Papers by Peter Mackenzie-Helnwein
Acta Geotechnica, 2012
An accurate and efficient low-order quadrilateral mixed up element suitable for dynamic analysis ... more An accurate and efficient low-order quadrilateral mixed up element suitable for dynamic analysis of fluid saturated porous media is presented. The element uses physical hourglass stabilization to facilitate single-point integration for the solid phase, and non-residual stabilization of the fluid phase to circumvent instability in the incompressible-impermeable limit due to the use of equalorder interpolation for the displacement and pressure fields. Element behavior is verified and demonstrated through several numerical examples.
Journal of Geotechnical and Geoenvironmental Engineering, 2012
This paper presents a procedure to account for the presence of a weaker layer of soil in a beam o... more This paper presents a procedure to account for the presence of a weaker layer of soil in a beam on nonlinear Winkler foundation analysis of a laterally loaded pile or drilled shaft. Three-dimensional (3D) finite-element (FE) models, considering a single pile embedded in a soil continuum, are used to compute representative p-y curves for various combinations of soil profile and pile diameter. Comparisons between the p-y curves resulting from homogenous and layered soil profiles, in which a weak soil layer is located between two stronger layers, provide the means to identify reductions in the ultimate lateral resistance and initial stiffness of the p-y curves representing the stronger soil owing to the presence of the weak layer. These reductions are characterized in terms of an exponential decay model. Dimensionless parameters are proposed as a means of implementing appropriate reductions for an arbitrary soil profile and pile diameter. Validation of the reduction procedure is conducted through a comparative study, in which the effects of liquefaction-induced lateral spreading on a pile are analyzed using pseudostatic nonlinear Winkler foundation and 3D FE approaches. Use of the reduction procedure is demonstrated with p-y curves defined by existing methods.
Computational Geosciences, 2014
In this paper, the material point method (MPM) is presented as a tool for simulating large deform... more In this paper, the material point method (MPM) is presented as a tool for simulating large deformation, gravity-driven landslides. The primary goal is to assess the interaction of these flow-like events with the built environment. This includes an evaluation of earthen mounds when energy dissipating devices are placed in the path of a snow avalanche. The effectiveness of the embankments is characterized using displacement, velocity, mass, and energy measures. A second example quantifies the force interaction between a landslide and a square rigid column. Multiple slide approach angles are considered, and various aspects of the impact force are discussed.
Geological Society of America Bulletin, 2016
A dearth of direct field observations limits our understanding of individual mechanical weatherin... more A dearth of direct field observations limits our understanding of individual mechanical weathering processes and how they interact. In particular, the specific contributions of solar-induced thermal stresses to mechanical weathering are poorly characterized. Here, we present an 11 mo data set of cracking, using acoustic emissions (AEs), combined with measurements of rock temperature, strain and other environmental conditions, all recorded continuously for a granite boulder resting on the ground in open sun. We also present stresses derived from a numerical model of the temperature and stress fields in the boulder, idealized as a uniform elastic sphere experiencing simple solar temperature forcing. The thermal model is validated using this study's data. Most observed cracking coincides with the timing of calculated maximum, insolationdriven, tensile thermal stresses. We also observe that most cracking occurs when storms, or other weather events, strongly perturb the rock surface temperature field at these times. We hypothesize that these weatheractuated thermal perturbations result in a complex thermal stress distribution that is superimposed on the background stresses arising from simple diurnal forcing; these additive stresses ultimately trigger measurable cracking. Measured locations of observed cracking and surface strain support this hypothesis in that they generally match model-predicted locations of maximum solar-induced tensile stresses. Also, recorded rock surface strain scales with diurnal temperature cycling and records progressive, cumulative extension (dilation), consistent with ongoing, thermal stress-driven subcriti-cal crack growth in the boulder. Our results therefore suggest that (1) insolation-related thermal stresses by themselves are of sufficient magnitude to facilitate incremental subcritical crack growth that can subsequently be exploited by other chemical and physical processes and (2) simple insolation can impart an elevated tensile stress field that makes rock more susceptible to cracking triggered by added stress from other weathering mechanisms. Our observed cracking activity does not correlate simply with environmental conditions, including temperature extremes or the often-cited 2 °C/min thermal shock threshold. We propose that this lack of correlation is due to both the ever-varying ambient stress levels in any rock at Earth's surface, as well as to the fact that ongoing subcritical crack growth itself will influence a rock's stress field and strength. Because similar thermal cycling is universally experienced by subaerially exposed rock, this study elucidates specific mechanisms by which solar-induced thermal stresses may influence virtually all weathering processes.
