Cohesive Zone Model Parameters Research Articles

On characterization of cohesive zone model (CZM) based upon digital image correlation (DIC) method

Cohesive zone model (CZM) has been extensively applied in numerical simulation of interfacial fracture behaviors such as composite delamination and fracture of adhesive joints. Accurate and efficient identification of CZM parameters signifies an important research topic to enable reliable analysis and design of these structures. This paper aims to derive the actual traction-separation law of CZM by means of experimental measurement with only digital image correlation (DIC) technique. A double cantilever beam (DCB) specimen was employed for experimental tests and a semi-empirical analytical model was developed to associate the traction-separation law with the global loading response. Explicit formulae of the initial stiffness, maximum traction stress and damage evolution were derived. In this study, the DCB specimens bonded with a ductile adhesive layer of Araldite® 2015 were fabricated and experimentally tested. The displacement distribution of the adhesive layer was quantified as the fundamental data for identification of CZM parameters. The initial stiffness, maximum traction stress and damage evolution of CZM were identified accurately. Based upon the extracted CZM parameters in the present work, a finite element (FE) model was developed and verified with the experimental results. Furthermore, a FE model with a traditional linear damage evolution was also established for comparison, thereby demonstrating the effectiveness of the proposed method for extracting the traction-separation law of CZM. By employing the present approach, the real traction-separation law can be better presented and accurate simulation of adhesive joint can be achieved.

Molecular dynamics simulation of enhanced interfacial cohesive behavior of Ni-coated MWCNT/Mg composites

ABSTRACT Debonding processes of different Ni-coated MWCNT/Mg interface models were studied by molecular dynamics simulations, and the stress–displacement relationships of different interface models were obtained by fitting a modified exponential cohesive zone law for interface. In addition, the debonding behaviors of Ni-coated and uncoated MWCNT/Mg interface were compared. The results show that CNT wall numbers and inner diameters as well as interface orientations have significant effects on the Ni-coated and uncoated MWCNT/Mg interface cohesive-zone model parameters. Compared with the above two kinds of uncoated MWCNT/Mg interface models, the average values of the peak stress and the work of separation for the Ni-coated MWCNT/Mg interface models are both increased significantly, showing that the addition of a Ni-modified coating on the surface of MWCNTs can effectively enhance the adhesion capacity of the MWCNT/Mg interface.

Finite element modeling of mode I fatigue delamination growth in composites under large-scale fiber bridging

In this study, the Turon et al. and Kawashita-Hallett fatigue damage models are extended with a trilinear cohesive law to simulate mode I fatigue delamination in the composites undergoing large-scale fiber bridging. The trilinear cohesive zone model (CZM) is constructed by superposition of two bilinear CZMs. In order to implement the developed models, a user defined element (UEL) subroutine is prepared in the Abaqus software and finite element analyses are conducted based on the envelope loading technique to simulate the 3D double cantilever beam (DCB) specimens under high-cycle fatigue loading. The finite element models are assessed through the experimental data from the literature and also five available fatigue damage models based on the bilinear CZM. It is concluded that the trilinear CZMs has more accuracy than the bilinear ones in prediction of fatigue delamination with fiber bridging effects. A parametric study is performed on the two extended models and sensitivity of models to the fitting parameters and quasi-static CZM parameters are comprehensively investigated.

