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1. Cyclic adaptive cohesive zone model to simulate ductile crack propagation in steel structures due to ultra‐low cycle fatigue

   https://doi.org/10.1111/ffe.13964  

   Ziccarelli, Andy ; Kanvinde, Amit ; Deierlein, Gregory ( February 2023 , Fatigue & Fracture of Engineering Materials & Structures)

   Micromechanics‐based continuum damage criteria have previously been developed to simulate the initiation of ductile fracture in structural steels under conditions with large‐scale plasticity where conventional fracture mechanics indices are invalid. Such models have been combined with methods to simulate ductile crack growth for monotonic loading. In this study, a micromechanics‐based adaptive cohesive zone model for simulating ductile crack propagation under monotonic loading is extended to handle cyclic loading. The proposed model adaptively modifies the cohesive traction–separation relationship for crack opening and closure, as the loading reverses between tension and compression. The approach is implemented into the finite element analysis platform WARP3D, and results of simulations that use the model are compared with data from coupon‐scale tests. The results demonstrate that the proposed model can accurately simulate the effect of crack propagation on specimen response, as well as other key aspects of observed behavior, including crack face closure and crack tunneling.

    

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2. Numerical Simulation of Fracture Initiation in Barre Granite using an Experimentally Validated XFEM Model

   Garg, P ; Hedayat, A ; Griffiths, D.V. ( June 2020 , 54th US Rock Mechanics/Geomechanics Symposium)

   null (Ed.)

   Fracturing in brittle rocks with an existing crack results in the development of a significant nonlinear region surrounding the crack tip called the fracture process zone. Various experimental and numerical studies have shown that the crack tip parameters such as the crack tip opening displacement (CTOD) and the fracture energy are critically important in characterizing the fracture process zone. In this study, numerical simulations of rock specimens with a center notch subjected to three-point bending were conducted using the extended finite element method (XFEM) along with the cohesive zone model (CZM) to account for fracture process zone. The input parameters of CZM such as the elastic and critical crack opening displacements were first estimated based on the results of three-point bending tests on the center notched Barre granite specimens. Displacements were measured using the two dimensional digital image correlation technique and used to characterize the evolution of the fracture process zone and estimate the parameters of the cohesive zone model. The results from the numerical simulations showed that CZM provided a good agreement with experimental data as it predicted all three stages of cracking from fracture process initiation to macro-crack growth. 

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3. Time dependent fracture of soft materials: linear versus nonlinear viscoelasticity

   https://doi.org/10.1039/D0SM00097C  

   Guo, Jingyi ; Zehnder, Alan T. ; Creton, Costantino ; Hui, Chung-Yuen ( January 2020 , Soft Matter)

   Toughness of soft materials such as elastomers and gels depends on their ability to dissipate energy and to reduce stress concentration at the crack tip. The primary energy dissipation mechanism is viscoelasticity. Most analyses and models of fracture are based on linear viscoelastic theory (LVT) where strains are assumed to be small and the relaxation mechanisms are independent of stress or strain history. A well-known paradox is that the size of the dissipative zone predicted by LVT is unrealistically small. Here we use a physically based nonlinear viscoelastic model to illustrate why the linear theory breaks down. Using this nonlinear model and analogs of crack problems, we give a plausible resolution to this paradox. In our model, viscoelasticity arises from the breaking and healing of physical cross-links in the polymer network. When the deformation is small, the kinetics of bond breaking and healing are independent of the strain/stress history and the model reduces to the standard linear theory. For large deformations, localized bond breaking damages the material near the crack tip, reducing stress concentration and dissipating energy at the same time. The damage zone size is a new length scale which depends on the strain required to accelerate bond breaking kinetics. These effects are illustrated by considering two cases with stress concentrations: the evolution of spherical damage in a viscoelastic body subjected to internal pressure, and a zero degree peel test. 

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4. A multiscale phase field fracture approach based on the non-affine microsphere model for rubber-like materials

   https://doi.org/10.1016/j.cma.2023.115982  

   Arunachala, Prajwal Kammardi ; Abrari Vajari, Sina ; Neuner, Matthias ; Linder, Christian ( May 2023 , Computer Methods in Applied Mechanics and Engineering)

   - (Ed.)

   Rubber-like materials have a broad scope of applications due to their unique properties like high stretchability and increased toughness. Hence, computational models for simulating their fracture behavior are paramount for designing them against failures. In this study, the phase field fracture approach is integrated with a multiscale polymer model for predicting the fracture behavior in elastomers. At the microscale, damaged polymer chains are modeled to be made up of a number of elastic chain segments pinned together. Using the phase field approach, the damage in the chains is represented using a continuous variable. Both the bond stretch internal energy and the entropic free energy of the chain are assumed to drive the damage, and the advantages of this assumption are expounded. A framework for utilizing the non-affine microsphere model for damaged systems is proposed by considering the minimization of a hypothetical undamaged free energy, ultimately connecting the chain stretch to the macroscale deformation gradient. At the macroscale, a thermodynamically consistent formulation is derived in which the total dissipation is assumed to be mainly due to the rupture of molecular bonds. Using a monolithic scheme, the proposed model is numerically implemented and the resulting three-dimensional simulation predictions are compared with existing experimental data. The capability of the model to qualitatively predict the propagation of complex crack paths and quantitatively estimate the overall fracture behavior is verified. Additionally, the effect of the length scale parameter on the predicted fracture behavior is studied for an inhomogeneous system. 

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5. How chain dynamics affects crack initiation in double-network gels

   https://doi.org/10.1073/pnas.2111880118  

   Zheng, Yong ; Matsuda, Takahiro ; Nakajima, Tasuku ; Cui, Wei ; Zhang, Ye ; Hui, Chung-Yuen ; Kurokawa, Takayuki ; Gong, Jian Ping ( December 2021 , Proceedings of the National Academy of Sciences)

   Double-network gels are a class of tough soft materials comprising two elastic networks with contrasting structures. The formation of a large internal damage zone ahead of the crack tip by the rupturing of the brittle network accounts for the large crack resistance of the materials. Understanding what determines the damage zone is the central question of the fracture mechanics of double-network gels. In this work, we found that at the onset of crack propagation, the size of necking zone, in which the brittle network breaks into fragments and the stretchable network is highly stretched, distinctly decreases with the increase of the solvent viscosity, resulting in a reduction in the fracture toughness of the material. This is in sharp contrast to the tensile behavior of the material that does not change with the solvent viscosity. This result suggests that the dynamics of stretchable network strands, triggered by the rupture of the brittle network, plays a role. To account for this solvent viscosity effect on the crack initiation, a delayed blunting mechanism regarding the polymer dynamics effect is proposed. The discovery on the role of the polymer dynamic adds an important missing piece to the fracture mechanism of this unique material. 

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