Progressive Cracking Research Articles

A novel creep contact model for rock and its implement in discrete element simulation

The creep behavior of rocks significantly impacts projects' structural safety, including mining, tunneling, and hydraulic engineering. Numerous creep models used in numerical calculations incorporate basic elements, such as Burger's model. However, relying solely on series or parallel combinations of these basic components fails to account for the accelerated creep stage, prompting efforts toward developing enhanced creep models. Despite noted progress, a limited number of studies on the full-process simulation of graded creep loading and the consequent generation of cracks. To overcome this problem, an innovative creep model is proposed based on both the Parallel Bond and the Kelvin-Voigt contact models. This proposed creep model simulates the graded loading creep test of rocks under different load conditions and also conducts a parameter sensitivity analysis for the impact of the proportionate discrepancy between the two contact models. The results indicate that this new creep model could successfully simulate the comprehensive creep process, effectively capture crack progression, and represent ultimate failure modes during the creep process. Notably, to ensure the precision of simulations, it is recommended that the proportion of the Kelvin-Voigt contact model within the total contact is at most 25%.

Machine learning based progressive crack fault monitoring on spur gear using vibration analysis

Machine learning based progressive crack fault monitoring on spur gear using vibration analysis

Damage Tolerance Behaviour of Stiffened Crown Panel of a Transport Aircraft Fuselage

Ever since the introduction of damage tolerance requirements in the aviation regulations, efforts continue to be made to prevent catastrophic failures due to damages present in the structure. It has also been realized that damage detection is the weakest link in the whole process of damage tolerance design to maintain continued airworthiness. The major components of the aircraft structure consist of both integral and riveted panels of sheets and stringers which are employed in fuselage skin panels, spar webs and stiffeners. In spite of all precautions, cracks or damages may arise in many of these primary structural members. These cracks cause stiffness degradation and reduce the total load-carrying capacity of the structure. In this paper, the damage tolerance behaviour of fuselage crown panel both integral and riveted stiffened panel configurations of Aluminium alloy 2024-T351 are studied using finite element based tools using crack growth analysis methods. The crack growth behaviour of both integral and riveted stiffened panels of aircraft fuselage having same geometrical configuration and subjected to uniformly distributed tensile loads is investigated. For this, a metallic stiffened panel with eight stringers, representative of crown panel of a transport aircraft fuselage is analysed with a centre skin crack propagating through the stringers. Stress intensity factors and fatigue crack propagation rates at the progressive crack tip of both types of the stiffened panels are computed by using Modified Virtual Crack Closure Integral (MVCCI) method. The stiffened panels fatigue crack growth rate was computed by using Paris law under constant amplitude fatigue loads. The analysis results show that integral stiffened panel causes higher stress intensity factor and less load bearing capability than riveted stiffened panel which has better damage tolerant capability in comparison to the integrally stiffened panel.

Machine learning based progressive crack fault monitoring on spur gear using vibration analysis

Machine learning based progressive crack fault monitoring on spur gear using vibration analysis

The influence of the hole-generation process on fatigue response of open-hole and assembled titanium samples

The holes required for a bolted connection can be produced in several ways. The defects around the hole and the hole surface finish influence the fatigue life of the produced part. Abrasive waterjet cutting is compared with conventional drilling to assess these influences on titanium alloy parts. Open-hole tensile tests show a preferential fatigue behavior for conventionally drilled samples because of the lower roughness. The originality of this work focuses on linking the surface texture and damage to the life expectancy of the open-hole samples and determining the failure steps/modes of the assembled samples. A model is proposed to track the crack progression that offers adequate life estimations for open-hole samples. Single-shear assembly bearing tests show no difference between the drilling processes, and failure is obtained by secondary bending for both drilling methods.

Predicting Stress Intensity Factor for Aluminum 6062 T6 Material in L-Shaped Lower Control Arm (LCA) Design Using Extended Finite Element Analysis.

