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  • 1.
    Carlsson, Jenny
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Isaksson, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Crack dynamics and crack tip shielding in a material containing pores analysed by a phase field method2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 206, p. 526-540Article in journal (Refereed)
    Abstract [en]

    Many naturally occurring materials, such as wood and bone, have intricate porous micro-structures and high stiffness and toughness to density ratios. Here, the influence of pores in a material on crack dynamics in brittle fracture is investigated. A dynamic phase field finite element model is used to study the effects of pores with respect to crack path, crack propagation velocity and energy release rate in a strip specimen geometry with circular pores. Four different ordered pore distributions are considered, as well as randomly distributed pores. The results show that the crack is attracted by the pores; this attraction is stronger when there is more energy available for crack growth. Crack propagation through pores also enables higher crack propagation velocities than are normally seen in strip specimens without pores (i.e. homogeneous material), without a corresponding increase in energy release rate. It is further noticed that as the porosity of an initially solid material increases, the crack tip is increasingly likely to become shielded or arrested, which may be a key to the high relative strength often exhibited by naturally occurring porous materials. We also find that when a pore is of the same size as the characteristic internal length then the pore does not localise damage. Since the characteristic internal length only regularises the damage field and not the strain end kinetic energy distributions, crack dynamics are still affected by small pores.

  • 2.
    Chen, Shaohui
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Isaksson, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    A note on the defect sensitivity of brittle solid foams2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 206, p. 541-550Article in journal (Refereed)
    Abstract [en]

    The fracture behavior of brittle solid foams of different densities and regularities is numerically analyzed in finite element models. The findings provide insight into the complex fracture phenomenon in cellular materials and reveal a size influence from a dominant microstructure on the global fracture mechanism. It is observed that a crack of length of about three times the average cell size in the foam is needed to obtain localization of nucleated fractures to the vicinity of the initial defect At cracks smaller than this critical size, the fractures nucleate at randomly positioned high-stressed regions in the foam far away from the initial crack, i.e. the structure is seemingly insensitive to the initial defect Further, it is found that irregular (i.e. randomly positioned cells) foams are more insensitive to defects than perfectly ordered foams if all other parameters are similar and thus indicate that classical fracture theories for solid foams have to be slightly modified.

  • 3.
    Englund, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Division of Scientific Computing. Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Numerical Analysis.
    Fast, accurate, and stable algorithm for the stress field around a zig-zag-shaped crack2003In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 70, p. 355-364Article in journal (Refereed)
  • 4.
    Espadas-Escalante, Juan José
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Isaksson, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    A study of induced delamination and failure in woven composite laminates subject to short-beam shear testing2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 205, p. 359-369Article in journal (Refereed)
    Abstract [en]

    Failure in woven composite laminates subject to global shear load is studied. Laminates are manufactured, tested and analyzed using X-ray computed tomography, scanning electron microscopy and finite element models. It is found that the stress distribution along the thickness direction is dependent on the layer shifting that alters different yarn interactions, which in turn, affects delamination and failure onset A suggested failure mechanism is in agreement with experimental observations.

  • 5.
    Espadas-Escalante, Juan José
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Isaksson, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Mesoscale analysis of the transverse cracking kinetics in woven composite laminates using a phase-field fracture theory2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 216, article id 106523Article in journal (Refereed)
    Abstract [en]

    A phase-field approach to fracture is used to simulate transverse cracking kinetics in composite laminates. First, a typical unidirectional tape laminate is modeled and the transverse cracking evolution with the consequent reduction in the in-plane modulus of elasticity is estimated. Then, a four-layered plain weave composite is modeled using different layer shifting configurations. Predictions in the transverse cracking evolution become improved as the shifting configuration of the laminate model become closer to experimental observations. Simulations predict that some cracks do not form perpendicularly to the loading direction, as it has been observed experimentally in similar locations. Only the fracture toughness and the in situ transverse strength of the ply are required without prior knowledge of the position of the cracks or an ad hoc criterion for crack evolution. All the simulations are compared qualitatively and quantitatively to experiments published elsewhere.

