite element modeling of the asymmetric cracked rotor

Fracture due to fatigue crack growth remains a significant failure mode in both brittle and ductile materials. When dealing with crack tip plasticity induced phenomena, characterized by high strain and stress field gradients, only highly refined meshes around the crack tip can produce accurate results. Therefore, optimized mesh parameters must be used, in order to achieve high quality models with low computational costs. In this study, artificial intelligence models and a numerical three-dimensional model for a middle tension specimen were combined to enhance crack closure and opening loads assessment. The numerical accuracy was analysed based on the estimated stress and strain fields, plastic zone shape and size and crack closure and opening load values. Two artificial neural networks were trained using four different crack lengths, mesh sizes and simulated plastic wakes. The networks were capable of stress and strain field predictions and crack opening and closure load determination. It was verified that the crack stress criterion is strongly correlated with the principal strain field and the displacement field around the crack tip, providing a viable way to analyse plasticity induced crack closure.

Crack detection and monitoring has become one of a focus center for the health monitoring of engineering structures under the influence of mechanical deformation.This paper discusses the detecting, tracking and monitoring mechanisms for the crack, by initiating a sensing system by using a standard notched specimen. Crack features such as magnitude of crack travel versus crack opening are displacement indicators that can vary due to the applied load on the normal line to the direction of the crack travel. More ever, the rate of the crack opening is considered to be proportional to the rate of the crack travel.Lines of three optical fibers of 125 nm are embedded and fixed at two ends and pass across the direction of the crack travel at an approximation of 2mm apart. The light of 2V from the source is passed through the optical fiber, across the crack tip. The crack opening forces the optical fiber to extend under tension, and hence the interruption of its diameter. This causes the decrease in the light intensity traveling through the fiber till the total failure of the fiber, to declare presence of crack. The embedded fiber optics on the specimen links the instrumentation optical system which permits the light beam throughput. Finally the converted light beam into voltage domain allowed us to closely analyze the crack tendency, and eventually be able to establish a model and sensor equation which governs the trend of the crack growth.

Influence of work-hardening on fatigue crack growth, effective threshold and crack opening behavior in the nickel-based superalloy Inconel 718

To measure crack propagation in compact tension specimen, many methods can be used. The electric drop potential measurement is one of them and allows the detection of crack initiation. In our case, CT specimens, which have been taken from a carbon steel pipe (Tu42C) used in the secondary circuit of French PWR, are employed for cyclic tearing test. The detection of crack closure and crack opening should provide information for energetic analysis. However, the electric signal is unusable due to the cyclic loading. Indeed, because of the clearance between the pin and the specimen, each direction loading change causes a discontinuity in the signal. The roughness of the lips surface or the crack closure during compression loading returns also an unusual signal. Moreover, local measurement is required and there is high strain level around the crack tips, so strain gages are not suitable. Thus, displacement field are measured with digital image correlation and a specific image acquisition is employed. These methods allow a direct measurement of strain fields on the surface of the specimen. Thereby an interpretation of the previous electric signal and the crack opening and closure detection is realizable. Then, F.E. simulation, with non-linear kinematic hardening and node release method, are performed. These simulations allow the check of crack opening and closure detection through the specimen thickness.

Characterizing the shape and heat production of open vertical cracks in burst vibrothermography experiments

Propagation of fatigue cracks is one of the major causes for catastrophic damage in rotor systems. Such cracks have an unusual property in that they can open and close continuously and synchronously with the shaft rotational speed which was termed as a breathing mechanism. This problem has not been adequately analyzed from nonlinear and rotor dynamic perspectives in the literature in spite of a large number of papers that have appeared over the last two decades. Separately from this, backward whirl (where the precession is opposite to the rotational direction) can be dangerous and can lead to catastrophic failures in rotor systems. Interestingly, a new backward whirl phenomenon at start-up and coast down operations in cracked rotor systems with open crack models was observed in a recent publication. This phenomenon has been numerically and experimentally verified with an open crack to directly appear after the passage through the critical speed. Building on these recent findings, there is an imperative need to further investigate this phenomenon in rotor systems associated with open and breathing crack models from nonlinear and rotor dynamic perspectives. For start-up and coast down operations at constant angular acceleration, the model of the cracked rotor system with open or breathing crack models becomes a linear time-varying (LTV) system. Here, we also numerically verify the existence of this new backward whirl (BW) phenomenon in a cracked rotor with a breathing crack model via numerical simulation. Results indicated that a wide zone of BW rotational speeds is observable after the passage through the critical speed due to appearance of the breathing crack in the considered Jeffcott rotor system.

