Showing papers in "Experimental Mechanics in 2021"

Multiscale DIC applied to Pantographic Structures

Abstract: Background: Since the mechanical behavior of pantographic metamaterials depends upon the properties of their microstructure, accurate descriptions of unit cells are needed. Objective: The present effort is motivated by this requirement to characterize the detailed deformation of unit cells formed of two orthogonal sets of 3 beams. Methods: Their deformations in a bias extension test were measured via digital image correlation performed at different scales. Results: Thanks to the gray level residuals, the microscale results were found in better agreement with the experiment than mesoscale and macroscale analyses. Fine analyses around the hinges showed that relative displacements occurred between the two beam layers. Conclusions: Such experimental analyses supply full-field data to validate models (e.g., as a starting point in homogenization procedures) for describing the mechanical behavior of pantographic metamaterials.

Overview of High-temperature Deformation Measurement Using Digital Image Correlation

Abstract: Developments in digital image correlation (DIC) in the last decade have made it a practical and effective optical technique for displacement and strain measurement at high temperatures. This overview aims to review the research progress, summarize the experience and provide valuable references for the high-temperature deformation measurement using DIC. We comprehensively summarize challenges and recent advances in high-temperature DIC techniques. Fundamental principles of high-temperature DIC and various approaches to generate thermal environment or apply thermal loading are briefly introduced first. Then, the three primary challenges presented in performing high-temperature DIC measurements, i.e., 1). image saturation caused by intensified thermal radiation of heated sample and surrounding heating elements, 2) image contrast reduction due to surface oxidation of the heated sample and speckle pattern debonding, and 3) image distortion due to heat haze between the sample and the heating source, and corresponding countermeasures (i.e., the suppression of thermal radiation, fabrication of high-temperature speckle pattern and mitigation of heat haze) are discussed in detail. Next, typical applications of high-temperature DIC at various spatial scales are briefly described. Finally, remaining unsolved problems and future goals in high-temperature deformation measurements using DIC are also provided. We expect this review can guide to build a suitable DIC system for kinematic field measurements at high temperatures and solve the challenging problems that may be encountered during real tests.

Dynamic Tensile Response of a Microwave Damaged Granitic Rock

Abstract: Understanding the dynamic tensile response of microwave damaged rock is of great significance to promote the development of microwave-assisted hard rock breakage technology. However, most of the current research on this issue is limited to static loading conditions, which is inconsistent with the dynamic stress circumstances encountered in real rock-breaking operations. The objective of this work is to investigate the effects of microwave irradiation on the dynamic tensile strength, full-field displacement distribution and average fracture energy of a granitic rock. The split Hopkinson pressure bar (SHPB) system combined with digital image correlation (DIC) technique is adopted to conduct the experiments. The overload phenomenon, which refers to the strength over-estimation phenomenon in the Brazilian test, is validated using the conventional strain gauge method. Based on the DIC analysis, a new approach for calculating the average fracture energy is proposed. Experimental results show that both the apparent and true tensile strengths increase with the loading rate while decreasing with the increase of the irradiation duration; and the true tensile strength after overload correction is lower than the apparent strength. Besides, the overload ratio and fracture energy also show the loading rate and irradiation duration dependency. Our findings prove clearly that microwave irradiation significantly weakens the dynamic tensile properties of granitic rock.

The Effect of the Ill-posed Problem on Quantitative Error Assessment in Digital Image Correlation

Abstract: This work explores the effect of the ill-posed problem on uncertainty quantification for motion estimation using digital image correlation (DIC) (Sutton et al. [2009]). We develop a correction factor for standard uncertainty estimates based on the cosine of the angle between the true motion and the image gradients, in an integral sense over a subregion of the image. This correction factor accounts for variability in the DIC solution previously unaccounted for when considering only image noise, interpolation bias, contrast, and the software settings such as subset size and spacing.

Experimental Validation of the Attenuation Properties in the Sonic Range of Metaconcrete Containing Two Types of Resonant Inclusions

Abstract: Metaconcrete is a new concept of concrete, showing marked attenuation properties under impact and blast loading, where traditional aggregates are partially replaced by resonant bi-material inclusions. In a departure from conventional mechanical metamaterials, the inclusions are dispersed randomly as cast in the material. The behavior of metaconcrete at supersonic frequencies has been investigated theoretically and numerically and confirmed experimentally. The feasibility of metaconcrete to achieve wave attenuation at low frequencies demands further experimental validation. The present study is directed at characterizing dynamically, in the range of the low sonic frequencies, the—possibly synergistic—effect of combinations of different types of inclusions on the attenuation properties of metaconcrete. Dynamic tests are conducted on cylindrical metaconcrete specimens cast with two types of spherical inclusions, made of a steel core and a polymeric coating. The two inclusions differ in terms of size and coating material: type 1 inclusions are 22 mm diameter with 1.35 mm PDMS coating; type 2 inclusions are 24 mm diameter with 2 mm layer natural rubber coating. Linear frequency sweeps in the low sonic range (< 10 kHz), tuned to numerically estimated inclusion eigenfrequencies, are applied to the specimens through a mechanical actuator. The transmitted waves are recorded by transducers and Fast-Fourier transformed (FFT) to bring the attenuation spectrum of the material into full display. Amplitude reductions of transmitted signals are markedly visible for any metaconcrete specimens in the range of the inclusion resonant frequencies, namely, 3,400-3,500 Hz for the PDMS coating inclusions and near 3,200 Hz for the natural rubber coating inclusions. Specimens with mixed inclusions provide a rather uniform attenuation in a limited range of frequencies, independently of the inclusion density, while specimens with a single inclusion type are effective over larger frequency ranges. With respect to conventional concrete, metaconcrete reduces up to 90% the amplitude of the transmitted signal within the attenuation bands. Relative to conventional concrete, metaconcrete strongly attenuates waves over frequency bands determined by the resonant frequencies of the inclusions. The present dynamical tests conducted in the sonic range of frequencies quantify the attenuation properties of the metaconcrete cast with two types inclusions, providing location, range and intensity of the attenuation bands, which are dependent on the physical-geometric features of the inclusions.

