Cohesive Parameters Research Articles

Micro-damage analysis and numerical simulation of composite solid propellant based on in situ tensile test

Abstract In order to quantitatively analyze the mesoscopic damage process of hydroxyl-terminated polybutadiene composite solid propellant under external load, periodic boundary conditions were applied to the representative volume element model based on sample composition and morphology, the mixed matrix containing aluminum powder was homogenized, and the hyperelastic matrix damage and bilinear/exponential particle–matrix interface cohesive model with initial damage were compiled through the secondary development of Abaqus. At the same time, a data interaction platform was constructed by means of Python and MATLAB, matrix and cohesion parameters were inverted according to the optimization algorithm and experimental data, and the whole process of propellant damage and fracture was simulated from the mesoscopic perspective. The results show that combining the adaptive particle swarm optimization algorithm and the Hooke–Jeeves algorithm can achieve the global optimal parameter inversion in 102 calculations, compared with the single local search algorithm, which can cut about 11% of the objective function values. Considering the matrix damage and the exponential cohesion model with initial damage, the optimal objective function value is 0.01635, which can more accurately simulate the propellant damage and fracture process compared with 0.02136 of a bilinear cohesion model.

High-fidelity computational modeling of scratch damage in automotive coatings with machine learning-driven identification of fracture parameters

As one of the key components of a vehicle, automotive coating systems are typical micro-size multilayer composites, which normally suffer from intricate scratch damage. Currently, it is a challenging task to reproduce the complex physical phenomena of automotive coatings with micron thickness using numerical simulations. The purpose of this work is to develop a computational framework for an accurate evaluation of such scratch damage. To achieve this end, a high-fidelity finite element (FE) model is proposed, where a rate-dependent cohesive zone model is developed to account for coating crack behaviors. A reverse identification approach based on the machine learning (ML) method is suggested to determine the rate-dependent cohesive parameters that are difficult to be measured via direct experiments. In the course of recognition, this work develops a two-step regional data augmentation strategy based on the existing single-shot recognition method. The developed strategy is capable of reducing the number of training data samples, while improving identification accuracy and robustness. With the identified cohesive parameters, the proposed finite element model is applied to simulate speed-dependent coating scratch behaviors. The good agreement between numerical and experimental results demonstrates the capacity of our developed computational framework for coating scratch problems.

Macro and micro damage analysis and parameter inversion of HTPB adhesive Interface based on DIC and FEMU

ABSTRACT In order to realize the quantitative analysis of the structural integrity and damage evolution of hydroxyl-terminated polybutadiene (HTPB) composite solid propellant under external load, the loading failure experiments were carried out on the macro rectangular specimen and the micro specimen, and the numerical analysis of the strain evolution process in the region of interest (ROI) of the specimen was carried out using digital image correlation (DIC). At the same time, with the help of finite element model updating (FEMU) method, the mechanical and cohesion parameters of the specimen were inversed using the combination of adaptive particle swarm optimization (APSO), back propagation (BP) and Hooke-Jeeves algorithm, and the simulation of the whole process of propellant damage and fracture was realized from the microscopic point of view. The results show that inhibiting the debonding of propellant/liner interface is the key to maintain the structural integrity. It starts with the damage and fracture of the propellant side. A scanning electron microscope (SEM) in-situ dynamic tensile test shows that the initial damage occurs at the strain of 27.368%, and the through-type crack propagates along the interface when the strain reaches 43.276%. In addition, the use of combinatorial optimization algorithm can realize the global optimal inversion of 16 parameters divided into three types in 100 complete calculations, reduce the optimal objective function value to 0.0251, and assist the finite element calculation to realize the quantitative analysis and accurate simulation of the experimental process.

