Cracked Specimens Research Articles

Impact of cracks on distribution of chloride-induced reinforcement corrosion

Surface crack width is regulated in codes to limit corrosion of reinforcement bars in concrete. However, the influence of surface crack width on corrosion damages is not directly inferable from previous research.In this work, data on corroded cracked concrete specimens in chloride environments was compiled. Detailed information was included, such as crack and pit locations, local corrosion pattern, etc. Five hypotheses on the influence of transversal cracks on corrosion damage were formulated, and statistical methods were used to test them on the dataset.Transversal cracks were good indicators of the position of corrosion pits. The corrosion rate of the pit increased in proximity of a crack. With time, pits grew in depth at a slower rate but increased in number. No clear correlation between surface crack width and corrosion damage was found. Results point out discrepancies in the collected data, arguing for the need of well-defined procedures for assessing crack and corrosion damage.Further, the statistical treatment allowed for identification of bias in existing data, which was used as a research planning tool to provide guidance on the design of additional experiments. Thus, recommendations for future experimental work required to reduce the bias are given.

Effect of cementitious capillary crystalline waterproof material on the various transport properties of cracked cementitious composites

The chemical reaction of CCCW endows it with the promise of improving self-healing behavior of crack of cementitious materials. However, there are few relevant studies about the self-healing behavior of CCCW in the different environments. Thus, the effect of CCCW on the various transport properties of cracked cementitious composites was explored in this paper, to clear the potential of CCCW in enhancing the durability recovery of cracked specimens. Four types of engineered cementitious composites (ECC) with various CCCW content were pre-cracked followed by curing in water, Ca(OH)2 solution and air. Water absorption and rapid chloride penetration tests were adopted to evaluate the various transport properties. And the closure behavior of crack was observed by the microscope. The composition and proportion of the self-healing products were analyzed by TG, XRD and FITR. Test results indicated that CCCW can promote the rapid reduction of water absorption of cracked specimens in early healing stage. The reduction range of charge passed of pre-cracked specimen with 4.5 % CCCW content were 8.03 % and 16.27 % higher than those of specimens without CCCW after curing in water and CH solution. CCCW facilitated lowering the various transport properties of cracked specimens, easier to restore to the uncracked state, especially in Ca(OH)2 solution. The healing degree of crack without CCCW with a width of 240.25 μm is only 36.61 %. However, the maximum crack width completely closed increased to 402.82 μm in Ca(OH)2 solution by adding 4.5 % CCCW content. After healing for 14 wet-dry cycles, the content of calcium carbonate in the healing products with CCCW was 12.19 % higher than that without CCCW. The formation of calcium carbonate was accelerated by CCCW in the entire healing process, increasing the number of healing products, thus helping to reduce the transport properties of cracked specimens.

Investigation of the size of the fracture process zone and the cleavage stress in cracked steel parts

AbstractThe study is devoted to the development of methods for predicting the brittle fracture of a steel part with a crack. To describe the limit state of the fracture process zone, a mathematical model of the fracture process zone in an elastic–plastic stress state and the generalized brittle fracture theory have been used. The cleavage stress and the size of the fracture process zone are used as parameters for the fracture toughness of the material. A finite element analysis approach was developed to determine these parameters by considering the elastic–plastic stress state of cracked specimens. An analytical procedure for calculating the above parameters for low‐carbon and low‐alloy steels is given. The proposed models allow the analytical calculation of critical values of the stress intensity factor for specimens with cracks at negative temperatures. The development of this study is linked to the improvement of the technology for determining the physical strength criteria of materials. The application of the proposed models makes it possible to create a methodology for predicting crack resistance of welded structures of arctic design considering their geometry and structural and technological characteristics.

Effect of Pre-Strain on Crack Growth Behavior of Zr702/TA2/Q345R Composite Plate

The influence of pre-strain on the fatigue crack growth behavior of Zr702/TA2/Q345R composite plate is studied by experiments and the finite element method. The crack perpendicular to the interface and the through-wall crack are considered at the same time. For the crack perpendicular to the interface, the monotonic plastic zone and cyclic plastic zone at the crack tip are used to study the influence of pre-strain on the plastic zone. Furthermore, the influence of pre-strain on the evolution of the plastic damage at the crack tip is analyzed in detail by studying the variation in the initial plastic energy and equivalent plastic strain. For the through-wall crack, the effect of pre-strain on the propagation behavior of cracks on both sides and the whole crack is studied systematically. The results show that the strengthening of the cracks on the Zr702 side is significantly higher than that on the Q345R side, so the inhibitory effect of pre-strain on the whole crack of the through-wall specimen is mainly due to the increase in the resistance to crack growth on the Zr702 side.

