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This article quantifies the evolution of bulk material defects in α -iron during cyclic elastic loading and investigates its subsequent effect on quasi-static and dynamic mechanical response. Fatigue test specimens were cycled at a constant stress amplitude, 70% of the yield stress, to cause microscopic defects throughout the material due to a macroscopic elastic load. Afterwards, wire-cut electrical discharge machining was used to obtain multiple sub-tensile specimens from within the gauge section of the fatigued material. X-ray computed tomography of the sub-tensile specimens was used to quantify distribution and location of voids that were induced at different stages of fatigue life. Then, tension tests were performed on the sub-tensile specimens at low (10−3 s−1) and high (103 s−1) strain rates to study changes in mechanical properties. Yield stress and ultimate tensile stress were found to change under both quasi-static and dynamic strain rates based on the amount of prior cyclic loading. Changes in mechanical behavior were correlated to the characteristics of observed defects that exhibited distinct grouping at a certain point during the fatigue life. Mechanical properties of the sub-tensile specimens after fatigue loading displayed different trends depending on the strain rate. Low strain rate deformation resulted in decreases of approximately 20% in the yield stress, ultimate tensile stress, and ductility due to increased fatigue loading. Conversely, during high strain rate deformation some samples with more fatigue loading displayed higher maximum flow stresses. Results demonstrate the significance of bulk material changes which occur at mechanical loads below the yield stress. The systematic experimental procedures developed here for investigating the accumulation, evolution, and effect on subsequent properties due to elastically induced microscopic defects could potentially be applied to a wide range of material systems.

Micro- and nanomanufacturing capabilities have rapidly expanded over the past decade to include complex three-dimensional (3D) structure fabrication; however, the metrology required to accurately assess these processes via part inspection and characterization has struggled to keep pace. X-ray computed tomography (CT) is considered an ideal candidate for providing the critically needed metrology on the smallest scales, especially internal features, or inaccessible regions. X-ray CT supporting micro- and nanomanufacturing often push against the poorly understood resolution and variation limits inherent to the machines, which can distort or hide fine structures. We develop and experimentally verify a comprehensive analytical uncertainty propagation signal variation flow graph (SVFG) model for X-ray radiography in this work to better understand resolution and image variability limits on the small scale. The SVFG approach captures, quantifies, and predicts variations occurring in the system that limit metrology capabilities, particularly in the micro/nanodomain. This work is the first step to achieving full uncertainty modeling of CT reconstructions and provides insight into improving X-ray attenuation imaging systems. The SVFG methodology framework is applied to generate a complete basis set of functions describing the major sources of variation in radiographs. Five models are identified, covering variation in energy, intensity, length, blur, and position. Radiographic system experiments are defined to measure the parameters required by the SVFGs. Best practices are identified for these measurements. The SVFG models are confirmed via direct measurement of variation to predict variation within 30% on average.

A model for calculating the X-ray reflectivity (XRR) of surfaces to extract both roughness and waviness features is presented. Expressions of reflectivity intensity are derived as a function of root-mean-square (RMS) roughness σ, RMS waviness σ L , and the cut-off frequency between the features ω0. Experiments were conducted at the Advanced Light Source at Lawrence Berkeley National Laboratory, beamline 8.3.2, on BK7 glass manufactured with a multi-step polishing process to validate the model, and were compared with atomic force microscopy (AFM), Fizeau interferometry and surface profilometry measurements. The parameter results and their deviations for XRR measurements were σ = 2.9 ± 0.2 nm and σ L = 14.6 ± 0.5 nm with a wavelength cut-off of 1/(18 ± 2) µm−1, while the results from the AFM, Fizeau and profilometry measurements were σAFM = 3.4 ± 0.4 nm, σ L,Fizeau = 21.6 nm, σprof = 4.0 ± 0.1 nm, and σ L,prof = 21.4 ± 0.1 nm with cut-offs for the profilometry and Fizeau measurements limited to frequencies of (1/16) µm−1 to (1/4) mm−1.

