Small-sized Cracks Research Articles

Consideration of the effects of frost heave force and train loads on the cracks and propagation pattern of ballastless track slabs in cold regions

The construction of ballastless railway tracks in cold regions is susceptible to frost heave forces, which may particularly impact the stability of track slab cracks and lead to crack propagation. In this paper, the frost heave tests are conducted on the concrete specimens with prefabricated cracks to observe the evolution of the frost heave force within the cracks. The frost heave force is subsequently applied to the finite element model of the concrete specimens to calculate the stress intensity factors (SIFs). In addition, the CRTS III ballastless track model is established to investigate the crack propagation patterns under frost heave and train loads. The results show that the frost heave force in concrete cracks during freezing and thawing is divided into five stages, which produces two peaks of frost heave force. Train loading caused the SIFs of the crack tips to open and close alternately and to rise significantly under the coupling effect of frost heave load. The SIF increases approximately linearly with crack width and is higher for transverse cracks than for longitudinal cracks. Small size cracks do not possess sufficient SIF values to exceed fracture toughness, implying no crack propagation. However, as the crack width is constantly increasing, the SIF will eventually exceed the fracture toughness. Therefore, strict control over track slab's crack width is necessary for CRTS III ballastless tracks.

Modeling the Crack Interference in X80 Oil and Gas Pipeline Weld.

Based on the numerical simulation method of the virtual crack closure technique (VCCT), an interference model was established to investigate the physical problem of two interacting cracks of different sizes in the welding zone of oil and gas pipelines. The obtained results of the current interference problem were compared with those of single crack case. Various crack configurations, such as different crack spacing and different crack sizes, were analyzed. The characteristic quantity fluid pressure load P during the crack propagation process, the peak value of the von Mises stress distribution field of the crack growth path, as well as the difference ∆Bx between the peak value of the magnetic induction intensity component at the crack and the value of the magnetic induction intensity component at its symmetrical position were calculated. The crack interaction scale factors, including γP, γMises, and γΔBx, were compared and analyzed. The numerical modeling results show that when the unequal-length double cracks interfere with each other, the cracks are more likely to propagate toward each other. The tendency of the double-cracks to propagate toward each other gradually weakens as the distance between the crack tips increases and is finally the same as that of single-crack cases. It was also found that the effect of large-sized cracks on small-sized cracks is greater than that of small-sized ones on large-sized ones. The numerical modeling results could be applied for the prediction and analysis of multicrack damage in oil and gas pipeline welds.

Damage detection in plates based on Lamb wavefront shape reconstruction

Many of the current studies in the area of damage detection using elastic wave propagation are based on deploying sensor networks with a large number of piezoelectric transducers to detect small-size cracks. A major limitation of these studies is that cracks are usually larger and have different shapes in real cases. Moreover, using a large number of sensing nodes for damage detection is both costly and computationally intensive. The paper proposes a novel elastic wave propagation method for damage detection based on the reconstruction of the shape of the reflected wavefront. The main advantage of the developed approach is that merely three sensors are necessary to localize and estimate the size of the cracks with various sizes and shapes. Several damage cases with different number of piezoelectric transducers and placement have been investigated experimentally. Extensive numerical simulations are performed to benchmark the performance of the proposed method.

Significant Improvement in Extrudability of Mg–9Al–0.8Zn–0.9Ca–0.6Y Alloy Through Mischmetal Addition

The combined addition of small amounts of Ca and Y to Mg–Al–Zn alloys has recently been found to greatly improve the ignition resistance, corrosion resistance, and mechanical properties of these alloys. However, Mg–Al–Zn–Ca–Y alloys with high Al content show poor extrudability, which is an obstacle to their use as extruded components in industrial applications. This study aims to improve the extrudability of a Mg–Al–Zn–Ca–Y alloy through the addition of a small amount of mischmetal (MM). To this end, a trace amount (0.5 wt%) of Ce-rich MM is added to a Mg–9Al–0.8Zn–0.9Ca–0.6Y (AZXW9110) alloy. MM addition leads to significant grain refinement of the homogenized billet, from 326 to 180 μm. When the MM-added alloy is extruded into a sheet at 350 °C with an extrusion ratio of 22.9, the maximum ram speed at which it is extrudable without the occurrence of hot cracking is 12 mm/s, substantially higher than the 2 mm/s possible without MM addition. The Ce and La atoms decomposed from the MM are dissolved in the Mg17Al12, Al8Mn4Y, Al2Ca, and Al2Y phases, apparently leading to an increase in the thermal stability of the phases and, consequently, to an improvement in the AZXW9110 alloy’s extrudability. At ram speeds above 2 mm/s, many small-sized cracks form at the edges of the extruded AZXW9110–0.5MM sheets. As the ram speed increases up to 6 mm/s, the number density of edge cracks gradually increases while their average spacing decreases, but beyond 6 mm/s both remain almost unchanged.

