Cohesive Interface Research Articles

Numerical modelling on effect of loading rate on uplift behavior of suction caissons

This paper details the development of a hydro-mechanical (HM) coupling model in ABAQUS to mimic the seepage and separation on the interface between clayey seabed and suction caissons subjected to uplift loading. This was achieved through introducing triple-node pore-water cohesive interface elements to the caisson-soil interface, thus enabling normal and tangential flow on the interface to be modelled properly. The model was validated against centrifuge tests conducted at different loading rates as well as companion numerical results, and then used to examine the uplift behavior of suction caissons under different drainage conditions. The results showed that the gradual transition of a localized failure mechanism to a global one, with the increase in loading rate, can be successfully captured. Besides, the development of negative pore water pressure inside the caisson illustrated strong sensitivity to the loading rate, when the dimensionless velocity group vD/cv fell within the range that denoted a partially drained condition. The model also proves to be capable of reproducing the separation of the soil plug from the inner wall and the necking behavior. A detailed parametric study discussed the influence of caisson wall roughness, caisson aspect ratio and typical clay properties on the uplift behavior of suction caissons.

Oblique Low-Velocity Impact Response and Damage Behavior of Carbon-Epoxy Composite Laminates.

The low-velocity impact behavior of carbon-epoxy cross-ply composites was numerically investigated, examining the effect of impact angle. A plastic continuum damage model, introducing the cohesive interface to describe delamination damage, was established and was validated by available experimental data. Impact histories, progressive deformation, stress transfer, and impact damage are respectively discussed. The results show that an increase in impact angle intensifies the action of tangential force, and gradually transfers energy absorption from normal plastic deformation to tangential deformation and friction, which dissipates more energy through relatively longer contact duration and larger impactor displacement. The delamination damage to upper layers is more affected by tangential loads, intensifying with the increase of the impact angle, and the damage area to the top interface is increased by 132.1% from 0° impact to 60° impact. Meanwhile, the delamination damage to lower layers is mainly determined by normal loads, weakening with the increasing impact angle overall, and the damage area of the lowest interface decreases by 36.6% from 0° impact to 60° impact.

English

While the current economic crisis clearly puts pressure on governments to cut spending and save money, and whilst the call for a more cohesive interface between the public and private sectors is currently resonating, public procurement has been marred by scandals, mismanagement, and possibly malpractices. According to the literature, corporate integrity drives transactional behaviors by acting as a hidden hand of social factors that are frequently beyond an individual's control. But scantly studied, as behavioral control mechanism in transaction costs. This paper studies the moderating effect of integrity on the relationship between transaction costs’ behavioral dimensions and competitive tendering in public procurement. Following a literature review, the paper takes a conceptual framework towards developing evidence of cascaded transaction costs’ behavioral dimensions, bounded rationality and opportunism. Formal research propositions amplify both concepts and its relationship with independent variable and dependent variables. The results are based on the empirical data collected from a self-administrative survey from public procurement practitioners in Tanzania. Descriptive and Hierarchical regression results reveals that public procurement practitioners’ perceived transaction costs to negatively when related with competitive tendering as moderated by integrity. This means, integrity can moderate the behaviors of public procurement practitioner which, in turn, accelerates efficiency of competitive tendering by reducing transaction costs.   Key words: Transaction costs, competitive tendering, integrity, public procurement.

Research on cohesive failure behavior and parameter sensitivity of rubber – SET under cohesive zone model