New Methodologies for Fracture Detection of Automotive Steels in Tight Radius Bending: Application to the VDA 238–100 V-Bend Test

Abstract: The VDA 238–100 tight radius V-bend test can be used to efficiently characterize the bendability and fracture limits of sheet metals in severe plane strain bending. Material performance in plane strain bending is critical for the selection of advanced high strength steels for energy absorbing structural components. The detection of failure based upon a reduction in the punch force can lead to erroneous predictions of failure for ductile or thin gage alloys in the VDA 238–100 test. New failure criteria were proposed and evaluated across a range of automotive steels. Four detection methods in the V-bend test were evaluated based upon the load drop, bending moment, novel stress metric and the strain rate for seven steels with strength levels from 270 to 1500 MPa. The appropriate failure threshold was identified from visual inspection of the surface during bending. The vertical punch force will decrease as a consequence of the mechanics in the V-bend test at intermediate bend angles even without fracture. The novel stress-based metric accounts for sheet thinning and could successfully identify “false positives” and punch lift-off when considering the strain-rate evolution. Failure detection using the VDA load threshold method may significantly under-report the bend performance of alloys with intermediate-to-high bendability or thin gauges. The proposed stress-based metric can reliably detect fracture for bend angles in excess of 160° and be readily calculated using the existing data. The VDA load threshold for failure can work well for materials that exhibit significant cracking.

Intermethod Comparison and Evaluation of Measured Near Surface Residual Stress in Milled Aluminum

Abstract: While near surface residual stress (NSRS) from milling is a driver for distortion in aluminum parts there are few studies that directly compare available techniques for NSRS measurement. We report application and assessment of four different techniques for evaluating residual stress versus depth in milled aluminum parts. The four techniques are: hole-drilling, slotting, cos(α) x-ray diffraction (XRD), and sin2(ψ) XRD, all including incremental material removal to produce a stress versus depth profile. The milled aluminum parts are cut from stress-relieved plate, AA7050-T7451, with a range of table and tool speeds used to mill a large flat surface in several samples. NSRS measurements are made at specified locations on each sample. Resulting data show that NSRS from three techniques are in general agreement: hole-drilling, slotting, and sin2(ψ) XRD. At shallow depths ( 0.04 mm) hole-drilling and slotting have the best repeatability (< 10 MPa). NSRS data from cos(α) XRD differ from data provided by other techniques and the data are less repeatable. NSRS data for different milling parameters show that the depth of NSRS increases with feed per tooth and is unaffected by cutting speed. Hole-drilling, slotting, and sin2(ψ) XRD provided comparable results when assessing milling-induced near surface residual stress in aluminum. Combining a simple distortion test, comprising removal of a 1 mm thick wafer at the milled surface, with a companion stress analysis showed that NSRS data from hole-drilling are most consistent with milling-induced distortion.

Time-Resolved Digital Image Correlation in the Scanning Electron Microscope for Analysis of Time-Dependent Mechanisms

Abstract: Background: Advancements in the Digitial Image Correlation (DIC) technique over the past decade have greatly improved spatial resolution. However, many processes, such as plastic deformation, have a temporal component spanning from fractions of a second to minutes that has not yet been addressed in detail, particularly for DIC conducted in-situ in the scanning electron microscope (SEM). Objective: To develop a methodology for conducting time-resolved digital image correlation in the SEM for analysis of time-dependent mechanical deformation phenomena. Methods: Microscope and electron beam scanning parameters that influence the rate at which time-resolved DIC information is mapped are experimentally investigated, providing a guide for use over a range of timescales and resolutions. Results: Time-resolved DIC imaging is demonstrated on a Ti-7Al alloy, where slip band propagation is resolved with imaging dwell times of seconds. The limits of strain resolution and strain collection speeds are analyzed. Conclusions: The new developed methodology can be applied to a wide range of materials loaded in-situ to quantify time-dependent plastic deformation phenomena.