Concrete Early-Age Crack Closing by Autogenous Healing

Autogenous healing is mainly produced by continuing hydration or carbonation. The aim of this research is to quantify the crack closing produced by autogenous healing for early-age concrete. This healing was evaluated for two crack size levels, 0.1 mm and 0.4 mm, under three healing conditions: water immersion, a humidity chamber, and wet/dry cycles. Crack closing was evaluated after 7, 14, 28 and 42 days under healing conditions. The internal status of the cracks was verified visually and using phenolphthalein. The results show that specimens stored in the humidity chamber did not experience healing, while specimens under wet/dry cycles and water immersion achieved the complete closing of small-sized cracks (under 0.15 mm). Autogenous healing showed higher speed under wet/dry cycles but higher final efficiency under water immersion. However, the inspection of the interior of the specimens showed that self-closing occurred mostly on the surface, and carbonation in the crack faces was only noticed very near the specimen’s surface. Additionally, this study proposes a preliminary strategy to model autogenous healing in concrete in terms of crack closing.

Extracting non-propagating oscillatory fields in concrete to detect distributed cracking.

Work to detect and locate distributed subsurface cracks in concrete by extracting non-propagating oscillatory fields is presented. The medium of interest is concrete, but the approach also applies to other types of inhomogeneous media. The theoretical basis of the work is first presented through a one-dimensional point-scatterer model that considers the wavefield set up by multiple distinct scatterers. More complex scattering scenarios are then investigated using numerical simulation. The numerical models consider two types of scatterers: elliptic large-scale particles distributed throughout a medium, and small-sized cracks localized within a damage zone. The theoretical and numerical analyses show that forward propagating waves undergo distinct scattering behavior within the crack damaged zone: non-propagating resonance-like oscillatory fields are set up within the cracked zone, and are distinct from the scatter caused by the large-scale particles. Frequency-wavenumber (f-k) domain analyses to extract the energy of non-propagating oscillatory fields and thus to detect and locate zones of distributed cracking are employed. The proposed approach is evaluated using numerical simulation and experimental data collected from concrete specimens that contain simulated distributed cracks. The results demonstrate that the location of distributed crack zones in discrete random media, such as concrete, can be successfully detected.

A Deep-Learning-Based Multiple Defect Detection Method for Tunnel Lining Damages

Diagnosing tunnel lining structural damages is vital to ensure safe tunnel operations. However, the detection of multiple defect is challenging task due to the size imbalance between cracks, spalling, and backgrounds. Currently, deep-learning-based methods for multiple defect are dependent on multiple-stage networks, which have limited their scalability and complex frame working processes. To accurately recognize the multiple defect at the pixel-level using only one-stage networks, a new method was proposed, which integrated the basic SegNet with a focal loss function, and was referred to as an FL-SegNet method. The focal loss function was adopted to address the problem of the size imbalance by down-weighing the losses assigned to the well-classified samples, and then the training was focused on the hard samples. Furthermore, comparative experiments were performed to evaluate the performances of the different methods. The experimental results demonstrated that FL-SegNet method was capable of accurately predicting the profiles of small-sized cracks and overlapping damages even under various noise conditions, and successfully outperformed the two-stream method and the basic SegNet method in this regard. The performance metrics (MPA and MIoU) of the FL-SegNet method were significantly higher than those of other multiple defect detection approaches in different scenarios (images with small-sized damages attained to 81.53% and 69.86%, increased by 11.99 % and 4.88% compared with two-stream method, and increased by 17.78% and 7.69% compared with basic SegNet). Therefore, this paper provides an effective solution for the future detection of multiple defect in tunnel linings.