The aim of this study is to solve a practical problem encountered in the automotive industry, especially the failure of a cracked lower control arm made of al 6062 T6 material during static and crash physical tests, and to characterize the behavior of cracked parts made of aluminum materials using the fracture mechanics parameters. As a first step, we carried out a numerical study and simulation using Abaqus/CAE 2020 software and the finite element method to determine the stress concentration and load limit capacity for different car weight cases. The von Mises stress variation shows crack initiation and propagation to be in the area of the lower control arm's attachment to the vehicle platform, where stress is concentrated. These numerical results are consistent with the experimental test results found by automotive manufacturers. Also, we find that the mechanical load that can support this part is below 4900 N for good performance. In the second step, we use the results of the first section to simulate the failure of a lower control arm with a crack defect. This paper investigates the stress intensity factor KI in mode I for different lengths (L) and depths (a) of the crack in the lower control arm using the extended finite element method (XFEM) under Abaqus/CAE. For crack failure initiation and progression, we relied on the traction separation law, specifically the maximum principal stress (MAXPS) criterion. The KI factor was evaluated for the materials steel and Al 6062 T6. The results obtained from the variation of the KI coefficient as a function of crack depth (a) and the thickness (t) show that the crack remains stable even when a depth ratio (a/t = 0.8) is reached for the steel material. However, the crack in the Aluminum 6062 T6 material becomes unstable at depth (a/t = 0.6), with a high risk of total failure of the lower control arm.

Effects of layer thickness on desiccation cracking behaviour of a vegetated soil

The objective of this study is to explore how different layer thicknesses affect the desiccation cracking behaviour of vegetated soil. During the experiment, an electronic balance was employed to quantify water evaporation, while a digital camera was utilized to capture the initiation and progression of soil surface cracking. Results indicate that in the early drying process, the rate of evapotranspiration in vegetated soil correlates positively with leaf biomass. For soil samples with the same layer thickness, the constant rate stage duration is consistently shorter in vegetated soil samples than in their bare soil counterparts. As the layer thickness increases, both vegetated and bare soil samples crack at higher water content. However, vegetated soil samples crack at lower water content than their bare soil counterparts. Vegetation significantly reduces the soil surface crack ratio and improves the soil crack resistance. The crack reduction ratio is positively correlated with both root weight and length density. In thicker vegetated soil layers, the final surface crack length noticeably declines.

High-fidelity simulations of low-velocity impact induced matrix cracking and dynamic delamination progression in CFRP beams

A high-fidelity finite element model is constructed to simulate the low-velocity line-impact experiments conducted by Bozkurt and Coker on [05/903]s CFRP beams. The simulations utilize a user-implemented three-dimensional continuum damage model with the LaRC05 criterion for matrix cracking, and a built-in cohesive zone model for delamination in ABAQUS/Explicit. The significant influence of boundary supports on the global impact response leads to proposing a heuristic boundary conditions approach using spring elements that replicate the experiment boundaries. Results then demonstrated an excellent agreement with the experiments in terms of the global impact response, the strain field, and damage pattern and sequence. Simulations reveal intersonic delamination with crack tip speeds around ∼5000 m/s, while the experimental crack speeds were measured as sub-Rayleigh speeds reaching ∼1000 m/s. When a crack tip definition based on the crack tip opening is introduced in the simulations, crack tip speeds in the range of 490–1500 m/s are measured which are within the range of experimental speeds, suggesting that the sliding mode might be physically hidden in the experiments. The potential use of delamination crack tip speeds as a benchmark for refining numerical simulations is demonstrated by increasing the effective interface toughness leading to a decrease in crack tip speeds with no noticeable effect on the global responses.

Effect of stacking sequence and interlayer toughening in ＋/−45 laminates

This paper investigates the combined effects of stacking sequence and interlayer toughening on the mechanical response and failure modes of ±45° carbon fiber reinforced laminates, comprising experimental and numerical studies. Two material systems (T800H/3631, T800S/3900) and three stacking sequences (4 double plies, 4 plies, 8 plies) have been analyzed. Digital Image Correlation (DIC) and a high resolution edge camera were utilized to capture the intricate crack patterns in each ply. This paper also addresses the challenge of the nonlinear in-plane shear characterization of the lamina, and proposes an amendment to the existing ASTM standard to obtain the in-situ non-linear response and the lamina shear strength with the consideration of residual stresses. The complex progression of micro-damage, matrix cracks and delamination as well as the final two-piece failure were captured by the enhanced semi-discrete damage modeling (eSD2M) framework. A parametric study was performed to reverse-engineer the most critical parameters such as interlayer strengths and toughnesses. The obtained values reflect the difference in the interlayer composition between the two material systems.