Two-photon lithography (TPL) is a polymerization based technique that enables additive manufacturing of millimeter scale parts with submicron features. TPL equipment is often based on retrofitted optical microscopes that lack precise registration capabilities. Consequently, slow and error-prone visual alignment to fiducials is necessary if registration to pre-existing features is required. Herein, we have designed, built, and tested precise kinematic fixtures that are repeatable to within ±315 nm (3σ value) and passively register the build surface to TPL equipment with an accuracy of ±1.7 μm. This enables one to sequentially print with multiple materials by building the structures directly on top of the kinematic fixtures. In addition, the same fixtures passively register to an X-ray computed tomography (CT) system to enable non-destructive 3D inspection that is integrated with the fabrication process. These fixtures (i) provide a practical means to handle micro-scale parts during non-destructive imaging, (ii) reduce the set-up time for X-ray CT from more than an hour to less than a few minutes, and (iii) eliminate operator uncertainty from the multi-material printing and imaging process. As such, these fixtures enable new printing and imaging functionalities that are critical for high-quality additive manufacturing of multi-material polymer parts with microscale and submicron features.

Two-photon lithography (TPL) is a high-resolution additive manufacturing (AM) technique capable of producing arbitrarily complex three-dimensional (3D) microstructures with features 2-3 orders of magnitude finer than human hair. This process finds numerous applications as a direct route toward the fabrication of novel optical and mechanical metamaterials, miniaturized optics, microfluidics, biological scaffolds, and various other intricate 3D parts. As TPL matures, metrology and inspection become a crucial step in the manufacturing process to ensure that the geometric form of the end product meets design specifications. X-ray-based computed tomography (CT) is a nondestructive technique that can provide this inspection capability for the evaluation of complex internal 3D structure. However, polymeric photoresists commonly used for TPL, as well as other forms of stereolithography, poorly attenuate X-rays due to the low atomic number (Z) of their constituent elements and therefore appear relatively transparent during imaging. Here, we present the development of optically clear yet radiopaque photoresists for enhanced contrast under X-ray CT. We have synthesized iodinated acrylate monomers to formulate high-Z photoresist materials that are capable of forming 3D microstructures with sub-150 nm features. In addition, we have developed a formulation protocol to match the refractive index of the photoresists to the immersion medium of the objective lens so as to enable dip-in laser lithography, a direct laser writing technique for producing millimeter-tall structures. Our radiopaque photopolymer resists increase X-ray attenuation by a factor of more than 10 times without sacrificing the sub-150 nm feature resolution or the millimeter-scale part height. Thus, our resists can successfully replace existing photopolymers to generate AM parts that are suitable for inspection via X-ray CT. By providing the "feedstock" for radiopaque AM parts, our resist formulation is expected to play a critical role in enabling fabrication of functional polymer parts to tight design tolerances.

X-ray phase contrast tomography (XPCT) is widely used for 3D imaging of objects with weak contrast in X-ray absorption index but strong contrast in refractive index decrement. To reconstruct an object imaged using XPCT, phase retrieval algorithms are first used to estimate the X-ray phase projections, which is the 2D projection of the refractive index decrement, at each view. Phase retrieval is followed by refractive index decrement reconstruction from the phase projections using an algorithm such as filtered back projection (FBP). In practice, phase retrieval is most commonly solved by approximating it as a linear inverse problem. However, this linear approximation often results in artifacts and blurring when the conditions for the approximation are violated. In this paper, we formulate phase retrieval as a non-linear inverse problem, where we solve for the transmission function, which is the negative exponential of the projections, from XPCT measurements. We use a constraint to enforce proportionality between phase and absorption projections. We do not use constraints such as large Fresnel number, slowly varying phase, or Born/Rytov approximations. Our approach also does not require any regularization parameter tuning since there is no explicit sparsity enforcing regularization function. We validate the performance of our non-linear phase retrieval (NLPR) method using both simulated and real synchrotron datasets. We compare NLPR with a popular linear phase retrieval (LPR) approach and show that NLPR achieves sharper reconstructions with higher quantitative accuracy., Comment: 2020 IS&T International Symposium on Electronic Imaging

Fiber-reinforced metal laminate (FRML) composites are currently used as a structural material in the aerospace industry. A common FRML, glass layered aluminum reinforced epoxy (Glare), possesses a set of mechanical properties which was achieved by designing its layup structure to combine metal alloy and fiber-reinforced polymer phases. Beyond static and dynamic mechanical properties at the material characterization phase, however, the need exists to develop methods that could assess the evolving material state of Glare, especially in a progressive failure context. This paper presents a nondestructive approach to monitor the damage at the material scale and combine such information with characterization and postmortem evaluation methods, as well as data postprocessing to provide an assessment of the failure process during monotonic loading conditions. The approach is based on multiscale sensing using the acoustic emission (AE) method, which was augmented in this paper in two ways. First, by applying it to all material components separately in addition to actual Glare specimens. Second, by performing testing and evaluation at both the laboratory scale as well as at the scale defined inside the scanning electron microscopy. Such elaborate testing and nondestructive evaluation results provided the basis for the application of digital signal processing and machine learning methods which were capable to identify data trends that are shown to be correlated with the evolution of failure modes in Glare.

