Crack Development Research Articles

Mechanical anisotropy in additively manufactured laminated high-entropy alloys: The role of interface geometry

The remarkable mechanical properties exhibited by laminated structures have generated significant interest in the realm of additively manufactured laminated high-entropy alloys (HEAs). Despite this burgeoning interest, the nexus between process, structure, and properties within laminated HEAs remains largely uncharted. There is a vast space for investigating the effect of the typical heterogeneous interface on the macroscopic mechanical properties. This study focuses on the influence of the characteristic heterogeneous interface on macroscopic mechanical properties of laminated HEAs, particularly anisotropy. Using the 3D-printed Fe50Mn30Co10Cr10-CoCrNi HEA as a model, we investigate the impact of interface geometry on mechanical characteristics. Tensile tests show that the reduced interface spacing increases yield strength. This laminated HEA displays significant anisotropy in strength and ductility, depending on the loading direction relative to the interface. Electron microscopic observations suggest that finer layer spacing enhances interface and dislocation strengthening, increasing yield strength. Anisotropic behaviors are confirmed to be mediated by interface orientation, explained in terms of deformation compatibility and crack development at the interface. This research offers fundamental insights into the relationship between heterogeneous interfaces and the mechanical properties in laminated HEAs. The knowledge is vital for designing, fabricating, and optimizing laminated HEAs through additive manufacturing, advancing their engineering applications.

Desiccation cracking behavior and its suppression methods in lateritic soil under drying and wetting cycles

In this study, the desiccation cracking behavior of lateritic soil caused by drying and wetting cycles was investigated and effective methods for mitigating crack development were proposed. Direct mixing and spray coating methods based on different additives were used to modify lateritic soil. Cyclic wet–dry tests were performed to analyze the influence of wet–dry cycles on the cracking behavior. Subsequently, uniaxial tensile tests were conducted to examine the strength degradation caused by crack formation and strength enhancement by additives. In addition, the modification mechanisms were revealed using electron microscopy. The results demonstrated that desiccation cracks developed significantly during the drying process, with some cracks closing upon wetting. However, most of the cracks reopened and expanded further during subsequent drying, leading to a steady increase in the crack rate during the wet-dry cycles. When using the direct mixing method, lignocellulose was the most effective additive for enhancing the crack resistance of lateritic soils. The optimal crack resistance was achieved with a lignocellulose content of 0.75%, resulting in an 18.2% increase in tensile strength. Conversely, when employing the spray coating method, PAC was found as the optimal additive, with a desirable concentration of 1.5–6.0%.

Crack and failure behaviors of sandstone subjected to dynamic loads visualized by micro-computed tomography

Microscopic dynamic failure behaviors of rocks are significant to rock engineering, which is still insufficiently understood. This study combines split Hopkinson pressure bar (SHPB) and micro-CT (computed tomography) to explore the microscopic failure characteristics of sandstone under impact loading. SHPB is responsible for the dynamic test, and micro-CT is responsible for pre- and post-test inspections. The results show that the pores and defect influence the dynamic strength but do not alter the overall trend of increased strength with a higher impact level. The dynamical crack development is then analyzed. Three types of cracks (i.e. I-, Y-, and H-type) are identified to describe the crack development. When rock is simply fractured, only I-type crack exists due to tensile failure, and it grows irregularly. As the strain rate increases, I-type crack is transformed into Y- and H-type crack due to shear failure. Crack coalesces at that moment, and the complexity increases along the impact direction. The coalescence occurs preferentially in the area with more pores, and around a third of pores are involved, where the maximum contribution area is in the middle of sample. Microcracks are formed inside the rock blocks, and rock grains and fragments fill in the cracks. The dynamic crack development is accompanied by microcracks, while rock grains and fragments result from the development of these microcracks. In addition, the influence of a semi-penetrating defect perpendicular to the impact direction is investigated. The defect can impede stress transfer and concentrate energy consumption. The findings are expected to enhance understanding of rock dynamics and support rock engineering development.

