Dynamic high strain rate deformation induces inertial and strain rate sensitive material response resulting in transient stress-strain states under dynamic equilibrium and non-equilibrium conditions. To reconstruct spatio-temporal stress fields in such situations, a novel method combining digital image correlation (DIC) full-field calculated deformations and an explicit dynamic finite element method (EFEM) is applied. The method utilizes finite element discretization of the specimen domain where measured boundary forces and DIC full-field measurements, including both displacements and accelerations, as input without requiring knowledge of material properties. The measured input in this non-parametric approach is used to calculate finite element internal forces, and hence the stress fields based on the measured accelerations, within each continuum finite element by solving the governing equations of motion using the EFEM framework. The current work focuses on application/validation by considering one-dimensional stress wave propagation in a ‘long’ specimen (50.8 mm) subjected to dynamic far-field compression loading by using a relatively short striker bar (101.6 mm) in a split Hopkinson Pressure bar (SHPB) to introduce transient stress states. EFEM stress field reconstruction method is first verified using simulated data from a 3D SHPB FE model within LS-DYNA of an aluminum specimen subjected to far-field compressive loading. The methodology is further assessed through experiments, using measured surface deformations on an aluminum specimen subjected to SHPB compression loading. EFEM stress field reconstructions using experimental data are similar to the FEA stresses. For softer materials, an additional experiment on a 3D printed polymeric specimen is performed, with EFEM stress field reconstructions in agreement with expected material behavior. Stress-strain data using the EFEM stress field reconstruction methodology more accurately describes the material behavior in all these cases when compared to conventional SHPB analysis based on the assumption of 1D wave propagation and homogeneous stress-strain states within the specimen.

UHMWPE cross-ply [0 degrees/90 degrees] thin film composites are emerging as an effective material for ballistic impact applications due to their high axial strength-to-weight ratio and favorable delamination properties. Mode-II interlaminar shear (ILS) fracture-driven delamination has been shown to significantly affect the ballistic performance of these composites as an energy absorption mechanism. Therefore, accurately characterizing ILS behavior in these composites across a range of strain rates is essential to inform computational models. Since these composites are significantly thinner and weaker in both interlaminar shear and transverse tension than traditional carbon fiber epoxy composites, experimental characterization of the ILS Mode II behavior presents several challenges, even at quasi-static rates. Standard test methods/specimens such as end-notched flexure (ENF) undergoing bending loads for Mode II characterization, are generally applicable to composites with stronger interfacial properties. Conversely, ENF specimens with weaker interfaces typically exhibit undesired plastic collapse under such conditions. To address this issue, a novel experimental method using a pre-cracked lap shear-type composite (PLSC) specimen bonded to rigid substrates is developed. When subjected to simple shear loading, the rigid substrates convert tension to simple shear on pre-cracked rectangular specimens, resulting in a more controlled variant of typical single lap shear test specimens. Experiments using the PLSC specimen are performed using solid-state extruded UHMWPE Tensylon (R) HSBD30A at quasi-static loading rates. Experimental results indicate the mechanical response is initially linear, with significant late-stage nonlinearity and sudden ultimate failure. Images of the PLSC fracture surfaces indicate failure originates within the interfacial material between the UHMWPE film layers. Based on experimental observations and the relatively low yield stress for the soft interface region, uniform shear tractions are assumed throughout the interface region prior to crack initiation. Using the measured loads, assumed uniform shear traction, and measured crack tip tangential relative displacements obtained via digital image correlation, a trapezoidal traction-separation law is assumed and constructed. The trapezoidal traction-separation model parameters include an initial interfacial stiffness of 4.708+1.418 MPa/& micro;m, a maximum shear traction of 3.267+0.646 MPa, a Mode II critical energy release rate of 69.6+18.8 J/m2, and a final displacement of 26.09+4.75 & micro;m. The validity of the experimentally constructed traction-separation model is confirmed via finite element simulations of the experiments using cohesive zone modeling for the interfacial response. The simulation results showed good correlation with experimental data in terms of average shear stress and crack tip displacement.