Atomistic-scale modeling of nano-clay-filled shape memory polymers

Shape memory polymers (SMPs) have been defined as materials that can recover an early programmed shape after subjecting to an external stimulus. The epoxy/clay nano-composites are developed as a mix of shape recovery behavior and improved thermomechanical properties. The epoxy/clay interface zone plays a big role in material failure. The existence of multiple material phases, as well as multiple interaction forces that interact phases with each other, make the interface zone very complicated to study and simulate. Furthermore, such a tiny size zone is very hard to characterize using the conventional experiment types of equipment. Therefore, the Molecular Dynamics method (MD) has come to light as an effective simulation method to study the atomistic scale dynamic material phenomena. In this work, the MD method is employed to evaluate the thermomechanical performance of the epoxy/clay nano-composites at the atomistic scale level. The estimated elastic parameters confirmed the improvement of the thermomechanical properties of the epoxy/clay nano-composites. Furthermore, the transition temperature and the thermal expansion coefficient are included inside the MD simulation for both the pure epoxy polymer and epoxy/clay nano-composites. The computed above-mentioned parameters are in fairly good agreement with the experimental observations that make these calculated parameters generally appropriate for use at a higher length scale level model. In addition, the response of the epoxy/clay interface zone under mechanical load was also investigated for different silicate layer orientations, namely, 0°, 20°, 45°, 70°, and 90° aligned to the external uniaxial load. The main cohesive parameters like cohesive stiffness, cohesive strength, and maximum separation are derived numerically for every silicate layer orientation. The resulting traction-separation curves of the interface zone provided a deep understanding of the material failure initiation. Moreover, the cohesive zone model parameters are estimated and parameterized in equations with the silicate layer orientations to use in a multiscale modeling framework.

Finite strain elasticity based cohesive zone model for mechanoluminescent composite interface: Part II. Interface damage characteristics

The need to estimate the micromechanical properties of elastico-mechanoluminescent (EML) composites was established in Part A of this paper. The particle–matrix interface was identified to be poorly understood towards building accurate EML models and to predict functional life estimates of the EML devices. Towards this, Part A of this paper defined the mechanical behavior of the interface as a softening spring through a bilinear cohesive zone model (CZM). Macroscale stress measurements were combined with material properties to estimate undamaged stiffness of the interface spring (). In Part B of this paper, the damage characteristics of the interface spring are estimated through an experimental approach to complete the CZM. This is achieved by first establishing the finite strain behavior of the macroscopic EML–PDMS composite from uniaxial tensile testing. Secondly, a controlled tear is induced in the composite and the microscale displacement fields around the tear tip are captured using digital image correlation technique. Tear tip displacements and finite strain behavior are combined to estimate cohesive stresses in the tear-tip region of the composite. The contribution of particle–matrix interfaces towards resisting propagation of the tear is isolated from experimental results to obtain interface-CZM damage parameters. The interface CZM parameters are subsequently utilized in an FEM model to simulate macroscopic composite behavior to find good correlation. The experimental framework established in this paper can be utilized to estimate interface parameters of any particulate-elastomer composites that exhibit finite strain behavior.

Use of a micromechanical approach to understand the mechanical behavior of solid propellants

The development of new generations of propellants with better energetic properties may be hampered by unsatisfactory mechanical behaviors in terms of strength and toughness. A micromechanical approach is adopted to provide a better understanding of the existing links between the constitutive phase behaviors and the local damage, and the macroscopic mechanical behavior of these materials. Three model materials have been made and tested in uniaxial tension. The stress-strain responses were recorded while monitoring their volume changes that quantify the macroscopic damage. A qualitative description of the local damage was obtained thanks to scanning electron microscopy images of samples under loading. The micromechanical approach consists in finite elements analyses on periodic microstructures of non-regular polyhedral particles embedded in a soft matrix. An original microstructure generation tool has been developed specifically in order to obtain highly filled isotropic microstructures. Debonding at the matrix/filler interface was taken into account with a cohesive-zone model (CZM). The impact of the CZM parameters is discussed, in an effort to make the link between the CZM parameters and how the local damage appears and develops, and between the cohesive behavior and the shape of the macroscopic stress-stretch responses of the heterogeneous materials.