Characterization and prediction of hygrothermally aged CFRP adhesive joint subjected to mode II load

Carbon fiber composite adhesives joints provide higher specific strength and fatigue life than traditional joints. However, carbon fiber composites and adhesives are polymeric materials that may lose their integrity in hygrothermal environment. Predicting the environmental degradation behavior of composite joints is challenging because of limited understanding of the complex failure behavior. Moreover, the experiments are time-consuming and therefore it is essential to use accelerated aging methods such as the open faced method. However, the challenge is how to incorporate the accelerated fracture test data into the fracture prediction of real joints. To address this, a methodology is presented for predicting mode II cohesive law parameters of degraded CFRP epoxy adhesive joints using a direct method, and an accelerated aging test. End notched flexure (ENF) specimens made with CFRP laminate and epoxy adhesive were used to study the mode II fracture. Accelerated and uniform aging was achieved using open-faced specimens aged at 40 °C and 82% relative humidity (RH). Fracture testing of aged open-faced specimens along with crack-tip images were used to determine the degradation in trapezoidal traction-separation law (TSL) with aging. These variations in trapezoidal TSL with aging were used to develop a 3D finite element (FE) model of a closed ENF specimen that captures the non-uniform degradation across the specimen width. The TSL parameters degraded significantly at the specimen edges compared to width center for the closed ENF specimens. Validation of the FE model with aged closed ENF joint showed good agreement.

A novel fatigue cohesive model for interface between ballastless track slab and self-compacting concrete

The interface between track slab and concrete is the weakest zone in a ballastless track. Under the long-term and repeated coupling effect of the environment and train loads, a large area of damage can occur at this interface, leading to the initiation and propagation of interlaminar crack, and the failure of the interlayer bonding area. To reveal the fatigue behaviors and degradation laws of the interface between the prefabricated track plate (C60) and self-compacting concrete (SCC) in China Railway Track System (CRTS) III slab track, a fatigue cohesive zone model is proposed for the analysis of the fatigue bonding performances of the interface C60-SCC, and the fatigue damage degradation criterion is established. Next, static and fatigue tests for C60-SCC composite specimens are conducted, and full-field strain distribution images of these specimens are obtained by the digital image correlation technique. The results show that: (1) the evolution of interfacial crack of C60-SCC specimens under monotonic loading was captured and static cohesive parameters were obtained; (2) the fatigue life curve and interfacial separating displacement curves were acquired, for predicting the interface fatigue life and calculating the degree of interface damage and residual fatigue life of CRTS III slab track in service; (3) the fatigue cohesive response of the interface C60-SCC under cyclic loading were determined for describing the fatigue behaviors of the interface in CRTS III slab track.

Effect of Through-the-Thickness Delamination Position on the R-Curve Behavior of Plain-Woven ENF Specimens.

In this study, the effect of through-the-thickness delamination plane position on the R-curve behavior of end-notch-flexure (ENF) specimens was investigated using experimental and numerical procedures. From the experimental point of view, plain-woven E-glass/epoxy ENF specimens with two different delamination planes, i.e., [012//012] and [017//07], were manufactured by hand lay-up method. Afterward, fracture tests were conducted on the specimens by aiding ASTM standards. The main three parameters of R-curves, including the initiation and propagation of mode II interlaminar fracture toughness and the fracture process zone length, were analyzed. The experimental results revealed that changing the delamination position in ENF specimen has a negligible effect on the initiation and steady steady-state toughness values of delamination. In the numerical part, the virtual crack closure technique (VCCT) was used in order to analyze the imitation delamination toughness as well as the contribution of another mode on the obtained delamination toughness. The numerical results indicated that by choosing an appropriate value of cohesive parameters, the trilinear cohesive zone model (CZM) is capable of predicting the initiation as well as propagation of the ENF specimens. Finally, the damage mechanisms at the delaminated interface were investigated with microscopic images taken using a scanning electron microscope.

