Cracking Stage Research Articles

Experimental study on the flexural performance of FRP grid-reinforced ultra-high-performance concrete I-beams with different steel fiber contents

To adapt to harsh engineering environments, the use of structural components with fiber-reinforced polymer (FRP) grid-reinforced ultra-high-performance concrete (UHPC) is considered a new development trend. Twelve FRP grid-reinforced UHPC I-beams are fabricated to investigate the effects of the thicknesses of FRP grids (0 mm, 1 mm, 3 mm, and 5 mm) and the steel fiber contents of UHPC (0%, 1%, and 2%) on their flexural performance. Four-point bending tests are conducted to obtain the failure modes, load-deflection characteristics, and strain responses of each testing beam, and a simplified calculation method for cracking loading and flexural bearing capacity is proposed. The results reveal that the increase in FRP grid thickness, the ultimate load bearing capacity of FRP grid-reinforced UHPC structural beams increases and the ductility also can be enhanced. The presence of steel fiber effectively inhibits the cracking and increases the first cracking load of the beam during the test process, although its influence on the ultimate bearing capacity is limited. Besides, until to the cracking stage, there is no noticeable change in the tensile strain of the FRP grids as the applied load increases. It has been used to simplify theoretical calculations, which shows good agreement with the experimental results.

Evolution mechanism of hydraulic fracture in concrete under dynamic loading

The influence of hydraulic loading within cracks on the evolution mechanism of concrete under dynamic loading was investigated in this paper. Several strain gauges were employed to stick along the crack tip during the experiments. The moving crack tip was captured using these gauges during fracture. In particular, during the stable cracking stage, consistent propagating crack-tip speeds were observed under varying water pressures. Initially, the measured speed measured 2.0 m/s, which later increased to 20.0 m/s. Based on hydraulic experiments, a relationship between the crack tip and the water front was established, allowing the derivation of pressure distribution within crack from experimental data collected under dynamic loading conditions. Furthermore, the fracture parameters, including initial toughness, critical toughness, and cohesive toughness, were calculated for concrete subjected to split loading. The cohesive toughness experienced a 13.9% reduction with the increasing hydraulic pressure to 0.4 MPa, highlighting the deterioration of cohesive fracture toughness due to water infiltration within the concrete crack. This study provides insights to improve the design of civil and hydraulic engineering structures, such as gravity dams.

Deep learning-assisted structural health monitoring: acoustic emission analysis and domain adaptation with intelligent fiber optic signal processing

Structural health monitoring aims to detect damage progression in materials. This study focuses on categorizing crack stages, a critical aspect of monitoring structural integrity. By leveraging acoustic emission (AE) monitoring, cracks can be analyzed in a data-driven manner. However, applying AE analysis poses several challenges, including discrepancies between simulated AE data from models and experimental data from the field, as well as class imbalance in crack progression data, with a scarcity of late-stage data. To bridge the gap between theory and experiments, our approach employs domain adaptation to synchronize simulated and actual AE data. The model learns robust domain-invariant features through meticulous experimentation across training epochs. Quantitative analysis of the model’s performance provides key insights. F1 scores vary with feature counts, and domain adaptation outperforms by 20% on highly imbalanced datasets. This emphasizes the model’s adaptability for precise crack classification, even with underrepresented damage classes. In summary, this study advances structural health monitoring by offering a solid AE analysis approach. Core contributions include reconciling simulated and experimental data discrepancies, tackling class imbalance, optimizing feature extraction, and demonstrating robust crack stage categorization. The insights gained highlight the merits of domain adaptation and data-driven AE analysis for predicting crack progression.