In mountainous areas, freezing is a prominent phenomenon for weathering processes in rock walls. A freezing front penetrates rock crack networks and causes its propagation. To study the evolution of rock mass stability, a suitable model of stress generated by freezing in open rock cracks is needed. This stress evaluated by the simple volume expansion model in a closed crack is too high to be realistic. In this paper, we present an assessment method for this stress and some results. Different experiments on notched limestone specimens submitted to freeze–thaw cycles were performed. Three different tight limestones (Larrys, Chamesson, Pierre de Lens) were tested. Actually, the stress generated by freezing begins to grow at the top of the notch where an ice plug is created and makes it possible for higher stresses to develop in deeper parts of the notch. Consequently, the stress induced by freezing depends on the geometry of the open crack represented by the notch. This value is, however, limited by the permeability of the surrounding rock matrix. A model of the stress evolution generated by freezing along an open crack was established and its envelope curve, named maximum stress, was parameterized. This maximum stress generated by freezing along the crack is completely defined by knowledge of the pore network of the limestone matrix and the geometry of the crack.

We present a new approach for mapping open cracks and tension fractures within rock slope instabilities and rock cliffs, which resides in high-resolution ground-based and airborne infrared thermography (IRT). The method is restricted to cold seasons, and its utility is demonstrated through three examples from the Flysch Belt of the Outer West Carpathians (rockslides at Kopce Hill and Mt. Kněhyně) and from the Northern Calcareous Alps (deep-seated gravitational slope deformations in Gschliefgraben/Mt. Traunstein). The approach is based on a contrast between temperatures deep within the rock mass, which at a depth of few meters represent local mean annual values, and winter-time temperatures of the ground surface. In winter, warmer, buoyant air from depth rises to the ground surface through open cracks and joints, and the temperature contrast can be detected by IRT. Our test survey was conducted in the beginning of February 2012, in order to achieve the best contrast between temperatures around open tension cracks and the adjacent ground. For temperature sensing, we used a FLIR B360 thermal camera; for airborne surveys in the ambient air, temperatures at the time of our surveys ranged from approximately −19 to −7 °C. IRT results conclusively revealed the presence of open cracks, loosened rock zones, and pseudo-karst caves over a distance sometimes greater than 1 km. The IRT approach proved useful for rapidly assessing the distribution of open cracks and tension fractures, key information required for assessing rockfall and rockslide hazard.

Comparing the floquet stability of open and breathing fatigue cracks in an overhung rotordynamic system

Summary Doped LiNiO2 has recently become one of the most promising cathode materials for its high specific energy, long cycle life, and reduced cobalt content. Despite this, the degradation mechanism of LiNiO2 and its derivatives still remains elusive. Here, by combining in situ electron microscopy and first-principles calculations, we elucidate the atomic-level chemomechanical degradation pathway of LiNiO2-derived cathodes. We uncover that the O1 phase formed at high voltages acts as a preferential site for rock-salt transformation via a two-step pathway involving cation mixing and shear along (003) planes. Moreover, electron tomography reveals that planar cracks nucleated simultaneously from particle interior and surface propagate along the [100] direction on (003) planes, accompanied by concurrent structural degradation in a discrete manner. Our results provide an in-depth understanding of the degradation mechanism of LiNiO2-derived cathodes, pointing out the concept that suppressing the O1 phase and oxygen loss is key to stabilizing LiNiO2 for developing next-generation high-energy cathode materials.

Bonding a fiber reinforced polymer (FRP) sheet to the tension-side surface of reinforced concrete (RC) structures is often performed to upgrade the flexural capacity and stiffness. Except for upper concrete crushing, FRP sheet reinforcing RC structure may fail in sheet rupture, sheet peeloff failure due to opening of a critical diagonal crack, or concrete cover delamination failure from the sheet end. Accompanying the occurrence of these failure modes, reinforcing effects of the FRP sheet will be lost and load-carrying capacity of the RC structures will be decreased suddenly. This study is devoted to developing a numerical analysis method by using a three-dimensional elasto-plastic finite element method to simulate the load-carrying capacity of RC beams failed in the FRP sheet peeloff mode. Here, the discrete crack approach was employed to consider geometrical discontinuities such as opening of cracks, slipping of rebar, and debonding of the FRP sheet. Comparisons between analytical and experimental results confirm that the proposed numerical analysis method is appropriate for estimating the load-carrying capacity and failure behavior of RC beams flexurally reinforced with a FRP sheet.