Prediction of Crack Propagation of Nano-Crystalline Coating Material Prepared from (SAM2X5): Experimentally and Numerically

The fracture and crack growth of materials can be practically and conveniently predicted through numerical analysis and linear elastics fracture mechanics. On this basis, the current study aims to present experimental work supported by a numerical technique for mimicking the crack propagation by Version 5.6 of COMSOL Multiphysics (version 5.6), used for the simulation of the coating made from Fe-based amorphous material with a thickness of 300 µm. The paper shows the effects of mixed-mode loading on cohesive zone parameters attained from load-crack mouth opening displacement (CMOD) curves. The microstructure dominates the fracture, which in mode I is altered from all-transgranular cleavage to nearly all-intergranular structure in mode II. Two common criteria for failure are linked to the mixed-mode results: Maximum energy release rate criterion (Maximum G) and maximum tensile stress criterion (Maximum S). However, distinguishing between the two criteria is made impossible by the large scatter in the data. The stress intensity factor is the basis for the. The stress intensity factor is the leading parameter facilitated by the singular element and should be estimated with accuracy. With the aim of comparing each criterion and illustrating the numerical schemes’ robustness, a number of examples are presented. It can be concluded that the Maximum G and Maximum S were successful and accurate in predicting the propagation of the Fe-based amorphous material prepared on mild steel.

Study on the developmental characteristics and mechanism of shale FPZs

The fracture process and behavior of shale play a key role in the success of hydraulic fracturing stimulation in shale reservoirs. In this study, the central straight cracked Brazilian disc (CSCBD) splitting test in three fracture directions (arrester, divider, and short transverse) is conducted to reveal the propagation behavior and mechanism of closed symmetric fractures in laminated shales, with a focus on the fracture process zone (FPZ). The developmental characteristics and anisotropy of the FPZ near the crack tip of Mode I fracture of shale are identified and quantified based on the digital image correlation (DIC) method. By monitoring the crack tip opening displacement (CTOD) during the whole process, the critical CTOD, as one of the pivotal cohesive parameters, is obtained, and the development of the FPZ can be phased into three stages by CTOD evolution patterns. Based on double-K fracture criterion, the initiation fracture toughness and unstable fracture toughness of shale fractures at different laminar configurations are also analyzed. Additionally, the cohesive fracture model in the shale FPZ is derived based on the power function form using the energy release rate method.

Mechanical response and gas flow characteristics of pre-drilled coal subjected to true triaxial stresses

Many gas pre-drainage boreholes are generally constructed in coal seams to ensure safe underground operation. True triaxial experiments were conducted to study the mechanical response and gas flow characteristics of pre-drilled coal to reveal the in-situ behavior at pre-drilled regions in coal seams. The mechanical parameters, failure modes, and flow rate evolutions of pre-drilled coal samples were collected. The gas flow rate of pre-drilled coal samples showed an S-shaped trend with the increase of strain, and no obvious descending progress was found in the flow rate-strain curves. The typical failure mode for pre-drilled coal samples was shear-type during the true triaxial stress compression. V-shaped macro-fractures around the boreholes were observed when boreholes were perpendicular to bedding planes and parallel to face cleats. In contrast, single shear macro-fractures around the boreholes were observed when boreholes were parallel to butt cleats. The pre-drilled coal samples exhibited lower peak strength values when boreholes were parallel to butt cleats, induced by the relatively low cohesion parameter. Pre-drilled coal's initial flow rates perpendicular to bedding planes and parallel to butt cleats directions were only 33.33% and 41.67% of values measured along face cleats. More complex macro-fractures were formed, and up to a 39.30% decrease in strength was observed in pre-drilled coal samples under true triaxial unloading stress paths. The variations in volumetric strain level and the flow rate evolution were further discussed with an anisotropic conceptual failure model. These mechanical responses and flow characteristics of pre-drilled coal would guide the design of borehole layout and prevention of coal dynamic disasters.