Comparative study on fracture evolution in steel fibre and bar reinforced concrete beams using acoustic emission and digital image correlation techniques

In recent decades, the demand for sustainable construction practices has increased, but raw materials such as reinforcing steel remain scarce. Therefore, steel fibres have emerged as a popular and sustainable choice in the construction industry, offering a cost-effective alternative to traditional steel bar reinforcement for both flatwork and elevated structures. The purpose of this study is therefore to compare the performance of fracture behaviour between steel fibre-reinforced concrete (SFRC) and steel bar-reinforced concrete (SBRC) beams subjected to three-point bending. The fracture process was monitored by using two non-invasive techniques: acoustic emission (AE) and digital image correlation (DIC). The damage level was identified by characterizing the parameter-based AE data such as hit rate, energy release, count, rise time, amplitude, and signal strength. DIC images were employed to visualise the crack propagation in parallel with the AE data. To further understand the fracture characteristics, the integration of 2D source localization of AE events (based on local AE fracture energies) with DIC results was investigated. The parameter-based AE results showed that SBRC beam experienced a high density of AE hits with large peak amplitude events that were accelerated during the pre-peak loading phase. The Ib-value analysis revealed that SBRC beam exhibited a higher degree of fracture magnitude during the primary crack development process than SFRC beam. Following the main cracking stage, SFRC beam demonstrated an improved post-cracking softening behavior and superior ability to arrest crack propagation compared to SBRC beam. The integration of local AE fracture energy and DIC results provided a novel approach for a better understanding of the fracture behaviour in both SFRC and SBRC beams. This study’s findings contribute to more precise monitoring of fracture evolution in SFRC and SBRC beams, ultimately improving the selection process for primary reinforcement in flatwork and elevated structures.

Techno-economic analysis of ammonia cracking for large scale power generation

The increasing interest in leveraging green ammonia to mitigate carbon emissions in fertilizer production is paralleled by an expanding acknowledgment of its potential as a fuel for decarbonizing the electricity sector, particularly in high-efficiency gas turbine power plants. Co-firing ammonia with hydrogen presents a promising method for integrating ammonia into existing infrastructures. Within this context, the development of efficient technology for ammonia cracking presents a potential avenue for deploying ammonia in gas turbines. The objective of this study is to conduct a preliminary techno-economic evaluation and uncertainty analysis of two cracking technologies namely a membrane reactor and a conventional FTR (Fired Tubular Reactor) for the co-firing of ammonia with hydrogen in a CCGT (Combined Cycle Gas Turbine) plant. The integration of a membrane reactor during the cracking stage demonstrates a remarkable improvement in the system's thermal efficiency, surpassing traditional approaches by over 25%. Additionally, it brings about an approximate 10% reduction in the levelized cost of hydrogen (LCOH), despite a higher initial capital expenditure (CAPEX). At the CCGT level, the discrepancy in levelized cost of electricity (LCOE) narrows, as it is strongly influenced by the cost of ammonia constituting 80% of the LCOE. Beyond LCOE, the widespread adoption of these systems also faces challenges due to material scarcity. Analysis reveals that revamping just 1 GWe of CCGT assets using membrane reactors would for example necessitate approximately 0.11% of the global palladium supply, and 10% of the global ruthenium production. Considering the limited availability of these resources, coupled with their high demand across multiple sectors and the possibility of external factors such as geopolitical tensions, this strategy seems unfeasible. To tap into this market, future research should prioritize the exploration of alternative membrane materials, such as carbon molecular sieves, and catalysts, like nickel.

Experimental studies and mathematical modeling of the catalytic conversion of biodiesel fuel into synthesis gas

In this work, steam reforming and autothermal reforming of methyl oleate (a model compound of biodiesel fuel) into synthesis gas on a structured Rh-containing catalyst were investigated. It has been shown that the conversion of methyl oleate proceeds through the stage of thermal cracking followed by the conversion of the resulting organic compounds with a shorter carbon skeleton. Based on the experimental results, a mathematical model was developed that takes into account the radial temperature gradient and represents an effective tool for quantitatively describing and optimizing the biodiesel conversion process.

Coefficients of the irregularity of the deformation of the reinforcement and the concrete of corrosion damaged reinforced concrete slabs

Introduction. In practice, to perform calculations to determine the bearing capacity of reinforced concrete slabs at the stage of cracking, the work of concrete between cracks in the tension zone is taken into account.
 
 This paper presents the results of full-scale tests of damaged precast reinforced concrete floor slabs to determine the coefficients V.I. Murashev – uneven deformations of reinforcement ψs in the tension zone and concrete ψb in the compression zone for corrosion-damaged slabs in the stage of cracking.
 
