Crack Density Research Articles

Performance evolution of low heat cement under thermal cycling fatigue: A comparative study with moderate heat cement and ordinary Portland cement

As a promising and environmentally friendly cementitious material for mass concrete applications, the long-term performance evolution of low-heat cement (LHC) and moderate-heat cement (MHC) under the alternating environmental temperature remains unclear. To address this gap, a comprehensive comparative study based on laboratory acceleration test was conducted by evaluating the physicochemical properties and cracking behavior of LHC and MHC in comparison to ordinary Portland cement (OPC) following 1200 thermal cycles (TC) at temperatures ranging from 5 to 45 °C. Cracking behavior is revealed by a quantitative identification analysis. The results indicate a negative linear relationship between porosity, crack density, and compressive strength, highlighting their competing mechanisms in determining the degree of damage under TC. TC sharply diminished the gel pores (<10 nm) and promoted the formation of capillary pores (50–1000 nm), which were the primary forms of pore coarsening observed. Notably, the different proportions of hydrated products such as HD-CSH, LD-CSH, and CH influence the toughness properties of the matrix and contribute to different resistances to cracking and the formation of cracks with varying fracture morphology, although they also impact the stability of the pore structure. Further analysis revealed different degrees of rehydration occurring within the first 200 cycles of both LHC and MHC, which contributed to the mitigation of performance degradation. Moreover, an increase in the water-to-cement (w/c) ratio had a tendency to coarsen the matrix and facilitate the formation of long and narrow cracks.

Preparation and mechanical characterization of multi-functional high-ductility engineered magnesium oxysulfate cement-based composites

Aiming at the integrated requirements of building energy saving and thermal insulation reinforcement in reinforcement engineering, the technology of preparing multi-functional high-ductility engineered magnesium oxysulfate cement-based composites (MOSC-ECC) by magnesium oxysulfate cement (MOSC) was proposed; the mechanism of industrial waste fly ash (FA) to improve the ductility of MOSC-ECC was explored; the effects of FA on the density, thermal conductivity and water resistance of MOSC-ECC were studied; the phase composition and microstructure of the hydration products of MOSC-ECC were analyzed. The results show that the incorporation of FA instead of light-burned magnesium oxide (LB-MgO) powder of equal mass can effectively reduce the cracking strength, fracture toughness, density and thermal conductivity of the matrix, and improve the ductility and multiple cracking ability of MOSC-ECC. The incorporation of hydrophobic agent made MOSC-ECC have excellent water resistance. The findings of this study provide useful knowledge for realizing the integrated design of thermal insulation and reinforcement of structures, which is of great significance for promoting the engineering application of ECC and the promotion of energy-saving green building materials.

Experimental investigations into surface crack density and micro hole dimensions during ultrashort pulse laser ablation

Experimental investigations into surface crack density and micro hole dimensions during ultrashort pulse laser ablation

Bridge percolation: electrical connectivity of discontinued conducting slabs by metallic nanowires.

The properties of nanostructured networks of conductive materials have been extensively studied under the lens of percolation theory. In this work, we introduce a novel type of local percolation phenomenon used to investigate the conduction properties of a new hybrid material that combines sparse metallic nanowire networks and fractured conducting thin films on flexible substrates. This original concept could potentially lead to the design of a novel composite transparent conducting material. Using a complementary approach including formal analytical derivations, Monte Carlo simulations and electrical circuit representation for the modelling of bridged-percolating nanowire networks, we unveil the key relations between linear crack density, nanowire length and network areal mass density that ensure electrical percolation through the hybrid. The proposed theoretical model provides key insights into the conduction mechanism associated with the original concept of bridge percolation in random nanowire networks.

