Browsing by Subject "Fracture mechanics"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Atomistically-informed Finite Element Simulations of Phase Transformations and Fracture in Materials
    (2020-01) Fan, Jiadi
    Multiscale material modeling is a powerful computational method to investigate materials at disparate length and/or time scales, and has been widely employed to study a large variety of problems in science and engineering. In this dissertation, an atomistically-informed finite element method (AFEM) is introduced, which involves two scales of calculations: the finite element method (FEM) and atomistic simulation. The FEM as a powerful tool to simulate material response in the continuum scale is widely used in solid mechanics field. However, phenomenological model is usually employed as constitutive law, which lacks the fundamental insights of material. Atomistic simulation can provide us with thermomechanical properties of material based on the interactions between atoms, but is limited to small model size due to the computational efficiency. In the AFEM presented in this dissertation, the material properties are calculated from the atomistic scale simulations, and are employed in the continuum scale FEM simulations as material parameters. Using such a modeling method, we can predict the large scale mechanical response of a system without losing atomistic insights of materials. The AFEM is implemented in an \emph{in situ} simulation of a diamond anvil cell to predict the phase transformation of silicon under pressure, and a cohesive element simulation of epoxy--graphene composite to study the fracture mechanism at small graphene loading.
  • Thumbnail Image
    Item
    Effects of compressive and tensile fields on the load carrying capacity of headed anchors.
    (2011-02) Piccinin, Roberto
    The results of research initiated in the early 1980s led to the replacement of plasticity-based design guidelines for the load-carrying capacity (concrete breakout) of headed anchors embedded in concrete with those developed using fracture mechanics. While provisions are available in the design codes that account for the presence of tensile fields causing concrete cracking, no provisions are available for anchors embedded in prestressed concrete. This thesis presents the results of linear and nonlinear elastic fracture mechanics analyses of the progressive failure of headed anchors embedded in a concrete matrix under compressive or tensile prestress. In addition, and because of the complete lack of experimental evidence, the results of a relatively large experimental investigation of the behavior of headed anchors embedded in compressively prestressed concrete are presented. Discrete crack finite element models and experiments predict an increase (decrease) in load-carrying capacity and post-peak dissipated energy with increasing compressive (tensile) prestress for all the embedment depths investigated. For extremely shallow cases, in which the embedment depth is less than the (typical) maximum aggregate size of concrete, it is shown that deterministic continuum-based models are not applicable. Overall, the results show that there is very little difference between the linear elastic and nonlinear elastic fracture mechanics approaches, this implying that the concrete breakout strength is governed by the strongest possible size effect. In addition to providing analytical support to the existing design approaches for the capacity of headed anchors embedded in cracked concrete (under tension), the present work provides an experimentally and analytically based preliminary easy-to-use design formula for the concrete breakout capacity of headed anchors in compressively prestressed concrete.
  • Thumbnail Image
    Item
    Laboratory Performance Test for Asphalt Concrete
    (Center for Transportation Studies University of Minnesota, 2015-06) Dave, Eshan
    The asphalt mixture design and acceptance procedures for Minnesota Department of Transportation are currently governed primarily by the mixture composition requirements put forth through use of various volumetric measures (such as, air content, asphalt film thickness, aggregate gradation etc.). The asphalt binder has been required to meet performance criteria through the Superpave asphalt binder specifications. This study looked at use of laboratory performance test for asphalt mixtures. The study was conducted in three phases, first phase focused on merging the asphalt mix design records with the pavement performance data to determine effects of mix design parameters on asphalt pavement cracking performance. Second and third phase used a series of field sections across Minnesota to conduct field performance evaluations as well as laboratory tests on field cored samples. The testing for second and third phase of the study focused on using disk-shaped compact tension (DCT) fracture energy test as a laboratory performance test. The findings form he first phase of study indicated that the asphalt binder type as defined by the Superpave performance grade (PG) plays an important role in affecting the field cracking performance, majority of mixture design parameters did not indicate a consistent effect on field cracking performance, this reinforces the need for use of laboratory performance test as a mixture design tool as well as acceptance parameter. The DCT testing results showed trends consistent with previous and other on-going research studies, whereby the asphalt mixtures with higher fracture energies corresponded with pavements with lower amount of transverse cracking.
  • Thumbnail Image
    Item
    Mechanistic Modeling of Unbonded Concrete Overlay Pavements
    (Minnesota Department of Transportation, 2012-01) Ballarini, Roberto; Liao, Minmao
    An unbonded concrete overlay (UBCO) system is a Portland cement concrete (PCC) overlay that is separated from an existing PCC pavement by an asphalt concrete (AC) interlayer. Current UBCO design procedures are based on empirical equations or highly simplified mechanistic models. To overcome the limitations, fracture mechanics concepts, specifically the finite element method-based cohesive zone model (CZM), are introduced in this research as a new paradigm for analyzing UBCOs with the ultimate goal of establishing a more rational design procedure. To illustrate the advantages of a fracture mechanics-based approach to design, specific attention is paid to but one type of failure associated with pavement structures: reflection cracking. The design against reflection cracking approach relies on a load-carrying capacity equivalency between the designed UBCO and a reference newly designed single layer PCC pavement. An illustrative fracture mechanics-based design procedure for UBCOs is developed and proposed by a large number of crack propagation simulations of both the UBCO composite and the reference single layer pavement. Preliminary comparisons of the results with field observations suggest that the fracture mechanics paradigm offers promise for improved design of UBCOs against reflection cracking and other potential loading conditions that could be analyzed using nonlinear fracture mechanics models. It is recommended that an experimental program be established to assess the accuracy of the model predictions, and additional experiments and three-dimensional fracture mechanics simulations be considered to provide additional insights as to whether UBCOs can be “thinned-up”.
  • Thumbnail Image
    Item
    Optimizing Asphalt Mixtures for Low-volume Roads in Minnesota
    (Minnesota Department of Transportation, 2023-08) Barman, Manik; Dhasmana, Heena; Manickavasagan, Vishruthi; Marasteanu, Mihai
    Minnesota has a large number of low-volume asphalt roads. These roads typically fail because of environmental factors, such as frigid temperatures, freeze-thaw cycles, and seasonal and daily temperature variations. The goal of this study was to suggest modifications to asphalt mixture designs currently used for low-volume roads in Minnesota to improve the resistance of the mixes against the environmentally driven distresses. The study was conducted by accomplishing multiple tasks, such as a literature review, online survey, fieldwork studying the cause of the asphalt pavement distresses, laboratory work comparing asphalt mixtures designed with Superpave-4, Superpave-5, and regressed air voids methods, and studying the field compaction of Superpave-5 mixes. The mechanical performance of the asphalt mixes was studied by conducting Disc-Shaped Compact Tension (DCT), Indirect Tensile Strength (ITS), and Dynamic Modulus (DM) tests. The study included both laboratory- and plant-produced mixes. The study found that asphalt layers for the low-volume roads did not get enough densification, which augments environmentally driven distresses, such as thermal cracks, and longitudinal joint cracks. The Superpave-5 method holds considerable promise for the design of asphalt mixtures for low-volume roads in Minnesota, which may likely increase the asphalt layer densification and mitigate some of the common distresses.