2D cohesive zone modeling of crack development in cementitious digital samples with microstructure characterization

This study employed the transmission X-ray microscope (TXM) characterization and three-dimensional (3D) cohesive zone modeling (CZM) techniques to investigate the internal-frost damage in cement paste samples. The microscale cement samples were tested under controlled freeze-thaw cycles. The TXM technique was applied to perform fast-image acquisition of capillary pores and micro-damage evolution at 30 nm resolutions. The constructed 3D nanostructures of tested specimens were used in two ways: digital sample generation for model simulation and model prediction on crack propagation. The thermodynamics principles were applied to calculate the ice crystallization pressure within saturated pores under subcooling temperatures. The 3D bilinear CZM techniques were applied to predict the internal-frost damage evolution under the calculated crystallization pressure exerted on pore walls. The CZM predicted crack propagation was favorably compared with the TXM captured micro-damage. The micromechanical modeli...

There have been studies for the crack propagation simulation such as the node release technique, the element elimination method, the cohesive zone model, the extended finite element method, etc. Among these methods, the cohesive zone model is known as an effective method for the crack initiation and propagation. The cohesive zone model is easy to implement in a finite element code and estimates accurately the experimental results. Unfortunately, it has some drawbacks. The crack path is already known for the crack propagation because the cohesive elements are already generated on the crack path. Additionally, it is difficult to generate the cohesive element because the thickness of the cohesive element is very thin. In this study, an effective method by using the superimposed finite element method is proposed to overcome these drawbacks. The superimposed finite element method is one of the local mesh refinement methods. A fine mesh is generated by overlaying the patch of the local mesh on the existing mesh called the global mesh. Thus, re-meshing is not required. When the crack propagates, the local mesh refinement by using the superimposed finite element method is operated using the local element patch. The mesh of the local element patch includes the crack and the cohesive elements are generated on the crack surface of the local element patch. Therefore, the crack propagation simulation can be performed along the new crack path. Also, some local mesh patches are classified as the direction of the crack propagation in the proposed method and the local mesh patch is generated using the hierarchical concept. Then, the generation of the local mesh patch is easy and efficient. Additionally, the re-meshing process is not required. Consequently, the proposed method improves the efficiency of the crack propagation simulation. The proposed method is applied to several examples.KeywordsCohesive Zone ModelSimulate Crack PropagationCohesive ElementsNode Release TechniqueLocal MeshThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Experimental study on crack propagation characteristics of unconventional reservoir rocks

In linepipe steels, there has been a growing interest in using damage mechanics that provides physical models of the fracture process which are embedded into a two- or three-dimensional finite element (FE) model. Among the various damage models, the cohesive zone model (CZM) has recently been used to simulate the ductile crack growth behavior in linepipe steels because of its computational efficiency and it requires only two parameters which can be determined in experiments. While CZM is not yet to be used as predictive tool, but it has a great application in crack arrestor design as well as in providing insight to ductile crack propagation. In this paper, the authors have demonstrated some practical applications of CZM in linepipe steels. The CZM was used to simulate the ductile crack propagation in full-scale pipes which was able to capture the global deformation as well as the experimental crack speed. The model was then used to determine the effect of anchor blocks at the end of the pipe in a large diameter full-scale burst test. Later, the model was used to simulate two small diameter pipe tests with steel crack arrestors to mimic two arrestor cases with one showing crack propagation and the other showing crack arrest. The CZM model was also applied to demonstrate the circumferential ring-off behavior of a small diameter pipe test with rigid crack arrestor. The arrestor model was then extended to simulate a large diameter full scale Mojave burst test with “soft crack arrestor (SCA)”. A single element FE model was developed to verify the SCA material which was later extended with stain-based failure criteria. Finally, ductile crack growth in full-scale pipe with SCA was demonstrated to show that the FE CZM model can be used to optimize the design of SCA.

This study combines the digital image processing (DIP) technique and extended finite-element method (XFEM) to analyze the crack initiation and propagation mechanism of an asphalt mixture in the indirect tensile test (IDT) at a mesoscopic level. Digital camera images (DCIs) of an asphalt mixture were used to generate mesoscale geometric information of the mixture structure, which was then processed by the free mesh generation technique to build a mesh model of the mixture mesostructure. By introducing a bilinear cohesive zone model (CZM) and the XFEM, the mesh model was used to simulate the complete failure process of the asphalt mixture in the IDT. After validation with laboratory test results and theoretical solutions, the model was used to investigate the crack initiation and propagation process and the stress distribution in the asphalt mixture. It was found that the developed finite-element model cannot only produce simulation results that are consistent with laboratory test results and theoretical solutions, but also provide detailed information, such as the stress distribution in between the aggregate and mastic. In the IDT, crack initiates in weak areas around the interface between asphalt mortar and aggregates. Multiple cracks would occur in the stress concentration areas, and then propagate along both ends at various rates. During crack propagation, crack width plays a leading role. Results of this study showed that the adopted mesostructure analysis method can overcome some shortcomings of conventional finite-element methods and, hence, may provide an effective way for improving the design of flexible pavements.