06 Şubat 2023, Pazarcık (Kahramanmaraş, Türkiye) depreminin neden olduğu jeomorfolojik deformasyon örnekleri

The left-laterally strike-slip Pazarcık fault is one of the East Anatolian Fault Zone (EAFZ) segments. On February 6, 2023, the ±85 km long Pazarcık fault generated a highly destructive Mw=7.7 earthquake. This study aims to explain the geomorphological deformations caused by the February 6, 2023, Pazarcık earthquake with typical examples. The surface rupture of the earthquake between Türkoğlu and Gölbaşı was followed precisely, and the changes in the earth's surface due to the left lateral strike-slip were determined, measured, and recorded. A DJI Phantom 4 and a DJI Mini Drone were used for aerial measurements and recordings during the fieldwork. Garmin e-Trex 10 handheld GPS and tape measure were used for terrestrial measurements. During the field studies, the surface rupture of the earthquake was investigated from a geomorphological perspective and mapped by taking location data. It was determined by the measurements that the left lateral offset distances in the surface fracture vary between 4.0-6.5m. One of the geomorphological deformations of the February 6, 2023 earthquake is transpressional ridges and/or transtensional depressions. Transpressional shortening and/or transtensional extension deformations due to a single surface rupture are the natural consequences of the curvilinear slip plane of the left-laterally strike-slip Pazarcık fault. Liquefaction samples with different characteristics were observed in the Sakarkaya alluvial fill area within the Gölbaşı depression. Rockfalls occurred on sandstone, mudstone, and limestone rock slopes weakened by the discontinuity due to the density of cracks outcropping in the valley where the surface rupture passes in the Kartal, Sakarkaya section. During field studies, slides and spreading were also observed. Typical examples of slide occurred on the unconsolidated fill ground on the south coast of Gölbaşı Lake with a slight slope towards the lake as a result of the vibration effect of the earthquake. In addition, the vibration effect of the earthquake caused lateral spreading deformations in the artificial fillings of road and road junction structures.

Hydrologic Impacts of a Strike-Slip Fault Zone: Insights from Joint 3D Body-Wave Tomography of Rock Valley

ABSTRACT The Rock Valley fault zone (RVFZ), an intraplate strike-slip fault zone in the southern Nevada National Security Site (NNSS), hosted a series of very shallow (&amp;lt;3 km) earthquakes in 1993. The RVFZ may also have hydrological significance within the NNSS, potentially playing a role in regional groundwater flow, but there is a lack of local hydrological data. In the Spring of 2021, we collected active-source accelerated weight drop seismic data over part of the RVFZ to better characterize the shallow subsurface. We manually picked ∼17,000 P-wave travel times and over 14,000 S-wave travel times, which were inverted for P-wave velocity (VP), S-wave velocity (VS), and VP/VS ratio in a 3D joint tomographic inversion scheme. Seismic velocities are imaged as deep as ∼700 m in areas and generally align with geologic and structural expectations. VP and VS are relatively reduced near mapped and inferred faults, with the most prominent lower VP and VS zone around the densest collection of faults. We image VP/VS ratios ranging from ∼1.5 to ∼2.4, the extremes of which occur at a depth of ∼100 m and are juxtaposed across a fault. One possible interpretation of the imaged seismic velocities is enhanced fault damage near the densest collection of faults with relatively higher porosity and/or crack density at ∼100 m depth, with patches of semiperched groundwater present in the sedimentary rock in higher VP/VS areas and drier rock in lower VP/VS areas. A relatively higher VP/VS area beneath the densest faults persists at depth, which suggests percolation of groundwater via the fault damage zone to the regionally connected lower carbonate aquifer. Potentially, the presence and movement of groundwater may have played a role in the 1993 earthquake aftershocks.

Unveiling fracture mechanics of a curved coating/substrate system by combined digital image correlation and numerical finite element analyses

A mechanism of coated, curved substrate failure is characterized via a C-ring compression test and multiscale modeling analysis. Coating crack formation is driven by sudden through-thickness cleavage in accordance with Mode I fracture type. The fracture is arrested at the coating/substrate interface. The cracks continue to widen as the underlying substrate experiences periodic regions of high stress under the crack and regions of relaxed stresses under the remaining intact coating. An analytical, closed form solution model developed via curved beam theory captures the hoop stress in the axis transverse to external loading and predicts the external load at crack initiation. After the onset of cracking, the system transitions to cleavage driven failure, which is simulated via element deletion failure in a small-scale explicit dynamics finite element model. The macroscale explicit dynamics finite element model, analytical solution, and bulk experimental results agree. The correlation between coating microstructure and deposition method is also investigated and reveals that high interfacial roughness and equiaxed grain morphology correspond to higher loader carrying capability and lower crack density while low interfacial roughness and smooth columnar grains correspond to higher strain to coating failure.