Solid expandable tubular (SET) technology is widely used in oil field. The cohesive failure of rubber reduces the suspension and sealing capacity of SET. Based on cohesive failure behavior between rubber and SET and morphological characteristics of rubber after expansion experiment. The cohesive zone model was introduced to describe cohesive behavior. The coupled three-dimensional finite element model of casing - rubber - SET was established. The effects of sensitive factors such as friction factors, rubber compression, rubber length, stiffness, damage initiation strength and fracture energy on cohesive failure behavior of SET rubber were researched. The results show that (1) In order to ensure that driving force in expansion process is small and meet the requirements of suspension force and sealing pressure after expansion. The friction coefficient of casing - rubber - SET system should be about 0.15. (2) On the premise of no cohesive failure occurs, the suspension force and sealing performance of rubber can be effectively improved by increasing theoretical compression and length of rubber. The optimum theoretical compression of rubber is about 50% of rubber thickness. The length of rubber should not exceed 60 mm. (3) Reducing stiffness and increasing damage initial strength of bonding material can reduce the risk of cohesive interface failure. The normal stiffness should not be greater than 5.4 N/m3 and the tangential stiffness should not be greater than 6.3 N/m3. The normal ultimate tensile stress should not be less than 9.6 MPa and the transverse shear stress strength should not be less than 9.7 MPa. It is not significant to increase bonding property of rubber by increasing fracture energy. The research results provide theoretical guidance for the structural design and bonding process parameters of SET rubber.

Plastic anisotropy and twin distributions near the fatigue crack tip of textured Mg alloys from in situ synchrotron X-ray diffraction measurements and multiscale mechanics modeling

In spite of many potential applications due to their superior mechanical properties, magnesium alloys still find many practical restrictions primarily due to our limited knowledge of their failure mechanisms. In situ and non-destructive strain measurements on the microstructural scales are critical in understanding the fatigue crack behavior, which has greater advantages than the macroscopic measurements based on replica technique and the small-scale mechanical testing that cannot easily provide a complete view on the synergy of many scale-dependent deformation and failure mechanisms. This work exploits the state-of-the-art synchrotron X-ray diffraction technique to attain in situ strain mapping results, with high spatial resolution, near a fatigue crack tip in the highly textured ZK60 Mg alloy. The accurate measurements in the plastic zone are compared to a micromechanical modeling framework, in which the steady fatigue crack is simulated by an irreversible, hysteretic cohesive interface model and the surrounding plasticity by Hill anisotropic plasticity. The general agreement in the anisotropic deformation fields is reached down to half of the plastic zone size towards the crack tip, while the discrepancy right at the crack tip can be used to develop an inverse model to extract the material-specific fatigue damage evolution in our future work. It is surprisingly noted that twin activities are localized in a narrow band from the tip to the wake of the fatigue crack, irrespective of the crack growth direction of this textured alloy. Such twin-crack interactions are understood here by our crystal plasticity finite element simulations with several representative crystallographic orientations.

Understanding the role of interfacial mechanics on the wrinkling behavior of compressible bilayer structures under large plane deformations

Layered soft structures under loading may buckle in order to release energy. One commonly studied phenomenon is the wrinkling behavior of a bilayer system consisting of a stiff film on top of a compliant substrate, which has been observed ubiquitously in nature and has found several applications. While the wrinkling behavior of the incompressible bilayer system has been explored thoroughly, the large deformation behavior of a compressible bilayer system had been virtually unexplored until very recently. On the contrary, it is well established where more than one material is concerned, there always exists an interphase region between different constituents whose mechanical modeling has presented itself as a long-lasting challenge. To address these gaps in the literature, herein we first propose a theoretical, generic, large deformations framework to capture the instabilities of a compressible domain containing an interface. The general interface model is employed such that at its limits, the elastic and the cohesive interface models are recovered. The instability behavior of a compressible bilayer domain undergoing large deformations for a wide range of cohesive stiffness values, stiffness ratios, compressibilities, and film thicknesses is systematically explored. In particular, it is shown that delamination of the film can also be captured via this interface model. In addition, this generic framework is examined for a coated beam and a coated half-space too. The results of the theoretical framework are thoroughly compared to numerical results obtained via finite element method simulations enhanced with eigenvalue analysis, and an excellent agreement between the two sets of results is observed. It is found that varying substrate Poisson’s ratio has a significant effect on the bifurcation behavior for higher cohesive stiffnesses. Remarkably, while in classical bilayers the critical stretch at wrinkling is independent of the film thickness, herein we discover a significant dependence of the critical stretch to the film thickness in the presence of the interface.