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.

At the present time, adhesive joints are widely used in the industry, replacing mechanical and welding joints in many applications. T-joints are used in many applications such as automotive, marine, and aerospace industries. Several methodologies have been proposed to reduce the stress concentration at bonding ends and to obtain a more uniform stress distribution along the bondline, such as optimization of the overlap length (LO) or the adhesives’ thickness, adding outer or inner chamfers, using adhesive fillets, and dual-adhesives. Aiming to improve the strength of T-joints, in this work the dual-adhesive technique is studied, consisting of using two adhesives, placing a more flexible adhesive at the overlap ends and a stiffer one in the central area. To test this method in T-joints, different adhesive ratios were applied. To evaluate this concept, a numerical study was carried out using the cohesive zone modelling (CZM) technique, in the ABAQUS® software, including a study of failure modes, stresses, strength or maximum load (Pm) and dissipated energy (U). With the present work, it was initially possible to validate the CZM technique with experimental results. The numerical work that followed showed that the strength of T-joints significantly depends on the combination of adhesives used and that it is possible to achieve increases in Pm and U in relation to single-adhesive joints (SAJ).

The Extended Finite Element Method (XFEM) has recently emerged as a highly effective tool for analyzing crack propagation in complex structures, but its use in pipeline fracture studies, particularly with cohesive zone models (CZM), is still developing. Current XFEM fracture criteria are not calibrated for pipeline steels, relying on fixed fracture stress or strain to initiate crack propagation. While the stress-based criterion works for brittle fractures, it fails for ductile ones, either accelerating cracks or preventing them altogether. The strain-based criterion better predicts both fracture types, but its numerical accuracy remains inadequate, highlighting a need for further research. This numerical study explores the use of XFEM to predict crack propagation in standard fracture specimens of single edge notch bending (SENB) made of X52 pipe steels. First, an XFEM-based cohesive zone model was developed to simulate the specimens. The maximum principal strain (MAXPE) and fracture energy (Gc) were selected as key damage parameters to characterize the fracture process, controlling crack initiation and resistance to crack propagation, respectively. These damage parameters were adjusted until the model closely matched experimental results (Load-crack tip opening displacement (CTOD)) for different initial notch sizes in (SENB) specimens. Subsequently, experimental results for (CTOD-R) and the strain distribution around the crack tip, both at crack initiation and during unstable crack propagation, were compared with the numerical model's predictions to validate the chosen XFEM input damage parameters. The research confirms the effectiveness of XFEM in predicting fracture characteristics.

The interaction of mechanical components experiencing relative movements and cyclic loads in a corrosive environment is known as fretting corrosion or tribocorrosion. In the current work, the mechanism of crack initiation and propagation in dovetail slots of Ti6Al4V samples (in contact with carbide rods) under fretting corrosion conditions was investigated. A newly developed test rig installed on a universal testing machine was used to conduct tests at 20 Hz frequency under 5 and 7.5 kN fretting loads. Tests were conducted at room temperature in 3.5% NaCl and phosphate-buffered saline solutions. Crack propagation in all samples was examined by a metallurgical microscope, and the detailed analysis of fractured samples was carried out by a scanning electron microscope. In comparison to dry conditions, early crack initiation and faster crack propagation were observed in salt and physiological solution environments. Colored spots and large amounts of chlorine, sodium, and oxygen were found around cracks, and plastically deformed regions in the 3.5% NaCl environment provided the evidence of a corrosive attack. Large amounts of oxygen, phosphorous, chlorine, potassium, and sodium were detected in the phosphate-buffered saline environment.

On the porosity induced by externally solidified crystals in high-pressure die-cast of AM60B alloy and its effect on crack initiation and propagation

Directionally solidified superalloy, which has elongated large grains, is used for gas-turbine blade application because of its high creep strength. Since the grain size is not small enough in comparison with the component size or the crack size, the inhomogeneous microstructure strongly affects the crack propagation behavior in such a structure. The authors investigate the effect of microstructural inhomogeneity on crack propagation morphology and crack propagation rate (da/dN) under a creep-fatigue condition. The macroscopic direction of the main crack is perpendicular to the loading axis, though the path is microscopically bumpy due to the effect of inhomogeneous multi-grains. Several subcracks are found around the main crack, and they occasionally coalesce with each other. In order to understand the local fluctuation of (da/dN) due to the inhomogeneous multi-grains, the grain shape and its crystallographic orientation are identified by means of a scanning electron microscope (SEM) and an electron back scattering diffraction (EBSD). The subcracks tend to initiate at the high angle grain boundaries oriented nearly perpendicular to the loading axis. The local fluctuation of da/dN is influenced by two factors. One is the cracking path; the intergranular crack shows higher da/dN. The other factor is the magnitude of Young’s modulus of the grain (in loading direction) at the crack tip. The higher Young’s modulus leads to a lower da/dN in transgranular cracking. The detailed observation of local displacement near the crack tip clarified that the magnitude of cyclic displacement inversely related to the Young’s modulus of the grain in the loading axis, and results in the high strain concentration at the crack tip in the grain with low Young’s modulus. The experiment result suggests that the strain around the crack tip governs the crack propagation rate though it is affected by the inhomogeneous microstructure.