 The aim of the work: to determine the coefficients of uneven deformation of reinforcement and concrete experimentally for corrosion damaged reinforced concrete floors at the stage of cracking.
 
 Materials and methods. The article describes the methodology for setting up an experiment to measure the deformations of reinforcement and concrete, slab deflections, crack opening widths, and describes the parameters of the testing installations.
 
 Results. The work presents the main results of experimental studies in graphical form.
 
 Conclusions. Research shows that with the appearance of cracks, the coefficient ψsx first increases and then stabilizes. The coefficient ψbx immediately after the formation of cracks first increases slightly, then decreases more gradually.

Effect of glass type and thickness on the static and blast response of LG panels

Laminated glass (LG) is created by bonding multiple glass layers using a polymeric interlayer, which reduces the risk of glass shards by keeping the pieces connected. The interlayer also absorbs cracking energy. This study examines the mechanical properties of three polymeric films (PVB, EVA, and SentryGlas) through tensile tests at different strain rates. The effect of glass type and thickness on the failure and deflection of LG windows is investigated using a full-scale water chamber. To address safety concerns and costs, a dynamic numerical model of LG windows is developed using ANSYS AUTODYN. The model's accuracy is validated against existing experimental data. The results show that tempered glass panels withstand higher pressure than annealed glass panels. Increasing glass thickness enhances the resistance of LG at the pre-cracked stage, with a 10% improvement for EVA and SentryGlas interlayers and a 153% increase for PVB interlayers. However, there is no significant impact on the LG at the cracked stage.

Effect of accelerated carbonation of fully recycled aggregates on fracture behaviour of concrete

Accelerated carbonation of recycled aggregates (RAs) enables fast carbon sequestration while improving engineering properties of prepared concrete. This paper presents a systematic experimental study on the fracture behaviour of concrete with carbonated recycled coarse and fine aggregates (RCA and RFA), in terms of fracture parameters and fracture process zone (FPZ). The relevant influencing mechanism was investigated by relating the macroscopic fracture responses to microstructural features. Results indicate that carbonated RFA mainly enhanced the initial fracture toughness of concrete, while carbonated RCA primarily increased its unstable fracture toughness. The enhancement of fracture energy was closely associated with carbonated RCA and RFA. The utilization of RFA resulted in the looseness and higher porosity of mortar matrix, which reduced the crack initiation resistance of concrete, mainly affecting its initial cracking stage. The incorporation of RCA reduced crack bifurcation and deflection of concrete, leading to a smaller width of FPZ, which made the initial descending segment of the tension softening curve (TSC) steeper and its tail segment shorter. The carbonation treatment of RAs improved the densification of the adhered old mortar and reduced the porosity of prepared concrete, rising the cracking resistance and inducing a wider FPZ. As a result, the initial descending segment of TSC became flatter and its tail segment tended to be longer.

Experimental study on the behavior of pre-loaded reinforced concrete (RC) deep beams with openings strengthened with FRP sheets

Purpose Recently, the repairing of reinforced concrete (RC) structures attracted great research attentions, but the research interests were mainly concentrated on common repairing types. To this end, in this paper, a repairing of pre-loaded RC beams strengthened by aramid reinforcement polymers (AFRP) is presented. Furthermore, the purpose of this paper is to study the behavior of pre-loaded RC Deep beams under sustained load. The AFRP has many advantages such as controlling stresses distribution around the openings, controlling failure modes, and enhancing the structural capacity of pre-cracked RC beams. Design/methodology/approach Four specimens were experimentally tested: one specimen without strengthening, which is considered as control specimen, one strengthened specimen using AFRP without pre-cracking and two specimens subjected to pre-cracking load before prior to AFRP application. Furthermore, after validation of experimental data by using ANSYS software, a parametric study was conducted to investigate the effect of pre-damage level on shear capacity of RC beams. For pre-cracked beams, loading was first applied until the cracking stage, followed by specimen repairing with epoxy injection, and then the specimens were loaded again until failure point. Findings The result showed that pre-damage level and AFRP strengthening have great influence on the ultimate strength and failure mode. In addition, the results obtained from experimental tests were compared with those from numerical validation with ANSYS and showed good agreement. Originality/value Based on ACI guidelines, an analytical equation for calculating the shear strength of strengthened RC beams with openings subjected to pre-damage was then proposed, and the calculated results were compared with those from the tests, with differences not exceeding 10%.

