CFRAC 2015
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Plenary Lectures

  • Claudia Comi (Politechnico di Milano, Italy)

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Title: Multi-phase modeling of chemo-mechanical degradation in concrete

Abstract: The long-term behavior of concrete structures may depend on various chemo-physical damage phenomena. Among them the degradation induced by the alkali-silica reaction and by the delayed ettringite formation are of particular concern for the safety assessment of several concrete dams and bridges built decades ago. The first chemical reaction, occurring between the amorphous silica contained in some types of aggregates and the alkali from the cement paste, produces a gel that, in the presence of water, exerts a pressure on the solid skeleton causing overall swelling and microcracks formation. In the second case, swelling and material degradation result from the reaction between the sulfate and the hydrated products of the cement paste. The kinetics of the reactions and, consequently, the severity of the damage depends on environmental factors (humidity, temperature, concentration of sulfate and pH of the media in contact with concrete) and intrinsic material properties (chemical composition of the cement paste, in particular aluminates content, size and chemical composition of the aggregates, pore distribution, diffusivity properties). The numerical description of these phenomena requires proper diffusion-reaction models coupled with damage models. Within the frame of the multi-phase theory of porous media, the lecture will focus on a chemo-elastic damage model developed for the description of the mechanical degradation of concrete structures affected by these phenomena. Several validation examples concerning laboratory tests and real structures will be discussed.

  • Christian Miehe (University of Stuttgart)

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Title: Phase Field Modeling of Brittle and Ductile Fracture in Multi-Field Environments
Click here to download the abstract.

  • Michael Ortiz (Caletch, USA)

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Title: Distributed Damage and Enhanced Permeability in Confined Brittle Materials under Triaxial Compression

Abstract: Distributed damage involving complex fracture patterns arises in a number of situations of interest, including: geological formations; confined structural ceramics and brittle-matrix composites; crushed concrete; and others. Applications such as fracking exploit such processes in order to enhance the permeability of rock for  purposes of oil and gas extraction. Attendant theoretical and computational challenges include: the characterization of the effective or macroscopic  behavior of the crushed material from its elastic and fracture properties; the prediction of the crack microstructures underlying the macroscopic behavior; and the prediction of the permeability enhancement due to said microstructures. A powerful theory of distributed damage in brittle materials deforming in triaxial compression may be based on explicit constructions of the crack microstructure such as recursive faulting.  These constructions account for the elasticity of the matrix, crack nucleation and the ensuing cohesive and frictional behavior of the cracks. A relaxation of the energy then describes explicitly the macroscopic material behavior averaged over all possible fine-scale structures. Once the optimal crack microstructure is known, standard theory gives the effective permeability of the fractured material. Validation calculations of the dynamic multi-axial compression experiments on sintered aluminum nitride (AlN) show that the theory correctly predicts the geometry the observed crack patterns and the brittle-to-ductile transition resulting under increasing confinement. Validation examples also demonstrate the predictive nature of the theory as regards the dependence of rock permeability on deformation and damage.                     

  • Krishnaswamy Ravi-Chandar (University of Texas, Austin, USA)

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Title: EXPERIMENTS AND NUMERICAL SIMULATIONS OF INITIATION AND GROWTH OF CRACKS UNDER MIXED MODE I + III LOADING

K.H. Pham and K. Ravi-Chandar
Center for Mechanics of Solids, Structures and Materials
The University of Texas at Austin, Austin, TX 78712-1221 

Abstract: The problem of initiation and growth of cracks under combined mixed-mode I+III loading has received much attention in the literature over the past few decades both from experiments and analysis, but suitable failure criteria are still not available. Specifically, the existence of a threshold ratio of mode III to mode II stress intensity factors below which fragmentation of the crack front (formation of daughter cracks) does not occur has been debated and the length scale associated with the spacing of the fragments when the do occur are still under debate. The continued growth of cracks under remote mode I + III loading is also of interest; it is observed that in some cases the fragmented cracks coalesce, while in others they maintain their independent development. We approach this problem through carefully designed experiments to examine the physical aspects of crack initiation and growth. This is then explored further through numerical simulations of the stress state that explore the influence of perturbations on the formation of daughter cracks. Finally, the direct numerical simulation of crack initiation and growth is explored using a phase-field model. The model is first validated for in-plane modes I + II through comparison to experiments, and then used to explore combined modes I + III.

  • Eric Van der Giessen (Univ of Groningen, Nederlands)

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Title: On Toughening in Polymer-Rubber Blends

Abstract: One of the popular ways to enhance the fracture toughness and impact resistance of inexpensive brittle polymers is to blend them with small rubber particles. This has led to well-known materials such as HIPS (High Impact Polystyrene) that find widespread application. However, many years after the inception of rubber-toughened blends, the insight in the origin of toughening is still largely qualitative. This talk will deal with computational models that have been proposed to gain a quantitative understanding. Attention will be focused on ABS (SAN blended with PB rubber particles), a blend which, depending on composition, can exhibit two key toughening mechanisms: extensive crazing between rubber particles versus facilitating shear yielding after cavitation of the rubber particles. I will review a suite of models, including micromechanical models for the individual deformation and fracture mechanisms, comprehensive microstructural models as well as non-linear homogenised theories for crazed or cavitated polymers.

 

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