Adaptive Multi-Fidelity (AMF) modelling of damage in composites under Low-Velocity impact and compression after impact

In this paper, the recently developed adaptive multi-fidelity (AMF) approach is extended for progressive damage analysis of composite laminate subjected to low-velocity impact (LVI) and compression after impact (CAI). This approach is more computationally efficient by utilizing a combination of low-fidelity shell elements and high-fidelity brick elements in a single concurrent model. A general set of criteria has been proposed to govern the transition of shell and brick elements from one to the other and vice versa, as dictated by the mechanics of damage evolution. When matrix cracks or interfacial delamination initiates, shell elements transit to brick elements and cohesive elements are inserted to capture the damage growth. The brick elements partitioned by non-critical cracks are reverted to smeared shell elements when the numerical crack density becomes saturated in a laminate. Geometrical non-linearity is adopted here to predict large deformation in the composite laminate for both implicit quasi-static and dynamic analyses. The performance of AMF models is benchmarked with high-fidelity models and experimental data reported in literature. The predicted results show reasonable agreement with experimental observation and high-fidelity models, and up to 51% improvement in computational efficiency is achievable.

Dynamic spall properties of an additively manufactured, high-entropy alloy (CoCrFeMnNi)

The dynamic spall properties of an additively manufactured (AM), CoCrFeMnNi high-entropy alloy (HEA) were investigated as a function of processing defects. Laser Powder Bed Fusion (LPBF) was used to additively manufacture HEA samples that were subsequently subjected to shock loading through plate impact experiments. Seven different combinations of laser power and scan speeds were explored, ranging from 180 to 280 W and 925–1350 mm/s, respectively. All samples were considered within the bounds of lack-of-fusion and keyholing defects based on initial experiments, but exhibited varying degrees of solidification cracking and microstructural changes. Grain size and grain aspect ratio were found to modestly decrease with faster scan speeds and lower laser powers, while cracking associated with the AM process increased with faster scan speeds and higher laser powers. Spall strength and spall damage did not systematically trend with microstructural characteristics such as grain size, but did exhibit a relationship with pre-existing crack density. Spall properties were found to generally degrade with increasing crack density. Evidence of defect compaction was not observed in soft recovered spall samples, thus pre-existing cracks were proposed to degrade spall properties by acting as stress concentrators and preferred damage nucleation sites during tensile impact loading. Here, the presence of manufacturing-related defects, such as cracks, dominated the dynamic response; however, in the absence of these defects, microstructural changes like grain size and texture would be expected to control dynamic behavior.

Electrode lifetime in resistance spot welding of coated sheets: Experiments and modeling

One of the most important challenges associated with resistance spot welding of coated sheets is short electrode life time. It has been established that the two factors-electrode tip growth and surface alloy formation-each affect negatively on electrode life when considered separately. This study's objectives were to examine the combined effects of these factors and ascertain how they relate to electrode life. Due to this, two electrodes were used to investigate the electrode life when resistance welding galvanized steel: a conventional Cu-Cr electrode with a compressive strength of 800 MPa and a dispersion strengthened copper electrode with a high volume percentage of alumina particles (3.5 %) with a compressive strength of 500 MPa. Electrode life time tests showed that the life of composite electrode (700 weld number) was reduced by about 20 % compared to Cu-Cr electrode (900 weld number). Electrode tip features showed that cracking in Cu-Cr electrode formed at about 200 weld number while in composite electrode this occurred at 600 weld number. The electrode tip cross section revealed the formation of four separate alloy layers at the tips of both electrodes, however the Cu-Cr electrode had significantly higher crack densities and a lower average thickness during life test (45 µm) than the composite electrode (55 µm). It was caused by the higher rate of electrode tip growth in Cu-Cr electrodes, which increased welding current and extended electrode life. A simple model was purposed for electrode life based on Joule heating effect that takes the effects of both electrode tip growth and electrode tip alloy formation in to account. According to the model and experiments when significant interaction occurs at the electrode tip, the some tip growth rate not only isn’t detrimental but also increases the electrode lifetime.