Effects of microstructure on crack initiation in super martensitic stainless steel under very‐high‐cycle fatigue at elevated temperature

AbstractThe present study envisaged the behavior and relevant crack initiation mechanism of super martensitic stainless steel (0Cr13Ni5Mo) at elevated temperature (ET) under very‐high‐cycle fatigue (VHCF). Fractographic analysis revealed that all the VHCF cracks of the failure specimens initiated from the surface and formed fine granular area (FGA) morphology at ET in an absence of vacuum. The initiation characteristic area (ICA) consisted of an initiation site, a relatively smooth area, and the annular continuous FGA. FGA was formed in the peaks and valleys of local microstructures, and the smooth regions connected these peaks and valleys dominated by shear stress. It was observed that the FGA was unevenly distributed on the crack surface related to the local microstructure and below the grain surface relevant to local strain distribution. The stress intensity factor (SIF) and the crack tip plastic zone of the ICA slightly changed with temperature.

Development of nanomodified self-healing mortar and a U-Net model based on semantic segmentation for crack detection and evaluation

In this study, a cement-based self-healing (SH) mortar mixed with multiwalled carbon nanotubes (MWCNTs) was developed, and the effect of MWCNT addition on the electromechanical properties of mortar was studied. In addition, a deep learning based U-shaped Network (U-Net) model was adopted for the crack healing evaluation in SH mortar specimens. The trained U-net model is capable to detect the cracks with the accuracy of 99 %. The MWCNT content in the mortar was varied from 0.05 % to 0.10 % by weight of the cement. A minerals-based SH admixture was incorporated at concentrations of 2.5 %, 5 %, and 7.5 % by weight of cement. Compression and flexural strength tests were performed to investigate the mechanical performance of the nanomodified mortar. Piezoresistive sensing properties of MWCNT/cement composite were monitored during flexural test. For crack healing evaluation, cracked mortar prisms were exposed to water for 28 days. Mechanical and piezoresistive sensing evaluation demonstrated improved mechanical performance of SH mortar with appreciable self-sensing properties. Specimens with 2.5 % admixture exhibited a crack sealing efficiency of approximately 90 %, while the cracks in specimens with 5 % and 7.5 % SH admixture content healed 100 % after 28 days of water exposure.

Experimental Investigation of Pre-Flawed Rocks under Dynamic Loading: Insights from Fracturing Characteristics and Energy Evolution.

Different fractures exist widely in rock mass and play a significant role in their deformation and strength properties. Crack rocks are often subjected to dynamic disturbances, which exist in many fields of geotechnical engineering practices. In this study, dynamic compression tests were carried out on rock specimens with parallel cracks using a split hopkinson pressure bar apparatus. Tests determined the effects of strain rate and crack intensity on dynamic responses, including progressive failure behavior, rock fragmentation characteristics, and energy dissipation. Based on the crack classification method, tensile-shear mixed cracking dominates the failure of rock specimens under the action of impact loading. Increasing the flaw inclination angle from 0°-90° changes the predominant cracking mechanism from tensile cracking to mixed tensile-shear cracking. The larger the loading rate, the more obvious the cracking mechanism, which indicates that the loading rate can promote the cracking failure of rock specimens. The fragmentation analysis shows that rock samples are significantly broken at higher loading rates, and higher loading rates lead to smaller average fragment sizes; therefore, the larger the fractal dimension is, the more uniform the broken fragments of smaller sizes are. Energy utilization efficiency decreases while energy dissipation density increases with increasing strain rate. For a given loading rate, the energy absorption density and energy utilization efficiency first decrease and then increase with increasing flaw inclination, while the rockburst tendency of rock decreases initially and then increases. We also find that the elastic-plastic strain energy density increases linearly with the total input energy density, confirming that the linear energy property of granite has not been altered by the loading rate. According to this inherent property, the peak elastic strain energy of the crack specimen can be calculated accurately. On this basis, the rockburst proneness of granite can be determined quantitatively using the residual elastic energy index, and the result is consistent with the intensity of actual rockburst for the specimens.

Response of thermal conductivity of loess after high temperature in northern Shaanxi burnt rock area, China.

