We use the graphical processing unit (GPU) to perform dynamic fracture simulation using adaptively refined and coarsened finite elements and the inter-element cohesive zone model. Due to the limited memory available on the GPU, we created a specialized data structure for efficient representation of the evolving mesh given. To achieve maximum efficiency, we perform finite element calculation on a nodal basis (i.e., by launching one thread per node and collecting contributions from neighboring elements) rather than by launching threads per element, which requires expensive graph coloring schemes to avoid concurrency issues. These developments made possible the parallel adaptive mesh refinement and coarsening schemes to systematically change the topology of the mesh. We investigate aspects of the parallel implementation through microbranching examples, which has been explored experimentally and numerically in the literature. First, we use a reduced-scale version of the experimental specimen to demonstrate the impact of variation in floating point operations on the final fracture pattern. Interestingly, the parallel approach adds some randomness into the finite element simulation on the structured mesh in a similar way as would be expected from a random mesh. Next, we take advantage of the speedup of the implementation over a similar serial implementation to simulate a specimen whose size matches that of the actual experiment. At this scale, we are able to make more direct comparisons to the original experiment and find excellent agreement with those results.

We present a method to reduce mesh bias in dynamic fracture simulations using the finite element method with adaptive insertion of extrinsic cohesive zone elements along element boundaries. The geometry of the domain discretization is important in this setting because cracks are only allowed to propagate along element facets and can potentially bias the crack paths. To reduce mesh bias, we consider unstructured polygonal finite elements in this work. The meshes are generated with centroidal Voronoi tessellations to ensure element quality. However, the possible crack directions at each node are limited, making this discretization a poor candidate for dynamic fracture simulation. To overcome this problem, and significantly improve crack patterns, we propose adaptive element splitting, whereby the number of potential crack directions is increased at each crack tip. Thus, the crack is allowed to propagate through the polygonal element. Geometric studies illustrate the benefits of polygonal element discretizations employed with element splitting over other structured and unstructured discretizations for crack propagation applications. Numerical examples are performed and demonstrate good agreement with previous experimental and numerical results in the literature. Copyright © 2014 John Wiley & Sons, Ltd.

In this work, we investigate the generalized displacement control method (GDCM) and provide a modification (MGDCM) that results in an equivalent constraint equation as that of the linearized cylindrical arc-length control method (LCALCM). Through numerical examples, we illustrate that the MGDCM is more robust than the standard GDCM in capturing equilibrium paths in regions of high curvature. Moreover, we also provide a geometric and physical interpretation of the method, which sheds light on the general class of path following methods in structural mechanics. (C) 2014 Elsevier Ltd. All rights reserved.

This paper presents a scheme for adaptive mesh refinement on unstructured polygonal meshes to better capture crack patterns in dynamic cohesive fracture simulations. A randomly seeded polygonal mesh leads to an isotropic discretization of the problem domain, which does not bias crack patterns, but restricts the number of paths that a crack may travel at each node. An adaptive refinement scheme is proposed and investigated through a detailed set of geometric studies. The refinement scheme is selectively chosen to optimize the number of paths that a crack may travel, while still maintaining a conforming domain discretization. The details of the refinement scheme are outlined, along with the criterion used to determine the region of refinement and the method of interpolating nodal attributes. Extrinsic cohesive elements are inserted when and where necessary, and follow the constitutive response of the Park–Paulino–Roesler cohesive model. The influence of bulk and cohesive material heterogeneity is investigated through the use of a statistical distribution of material properties. The adaptive mesh modifications are handled through a compact topological data structure. Numerical examples highlight the features of adaptive refinement in capturing physical fracture patterns while addressing computational cost. Thus, the present approach is a step towards obtaining accurate dynamic fracture patterns and fields with polygonal elements.

Low-temperature cracking (LTC) is a major distress and cause of failure for asphalt pavements located in regions with cold climate; however, most pavement design methods do not directly address LTC. The thermal cracking model (TCModel) utilised by American Association of State Highway and Transportation Officials Mechanistic-Empirical Pavement Design Guide relies heavily on phenomenological Paris law for crack propagation. The TCModel predictions are primarily based on tensile strength of asphalt mixture and do not account for quasi-brittle behaviour of asphalt concrete. Furthermore, TCModel utilises a simplified one-dimensional viscoelastic solution for the determination of thermally induced stresses. This article describes a newly developed comprehensive software system for LTC prediction in asphalt pavements. The software system called ‘IlliTC’ utilises a user-friendly graphical interface with a stand-alone finite-element-based simulation programme. The system includes a preanalyser and data input generator module that develops a two-dimensional finite element (FE) pavement model for the user and which identifies critical events for thermal cracking using an efficient viscoelastic pavement stress simulation algorithm. Cooling events that are identified as critical are rigorously simulated using a viscoelastic FE analysis engine coupled with a fracture-energy-based cohesive zone fracture model. This article presents a comprehensive summary of the components of the IlliTC system. Model verifications, field calibration and preliminary validation results are also presented.