This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as acoustic emission. To achieve this goal, a ductile fracture problem is investigated computationally using the finite element method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with acoustic emission, a crack increment is produced given a pre-determined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the digital image correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to acoustic emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture.

The evolution of crack tip displacement and strain fields during uniaxial, room temperature, low-cycle fatigue experiments of Nickel superalloy compact tension specimens was measured by a digital image correlation approach and was further used to validate a cyclic plasticity model and corresponding deformation calculations made by a finite elements methodology. The experimental results provided data trends for the opening displacements and near crack tip strains as function of cycles. A finite element model was developed to capture test conditions for a measured crack size. The model captures crack tip plasticity by using a constitutive model calibrated against stress-strain measurements performed on a round bar. Similar quantities were extracted from the model predictions to compare with the digital image correlation measurements for model validation purposes. This type of direct comparison demonstrated that the computational model was capable to adequately capture the crack opening displacements at various stages of the specimen's fatigue life, providing in this way a tool for quantitative cyclic plasticity model validation. In addition, this integrated experimental-computational approach provides a framework to accelerate our understanding related to interactions of fatigue test data and models, as well as ways to inform one another.

The objective of this investigation is to quantify the evolution of microscopic features, including porosity, precipitates, and grain structure with respect to post-processing heat-treatments and their influence on the quasi-static and dynamic behavior of additively manufactured Inconel 718 built by laser powder bed fusion technique. In laser powder bed fusion, porosity, phase formation, and grain configuration could arise from different sources, including building powder or from the processing (e.g., laser pattern and solidification). The effect of these microscopic features on mechanical properties has drawn the scientific community's attention to developing thermal post-processing procedures targeting porosity closure, microstructural recrystallization, and precipitation of strengthening phases, thus improving material performance. One major challenge is to understand whether one can generalize the effect of certain microscopic features on mechanical properties under different states of stress and whether observed trends remain the same across strain rates? To address these questions, a comprehensive characterization agenda, which includes X-ray Micro-Computed Tomography, Scanning Electron Microscopy, Electron Backscatter Diffraction and mechanical testing at two strain rates ( 10 − 4 s − 1 and 10 3 s − 1 ) were implemented on as-built, stress relieved, hot isostatically pressed, and solution treatment plus aging test samples. Results show that, for the given specimen size, precipitate morphology and distribution changes from heat-treatment have a more significant effect than the porosity. In addition, it is demonstrated that there is a considerable enhancement in the magnitude of the plastic flow stress after applying the stress relief heat-treatment, while the subsequent heat-treatments do not have the same effect. When the state of stress is changed to compression, a different trend is observed that is relatively consistent increases in the magnitude of the flow stress. However, an opposite observation is seen under dynamic loading conditions indicating compressive mechanical behavior is more sensitive to the underlying microstructure than tensile behavior. Furthermore, strengthening mechanisms as a result of implemented heat-treatment is also discussed.

Blur in X-ray radiographs not only reduces the sharpness of image edges but also reduces the overall contrast. The effective blur in a radiograph is the combined effect of blur from multiple sources such as the detector panel, X-ray source spot, and system motion. In this paper, we use a systems approach to model the point spread function (PSF) of the effective radiographic blur as the convolution of multiple PSFs, where each PSF models one of the various sources of blur. In particular, we model the combined contribution of X-ray source and detector blurs while assuming negligible contribution from other forms of blur. Then, we present a numerical optimization algorithm for estimating the source and detector PSFs from multiple radiographs acquired at different X-ray source to object (SOD) and object to detector distances (ODD). Finally, we computationally reduce blur in radiographs using deblurring algorithms that use the estimated PSFs from the previous step. Our approach to estimate and reduce blur is called SABER, which is an acronym for systems approach to blur estimation and reduction.