Spontaneous combustion of coal seams can produce a high temperature of about 800 ℃, which greatly changes the thermal conductivity of the overlying loess layer. The thermal conductivity of loess plays an important role in ecological restoration design and the calculation of roadbed and slope stability. In this study, loess in northern Shaanxi, China was taken as the research object to measure the mass-loss rate and heat conduction parameters of loess specimens after high temperature. The test results show that, between 23 and 900°C, with temperature increasing, the mass-loss rate is reduced. And the heat conduction coefficient (λ), specific heat capacity (c), and thermal diffusion coefficient (α) decreased by 48.9%, 23.1%, and 35.6%. This is due to the air thermal resistance effect caused by the increase of pores and cracks in loess specimens after high temperature.

Crack Growth in Ni-Cr-Mo-V Steel Using ΔCTOD Elastic–Plastic Model

Many studies have shown that the linear elastic fracture mechanics (LEFM) method based on the stress intensity factor range (ΔK) has limitations that cannot be ignored. Due to neglecting the influence of plastic deformation near the crack tip, LEFM shows apparent deviations in evaluating the fracture behaviour. Therefore, in this study, the crack tip opening displacement range (ΔCTOD) is chosen as an alternative to ΔK and based on the elastic–plastic fracture mechanics (EPFM) to develop a new fracture behaviour assessment approach for marine structures. Firstly, a ΔCTOD model based on the HRR (Hutchinson, Rice, and Rosengren) solution is proposed considering the crack closure effect. Secondly, a series of compact tension (CT) specimen crack growth experiments under constant amplitude loading is carried out. According to the experimental results, the prediction accuracy of the HRR model and traditional Irwin and Dugdale models is compared and analysed. The rationality of ΔCTOD as an alternative to ΔK is verified. The results show that ΔCTOD can describe the crack propagation behaviour well. The proposed HRR model shows better accuracy and a more comprehensive application range than the traditional models, which has a guiding significance for studying fracture behaviour for marine structural applications.

Damage detection for structural health monitoring using ultra-sensitive flexible piezoelectret sensors

Damage detection is of great importance for structural construction management and safety. In this study, an ultra-sensitive flexible piezoelectret sensor for damage monitoring in structural health monitoring (SHM) is designed and fabricated, based on excellent electromechanical properties and piezoelectric response characteristics of electret films. The charge stability of the piezoelectret sensor was demonstrated by continuous potential measurement experiments. The experimentally measured piezoelectret coefficient is 6610 pC/N, which is about 10 times that of ordinary piezoelectric ceramics and proves the high sensitivity of piezoelectret. A series of comparative experiments on the piezoelectret sensor and piezoelectric ceramics demonstrate the high performance of the former. According to the damage detection test results, the damage index of cement mortar specimens was obtained. The linear relationship between the damage index and the crack widths proves the excellent performance of the piezoelectret sensor. A piezoelectret array is also used to detect cracks in cement mortar specimens, and can roughly estimate the depth of the fracture based on the damage index (DI) value of the sensing array.

Flexural Behavior of One-Way Slab Reinforced with Grid-Type Carbon Fiber Reinforced Plastics of Various Geometric and Physical Properties

Textile-reinforced concrete (TRC) has many advantages, including corrosion resistance, but TRC is a novel composite material and there is limited experimental research on the flexural behavior of TRC members. This paper aims to experimentally evaluate the flexural behavior of TRC slabs reinforced with nine types of grid-type carbon fiber-reinforced plastic (CFRP) (hereafter referred to as carbon grid) with varying cross-sectional areas, spacings, tensile strengths, and elastic moduli of longitudinal strands. The experimental results show that the maximum load tends to be higher in specimens reinforced with carbon grids with small cross-sectional areas and spacings of strands but high tensile strength. Cross-sectional area and spacing were also revealed to influence the crack-formation stage behavior. On the other hand, stiffness decreased to approximately 8% or lower than the initial stiffness, with cracking in all carbon grid-reinforced specimens; post-peak behavior also exhibited dependency on tensile stress acting on the carbon grids under the maximum load, based on 80% of the tensile strength.

High fracture toughness in van der Waals-layered MoTe2: Disappearance of stress singularity

In this study, we demonstrated that van der Waals-layered MoTe2, where two-dimensional atomic layers are closely laminated by weak interactions, has higher fracture toughness in out-of-plane cracks than that in in-plane cracks owing to structural anisotropy. In situ electron microscopy fracture toughness tests were performed on micro-sized specimens. The apparent fracture toughness of the out-of-plane crack based on the continuum assumption was approximately twice that of the in-plane crack. In the out-of-plane crack specimen, as the loading progressed, interlaminar slip occurred between layers, and this discrete nature caused the stress singularity to disappear, leading to high fracture toughness.