Nonlinear problems are prevalent in structural and continuum mechanics, and there is high demand for computational tools to solve these problems. Despite efforts to develop efficient and effective algorithms, one single algorithm may not be capable of solving any and all nonlinear problems. A brief review of recent nonlinear solution techniques is first presented. Emphasis, however, is placed on the review of load, displacement, arc length, work, generalized displacement, and orthogonal residual control algorithms, which are unified into a single framework. Each of these solution schemes differs in the use of a constraint equation for the incremental-iterative procedure. The governing finite element equations and constraint equation for each solution scheme are combined into a single matrix equation, which characterizes the unified approach. This conceptual model leads naturally to an effective object-oriented implementation. Within the unified framework, the strengths and weaknesses of the various solution schemes are examined through numerical examples.

Thermally induced cracking in asphalt pavements remains to be one of the prominent distress mechanisms in regions with cooler climates. At present, the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) is the most widely deployed pavement analysis and design procedure. For thermal cracking predictions, MEPDG utilizes a simplified one-dimensional stress evaluation model with a simple Paris-law (i.e. linear elastic fracture mechanics) based crack propagation procedure. The user-friendly graphical interface for MEPDG makes it an attractive design procedure of choice, however, the over simplicity of the model and lack of a physicsbased representation to accurately capture the nonlinear fracture behavior of ratedependent asphalt concrete reduce(s) the reliability of predictions. This study presents an interactive thermal cracking prediction model that utilizes a nonlinear finite element based thermal cracking analysis engine which can be easily employed using a user-friendly graphical interface. The analysis engine is comprised of (1) the cohesive zone fracture model for accurate simulation of crack initiation and propagation due to thermal loading and (2) the viscoelastic material model for time and temperature dependent bulk material behavior. The graphical user interface (GUI) is designed to be highly interactive and user-friendly in nature, and features screen layouts similar to those used in the AASHTO MEPDG, thus minimizing transition time for the user. This paper describes the individual components of the low temperature cracking prediction software (called LTC Model) including details on the graphical user interface, viscoelastic finite element analysis, cohesive zone fracture model, and integration of various software components for thermal cracking predictions.

With shrinking maintenance budgets and the need to ‘do more with less,’ accurate, robust asset management tools are greatly needed for the transportation engineering community. In addition, the increased use of recycled materials and low energy production techniques such as warm-mix asphalt are leading to increased needs for preventive and rehabilitative maintenance activities. The timing of such activities will greatly affect the total discounted life-cycle cost of the pavement system. Low-temperature cracking of hot-mix asphalt (HMA) pavements continues to be a leading cause of premature pavement deterioration in regions of cold climate and/or where significant thermal cycling occurs. Recent advances in fracture testing and modeling of hot-mix asphalt (HMA) materials have greatly aided in the understanding of the key mechanisms behind this important pavement distress, which can greatly reduce pavement lifespan and the lifespan of subsequent rehabilitation cycles. However, there is a need for implementation of new models into a standalone program which can be readily utilized by researchers and practitioners. Moreover, the complete integration of material selection, material design, pavement design and pavement performance into a more holistic asset management system has been hampered by the lack of accurate, user-friendly performance prediction models for pavements. The objective of this project was to complete the development of a user-friendly interface that provides simplified access to sophisticated low-temperature cracking prediction models. This stand-alone program will greatly accelerate the transfer of this technology to practitioners and other interested scientists and engineers (pavement designers, analysts, and researchers). The program is designed to be compatible with the existing thermal cracking model used in the Mechanistic Empirical Pavement Design Guide (MEPDG), and with the new thermal cracking model being developed under the National Pooled Fund Study on Low Temperature Cracking. As part of this report, the key developments associated with the new mechanics-based thermal cracking model are presented. The funding provided by this NexTrans supplement allowed the development of the thermal cracking software to include aspects which will facilitate its seamless integration into an overarching pavement management software program.