A Numerical Model for Understanding the Development of Adhesion during Drying of Cellulose Model Surfaces

Adhesion is crucial for the development of mechanical properties in fibre-network materials, such as paper or other cellulose fibre biocomposites. The stress transfer within the network is possible through the fibre-fibre joints, which develop their strength during drying. Model surfaces are useful for studying the adhesive strength of joints by excluding other parameters influencing global performance, such as geometry, fibre fibrillation, or surface roughness. Here, a numerical model describes the development of adhesion between a cellulose bead and a rigid surface using an axisymmetric formulation, including moisture diffusion, hygroexpansion, and cohesive surfaces. It is useful for studying the development of stresses during drying. A calibration of model parameters against previously published contact and geometry measurements shows that the model can replicate the observed behaviour. A parameter study shows the influence of cohesive and material parameters on the contact area. The developed model opens possibilities for further studies on model surfaces, with quantification of the adhesion during pull-off measurements.

Experimental investigation of mechanical behavior of geosynthetics in different soil plasticity indexes

Nowadays, in-depth research is done to evaluate the behavior of geosynthetics in cohesive soil. However, it is still unclear how the plasticity index (PI) of soil affects the mechanical behavior of reinforced cohesive soil. The behavior of distinct geosynthetic types in soils having different PI is likewise uncertain. To bridge this gap, the mechanical behavior of three cohesive soils designated as S1, S2, and S3 reinforced with four types of geosynthetics (woven/non-woven geotextile, geocomposite, and geogrid) were studied by executing multiple triaxial compression and interface direct shear tests with and without reinforcement. The results show that the soils reinforced with woven geotextile (GTW) produce higher shear strength than other reinforcements because of their higher interface frictional resistance and tensile strength. It was found that three layers of GTW to S1 soil increased its stiffness, cohesion, and shear strength parameters by 150%, 307%, and 161%, respectively. On the other hand, the increase in shear strength caused by adding three layers of GTW to S2 and S3 soil was only about 59% and 37%, respectively. The interface properties indicate a strong interaction between GTW-S1 soil lasted, whereas a weak interaction between GTW-S2 or S3 soil existed. These findings demonstrate that an increase in the PI of soil significantly reduces the performance of reinforcements due to the reduction in interface friction, lateral constraint, and interlocking effect. The geosynthetic reinforcement performed better in lower plastic soil rather than in higher.

Degradation prediction of mode I cohesive laws of hygrothermally aged CFRP adhesive joint using a direct method and accelerated aging

AbstractA framework has been proposed to predict the mode I cohesive law parameters of degraded composite adhesive joint using direct method and accelerated aging tests. Double cantilever beam (DCB) specimens made with CFRP and epoxy were used to study mode I fracture. Accelerated and uniform aging was achieved using open‐faced specimens aged at 40°C and 82% relative humidity. Fracture testing of aged open‐faced specimens along with crack‐tip images were used to determine the degradation in traction‐separation law (TSL) with an aging. These correlations with bilinear TSL with aging were then used to develop a 3D finite element model of a closed DCB specimen that captures the nonuniform degradation across the specimen width. The TSL parameters degraded significantly at the edges compared with width center for closed DCB specimens. Reasonable agreement was observed between the predictions and the maximum load response of the closed joint.

Uncertainty Characterization for Soil Cohesion in a Project Site in Nasiriyah Using Bayesian Methods

High uncertainties arias through the characterization of soil parameters because of the lack of data obtained from geotechnical reports. Reducing these uncertainties may improve the characteristic values of soil parameters. This research aims to probabilistically characterize a soil's cohesion parameter in Nasiriyah. The Bayesian approach has been applied to soil data obtained through a project in Nasiriyah. The soil at the site is classified as lean clay, and the soil cohesion has been evaluated using two Bayesian methods: the ordinary, normal distribution method (OND) and the Marcove Chain Monte Carlo-based Bayesian approach (MCMC) method. The previous knowledge utilized in the Bayesian approach was based on 20 boreholes, and the subjective probability approach has functioned in the prior probability distribution. The OND method deduced a mean value of cohesion of (195.9 kPa) and a standard deviation of (14.68 kPa), (COV) 7.49%. It was noted that the probability distribution has a more significant effect than the previous distribution on the posterior distribution. The MCMC method summarized the probabilistic description of the soil characteristic, through which it reached the mean and the subsequent standard deviation (167.49) kPa (109.8) kPa, respectively, and the coefficient of Variation (COV) was 65.6%. It is considered the most appropriate and common method, especially in high-dimensional data when the results are not well known because it can provide a probabilistic value for the not well-known data.