Experimentally achieved mechanical properties of nanotube–epoxy composites fail to match theoretical expectations; shortcomings are mainly attributed to poor dispersion. The elastic modulus of a well-dispersed single walled carbon nanotube (SWNT)-ionic liquid-epoxy composite was evaluated in tension and compared to predictions by a micromechanics homogenization model. The model takes into account the mechanical properties of the constituent phases in addition to SWNT aspect ratio, spatial distribution, dispersion, and agglomeration. These parameters were evaluated using information obtained via scanning and transmission electron microscopy. The Young’s modulus of the composite shows excellent agreement with the model at low concentrations, while discrepancies at high SWNT concentrations are possibly due to composite processing limitations. At high concentrations the uncured composite mixture is above the rheological percolation threshold. As the polymer network reaches its maximum capacity for well-dispersed SWNTs, increasing volume fraction does not result in further significant reinforcing effects.

This article demonstrates the effectiveness of acoustic nondestructive inspection methods to monitor the condition of the bond-line in carbon fiber reinforced polymer stiffened panels. In particular, acoustic emission and guided ultrasonic waves were utilized experimentally to identify the onset of debonding in the stiffened panels and track its evolution under constant amplitude fatigue loading. Before testing, numerical simulations of wave propagation were carried out to evaluate the wave characteristics and optimize the placement of sensors along the spar. A damage index based on numerically produced guided waves was developed to estimate the extent of debonding which was tested and proved effective using actual experimental data. The panels were subjected to both quasi-static and fatigue loading conditions, while continuously recording acoustic emission and triggering guided ultrasonic waves at predetermined load levels. The acoustic emission activity associated with debonding was found to have a dual dominant frequency content and a low frequency centroid, which were also confirmed with wavelet analysis. In view of potential aerospace applications of the investigation presented herein, pattern recognition algorithms were also implemented and showed great potential for real-time detection of such damage.

This article provides experimental evidence on the pronounced three-dimensional surface effects caused by tension twinning at early stages of plasticity of a magnesium alloy. To achieve this goal, multiscale mechanical testing was combined with strain field, texture and surface morphology quantification. The current investigation correlates crystallography-driven, grain-scale surface roughening to the spatially heterogeneous micro- and macrostrain. It was found that tension twinning causes considerable surface extrusions and intrusions across multiple grains that are directly related to severe strain inhomogeneities.

The next generation of acoustics-based non-destructive evaluation for structural health monitoring applications will depend, among other reasons, on the capability to effectively characterize the transient stress wave effects related to acoustic emission (AE) generated due to activation of failure mechanisms in materials and structures. In this context, the forward problem of simulating AE is addressed herein by a combination of experimental, analytical and computational methods, which are used to form a data-driven finite element (FE) model for AE generation and associated transient elastic wave propagation. Acoustic emission is viewed for this purpose as part of the dynamic process of energy release caused by crack initiation. To this aim, full field experimental data obtained from crack initiation monitored by digital image correlation is used to construct a traction-separation law and to define damage initiation parameters. Subsequently, 3D FE simulations based on this law are performed using both a cohesive and an extended finite element modeling approach. To create a realistic computational AE source model, the transition between static and dynamic responses is evaluated. Numerically simulated AE signals from the dynamic response due to the onset of crack growth are analyzed in the context of the inverse problem of source identification and demonstrate the effects of material and geometry in crack-induced wave propagation.

Two-photon polymerization (TPP) is a laser writing process that enables fabrication of millimeter scale three-dimensional (3D) structures with submicron features. In TPP, writing is achieved via nonlinear two-photon absorption that occurs at high laser intensities. Thus, it is essential to carefully select the incident power to prevent laser damage during polymerization. Currently, the feasible range of laser power is identified by writing small test patterns at varying power levels. Herein, we demonstrate that the results of these tests cannot be generalized, because the damage threshold power depends on the proximity of features and reduces by as much as 47% for overlapping features. We have identified that this reduction occurs primarily due to an increase in the single-photon absorptivity of the resin after curing. We have captured the damage from proximity effects via X-ray 3D computed tomography (CT) images of a nonhomogenous part that has varying feature density. Part damage manifests as internal spherical voids that arise due to boiling of the resist. We have empirically quantified this proximity effect by identifying the damage threshold power at different writing speeds and feature overlap spacings. In addition, we present a first-order analytical model that captures the scaling of this proximity effect. Based on this model and the experiments, we have identified that the proximity effect is more significant at high writing speeds; therefore, it adversely affects the scalability of manufacturing. The scaling laws and the empirical data generated here can be used to select the appropriate TPP writing parameters.

