Crack Development Research Articles

Prediction on seismic performance levels of reinforced concrete beams by considering crack development

The plastic rotation angle or deflection is typically used as the indicator to evaluate the earthquake-induced damage and classify the seismic performance levels of reinforced concrete (RC) beams. Herein, one of the most important issues is to determine the seismic performance level limits of RC beams. Whereas, different countries provided their specific method to determine the limit values by considering the loading-carrying capacity of RC beams, which could not be used to describe the earthquake-induced seepage of structures, especially for underground structures. Therefore, in this study, the seismic performance level limits of RC beams were predicted by using the machine learning methods and considering the development of cracks. Firstly, the seismic performance level limits of RC beams were presented after discussing the methods in different codes and the development of cracks. Then an earthquake performance test database of RC beams was established after collecting 452 test results of RC beams, and Pearson correlation analysis was conducted for feature selection to determine the input mechanical parameters and dimensional parameters for machine learning. Meanwhile, the correlation between the inputs and limit values was analyzed using the mutual information method. Regression models of seven machine learning methods were then established to predict the performance level limits of RC beams, and the hyperparameters of the machine learning models were optimized with the TPE optimization algorithm and cross-validation. The generalization ability of the prediction models was evaluated and the accuracy of predicted results by different methods was analyzed. Finally, the predicted seismic performance level limits of RC beams could be used to evaluate the earthquake-induced damage of RC beams by combining them with the seismic behavior of RC beams.

Fracture characteristics and microscopic mechanism of slag-based marine geopolymer mortar prepared with seawater and sea-sand

This study utilized seawater, sea sand, and ternary solid waste to produce marine geopolymer mortar (MGM). 23 groups of MGM samples with varying alkali content and basalt fiber (BF) and polypropylene fiber (PPF) volume fractions, as well as marine cement mortar (MCM), were tested for fracture characteristics. Three initial crack length ratios (a0/h) (0.2, 0.3, and 0.4) were designed. The analysis included load-crack mouth opening displacement (P-CMOD) curves for all specimens, focusing on critical crack length (ac), critical crack tip opening displacement (CTODc), fracture toughness (KSIC), fracture energy (GF), and facture brittleness index (FBI). Results indicated that increasing alkali content enhanced strength and elastic modulus, peak fracture load, critical crack extension length (Δac), and CTODc, while decreasing final CMOD. This was attributed to the increased alkali content, which enhanced the mortar’s density. However, excessive alkali content also led to increased shrinkage damage within the matrix, making it prone to stress concentrations and resulting in brittle failure. Based on the research, it is recommended that the alkali content be controlled within the range of 4–5 %. Hybrid fibers showed superior overall toughness enhancement, increasing KSIC, GF, and FBI by 54.3 %, 95.6 %, and 427.4 %, respectively. This improvement was due to the synergistic effect of BF and PPF, which increased the specimen’s elastic modulus, reduced crack propagation speed, and sustained load-bearing capacity during the later stages of crack development. The fracture toughness of conventional geopolymer mortar was positively correlated with the Ca/Si ratio and was lower than that of MGM. In contrast, MGM’s Ca/Si ratio showed an inverse correlation with KSIC, indicating that phase assemblage governs the fracture resistance of MGM. This is because Cl- in seawater intensified geopolymerization reactions. Interwoven gel structures in MGM, with surface depositions like magnesium aluminosilicate hydrate (M-A-S-H), magnesium silicate hydrate (M-S-H), and silica gel, filled pores, and improved homogeneity, resulting in its superior fracture toughness over conventional geopolymer mortar.