Smart Digital Image Correlation Patterns via 3D Printing

Abstract: Digital Image Correlation (DIC) is a popular experimental technique for measuring full-field deformations in materials. Accurate motion and displacement field reconstruction in DIC depend heavily on the intrinsic material texture or speckle patterns painted on the material prior to deformation. Many traditional techniques such as spray painting, ink stamping, or manual texturizing have provided adequate performance but are often challenging to apply on highly compliant, porous or non-planar surfaces. To address this challenge we present a new, robust and efficient technique to print DIC speckle dot patterns on both planar and non-planar sample surfaces using a custom-modified 3D printer in an automated fashion. In this new technique, a 3D printer is modified by replacing the conventional extrusion head with a syringe filled with ink. The motion of the 3D printer is controlled via customizable G-code scripts, precisely controlling both the extrusion volume and spatial positioning of the print head in a well-controlled and predictable fashion. The printed speckle dots have radii on the order of O( $$10^2$$ ) $$\upmu$$ m, and the subsequent DIC reconstructed deformations have an accuracy on the order of O( $$10^{-2}$$ ) pixels and O( $$10^{-4}$$ ) in measuring displacements and strains, respectively. Furthermore, we demonstrate that this technique has the capability to print suitable patterns for tracking large and heterogeneous deformations in highly compliant and porous materials, as well as materials with significant 3D topographies.

Cool, Dry, Nano-scale DIC Patterning of Delicate, Heterogeneous, Non-planar Specimens by Micro-mist Nebulization

Abstract: Background: Application of patterns to enable high-resolution Digital Image Correlation (DIC) at the small scale (μm/nm) is known to be very challenging as techniques developed for the macro- and mesoscale, such as spray painting, cannot be scaled down directly. Moreover, existing nano-patterning techniques all rely on harsh processing steps, based on high temperature, chemicals, physical contact, liquids, and/or high vacuum, that can easily damage fragile, small-scale, free-standing and/or hygro-sensitive specimens, such as MEMS or biological samples. Objective: To present a straightforward, inexpensive technique specially designed for nano-patterning highly delicate specimens for high-resolution DIC. Methods: The technique consists in a well-controlled nebulized micro-mist, containing predominantly no more than one nanoparticle per mist droplet. The micro-mist is subsequently dried, resulting in a flow of individual nanoparticles that are deposited on the specimen surface at near-room temperature. By having single nanoparticles falling on the specimen surface, the notoriously challenging task of controlling nanoparticle-nanoparticle and nanoparticle-surface interactions as a result of the complex droplet drying dynamics, e.g., in drop-casting, is circumvented. Results: High-quality patterns are demonstrated for a number of challenging cases of physically and chemically sensitive specimens with nanoparticles from 1 μm down to 50 nm in diameter. It is shown that the pattern can easily be scaled within (and probably beyond) this range, which is of special interest for micromechanical testing using in-situ microscopic imaging techniques, such as high-magnification optical microscopy, optical profilometry, atomic force microscopy, and scanning electron microscopy, etc. Conclusions: Delicate specimens can conveniently be patterned at near-room temperature ( $\sim $ 37 ∘C), without exposure to chemicals, physical contact or vacuum, while the pattern density and speckle size can be easily tuned.

Machine Learning Neural-Network Predictions for Grain-Boundary Strain Accumulation in a Polycrystalline Metal

Abstract: Microstructural features such as grain boundaries play a significant role in the macroscopic plastic response of polycrystalline metals. However, a quantitative link between plastic strain accumulation at grain boundaries and material response in plasticity dominated phenomena is still lacking. Here we seek to develop predictive relations between a material’s granular microstructure and the accumulation of plastic strains at the microstructural level during plastic deformation. A single-input neural network approach was applied to predict the residual plastic strain fields at regions surrounding grain boundaries of an austenitic stainless steel. The neural network was trained on data obtained by applying a very-high resolution digital image correlation (DIC) experimental technique that allows the measurement of grain-scale strains aligned to the underlying microstructure obtained from electron backscatter diffraction (EBSD) scans. The neural-network-predicted and the DIC-measured strain fields showed good correlation for most of the tested cases. Best individual agreement was found when each microstructure was used to predict fields in its own case. However, best overall average predictions were seen when multiple samples were used for the network training. The results showed that the local geometrical angle between a grain boundary and the loading axes is in many cases a good predictor for the accumulation of strains at the given boundary. The expected limitations of this single parameter approach (grain boundary angle alone cannot be a good predictor for varying strains along a straight grain boundary, for example) were seen as the reason for the situations where predictions were not as good.

Obtaining a Wide-Strain-Range True Stress–Strain Curve Using the Measurement-In-Neck-Section Method

Abstract: Measuring true stress–strain curve over a large-strain-range is essential to understand mechanical behavior and simulate non-linear plastic deformation. The digital image correlation (DIC) technique, a non-contact full-field optical measurement technique, is a promising candidate to obtain a long-range true stress–strain curve experimentally. This paper proposes a method for measuring true stress–strain curves over a large-strain-range during tensile testing using DIC. The wide-strain-range true stress–strain curves of dual-phase and low carbon steels were extracted on the transverse direction in the neck region. The axial strain on the neck section was estimated by averaging the inhomogeneous deformation on the cross-section of the tensile specimen. The true stress was calculated from the engineering stress and the cross-sectional area of the neck. The validity of the proposed method was assessed by comparing the experimental load–displacement responses during tensile testing with the finite element method (FEM) simulation results. The stress and strain on the neck section estimated using the FEM and DIC, respectively, were proven to satisfy the uniaxial condition and successfully obtained. The experimental results agree well with the FEM results. The proposed concept can be applied to various deformation modes for accurately measuring long-range true stress–strain curves.

