Challenges in Mechanics of Time Dependent Materials, Mechanics of Biological Systems and Materials & Micro-and Nanomechanics, Volume 2

Profuse mechanical twinning is a central element in the mechanical behavior of Magnesium alloys. In the attempt to further employ this lightest class of structural metals in engineering applications, this micro-mechanism underlies significant challenges due to its abrupt and unipolar activity. A significant trait in mechanical twinning is autocatalytic activity, crudely, twinning in one grain triggering the twin in the next. In a sharply textured Magnesium polycrystal, this entails a collaborative twinning front that abruptly advances across a vast number of grains, producing a Lüders-like banded deformation. An area-scanning version of DIC with optical microscopy has been instrumental in studying this deformation regime in situ, by producing quantitative strain data that bridges grain and sample length scales. These measurements elucidate the multi-scale nature of the bands, clearly showing that a sample-scale collaborative twin band is composed of a fine mesh of microscopic strain localization bands at the grain scale. To construct the physical connection of this strain localization mesh to the underlying collaborative twinning field, however, one typically requires a separate microscopy study where the twins are observed with electron microscopy. Recently, we have shown that a detailed microscopic image of the twin band field can be derived from the same images that are recorded for DIC analysis. This is achieved via an imaging mode called residual intensity that refers to a material-point-aligned subtraction of reference and deformed images—only the latter containing the local intensity offsets imposed by the twin bands. The result is a pixel-resolution imagery of the twins that can be superimposed over the calculated DIC strain fields. To maximize the benefits of this approach, DIC is conducted with high resolution optics and a maximal magnification for optical DIC with ~250 data points per grain (average grain size 13 μm).

In the current effort, we will make a more detailed analysis of the band boundary and its advance, similarly utilizing strain and residual intensity fields to obtain combined deformation and morphology data. As observed over residual intensity maps, the macroscopic band boundary comprises a comb-like array of twins that protrude into the dormant material. We detail the deformation/twin fields as the band advances normal to this boundary, namely, when the next section of the dormant material undergoes collaborative twinning.

Author studied model beams (with equal overhang) simply supported beam like a bridge girder. The objective of this study is to understand TDM (Time Dependent Materials) response due to transverse impact experimentally.

Two types of experiments were carried out.

First dynamic photoelasticity was used to study transverse impact on urethane rubber beam (PSM-4) by free fall of a striker from a height of 176.4 mm employing Fastax framing camera (12,000 frames per second). Isochromatic fringe photographs were recorded in a light-field polariscope for central and non-central impact on simply supported beam with equal overhang for three different types of spans, namely, 90 mm, 120 mm and 150 mm with three different mass of strikers 10.52 g, 14.02 g and 17.53 g, respectively. Beam-striker weight ratio (2.675), height of fall, and dimensions of beam were kept constant.

From these experiments author finds beam span of 120 mm is critical from designer’s point of view.

Author carried out second set of experiments with 120 mm beam span made of different materials to understand time dependent material response due to central impact.

A contact force transducer was fabricated (Goldar et al., Proc. 40th Anniversary Meeting, Spring Meeting, Society for Experimental Mechanics, Cleveland, Ohio, USA, pp 187–190, 1985) (with quartz crystals) to measure impact force using charge amplifier and storage oscilloscope. Second set of experiments were conducted on beams made of three different materials, namely, PMMA, Aluminium and layer composite PMMA-AL-PMMA with 120 mm beam span. The mass of striker was 41 g for central impact only.

Using electrical resistance strain gauges, contact force transducer peak-tensile strain vs. time and contact force vs. time were recorded to study time dependent material response.

To understand TDM response a nomogram indicating normalized Hertz’s constant, striker-beam weight and peak-tensile strain for the beams made of different materials were plotted.

In addition peak-tensile strains vs. time in PMMA beam impacted centrally by different strikers were also recorded.

Another nomogram indicating normalized Hertz’s constant, striker-beam weight ratio and peak-tensile strain for the PMMA beam under central impact were plotted.

Presence of small-amplitude ‘precursors’ and small-amplitude higher frequency oscillations in the strain-histories recorded and identified.