  • 6.
    Isaksson, Per
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Dumont, P. J. J.
    Approximation of mode I crack-tip displacement fields by a gradient enhanced elasticity theory2014In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 117, p. 1-11Article in journal (Refereed)
    Abstract [en]

    Gradient theories are capable of describing deformation of heterogeneous elastic materials better than classical elasticity theory since they are able to capture internal length effects. Here, crack-tip displacement fields at the tip of a mode I crack in gradient enhanced elastic materials are derived in closed form and contrasted with experiments. Heterogeneous materials, represented by discrete fiber networks, are analyzed in finite element models to judge the theory. It is shown that using a classical continuum approach to describe macroscopic singular-dominated deformation fields in heterogeneous materials lead to erroneous results because a structural effect that alters the displacement field becomes pronounced and results in severe blunting of crack-tips. A key conclusion is that the average segment length in the material gives the internal length scale parameter, used in the gradient enhanced continuum theory, hence allows for bridging between scales.

  • 7.
    Isaksson, Per
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Hägglund, R.
    Crack-tip fields in gradient enhanced elasticity2013In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 97, p. 186-192Article in journal (Refereed)
    Abstract [en]

    Nonlocal and gradient theories are capable of describing deformation of heterogeneous elastic materials better than classical elasticity theory. Crack-tipstress and strain fields in a gradient enhanced elastic material are derived on closed form. Physical requirements of finite stresses and strains at infinity and at the tip are applied to remove singularities. A fracture criterion is derived that links applied remote macroscopic stress via microscopic cohesive stress in the vicinity of thecrack-tip to the Griffith's energy. A comparison to a classical nonlocal theory by Eringen is made. It is believed that the solutions will help engineers to deal with fracture analyses in elastic brittle heterogeneous materials. 

  • 8.
    Joffre, Thomas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Chen, Song
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Isaksson, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Microscopic strain fields at crack tips in porous materials analyzed by a gradient-enhanced elasticity theory2016In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 168, p. 160-173Article in journal (Refereed)
    Abstract [en]

    The microstructural influence on the strain field at opening mode crack tips in porous materials, and especially its practical implication for understanding macroscopic failure, i.e. on a scale above, is investigated. Theoretical subscale microstrain fields are approximated using a gradient-enhanced elasticity theory and compared to microstrain fields computed in discrete high-resolution finite element microstructural models having varying pore densities but similar macroscopic geometry and boundary conditions as the theoretical gradient-enhanced model. The numerical elastic microstrain and microstress fields are non-singular in strong contrast to the singular macroscopic fields in classical linear elastic fracture theories. Experimentally approximated microstrain fields, estimated with a digital image correlation algorithm on images obtained in X-ray computational tomography fracture tests on a small wood specimen, are used to contrast the.numerical analyses. A key observation is that an internal length parameter, used in the gradient-enhanced model, seems to be linked to the average pore diameter, allowing for direct bridging between scales.

  • 9.
    Sun, Fengzhen
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Li, Hu
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Gamstedt, E. Kristofer
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics.
    Polymer fracture and deformation during nanosectioning in an ultramicrotome2017In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 182, p. 595-606Article in journal (Refereed)
    Abstract [en]

    The fracture and deformation behaviour of a thermoplastic in the nanosectioning process is investigated by using an ultramicrotome instrumented with force transducers. For sections with thickness values in the order of 10–100 nm, the specific work of surface formation of polymethyl methacrylate is found to be 6.3 J m−2, which is considerably smaller to macroscopic fracture toughness, but relatively close to the theoretical specific surface energy of 1.5 J m−2 of breaking the covalent bonds. Periodic transverse features, spaced a few hundred nanometres on the sectioned surfaces, are observed by atomic force microscopy for sections above a critical thickness. It reveals that a transition of the deformation mode occurs at a certain thickness, which is in concert with sectioning experiments for other materials showing adiabatic shear bands.

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