This paper deals with the improvement of fracture properties of epoxy nanocomposites using graphene oxide (GO) nanosheets. Epoxy/GO samples were fabricated in the form of compact tension shear (CTS) at filler contents of 0, 0.25 and 0.5 wt.%. Using Arcan fixture and an out-of-plane fixture as well as by varying the loading angle, a full combination of fracture modes (I/II/III) was created to evaluate the effect of nanofiller on fracture toughness. It was observed that the fracture toughness of the samples in all cases was significantly increased by the addition of GO. The highest effectiveness was achieved by the addition of 0.25% GO when the samples were tested under the combined mode I/II and pure mode III, which resulted in a 69 and 65% increase in the fracture toughness of the epoxy system, respectively. Also, a microstructural analysis using scanning electron microscopy (SEM) showed how GO prevented crack propagation in the composite. Microstructure analysis was also performed using scanning electron microscopy (SEM), which showed how GO inhibits crack propagation in the nanocomposite. The fracture mechanisms caused by the addition of GOs, including crack deflection, bridging, and secondary cracks, are presented and discussed based on SEM images. Therefore, GOs are able to provide effective fracture toughness to epoxy resins and are a highly efficient filler for use in epoxy-based nanocomposites.

Mixed mode fracture analysis in a ductile adhesive using semi-circular bend (SCB) specimen

Thermal fatigue tests were conducted for SB410 steel and SUS310S steel for boiler. A laboratory made thermal fatigue testing apparatus was used. This apparatus consisted principally of a heating device using oxygen and LPG gas, a temperature control device for the heated zone and a rapid cooling device for the specimen. During thermal fatigue tests the heating and cooling cycles were repeatedly loaded on the specimen placed on the specimen holder. City water was used as a cooling medium. The heating temperatures were 673K, 773k and 873K, and cooling water was sprayed on to the specimen surface through a nozzle. As it was difficult to measure the surface temperature of the specimen, small holes for a thermocouple were prepared to measure the temperature of the notch during the thermal fatigue tests. Thus the measured temperatures were used as the testing temperatures. The plate specimens with an electric discharged notch at Open image in new window Figure 1. Thermal fatigue test specimen the bottom of the mechanical U notch. The thicknesses of the specimens were 24mm and 12mm. The thermal fatigue crack initiation tests and thermal fatigue crack propagation tests were conducted up to 100 cycles for specimens with 24mm thick and 300 cycles for specimens with 12mm thick. Figure 1 shows the figure and the size of the thermal fatigue test specimen. The thermal fatigue crack length was measured at every ten cycles by use of a viewing microscope with magnification of 20 after interrupting thermal fatigue tests. The thermal fatigue cracks were examined by an optical microscope and thermal fatigue fracture surfaces were examined by a JEOL scanning electron microscope (JSM5500S). Figure2 shows crack propagation curves for SB410 steel with 12mm thick. The higher the testing temperature the smaller the number of heat cycles is. The higher the testing temperature the faster the crack propagation rate is. The same phenomenon was observed for SUS310S. The thermal fatigue crack propagation rate of SB410 steel was faster than that of SUS310S steel. Plural thermal fatigue cracks were observed on the bottom of the electric discharged notch for SB410 steel. The crack branching was observed on the specimen tested at 873K. Striation was observed on fracture surfaces of SB410 steel. Striation was predominant on fracture surfaces of SUS310S.The striation spacing per cycle ,S obtained from the measured striation spacing S versus K curve was well coincident with the da/dN K curve in the high crack propagation rate. It can be concluded that thermal fatigue crack of boiler steels propagate in association with striation. Open image in new window Figure2. Thermal fatigue crack propagation curves of SB410 steel with 12mm thick

The present paper reports the comminution of cementitious composites in a laboratory jaw breaker. Two samples of cement paste, one mortar sample, and samples of five different concrete mixtures were investigated. Water/cement ratio and maximum aggregate size were varied. Based on the sieve analysis, the densities of the fragment distributions were estimated and discussed. All fragment samples followed a logarithmic normal distribution. Ordinary concrete materials and cement paste samples showed bimodal distribution density functions, whereas mortar and lightweight concrete could be characterised by unimodal distribution density functions. The modal values were of comparative order for all materials, but the type of the distribution density function depended upon material structure. A preliminary explanation for the different responses of the materials is given based on SEM images of the fragments.