Advancement of Optical Methods and Fracture and Fatigue, Volume 3

Blast exposure can result in tympanic membrane (TM) rupture and middle ear and inner ear damage. However, the dynamics of the blast-induced TM rupture are unclear, leaving questions about how the blast-induced stresses and strains on the TM surface produce TM rupture unanswered. Our group is developing quantitative high-speed optical techniques for real-time characterization of the fracture mechanics of the TM. Here we describe our efforts to produce a shock tube acoustic loading device that produces repeatable and accurate supersonic blasts with peak pressures capable of rupturing TM samples. The design of the shock tube was decided through structural analysis, and the pressures produced by the blast waves were verified with multiple high-speed pressure sensors. Schlieren imaging combined with a high-speed camera is used to “visualize” blast waves emitted from the shock tube opening and analyze how they propagate and interact with testing samples. This information is necessary to understand the influence of these waves on the dynamic behavior of the TM and how they induce damage of the TM.

Magnetic resonance imaging (MRI) provides a better visualization for diagnosis, interventional radiology, and surgery and enables the ability of intra-operative precise surgical procedures such as deep brain tumor ablation. However the limitation of using an MRI scanner with strong magnetic fields and constrained space inside the bore creates the need of using a robotic system to assist surgeons with MR-guided robot-assisted procedures. Piezoelectric ultrasonic motors are a novel class of actuators often used to drive these robots to operate in MRI environments. Hollow-type motors address the advantages of compact design with an end effector assembly, low rotary inertia, fast response time, and eliminating mechanical nonlinearities introduced by gearboxes. In this chapter, we present a custom-made ultrasonic hollow cylindrical motor with toward center amplitude and along circumference traveling waves to frictionally couple with the rotor. A finite element model of hollow stator using COMSOL Multiphysics is created in frequency- and time-dependent domains and validates the FEM simulation with time-averaged digital holography. Results show that the holographic images resulting in vibration patterns matched the FEM simulation on eigenfrequencies of excitation with the largest difference of 6.1% and yield the lowest Pearson correlation coefficient of 0.7113 with toward the center displacement curve compared to the FEM simulation results. This chapter shows the feasibility of a new type of ultrasonic motor used in the MRI environment.

This study deals with the measurement of the displacement and discontinuous strain distributions around fiber and matrix interface of CFRP. Not only finite element global digital image correlation (FE global DIC) but also global DIC and local DIC are used for this purpose, and the results are compared. FE global DIC obtains nodal displacements by considering both the correlation coefficient of brightness value and the nodal forces of a mesh. The effectiveness of FE global DIC is demonstrated by applying it to the specimen simulating the section of CFRP. The measurement results by FE global DIC show the discontinuous strain distribution around the fiber-matrix interface. Therefore, these results reveal the effectiveness of FE global DIC to measure the strain field of a cross section in CFRP.

Reflection photoelastic strain analysis (RPSA) is a well-established experimental strain analysis technique that provides users with immediate qualitative and quantitative information. However, the technique suffers from extensive and complicated application processes that reduce the usability of RPSA. This research demonstrates the use of a new practical photoelastic coating material that improves upon current application processes while maintaining a sensitivity to strains comparable to commercial materials. By developing a standard operating procedure for use alongside digital photoelastic methods, the new photoelastic coating possesses a strain-optic coefficient of 0.080 ± 0.003 (a medium-high sensitivity coating) with the ability for automatic thickness corrections between applied thicknesses of 50 and 100 μm. Additionally, this research aims to investigate the applications of a dual-technique coating by introducing a mechanoluminescent phosphor. The strain-induced light field can be observed simultaneously with photoelasticity providing the potential for an innovative full-field strain measurement technique. This research will investigate the feasibility of this dual-technique coating along with the representation of the strain-induced mechanoluminescent light.

Digital fringe projection method is a noncontact surface profiling technique. For accurate measurement, phase-shifting technique is usually adopted. However, the specimen may be moved during the time of image capture, and not suitable for dynamic measurement. In order to overcome this drawback, a digital fringe projection system consisting a color projector, a color CCD camera, and a PC is proposed. The phase shift is introduced between two computer-generated fringe patterns with different colors, which is projected onto the object to be measured simultaneously. The projected fringe patterns are captured by the color CCD camera and separated and processed by a two-step fringe analysis scheme using spiral phase transform and optical flow techniques. The proposed system was applied to a real specimen. The test results are reported.