new UQ features, added the CWE interface to the tool to aid beginners perform CFD simulations, in... more new UQ features, added the CWE interface to the tool to aid beginners perform CFD simulations, integrate with wind tunnel datasets
Computer Methods in Applied Mechanics and Engineering
Abstract Phenomena involving general solid–water interactions such as flows with debris are chall... more Abstract Phenomena involving general solid–water interactions such as flows with debris are challenging to model numerically because they are not easily represented using solid- or fluid-oriented methods. The material point method (MPM) provides a unified multi-material interaction platform potentially capable of modeling complex solid–water flow phenomena. However, it is necessary to address volumetric locking for (nearly) incompressible materials when modeling fluids, while also stabilizing integration errors that arise in standard MPM. This paper examines these challenges in depth, and presents a flux-based smoothing algorithm designed to address integration-error-induced destabilization via controlled strain energy dissipation. The effectiveness of the algorithm is demonstrated with two simple but fundamental fluid/solid problems, and with an application to a complex solid–water dynamic interaction problem. Results show the flux-based smoothing algorithm is capable of stabilizing the side-effects of numerical integration errors, while at the same time remaining inactive if there is no integration-error-induced oscillation. Based on this study, the flux-based smoothing algorithm is suggested as a stabilization scheme for MPM when using constant-interpolated hybrid elements.
Journal of Computational Physics, 2012
ABSTRACT The material point method exhibits kinematic locking when traditional linear shape funct... more ABSTRACT The material point method exhibits kinematic locking when traditional linear shape functions are used with a rectangular grid. The locking affects both the strain and the stress fields, which can lead to inaccurate results and nonphysical behavior. This paper presents a new anti-locking approach that mitigates the accumulation of fictitious strains and stresses, significantly improving the kinematic response and the quality of all field variables. The technique relies on the Hu–Washizu multi-field variational principle, with separate approximations for the volumetric and the deviatoric portions of the strain and stress fields. The proposed approach is validated using a series of benchmark examples from both solid and fluid mechanics, demonstrating the broad range of modeling possibilities within the MPM framework when combined with appropriate anti-locking techniques and algorithms.
Acta Geotechnica, 2014
ABSTRACT This paper presents an analysis of sand column collapse phenomena using the Material Poi... more ABSTRACT This paper presents an analysis of sand column collapse phenomena using the Material Point Method and a non-associative variant of the Matsuoka–Nakai constitutive framework, resulting in a simulation environment capable of emulating the mechanical response of a family of granular materials representing typical sands with a wide range of friction and critical state angles. A series of two-dimensional sand column collapse configurations are analyzed, and results evaluated qualitatively in light of outcomes from existing experimental and numerical simulations from the literature. Key geometric relationships are identified linking the final deposit geometry to the initial configuration via the aspect ratio, a. This work shows the internal friction angle influences the final geometry significantly and discusses various aspects of the collapse dynamics. The net deformation, stress, and base contact/reaction forces are examined in detail
Journal of Geotechnical and Geoenvironmental Engineering, 2011
ABSTRACT This paper presents a kinematic analysis of a single pile embedded in a laterally spread... more ABSTRACT This paper presents a kinematic analysis of a single pile embedded in a laterally spreading layered soil profile and discusses the relevancy of conventional analysis models to this load case. The research encompasses the creation of three-dimensional (3D) finite-element (FE) models using the OpenSees FE analysis platform. These models consider a single pile embedded in a layered soil continuum. Three reinforced concrete pile designs are considered. The piles are modeled using beam-column elements and fiber-section models. The soil continuum is modeled using brick elements and a Drucker-Prager constitutive model. The soil-pile interface is modeled using beam-solid contact elements. The FE models are used to evaluate the response of the soil-pile system to lateral spreading and two alternative lateral load cases. Through the computation of force density-displacement (p-y) curves representative of the soil response, the FE analysis (FEA) results are used to evaluate the adequacy of conventional p-y curve relationships in modeling lateral spreading. It is determined that traditional p-y curves are unsuitable for use in analyses where large pile deformations occur at depth. DOI: 10.1061/(ASCE)GT.1943-5606.0000468. (C) 2011 American Society of Civil Engineers.