Recent Advances in Nanotribology of Ionic Liquids

Abstract: Ionic liquids (ILs) have recently attracted considerable attention in tribology owing to their unique physico-chemical properties and promising lubrication performance when used in a wide range of material pairs. The aim of this review article is to summarize recent advances in our knowledge related to the lubrication mechanisms of neat ILs, with a particular focus on nanoscale studies dealing with the behavior of ILs in the boundary lubrication regime. We first discuss the current state-of-the-art concerning the normal pressure-dependent lubrication mechanism of ILs and then focus on the dynamic behavior of ILs upon nanoconfinment. Finally, we summarize recent research efforts aiming to control the tribological response of ILs by changing the surface charge density, evaluate the effects of impurities on the lubricity of ILs, and shed light on the IL tribochemistry at small length scales. While the field of IL-mediated lubrication has made significant progress, several open questions still remain, including the effects of temperature, impurities, and surface roughness on the friction response and dynamic behaviors of nanoconfined ILs. Additionally, a mechanistic understanding of the tribochemical reactivity of ILs is still lacking. To harness the full potential of ILs for tribological applications, significant work is still required to establish links between the IL structure, lubrication mechanism(s), and performance. These advancements will be instrumental for the predictive design, development, and implementation of ILs with enhanced tribological properties in next-generation lubricants for a variety of applications across several sectors, including manufacturing and transportation.

Laser-Ultrasonic Characterization of Strongly Anisotropic Materials by Transient Grating Spectroscopy

Abstract: Transient grating spectroscopy (TGS) is a laser-ultrasonic method allowing measurement of the surface acoustic wave (SAW) velocity in an examined material for a given direction of the wave vector. We explore the capability of TGS for determination of shear elastic coefficients ( $$c^\prime$$ and $$c_{44}$$ ) of strongly anisotropic cubic materials. TGS is tested on a set of single crystals with an anisotropy factor up to $$A=25$$ . Using a numerical simulation based on a Ritz-Rayleigh approach, we show that strong anisotropy may lead to significant coupling of SAWs with bulk shear waves, which complicates TGS measurements in specific directions. Based on the obtained TGS data, we discuss the possibility of also using the TGS technique for assessing the longitudinal elastic coefficient ( $$c_L$$ ). Despite the energy focusing and other effects originating from the strong anisotropy, the TGS method can be used to reliably determine the directional dependence of the SAW velocity in these materials, and the resulting experimental datasets are sufficient for inverse determination of both the soft shear elastic constant ( $$c^\prime$$ ) and the hard shear elastic constant ( $$c_{44}$$ ). The longitudinal coefficient can be determined with lower accuracy. TGS is a suitable experimental tool for contactless characterization of strongly anisotropic materials.

Optimal Aperture and Digital Speckle Optimization in Digital Image Correlation

Abstract: Digital image correlation (DIC) method is a non-interference and non-contact full-field optical measurement technology. DIC’s measurement accuracy is largely determined by the image acquisition system and the quality of the speckle pattern. Although the optimal speckle pattern has been designed theoretically, the optimization process does not consider the influence of the imaging system. Furthermore, the optimal aperture has not been yield. The objective of this paper is to improve the measurement accuracy through optimization of aperture and speckle pattern. In this paper, speckle images with different apertures and different speckle generation parameters were captured, and the corresponding measurement errors were evaluated. (1) The influence of aperture and the optimization of speckle are independent of each other. The optimal speckle generation parameters (speckle size and density) are determined experimentally, which turn out to be consistent with existing theoretical models. (2) The optimal aperture is 5.6, because excessively large F-number causes the loss of high-frequency information and excessively small F-number leads to significant lens aberrations, both of which will reduce the measurement accuracy. The optimal aperture and speckle generation parameters (speckle diameter and density) are found and proved to be uncorrelated through actual experiments in practical conditions. These results provide experimental basis for the selection of parameters in actual engineering applications.

A Contactless Approach to Monitor Rail Vibrations

Abstract: Continuous welded rails (CWR) are subjected to thermal effects that may lead to buckling or fracture during warm or cold seasons, respectively. The modal characteristics (frequency and mode shapes) of CWR may reveal important information about the thermal stress that can be used to prevent rail failures. The primary objective of this study is to prove a contactless method to monitor the vibration and to extract the modal characteristics of rails using a high-speed camera and advanced image processing. This study is the first step towards a general noninvasive monitoring paradigm aimed at measuring axial stress in CWR. To prove the principles of the proposed paradigm, a finite element model of an unrestrained rail segment under varying length, boundary conditions, and axial stresses was formulated. The results of the model were then used to interpret the experimental results relative to a 2.4 m-long rail subjected to compressive loading–unloading cycles. During the experiment, the rail was subjected to the impact of an instrumented hammer and the triggered vibration was recorded with a high-speed camera. The videos were then processed using the phase-based displacement extraction, motion magnification, as well as dynamic mode decomposition techniques to extract the modal characteristics of the specimen. The results show that the frequencies extracted from the images matched well those obtained with two conventional accelerometers bonded to the rail while the mode shapes extracted from the videos matched those predicted numerically. Additionally, the numerical analysis enabled the interpretation of some unexpected experimental results. The results presented here proved that the proposed method to infer axial stress in CWR requires proper modeling in order to link the modal characteristics of the rails to the axial stress. In the future, the finite element formulation presented here will be expanded to model CWR under given cross-ties and fasteners conditions in order to link the modal characteristics of the rail of interest to its axial stress.