Performing both split Hopkinson pressure bar (SHPB) experiments and finite element simulations, this study investigates the effect of specimen length on dynamic force equilibrium, stress uniformity, and stress concentrations in square cross-section test specimens. While geometric mismatch at the load introduction induces axial stress concentrations, friction at the bar-specimen interfaces and lateral inertia due to Poisson's ratio (nu) effects induce shear stresses, resulting in complex multi-axial stress states near the specimen ends and delayed force equilibrium. A semi-analytical formulation based on composite material shear lag theory is employed to estimate a characteristic transfer length L-t =W-s.root 1+upsilon /4 over which the axial stress concentrations and shear stresses vanish from corners in a compression SHPB square cross-section specimen of width, W-s. The transfer length W-s formula is combined with FE and SHPB experimental measurements to determine a critical specimen length, L-scrit = 2L(t) and associated aspect ratio L-scrit/(Ws)=root 1+upsilon /2. These findings are validated by performing SHPB FE simulations and experiments for plastically deforming Al6061 specimens of three different lengths, L-s similar to L-scrit, 2L(scrit) and 3.5L(scrit) (L-s/W-s similar to 0.47, 1.18 and 2) and a width of 12.7 mm. The critical stress transfer length predicted by the FE models and estimated from experimental digital image correlation (DIC) displacements match the analytical predictions. The number of wave transits, n, needed for specimen end forces to Eq.uilibrate in square shaped specimens ranges from 12 <= n <= 18 and are 3X higher than what's reported in the literature (n = 4 to 6) for cylindrical specimens. For L-s< L-scrit, axial strains calculated using classical SHPB equations based on 1D wave propagation theory overestimates the actual strains resulting in significant errors. For L-s> L-scrit, average stresses and strains estimated using the classical SHPB Eq.uations represents the actual/expected material response reasonably well. When using strains measured via DIC, the estimated stress-strain response matches the actual/expected material response for L-s >= L-scrit. Considering stress-strain-strain rate uniformity, the optimal specimen length is shown to be L-s similar to 2L(scrit) resulting in an optimal aspect ratio of L-s/W-s = 2L(scrit)/W-s = sqrt(1+nu) = 1.14 for a Poisson's ratio of 0.30.

The in-situ bond strength and toughness (IBST) or tackiness of uncured carbon/epoxy thermoset prepreg tows is a critical parameter that governs the quality of adhesion of automated fiber and tow placement (AFP/ATP) processes. Lack of adhesion at short contact times results in defects such as wrinkles and tow folds, especially along curved placement paths where tows undergo locally mixed-mode compression shear loading during deposition. While recent studies have made advances in the characterization of Mode I IBST for short (10-50 ms) and long (0.1-10 s) contact timescales, the in-situ mixed-mode tackiness response of prepreg tows remains relatively unexplored for its direct relevance to curved-path manufacturing. A novel experimental technique is presented for quantifying the mixed-mode IBST of uncured IM7-G/8552 carbon/epoxy thermoset prepreg tows at short ATP relevant contact millisecond (ms) timescales. The test fixture employs two rigid platens with a single tow attached to each surface at an angle of 45°, a micron-scale Teflon insert to ensure a crack initiation point under controlled mixed-mode separation. The experiments are carried out at bonding temperature Tb = 40°C, contact pressure Pc = 230 kPa, and three contact times tc =10 ms, 30 ms, and 1 s. A constant debonding rate of dbr = 5 mm/s is used in all experiments. Force-displacement data is used to derive fracture mechanics-based traction-separation relations for each condition. The results demonstrate that mixed-mode peak traction (σmax) and critical energy release rate (GMC) increase with contact time (tc), consistent with previously reported Mode I trends. In particular, the mixed-mode IBST values are all higher than the corresponding Mode I values for all contact times examined, which indicate enhanced resistance to interfacial debonding due to additional shear contributions. These findings indicate that tackiness characterization with shear loading better describes the adhesion condition that is encountered upon tow steering and curved-path placement during AFP/ATP processing. The development of this experimental method provides a simpler means into measurement of short-time, mixed-mode tackiness of uncured thermoset prepregs relevant to real time curved-path ATP manufacturing conditions. Through a direct comparison of Mode I and mixed-mode IBST, this research highlights the role of mixed-mode adhesion in tow fold defect suppression and wrinkle prevention. This serves as a basis for process optimization, defect prediction, improved quality of the bond, and maximum structural performance of composite laminates fabricated by automated placement techniques.