Effect evaluation of microbial mineralization for repairing load-induced crack in concrete with a cyclic injection-immersion process

This study presents the experimental investigations on repairing the load-induced cracks in concrete with microbial mineralization technology. Two groups of load-induced cracked concrete specimens, splitting specimens and bending specimens, with crack widths in range of 0.05–0.10 mm and 0.10–0.30 mm were prepared, respectively. Bacillus pasteurii (DSM 33) was selected to repair these cracks using microbially induced calcium carbonate precipitation (MICP) method. The repair effectiveness on crack healing was evaluated by means of visual observations, ultrasonic measurement tests, rapid chloride migration (RCM) tests and splitting tensile strength tests. The results indicated that after MICP treatment the resistance of repaired cracked specimens to chloride penetration could be effectively improved. Comparison of repair effectiveness between splitting specimens and bending specimens showed that the former had slightly better crack recovery than the latter. For both cracked specimens, the improvement coefficient which can reflect the repair effectiveness decreased from nearly 90–70% with increasing crack width from 0.05 mm to 0.23 mm. Scanning electron microscope (SEM) and energy dispersive spectrum (EDS) tests revealed that the formation of microbial induced CaCO3 precipitation would accumulate and fill the concrete crack, which is believed to be the reason of improving resistance to chloride ingression in the repaired cracked specimens. Besides, for repaired splitting cracked cylinders, their mean splitting tensile strengths could recover around 65.8% and 52.6% for crack width intervals of 0.05–0.10 mm and 0.10–0.30 mm, respectively.

Cluster-Based Joint Distribution Adaptation Method for Debonding Quantification in Composite Structures

Adhesive bonding is widely adopted in aeronautic structures to join composite materials or to repair damaged substrates. However, one of the most common failure modes for this type of joint is debonding under fatigue loading. In the past years, it has been proven that deboning quantification is feasible, given that abundant experimental data are available. In this context, using domain adaptation to assist diagnostic tasks based on labeled data from similar structures or simulations would be thoroughly beneficial. However, most domain adaptation methods are designed for classifications and cannot efficiently address regressions. A fuzzy-set-based joint distribution adaptation for regression method has been developed by the authors, tackling regression problems but being limited to single outputs. The novelty presented in this paper exploits clustering techniques to approach multi-output problems, adopting a modified multikernel maximum mean discrepancy to improve the domain discrepancy metric. The proposed method is applied to cracked lap shear specimens to assist debonding quantification. Several domain adaptations are investigated: from simulations to experiments, and from one specimen to another, proving that the accuracy of damage quantification can be improved significantly in realistic environments. It is envisioned that the proposed approach could be integrated into fleet-level digital twins for nominally identical but heterogeneous systems.

Application of the Deformation Fracture Criterion to Cracking of Disc Specimens with a Central Narrow Slot

Abstract Using the method of singular integral equations, the elastic-plastic problem for cracked Brazilian disk was solved. Based on the Dugdale model and deformation fracture criterion, the relationships between critical load, notch tip opening displacement and length of the plastic strips were established. Also, the comparison between the present solution for the finite domain and the known solution obtained for the semi-infinite notch in the elastic plane was performed.

Glass laminate aluminum reinforced epoxy under non-proportional multiaxial fatigue loading: Experimental testing and new fatigue apparatus development

Predominately, the structures and components in aeronautic, automotive and other important implementations suffer from non-proportional multi-axial fatigue loading. One of the best selected materials to fabricate these structures and components are fiber metal laminates (FML). In this research, fatigue crack-propagation in glass-laminate aluminum reinforced-epoxy (GLARE) specimen under the effect of special case of multi-axial fatigue loading (constant shear with repeated tensile stresses) has been studied analytically, numerically and experimentally. GLARE_2 A 2/1 specimen was fabricated using vacuum assisted resin transfer molding technique after special preparation aluminum surface procedure. New multi-axial fatigue apparatus including mechanical, hydraulic and control systems was manufactured to investigate crack growth experimentally. Analytical model using Paris relation was adopted to predict crack growth through aluminum layers of GLARE specimen. The rate of crack propagation is related to the mixed-mode equivalent stress intensity factor, which is estimated by the superposition for both fiber-bridging and far-field stress intensity factors. The crack opening contour, distribution of fiber-bridging stress, delamination growth and its shape were evaluated in simultaneous manner to find the fiber-bridging stress intensity factor. ABAQUS 2021 software has been used to simulate crack and delamination growth in GLARE specimen with the aforementioned fatigue loading. The extracted results from analytical and numerical models such as crack growth, crack opening, delamination shape and crack angle have been compared with that measured experimentally. The comparison indicates that there is good agreement between the calculated and measured results (with maximum deviation of 8.74% for crack growth).