Interlaboratory Study of Digital Volume Correlation Error Due to X-Ray Computed Tomography Equipment and Scan Parameters: an Update from the DVC Challenge

Abstract: The quality of Digital Volume Correlation (DVC) full-field displacement measurements depends directly on the characteristics of the X-ray Computed Tomography (XCT) equipment, and scan procedures used to acquire the tomographic images. We seek to experimentally study the effects of XCT equipment and tomographic scan procedures on the quality of these images for DVC analysis, and to survey the level of DVC error that may be achieved using standard XCT operating procedures. Six participants in an interlaboratory study acquired high-quality XCT scans of a syntactic foam before and after rigid body motion. The resulting images were correlated using commercial DVC software to quantify error sources due to random image noise, reconstruction artifacts, as well as systematic spatial or temporal distortion. In the absence of rigid body motion, the standard deviation of the displacement measurements ranged from 0.012 to 0.043 voxels using a moderate subvolume size, indicating that subvoxel measurement resolution could readily be achieved with a variety of XCT equipment and scan recipes. Comparison of consecutive scans without rigid body motion showed transient dilatational displacement gradients due to self-heating of the X-ray source and/or thermal expansion of the foam. Evaluation of the scans after rigid body motion showed significant, machine-specific spatial distortion in the displacement fields of up to 0.5 voxels; new approaches to remove this error need to be developed. Analysis of the scan protocols used in the interlaboratory study, as well as a complementary parametric sensitivity study, showed that the DVC error was strongly influenced by the XCT equipment, but could be mitigated by adjusting the total scan duration.

Effect of Glycation on Interlamellar Bonding of Arterial Elastin

Abstract: Interlamellar bonding in the arterial wall is often compromised by cardiovascular diseases. However, several recent nationwide and hospital-based studies have uniformly reported reduced risk of thoracic aortic dissection in patients with diabetes. As one of the primary structural constituents in the arterial wall, elastin plays an important role in providing its interlamellar structural integrity. The purpose of this study is to examine the effects of glycation on the interlamellar bonding properties of arterial elastin. Purified elastin network was isolated from porcine descending thoracic aorta and incubated in 2 M glucose solution for 7, 14 or 21 days at 37 °C. Peeling and direct tension tests were performed to provide complimentary information on understanding the interlamellar layer separation properties of elastin network with glycation effect. Peeling tests were simulated using a cohesive zone model (CZM). Multiphoton imaging was used to visualize the interlamellar elastin fibers in samples subjected to peeling and direct tension. Peeling and direct tension tests show that interlamellar energy release rate and strength both increase with the duration of glucose treatment. The traction at damage initiation estimated for the CZM agrees well with the interlamellar strength measurements from direct tension tests. Glycation was also found to increase the interlamellar failure strain of arterial elastin. Multiphoton imaging confirmed the contribution of radially running elastin fibers to resisting dissection. Nonenzymatic glycation reduces the propensity of arterial elastin to dissection. This study also suggests that the CZM effectively describes the interlamellar bonding properties of arterial elastin.

Tricuspid Chordae Tendineae Mechanics: Insertion Site, Leaflet, and Size-Specific Analysis and Constitutive Modelling

Abstract: Background: Tricuspid valve chordae tendineae play a vital role in our cardiovascular system. They function as “parachute cords” to the tricuspid leaflets to prevent prolapse during systole. However, in contrast to the tricuspid annulus and leaflets, the tricuspid chordae tendineae have received little attention. Few previous studies have described their mechanics and their structure-function relationship. Objective: In this study, we aimed to quantify the mechanics of tricuspid chordae tendineae based on their leaflet of origin, insertion site, and size. Methods: Specifically, we uniaxially stretched 53 tricuspid chordae tendineae from sheep and recorded their stress-strain behavior. We also analyzed the microstructure of the tricuspid chordae tendineae based on two-photon microscopy and histology. Finally, we compared eight different hyperelastic constitutive models and their ability to fit our data. Results: We found that tricuspid chordae tendineae are highly organized collageneous tissues, which are populated with cells throughout their thickness. In uniaxial stretching, this microstructure causes the classic J-shaped nonlinear stress-strain response known from other collageneous tissues. We found differences in stiffness between tricuspid chordae tendineae from the anterior, posterior, or septal leaflets only at small strains. Similarly, we found significant differences based on their insertion site or size also only at small strains. Of the models we fit to our data, we recommend the Ogden two-parameter model. This model fit the data excellently and required a minimal number of parameters. For future use, we identified and reported the Ogden material parameters for an average data set. Conclusion: The data presented in this study help to explain the mechanics and structure-function relationship of tricuspid chordae tendineae and provide a model recommendation (with parameters) for use in computational simulations of the tricuspid valve.