Residual stress and thickness effects on fracture behavior of trilayer films during uniaxial loading

The effects of residual stress and film thickness on crack evolution in metallic trilayers deposited on polyimide substrate have been investigated during uniaxial tensile loading. Four different systems, consisting of Mo/Cu/Cr layers deposited on polyimide, with nominal thicknesses of 100/100/100 or 100/500/100 nm, were developed using two physical vapor deposition techniques, resulting in opposite as-deposited high residual stress for the top Cr layer (about +2.7 GPa versus -2.6 GPa). First, it was found that the higher the compressive residual stress of the top Cr layer and the thickness of the middle Cu layer, the higher the critical strain to initiate crack propagation in the trilayer film system. Second, it was found that the higher the compressive residual stress of the top Cr layer and the thickness of the middle Cu layer, the lower the maximum crack density in the trilayers. The maximum crack density in the thicker system with Cr in a tensile residual stress was similar to the maximum crack density in the thinner system with the Cr top layer in a compressive residual stress.

Study on Dynamic Crack Expansion and Size Effect of Back–Filling Concrete under Uniaxial Compression

With the continuous expansion of the application range of gob–side entry retaining technology, the depth, height, and advancing speed of coal seams also increase, which brings great problems to the stability control of surrounding rock structures of gob–side entry retaining. As one of the main bearing structures of the surrounding rock, the stability of the roadway–side support body is a key factor for the success of gob–side entry retaining. In order to study the deformation characteristics and instability mechanism of roadway-side support body, based on the roadway–side support materials of gob-side entry retaining, the dynamic expansion test of back–filling concrete cracks under uniaxial compression was carried out. The YOLOv5 algorithm was applied to establish the fine identification and quantitative characterization method of macroscopic cracks of the samples, and the dynamic expansion rule of roadway-side support body cracks and its dimensional effect were revealed by combining the fractal theory. The results show that the F1 value and average precision mean of the intelligent dynamic crack identification model reached 75% and 71%, respectively, the GIoU loss value tends to fit around 0.038, and the model reached the overall optimal solution. During the uniaxial compression process, micro cracks on the surface of the back–filling concrete first initiated at the end, and after reaching the yield stress, the macroscopic cracks developed significantly. Moreover, several secondary cracks expanded, pooled, and connected from the middle of the specimen to the two ends, inducing the overall instability of the specimen. The surface crack expansion rate, density, and fractal dimension all show stage change characteristics with the increase in stress, and the main crack expansion rate has obvious precursor characteristics. With the increase in the size, the decrease in crack density after back–filling concrete failures gradually decreases from 93.19% to 4.08%, the surface crack network develops from complex to simple, and the failure mode transits from tensile failure to shear failure. The above research results provide a basic experimental basis for design optimization and instability prediction of a roadway–side support body for engineering-scale applications.

A thermo-mechanical model for hot cracking susceptibility in electron beam powder bed fusion of Ni-base superalloys

Hot cracking is a major challenge in the additive manufacturing of Ni-base superalloys. Most cracking indicators only consider the alloy composition and its effects on solidification behavior, strength, and ductility. We propose a new model that extends these classical cracking indicators by additionally incorporating the transient thermo-mechanical state during melting and solidification. The model is derived from insights into the cracking of thin layers and is mainly based on the elastic strain energy available to drive crack opening in a critical temperature interval. The underlying thermo-mechanical process simulations are implemented in a custom computational framework designed to efficiently model powder bed fusion. The model predictions are validated against experimentally measured crack densities in CMSX-4 processed by electron beam powder bed fusion. The proposed model allows for rationalizing the dependence of cracking susceptibility on the thermal history, which is controlled by scan speed and beam power. Finally, the model is applied to develop a crack mitigation strategy by reducing the built-up strain energy density. This is achieved by heating the melting area to a high temperature before melting, thereby reducing the temperature gradient.