The role of microstructure in the impact induced temperature rise in hydroxyl terminated polybutadiene (HTPB)–cyclotetramethylene-tetranitramine (HMX) energetic materials using the cohesive finite element method

In this work, microstructure dependent impact-induced failure of hydroxyl-terminated polybutadiene (HTPB)–cyclo-tetra-methylene-tetra-nitramine (HMX) energetic material samples is studied using the cohesive finite element method (CFEM). The CFEM model incorporates experimentally measured viscoplastic constitutive behavior, experimentally measured interface level separation properties, and phenomenological temperature increase due to mechanical impact based on viscoplastic and frictional energy dissipation. Nanoscale dynamic impact experiments were used to obtain parameters for a strain-rate dependent power law viscoplastic constitutive model in the case of bulk HTPB and HMX as well as the HTPB–HMX interfaces. An in situ mechanical Raman spectroscopy (MRS) setup was used to obtain bilinear cohesive zone model parameters to simulate interface separation. During analyses, the impact-induced viscoplastic energy dissipation and the frictional contact dissipation at the failed HTPB–HMX interfaces is found to have a significant contribution toward local temperature rise. Microstructures having circular HMX particles show a higher local temperature rise as compared to those with diamond or irregularly shaped HMX particles with sharp edges indicating that the specific particle surface area has a higher role in temperature rise than particle shape and sharp edges. Regions within the analyzed microstructures near the HTPB–HMX interfaces with a high-volume fraction of HMX particles were found to have the maximum temperature increase.

Fast ductile fracture: Effect of inertia on propagation resistance and CTOA in pipe steels

In this paper, the effect of inertia on the steady-state velocity of propagating cracks in modern high toughness pipeline steel was investigated. The steel grades examined in this work were American Petroleum Institute Standard X70 and X100. A tensile plate model, simplified from the geometry of a pipe, was studied using the finite element code ABAQUS 6.14–2. The cohesive zone model (CZM) was used to simulate crack propagation. The CZM parameters were calibrated based on matching the crack tip opening angle (CTOA) measured from a drop-weight tear test (DWTT) finite element model to the experimental CTOA of the material. The CZM parameters were then applied to the tensile plate model. The effect of inertia on the steady-state crack velocity was systematically assessed by altering the density of the material used with the plate model. To isolate the influence of inertia, the effect of strain rate on the fracture process and material hardening was neglected and the CZM parameters were assumed constant. It was found that the steady-state crack velocity decreased with increasing mass density; this is consistent with the effect of backfill in pipeline burst tests. The CTOA extracted from the FE model was not affected by the density.

Numerical investigation on fatigue crack growth of fracturing pump head using cohesive zone model

This paper concerns with the fatigue crack growth of the fracturing pump head. The bi-linear cohesive zone model (CZM) with irreversible damage evolution equation was employed to simulate the fatigue crack growth of the fracturing pump head. Through comparing the data of the fracture toughness tests and fatigue crack tests with numerical results, the selected cohesive zone model parameters for alloy steel 30CrNi2MoV were reliable. In order to reduce dependence of computational resources a simplified method of the fracturing pump head was proposed to predict the fatigue crack growth. The fatigue crack extension along the crack front on the fracturing pump head are discussed in detail. The variation of the fatigue crack growth rate da/dN along the crack front is periodicity. The results show that cohesive zone model is suitable for prediction of the fatigue crack growth on the fracturing pump head.

Mode I Fracture Toughness Evaluation of Adhesively Bonded Joints via J‐Integral and DIC

AbstractAmorphous polymers, such as epoxy resins, are commonly used in the realization of adhesively bonded joints. In this paper an evaluation of mode I fracture toughness of bonded joints is presented. Moreover, an identification of cohesive zone model parameters via Rice's J‐integral is described. Experimental tests are performed on double cantilever beam specimens and relative displacements between adherends are acquired by using the digital image correlation technique. The obtained interface law can be implemented in a finite element code for simulating the decohesion process of complex bonded structures.