Two-Dimensional Meso-Scale Simulation of Hydraulic Fracture in Concrete

A finite element (FE) modeling approach is developed for meso-scale hydraulic fracture in concrete. The FE models, consisting of aggregates, mortar and pores, are first generated by either random aggregate generation and packing or direct conversion of real micro-scale X-ray computed tomography images. The cohesive interface elements coupling fluid pressure freedoms are then inserted into the solid FE meshes by a developed Python code to simulate hydro-cracking. The developed approach was validated by simulating a few examples. It was found that the contents and distribution of aggregates and the fluid viscosity, the confining pressure ratios, the inclined angels of natural cracks and pores have significant effects on the hydraulic fracture resistance and the crack patterns.

Prediction of fracture toughness due to delamination in cross-ply composite laminates considering interlaminar and intralaminar cracking and inhomogeneous fiber-matrix modeling

In this study, a finite element strategy is implemented for more accurate simulation of nonlinear failure behavior in multidirectional laminated composites. The considered approach is based on the parameters which could affect the SERR2. Accordingly, interaction between delamination and matrix cracking as well as mixed stress distribution around the fiber due to mismatch in properties of adjacent plies with different fiber angle orientation influence the strain energy release rate. Thus, in the present research the prediction of the damage behavior of cross-ply laminates is improved considering the effects of matrix cracking and inhomogeneous fiber-matrix modeling. For simulation of the delamination and the matrix cracking damage, cohesive interface elements are implemented. They have been used perpendicularly inside the 90o plies for the matrix cracking. CZM3 approach, ABAQUS subroutine and Python programming were utilized for automatic modeling of matrix cracking. The resultant load–displacement curve in DCB4 test of cross-ply laminates indicated that by considering delamination and matrix cracking simultaneously, load–displacement curve of cross-ply laminates became nonlinear earlier and the obtained results had an acceptable compatibility with experimental results. The obtained results for double cantilever beam (DCB) test specimen with cross-ply lay-up indicate that the numerically reproduced GIc parameter of 90/0 interface can be improved about 20% using this simulation. Moreover, by simulating periodic matrix cracking inside 90o plies, crack jumping, stick-slip phenomenon and local hardening behavior are anticipated such as experiments.

Cohesive fracture evolution within virtual element method

The paper presents a numerical procedure for the nucleation and evolution of cohesive fracture in two-dimensional bodies. The procedure is based on the development of a 12-node virtual element. Initially, the construction bases of the virtual element method (VEM) are illustrated, with specific reference to a four sides 12-node element. The recovery of the stress via complementary energy within the single element is described. The fracture is introduced in the two-dimensional body by splitting the virtual element, called parent element, into two slave elements joined by a cohesive interface. Thus, a damage interface model characterized by bilinear softening and accounting for mode I, mode II and mixed mode of fracture is presented. Then, the numerical procedure for crack nucleation and evolution is proposed. The crack direction is individuated by the maximum tensile principal nonlocal stress computed averaging on the mesh, by means of a weight function, the stress field evaluated via complementary energy within each element. Once the direction is determined, the (parent) 12-node element is split into two (slave) elements joined by the interface. Several numerical applications are developed, illustrating the performance of the 12-node element and the its accuracy in evaluating the stress field. Then, computations concerning typical fracture tests are performed, comparing the obtained results with available experimental evidences and with the ones carried out by using different numerical techniques.

A Numerical Investigation of Thermal-Induced Explosive Spalling Behavior of a Concrete Material Using Cohesive Interface Model

Thermal-induced spalling is a typical failure behavior of concrete materials exposed to high temperatures. This study uses Abaqus to establish a numerical model of concrete material comprising aggregates and mortar matrix. Cohesive elements considering heat conduction are embedded into this numerical model to simulate the thermal-induced explosive spalling failure process of the concrete material. Simulation results show that the heat gradually transfers from the outer boundaries to the inner areas with increasing temperature. Thermal stresses concentrate in the aggregates-mortar interfaces, where thermal-induced cracks initiate and propagate. The occurrence of thermal-induced cracks reduces the heat conductivity of mortar, reduces thermal stresses and leads to severe spalling failure in the concrete material. This research provides a practical scheme for the numerical simulation of the thermal-induced spalling behavior of concrete materials.