From macro-morphology to micro-mechanics: a deep dive into hydraulic fracturing of compacted bentonite

A series of injection tests with compacted Gaomiaozi bentonite of varying dry density were performed using an innovative testing apparatus for hydraulic fracturing visualization, with emphases placed on fracturing macro-morphological dynamics and micro-mechanical mechanisms. Results showed that fracturing dynamics mainly consisted of three stages, namely the hydrating stage, the cracking stage, and the fracturing stage. As dry density increased, the circular hydration zone expanded, the cracking network became more intricate, and the fracturing pattern transitioned from long and clear to short and fuzzy. Breakthrough time and breakthrough pressure increased exponentially with increasing dry density, while injection rate decreased exponentially and breakthrough water volume increased linearly. The fracturing mechanism in compacted bentonite was turning from tensile to shear mode with increasing dry density. By comparing the shear strength with the sum of swelling pressure and tensile strength, a simple criterion for determining the limit state between tensile and shear modes was established, which can be popularized in other soils without swelling potential but under different confining pressures. Finally, a novel calculation method for breakthrough pressure was proposed based on cavity expansion theory, taking into account swelling effects and fracturing failure modes.

Numerical prediction of fretting fatigue crack growth in scaled railway axles considering fretting wear evolution

To predict the fretting fatigue crack growth (FFCG) life of railway axles, a series of interrupted fatigue experiments were conducted on scaled railway axles. Subsequently, the evolutions of fretting wear and fatigue cracks in the press-fitted region were analyzed. Based on the test results, finite element models incorporating fretting wear evolution were established, and the FFCG was investigated using the maximum tangential stress criterion, cyclic resistance curve, and the modified NASGRO equation. The analysis revealed that the fretting wear evolution leads to stress redistribution at the press-fitted region, thereby promoting FFCG. When considering fretting wear evolution, the equivalent stress intensity factor (SIF) range of the crack remains above the threshold value throughout the short crack stage. However, neglecting fretting wear evolution results in the SIF range being below the threshold value for cracks shallower than 0.30 mm. This implies that considering fretting wear evolution enables life prediction throughout the short crack stage. As the crack length increases, the influence of fretting wear evolution on crack growth gradually diminishes. By accounting for fretting wear, a more accurate stress distribution in the press-fitted region can be obtained, leading to a more precise and conservative prediction of crack growth life.

Statistical modeling of microstructurally short crack growth in high cycle fatigue

Capturing the behavior of short cracks under fatigue conditions has become a challenge for researchers. The early stage of crack propagation is influenced by microstructural features, such as grain boundaries and orientations, which leads to significant statistical variations. Models such as the ones developed by Navarro and De Los Rios have been able to describe the process by considering the average properties of the material and estimating the number of cycles related to the short crack stage. However, fatigue should be seen as an extreme value process. In the present work, a more realistic model was developed to consider the statistical nature related to grain orientations to estimate fatigue life dispersion. Eleven crack propagation criteria have been used to compute the number of cycles spent in the short crack stage. Parameters such as the Schmid factor, the potential crack length, and the various angles describing the misorientation between adjacent grains were considered. One thousand random scenarios for short crack growth were studied for each propagation criterion and some statistical analyzes were run to estimate the worst-case scenario out of one million. Results can be well described using the generalized extreme value distribution, capturing the stochastic nature of the fatigue life.