Seismic behavior and failure analysis of prefabricated foamed concrete composite walls for passive houses

In view of the lack of research on the failure mode and seismic behavior of high-performance insulation walls utilizing lightweight concrete as the structural material, this study presents experimental findings from a seismic behavior investigation of prefabricate foamed concrete composite walls (PFCCWs) for passive house in China. The PFCCWs incorporate low thermal conductivity and high-strength foamed concrete, polyurethane, decorative magnesium straw slabs to provide excellent thermal and mechanical properties for prefabricated passive houses in hot summer and cold winter area. The seismic behavior of PFCCWs under different influence factors were elaborated using quasi-static test data and observation of external damage. The lightweight and porous nature of foamed concrete leads to significant crack formation during failure. The findings suggest that the skin effect of magnesium straw slabs and polyurethane impedes crack development and improve the load-bearing capacity with ductility enhancement. Additionally, reducing the aspect ratio and substituting post-cast column concrete with common concrete are effective to improve the seismic behavior. The window opening changes the worst damage area of the wall and notably weakens the seismic behavior. The enhanced integrity of cast-in-place wall leads to the difference in the failure mechanism and increases in load-bearing and deformation capacities. Meanwhile, design suggestions for the foamed concrete insulation walls are also proposed by comparing the failure mode and hysteretic response of various wall types.

Analysis of dynamic response and impact resistance of corrugated plate-reinforced concrete under simulated rockfall

In this study, corrugated plate material is used as the internal mold for the reinforced concrete structure, proposing a combined shelter structure designed to protect against rockfall hazards in mountainous areas. The research focuses on the impact of low-velocity rockfall (below 150 km/h) directly hitting both the reinforced concrete structure and the corrugated plate and reinforced concrete combined shelter structure. The accuracy of the numerical simulation results is validated through comparative analysis with indoor model experiments. Further investigation reveals the impact resistance and failure characteristics of RC (Reinforced Concrete) structures, CPRC(D) (Corrugated plate double-layer reinforced concrete composite structure) structures and CPRC(S)（Corrugated plate single-layer reinforced concrete composite structure）, with the residual resistance index (RRI) introduced for the quantitative analysis of the residual load-bearing capacity of the protective structures post-impact. The results indicate that the shape of impact craters and the maximum impact force obtained through numerical simulations closely match the experimental findings, demonstrating the validity of the simulation parameters. Under the same impact energy, the maximum peak stress at various points in the CPRC structure is reduced by more than 50 % compared to the RC structure. Damage to the RC structure under rockfall impact is primarily characterized by central penetration failure and extensive development of diagonal cracks. In contrast, the CPRC structure effectively suppresses penetration, dent deformation, and crack propagation, significantly reducing the volume of concrete damage and the proportion of rebar damage, with the residual resistance index consistently remaining above 0.27.The corrugated plate and reinforced concrete combined structure outperforms the reinforced concrete structure in several impact resistance factors, including maximum impact force, effective stress extremes, structural damage, and penetration depth. Additionally, after reinforcement with corrugated plates, the CPRC structure maintains a high level of impact resistance in most scenarios, even when the lower layer of rebar is eliminated, providing a theoretical basis and preliminary feasibility proof for the lightweight modification of the structure.

Experimental and simulation study on seismic performance of seawater sea-sand PVA cementitious composite columns with GFRP bars

The seismic properties of seven seawater sea-sand (SWSS) PVA cementitious composite columns with Glass Fiber Reinforced Polymer (GFRP) bars were studied. The effects of four parameters, namely polyvinyl alcohol (PVA) fiber content, axial compression ratio, cementitious composite strength and stirrup spacing, on the seismic performance of composite columns were analyzed. The failure of the specimen was observed, and the hysteresis curve, skeleton curve, stiffness degradation, energy dissipation capacity, ductility and strain of the specimen were analyzed. The test results show that the large chip-based spalling will occur when the specimen without fiber incorporation is damaged, and the incorporation of PVA fiber, the reduction of stirrup spacing and the reduction of axial compression ratio will delay the rate of crack formation and development. In addition, through cross-section analysis and considering the reinforcement effect of PVA fibers, a proposed formula for calculating the horizontal bearing capacity of composite columns is proposed. Finally, the finite element model of SWSS PVA cementitious composite columns with GFRP bars is established, and it is found that increasing the strength grade of cementitious materials can greatly improve the seismic performance of composite columns. The conclusions could be references for the engineering application. In the context of the global emphasis on green development and environmental protection, seawater and sea-sand cementitious composites and components with fiber reinforcement will have a wide range of application prospects in the construction of coastal areas.