Crack development and fracture performance of Recycled Powder Engineered Cementitious Composites (ECC): Experimental investigation

In this paper, recycled concrete powder (RCP) and recycled brick powder (RBP), with particle sizes ranging from 10 to 130 μm, are used to completely replace the fine aggregate quartz sand at a ratio of 1:1. On this basis, recycled glass powder (RGP), with a discussed particle sizes range of 38–300 μm, is used to partially substitute the cementitious materials. Polyethylene (PE) fibers are used as the reinforcement in the Engineered Cementitious Composites (ECC). Using the particle size range and substitution rate of RGP as variables, the crack development, crack mouth opening displacement (CMOD), fracture toughness and fracture energy of Recycled glass powder ECC (RGP-ECC) were discussed through a three-point bending experiment on a single-edge notch beam. A relatively comprehensive evaluation was made on the fracture performance and behavior of recycled ECC, and a suitable mix ratio of recycled powder ECC was given. The results show that the addition of various recycled powders will not affect the multi-slit cracking behavior of ECC, and the cracks can still spread densely, giving the specimen greater deformation capacity and CMOD. The total substitution of fine aggregate by RCP and RBP has a positive impact on the fracture toughness KICini, KICun and fracture energy (GF), but the addition of RGP has some minor negative effects. Among the particle size range and substitution rate of RGP, the influence of substitution rate is more obvious, and the fracture performance will show a downward trend after growth as the substitution rate increases. Choosing the appropriate RGP substitution rate and particle size range can maximize the recycling of construction waste while ensuring fracture performance.

Study on overburden failure characteristics and displacement rule under the influence of deep faults

Deep faults significantly impact the structural stabilities and deformation behaviors of their overburden rocks, which are key factors in underground engineering and geological hazard research. Considering the problem of deep-fault-overburdened breaking during mining of super-thick coal seams and taking the Yaoqiao Coal Mine as the research object, the mining fracture evolution characteristics and overburden displacement law of the non-structured and fault-bearing overburden corresponding to fully mechanized caving mining are compared and analyzed using numerical simulations and physical similarity simulations. The results of this study show the following: 1) The fracture height of the overlying rock presents a specific change law with advancement of the working face; the initial rapid increase to a maximum height of 74 m is achieved when the working face advances to 90 m; with the development of the plastic zone indicating past yield, the fracture height decreases to 54 m and becomes stable, and the final caving angle of the fracture stabilizes at 70°. 2) In coal mining under normal fault conditions, when the working face advances from the upper to lower walls, the roof forms a masonry beam structure that slows down fault activation and crack development. When moving away from the fault, the overburden movements and water-conducting cracks are fewer, and the crack height is lower than that without faults. When approaching the fault, the influence of the faults in the fracture zone increases, and the height of fracture development reaches the maximum value after crossing the fault, highlighting the significant influence of the fault on fracture development. 3) Through a similarity simulation test, it is shown that the overburden caving zone is further compacted by the overburden rock and that the roof collapses in a large range, resulting in rapid upward development of the overburden rock cracks and separation of the central overburden rock cracks that are gradually compacted and closed. These findings are expected to have important theoretical and practical significance for deep underground engineering design, geological disaster prevention, and fault activity monitoring.

Impact of the water-wedge deterioration effect of pulsating water injection on AE waveforms and crack propagation during fracturing processes

Pulsating hydraulic fracturing (PHF) is a promising technology for enhancing the extraction of coalbed methane. In this study, the effects of pulsating water response of coal fractures were investigated. Prior to quasi-static fracturing, coal samples were subjected to pulsating water injection for 120 min. The fracturing process was monitored using an acoustic emission (AE) system and compared with direct fracturing without pulsating water injection. The Hilbert–Huang transform theory was employed for the analysis. The AE signals were examined in time and frequency domains, across various parameters, including intrinsic mode function (IMF) components, energy, three-dimensional spectrum, location features, and crack development. The results indicated a 37.82 % reduction in initiation pressure, accompanied by decreased AE counts and the promotion of gradual fracture initiation. Initially, the AE waveform amplitudes decreased, followed by an increase in distinctive patterns. Water injection primarily reduced the amplitudes during unstable fracturing, shifting the extreme IMF component forward. Moreover, it decreased the peak energy, shortened the energy release duration, and improved the signal-to-noise ratio during crack propagation. Water injection also narrowed the frequency distribution range, concentrated the energy distribution, expanded the extent of the fracture, increased the number of location points, and induced more macroscopic cracks within the coal body.