Friction stir welding introduces spatial variations in local thermo-mechanical history, resulting in heterogeneity in both the local grain structure and elasto-plastic behavior. Such spatial heterogeneities make it difficult to extract the entire distribution of local material behavior. This information is required for effective and accurate simulations of structural response (e.g., localized yielding, fracture initiation, failure) under both quasi-static and dynamic loading conditions. At higher strain rates, the problem is even more challenging because of the need to use ultra-high speed (UHS) cameras.

Digital image correlation is used to obtain full-field displacement measurements in a variety of applications. Since DIC measurements are obtained by comparing digital reconstructions of images captured during an experiment, noise in digital image data will introduce error in the displacement measurements which then propagate into derived field quantities such as displacement gradients, strains, velocity, and acceleration. The propagation of displacement error into velocities and accelerations is of significance in high-rate loading experiments such as direct impact or split Hopkinson pressure bar tests where accurate pointwise velocity and acceleration measurements are important, especially during the early transient stages of loading. While numerous studies have focused on quantifying displacement uncertainty in DIC, displacement uncertainty propagation into the derived velocity and acceleration fields has not been well explored. Employing well-known first and second order differentiation formulae with DIC-measured displacements to determine pointwise velocity and acceleration metrics, the enclosed study presents an error analysis and uncertainty quantification for these field quantities as a function of displacement noise and camera frame rate. When using central difference algorithms, theoretical analysis indicates that (a) uncertainty in acceleration is directly proportional to the product of frame rate squared and standard deviation in displacement and (b) uncertainty in velocity is directly proportional to the product of frame rate and standard deviation in displacement. The uncertainty analysis noted above is supported by results obtained for (a) numerical rigid body translation experiments and (b) a series of both static and wave-induced quasi-rigid motion dynamic experiments where images are obtained using a highspeed camera at frame rates from 100,000 to 5 million frames per second.

Additive manufacturing approaches enable designing and fabricating structures with complex geometries, such as triply periodic minimal surface (TPMS) lattices with unique mechanics. TPMS structures are pursued for impact mitigation for civilian and military applications. Herein, additive manufacturing of TPMS structures (gyroids, Schwarz diamond, and Schwarz primitive) is done using hyperelastic photocurable resin with glass microballoon reinforcements and the strain rate effects on the mechanical responses are investigated. A successful optimization of vat photopolymerization 3D printing is done to realize TPMS structures with modified photocurable resin with up to 20.6 vol% (20 wt%) glass microballoons. An exploratory investigation is performed using a split‐Hopkinson pressure bar to test the impact response of bulk samples and TPMS structures. It is found that glass‐reinforced hyperelastic resins exhibit favorable mechanical and structural behaviors, motivating comprehensive experimental regimens as a function of strain rates, including quasi‐static and low‐ and moderate‐velocity loading scenarios. The results highlight the affinity of gyroid structures to self‐contact and relative sliding, enhancing the performance at low strain rates. The primitive TPMS structures outperform the remaining counterparts in the impact loading scenarios based on the structural performance. The outcomes of this research evidence the potential of 3D‐printed TPMS structures with glass‐reinforced hyperelastic photocurable resins for improved impact efficacy.

This study investigates the Mode I in-situ bond strength & toughness (IBST) or tackiness response of uncured IM7/G-8552 carbon epoxy thermoset prepreg tows across short (10-50 ms) and long (100 ms-10 s) contact timescales. The IBST test setup employs two rigid platens with individual tows adhered to each surface. Using a micron-scale Teflon film to create a bond-free region and induce crack initiation between the contacting tows during separation, load and displacement measurements are used to obtain fracture mechanics-based tractionseparation laws for a broad range of contact pressures and contact times. Experiments are conducted at three contact pressures Pc = 100 kPa, 230 kPa and 400 kPa and eight contact times tc = 10 ms, 30 ms, 50 ms, 100 ms, 500 ms, 1 s, 5 s, and 10 s under constant bonding temperature, Tb = 40 degrees C and debonding rate, dbr = 5 mm/s. Results demonstrate that, for all the pressures studied, both peak stress, 6max, and critical energy release rate, GIC, are significantly lower at short contact timescales, with 6max and GIC reduced by 55-70 % and 60-75 %, respectively, compared to long contact timescales. At long contact timescales, increasing the pressure beyond 230 kPa has a limited effect on bond formation, while bond formation at short contact timescales shows a stronger dependency on contact pressure. Scaling analysis of the results from 10 ms to 1 s reveals relationships of 6max proportional to tc1/3.0 and GIC proportional to t1/2.75 c at Pc = 230 kPa, and 6max proportional to t1/3.4 c and GIC proportional to t1/3.1 pressures, suggesting a reasonable correlation to the theoretical degree of bonding models based on degree of intimate contact and interdiffusion. These findings highlight the influence of contact time and pressure on IBST, providing valuable insights for optimizing the process conditions during automated fiber/tow placement manufacturing.