The combined effect of cohesion and finite size on the collapse of wet granular columns.

The collapse of low-saturation liquid-containing granular materials is prevalent in nature and industrial processes, and understanding the associated transient dynamics is extremely important for exploring such complex flow processes. In this paper, the collapse of a finite-size wet granular column is systematically studied and the determinants affecting its dynamics are analyzed based on the discrete element model for wet particles and the corresponding small-scale experiments. With the aid of parametric analysis, the dimensionless cohesion parameter containing the system size and grain-scale bond number is proposed, and its relevance in characterizing column stability and collapse dynamics of wet granular materials is further confirmed. For the collapse of wet granular columns with a fixed aspect ratio, the initial height contained in the cohesion parameter is verified to be a manifestation of the finite size effect, which is present in a wet granular collapse and is coupled with the cohesive effect. Such a coupling effect is taken into account in our proposed scaling laws that can be applied to uniformly describe the deposit morphology of wet granular columns after collapse.

AN INDENTATION METHOD FOR ADHESIVE STRENGTH EVALUATION OF POLYMER COATINGS

An indentation method was developed for adhesive strength evaluation of a polymer coating using the example of an epoxy composition with titanium alkoxide deposited on the surface of low-carbon steel. The method is based on the numerical implementation of the fracture mechanics approach in experimental data processing. It was found that the penetration of a Rockwell indenter perpendicular to the coating surface causes annular delamination buckling of the coating around the indentation, which is due to adhesive bond breaking as a result of radial shear when the coating material is forced out from under the indenter. A controlled parameter in finite element modeling was the experimentally determined width of the coating delamination zone at a fixed indentation depth at which delamination occurred. The adhesive contact conditions were set using the bilinear law of the cohesive zone model, which describes the relationship between the adhesive shear stress and adhesive bond stretching during shear of interacting surfaces in the contact plane. The limiting specific surface energy of adhesive failure was used as a quantitative evaluation criterion for adhesive strength. The optimal value of the limiting specific surface energy of adhesive coating failure was determined for the cohesive zone model parameters that provide the best convergence of the numerical and experimental data.

Computational Modeling of Intergranular Crack Propagation in燼n營ntermetallic Compound Layer

A micromechanical model is presented to study the initiation and propagation of microcracks of intermetallic compounds (IMCs) in solder joints. The effects of the grain aggregate morphology, the grain boundary defects and the sensitivity of the various cohesive zone parameters in predicting the overall mechanical response are investigated. The overall strength is predominantly determined by the weak grain interfaces; both the grain aggregate morphology and the weak grain interfaces control the crack configuration; the different normal and tangential strengths of grain interfaces result in different intergranular cracking behaviors and play a critical role in determining the macroscopic mechanical response of the system.