Study of Rock Damage Constitutive Model Considering Temperature Effect Based on Weibull Distribution

The deformation and damage process of rocks is accompanied by crack extension and penetration. The rock strength criterion, as a macroscopic characterization of the rock strength microelement, is the basis for establishing the damage constitutive modeling of rock. Aiming at the problem of the Hoek–Brown (H–B) strength criterion having a large strength prediction value under high confining pressure, the H–B strength criterion is corrected by considering the influence of the initial cracks on the development of the rock strength, and its applicability is verified. Based on the damage theory, assuming that the rock strength microelement obeys the Weibull distribution and considering the influence of residual strength, the damage correction coefficient is introduced, and a thermal damage statistical constitutive model that can reflect the whole process of the development of initial cracks inside the rock is established. The degree of penetration up to the damage is established, and the method of determining the parameters of the model is given. The theoretical curves of the established model are compared and analyzed with the curves of a conventional triaxial compression test of rock samples, and the study shows that the statistical constitutive model of the thermal damage of rock, established based on the modified H–B strength criterion, can better simulate the stress–strain relationship of rock under a conventional triaxial test. It also verifies the reasonableness and applicability of the model, which is expected to provide a basis for the exploitation of deep resources and the safety assessment of underground engineering.

The study of the natural frequency evolution and numerical simulation of retrogressive landslides

A retrogressive landslide is influenced by the cyclical fluctuations in reservoir water levels is considered a common natural disaster. Tension cracks are important indicators for assessing landslide status in the case of retrogressive landslides. Displacement monitoring is a commonly used method and provides an intuitive reflection of the landslide deformation; however, it does not directly indicate the depth of the tension cracks. Based on the principles of vibrational dynamics, a retrogressive landslide is proposed to be initially classified as a single-mass spring oscillator model before the development of cracks. Following the development of tension cracks, the model can be classified as a double-mass spring oscillator model. The model patterns are verified through numerical simulations using ABAQUS. Based on the numerical simulations, with an increase in the number of reservoir water cycle fluctuations, the displacement and stress of the landslide exhibit periodic growth. However, during displacement growth, the tension cracks do not necessarily increase. As the tension cracks deepen, the landslide transitions from a single-mass spring oscillator model to a double-mass spring oscillator model, with the appearance of a second-order natural frequency. Moreover, as the tension cracks deepen, the numerical values of the natural frequency change. The maximum change in first-order natural frequency is 3.5 Hz. The maximum change in second-order natural frequency is 4.5 Hz. The variation in the natural frequency can reflect the depth of development of the landslide's tension cracks and, consequently, indicate changes in the stability state of the landslide.

The development and performance evaluation of diagonal tension cracks control devices

ABSTRACT The cause of diagonal tension cracks is the drying shrinkage of the initial concrete, which causes them to appear at the corners of square openings, which are the most vulnerable areas to stress. Looking specifically at the cause from a structural aspect, the concentrated stress exceeds the tensile strength of the concrete, causing cracks to occur at the corners. Typically, diagonal tension reinforcement bars are arranged to prevent diagonal tension cracks, however, they have often failed to be practical solutions because they do not entirely prevent the continued occurrence of cracks. Consequently, the present study developed diagonal tension cracks control devices that are capable of preventing diagonal tension cracks. Next, comparative experiments on existing diagonal tension reinforcement bars with respect to drying shrinkage were conducted to verify the utility of the diagonal tension crack reinforcement materials through performance evaluation. According to the results, the maximum elastic strain at the top and bottom opening edges was 29.2% and 35.8%, respectively, which was a large reduction compared to the 6.7% and 4.9% of the existing diagonal tension reinforcement. Considering the above points, it is expected that the developed control device will be very practical in terms of industry.