Quantification of tiny damage variation in asphalt mixture during the low-temperature thermal cycle

The performance evaluation of asphalt materials during low-temperature thermal cycles remains a challenge, particularly in quantifying the early tiny damage development and recovery processes. To address this issue, a high-sensitivity nonlinear ultrasonic method - the second harmonic generation (SHG) technique in the form of Rayleigh surface waves, is proposed to investigate these behaviors in asphalt mixture. The decrease in thermal contraction value and stiffness modulus with cycles verifies the damage behavior at the macro level. Turning points in crack length, crack density, and nonlinear parameter during the thermal cycle indicate the generation of cracks from − 20 ℃ to − 30 ℃ during the cooling process. The decrease of crack length and crack density in the heating phase demonstrates the recovery of asphalt. The nonlinear ultrasonic parameter β'(c) caused by crack is successfully separated from the total nonlinear parameter β', and the recovery index (RI)is defined to quantify the recovery behavior. Both of them are highly consistent with crack length and crack density. While the sensitivity index (SI) of nonlinear parameter is hundreds or thousands of times greater than other measurements of damage development. Furthermore, a qualitative correlation between the nonlinear parameters and plastic strain has been established. This breakthrough widens the scope of studying low-temperature weak damage in asphalt mixtures, moving beyond the confines of crack analysis.

Assessing the fatigue damage of concrete structures using automatically classified crack severity level information

The aim of this paper is to assess the fatigue damage of concrete structures using automatically classified crack severity level information. To solve the problem that the label data used for the automatic classification of regional complex crack severity levels can only be labeled by intuition, this paper proposes a quantitative index of regional complex crack severity levels that can consider the regional crack width, height, and density information. The testing set F1 score of the Swin Transformer classification model trained by the label data reaches 0.9903. To investigate the feasibility of assessing the concrete structure fatigue damage based on the crack change information, the paper adopted the scanning shooting method to acquire the concrete beam panoramas at different cyclic loading times in the fatigue test, and automatically classified the crack severity levels in different regions of the panoramas using the Swin transformer. The test results show that the information on the change in the number of regions with the same crack severity level on the concrete structure surface can be used to estimate approximately the fatigue life percentage interval that the concrete structure is currently in, which provides a reference for the development of maintenance measures and safety warnings.

EXPERIMENTAL INVESTIGATION AND PARAMETRIC OPTIMIZATION OF ELECTRO DISCHARGE MACHINING FOR TI-5553 ALLOY USING GREY RELATION ANALYSIS COUPLED WITH TAGUCHI METHOD

In this paper, the machinability aspects of Ti-5553 have been experimentally investigated during Electro-Discharge Machining using three different geometrical shapes of copper electrodes i.e. cylindrical, triangular, and square. TIMETAL (Ti-5553) is a titanium-based alloy extensively used for special aerospace and marine applications due to its high strength and higher hardenability. By changing each of the aforementioned process parameters at three distinct levels, experiments based on the [Formula: see text] orthogonal array (OA) design of the experiment (DOE) were carried out. The machining performance has been carried out by utilizing different input parameters i.e. current, pulse off time ([Formula: see text]), flushing pressure ([Formula: see text]), tool shape ([Formula: see text]), and voltage (V) to examine machining performance characteristics like material removal rate (MRR), tool wear rate (TWR), surface roughness (Ra), white layer thickness (WLT), and surface crack density (SCD). Surface integrity in contents of surface morphology and surface topography is discussed herein. In this MRR, TWR, and Ra have been optimized by using a methodology (combining Grey relational analysis method and Taguchi’s philosophy) and found the optimal parameters that will have more MRR and less TWR, Ra. Later SCD and WLT were investigated by employing the optimal setting (A3, B2, C2, D2, E1, F3) and arbitrary setting (A3, B2, C1, D3, E3, F2) and concluded that SCD and WLT are less by utilizing optimal settings for machining as compared to arbitrary settings. Among all three different geometrical shapes of copper electrodes, the cylindrical tool has been found to be the best suitable tool for machine Ti-5553.

