David S. Kammer

Last Name

Kammer

First Name

David S.

ORCID

0000-0003-3782-9368

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09650 - Kammer, David / Kammer, David

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Publications 1 - 10 of 99

Transonic and Supershear Crack Propagation Driven by Geometric Nonlinearities
Pundir, Mohit; Adda-Bedia, Mokhtar; Kammer, David S. (2024)
Physical Review Letters
Linear elastic fracture mechanics theory predicts that the speed of crack growth is limited by the Rayleigh wave speed. Although many experimental observations and numerical simulations have supported this prediction, some exceptions have raised questions about its validity. The underlying reasons for these discrepancies and the precise limiting speed of dynamic cracks remain unknown. Here, we demonstrate that tensile (mode I) cracks can exceed the Rayleigh wave speed and propagate at supershear speeds. We show that taking into account geometric nonlinearities, inherent in most materials, is sufficient to enable such propagation modes. These geometric nonlinearities modify the crack-tip singularity, resulting in different crack-tip opening displacements, cohesive zone behavior, and energy flows towards the crack tip.
Non‐Precursory Accelerating Aseismic Slip During Rupture Nucleation
Wang, Xiaoyu; Dal Zilio, Luca; Morgan, Julia K.; et al. (2023)
Journal of Geophysical Research: Solid Earth
Accelerating aseismic slip events have been commonly observed during the rupture nucleation processes of the earthquake. While that accelerating aseismic slip is usually considered strong evidence for precursory activity, it remains unclear whether all accelerating aseismic slip events are precursory to an incoming earthquake. Two contrasting nucleation models have been introduced to explain the observations associated with the nucleation of unstable slip: the pre-slip and cascade nucleation models. Each of these two-end members, however, has its own limitations. In this study, we employ Discrete Element Method simulations of a 2-D strike-slip fault to simulate various rupture nucleation and triggering processes. Our simulation results manifest that the final seismic event is a product contributed by multiple pre-slip nucleation sites, which may interact, causing clock advance or cascade nucleation rupture processes. We also introduce a strengthening perturbation zone to investigate the role of a single nucleation site in an imminent seismic event. The simulation results reveal a new type of non-precursory aseismic slip, representing the region favoring the generation of the precursory slip process but not correlating to the incoming main event, which differs from the previous interpretation of precursory slip. Furthermore, we include weakening perturbation zones in some simulations to demonstrate how small earthquakes may or may not trigger a nucleation site depending on spatial and temporal conditions. Our simulation results imply that such non-precursory but accelerating aseismic slip events may suggest a fault segment that appears weakly coupled but possesses the potential to be triggered seismically.
Learning physics-consistent material behavior from dynamic displacements
Han, Zhichao; Pundir, Mohit; Fink, Olga; et al. (2025)
Computer Methods in Applied Mechanics and Engineering
Accurately modeling the mechanical behavior of materials is crucial for numerous engineering applications. The quality of these models depends directly on the accuracy of the constitutive law that defines the stress–strain relation. However, discovering these constitutive material laws remains a significant challenge, in particular when only material deformation data is available. To address this challenge, unsupervised machine learning methods have been proposed to learn the constitutive law from deformation data. Nonetheless, existing approaches have several limitations: they either fail to ensure that the learned constitutive relations are consistent with physical principles, or they rely on boundary force data for training which are unavailable in many in-situ scenarios. Here, we introduce a machine learning approach to learn physics-consistent constitutive relations solely from material deformation without boundary force information. This is achieved by considering a dynamic formulation rather than static equilibrium data and applying an input convex neural network (ICNN). We validate the effectiveness of the proposed method on a diverse range of hyperelastic material laws. We demonstrate that it is robust to a significant level of noise and that it converges to the ground truth with increasing data resolution. We also show that the model can be effectively trained using a displacement field from a subdomain of the test specimen and that the learned constitutive relation from one material sample is transferable to other samples with different geometries. The developed methodology provides an effective tool for discovering constitutive relations. It is, due to its design based on dynamics, particularly suited for applications to strain-rate-dependent materials and situations where constitutive laws need to be inferred from in-situ measurements without access to global force data.