Parkinson’s disease is a neurodegenerative disease that affects nearly a million people in the United States. Currently there is no cure for the disease, but there are many attempts to manage the symptoms. One effort uses assistive devices, which can help patients cope with the most common symptom: hand tremors. The goal was to design a noninvasive, adjustable device that effectively suppresses hand tremors. To begin, theoretical models were developed to gain an understanding of the governing principles involved in the hand–device interface. Simulated models gave insight to design considerations. Research moved into prototyping which involved sketching, modeling, and fabrication. Alongside prototyping, initial testing was performed to view qualitative tremor suppression. From this iterative process, two designs (The String and The Pin) met the essential criteria. These devices went through further testing at various adjustment settings to quantify their tremor suppression capabilities. The desired capabilities involved effectively suppressing frequencies from 4 to 12 Hz, since Parkinsonian tremors range from 4 to 6 Hz (Zach et al., J Parkinsons Dis 5:471–474, 2015) and essential tremors occur at higher frequencies. The String design had an adjustable effective range of 4.7–11.5 Hz whereas The Pin was only adjustable to be effective from 3.2 to 6.2 Hz. The effectiveness was defined as a reduction in tremor response by an order of magnitude. Additionally, once fixed to address a certain tremor frequency, The String design was able to effectively suppress a wider range of frequencies when compared to The Pin design (U + 00B1 2.3 Hz and U + 00B1 0.55 Hz).

In this work, an Atomic Force Microscope (AFM) technique known as Contact Resonance (CR) Spectroscopy is used to measure the complex modulus, as a function of frequency, of a Delrin™ (Polyoxymethylene) sample. These CR experiments, along with AFM creep and relaxation experiments, are conducted at different temperatures, and a time temperature superposition scheme is developed and applied to construct storage and loss modulus master curves over a wide range of frequencies. Large arrays of classical, integer-order, differential equation-based viscoelastic models are typically used to extract the material parameters from the master curves, however, the resulting models exhibit ringing phenomena, poor extrapolation properties, and are numerically cumbersome due the large number of model parameters that are needed. To avoid these pitfalls, we apply fractional viscoelastic models to describe both the time and frequency dependent experimental data, and extract the corresponding mechanical parameters. Fractional viscoelastic models are based on differential equations with fractional derivatives. These models require fewer fitting parameters, compared to their integer-order counterparts, and naturally capture the power-law time domain responses observed in the Delrin™ material.

This study establishes a method for measuring displacement and strain of rubber materials with large and fast deformations using digital image correlation. Using the proposed method, the behavior of the crack tip, which is important for elucidating the growth of cracks is evaluated. A tensile load is applied to the rubber test piece containing the initial crack, and the state is photographed with a digital camera. The results show that oscillating variations displacement and strain rates near the crack from the start of crack growth to fracture are observed.

Here, we report in situ AFM tapping-phase study of polyurea’s nano and mesoscale phase transitions within the submicron-size field of view. To this end, we designed and assembled a novel in situ AFM loading device that keeps a reference point stationery within the observation window. Using this device, we acquired sequential AFM-tapping-mode phase images of polyurea’s nanophase evolution during relaxation under various fixed tensile strains up to 200%. We found that initial hard nano-domains fragment upon rapid loading and the fragmented hard phases go through various nano and mesoscale phase transitions. These fragmented pieces are recombined to form coarsened bicontinuous clusters during the relaxation process. The AFM in situ testing enables us to better understand dynamic-bond characteristics of segmented block copolymers. Interplay between the dynamic-bond characteristics of supramolecular interactions and the hard/soft-phase load/deformation sharing characteristics is believed to predict the self-healing and dynamic toughening mechanisms of polyurea.