This study provides an in-depth analysis of the mechanical behavior of rotating-square auxetic structures under various strain rates. The structures are fabricated using stereolithography additive manufacturing with a flexible resin. Mechanical tests performed on structures include quasi-static, intermediate, and high strain rate compression tests, supplemented by high-speed optical imaging and two-dimensional digital image correlation analyses. In quasi-static conditions (5 x 10(-3) s(-1)), multiscale measurements reveal the correlation between local and global strains. It is shown that cell hinges play a significant role in structural deformation and load-bearing capacity. In drop tower impact conditions (intermediate strain rate of ca. 200 s(-1)), the auxetic structures display significant strain rate hardening compared to loading at quasi-static rates. The thin-hinge structures maintain a Poisson's ratio of approximately -0.8, showing higher auxeticity than slow-rate compression tests. High strain rate conditions (ca. 2000s(-1)) activate additional deformation mechanisms, including a delayed state of equilibrium exemplified by a heterogeneous distribution of lateral strains, possibly due to stress wave interactions and inertial stresses. The study further reveals nonlinear correlations between Poisson's ratio, strain, and strain rate, indicating reduced auxeticity at higher strain rates. These observations are discussed in terms of complex wave interactions and the strain rate hardening characteristics of the base polymer.

In-situ bond strength toughness (IBST), commonly referred to as tow tackiness, is a first order property affecting the adherence quality and defect formation (e.g., wrinkles, folds) during automated tow placement (ATP) processing of uncured thermoset polymer matrix composite (PMC) tows. Tow-tow IBST develops over characteristic millisecond timescales (tc≤50ms) due to the rapid tow placement velocity of ∼1 m/s. In this paper, an experimental method is presented to determine millisecond timescale tow-tow mode-I IBST in terms of fracture mechanics-based traction-separation relationship. The test specimen consists of two tows bonded to rigid platens with a pre-crack between them to induce crack initiation. Experiments are performed using IM7-G/8552 carbon/epoxy prepregs for both long and short timescales at a constant debonding rate of 5 mm/s, contact pressure of 0.23 MPa, and bonding temperature of 40 °C. In addition, high-speed two-dimensional digital image correlation (2D-DIC) is used to image and measure the tow-tow interfacial deformation during debonding. The peak traction and apparent energy release rate are found to significantly decrease (about 60 %) at the short millisecond timescale contact hold times compared to longer (i.e., second) timescale.

Due to their exceptional impact resistance capabilities, density-graded cellular materials have immense potential in applications where crashworthiness requirements are of prime importance. Under impact loading, the deformation in these materials is characterized by compaction front propagation. Previous studies have utilized numerical techniques to solve the equations governing compaction wave propagation for cellular materials with density gradation. In this study, analytical solutions are formulated using compaction front position as the independent variable. The expressions for the velocity of the impinging rigid mass, energy absorption capacity, incident stress, and transmitted stress are determined. The analytical solutions are shown to be in excellent agreement with the cell-based finite element solutions. The effect of density gradient on energy absorption and stresses is studied. The impact resistance factor is employed to assess different density-graded cellular materials for their effectiveness in impact mitigation. This study demonstrates that density-graded cellular materials can offer superior impact protection to objects at both the incident and transmitted ends. Lower density toward the incident end enhances impact resistance to objects located there, and likewise, lower density at the transmitted end offers more impact resistance to objects at that end.