Unified Models for Bond Strength and Bond-Slip Response of Deformed Rebars in Cracked Concrete

Cracks in reinforced concrete structures pose a potential risk to their long-term stability and safety, necessitating an accurate analysis of the bond failure mechanism between the reinforcement and the cracked concrete. However, currently, there is a lack of comprehensive bond prediction models for the entire cracking stage, and limited understanding of the bond slip response. To address the aforementioned issues, an improved model incorporating internal pressure, crack width, and failure modes, aiming to provide a more precise estimation of the bond strength between deformed bars embedded in concrete under various cracked cases. The proposed model demonstrates superior performance in predicting cracked cases and failure modes. The performance of the proposed model in predicting cracked cases and failure modes is superior to existing models, as demonstrated by a comparative analysis. Additionally, a comprehensive model is proposed to analyze the bond-slip behavior in cracked concrete. This model introduces a bond reduction factor for cracks, aiming to determine the critical bond length required for bond strength.

Experimental study on hydrogen production from heavy tar in biomass gasification furnace catalyzed by carbon-based catalysts

High boiling point heavy tar compounds such as pyrene and naphthalene are key factors causing corrosion and blockage in the pipelines of biomass thermal utilization equipment, and catalytic reforming represents an effective means to address heavy tar issues. Biomass carbon, characterized by its cost-effectiveness, high efficiency, and environmental friendliness, is envisioned as a catalyst carrier with extensive application prospects. In this study, the tar produced by a biomass fixed-bed gasifier was investigated, with an initial analysis of the pyrolysis process and definition of its different stages conducted. It was discovered that the condensation and secondary cracking stages are the predominant phases for the formation of heavy tars, including polycyclic aromatic hydrocarbons (PAHs). Carbon-based carriers were prepared using apricot shell as the raw material, and biomass carbon-based catalysts were synthesized via impregnation methods. The heavy tar extracted from the original tar through distillation was utilized in the hydrogen production tests of steam catalytic reforming with biomass carbon-based catalysts. The investigation into the impact of temperature, steam-to-tar mass ratio (S/T), and tar-to-catalyst mass ratio (T/C) on the catalytic reforming of tar has been conducted. In considering a variety of parameters, including hydrogen production rate, tar conversion rate, and the volume of hydrogen generated, it was found that under the conditions of 800 °C, S/T = 3, and T/C = 2, the Ni/C catalyst yielded the highest amount of hydrogen (91.52 g H2/kg tar) with a tar conversion rate of 93.30 %. The Ni-Co/C composite catalyst exhibited stronger catalytic activity and superior deactivation resistance compared to the monometallic catalysts. For the latter, steam regeneration methods effectively restored the catalytic activity of the carbon-based catalysts, enhancing the hydrogen yield rates of Ni/C and Co/C catalysts by 2.74 and 1.55 times respectively.

Strengthening of RC beams with inadequate lap splice length using cast-in-situ and anchored precast ECC ferrocement layers mitigating construction failure risk

Design standards necessitate the use of a sufficient lap splice length when dealing with longer spans of Reinforced Concrete (RC) members. An inadequate lap splice length results in a reduction of both the flexural strength and ductility of reinforced concrete beams. The failure risk of these beams is considered a potential threat which is experimentally identified, numerically analyzed, and carefully mitigated in this research to increase building safety and sustainability to avoid risk of construction failure. The ongoing experimental and numerical research focuses on examining the structural performance of RC beams with insufficient lap splice length, which have been fortified through various applications of Engineered Cementitious Composite (ECC) ferrocement layers. A total of eleven beams, divided into four groups, were purposefully designed and tested under bending loads until they reached the point of collapse. The investigated parameters included the casting technique, installation strategy, and anchorage length (La) for such layers. Cast in situation and pre-casting scenarios were utilized in order to cast the strengthening ECC layers. The La for all layers was designed to be the main studied variable beside type of casting and installation technique. Three La values were studied: 30D, 40D, and 50D where D is the bar diameter of main reinforcement steel in tension side. It was found that, the application of chemical anchor bolts in cast in situation ECC ferrocement layers credited the higher enhancement in both cracking and ultimate stages by about 92–137% and 114–164% for cracking and ultimate levels, respectively. Then, the experimentally measured outcomes were employed for developing nonlinear Finite Element Models (FEMs) to simulate the performance of such ECC ferrocement layers that employed. The variance between numerical results and experimental counterparts ranged in between 6% and 9%, achieving an acceptable variance.