Experimental study on the flexural behavior of reinforced concrete beam strengthened with novel prefabricated HSSM-UHPC thin plate

Ultra-high performance concrete (UHPC) has been used to strengthen reinforced concrete (RC) structures recently. Although UHPC has high compressive strength, the tensile strength is still low and, the high curing condition on site requirements of UHPC may hinder the further application in complex condition. In this study, a novel prefabricated UHPC thin plate, which is reinforced with high-strength steel wire strand meshes (HSSM), is proposed for rapidly strengthening RC beams. In order to understand the flexural performance of RC beams strengthened with the prefabricated HSSM-UHPC thin plate, four-point bending tests were conducted on three beams including one unstrengthened RC control beam (CB), one cast-in-place HSSM-UHPC strengthened beam (SUB) and one prefabricated HSSM-UHPC strengthened beam (PSUB). The failure modes of the beams were investigated and, the crack development process of the test beams was detailly observed by digital image correlation (DIC). Experiment results showed that compared with CB, the cracking, peak load of SUB and PSUB increased by 25.30%, 28.5% and 19.8%, 26.4%, respectively. However, the deflection ductility of the strengthened beams slightly decreased. Finally, calculation model and theoretical equation for predicting the cracking as well as peak flexural capacity of HSSM-UHPC plate strengthened beams were proposed. Comparison between theoretical and test results showed that the proposed model can accurately predict the cracking and peak capacity of the strengthened beams, and can be used for strengthening design.

Tests on seismic and shear performance of RC shear walls under alternating axial tensile and compressive loads

Under strong earthquakes, reinforced concrete (RC) shear walls at the bottom of high-rise buildings may experience an internal force state of alternating tension-shear (-bending) and compression-shear (-bending). In order to clarify the seismic and shear performance of shear walls under alternating axial tensile and compressive loads (referred to as alternating axial loads), cyclic tests on five shear-controlled large-scale RC shear walls were conducted in this study. The tests were conducted with synchronized axial loads and horizontal displacements, and a quantitative analysis of the influence of the alternating axial loads was achieved by setting two control specimens. The crack development, failure modes, load-carrying capacity, deformation capacity, and energy dissipation of the specimens were measured and analyzed. The test results indicate that different axial load cases altered the crack pattern and failure mode. With an increase in the target axial tensile load, both tension- and compression-shear capacities of the specimens under alternating axial loads decreased, with the former experiencing a more significant reduction. In comparison to the tension-shear control specimen, it was observed that the alternating axial loads significantly reduced the displacement ductility of shear walls in the tension-shear state, resulting in substantially lower energy dissipation capacity. In addition, based on the tests in this study and the collected shear wall tests, the shear strength formulas in the current codes ACI 318-19 and JGJ 3–2010, as well as a shear model previously proposed by the authors, were evaluated. Based on the test results, a reduction factor was fitted to account for the effect of alternating axial loads on the compressive-shear capacity of RC walls.

Insights into Structural Changes During Scratch Deformation of Ductile Coating Systems: Plastic Flow, Microstructural Transformations, and Crack Development

This study investigates the structural changes occurring during the scratch deformation of ductile materials, focusing on a composite aluminum (Al) coating on a steel substrate fabricated using a solid-state aluminizing technique. By analyzing the results of the scratch test with transmission electron microscopy (TEM) and focused ion beam (FIB) techniques, the research provides a detailed examination of plastic flow, microstructural transformations, and crack development. Results demonstrate that strong adhesion at the interface and similar mechanical properties between steel and Al layers facilitate co-deformation, crucial for maintaining the integrity of the composite structure. The study features the formation of a rolling-like laminated structure and significant grain refinement in both the steel and Al components, driven by plastic deformation. Observed atomic-scale intermixing and the formation of an amorphous interlayer highlight extensive material interactions during deformation. The visualization of crack progression using FIB allows for an in-depth understanding of the fracture process. Subsurface cracks initiate within the thin Al interlayer and propagate toward the surface, evolving through the coalescence of nanopores into microvoids and larger cavities, ultimately leading to crack propagation. Secondary internal cracks may arise due to stress fields generated by the primary crack, contributing to a complex network of internal cracks.