To compare finite element analysis (FEA) predictions and stereovision digital image correlation (StereoDIC) strain measurements at the same spatial positions throughout a region of interest, a field comparison procedure is developed. The procedure includes (a) conversion of the finite element data into a triangular mesh, (b) selection of a common coordinate system, (c) determination of the rigid body transformation to place both measurements and FEA data in the same system and (d) interpolation of the FEA nodal information to the same spatial locations as the StereoDIC measurements using barycentric coordinates. For an aluminum Al-6061 double edge notched tensile specimen, FEA results are obtained using both the von Mises isotropic yield criterion and Hill’s quadratic anisotropic yield criterion, with the unknown Hill model parameters determined using full-field specimen strain measurements for the nominally plane stress specimen. Using Hill’s quadratic anisotropic yield criterion, the point-by-point comparison of experimentally based full-field strains and stresses to finite element predictions are shown to be in excellent agreement, confirming the effectiveness of the field comparison process.

A direct approach based on finite element formulation is described to determine material property distribution in a nominally heterogeneous material subject to tensile/compression loading. The formulation is developed for plane stress applications using basic theoretical constructs, resulting in a computational framework that has a matrix form [A] {E} = {F}, where the [A] matrix components are known functions of measured strain components and nodal coordinates, {F} components are known functions of body forces, applied loads and reactions and {E} components are the unknown material properties at discrete locations. A methodology for material property identification is outlined, involving measured strain components at discrete locations amid varying levels of random noise. The presented results illustrate the accuracy of the approach as well as it's sensitivity to noise.

In-situ bond strength toughness (IBST), commonly referred to as tow tackiness, is a first order property affecting the adherence quality and defect formation (e.g., wrinkles, folds) during automated tow placement (ATP) processing of uncured thermoset polymer matrix composite (PMC) tows. Tow-tow IBST develops over characteristic millisecond timescales (tc <= 50ms) due to the rapid tow placement velocity of -1 m/s. In this paper, an experimental technique is presented to determine millisecond timescale towtow mode-I IBST in terms of fracture mechanics-based traction-separation relationship. The test specimen consists of two tows bonded to rigid platens with a pre-crack between them to induce crack initiation. Experiments are performed using IM7-G/8552 carbon/epoxy prepregs for both long and short timescales at a constant debonding rate of 5 mm/s, contact pressure of 0.23 MPa, and bonding temperature of 40 degrees C. In addition, high-speed two-dimensional digital image correlation (2D-DIC) is used to image and measure the tow-tow interfacial deformation during debonding. The peak traction and apparent energy release rate are found to significantly decrease (about 60 %) at the short millisecond timescale contact hold times compared to longer (i.e., second) timescale. (c) 2024 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.

A novel hybrid experimental-computational study is performed to predict the flow fields and pressure distributions on the measured three-dimensional shapes of flexible, three-tab asphalt roofing shingles undergoing increasing uplift when exposed to hurricane velocity winds for two hours. To quantify the evolution of shingle shapes, StereoDIC analysis is used to measure the transient, full-field deformed shapes of full-sized, three-tab asphalt shingles that did not separation or failure when subjected to hurricane velocity winds for two hours. Based on physical observations during wind loading, the authors performed steady state computational fluid dynamics (CFD) simulations to predict the full-field pressure distributions on as-measured, uplifted three-dimensional shingle shapes at selected time instances during wind loading.Simulation predictions clearly show flow recirculation regions on both the front and top of the shingles that remain attached throughout wind loading and control the full-field uplift pressure distribution. For low velocity flow with maximum uplift ≤ 8.4 mm,, CFD-predicted pressures are in good agreement with prior measurements. For both low and high-speed flows, the model predictions indicate that high pressures are formed at the leading-edge, upstream of the sealant layer, with maximum pressure occurring near the tab cutouts along the leading-edge of the shingle, providing a physical basis for the observed higher uplift and increased potential for shingle failure in these regions. The combined experimental-computational studies provide a contemporary way to eliminate the difficulties associated with attachment of pressure sensors to flexible materials that can alter shingle response, providing the basis for future design improvements by delineating the physical processes controlling pressure loading and shingle uplift in hurricane velocity winds.