Novel Double-Half Spot Weld Testing Technique For Damage Progress And Failure Analysis Using Digital Image Correlation Techniques

Abstract: In-situ spot weld failure analysis under complex loading conditions is significantly restricted by the enclosed space around the weld nugget. Failure can be only observed at the post-failure stage which provides only limited information of the manner in which damage progresses and leads to crack propagation. Novel testing and analysis techniques were developed to observe the through thickness evolution of cracks and failure in real-time. Two new double-half weld (DHW) specimens for normal and shear load testing, compatible with digital image correlation (DIC) strain measurement, were developed that can be used to directly observe and record damage processes and cracking within spot welded samples. The test samples were analyzed with a speckle pattern to create strain maps and with a macro-etched surface to observe in-situ crack propagation. This novel testing method was used to determine complex failure mechanisms/modes for spot welds in hot stamped, ultra-high strength Al-Si coated 22MnB5 steel. Interfacial and pull-out failure mechanisms in normal and shear loading were observed though the thickness of spot weld and were recorded for the first time. Interfacial failure is initiated by strain localization at the weld notch followed by corona debonding and crack propagation towards the weld nugget. In pull-out failure, the crack follows the shape of a transient softened zone at the fusion boundary, while, interfacial failure is dominated by ductile shearing at the faying surface of the sheets. The results for the DHW test geometries reveal complex failure mechanisms/modes in spot welds that can be quantified using DIC analysis.

Radiofrequency Ablation Alters the Microstructural Organization of Healthy and Enzymatically Digested Porcine Mitral Valves

Abstract: Myxomatous mitral valve degeneration is a common cause of mitral regurgitation and is often associated with mitral valve prolapse. With no known targets to pharmacologically treat mitral valve prolapse, surgery is often the only treatment option. Recently, radiofrequency ablation has been proposed as a percutaneous alternative to surgical resection for the reduction of mitral valve leaflet area. Using an in vitro model of porcine mitral valve anterior leaflet enlargement following enzymatic digestion, we sought to investigate mechanisms by which radiofrequency ablation alters the geometry, microstructural organization, and mechanical properties of healthy and digested leaflets. Paired measurements before and after radiofrequency ablation revealed the impact of ablation on leaflet properties. Multiphoton imaging was used to characterize changes in the structure and organization of the valvular extracellular matrix; planar biaxial mechanical testing and constitutive modeling were used to estimate mechanical properties of healthy and digested leaflets. Enzymatic digestion increased leaflet area and thickness to a similar extent as clinical mitral valve disease. Radiofrequency ablation altered extracellular matrix alignment and reduced the area of digested leaflets to that of control. Additionally, enzymatic digestion resulted in fiber alignment and reorientation toward the radial direction, causing increased forces during ablation and a structural stiffening which was improved by radiofrequency ablation. Radiofrequency ablation induces radial extracellular matrix alignment and effectively reduces the area of enlarged mitral valve leaflets. Hence, this technique may be a therapeutic approach for myxomatous mitral valve disease and is thus an avenue for future study.

A Novel Method for In Vitro 3D Imaging of Dissecting Pressurized Arterial Segments Using X-Ray Microtomography

Abstract: It is commonly admitted that a dissection initiates with an intimal tear or at least a defect inside the arterial wall Nevertheless, few studies investigated the initiation sequence due to the difficulty to monitor this process The objective of this work was to observe and investigate the mechanisms leading an intimal tear to propagate into a dissection A custom-made tension-inflation device fitting inside an X-ray microtomography setup was designed A notch was created inside a porcine carotid artery before performing the tension-inflation test The X-ray tomography setup allowed observing the wall-structure and the notch behavior during the inflation of the carotid artery A quantitative description of the notch morphology was performed, suggesting the prevalence of high shear stress in the region of the crack tip as a possible trigger for propagation of a dissection The present experimental approach allowed understanding better the mechanisms leading to dissection and constitutes a first step toward the improvement of failure modeling and risk assessment of this disease

Residual Intensity as a Morphological Identifier of Twinning Fields in Microscopic Image Correlation

Abstract: In the microscopic observation of deforming metals, it is well known that crystallite defects that accommodate strain can occasionally become visible, namely, they introduce image contrast to their locality. For microscopic digital image correlation (DIC) applications, this is typically known as a disturbance. Here, we explore a potential upside of these image-intensity offsets, to present a new mode of differential imaging that exclusively displays the underlying plasticity agents. For this, the intensity-offset signal is isolated with residual intensity, essentially the differential between reference- and deformed-configuration intensity of each material point. The premise is showcased over an autocatalytic twin band in Magnesium AZ31, with an advanced DIC instrument that utilizes bright-field optical microscopy. With robust area-scanning that utilizes in-situ corrective measures, a material field of around 5000 grains (13 μm average size) is sampled with a maximal intragranular resolution (~250 data points per grain) for this technique. For added robustness against the intensity alterations, a DIC algorithm (Augmented Lagrangian DIC) that enforces global kinematic compatibility constraints is utilized. The calculated residual intensity map yields a detailed image of the twin networks that show a strong positional alignment with the strain localizations. At the band boundary, the twins (and their accompanying strain localization) protrude into the dormant material in a comb-like pattern. With a combination of high-resolution optics and defects that alter the surface topography, residual intensity presents a new in-situ microscopy mode that is tied to the DIC analysis. This principle also offers potential micro-deformation imaging capabilities for various other material-microscopy combinations.