Mode I cohesive zone model parameters identification and comparison of measurement techniques based on uncertainty estimation

Adhesive bondline mechanical behaviour is frequently described with cohesive zone models (CZM). For mode I loading condition these phenomenological laws simply represent the evolution of the peel stress as a function of the two adherends relative displacement normal to the joint. Generally, these laws are identified rather than really measured using experimental data obtained from crack initiation and propagation experiments such as the Double Cantilever Beam Test (DCB). The uncertainty on parameter estimation are generally not indicated, as for a DCB test it is only the critical energy release rate that has the most influence on the results. However, the uncertainties on the other parameters prevent the use of the identified TSL for other mechanical tests where mode I solicitations are predominant. In this article, the purpose is to evaluate the methodologies reliability for the assessment of mode I CZM. To do so, several methods used to evaluate CZM parameters are compared in terms parameter estimation reliability. Synthetic noisy data are considered for a χ² function minimisation. Then, sensitivity calculations are performed to determine the estimated parameters standard deviation. By applying this procedure on different type of synthetic measurements (respectively P(Δ), J(δ,θ), backface strain and DIC) the ability of these different techniques to capture the best parameters for a chosen CZM shape can be rigorously evaluated.

Development of a computational model for acute ischemic stroke recanalization through cyclic aspiration.

Acute ischemic stroke (AIS), the result of embolic occlusion of a cerebral artery, is responsible for 87% of the 6.5 million stroke-related deaths each year. Despite improvements from first-generation thrombectomy devices for treating AIS, 80% of eligible stroke patients will either die or suffer a major disability. In order to maximize the number of patients with good outcomes, new AIS therapies need to be developed to achieve complete reperfusion on the first pass. One such therapy that has shown promise experimentally is the application of cyclic aspiration pressure, which led to higher recanalization rates at lower pressure magnitudes. In order to investigate AIS and cyclic aspiration recanalization, an improved computational modeling framework was developed, combining a viscoelastic thromboembolus model with a cohesive zone (CZ) model for the thromboembolus-artery interface. The model was first validated against experimental displacement data of a cyclically aspirated thromboembolus analog. The CZ model parameters, including the addition of a damage accumulation model, were then investigated computationally to determine their individual effects on the thromboembolus and CZ behavior. The relaxation time and the damage model critical opening length were shown to have the greatest effect on the CZ opening and led to increased displacement that accumulated with repeated loading. Additional simulations were performed with parameters relevant to AIS including internal carotid artery dimensions and thromboemboli mechanical properties. In these AIS cases, more upstream CZ opening was observed compared to the thromboembolus analog cases and greater displacement was achieved with the lower-frequency aspiration (0.5 vs 1Hz).

Development of 3D Meso-Scale finite element model to study the mechanical behavior of steel microfiber‐reinforced polymer concrete

In this study, 3D Meso-scale finite-element model is presented to study the mechanical behavior of steel microfiberreinforced polymer concrete considering the random distribution of fibers in the matrix. The composite comprises two separate parts which are the polymer composite and steel microfibers. The polymer composite is assumed to be homogeneous, which its mechanical properties are measured by performing experimental tests. The steel microfiber-polymer bonding is simulated with the Cohesive Zone Model (CZM) to offer more-realistic assumptions. The CZM parameters are obtained by calibrating the numerical model using the results of the experimental pullout tests on an individual microfiber. The accuracy of the results is validated by comparing the obtained results with the corresponding values attained from testing the steel microfiber.reinforced polymer concrete incorporating 0, 1 and 2% by volume of microfibers, which indicates the excellent accuracy of the current proposed model. The results show that the microfiber aspect ratio has a considerable effect on the mechanical properties of the reinforced polymer concrete. Applying microfibers with a higher aspect ratio improves the mechanical properties of the composite considerably especially when the first crack appears in the polymer concrete specimens.