Optimal energy collection with rotational movement constraints in concentrated solar power plants

In concentrated solar power (CSP) plants based on parabolic trough collectors (PTC), the sun is tracked at discrete time intervals, with each interval representing a movement of the collector system. The act of moving heavy mechanical structures can lead to the development of cracks, bending, and/or displacement of components from their optimal optical positions. This, in turn, diminishes the overall energy capture performance of the entire system. In this context, we introduce two combinatorial optimization problems of great interest to PTC plants. The minimum tracking motion (MTM) problem is aimed at detecting the minimum number of movements needed while maintaining production within a given range. The maximal energy collection (MEC) problem seeks to achieve optimal energy production within a predetermined number of movements. Both problems are solved assuming scenarios where the energy collection function contains any number of local maximums/minimums due to optical errors of the elements in the PTC system. The MTM and MEC problems are solved in O(n) time and O(n2mω∗) time respectively, where n is the number of steps in the energy collection function, m the maximum number of movements of the solar structure, and ω∗ the maximal amplitude angle that the structure can cover. The advantages of the solutions are shown in realistic experiments. While these problems can be solved in polynomial time, we establish the NP-hardness of a slightly modified version of the MEC problem. The proposed algorithms are generic and can be adapted to schedule solar tracking in other CSP systems.

In-situ graft fibrous PPy@PEI enables sensitive and all-seasoned crack monitoring for ancient architectures based on flexible pressure sensors

The non-destructive, real-time, and automatic monitoring for the development of cracks in ancient architectures is crucial to the preservation of precious ancient heritages. Here, light-weight crack sensing constructed with ultra-thin and high-performance flexible pressure sensors is proposed to fulfill this demand. The signal drift and poor reversibility commonly observed in widely-reported sensors based on elastomer matrix, which originates from the creep and relaxation behaviors, are addressed using piezoresistive polypyrrole-grafted polyetherimide fibers. The corresponding flexible piezoresistive sensors exhibit great sensitivity (9.45 kPa−1 in 0 ∼ 1 kPa and 3.48 kPa−1 in 3 ∼ 10 kPa), outstanding static stability (5.13 % signal drift after 48 h under 1 kPa), dynamic durability (4.72 % degradation after ∼30,000 cycles) and excellent robustness across an ultra-wide temperature range of −70 ∼ 150 ℃. Based on this highly reliable pressure sensor, the proposed non-destructive crack sensors integrated into an Internet of Things (IoT) platform display outstanding stability during their service in the Forbidden City. By virtue of the low energy consumption, one lithium battery can power the monitoring system for more than one year.

Effect of magnetic induction intensity and steel fiber rate on strength improvement of cementitious filling composites

The underhand cut-fill backfill mining manner (UCFBMM) is widely employed in nonferrous metallic mines, and stability of the artificial backfill top plate directly determines the safety of operators and equipment, which puts forward higher requirements on fill’s bending properties. The purpose of the current research is to advance steel-fiber reinforced cemented tailings backfills’ (SFCTBs’) mechanical features by employing steel fibers. 17 groups of specimens were prepared by considering the effects of diverse steel fiber doping and magnetic induction strength on SFCTB’s bending properties. Steel fiber reinforced SFCTB’s bending evolution was studied by utilizing three-point bending tests, SEM interpretations, and X-ray computed tomography. Lab findings exhibited that directional distribution of steel fibers was effective in increasing SFCTB’s flexural strength, and the flexural strength was positively correlated with fiber doping and magnetic induction strength. The presence of steel fibers and their directional distribution well inhibited crack development, and thus the crack resistance and toughness of SFCTBs, enabling them to bear greater tensile force. The unadulterated steel fiber reinforced SFCTBs showed sudden fracture when reaching the peak load, while the steel fiber adducted SFCTB specimens showed the characteristics of good ductility. Due to gravity and magnetism, steel fibers were distributed at the bottom of SFCTBs and showed a directional distribution along the length of the specimens. Besides, SFCTBs produced further CSH-gels and Aft in process of hydration. The overall outcomes of the current study could run an academic guide to construction of artificial backfilling roofs in UCFBMM in terms of cost and operational stages.