Mechanical Stimuli for Left Ventricular Growth During Pressure Overload

Abstract: The mechanical stimulus (i.e., stress or stretch) for growth occurring in the pressure-overloaded left ventricle (LV) is not exactly known. To address this issue, we investigate the correlation between local ventricular growth (indexed by local wall thickness) and the local acute changes in mechanical stimuli after aortic banding. LV geometric data were extracted from 3D echo measurements at baseline and 2 weeks in the aortic banding swine model (n = 4). We developed and calibrated animal-specific finite element (FE) model of LV mechanics against pressure and volume waveforms measured at baseline. After simulation of the acute effects of pressure-overload, the local changes of maximum, mean and minimum myocardial stretches and stresses in three orthogonal material directions (i.e., fiber, sheet and sheet-normal) over a cardiac cycle were quantified. Correlation between mechanical quantities and the corresponding measured local changes in wall thickness was quantified using the Pearson correlation number (PCN) and Spearman rank correlation number (SCN). At 2 weeks after banding, the average septum thickness decreased from 10.6 ± 2.92 mm to 9.49 ± 2.02 mm, whereas the LV free-wall thickness increased from 8.69 ± 1.64 mm to 9.4 ± 1.22 mm. The FE results show strong correlation of growth with the changes in maximum fiber stress (PCN = 0.5471, SCN = 0.5111) and changes in the mean sheet-normal stress (PCN = 0.5266, SCN = 0.5256). Myocardial stretches, however, do not have good correlation with growth. These results suggest that fiber stress is the mechanical stimuli for LV growth in pressure-overload.

An Experimental Methodology to Characterize the Plasticity of Sheet Metals from Uniaxial to Plane Strain Tension

Abstract: Although accurate knowledge of material behavior in plane strain tension is important for the modelling of sheet metal forming processes, it is often overlooked in yield function calibration because of experimental characterization challenges. Plane strain notch tensile tests, though experimentally convenient, are subject to stress and strain gradients across the gauge width that complicate the analysis. A novel experimental integration methodology was developed to exploit these stress and strain gradients to locally calibrate the arc of an anisotropic yield surface from uniaxial-to-plane strain tension. Constraining the anisotropic yield surface at the plane strain point, to be consistent with pressure-independent plasticity, enables the local arc to be governed by a single parameter. The arc shape is largely independent of the choice of yield function and can be optimized using a cutting line approach and full-field optical strain measurements. The accuracy of the method was evaluated using finite-element simulations of isotropic and anisotropic materials with different hardening behaviors. The methodology was applied to a dual phase DP1180 steel and AA5182-O aluminum alloy in the rolling, transverse, and diagonal directions. Data along each of the three locally calibrated arcs was included in calibrations of Yld2000 and Yld2004 yield surfaces. The plane strain yield strength and arc shape had significant implications on the calibration of advanced anisotropic yield criteria. The yield exponent of the DP1180 agreed with the common value of six for BCC metals while the AA5182 yield surface approximated a Tresca-shape with local yield exponents in excess of 20.

Investigation of Thermal Gradient Mechanical Fatigue Test Methods for Nickel-based Superalloys

Abstract: Temperature gradients significantly affect the material fatigue process. A reliable and robust test procedure is needed for quantifying the effects of temperature gradients on the evolution of fatigue damage in nickel-based superalloys. The present study aims to develop a radiation heating system for universal material testing machine for simulating thermal gradient mechanical fatigue in turbines. The developed heating system mainly consists of halogen lamps, reflectors, cooling subsystems, and control units. Based on extensive experimental and computational investigations, a thermal model is developed for accurate thermo-mechanical fatigue testing under given temperature gradients in the heating system, and it is applied to variable temperature fatigue tests. The detail procedure for determining heat transfer coefficients is presented based on experimental and computational results. TGMF tests are successfully performed with the developed radiation heating system. The temperature gradients are found to reduce the TGMF life significantly in comparison to that of the TMF life. It is confirmed that the developed thermal gradient mechanical fatigue methodology can be applied to different thermo-mechanical fatigue tests with various temperature gradients.

Drone-Based StereoDIC: System Development, Experimental Validation and Infrastructure Application

Abstract: Digital Image Correlation (DIC) is widely used for remote and non-destructive structural health evaluation of infrastructure. Current DIC applications are limited to relatively small areas of structures and require the use of stationary stereo vision camera systems that are not easy to transfer and deploy in remote areas. The enclosed work describes the development and validation of an Unmanned Aircraft System (UAS, commonly known as drone) with an onboard stereo-vision system capable of acquiring, storing and transmitting images for analysis to obtain full-field, three-dimensional displacement and strain measurements. The UAS equipped with a StereoDIC system has been developed and tested in the lab. The drone system, named DroneDIC, autonomously hovers in front of a prestressed railroad tie under pressure and DIC data are collected. A stationary DIC system is used in parallel to collect data for the railroad tie. We compare the data to validate the readings from the DroneDIC system. We present the analysis of the results obtained by both systems. Our study shows that the results we obtain from the DroneDIC system are similar to the ones gathered from the stationary DIC system. This work serves as a proof of concept for the successful integration of DIC and drone technologies into the DroneDIC system. DroneDIC combines the high accuracy inspection capabilities of traditional stationary DIC systems with the mobility offered by drone platforms. This is a major step towards autonomous DIC inspection in portions of a structure where access is difficult via conventional methods.