Metal additive manufacturing allows for the fabrication of parts at the point of use as well as the manufacture of parts with complex geometries that would be difficult to manufacture via conventional methods (milling, casting, etc.). Additively manufactured parts are likely to contain internal defects due to the melt pool, powder material, and laser velocity conditions when printing. Two different types of defects were present in the CT scans of printed AlSi10Mg dogbones: spherical porosity and irregular porosity. Identification of these pores via a machine learning approach (i.e., support vector machines, convolutional neural networks, k-nearest neighbors’ classifiers) could be helpful with part qualification and inspections. The machine learning approach will aim to label the regions of porosity and label the type of porosity present. The results showed that a combination approach of Canny edge detection and a classification-based machine learning model (k-nearest neighbors or support vector machine) outperformed the convolutional neural network in segmenting and labeling different types of porosity.

In this work, compression tests have been carried out at different strain rate, from 10⁻³ to approximately 10³ s⁻¹, on agglomerated cork material. The quasi-static and low strain rate tests have been conducted by means of servo-pneumatic machine, whereas the high strain rate tests have been conducted by means of polymeric Hopkinson bar. The experimental results show a stress–strain relationship that is characterized by a typical S-shaped curve. As expected, the strength is observed to increase when the material is deformed at increasing strain rate. In addition, the properties during the relaxation phase have been considered as well, showing that the stress response is characterized by a rapid decrement while the deformation is almost completely recovered. The global mechanical behavior is found to be very well reproduced by a combination of constitutive models, which include compressible hyperelastic modeling and large-strain viscoelasticity. The matching between the experimental and analytical data is very precise in the monotonic loading phase. Moreover, considering a damage model of the Mullins type it is possible to reproduce reasonably well also the unloading phase.

Understanding the dynamical behavior of an oscillating bubble near a hydrogel–water interface is an interesting and important multiphase problem arising in many medical treatments including minimizing tissue damage during ultrasound and laser surgeries, guiding targeted drug deliveries, to name a few. Here, by using ultrahigh-speed videography, we captured the complex interaction of an inertial cavitated bubble at a soft hydrogel–water interface penetrating the gel-fluid (water) boundary. Next, we experimentally measured and numerically modeled the nonlinear bubble dynamics near the hydrogel–water interface. Here we present our experimentally observed interface penetration process including annular jetting and shock wave propagation toward the water side. On the gel side we observed the induction of significant large and complex deformations induced by the cavitation bubble. We provide a comprehensive analysis of these phenomena including a quantitative estimate of the associated material strains and damage during this high strain-rate penetration process.

The effect of drying on the tensile behavior of a dual cross-linked poly(vinyl alcohol) (PVA) hydrogel is studied here. This gel contains about 90% water when fully hydrated. The mass–volume relationship of the gel is measured using a microbalance with a density kit. Our results show that as the gel dries the volume is linearly proportional to the mass. The impact of drying on the gel’s mechanical properties is measured in uniaxial tension tests, which include loading-unloading tests at three different constant stretch rates, a complex loading history test and a stress-relaxation test. Data from specimens with different hydration levels can be described by a constitutive model of the gel. The results show that the model parameters are strongly dependent on hydration level and that as the gels dry, the gels are much stiffer than those in the fully hydrated state.

Unlike traditional structural materials, soft solids can often sustain very large deformation before failure, and many exhibit nonlinear viscoelastic behavior. Modeling nonlinear viscoelasticity is a challenging problem for a number of reasons. In particular, a large number of material parameters are needed to capture material response and validation of models can be hindered by limited amounts of experimental data available. We have developed a Gaussian Process (GP) approach to determine the material parameters of a constitutive model describing the mechanical behavior of a soft, viscoelastic PVA hydrogel. A large number of stress histories generated by the constitutive model constitute the training sets. The low-rank representations of stress histories by Singular Value Decomposition (SVD) are taken to be random variables which can be modeled via Gaussian Processes with respect to the material parameters of the constitutive model. We obtain optimal material parameters by minimizing an objective function over the input set. We find that there are many good sets of parameters. Further the process reveals relationships between the model parameters. Results so far show that GP has great potential in fitting constitutive models.