Study on the effect of post curing on the mode II fracture energy of structural adhesive using a parameter identification approach

In this work, a parameter identification approach was developed based on a combined experimental-numerical approach to determine the optimal set of adhesive parameters in adhesively bonded specimen subjected to various post curing treatments. End-Notched Flexure (ENF) testing was conducted to characterise the Mode II fracture property under both recommended and post curing conditions, providing benchmark data for the numerical analysis stage. Experimental results revealed that Mode II fracture energy was effectively affected by the post curing history, with higher temperature and longer curing duration leading to enhanced fracture resistance. The numerically identified Cohesive Zone Model (CZM) parameters using multi-island genetic algorithm provided good correlation in Mode II fracture energies between Finite element (FE) modelling and experimentally measured values, thus extensive experimental characterisation work to determine the adhesive parameters can be effectively eliminated.

Nanomechanics Modeling of Interface Interactions in Asphalt Concrete: Traction and Shearing Failure Study

The interface bonding strength is critical for asphalt concrete performance under external load applications. A thorough understanding of the load transfer mechanism bridging the nanoscale interfacial details and the macroscale properties is required to accurately predict the performance of asphalt concrete. This research presents a multiscale analysis procedure for the modeling of interface behaviors, in which material properties are evaluated by atomic scale interactions, emphasizing the complex shearing and separation mechanisms under various loading modes. The representative model system was established based on multiscale experimental characterization of the tight-bonding interface between asphalt and aggregate. Interfacial load transfer and failure studies were conducted for investigating the effect of tension and compression on shearing mode separation. The cohesive zone model parameters, such as peak traction and energy of separation were evaluated for each loading mode. Different boundary conditions were applied to obtain the representative volume element (RVE) and connection to continuum level properties. Results indicated that depending on the various loading modes, the failure of the nanoscale interface system may occur within the asphalt phase or at the interface. These results set the basis for continuum length-scale micromechanical models which may be used to determine the bulk material response, incorporating interfacial phenomena. The findings presented in this paper are consistent with observations reported in previous studies and expand on the understanding of the relationship between molecular structures and combined shearing separation failure properties of asphalt concrete interfaces.

Impact modelling of single-lap bonded joints by cohesive zone models

The main purpose of this work is to study the influence of the cohesive parameters in predicting the strength of single-lap joints (SLJ) subjected to impact loads. The influence of the cohesive parameters was studied by varying each of these parameters while keeping the remaining ones. The Araldite® AV138 (high stiffness) and Nagase Chemtex® XNR6852 E-2 (high toughness) are the chosen adhesives to understand this influence using adhesives with different properties. It was concluded that cohesive zone models (CZM) accurately predict the joints’ impact strength, and that the CZM parameters highly influence the joints’ strength.

Analysis of automotive mixed‐adhesive joints weakened by moist conditions: Experimental characterization and numerical simulation using cohesive zone model

AbstractThis paper is mostly concerned with an experimental and numerical study to clarify the behaviour and failure in the mono‐adhesive joints and mixed‐adhesive joints under different environmental conditions (dry [E0], 75.3% relative humidity [E1], 84.2% relative humidity [E2], and submerged in tap water [E3] at 25°C) and different strain rates (1 and 100 mm/min). Experimental investigations are compared with the numerical analysis, which is carried out by bilinear cohesive zone model (CZM). Through this work, degradations of cohesive parameters are calculated by using open‐faced double cantilever beam (DCB) and end notch flexure (ENF) specimens. The experimental data show that the cohesive parameters of Araldite 2015 have negative correlation to moisture content. Although Araldite AV138 parameters experience a decrease in mode I, in mode II, its cohesive parameter increases. First, it is found that the decrease in experimental failure load of mixed‐adhesive joints with regard to the dry condition is recorded as 32.6%, 54%, and 59.1% for E1, E2, and E3 conditions, respectively. Following this further, single lap joint (SLJ) that has non‐uniform moisture content distribution is modelled by engaging specific CZM parameters related to the specific moisture content. The results show a good agreement between experimental and numerical data.