Mechanical and Deformation Performance of Masonry Walls with Low-Strength Mortar Retrofitting Using Spray-on Polyurethane Coating

This study aimed to investigate the feasibility and effectiveness of spray-on polyurethane coating as a seismic strengthening method for rural masonry buildings. Three masonry wall specimens were tested under cyclic lateral loading, including a control specimen, a single-side strengthened specimen, and a double-side strengthened specimen. Digital image correlation (DIC) techniques were used to evaluate full-field strain, crack width, and failure progression in a non-contact manner. The seismic performances were compared in terms of failure mode, hysteretic behavior, skeleton curve, deformation performance, energy dissipation capacity, and stiffness degradation. Results indicated that a spray-on polyurethane coating effectively delayed the onset and progression of cracks, postponing the peak load and slowing strength and stiffness degradation. Compared to the unstrengthened specimen, the bearing capacity, ultimate displacement, and cumulative energy dissipation of the single-side strengthened specimen increased by 20%, 60%, and 514%, respectively. Compared to the single-side strengthened specimen, the double-side strengthened specimen BW-D exhibited improved integrity, deformation capacity, and energy dissipation capacity. Its ultimate displacement and cumulative energy dissipation increased by 28% and 10%, respectively.

Crack Propagation and Fatigue Performance of Partial Posterior Indirect Restorations: An Extended Finite Element Method Study.

Dental ceramics are susceptible to slow, progressive crack growth after cyclic loading. The purpose of this study was to investigate the progressive patterns of cracks in two different types of CAD/CAM ceramic materials used with three different partial posterior indirect restoration (PPIR) designs and to determine the materials' failure risk using a fatigue test. Standard initial cracks were formed in Standard Tessellation Language (STL) files prepared for three different PPIRs. The materials chosen were monolithic lithium disilicate (LS) and polymer-infiltrated ceramic networks (PICNs). The extended finite element method (XFEM) was applied, and the fatigue performance was examined by applying a 600 N axial load. The cracks propagated the most in onlay restorations, where the highest displacement was observed. In contrast, the most successful results were observed in overlay restorations. Overlay restorations also showed better fatigue performance. LS materials exhibited more successful results than PICN materials. LS materials, which can be used in PPIRs, yield better results compared to PICN materials. While inlay restorations demonstrated relatively successful results, overlay and onlay restorations can be specified as the most and the least successful PPIR types, respectively.

On the decay of fracture toughness of sandstone subjected to freeze–thaw cycles

AbstractIn cold regions, progressive cracking, caused by freeze–thaw cycles, plays an important role in deteriorating engineering rock mass. Employing central cracked Brazilian disks (CCBDs) of sandstone, this paper aims at investigating the variations of falling rate among different modes of fracture toughness exposed to freeze–thaw cycles and the applicability of fracture criterion in predictions. Results revealed that different modes of fracture toughness exhibited comparable descending trends after varied freeze–thaw cycles. There was an agreement between the predictions via the generalized maximum tangential stress criterion (GMTS) and the test results. In specified conditions, the GMTS criterion incorporating fracture process zone of 0 freeze–thaw cycle can be applied to estimations for different freeze–thaw cycles. With increasing freeze–thaw cycles the fractal dimensions of mode I and mixed‐mode I–II fracture surfaces displayed a growing trend, indicating that the fracture surfaces became rougher and more complicated.

Meso-scale numerical simulation of the mechanical behaviour of brick masonry in earth mortar

The paper discusses the mechanical behaviour of masonry composed of fired solid bricks and earth mortar. Such masonry is a typical construction technique in South and Southeast Asia. As its mechanical behaviour is complex due to high anisotropy attributed to noticeable difference in stiffness and strength between bricks and earth mortar, it is still considered challenging to comprehend the mechanical behaviour of such masonry. Taking into account this background, the present research addresses the formulation of the mechanical behaviour of brick masonry in earth mortar. Numerical analysis was performed to reproduce uniaxial compression and diagonal compression tests, referring to empirical evidences on interface behaviour between bricks and earth mortar. Meso-scale analysis was performed, applying a multi-surface failure criterion composed of cracking, shearing and crushing to unit-joint interfaces. The analysis showed very similar stress-strain relations and failure patterns to the tests. The simulation of the uniaxial compression test presented that the distributions of compressive stress shifted along with the progress of cracks. In the reproduction of the diagonal compression test, vertical stress was irregularly distributed after diagonal stepwise cracks appeared. Two comparative studies were conducted to examine the shear performance of brick masonry in earth mortar. First, sensitivity analysis indicated the noticeable influences of compressive strength, friction angle and facture energy of unit-joint interfaces. Second, macro-scale analysis was conducted, using a homogeneous material model. The observed diagonal crack patterns were similar to the test. The paper contributes to the understanding of the mechanical behaviour of brick masonry in earth mortar.

Comparative Analysis of Reinforced Asphalt Concrete Overlays: Effects of Thickness and Temperature.