The presence of secondary creep in historic masonry constructions: A hidden problem

Abstract Top-ranked European research groups have been working for years on the problem of identifying the behavior of load-bearing elements due to creep in historic masonry structures. All analyses carried out were performed in unexpected structural collapses, related with earthquakes or not, in several regions all around Europe. These collapses are related with the presence of appreciable size cracks, which appear more or less vertical in towers, vaults and pillars. Although these cracks can be easy detected through mortar before the collapse, the proximity of the structure failure is not fully defined nowadays. The paper deals with the presence of another crack typology in stonework masonry blocks, which has not yet been taken into consideration. Despite they sometimes can through the blocks vertically, this second type cracks are difficult to detect owing to their reduced size and because they are under surface patina. In this sense, several real cases are presented where these cracks were found. In the laboratory work sandstone masonry blocks were subjected to a cyclic load to generate this kind of cracks, with the aim to propose a method to evaluate them in situ, before the building collapse. The proposed methodology is based on the procedures employed in the field of the quantitative metallography. Although the relation between both types of cracks is not clear, the possibility that small ones coalesce to the bigger ones is suggested. The small typology cracks appear due to long-term loads, which are associated with cyclical loads. In other words, it is a fracture caused by creep-fatigue interaction. This work aims to point out the probable existence of these cracks hidden under stonework masonry blocks surface, in a great number of historical constructions. These small size cracks would correspond to the secondary creep. When these cracks reach the edge of the blocks, bigger cracks appear in the mortar, causing the tertiary creep.

Identification of Small-Sized Cracks on Cross-Stiffened Plate Structures for Ships

Abstract In this paper we discuss the results of experimental and numerical studies carried out for monitoring the integrity of a 1∕20th model of the side shell of a ship’s structure, using its vibration responses. The model was tested for different crack development scenarios under random exciting force having a dominant spectral frequency much lower (≈2Hz) than the first natural frequency of the structure (≈580Hz). Sensor locations were justified based on modal responses under intact and damaged conditions. A damage detection procedure using the root mean square response modal amplitudes is also presented.

Calculation of the ECT signal of a minute crack by a FEM-BIM hybrid method

A hybrid method is presented for the calculation of the interaction of a complicated shape ferrite-core eddy current testing probe with extremely small size cracks in a plate. The method is obtained by blending the boundary integral (BIM) and finite element (FEM) methods preserving their attractive properties, that is, the fast and accurate evaluation of the defect field and the versatility in specimen and probe geometry, respectively. FEM is applied for the computation of the electric field induced in the specimen without cracks. Using the obtained, so-called, incident field the BIM is used for the calculation of the field perturbation due to the presence of the cracks in the tested specimen. With the application of the presented hybrid calculations, a very fast and accurate method is developed for the solution of a problem that would pose considerable difficulties for either the FEM or the BIM if they were applied purely alone in the conventional way. The results of the calculations are compared with a large number of experimental data. The very good correlation between the measured and simulated probe responses proves the applicability of the presented method.

Elastic-Plastic Analysis of Small Cracks in Tubes

Analytical methods for predicting the elastic or elastic-plastic behavior of large circumferential through-wall cracks in tubes subjected to bending, tension, or combined bending and tension are well developed. Gilles and Brust (1991) and Gilles et al. (1991) summarize five such methods and provide a number of comparisons between analytically predicted results and experimental data. These techniques consist of developing a method for estimating the value of the J-integral. Classical J-tearing theory is utilized for the analyses. A method has also been recently developed to estimate J for a through-wall crack in a pipe weld (Rahman et al., 1991; Rahman and Brust, 1992). Unfortunately, the ability of these J-estimation techniques to predict the crack growth behavior for small cracks ({<=} 12 percent of the circumference) has not been established, even though such small cracks are often the concern in practical structures. Indeed, the finite element solutions compiled in the GE/EPRI handbook (Kumar et al., 1984) appear quite inadequate for small-size cracks (Gilles and Brust, 1991; Gilles et al., 1991). This paper presents the results of a series of finite element solutions for small cracks tabulated in the spirit of the handbook (Kumar et al., 1984). The theoretical background for the developmentmore » and use of simplified elastic-plastic fracture methods is fully discussed in Gilles and Brust (1991). Here, the authors provide the solutions for J-integral and the crack opening displacements for small crack problems under pure bending. Ongoing work will complete these solutions for tension and for combined tension-bending loads.« less

Influence of repeated plastic shear strain in reversed torsional fatigue of low-pressure-turbine rotor steel.

Alternating two-load level torsional fatigue tests were performed using 3.5 Ni-Cr-Mo-V steel to investigate the influence of overloads on the fatigue damage in the crack initiation and propagation stages. The base block consisted of strain cycling of Δγp = 2% as the overloads and constant stress cycling of τH = 260 MPa, higher than the fatigue limit, or τL = 220 MPa, lower than the fatigue limit, as the baseline loading. Constant stress and strain cycling were also performed. It was found that torsional fatigue cracks tended to branch off under constant stress cycling, but did not branch off under constant strain cycling. Overloads always reduced the crack initiation life and the fatigue life. In the torsional fatigue, small-size cracks were easily initiated, but they propagated slowly in comparison with push-pull fatigue.