Experimental and Numerical Investigation of the Tensile Performance and Failure Process of a Modified Portland Cement

A better understanding of the tensile performance and tensile failure mechanism of cement paste is significant in preventing rock reinforcement failure. Therefore, this paper aims to reveal the tensile performance and failure mechanism of a modified Portland cement: Stratabinder HS cement. To achieve this objective, the split tensile test was conducted on specimens followed by simulating the failure mechanism numerically. The results indicated that the water–cement rate significantly influenced the tensile performance of the cement paste. When the water–cement rate increased from 0.35 to 0.42, the tensile strength declined from 1.9 MPa to 1.5 MPa. It was also observed that vertical tensile failure constantly occurred regardless of the water–cement rate. During the testing process, tensile cracks and shear cracks occurred. The increasing rate in the number of specimen cracks was dependent on the tensile stress state. Before the tensile stress reached the peak, the crack quantity increased slightly. After the peak, the crack quantity increased dramatically. During the vertical loading process, horizontal tensile stress occurred in the specimen. This horizontal tensile stress zone showed a diamond shape. The higher the tensile stress is, the larger the area of the horizontal tensile stress zone. When the tensile strength was reached, horizontal tensile stress mainly concentrated at the vertical centre of the specimen. This finally led to tensile failure of the specimen. This paper indicated that the water–cement rate was the key factor in evaluating the tensile strength of the Stratabinder HS cement.

A fatigue crack growth prediction model for cracked specimen under variable amplitude loading

The objective of this study was to propose a crack growth prediction model for cracked specimens under large and variable amplitude loading conditions. In this study, the concept of change in net-section strain energy was applied to simulate elastic–plastic fracture behavior, and three-dimensional fitting of the results with respect to changes in the net-section strain energy, load ratio, and crack growth rate was performed to create the crack growth prediction model. The validity of the generated model was examined using comparisons with experimental data as well as simulation results from existing crack growth models under several variable loading conditions.

Residual stress distribution and fatigue performance investigations of electromagnetic force-based dynamic cold expansion open-holed sheet

In this study, residual stress distributions and fatigue performances of electromagnetic driving dynamic cold expansion (DCE) open-holed sheets were investigated. Fatigue behaviors were tested, and residual stress fields were obtained experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distribution was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The stress distribution differences led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Uniform stress distributions along axial direction in DCE specimens contributed to random initiations and horizontal propagations of fatigue cracks, while the fatigue cracks in SCE specimens usually initiated at the mandrel entrance layer and propagated to the mandrel exit layer. Moreover, the predicted fatigue life results agreed well with experimental data.

Initiation and propagation behaviour of cohesive cracks at high temperatures: A dynamic experimental study

Temperature, as an important factor, affects the fracturing effect in deep reservoirs, so it is urgent to carry out high-temperature crack propagation experiments. Here, by combining a high-speed camera and DIC technology, Brazilian splitting tests of three types of rocks at different temperatures are carried out to study the crack initiation and propagation characteristics from a mesoscopic perspective. A new way to identify the cohesive crack tip is proposed based on the high-temperature cohesive zone model and opening displacement patterns. In addition, the dynamic propagation process and changes in the characteristics of cohesive cracks at different temperatures are analysed. The experimental results demonstrate that dynamic process of high-temperature cohesive crack initiation and propagation can be subdivided into three stages: stress concentration, cohesive microcrack initiation and propagation and cohesive macrocrack penetration. Interestingly, multiple interconnected rather than single cohesive microcracks develop in the centre region of sandstone and limestone specimen surfaces at 260 ℃. Notably, the duration of the stress concentration stage of sandstone and limestone shows an increasing trend with temperature. The average propagation speed and the opening displacement change rate of the cohesive crack in sandstone, limestone and shale specimens show a declining trend with increasing temperature. The results can provide some insights into the high-temperature crack propagation mechanism in deep reservoirs.