Numerical Analysis of Structural Performance of Concrete-GFRP Composite I-Beam

The concrete-GFRP composite beams have received extensive attention in civil engineering. However, the ambiguity of the fracture, debonding of the interface, and the GFRP profile limit the precise design of the composite beam. This article presents a comprehensive numerical study for the structural performance of composite pultruded GFRP beams to provide a better understanding of the mechanism of interfacial debonding and GFRP matrix fracture. The failure and delamination process of pultruded GFRP for anisotropy of materials is modeled using the Hashin criteria. The bond–slip behavior between the concrete slab and the top flange of the GFRP I-beam is simulated by the bilinear cohesive interface element. The availability and accuracy of the finite element model are verified by comparison with the four-point bending test results of the pure GFRP I-beam and composite beams as well. Based on the proposed comprehensive finite element model, the effects of the strength, thickness, and width of the concrete slab and the shear-span ratio of the beam on the structural behavior of the composite beam are studied. According to the parametric analysis, the excessive high strength of concrete, the width, and/or thickness of the concrete slab would lead to shear failure of the slab rather than significantly increasing the ultimate load of the composite beam. When having a small shear-span ratio, the matrix fracture and delamination will occur in the web of the GFRP profile. In addition, the height of the I-profile web has a significant effect on the stress and strain distribution of the composite beam. These parametric analyses could provide the numerical basis for the design of the GFRP composite beams.

Microbial ecology and functional coffee fermentation dynamics with Pichia kudriavzevii

Specialty coffee can be developed by the application of explicit microorganisms or starters to obtain desired fermentation. In the present study, natural fermentation (NF) of Arabica coffee was carried out spontaneously, the other set was inoculated with Pichia kudriavzevii (Y) starter culture (isolated, identified and mass cultured). The effect of microbial fermentation, metagenomics, production of functional metabolites, volatiles and their sensorial aspects were studied. The bioprocess illustrated cohesive interface of coffee nutrients and microbial communities like Mycobacterium, Acinetobacter, Gordonia, etc., in NF, Lactobacillus and Leuconostoc were prevailing in Y. The Pichia and Rhodotorula dominated in both the groups. The bioactivity of bacteria and fungi induced complex changes in physicochemical features like pH (4.2–5.2), Brix° (9.5–3.0), and metabolic transition in sugar (3.0–0.7%), alcohol (1.4–2.7%), organic acids modulating flavour precursors and organoleptics in the final brew. In the roasted bean, Y exhibited higher sugar (42%), protein (25%), polyphenol (3.5%), CGA (2.5%), caffeine (17.2%), and trigonelline (2.8%) than NF. The volatile profile exhibited increased flavour molecules like furans, ketones, and pyrazines in Y, besides lactone complexes. The organoleptics in Y were highlighted with honey, malt and berry notes. P. kudriavzevii coffee fermentation could be beneficial in specialty coffee production and enhancement of distinct characteristic flavours.

3D meso-scale investigation of ultra high performance fibre reinforced concrete (UHPFRC) using cohesive crack model and Weibull random field

Ultra high performance fibre reinforced concrete (UHPFRC) is composed of fibres, mortar, fibre-mortar interfaces, and pores randomly distributed in space, and thus has heterogeneous mechanical properties and complicated failure mechanisms at micro/meso-scales. This study aims to develop a highly effective 3D finite element modelling approach to simulate both the mortar heterogeneity and the complicated failure mechanisms in UHPFRC materials and structural members. In this approach, Weibull random fields are used to characterize random properties (such as moduli, strength and fracture energy) of the mortar caused by defects such as pores and microcracks; pre-inserted cohesive interface elements with softening constitutive laws are applied to model multi-cracking behaviour; specific probability density distributions are adopted to describe the random orientation of fibres in the mortar, and orientation-dependent bond-slip relations are developed to model fibre-mortar interactions. The fibres are rigidly embedded in the mortar so that non-conforming finite element meshes can be used conveniently. The modelling approach has been validated by extensive simulations of single fibre pull-out tests and direct tensile tests of UHPFRC specimens with thousands of fibres in agreement with experimental data. It is found that the modelling approach is highly efficient in capturing the fibre crack-bridging effect, simulating the damage and fracture initiation and evolution, and quantifying the load-carrying capacities with statistical analyses considering fibre orientation and mortar heterogeneity.