International Journal for Numerical Methods in Engineering, 2010
Journal of Biomechanics, 2016
Finite element (FE) foot models can provide insight into soft tissue internal stresses and allow ... more Finite element (FE) foot models can provide insight into soft tissue internal stresses and allow researchers to effectively conduct parametric analyses. Accurate plantar soft tissue material properties are essential for the development of FE foot models for clinical interventions. The aim of this study was to identify the first-order and second-order Ogden hyperelastic material properties of the subcalcaneal fat using an inverse FE analysis. The cylindrical soft tissue FE model was developed based on a priori in vitro dynamic compression experiment. The model simulated a 1Hz triangle wave displacement to apply a compressive strain up to 48%. The hyperelastic properties were identified by systematically varying the material parameters to minimize the difference between the model predicted force and the target experimental data. Optimal material properties were obtained (μ1=0.0235kPa and α1=12.07 for the first-order Ogden model and μ1=-4.629×10(-6)kPa, α1=-16.829; μ2=-1.613kPa and α2=-1.043 for the second-order Ogden model). The second-order Ogden model was superior in capturing the highly nonlinear force-deformation response when compared to the first-order model (root mean square error (RMSE) 0.169N vs. 0.570N). The material sensitivity analysis indicated that the predicted force was strongly affected by the Poisson׳s ratio (12-fold increase in RMSE when reducing Poisson׳s ratio by 10% from the baseline) and the coefficient α1 (3.2-fold and 32-fold increase in RMSE for both first-order and second-order Ogden models when increasing α1 by 10% from the optimal value).
The goal of this contribution is the development of a plane stress orthotropic plasticity materia... more The goal of this contribution is the development of a plane stress orthotropic plasticity material model for clear spruce wood. Such a model has to consider an initially linear elastic domain as well as hardening and softening behaviour at higher states of stress and strain, respectively. Combining the advantage of a smooth single-surface plasticity model with the identification of distinct hardening and softening laws for tension and compression loading in longitudinal and radial direction is the key to the novel mathematical formulation presented in this contribution. By associating characteristic strength values of the single-surface model with the respective characteristic failure modes, one can define a set of nonlinear evolution laws for six parameters of the criterion by TSAI & WU . Such a modification redefines the meaning of the original failure envelope as a yield surface. Depending on the stress state, plastic loading causes hardening or softening. This is represented by changing size and location of the elliptic yield surface in the orthotropic stress space . The verification of the developed model is given for the entire plane stress in the orthotropic stress space by means of back-calculation of the biaxial experiments by EBERHARDSTEINER .
While the mechanical description of wood in traditional applications was mainly uniaxial (identif... more While the mechanical description of wood in traditional applications was mainly uniaxial (identified as either longitudinal direction L or perpendicular to grain directions R and T ), a realistic characteriza- tion of stress states in modern wood based materials and even in and near joints in classical applications require a multi-axial mechanical description. A remarkable number of uniaxial tests are reported in the literature. Very little, though, is available for mixed modes of loading. This was the motivation for the design of a suitable test for wood subjected to arbitrary biaxial loading under plane stress conditions. Based on a series of approximately 450 individual tests, the mechanical behavior of clear spruce wood under multiaxial loading has been thoroughly investigated. In extension to common strength tests, the biaxial load-displacement behavior has been monitored. This enables the identification of both the pre-failure behavior and the failure location. The presentation will cover the basic layout of the experiment, an overview of the respective ex- perimental observations, a brief discussion of characteristic tests, and a description of failure locations based on the obtained results and the failure criterion according to Tsai and Wu.
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Papers by Peter Mackenzie-Helnwein