The mechanical behavior and performance of engineered components are severely affected by the nucleation of surface and internal cracks or voids that could potentially lead to detrimental mechanical failures. In this paper, progress on the development of a high-resolution nondestructive method for detection and quantification of micro-surface and subsurface defects is presented. The experimental process comprises the use of manufactured samples with engineered cracks stimulated by impact-induced surface traveling waves. Micro- to nanoscale distortions of the traveling wavefronts are quantified in full field-of-view (FOV) using high-speed digital holographic interferometry (DHI). The experimental results are compared and validated with laser Doppler vibrometer (LDV). We report the performance of the currently under development approach for detection and quantification of surface cracks found in engineered components using high-speed DHI.

The standard digital image correlation (DIC) algorithm samples the region of interest (ROI) on a regular grid to describe the displacement field on a specimen. In each sampling point, a local problem is solved. Thus, apart from the oversampling usually used when selecting the grid parameters, the solutions related to each point are statistically independent. This approach is general but usually over-restraining because the measured displacement fields are usually continuous. Using global shape functions may thus give some advantages in terms of accuracy and computational time. The standard way to achieve this objective is using a finite element-like approach, but other options are possible.

This work focuses on using radial base functions: various approaches to globally describe the displacement fields are possible, and different families of functions exist. We intend to compare some of them, looking at the performance (accuracy), implementation complexity, and computational time.

High-speed digital holographic interferometry (DHI) and scanning laser Doppler vibrometry (SLDV) are optical full field-of-view (FOV) methods for measurement of dynamic deformations. In this paper, we present and discuss a comparative study of the performance between the two methods as applied to experimental modal analysis. The designed experimental procedure uses a highly repeatable metallic turbine blade stimulated by 25 kHz bandwidth excitation signals using piezoelectric shakers. As part of the comparison analysis, measurement acquisition time consumptions, errors and deviations for response magnitudes, frequencies, and damping parameters are studied. We report advantages and disadvantages of each method and the suitability of each technique in various applications.

Image-based techniques, such as DIC and grid method, have been developed to measure strain and displacement. The equipment used can be prohibitively expensive, and most of these techniques require marking the sample to track their motion/deformation. However, speckling samples is not always possible to get full-field measurements. Fiducial markers are primarily used in this case. This study explores a color tracking technique to track colored fiducial points on materials that cannot be speckled via standard techniques – such as fiber tows – and explores how to perform these measurements at a lower cost by using a conventional smartphone. A moving pendulum setup is used as an example for image tracking with a green ball tied to a tow of nylon fibers. The imaging process uses the color differences to automatically detect the point. This noncontact method showed positive results in tracking displacement with minimal noise and is being further explored for strain measurements and as an educational tool.

In situ and ex situ inspection methods are needed to guarantee the functionality, performance, and structural integrity of additive manufactured (AM) components. Ex situ methods such as infrared thermography, X-ray computed tomography, and ultrasound are used to characterize the functionality and performance of components post-fabrication. In situ methods are used to detect manufacturing defects during the manufacturing processes in real time and to optimize quality control. In this paper, two full-field-of view methods for metrology in situ are described: structured light fringe projection (FP) and optical coherence tomography (OCT). Representative results show that shape measurements with FP can be applied to characterize creep and stress relaxation, while OCT has capabilities to measure surface and subsurface topologies to quantitatively identify subsurface voids and fabrication irregularities in real time. Our investigations indicate that FP and OCT can be miniaturized and integrated into AM environments. Furthermore, in situ measurements can enable the development of AI methods to enhance process monitoring and control real time.