Image-based tracking of the crack propagation within irregular lattices
Lingua, Alessandra; Sanner, Antoine; Hild, François; et al. (2025)
CFRAC 2025: The Eighth International Conference on Computational Modeling of Fracture and Failure of Materials and Structures
Architected materials offer disruptive opportunities to combine low density with high toughness or strength. Tuning their architecture allows for further unlocking of features not intrinsic to the base material, such as negative Poisson’s ratio, local toughening, and ultra-stiffness. Due to the high slenderness of lattice struts, it is challenging to characterize how engineering the topology alters the local stress state and modifies the macroscopic failure of functionalized architected materials. We propose an approach based on digital image correlation (DIC) to unravel the crack initiation and propagation mechanisms of 2D heterogeneous lattices under mode I loading. To identify the dominant strut failure modes, we 3D-printed compact tension specimens with stretching and bending-dominated topology (triangular, hexagonal, and Kagome). Using FE-based DIC, we measured the nodal displacements of the joints and extracted the strain distribution within the struts by sub-lattice scale meshing. Based on the gray level residuals, we determined failure criteria for the strut in terms of strain thresholding, and we tracked the crack tip advancement during testing. The critical stress intensity factor and energy release rates were obtained by a priori knowledge of the detected crack path and integrated DIC. Leveraging these local measurements, we assessed the influence of heterogeneities on the lattice strain distribution and thus damage nucleation and propagation mechanisms, resulting in increased work to failure. Ultimately, we highlight how image-based characterization approaches enlighten the effects of the topology alterations on the fracture behavior of lattice materials and provide realistic failure criteria for damage modeling. This insight is essential to design architected materials with engineered properties and customized fracture for real-world applications
Scaling, saturation, and upper bounds in the failure of topologically interlocked structures
Feldfogel, Shai; Karapiperis, Konstantinos; Andrade, Jose; et al. (2023)
International Journal of Solids and Structures
Topological Interlocking Structures (TIS) have been increasingly studied in the past two decades. However, some fundamental questions concerning the effects of Young’s modulus and the friction coefficient on the structural mechanics of the most common type of TIS application – centrally loaded panels – are not yet clear. Here, we present a first-of-its-kind parametric study that aims to clarify how these two parameters affect multiple aspects of the behavior and failure of centrally-loaded TIS panels. This includes the evolution of the structural response up to and including failure, the foremost structural response parameters, and the residual carrying capacity. We find that the structural response parameters in TIS panels scale linearly with Young’s modulus, that they saturate with the friction coefficient, and that the saturated response provides an upper-bound on the capacity of centrally loaded TIS panels reported in the literature. This, together with additional findings, insights, and observations, comprise a novel contribution to our understanding of the interlocked structural form.
Creep fronts and complexity in laboratory earthquake sequences illuminate delayed earthquake triggering
Cebry, Sara Beth L.; Ke, Chun‐Yu; Shreedharan, Srisharan; et al. (2022)
Nature Communications
Earthquakes occur in clusters or sequences that arise from complex triggering mechanisms, but direct measurement of the slow subsurface slip responsible for delayed triggering is rarely possible. We investigate the origins of complexity and its relationship to heterogeneity using an experimental fault with two dominant seismic asperities. The fault is composed of quartz powder, a material common to natural faults, sandwiched between 760 mm long polymer blocks that deform the way 10 meters of rock would behave. We observe periodic repeating earthquakes that transition into aperiodic and complex sequences of fast and slow events. Neighboring earthquakes communicate via migrating slow slip, which resembles creep fronts observed in numerical simulations and on tectonic faults. Utilizing both local stress measurements and numerical simulations, we observe that the speed and strength of creep fronts are highly sensitive to fault stress levels left behind by previous earthquakes, and may serve as on-fault stress meters.
Structural build-up at rest in the induction and acceleration periods of Portland Cement
Michel, Luca; Reiter, Lex; Sanner, Antoine; et al. (2024)
Cement and Concrete Research