Mesoscopic Analysis of Fatigue Damage Development in Asphalt Mixture Based on Modified Burgers Contact Algorithm in Discrete Element Modeling.

This study is aimed at examining the mesoscopic mechanical response and crack development characteristics of asphalt mixtures using the three-dimensional discrete element approach via particle flow code (PFC). The material is considered an assembly of three phases of aggregate, mortar, and voids, for which three types of contact are identified and described using a modified Burgers model allowing for bond failure and crack formation at contact. The laboratory splitting test is conducted to determine the contact parameters and to provide the basis for selecting three different load levels used in the indirect tensile fatigue test and simulation. The reliability of the simulation is verified by comparing the fatigue lives and dissipated energies against those from the test. Under cyclic loading, the internal tensile and compressive force chains vary dynamically as a response to the cyclic loading; both are initially concentrated beneath the top loading strip and then extend downward along the loading line. The compressive chains are oriented roughly vertically and develop an elliptic shape as damage grows, while the tensile chains are mostly horizontal and become denser. An analysis based on the histories of the numbers of different contact types indicates that damage mainly originates from bond failures among the aggregate particles and at the aggregate-mortar interfaces. In terms of location, cracking is initiated below the loading point (consistent with observations from the force chains) and propagates downward and laterally, leading to the macrocrack along the vertical diameter. The findings provide a mesoscopic understanding of the fatigue damage initiation and propagation in asphalt mixture.

Research on the Crushing of Reinforced Concrete Two-Way Slabs by Pulse Power Discharge Technology

The application of pulse power discharge (PPD) technology in the crushing and dismantling of concrete structures has characteristics related to both green and environmental protection, as well as safety and reliability, with broad application prospects in the construction and municipal engineering fields in dense urban areas. Nevertheless, the research into using this technology to break reinforced concrete (RC) slabs is very limited, while the influence of key parameters on the crushing effect of reinforced concrete slabs is not clear. To solve this problem, a finite element model of an RC slab was established by ABAQUS. The effect of a shock wave generated by PPD on the surrounding concrete was simulated by an explosion-load equivalent, and the development process of concrete crack was simulated by a cohesive force model. Based on the results of the model analysis, the effects of reinforcement spacing, as well as diameter and concrete strength on the crushing effect of RC slabs were investigated. The results show that the increase in reinforcement diameter and the decrease in reinforcement spacing have a significant effect on limiting the development of cracks. According to the development of cracks, they can be divided into three types: edge cracks, cracks between central holes, and cracks between edge holes. The influence of reinforcement spacing and diameter on the first two crack widths is the most obvious. The increase in concrete strength also reduces the width of cracks. Based on the analysis results, the calculation expressions of the crushing effect of the PPD technique on RC slabs were established, which provides theoretical support for the popularization and application of this technique.

Experimental and numerical simulation study on flexural performance of high-strength reinforced concrete beams under static loading

In recent years, high-strength steel bars (HSSB), such as hot-rolled ribbed bars (HRB) and heat-treated ribbed bars (HTRB), have been promoted and used in reinforced concrete (RC) structures worldwide. There is still some controversy in existing research regarding the crack development process, failure modes, and the applicability of current code-specified calculation formulas for high-strength RC beams. The study of the mechanical properties of high-strength RC beams is of great significance in promoting the development of construction engineering science. In order to investigate the flexural behavior of high-strength RC beams, four-point bending static loading tests were carried out on 11 groups of RC beams with different parameters (reinforcement strength, concrete strength, and reinforcement ratio). The test results indicate that the crack development process and failure mode of high-strength RC beams exhibit obvious bending failure characteristics. Increasing reinforcement strength and reinforcement ratio significantly improves the bearing capacity of the beams, while the impact of increasing concrete strength on the beam's capacity is relatively small. The applicability of the calculation formulas in the current codes was analyzed, it was found that the yield moment formula in the current code accurately predicts the yield moment of high-strength RC beams, and the formula for calculating the maximum crack width was modified by proposing the maximum crack width adjustment factor kω. Subsequently, the failure process of high-strength RC beams was simulated using explicit dynamic finite element software LS-DYNA. Parameterized analyses were conducted to broaden the research scope. The ultimate moment prediction factor ku was proposed and a formula that can accurately predict the ultimate moment of high-strength RC beams was established.