Using the digital image correlation method for on-site measurement, limited by continuous operation conditions and environment, two inevitable challenges are introduced. First, the reference images at the balanced positions needed for DIC analysis are always unknown; and environmental disturbances contaminate the continuously recording images and distort the measurement results. To overcome the two challenges, a preliminary attempt is to adopt the optical flow algorithm to redefine the reference image and reject the outlier video images to suppress the possible significant departures introduced by unknown reference images and environmental disturbances on the DIC-determined dynamic displacement and strain field. In this study, the balloon inflation experiment evaluates the improvement by adopting the optical flow method. The results show that the optical flow can provide additional information that helps redefine the reference image and reject unwanted image frames. After adopting the redefined reference image for DIC calculation, the averaged displacement and strain intuitively show that the balloon is in extension. Meanwhile, the overall frames’ averaged displacement and averaged strain can conduct a 3.6% difference for displacement and 5.6% for strain in magnitude as the redefined one is used. According to the results, selecting reference images needs further study to improve the measurement accuracy, particularly for continuous displacement and strain measurement using DIC.

Camera calibration is known to be a difficult problem, mainly because the quantities to be identified vary over several orders of magnitude and affect the accuracy of the result in different ways. Various approaches have been proposed in the literature, e.g., the Tsai approach or the Zhang algorithm, but although they differ significantly, they all rely on the pinhole camera model.

The calibration of a telecentric lens is worth attention because it implies a different procedure for the design of the optics itself. In this article, we propose a solution for telecentric lens calibration.

Among factors affecting the fatigue resistance of rubber materials, network appears as one of the most important. Network forms during the vulcanization process, such that the nature and crosslinking length depend on the sulfur and accelerator content, as well as on the processing conditions (time, temperature, pressure). Although this topic has been widely studied, the relationship between the active chain density and the fatigue properties of the rubber has not yet been fully understood.

The present study addresses the effect of network on the fatigue properties of natural rubber (NR). Several NR compositions have been defined, and different times and temperatures of vulcanization have been considered. Our results highlight and quantify the effect of the active chain density on the fatigue resistance, revealing the key significance of this parameter.

Welded rails are widely used in the railway industry nowadays for better performance over jointed rails. Cracks initiate in these rails during service increasing its vulnerability to failure in adverse thermal gradients. It is desirable to study the adverse cases using simplified models to get meaningful insights for prototype analysis. Toward this, photoelastic experiments are carried out on simplified planar models for certain crack configurations in tension and compression zones under contact loading. The stress intensity factors (SIFs) are evaluated using over-deterministic nonlinear least squares method. The same cases are numerically simulated using Abaqus^® and compared with experimental results for validation. Configurations are identified for their criticality based on the evaluated SIFs. A symmetrically loaded bottom crack configuration is taken first to gauge the difference in numerical and experimental SIFs. Next, for two different configurations with same load, it is noted that crack in the compression zone shows higher SIF compared to that in tension zone. It is shown that the numerical results match with experiments for crack in compression zone when it is remodelled as a crack with a finite root radius. This study finds application in rail-fracture analysis considering multiple crack interactions.

Steel structures may experience localized plastic strains arising from a wide range of service anomalies. Regions of accumulated plastic strain are more prone to accelerated stress corrosion cracking and reduced fatigue life. In this work, we systematically analyzed intergranular corrosion (IGC) under combined oscillatory mechanical loading and active electrochemical environment in a specially designed experimental apparatus. Loading cycles were designed to mimic both the low-amplitude high-frequency vibration loads and the low-frequency high-amplitude structural duty cycles. Electrochemical potentials were maintained for active dissolution in moderately alkaline carbonate-bicarbonate solutions and under pre-accumulated plastic strain ranging from 0% to 4.0%. We report the strain-dependent morphological evolution during the initial stage of IGC of X70 steel in sodium bicarbonate solution in the potential range of high-stress corrosion cracking (SCC) susceptibility. At potentials in the range of SCC susceptibility, IGC creates triangular wedges of porous corrosion products centered at grain boundary triple junctions. The wedge shapes and the total integrated charge from the polarization curves were greatly affected and correlated with the level of the accumulated plastic strains and the load profile. To model these interactive threats on the remaining fatigue life of the structure, a three-dimensional elastoplastic continuum damage mechanics model for multiaxial fatigue is developed. The modeling framework employs the thermodynamic formulation for the elastic and plastic continuum damage evolution laws proposed by LeMaitre and Chaboche and is implemented into an ABAQUS user material subroutine (UMAT). The model accounts for both the pre-accumulated plastic strain and the induced elastoplastic fatigue strains to accelerate the evolution of damage accumulation. The experimentally observed acceleration of wedge propagation under different electrochemical rates is also integrated into the damage evolution equation. The initial model predictions show up to 95% of life reduction for a pre-accumulated plastic strain of up to 5%. These findings can be used to advance the understanding of the combined effect of damage and corrosion on the remaining fatigue life of energy materials.