Influence of loading rate on the mode II fracture characteristics of SCC samples: Experiments and numerical simulations

This paper explores the influence of loading rate on mode II fracture failure in rock. Impact experiments were performed on samples of SCC at five different impact pressures via an SHPB system. This study reveals the correlations among the peak load, fracture toughness, and dynamic elastic modulus of rock mode II fractures under diverse loading rates, as well as the alterations in fracture trajectories. Simultaneously, PFC3D discrete element software has been adopted to numerically simulate the experiments and analysis of the fracture process of rocks and the variations in crack quantity and energy from a microscopic perspective. The results suggest that dynamic mode II fracture tends to increase as the loading rate increases and that the loading rate affects the fracture failure trajectory of a sample. Concurrently, as loading increased, the percentage of shear cracks in the samples gradually decreased, whereas the proportion of tensile cracks gradually increased, indicating that the samples experienced compressive stress after mode II fracture occurred at high loading rates. The proportion of energy absorbed by the samples for crack initiation and development, as well as the kinetic energy of the particles, initially tends to decrease but then increases with increasing loading rate, which is related to whether the loading rate generates secondary cracks. It is hypothesized that there exists a critical loading rate that triggers secondary cracks in SCC samples.

Experimental study on residual compressive bearing capacity of cracked reinforced concrete columns by SCDA

Abstract The static demolition of structures via soundless chemical demolition agent (SCDA) is attracting increasing attention because of its environmental friendliness and safety, especially in the dense urban areas. However, the research on the application of SCDA in demolishing the reinforced concrete is still scarce, seriously limiting the development of static demolition. In this work, with the combined efforts of experiments and theoretical analysis, we investigate the critical role of borehole layouts in the static demolition of reinforced concrete (RC) by determining the occurrence and expansion of cracks during static demolition, and the resulting residual mechanical properties of RC. The results show that the growth of cracks is significantly dependent on the reaction of SCDA. Additionally, the initiation time of cracking exhibits minimal sensitivity to variations in the layouts pattern of boreholes, while the progression of cracking is predominantly governed by the layouts of boreholes. We demonstrate that cracks formed in static demolition process can significantly reduce the strength of RC with about the remaining bearing capacity is only 10% to 33% of the original and change its failure mode. Further, it is point out that the key factor controlling the failure of RC under axial compression (i.e., the integrity of structures), and the RC with larger crack size and less structural integrity is more prone to fail. The results and findings in this work further clarify the role of borehole layouts and are of great significance in improving the efficiency of static demolition.

Experimental investigation on the frost resistance of RCC layers with various interface treatments

This research investigates the influence of various interface treatment methods on the frost resistance performance of Roller Compacted Concrete (RCC) layers. Freeze-thaw cycle tests were conducted on RCC with interfaces treated using cement mortar, expansion agent mortar, and nano-SiO2 mortar. The study evaluated shear failure modes, shear strength, crack expansion, pore structure, and liquid uptake of the RCC interface after freeze-thaw cycles under different treatment methods. Results reveal that expansion agent mortar and nano-SiO2 mortar improve the shear strength of RCC after freeze-thaw cycles more effectively than cement mortar. The freeze-thaw cycles increase the width and expansion rate of cracks at the RCC interface during the shear process, while mortar treatments help delay crack development. Additionally, the pore volume at the RCC interface gradually increases, and the uniformity of pore distribution decreases under freeze-thaw cycles. During liquid uptake, mortar treatments slow down the upward migration of liquids by improving the pore structure at the interface. The liquid uptake of RCC exhibits a distinct linear relationship with its shear strength and pore structure. As liquid uptake time increases, water first fills larger pores before gradually entering smaller ones. Freeze-thaw cycles cause an increase in pore volume and enlargement, which leads to greater liquid uptake. Based on the damage mechanism of the RCC interface under freeze-thaw conditions, a model of shear strength degradation due to freeze-thaw cycles was established, aligning well with experimental results. Mechanism analysis indicates that nano-SiO2 and expansion agents enhance frost resistance by filling interface pores and cracks through reactions with certain compounds, thereby improving the microstructure.