Reflection cracking in asphalt concrete (AC) overlays is a common form of pavement deterioration that occurs when underlying cracks and joints in the pavement structure propagate through an overlay due to thermal and traffic-induced movement, ultimately degrading the pavement's lifespan and performance. This study aims to determine how alterations in overlay thickness and temperature conditions, the incorporation of chopped fibers, and the use of geotextiles influence the overlay's capacity to postpone the occurrence of reflection cracking. To achieve the above objective, a total of 36 prism specimens were prepared and tested using an overlay testing machine (OTM). The variables considered in this study were the thickness of the overlay (40, 50, and 60 mm), temperature (20, 30, and 40 °C), mix type (reference mix and mix modified with 10% chopped fibers by weight of asphalt cement), and the inclusion of geotextile fabric at two positions (one-third of the depth from the base and at the bottom). The research outcomes revealed that a decreased temperature and thicker overlay led to a higher resistance to crack initiation and full propagation, as indicated by the values of critical fracture energy (Gc) and crack progression rate (CPR). Furthermore, the study observed the enhanced crack resistance of overlays in the presence of geotextiles, whether at the bottom or one-third of the depth from the bottom, with superior performance of the former. Despite a slight enhancement in certain properties, the incorporation of chopped fibers in the overlays did not substantially improve the overall performance compared to the reference specimens. Overall, the study provides valuable insights into the variables that influence the ability of AC overlays to mitigate reflection cracking. These findings will aid engineers and designers in making informed decisions regarding overlay design and construction.

NONLINEAR FINITE ELEMENT ANALYSIS OF FULL-SCALE EXTERNAL POST-TENSIONED CONCRETE BOX GIRDER AT JACKING AND SERVICE

This paper presents the modelling of a 47-meter of simply supported full-scale external post-tensioned concrete box girder in a bridge using in-house software 3D-NLFEA. The Girder was designed with sections, adequate reinforcements, and the jacking force according to the codes to resist the live load. The objective of this work is to assess the global structural response i.e., the load and deflection capacity under service loading. The detailed response of anchorage and deviator segments concerning their micro-cracking behavior during jacking transfer and service load conditions was also discussed. The constitutive model of materials applied to the modelling covered the hardening and softening of concrete under tension and compression, the nonlinearity of mild steel reinforcement, and prestressing tendons. The first stage was to simulate the self-weight of the girder simultaneously with the application of jacking forces from the tendons. The second stage was to load the girder under three-point bending to replicate the typical wheel axle from vehicles. The load was in the form of displacement-controlled and it was applied at the top midspan. From the results obtained, it was shown that under service limit state conditions, the load-displacement curve of the girder was linear at elastic. Cracks in allowable limit manifested in several parts of anchorage and deviator segments by the end of prestressing sequences. This highlighted the importance of providing crack control reinforcement to alleviate the progression of cracks under jacking service load conditions.

Autonomous healing of fatigue cracks via cold welding.

Fatigue in metals involves gradual failure through incremental propagation of cracks under repetitive mechanical load. In structural applications, fatigue accounts for up to 90% of in-service failure1,2. Prevention of fatiguerelies on implementation of large safety factors and inefficient overdesign3. In traditional metallurgical design for fatigue resistance, microstructures are developed to either arrest or slow the progression of cracks. Crack growth is assumed to be irreversible. By contrast, in other material classes, there is a compelling alternative based on latent healing mechanisms and damage reversal4-9. Here, we report that fatigue cracks in pure metals can undergo intrinsic self-healing. We directly observe the early progression of nanoscale fatigue cracks, and as expected, the cracks advance, deflect and arrest at local microstructural barriers. However, unexpectedly, cracks were also observed to heal by a process that can be described as crack flank cold welding induced by a combination of local stress state and grain boundary migration. The premise that fatigue cracks can autonomously heal in metals through local interaction with microstructural features challenges the most fundamental theories on how engineers design and evaluate fatigue life in structural materials. We discuss the implications for fatigue in a variety of service environments.

Study of the Overlying Strata Movement Law for Paste-Filling Longwall Fully Mechanized in Gaohe Coal Mine