Damage evolution and crystalline orientation effects in ultrafast laser micro/nano processing of single-crystal diamond

Single-crystal diamond is a typical difficult-to-machine material because of its extreme hardness and high brittleness. Compared with traditional machining techniques, ultrafast laser is believed to enable materials processing with high efficiency, high quality and high spatial resolution. To understand the formation mechanism and crystalline orientation effects of damage in ultrafast laser processing diamond, single-path scans were performed in different crystal directions, and groove shape, surface morphology, subsurface damage, phase transformation, and laser-induced periodic surface structure (LIPSS) were systematically analyzed. The experimental results illustrate that the damage evolution can be divided into three different stages according to the pulse number, named weak ablation stage, severe cracking stage, and severe phase transformation stage. Crystal orientation significantly affects the groove shapes, cracks, and phase transformation, and these differences are related to the cleavage energy and atomic rearrangement energy of the crystal plane deposited by the laser energy during processing. At low pulse numbers, the surface graphitization is initiated easier along the <110> direction, which makes the absorption rate of laser energy locally enhanced, further driving the anisotropy of the processing damage at moderate and high pulse numbers. This work provides a new perspective on ultrafast laser processing of single-crystal diamond, which is crucial for ultra-precision and low-damage fabrication of diamond-based functional micro/nano devices.

Study on stage characteristics of hydraulic concrete fracture under uniaxial compression using acoustic emission

ABSTRACT Acoustic emission (AE) can monitor and evaluate the cracking situations of hydraulic structures for it is real-time and non-destructive. However, a current challenge of this approach is to accurately identify the various stages during the cracking process. The innovation of this paper lies in the application of AE data in analyzing the stage characteristics during the whole process of hydraulic concrete fracture. First, the slope changing of the cumulative ringing count curve was used to determine the cracking stages of hydraulic concrete. Second, the function of inter-event times (denoted as F-function) was used to further investigate the AE characteristics proximity to the final fracture. Last, the spatial-temporal-energy evolution of cracks was studied. The results show that: (1) The whole process of hydraulic concrete failure shows an obvious stage characteristic from the perspective of AE feature and it can be distinguished to five stages by the slope change of the cumulative ringing count curve; (2) The F-function can be used to identify the last two cracking stages, and it is capable of early warning the macroscopic cracking in ahead of b-value; (3) The spatial-temporal-energy distribution of cracks helps to better understanding the generation and evolution for cracks inside hydraulic concrete.

Effect of notch depth ratio and specimen width on mode II fracture properties of ECC

Engineered Cementitious Composites (ECCs) can be used as repair and reinforcement materials, which can improve the shear performance of the structure. Hence, doubled-notched and unnotched half-symmetric loading tests are used to study the mode II fracture properties of ECC and concrete, and the fracture failure morphology and fracture parameters of ECC and concrete are compared. The effects of the notch depth ratios of 0, 0.1, 0.3, and 0.5 and the specimen widths of 200 mm, 250 mm, and 300 mm on the mode II fracture properties of ECC and concrete are investigated. The mode II crack development of ECC can be divided into four stages: microcrack development stage, multiple cracking stage, main crack formation stage, and crack instability development stage. It can be found that ECC not only has high ultimate tensile strain and energy dissipation capacities, but also has obviously better ability to resist the unstable propagation of shear cracks than concrete. ECC can meet the high shear resistance requirements of some parts of the structure.

The effect of the degree of cracking on the state of stress and cracks width in elements with restrained boundary conditions

AbstractA new and more precise model of cracking of reinforced concrete elements subjected to imposed strains is presented. A key question is how in the stress classic approach to take account of the effect of element relaxation (after each cracking episode) on cracks width reduction and other cracking criteria. Such a model would enable the calculation of the maximum cracks width depending on the cracking stage (i.e., the stage of cracks formation or that of stabilized cracks). This phenomenon is important particularly in semi‐massive structures in which the condition of watertightness must be fulfilled. The model has been validated based on the experimental studies available in the literature. For higher levels of strain the cracks widths have been found seriously underestimated after European standards (especially in elements with higher reinforcement ratio) while the calculations performed following the proposed model indicate proper estimation of cracks width.