Experimental investigation of failure mechanism of fissure-filled sandstone under hydro-mechanical conditions

Fissure fillings are critical to the hydro-mechanical properties of jointed rock masses in rock engineering. In this study, triaxial seepage tests were performed on standard cylindrical fissure-filled sandstone. The characteristics of stress–strain relationships, absorption and consumption of energy, variations in deformation resistance, and permeability evolution during the experimental process, along with the crack development observed in post-failure computed tomography scan images of the sandstone specimens were analyzed. The results demonstrate that the fillings improve the energy capacity and reduce the damage accumulation of sandstone specimens, with sand-filled specimens performing better than mud-filled specimens, especially at lower bridge angles. The fillings can reduce the depth of crack extension and lessen the influence of prefabricated fissures on sandstone failure, with this effect diminishing as the rock bridge angle increases. Permeability decreases in the pre-peak failure stage as the fillings improve the deformation resistance of the sandstone specimens. In the post-peak failure stage, the fillings and rock debris generated by the sandstone failure move within the developed fractures, causing significant fluctuations in permeability. These findings deepen the understanding of the hydro-mechanical properties of jointed rocks and provide a scientific basis for stability analysis in rock engineering.

Enhancing dynamic mechanical properties of cemented lithium mica tailings backfill with alkaline rice straw fibers: Experimental investigation and microscopic analysis

Cemented lithium mica tailings backfill (CLMTB) faces dynamic loads from mining operations, affecting its stability. This study investigates the use of alkaline rice straw fibers (ARSF) to enhance CLMTB's dynamic properties. Dynamic impact tests were conducted on CLMTB specimens with varying ARSF contents, revealing that dynamic compressive strength (DCS) increased with ARSF up to 0.45% before decreasing. DCS improved with rising strain rates. The research includes analyses of stress-strain behavior, failure patterns, and energy dissipation during impacts, with scanning electron microscopy revealing the microstructural effects of ARSF on CLMTB. The findings suggest that ARSF effectively mitigates the damaging impacts of loads, significantly reducing crack development in CLMTB.

Experimental investigations on the failure characteristics of the slit-contained circular opening under biaxial compression: Insights into the rockburst prevention

The slit-cut method for the rockburst prevention and control is believed effective with its easiness in operation, adjustment and compatibility. However, there is limited advance knowledge of the physics of the slit-cut method, which is vital for the engineering designs. In this study, the biaxial compression tests with a synchronous AE (acoustic emission)-DIC (digital image correlation) monitoring are creatively carried out on the slit-contained circular opening specimens with different slit configurations to demonstrate the academic thoughts and mechanisms of the slit-cut method. The experiments document the two typical failure types namely the internal crack propagation and the dynamic rockburst, which occupy different AE hit rate characteristics and different entropy properties. With the occurrence of rockburst, the AE hit rate presents a bouncing ascend-descend trend, and a higher disorder and chaos is faithfully exhibited. Depending on the slit parameters, the slit-cut method can efficiently mitigate rockburst in terms of the occurrence frequency and magnitude. The underlying mechanism lies in the enhancement of the shear mechanism and the development of the internal cracks through which the stored energy can be greatly dissipated. However, due to the unrestricted shear failure in the slit-contained opening specimens, the opening-scale inward instability can be triggered by the internal crack coalescence, thus posing a threat to the safety of the opening. Implementing the slit-cut method with a consideration of the in-situ stress conditions is evidently essential for the safe excavation.