Viscoelastic-damage constitutive model and mechanical characterisations of Aluminium/polytetrafluoroethylene under impact loading

As a typical structural energy-containing material, aluminium/polytetrafluoroethylene (Al/PTFE) exhibits excellent mechanical properties under conventional conditions and releases large amounts of energy through chemical reactions under impact loading. In this research, a viscoelastic-damage constitutive model suitable for polymer-based reactive materials was developed, which consisted of an elastic element, two Maxwell elements, and a SCRAM damage element. The effect of damage on the stress–strain relationship was obtained by analysing the crack evolution process and deformation field. A modified Split Hopkinson pressure bar experiment investigated the mechanical characteristics of aluminium/polytetrafluoroethylene under uniaxial/multiaxial impact loading. The results indicated that the viscoelastic-damage constitutive model could describe the effects of the initial crack size and initial crack density on the mechanical properties of aluminium/polytetrafluoroethylene. With the decrease in initial crack length and density, the damage to the aluminium/polytetrafluoroethylene under impact loading was limited, which caused the dynamic mechanical properties to increase. When the pre-stress increases from 0 MPa to 10 MPa, the dynamic ultimate compressive stress of aluminium/polytetrafluoroethylene increases from 77.2 MPa to 89.8 MPa at 2910 s−1 strain rate. A comparison of the experimental and calculated strain–stress curves shows that the established constitutive model is feasible to predicting the mechanical behaviour of aluminium/polytetrafluoroethylene.

A multi-scale model from microscopic cracks to macroscopic damage of concrete at elevated temperatures

A multi-scale model is established to describe the relationship between the macroscopic damage evolution and microscopic cracks behaviors of concrete at elevated temperatures. The evolution equation of the ideal microscopic crack system of concrete at elevated temperatures is deduced for construct the model, which can predict the microscopic crack density and macroscopic damage of concrete at elevated temperatures. The multi-scale model fuses some advantages of the traditional microscopic and macroscopic damage models. Finally, multi-scale damage of a concrete block under high temperature is predicted and compared with the corresponding experimental results, which is utilized to support the ability of the developed model. The results show that the developed multi-scale model can be used to evaluate fire damage of concrete structures in macro-scale as well as explain its physical mechanisms in micro-scale.

Numerical Analysis of Interbedded Anti-Dip Rock Slopes Based on Discrete Element Modeling: A Case Study

Varying geological conditions and different rock types lead to complex failure modes and instability of interbedded anti-dip rock slopes. To study the characteristics of failure evolution of interbedded anti-dip slopes, a two-dimensional particle flow code (PFC2D) based on the discrete element method (DEM) was utilized to establish an interbedded anti-dip rock slope numerical model for the Fushun West Open-pit Mine based on the true geological conditions and field investigations. The slope model with an irregular surface consists of interbedded mudstone and brown shale as two different rock layers, and a number of small-scale rock joints are randomly distributed in the rock layers. The influence of different inclination angles (20° and 70°) of the rock layer and slope angles (60° and 80°) on the stability of interbedded anti-dip rock slopes was considered. The evolution of the failure progress was monitored by the displacement field and force field. The simulation results showed that the rock joints in the rock stratum promoted crack initiation and increased the crack density but did not change its shear-slip failure mode. A large inclination angle of the rock layers and slope angle can lead to topping slip failure along the slip zone. However, shear-slip instability generally occurs in interbedded anti-dip rock slopes with small inclination angles of the rock layer and small slope angles. These results can contribute to a better understanding of the failure mechanism of interbedded anti-dip rock slopes under different geological conditions and provide a reference for disaster prevention.

In situ mechanical testing of hard yet tough high entropy nitride coatings deposited on compliant steel substrates

High entropy nitrides are an interesting material class for wear resistant coatings. Their fracture properties and toughness are of particular interest for future application in tool coatings. Therefore, in situ tensile testing and micro-cantilever experiments were performed on (AlCrTaTiZr)N, (AlCrNbSiTiV)N, (HfNbTaTiVZr)N and (AlTi)N coatings inside a scanning electron microscope. Parallel cracks form during tensile testing from which the fracture toughness was determined via a modified Hsueh and Yanaka model. For this the misfit strain between coating and substrate was treated as an unknown parameter and was fitted to the beginning part of the underlying crack density – applied strain curves. The validity of the model was evaluated by comparing the results with fracture toughness values derived via micro-cantilever bending experiments. A good fit between the data could be observed for three out of four tested coatings. The limitations of the underlying analytical model are presented and discussed. The tested high entropy nitride coatings displayed a promising combination of high hardness and fracture toughness.