The murine aorta is a complex, heterogeneous structure that undergoes large and sometimes asymmetrical deformations under loading. For analytical convenience, mechanical behavior is predominantly described using global quantities that fail to capture critical local information essential to elucidating aortopathic processes. Here, in our methodological study, we used stereo digital image correlation (StereoDIC) to measure the strain profiles of speckle-patterned healthy and elastase-infused, pathological mouse aortas submerged in a temperature-controlled liquid medium. Our unique device rotates two 15-degree stereo-angle cameras that gather sequential digital images while simultaneously performing conventional biaxial pressure-diameter and force-length testing. A StereoDIC Variable Ray Origin (VRO) camera system model is employed to correct for high-magnification image refraction through hydrating physiological media. The resultant Green-Lagrange surface strain tensor was quantified at different blood vessel inflation pressures, axial extension ratios, and after aneurysm-initiating elastase exposure. Quantified results capture large, heterogeneous, inflation-related, circumferential strains that are drastically reduced in elastase-infused tissues. Shear strains, however, were very small on the tissue's surface. Spatially averaged StereoDIC-based strains were generally more detailed than those determined using conventional edge detection techniques.

Voronoi cellular structures (VCSs) are multifunctional materials that can be customized to specific design requirements by suitable density gradation. By locally changing the density, their response can be modulated, and therefore, they can be readily adapted to tailored requirements. Despite these merits, their fabrication is a challenge due to the complexities in the topology of their structure. Conventional manufacturing methods are inadequate to produce the designs dictated by specialized requirements. In this study, the technique for the fabrication of 3D VCSs with spatially varying density is developed using additive manufacturing. Open-celled VCSs are additively manufactured using photopolymer jetting technology that allows the creation of complex parts with high accuracy. The data processing required for generating 3D Voronoi models and managing their complex geometrical features is performed using Python scripts. Uniform-density and density-graded VCSs are fabricated by controlling the cell size, which directly influences their local density. Their dynamic response under the impact of a rigid mass is experimentally determined using a drop tower setup. The compression in the specimen is measured using digital image correlation with the help of a high-speed camera. It is observed that the deformation behavior of VCSs can be tailored by local density variation.

High performance carbon and glass fibers are widely used as reinforcements in composite material systems for aerospace, automotive, and defense applications. Modifications to fiber surface treatment (sizing) is one of the ways to improve the strength of fibers and hence the overall longitudinal tensile strength of the composite. Single fiber tensile tests at the millimeter scale are typically used to characterize the effect of sizing on fiber strength. However, the characteristic length-scale governing the composite failure due to a cluster of fiber breaks is in the micro-scales. To access such micro-scale gage-lengths, we aim to employ indenters of varying radii to transversely load fibers and use scanning electron microscope (SEM) with digital image correlation (DIC) to measure strains at these lengthscales. The use of DIC technique requires creation of a uniform, random, and high contrast speckle pattern on the fiber surface such as that shown in Figure 1. In this work, we investigate the formation of sub-microscale speckle pattern on carbon fiber surface via sputter deposition and pulsed laser deposition techniques (PLD) using Gold-Palladium (Au-Pd) and Niobium-doped SrTiO3 (Nb:STO) targets respectively. Different processing conditions are investigated for both sputter deposition: sputtering current and coating duration, and PLD: number of pulses respectively to create sub-micron scale patterns viable for micro-DIC on both sized and unsized carbon fibers. By varying the deposition conditions and SEM-imaging the deposited patterns on fibers, successful pattern formation at sub-micron scale is demonstrated for both as-received sized and unsized IM7 carbon fibers of average diameter 5.2 μm via sputter deposition and PLD respectively.

Failures of roof asphalt shingles under high winds have been reported to occur at wind speeds lower than the shingle performance classification. The understanding of the progressive failure mechanisms of shingles subjected to high-wind pressures has been hindered by difficulty in accurately quantifying uplifts and wind-induced pressures with high spatial resolution, especially for large-scale wind test setups that simulate realistic scenarios. This paper reports on an experimental study to assess the use of three-dimensional digital image correlation ('StereoDIC') to accurately measure full-field surface deformations of shingles installed on full-scale roof structures and subjected to realistic high winds. Deformation measurements were acquired on shingles installed on typical wood-frame roof panels and subjected to winds with speed up to 257 km/h (160 mph) using an outdoor wind tunnel. The specimens were designed to study the proposed test setup and StereoDIC measurement method for two different combinations of shingle surface coloration (and thus speckle pattern) and field of view. The experiments produced accurate local and full-field shingle deformation measurements. Different datasets were used to gain new insight into the influence of salient aspects of the wind test setup and deformation measurement method on the understanding of the temporal evolution of shingle uplift and failure mechanisms.