A micromechanical approach relying on 3D image-based finite element (FE) models, together with experimental testing, is employed to investigate the stress distribution in an L6 ordinary chondrite's microstructure subjected to thermal and mechanical loadings. An in-situ X-ray micro-computed tomography (micro-CT) setup capable of applying thermal and mechanical loads shed light on crack initiation mechanisms in chondrite samples. Micro-CT images were further used to reconstruct realistic microstructural models of the samples, which were transformed into FE models using an automated non-iterative mesh generation algorithm. FE approximation of stress fields showed that under mechanical loads, the stress often concentrates in matrix-particle interfacial regions with higher curvatures. This could result in the nucleation and eventually propagation of cracks in such regions. However, when the thermal cycling takes place, the stress concentrates more uniformly along with particle interfaces, resulting in their debonding from the surrounding matrix. Both observations were verified by experimental data obtained from in-situ micro-CT imaging.

Damage is an inherently dynamic and multi-scale process. Of interest herein is the monitoring and quantification of progressive damage accumulation in a newly developed glass fiber reinforced polymer composite subjected to both tensile and fatigue loading conditions. To achieve this goal, the potential of data fusion in structural damage detection, identification and remaining-life estimation is investigated by integrating heterogeneous monitoring techniques and extracting damage-specific information. Damage monitoring is achieved by the use of a hybrid non-destructive testing system relying on the combination of acoustic emission, digital image correlation and infrared thermography. Full-field strain and temperature differential maps reveal appearance and development of damage “hot spots” at prescribed strain/load increments that also correlate well with distinct changes in the recorded acoustic activity. The use of non-destructive and mechanical testing data further allows the quantification of the observed hysteretic fatigue behavior by providing measurements of the: (i) stiffness degradation, (ii) energy dissipation, and (iii) average strain as a function of fatigue cycles. Furthermore, analysis of the real time recorded acoustic activity indicates the existence of three characteristic stages of fatigue life that can be used to construct a framework for reliable remaining life-predictions.

Digital image correlation and acoustic emission were used simultaneously to investigate the near-yielding behavior of textured samples of a magnesium alloy. Distinct band evolution near yielding was monitored in situ and directly related to profuse localized twin formation, verified by ex situ electron backscatter diffraction. The presented results demonstrate for the first time quantitatively in both space and time the dominant role of twinning in the early stages of plastic deformation of magnesium alloys.

These are slides about in-situ X-ray CT results of damage evolution in L6 ordinary chondrite meteorites. The following topics are covered: mechanical and thermal damage characterization, list of Grosvenor Mountain (GRO) meteorite samples, in-situ x-ray compression test setup, GRO-chipped reference at 0 N - existing cracks, GRO-chipped loaded at 1580 N, in-situ x-ray thermal fatigue test setup, GRO-B14 room temperature reference, GRO-B14 Cycle 47 at 200°C, GRO-B14 Cycle 47 at room temperature, conclusions from qualitative analysis, future work and next steps. Conclusions are the following: Both GRO-Chipped and GRO-B14 had existing voids and cracks within the volume. These sites with existing damage were selected for CT images from mechanically and thermally loaded scans since they are prone to damage initiation. The GRO-Chipped sample was loaded to 1580 N which resulted in a 14% compressive engineering strain, calculated using LVDT. Based on the CT cross sectional images, the GRO-B14 sample at 200°C has a thermal expansion of approximately 96 μm in height (i.e. ~1.6% engineering strain).