Capillary suction predictions in granular materials subjected to 1D compression and triaxial loading with emphasis on projectile penetration

The role of partial saturation in penetration resistance of projectiles in granular materials is not clear due to experimental constraints imposed by high cost and special considerations in equipment design. In this work, granular material near the tip and far-field of the projectile is numerically simulated based on 1D compression and triaxial stress paths, respectively, using the finite discrete element method. The crushing of grains in 1D compression simulations is implemented by pre-inserting cohesive interface elements in regular finite element mesh. The capillary suction is numerically predicted by extracting the deformed granular assembly microstructure at different loading steps as an input to the pore morphology method. The results demonstrate the development of high capillary suction in 1D compression loading due to the significant crushing of grains. The evolution of capillary suction is negligible during triaxial loading compared to the 1D compression loading. This suggests that future simulations related to projectile penetration in partially saturated granular materials should account for coupled hydromechanical effects near the tip whereas the far-field can be approximated as a dry material. Finally, the capillary suction corresponding to extreme comminution near the projectile tip is estimated from a 3D assembly of spherical grains with a mean grain size of 1 µm.

A symmetric tangent stiffness approach to cohesive mechanical interfaces in large displacements

The present article proposes a formulation for a cohesive interface element in large displacement conditions. Theoretical and computational aspects, useful for an effective and efficient finite element implementation, are examined in details. A six-node (or higher) isoparametric interface element for two dimensional cohesive fracture propagation problems is developed. The element operators are consistently derived by a variational approach enforced in the current configuration, where a current frame is defined with axes tangential and normal to the middle line of the interface opening displacement gap. Under the constitutive assumption of small value of the modulus of the vector product between the Cauchy traction and the displacement jump vector, explicit expression of the interface nodal force vector and of the consistent tangent stiffness matrix are derived in a closed form. At difference with most of the available formulations, the proposed approach allows to naturally achieve a symmetric tangent stiffness matrix, provided that all the involved terms are maintained in the development, namely without neglecting second-order partial derivatives involved in the exact linearization procedure.

Experimental study and numerical modeling of ENF scheme: Comparison of different beam approaches

In the present paper five beam models are presented to reproduce the experimental response of the End Notch Flexure (ENF) test. In particular, a specific ENF scheme is considered; it is made of two symmetric laminates partially bonded by a adhesive layer, whose thickness is not negligible. The failure occurs at the interface between one laminate and the adhesive, leading to an unsymmetrical failure; moreover, because of the presence of the adhesive layer, the two arms representing the initial defect of the ENF are not in contact. The experimental setup is described and five beam models are introduced to simulate the nonlinear response of the ENF scheme, characterized by an increasing improved kinematics. Failure is studied considering both linear and nonlinear elastic fracture mechanics, introducing in the latter case a very simple cohesive interface model. Solutions obtained by the five beam models are compared with the ones recovered with finite element (FE) analyzes and with the results of the experimental tests, remarking the reliability, the simplicity and the computational efficiency of the beam schemes. It is found that the improved kinematics and the cohesive fracture evolution play an important role in accurately reproducing the response of the ENF scheme.