Effect of wood and peanut shell hydrochars on the desiccation cracking characteristics of clayey soils

Soil cracking can significantly alter the water and nutrient migration pathways in the soil, influencing plant growth and development. While biochar usage has effectively addressed soil cracking, the feasibility of using less energy-intensive hydrochars in desiccating soils remains unexplored. This study investigates the impact of wood and peanut shell hydrochars on the desiccation cracking characteristics of clayey soil. A series of controlled environmental laboratory incubations with regular imaging was conducted to determine crack development's dynamic in unamended and hydrochar-amended soils. The results reveal that the addition of wood hydrochar at 2% and 4% dosage reduced the crack intensity factor (CIF) by 22% and 43%, respectively, compared to the unamended control soil. Similarly, the inclusion of peanut shell hydrochar at 2% and 4% lowered the CIF by 22% and 51%, respectively. The presence of hydrophilic groups on the surface of hydrochars, such as O–H, CH, and C–O–C, enhanced the water retention capacity, as confirmed by Fourier-transform infrared analysis. The CIF decrease is attributed to mitigated water evaporation rates, enabled by enhanced water retention within the hydrochar pore spaces. These findings are supported by scanning electron microscopy analyses of the hydrochar morphology. Despite CIF reduction with hydrochar incorporation, the crack length density (CLD) increased across all hydrochar-amended series. In contrast to unamended soil which exhibited pronounced widening of large cracks and extensive inter-pore voids, the incorporation of hydrochar resulted in higher CLD due to the formation of finer interconnecting crack meshes. Consequently, the unamended control soil suffered greater water loss due to heightened evaporation rates. This study sheds new light on the potential of hydrochars in addressing desiccation-induced soil cracking and its implications for water conservation.

Deterioration effect of creep on fatigue properties in QCr0.8 alloy: Damage mechanism and fatigue-creep life evaluation

The QCr0.8 alloy used in the thrust chamber liner wall of the reusable liquid rocket engine is subjected to high-temperature fatigue and high-temperature creep loading. This work investigates the effect of fatigue-creep interaction (FCI) on the fatigue life of QCr0.8 alloy. Experimental results reveal accelerated cyclic softening and decelerated stress relaxation in the alloy under FCI conditions, attributed to creep deformation and stress amplitude growth. The damage evolution mechanism of FCI is revealed by the results of the energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The decrease in fatigue-creep life of the QCr0.8 alloy is attributed to the fracture of the outer surface oxide layer, as well as the coupling development of creep voids and secondary cracks. Finally, a new fatigue-creep life prediction model has been developed, taking into account the increase in plastic strain energy resulting from stress relaxation and creep deformation behaviors. The study results provide substantial theoretical justification for the utilization of QCr0.8 alloy in the thrust chamber wall of reusable liquid rocket engines.

Static Bending Mechanical Properties of Prestressed Concrete Composite Slab with Removable Rectangular Steel-Tube Lattice Girders

In recent years, with the development of building technology, the Chinese construction industry has begun to gradually promote the prefabricated buildings to save on construction costs. Among them, composite slabs, as essential components of prefabricated buildings, have been widely used by designers mainly in favor of their low cost. However, is it possible to further reduce the cost without affecting the quality? Researchers think so if the operation cycle of support from the bottom of composite slabs can accelerate and the mechanical properties of their bottom plate can be optimized. To prove this hypothesis, researchers proposed a new type of prestressed concrete composite slab with removable rectangular steel-tube lattice girders (referred to as CDB composite slabs), whose bottom plate consists of a temporary structure composed of a prestressed concrete prefabricated plate and removable rectangular steel-tube lattice girders. Through static bending performance tests on three prefabricated bottom plates and one composite slab, researchers measured corresponding load-displacement curves, load-strain curves, crack development, and distribution, etc. The test results show that the top chord rectangular steel tubes connected to the bottom plate concrete through web reinforcement bars significantly improve the rigidity, crack resistance, and load-bearing capacity of the bottom plate and possess better ductility and out-of-plane stability. The number of supports at the bottom of the bottom plate is effectively reduced, with the maximum unsupported span reaching 4.8 m. Beyond 4.8 m, only one additional support is needed, and the maximum support span can be up to 9.0 m, which provides space for cost reduction. The cooperative load-bearing performance of the prefabricated bottom plate and the post-cast composite layer concrete is good. The top chord rectangular steel tubes are easy to dismantle and can be reused, which reduces the steel consumption by about 24% compared to that used for the same size of ordinary steel-tube lattice-girder concrete composite slabs. It can greatly decrease the cost. In conclusion, the results have shown that the new method researchers proposed here is practically applicable and also provides great space to save on financial costs.