Biofilm formation is a significant problem in America, accounting for 17 million infections, and causing 550,000 deaths annually. An understanding of factors that contribute to strong biofilm surface adhesion at implant interfaces can guide the development of surfaces that prevent deleterious biofilms and promote osseointegration. The aim of this research is to develop a metric that quantifies the adhesion strength differential between a bacterial biofilm and an osteoblast-like cell monolayer to a medical implant-simulant surface. This metric will be used to quantify the biocompatible effect of implant surfaces on bacterial and cell adhesion. The laser spallation technique employs a high-amplitude short-duration stress wave to initiate spallation of biological films. Attenuation of laser energy results in failure statistics across increasing fluence values, which are calibrated via interferometry to obtain interface stress values. Several metrology challenges were overcome including how membrane tension may influence laser spallation testing and how to determine stress wave characteristics when surface roughness precludes in situ displacement measurements via interferometry. Experiments relating loading region within biofilm to centroid of biofilm revealed that location played no role in failure rate. A reflective panel was implemented to measure stress wave characteristics on smooth and rough titanium, which showed no difference in peak compressive wave amplitude. After overcoming these metrology challenges, the adhesion strength of Streptococcus mutans biofilms and MG 63 monolayers on smooth and rough titanium substrates is measured. An Adhesion Index is developed by obtaining the ratio of cell adhesion to biofilm adhesion. This nondimensionalized parameter represents the effect of surface modifications on increases or decreases in biocompatibility. An increase in Adhesion Index value is calculated for roughened titanium compared to smooth titanium. The increase in Adhesion Index values indicates that the increase in surface roughness has a more positive biological response from MG 63 than does S. mutans. In this work further experiments quantifying impact of various surface coating including blood plasma, and adhesion proteins found within the extracellular matrix to expand the Adhesion Index.

The efforts reported here are focused on mitigating unnecessary mass in aerospace hardware. The approach to remove this undesired mass from the design is to leverage both the dynamic strength of materials and the frequency dependency of strain. Analytically predicted dynamic responses of structures are often applied as static loads in stress analyses that ultimately dictate the weight of a structural design. Assuming a dynamic response is a static load and then comparing resulting stress predictions to a static strength property is a long-standing engineering practice. Doing so is known to be, or is assumed to be, conservative. However, little indication of the order of magnitude of embedded conservatism has been identified. NASA/MSFC efforts in 2011, 2019, 2020, and now in 2021 have begun to qualitatively show the order of magnitude of that conservatism. A quick turnaround engineering method is pursued to leverage the subject facets of physics for the purpose of decreasing the weight of flight hardware. Tests performed using simple beams and significant observations are described.

The cladding acts as the primary protection barrier for nuclear fuel in nuclear reactors. Following the aftermath of loss of coolant accident at Fukushima Daiichi plant in 2011, considerable international efforts have been directed toward developing newly Accident-Tolerant Fuel and cladding (ATF) materials for light water reactors (LWRs) as alternatives to zirconium-based alloys. The main purpose of ATF materials is to maintain the integrity of the core components under normal operation and to slow core degradation at a high-temperature steam environment under severe accident scenarios such as loss of coolant. Preliminary investigations suggest that Iron-Chromium-Aluminum (FeCrAl) based alloy system can be utilized for cladding applications in LWRs due to their excellent high-temperature steam oxidation, corrosion performance and high-temperature strength. To promote a better understanding about failure behavior, stress rupture tests for thin-walled tubing FeCrAl C26M2 grade have been conducted at temperatures ranging from 480 to 650 °C. The tubes were pressurized by inert Argon gas in the pressure range of 500–6200 psi (corresponding to hoop stresses of 170–376 MPa using the thin-tube approximation). Time to rupture, uniform strain, steady-state creep rate were determined and analyzed as functions of the temperatures and applied pressures. An apparent decrease of burst time was observed with increasing pressure/temperature. Using burst data of FeCrAl alloy obtained in this study, Larson–Miller Parameter and Monkman–Grant relationship were developed and compared with other ferritic alloys. Further, high temperature deformation behavior was found to obey the power-law creep with stress exponent of 5.2 ± 0.9 and an activation energy of 290 ± 32 kJ/mol. In addition, the FeCrAl tubing was found to fail by two different fracture modes: either by direct open-up or small crack and pinhole formation depending on applied temperature and pressure. The results are discussed and compared with commonly used cladding materials.