Complete analytical solutions for double cantilever beam specimens with bi-linear quasi-brittle and brittle interfaces

In this work we develop a complete analytical solution for a double cantilever beam (DCB) where the arms are modelled as Timoshenko beams, and a bi-linear cohesive-zone model (CZM) is embedded at the interface. The solution is given for two types of DCB; one with prescribed rotations (with steady-state crack propagation) and one with prescribed displacement (where the crack propagation is not steady state). Because the CZM is bi-linear, the analytical solutions are given separately in three phases, namely (i) linear-elastic behaviour before crack propagation, (ii) damage growth before crack propagation and (iii) crack propagation. These solutions are then used to derive the solutions for the case when the interface is linear-elastic with brittle failure (i.e. no damage growth before crack propagation) and the case with infinitely stiff interface with brittle failure (corresponding to linear-elastic fracture mechanics (LEFM) solutions). If the DCB arms are shear-deformable, our solution correctly captures the fact that they will rotate at the crack tip and in front of it even if the interface is infinitely stiff. Expressions defining the distribution of contact tractions at the interface, as well as shear forces, bending moments and cross-sectional rotations of the arms, at and in front of the crack tip, are derived for a linear-elastic interface with brittle failure and in the LEFM limit. For a DCB with prescribed displacement in the LEFM limit we also derive a closed-form expression for the critical energy release rate, G_c. This formula, compared to the so-called ‘standard beam theory’ formula based on the assumptions that the DCB arms are clamped at the crack tip (and also used in standards for determining fracture toughness in mode-I delamination), has an additional term which takes into account the rotation at the crack tip. Additionally, we provide all the mentioned analytical solutions for the case when the shear stiffness of the arms is infinitely high, which corresponds to Euler–Bernoulli beam theory. In the numerical examples we compare results for Euler–Beronulli and Timoshenko beam theory and analyse the influence of the CZM parameters.

A cohesive zone model for the characterization of adhesion between cement paste and aggregates

The influence of an Interfacial Transition Zone on mechanical behavior of concrete has been established by many authors. Several experimental studies have highlighted the weak mechanical properties of Interfacial Transition Zone due mainly to higher porosity than other phases, or to the presence of initial damage at the interface between paste and aggregates. Thus, it is necessary to take this zone into account when modelling the mechanical behavior of concrete, particularly outside of the elastic domain. Various analytical or numerical methods have been proposed, and some of these replace the physical representation of an Interfacial Transition Zone, by an imperfect interface between paste and aggregates, and an adhesion model with specific mechanical properties. In this work a cohesive zone model coupling adhesion, friction and unilateral contact is applied to a finite element model of a cement paste-aggregate composite, designed to test interface mechanical properties. Cohesive zone model parameters are calibrated using an experimental study of the same composite, submitted to mechanical loading during hydration and leaching of cement paste. This numerical approach allows modelling of the mechanical behavior of a composite until the tensile strength is reached. Moreover evolution of cohesive zone model parameters during hydration and leaching, shows a partial adhesion on contact surfaces between cement paste and aggregate. This could explain initial interface damage.

Graphene Reinforced Epoxy Adhesive For Fracture Resistance

The fracture resistance of construction adhesive has attracted tremendous interests in the past decades. This paper conducts an experimental study on the mode I fracture resistance of epoxy construction adhesive reinforced with graphene nanoplatelets (GNPs) through double cantilever beam (DCB) specimens. The experimental results show that the mode I fracture toughness of nanocomposites increases compared with the neat epoxy. It is worth noting that the mode I fracture toughness of nanocomposites at a graphene content of only 0.25 wt% exhibits a 5 times enhancement compared with neat epoxy adhesive. When the graphene content continues to increase, the mode I fracture toughness of adhesive decreases as the aggregation of graphene in adhesive. The mechanical behavior of the DCB specimens with different nanocomposites adhesive are predicted using finite elements analysis (FEA). The mode I fracture properties of nanocomposites obtained from the experimental results are used as cohesive zone model parameters in FEA. The prediction agrees very well with the experimental results.