Structural build-up in fresh cement paste at rest is characterized by time evolutions of storage modulus and yield stress, which both increase linearly in time during the induction period of hydration, followed by an exponential evolution after entering the acceleration period. Here, we investigate structural build-up by coupling calorimetry and oscillatory shear measurements of Portland Cement at different w/c ratios and in the absence of admixtures, capturing how the storage modulus evolves with changes in cumulative heat. This allows the decoupling of hydration kinetics from the mechanisms dictating build-up at rest. We obtain an exponential relation between stiffness and heat, with the same exponent in both the induction and acceleration periods. This suggests that, at least in the absence of admixtures, the same mechanism dictates build-up at rest in both periods. Since it is understood that C-S-H dictates build-up at rest in the acceleration period, we deduce that the same mechanism holds in the induction period.
Nucleation of frictional sliding by coalescence of microslip
Schär, Styfen; Albertini, Gabriele; Kammer, David S. (2021)
International Journal of Solids and Structures
The onset of frictional motion is mediated by the dynamic propagation of a rupture front, analogous to a shear crack. The rupture front nucleates quasi-statically in a localized region of the frictional interface and slowly increases in size. When it reaches a critical nucleation length it becomes unstable, propagates dynamically and eventually breaks the entire interface, leading to macroscopic sliding. The nucleation process is particularly important because it determines the stress level at which the frictional interface fails, and therefore, the macroscopic friction strength. However, the mechanisms governing nucleation of frictional rupture fronts are still not well understood. Specifically, our knowledge of the nucleation process along a heterogeneous interface remains incomplete. Here, we study the nucleation of localized slip patches on linear slip-weakening interfaces with deterministic and stochastic heterogeneous friction properties. Using numerical simulations, we analyze the process leading to a slip patch of critical size for systems with varying correlation lengths of the local friction strength. Our deterministic interface model reveals that the growth of the critical nucleation patch at interfaces with small correlation lengths is non smooth due to the coalescence of neighboring slip patches. Existing analytical solutions do not account for this effect, which leads to an overestimation of global interface strength. Conversely, when the correlation length is large, the growth of the slip patch is continuous and our simulations match the analytical solution. Furthermore, nucleation by coalescence is also observed on stochastic interfaces with small correlation length. In this case, the applied load for a given slip patch size is a random variable. We show that its expectation follows a logistic function, which allows us to predict the strength of the interface well before failure occurs. Our model and observations provide new understanding of the nucleation process and its effect on the static frictional strength.
Properties of the shear stress peak radiated ahead of rapidly accelerating rupture fronts that mediate frictional slip
Svetlizky, Ilya; Pino Muñoz, Daniel; Radiguet, Mathilde; et al. (2016)
Proceedings of the National Academy of Sciences of the United States of America
Early-age workability loss in LC3 systems
Michel, Luca; Zunino, Franco; Flatt, Robert J.; et al. (2023)
Further Reduction of CO2-Emissions and Circularity in the Cement and Concrete Industry, 16th International Congress on the Chemistry of Cement 2023 - ICCC2023 (Bangkok 18.-22.09.2023)
The production of cement represents nearly 8% of yearly CO2 emissions worldwide. Climate change being the most important challenge modern society has to face in the coming decades, alternative formulations of cementitious materials must be brought to the market. Limestone Calcined Clay Cements (LC3) are one viable option, presenting a high potential of reduction of CO2 emissions while being easily transferable to existing technologies on the production scale. However, the main disadvantage of these systems from an implementation viewpoint is their loss of workability at early age. The origin of this workability loss is usually linked to water absorption by the clay particles, resulting in an effective decrease of w/c ratio. Here we show that the structuration at rest of such pastes cannot be brought back to absorption of water by clay particles. Rheometer measurements are performed on LC3 samples at different w/c ratios. One additional sample is prepared with metakaolin presaturated in water to assess the effect of water absorption on workability. The trends obtained show that no significant reduction of w/c ratio takes place in LC3 systems, thus challenging the proposed mechanism of water absorption by clay particles.

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David S. Kammer

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