Pentagalloyl Glucose (PGG) Partially Prevents Arterial Mechanical Changes Due to Elastin Degradation

Abstract: Elastic fibers are composed primarily of the protein elastin and they provide reversible elasticity to the large arteries. Degradation of elastic fibers is a common histopathology in aortic aneurysms. Pentagalloyl glucose (PGG) has been shown to bind elastin and stabilize elastic fibers in some in vitro studies and in vivo models of abdominal aortic aneurysms, however its effects on native arteries are not well described. Perform detailed studies of the biomechanical effects of PGG on native arteries and the preventative capabilities of PGG for elastin degraded arteries. We treated mouse carotid arteries with PGG, elastase (ELA), and PGG + ELA and compared the wall structure, solid mechanics, and fluid transport properties to untreated (UNT) arteries. We found that PGG alone decreased compliance compared to UNT arteries, but did not affect any other structural or biomechanical measures. Mild (30 s) ELA treatment caused collapse and fragmentation of the elastic lamellae, plastic deformation, decreased compliance, increased modulus, and increased hydraulic conductance of the arterial wall compared to UNT. PGG + ELA treatment partially protected from all of these changes, in particular the plastic deformation. PGG mechanical protection varied considerably across PGG + ELA samples and appeared to correlate with the structural changes. Our results provide important considerations for the effects of PGG on native arteries and a baseline for further biomechanical studies on preventative elastic fiber stabilization.

Bending of Soft Micropatterns in Elastohydrodynamic Lubrication Tribology

Abstract: Soft tribology is increasingly important in the design and engineering of materials used in robotics, haptics, and biomechanics studies. When patterned surfaces are part of a lubricated tribopair that undergoes sliding and compressive deformation, the patterns experience a bending strain that affects the lubrication film thickness and elastohydrodynamic friction. The contribution of bending patterns to soft tribology is not well understood because earlier studies focused on hard tribopairs with effectively flat surfaces. We investigate and model the differences in lubricated friction for poly(dimethyl siloxane) (PDMS) elastomer and PEGDA/alginate double network hydrogel patterns in order to determine the effect of height-to-width aspect ratio and bending angle on the elastohydrodynamic friction. Photoresists of two different viscosities are spin coated onto silicon substrates to fabricate molds with pattern heights ranging from 20 μm to 50 μm. Tribological characterization of the tribopairs in the elastohydrodynamic lubrication regime shows that the patterns generate a friction peak that is independent of aspect ratio for short patterns but displays a “power-law fit” decrease with increasing aspect ratio for taller patterns. Two independent models are used to estimate the theoretical bending and deflection angles for the tribopairs. The decrease in lubricated friction is attributed to taller patterns having large bending angles and a reduced effective surface for fluid load bearing. Results suggest that the bending of micropatterns could be harnessed to engineer lubricated friction in a variety of applications.

3D Strain Field Evolution and Failure Mechanisms in Anisotropic Paperboard

Abstract: Experimental analyses of the 3D strain field evolution during loading allows for better understanding of deformation and failure mechanisms at the meso- and microscale in different materials. In order to understand the auxetic behaviour and delamination process in paperboard materials during tensile deformation, it is essential to study the out-of-plane component of the strain tensor that is, in contrast to previous 2D studies, only achievable in 3D. The main objective of this study is to obtain a better understanding of the influence of different out-of-plane structures and in-plane material directions on the deformation and failure mechanisms at the meso- and microscale in paperboard samples. X-ray tomography imaging during in-situ uniaxial tensile testing and Digital Volume Correlation analysis was performed to investigate the 3D strain field evolution and microscale mechanical behaviour in two different types of commercial paperboards and in two material directions. The evolution of sample properties such as the spatial variation in sample thickness, solid fraction and fibre orientation distribution were also obtained from the images. A comprehensive analysis of the full strain tensor in paperboards is lacking in previous research, and the influence of material directions and out-of-plane structures on 3D strain field patterns as well as the spatial and temporal quantification of the auxetic behaviour in paperboard are novel contributions. The results show that volumetric and deviatoric strain, dominated by the out-of-plane normal strain component of the strain tensor, localize in the out-of-plane centre already in the initial linear stress-strain regime. In-plane strain field patterns differ between samples loaded in the Machine Direction (MD) and Cross Direction (CD); in MD, strain localizes in a more well-defined zone close to the notches and the failure occurs abruptly at peak load, resulting in angular fracture paths extending through the stiffer surface planes of the samples. In CD, strain localizes in more horizontal and continuous bands between the notches and at peak load, fractures are not clearly visible at the surfaces of CD-tested samples that appear to fail internally through more well-distributed delamination. In-plane strain localization preceded a local increase of sample thickness, i.e. the initiation of the delamination process, and at peak load, a dramatic increase in average sample thickening occurred. Different in-plane material directions affected the angles and continuity of the in-plane strain patterns as well as the sample and fracture properties at failure, while the out-of-plane structure affected how the strain fields distributed within the samples.