Effect of In Situ Polymerization of Super Absorbent Polymers on the Protection of Earthen Heritage Sites in Semi-Arid Regions

As precious cultural heritage, earthen sites are susceptible to various natural factors, leading to diverse forms of degradation. To protect earthen sites, the effects of super absorbent polymers (SAPs) on soil water retention, physical properties, and color compatibility at different concentrations were studied. After applying SAP treatment to an earthen site with different degrees of weathering, we drew the following conclusions. SAP-2 improved soil water retention capacity, increased soil water content, and slowed down the precipitation of soluble salts. At the same time, SAP-2 had the least effect on soil color difference and reduced the development of cracks by filling soil pores and enhancing the cohesion between soil particles, thus giving the earthen sites better weathering resistance. Therefore, the results provide a useful reference for the surface cracking of earthen sites in semi-arid areas and the degradation caused by flaking and block spalling.

Enhanced ductility via high-density nanoprecipitates driven by chemical supersaturation in a flash-heated precipitation-strengthened high-entropy alloy

Precipitation strengthening is an effective method to strengthen high-entropy alloys (HEAs); however, the incompatible plastic deformation caused by precipitates accelerates the initiation and development of cracks, reducing ductility. Therefore, it is necessary to spare no effort to overcome this strength–ductility trade-off. However, improving the ductility of HEAs without losing the precipitation-strengthening effect remains a daunting challenge. The strategy of this study is to increase chemical supersaturation without introducing other defects to drive the formation of high-density nanoprecipitates to share stress, reduce localization, and alleviate incompatible plastic deformation, thereby fully utilizing the strain-hardening ability to improve the ductility of HEAs without reducing their strength. Specifically, common Ni35(CoCrFe)55Al5.5Ti4.5 (at.%) precipitation-strengthened HEAs were selected as the experimental object. Short-term high-temperature flash heating causes the dissolution of the initial nanoprecipitates, and the separated atoms form chemically supersaturated microregions in the matrix between the initial nanoprecipitates after insufficient short-range diffusion, resulting in the formation of high-density new nanoprecipitates. The density of the nanoprecipitates reaches 5.0 × 1024m−3, representing a two-order increase in magnitude compared to the aging state (1.9 × 1022m−3). By increasing the density of nanoprecipitates, multiple nanoprecipitates share stress together. Sufficient slippage of dislocations maximizes the potential excellent strain-hardening ability, considerably improving the ductility of HEAs. Moreover, the strategy of improving the ductility by increasing the density of nanoprecipitates can be applied to many other precipitation-strengthened alloy systems.

Application of vibrothermography to visualise hidden cracking process in concrete at the component scale

A thorough understanding of the cracking process in concrete structures, especially the initiation and expansion processes in the hidden stage, plays a decisive role in the maturity and completion of fracture mechanics theory. However, there exist few simple methods to detect the multi-crack development under the condition of unknown initiation area at the component scale and associated behaviours of the cracks during crack development. In this study, ultrasonic thermal excitation to visualise the multi-point hidden cracking process in concrete components was studied and hidden cracking detecting experiments were conducted. Multi-point initiation and expansion within the range of the components were achieved by destroying a suitably reinforced concrete beam using four-point bending tests with step-by-step cyclic loading. The cracking processes were monitored in real time via the auxiliary use of strain gauge groups and digital image correlation (DIC). The loading process was controlled based on the strain rules, and typical cracking stages were observed. The ultrasonic thermal excitation visualisation effect at typical cracking stages was studied via local monitoring using DIC. The results show that the ultrasonic thermal excitation visualisation method and the corresponding device proposed herein can be used to monitor hidden cracking processes in concrete at the component scale, while multi-point initiation and expansion of 0.01-mm-level crack groups can be detected in a simple and efficient manner.