With the ever-increasing usage of lightweight, high strength, and high stiffness per unit weight polymer-composite-based structural components combined with sensitive electronic components in land and air vehicles, there has been a growing need to develop composite structures that have integrated EMF shielding, as well as reliable detection and quantification of composite fatigue damage. Of particular importance for damage detection are the damage precursors that are initiated during early stages of structural fatigue life. While existing techniques, such as the acoustic emission (AE) and ultrasonic pitch-catch, etc. have been quite useful, those techniques only provide damages at the bulk level, missing the precursors generally created at microstructural level. Additionally, there have been increasing threats in damage and detection of platform by electromagnetic (EM) radiation. To address these challenges and also to develop structurally compatible noninvasive techniques, there is a need to develop composite structures with functional materials that can be used for sensing damage, morphing, shape sensing, and even EM shielding, potentially using non-contacting techniques. To realize these structures, we have employed “layered jamming” principles, where we can combine layers of different materials with various functionalities whose response can be controlled via different mechanisms, such as vacuum pressure. We have designed and fabricated a prototype-layered jamming functional polymer-based composite structure that behaves like a shape memory material using bistable composites for morphing capabilities combined with metal layers with carbon-coated paper that be used for flexural sensing, as well as paper layers to control the mechanical properties of the structure. Results indicate that it is viable to realize fatigue-tolerant structures with morphing, shape sensing, and structural health-monitoring capabilities through the combination of bistable composites and layered jamming structures.

Phase-field models (PFM) are regularized versions of the variational approach applied to fracture problems. Albeit, different numerical methods are available which can be used to solve fracture problems; the robustness of the PFM for fracture lies in their capability to handle cracks or discontinuity. Many researchers have already explored the ability of the PFMs to capture the crack initiation and propagation. In this study, as part of the validation, the crack propagation in the PFM for brittle fracture is compared with experiments for a single-edge crack specimen made of epoxy material. The isochromatic fringes generated by post-processing stress data from the PFM are compared with the experimental isochromatics to identify modelling parameters. A preliminary study of PFM with the aid of photoelasticity shows that the length scale parameter (l_o) has a significant role in modelling.

The catastrophic failure response of brittle solids is governed by the mechanics of crack nucleation and growth, which have been observed to be rate- and orientation-dependent. One property that is characteristic to this process is the failure strength. At low to intermediate strain rates, the failure strength has been observed to be nearly constant and equal to the strength observed under quasi-static conditions; however, at high-enough strain rates, the failure strength has been observed to become rate-dependent. The main objective of the present work is to interrogate the effects of loading rate and orientation on the failure strength of uniaxially compressed α-quartz at very high strain rates to ascertain the transition into rate sensitivity. For doing this, a miniature Kolsky bar is used to perform dynamic compression experiments on α-quartz at strain rates in the order of 10³–10⁴/s, and X-ray phase contrast imaging (XPCI) is used to directly visualize and quantify the cracking process. Experiments are carried out on nominally 1 mm and 2 mm rectangular α-quartz specimens compressed on the {-1,-1,2,0} and {-2,2,0,3} family of planes, resulting in strain rates of approximately 2000 - 5000/s to 20,000/s at the time of failure. The results show no appreciable orientation effects, suggesting that the loading configuration rather than the crystal orientation relative to the loading direction controls the orientation of crack propagation. However, the stress history and XPCI reveal that the failure strength is appreciably rate-sensitive within the present loading rate regimes. The stress history for both configurations exhibits an increasing average failure strength from around 2 GPa to 3 GPa as loading rates increase from 2000 - 5000/s to 20,000/s. The stress history and XPCI data are expected to provide crucial insight into the rate-dependence of the damage mechanisms occurring in this material.