CFD–DEM Simulation of Dust Deposition on Solar Panels for Desert Railways

With the greening of the railway energy supply chain, large-scale photovoltaic power stations will be the best choice to integrate with the railways. Understanding the deposition mechanisms and rules of dust grains on photovoltaic panels is of great guiding significance for the operation of photovoltaic (PV) power stations. In this paper, based on computational fluid dynamics (CFD) combined with the discrete element method (DEM), the dynamic dust deposition process on solar panels was simulated, and the flow field around solar panels and the movement of dust particles in the wind were calculated. The simulation results of clay particles (d = 10 mm) and fine sand particles (d = 100 mm) under different wind speeds showed that the clay particles could follow the air flow properly, and their deposition rate was only 4.6%, while the deposition rate of the fine sand particles was up to 32%, which was determined by the inflow wind speed and cohesion parameters. An image of the non-uniform distribution of particles on the panels was given in this paper for the first time. This will provide a basis for a more accurate assessment of the impact of dust accumulation on PV output in real-world environments. These results provide a critical reference for railway photovoltaic power supply development in desertification areas.

Application of the FDEM Based on the CZM in Simulating Three-Point Bending Test of Frozen Soil

The combined finite–discrete element method (FDEM) based on the cohesive zone model (the CZM) achieves cracking simulation by inserting cohesive elements between solid elements. In this study, three-point bending fracture tests of frozen soil were simulated by using the FDEM based on the CZM. Firstly, the sensitivity of the cohesive model parameters was analyzed. Secondly, through a series of simulations of the three-point bending test of frozen soil, it was found that the model with reasonable values of the CZM parameters had a good adaptability to the three-point bending cracking test of frozen soil, as the model not only reflects the load-displacement curve, but also has good correspondence with the fracture pattern compared with the test. Finally, the relationship between the CZM parameters and the specimens’ temperature under two loading rates of 1 mm/min and 0.1 mm/min was analyzed, and it was found that the CZM parameters had a good linear relationship with the specimens’ temperature. This paper is expected to provide a new possibility for the numerical simulation of frozen soil cracking.

Multiscale modeling for dynamic compressive behavior of polymer bonded explosives

The impact damage behavior of polymer bonded explosives (PBXs) is critical to ensure the safety of explosive systems. PBX 1314 (60 wt% RDX, 16 wt% aluminum, and 24 wt% HTPB) is a popular propellant owing to its characteristics of high energy density and low sensitivity. In this study, the compressive damage evolution rules and the strain rate effect of PBX 1314 were investigated by using a multiscale method. To acquire damage evolution data during loading, a finite element model based on real crystal morphology, with a cohesive zone model (CZM) for describing the damage, was developed. A novel hybrid/inverse optimization strategy was developed to calibrate the cohesive parameters of PBX 1314, and the accuracy of the parameters was verified basis experimental results. Using an as-developed bilinear model, the shift of damage pattern with the variation of strain rate was observed, and the nonlinearity of the stress–strain curve was presented at the mesoscale. The crack parameters and distribution of damaged elements in each mesoscopic component were acquired to quantitively characterize the strain rate dependence of PBX 1314, and the mechanism of crack propagation and the shift of damage mode under various strain rates was illustrated. The findings of this study provide insights for understanding the nonlinearity of macroscopic mechanical behavior and the influence of strain rate on the mesoscopic damage mode of PBXs with high binder content.

Probabilistic Analysis of Strip Footings Using the Subset Simulation Approach and the Influence of Spatial Variability of Two Clay Layers

Abstract This article studies the probabilistic analysis at the ultimate limit state (ULS) of a strip footing resting on a purely cohesive soil composed of two layers with spatial variability of the upper or lower layer and treats the simultaneous effect of the variability of the two layers on the probability of failure Pf value using the subset simulation approach. The results are examined by comparing the Pf values obtained with the Pf value corresponding to the reference case of two homogenous clay layers. In general, the subset simulation approach is used in cases of uncertain parameters modeled by random variables. But in this paper, it is employed with uncertain parameters modeled by random fields because the spatial variability of soil properties strongly affects the behavior of geotechnical structures and causes a significant change in the variability of their responses. The soil cohesion parameter is modelled as a non-Gaussian (log-normal) anisotropic random field using a square exponential autocorrelation function by the optimal linear expansion estimation (EOLE) and the deterministic model is based on numerical simulations using the finite difference software FLAC3D.