Effect of PVA fibers grafted with MWCNTs on the shrinkage behaviors of engineered geopolymer composite (EGC)

Geopolymer have been considered as a promising alternative for the development of high-ductility and eco-friendly engineered materials. In this study, modified PVA fibers grafted with MWCNTs(PM) are used as additives to investigate their impact on the drying shrinkage, autogenous shrinkage and chemical shrinkage of Engineered Geopolymer Composites(EGC). Additionally, various micro-structure techniques, including X-ray Diffraction(XRD) and Scanning Electron Microscopy(SEM), are employed to examine the morphology and chemical composition of EGC. The experimental findings indicates that PVA fibers and MWCNTs both help to mitigate the drying shrinkage and autogenous shrinkage of the EGC. When the PVA fiber content is 2.0 % and the MWCNT content is 1.0 ‰, the 90-day drying shrinkage and autogenous shrinkage values of the EGC are minimized, reducing by 31.67 % and 34.16 %, respectively, in comparison to the control group. However, the incorporation of MWCNTs increases the chemical shrinkage of the geopolymer matrix, with the chemical shrinkage value reaching its maximum when the MWCNT content is 1.0 ‰. Micro-structural analysis reveals that the inclusion of PM leads to an improvement in the quantity of reaction products without affecting their type. Furthermore, the incorporation of PM refines the overall pore structure, restricts the crack development and enhances the interfacial bonding effect of EGC. Notably, a correction prediction model for drying shrinkage strain and autogenous shrinkage strain, incorporating the influence coefficients of PVA fibers and MWCNTs, is constructed under constant humidity and temperature conditions. This significantly improves the prediction accuracy of drying shrinkage and autogenous shrinkage of EGC.

Reactive molecular dynamics of the fracture behavior in geopolymer: Crack angle effect

Reactive molecular dynamics was applied in this study to construct the sodium aluminosilicate hydrate (N-A-S-H) and tensile fracture models with various crack angles. The impact of crack angle on the behavior of N-A-S-H fractures was explored while considering structural mechanical properties and energy evolution. Furthermore, the fracture toughness and brittleness index for various crack angle models were calculated. The findings indicated that the ultimate strength and elastic modulus of the fracture models grew linearly with the increase in crack angle. The fracture toughness value progressively grew while the model’s elastic energy efficiency and new surface energy efficiency decreased simultaneously. The 45° crack model possessed the largest oblique crack development surface in the fracture process due to the coupling effect of tensile and shear stress. Its elastic energy efficiency decreased as well the most, while the new surface energy efficiency increased and the fracture toughness value dropped sharply. It is crucial to place a stronger emphasis on spotting cracks both in the in-service structures or structures being demolished. This ensures optimal performance and safety by enabling more effective adjustments to the direction of external forces and energy input.

Research on the strength influence and crack evolution law of layered backfill based on macro and meso mechanical response

This article analyzes the effects of inclined angle (IA), filling times (FTs) and cement tailings ratio (CTR) on the uniaxial compressive strength (UCS) and crack development law of layered cemented backfill (LCB) using various research methods, including indoor experiments, numerical simulation, and computed tomography (CT) scanning. The research shows: (1) In the indoor experiments, it was observed that increasing the IA, increasing the number of FTs, and decreasing the CTR had a significant weakening effect on the UCS of the specimens. The greater the IA, the more pronounced the separation and dislocation of the layered contact surface, resulting in a significant increase in macroscopic cracks at the bottom. As the FTs increases, the center of gravity of the failure of the LCB gradually moves downwards. This causes the damage and failure time to advance, and the peak stress to change from one main crack to multiple main cracks. (2) During the numerical simulation, compressive stress accumulates and increases around the pores in the layered contact surface. Microcrack aggregation occurs on the contact surface, and downward expansion and extension. The peak strain energy decreases as the IA increases with the FTs. The number of shear cracks increased more at 20° to 30° than at 0° to 20°. The strain energy of the 0° specimen is the highest, while that of the 30° specimen is the lowest. (3) The CT scan shows that the LCB is affected by stress conduction hysteresis. When the bottom backfill is in the stage of crack evolution, the top backfill is in the stage of compaction.When the LCB reaches the peak stress, both the bottom and middle layers of the backfill lose load-bearing capacity, and the top layer of the backfill has no cracks. This paper provides reference value and research ideas for the destruction of backfill that contain layered defective structures.