Experimental Study on the Effect of Wetting-Drying Cycles on Crack Development and Radon Retardation of the Covering Soil in Uranium Mill Tailing Impoundments.

The majority of uranium mill tailing impoundments in the southern part of China are located in humid subtropical regions where persistent rainfall and rapid evaporation of water after rain often occur. Under the prolonged influence of alternating wet and dry conditions, the covering soil layer of uranium mill tailing impoundments develops cracks, leading to the issue of degradation or even failure of the radon retardation effect. A beach surface of uranium mill tailing impoundments in the southern part of China is selected as the research object. Through use of a self-made simulation test device, a degradation experiment of uranium mill tailing covering soil models under wetting-drying cycles was conducted indoors. The experimental results indicate that with a constant amplitude of wetting-drying cycles, microcracks characterized by a narrow width and high abundance were mainly generated in the early-to-mid-stage of wetting-drying cycles. The main cracks, characterized by their wide width and less abundance, were mainly formed in the mid-to-late stage of wetting-drying cycles. After seven wetting-drying cycles, the total length of cracks showed a "stair-step" increase and the surface crack ratio exhibited a trend of moving from rapid growth to stable growth and then to a slight decline. The cumulative damage degree showed a rapid increase to stable growth with an increase in the number of wetting-drying cycles. Grey relational analysis revealed that, compared to other surface crack indicators, radon exhalation rate was the most closely correlated with the surface crack ratio. With a constant amplitude of wetting-drying cycles, the radon exhalation rate underwent four stages as the number of wetting-drying cycles increased: stage I witnessed a rapid increase, stage II witnessed a rapid decrease, stage III witnessed a gradual increase, and stage IV witnessed a stable or even slight decrease. With a constant number of wetting-drying cycles, the radon exhalation rate correspondingly increased with the amplitude of wetting-drying cycles, particularly noticeable when the alternation amplitude was 30 ± 20%. From the early mid-stage to the late stage of wetting-drying cycles, the curves of the radon exhalation rate, surface crack ratio, and cumulative damage degree tended to be consistent, showing a gradual increase. The research provided in this study offers valuable insights into radon control measures and environmental assessments on the beach surface of uranium mill tailing impoundments.

Wood properties influencing surface cracking and moisture dynamics of untreated Norway spruce exposed outdoors

Untreated wood has excellent environmental benefits due to the lack of treatments; however, its durability needs to be great enough to provide a sufficient service life to not override the environmental benefits. The aim of this study was to investigate some wood properties of untreated, unfinished Norway spruce and their influence on moisture dynamics and crack development under natural exposure. Three field-trials were carried out, all under natural exposure during various exposure times. The specimens differed in their exposure direction (north/south), composition (heartwood/ sapwood), density, and thickness. Moisture measurements were carried out either by use of sensors or weighing the specimens, while the crack formation was measured using digital calipers. Generally, high-density spruce exhibited more rapid moisture fluctuations than low-density; this agreed well with the increased crack development observed in the field-trials. More cracks were observed for specimens containing sapwood rather than heartwood. This was likely caused by an increase in moisture uptake, generating greater moisture gradients. The results also showed that the crack tendency was greater in specimens within the high-density group placed facing south, which is likely due to an increase in moisture variation, and perhaps also faster UV-deterioration. No clear correlation between crack tendency and thickness was found.