Green mining plays a vital role in achieving environmentally friendly and ecologically sound mining practices. In domestic mining areas, the coal mining method is gradually transitioning from collapse mining to filling mining. Paste filling has been proven effective in controlling surface deformation, although the understanding of its underlying control mechanisms remains incomplete. This study focuses on the E1302 paste-filling working face at Shanxi Gaohe Energy Co., Ltd. and conducts a comprehensive investigation into the movement patterns of overlying strata in longwall fully mechanized mining with paste filling. Through mathematical analysis, a mechanical model for overburden movement in paste-filling faces is established, and the movement behavior of overburden is studied through numerical simulations. Field measurements are conducted to analyze the primary influencing factors of overburden movement, while surface subsidence monitoring is employed to analyze the subsidence characteristics of paste-filling faces. The research reveals that the deflection formula for the roof behind the paste-filling face follows a unitary quartic equation. The key factors influencing significant roof subsidence in filling faces include the filling step distance, filling body strength, and filling rate. Compared to traditional caving mining, filling mining exhibits reduced stress concentration, a smaller range of stress influence, and less deformation in the surrounding rock. The coefficient of gentle subsidence for the overlying rock in filling mining is approximately one-tenth of that in caving mining. The development of cracks in filling mining can be divided into three stages: initial crack propagation, crack recompaction, and stable maintenance of cracks. Notably, the progression of advanced cracks assumes a “sail-shaped” pattern, and the area of crack recompaction is located above the rear side of the excavation. Cracks behind the working face only appear in the basal roof rock layer. When the filling rate in longwall fully mechanized mining with paste filling exceeds 94%, the top plate of the filling working face remains intact but exhibits bending and sinking. The sinking of the top plate increases exponentially with the filling step distance, and approximately 80% of the filling body’s deformation occurs within 20 m after filling. Following backfilling mining, the stability period of the overlying rock is significantly shortened compared to caving mining, resulting in a relatively gentle movement without an active surface movement phase. After six months of backfilling, the overlying rock settles steadily and consistently. The subsidence coefficient for backfilling mining is 0.065, with a maximum surface subsidence of 215 mm. These findings highlight the successful control of surface subsidence. The research outcomes provide an effective theoretical foundation and research direction for predicting overburden movement and surface subsidence in paste-filling faces.

Numerical analysis of hydro-thermal fracturing in saturated rocks by considering material anisotropy and micro-structural heterogeneity

This study is devoted to numerical modeling of cracking process induced by temperature change in saturated porous rocks in the context of geological disposal of radioactive waste. Effects of material anisotropy and heterogeneity are taken into account. The macroscopic elastic properties are determined from two steps of homogenization by considering pores and mineral inclusions at two different scales. An extended phase-field model is proposed to describe the initiation and propagation of localized cracks. Two damage variables are introduced to conveniently represent both tensile and shear cracks. New damage evolution criteria are defined by incorporating the pore pressure effect. Three application examples are presented. By assuming a random distribution of pores and inclusions, the efficiency of the proposed model for capturing the progressive cracking process is first verified in a triaxial compression test. The thermal cracking process in an anisotropic and heterogeneous sample is then investigated. The respective influences of elastic anisotropy and spatial variability of pores and inclusions are outlined. Finally, the proposed model is applied to a series of real laboratory thermal cracking tests. Both hydromechanical responses and cracking evolution patterns are investigated. Numerical results are compared with experimental measurements. The main mechanisms involved in the thermal cracking process are highlighted.

Research on curing kinetics of multiaxial warp‐knitted composites

AbstractIn this paper, the differential scanning calorimetry (DSC) was used to test the crosslinking reaction system between epoxy resin and curing agent, and the curing kinetic model of the resin system was established by means of phenomenal model. The optimal curing process was obtained based on DSC curve. The quadriaxial warp‐knitted composite specimens were prepared by vacuum‐assisted resin infusion (VARI). The effects of curing process on mechanism properties and failure mechanism of the composite specimens were studied experimentally. Results show that the cross‐linking reaction between epoxy resin and curing agent agrees with the autocatalytic reaction model and the curing kinetics model could be used to characterize the curing process of the matrix. The optimum curing process could be used to improve the interface properties between the matrix and the fiber, and the crack progress will be inhibited as the specimen bearing the external force, and the mechanical properties of the specimen is also improved significantly. Result could provide a theoretical basis for the study of curing kinetics of epoxy resin, the exploration of curing process of composite materials and improve the strength of composits.

Microstructural effects on short fatigue crack propagation in cast Al-Si alloys

The role of microstructure on the short fatigue crack behaviour of cast Al-Si alloy under high cycle loading conditions is investigated by tracking the progress of cracks through fine, medium and coarse microstructured samples. Generally, in the early stage of crack growth, short cracks mainly grow transgranularly, with cracks growth close to low index crystallographic planes (mainly {100}) in some grains and frequently inhibited at grain boundaries. In the late stage, the crack growth mode gradually shifts from crystallographic stage-I to non-crystallographic stage-II. The short cracks grow faster and fluctuate significantly according to local microstructure. Our results show that finer microstructures offer greater crack nucleation and propagation resistance.