Uncertainty in debonding of layered structures with adhesive layers

The impact of uncertainty on the stochastic debonding process in adhesively bonded layered structure is examined in this paper. The emphasis and focus of this work are put on the effect of explicitly modeling the adhesive layer within the stochastic-structural framework. A pull-out test of a single composite strip adhesively bonded to a rigid substrate is considered as a model of the layered configuration. The stochastic analysis includes physical uncertainty attributed to 10 different input parameters that are represented by random fields. Three main methodologies are integrated. The first one combines a high-order modeling of the deformable adhesive layer and the modeling of its two interfaces by the cohesive interface approach. This enables the analysis to look into the stress fields in the adhesive layer and the tractions that develop across its two interfaces. The second methodology quantifies the impact of uncertainty on the structural response by the nonlinear perturbation based stochastic finite elements method. Following the highly nonlinear character of the problem and the snap back folds involved with the response, the third methodology is integrated by means of an extended pseudo-arc-length procedure that is formulated to solve the nonlinear problem. The stochastic analysis points at the different role played by each component of the structure and each uncertain parameter in the stochastic structural response. It also describes the way these uncertainties evolve along the structural process. The results of the high order model are compared to those of a simplified model in which the adhesive layer and its two interfaces are replaced by a single cohesive interface representing the entire bonding layer. The comparison points at significant differences between the two modeling approaches, evident both in the expected (or deterministic) behavior of the structure and in its stochastic scattering measures.

Cracking analysis in Ultra-High-Performance Fiber-Reinforced Concrete with embedded nanoparticles via a diffuse interface approach

Several recent experimental investigations have clearly demonstrated that the embedding of a little quantity of nanoparticles within the concrete mixture can lead to a notable increase in strength and toughness properties, especially in the case of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC), for which nanoparticles are adopted in a synergistic combination with randomly distributed short-fiber reinforcements. Common fracture and damage models often are not able to capture the complex failure behavior of such structures when subjected to general loadings, which usually promote multiple crack propagation under mixed-mode conditions. In the present work, a crack propagation analysis in nano-enhanced UHPFRC structures is performed, by using a refined diffuse interface approach, according to which a standard finite element mesh is enriched with cohesive interface elements placed along all the internal boundaries and representing potential crack segments. A trilinear traction-separation law has been proposed and properly calibrated to incorporate the toughening effects provided by the embedded nanoparticles. The reliability of the proposed fracture model is assessed by performing complete failure simulations of UHPFRC specimens containing different fractions of nanoparticles (i.e., graphite nanoplatelets) and by comparing the related numerical outcomes with the available experimental data. The present results have shown the numerical accuracy of the proposed approach, also highlighting the role of the embedded nanoparticles in the overall load-bearing capacity as well as in the crack width control for UHPFRC structures.

A Cohesive fracture approach for the nonlinear analysis of load-induced degradation of vibration characteristics in RC beams

In this work, the effects of damage on the mechanical and vibrational response of reinforced concrete structures is investigated. The proposed model is based on a cohesive interface approach able to simulate the diffuse cracking behavior typical of reinforced concrete structures, in conjunction with an embedded truss model, able to simulate interaction phenomena between steel reinforcement and surrounding concrete. The mathematical model is developed to describe in a realistic way the crack pattern and its evolution and to determine the main factors that can influence the static and dynamic response of damaged reinforced concrete structures. Their static and dynamic properties were evaluated for increasing levels of damage after the removal of the load, starting from the load level at the first crack nucleation to the load level of incipient collapse. In order to verify the accuracy and reliability of the proposed computational model, the numerical results, obtained in terms of variations of the vibrational characteristics of the system will also be compared with experimental data reported in literature. Results show the applicability and reliability of damage identification procedures based on both mathematical models and experimental data for structural systems such as reinforced concrete beams common in girder bridges.

Comparative finite element modelling approaches for the seismic vulnerability analysis of historical masonry structures: the case study of the Cathedral of Catanzaro (Italy)

The vulnerability assessment of an ancient cathedral located in Southern Italy is performed by using both conventional and advanced finite element models. The structural safety under both static and seismic loading conditions, also including wind and temperature loadings, is mainly assessed via a response spectrum analysis performed on an accurate 3D global model of the cathedral ad hoc developed. Moreover, additional pushover analyses are conducted using an advanced nonlinear model, for a better understanding of the seismic behaviour of the analysed structure. In particular, an innovative diffuse cohesive interface approach has been adopted and suitably compared with a well-established damage model, also in terms of associated mesh dependency issues. Finally, a critical discussion of the numerical results obtained via all these different models is reported, with the aim of comparing their predictive capabilities in terms of both global and local structural safety.