Nanodiamond, due to its superior hardness and Young's modulus in combination with its large surface area holds great potential for the mechanical reinforcement of polymer matrices. However, it is still not possible to take full advantage of these properties in polymer matrix composites. The main reasons are poor dispersion, agglomeration and a weak interface between nanodiamond and the polymer. Aminated nanodiamond can be used to form a strong covalent interface with an epoxy polymer. We show how Young's modulus can be further improved if an interference of nanodiamonds amino groups with the epoxy stoichiometry is taken into account. Dispersion of nanodiamond is improved by slightly adjusting the manufacturing process, thus increasing Young's modulus of the composites. Predictions by a micromechanics homogenization method that consider nanodiamond agglomeration and dispersion suggest that Young's modulus can be further increased by slightly improving both parameters. For the first time we show that epoxy can be cured solely by nanodiamonds amino groups without any additions of curing agent, resulting in a composite's Young's modulus measured by nanoindentation of up to ∼18 GPa – a 700% improvement over neat epoxy.

A novel structural health monitoring approach consisting of guided ultrasonic waves, acoustic emission, and digital image correlation, as well as real-time and postmortem analyses, was implemented to monitor and quantify crack growth in Al 2024 compact tension specimens, designed, and precracked according to ASTM E647-08. Tensile loads were applied according to ASTM E1290-08. Guided ultrasonic waves were generated with pulses centered at three different frequencies and were recorded using piezoelectric transducers. Guided ultrasonic waves were also modeled using finite element wave propagation models. The same transducers were further used for online acoustic emission monitoring. A digital image correlation system continuously monitored the crack growth and provided full-field surface strains. The application of this integrated structural health monitoring approach resulted in reliable damage detection and quantified crack growth measurements. In addition, a novelty detector based on the Mahalanobis distance was implemented in a data fusion scheme to assess the extent of damage. The reported results constitute a proof-of-concept investigation of a novel structural health monitoring approach based on the combination of real-time optical and acoustic nondestructive testing.

A mathematical formulation is presented to compute the dispersion characteristics of tubular thin walled waveguides with arbitrary shape that are in contact with perfect fluids. The wave propagation problem is described in the frequency-wavenumber domain by using a Semi-Analytical Finite Element (SAFE) formulation for the thin walled waveguide and a regularized 2.5D Boundary Element Method (BEM) for the fluid. The wave dispersive equation is obtained by imposing continuity and equilibrium conditions on the fluid-structure interface, where the generalized Snell-Descartes law is also enforced, resulting into a nonlinear eigenvalue problem in the complex axial wavenumber. Leaky and non-leaky poles are then found by means of a contour integral algorithm. Two numerical examples are finally presented in order to show the accuracy of the method, consisting in a fluid-filled pipe and an immersed tubular section of rectangular shape. doi: 10.12783/SHM2015/113

The reliable characterization of fatigue behavior and progressive damage of advanced alloys relies on the monitoring and quantification of parameters such as strain localizations as a result of both crystallographic deformation mechanisms and bulk response. To this aim, this article attempts to directly correlate microstructural strain at specific fatigue life to global strain as well as surface roughness in Magnesium alloys. Strain at the grain scale is calculated using Digital Image Correlation (DIC), while surface topography gradients are computed using roughness data at different stages of the fatigue life. The results are further correlated to Electron Back Scatter Diffraction (EBSD) measurements which reveal the profuse and spatially inhomogeneous nature of the crystallographic deformation mechanisms related to yielding and fatigue crack initiation. Emphasis is given on using multimodal NDE data to formulate first a description of the current state of the material subjected to fatigue loading and on identifying conditions that can probabilistically drive the affected by both local and global response, governing degradation process.Copyright © 2014 by ASME

This article presents the advantages of combining Acoustic Emission (AE) and Digital Image Correlation (DIC) in nondestructive testing (NDT) applications focusing on in situ damage monitoring. This data-fusion approach is used herein to characterize the mechanical and damage behavior of a fiber metal laminate (Glare 1A) tested in both tension and fatigue. Furthermore, the approach is used to investigate the structural behavior of partially grouted reinforced masonry walls. The obtained AE datasets were post-processed, in combination with DIC and mechanical information, using signal processing and pattern recognition techniques to investigate progressive failure of the Glare 1A. In the case of the masonry wall specimens, DIC clearly identified critical damage areas as a function of applied loading, while AE was capable to monitor the damage process and reveal changes in the overall behavior. The presented analysis demonstrates the potential of integrating AE and DIC in data-driven damage mechanics investigations at multiple time and length scales.

A cross-validated nondestructive evaluation approach was employed to in situ detect the onset of damage in an Aluminum alloy compact tension specimen. The approach consisted of the coordinated use primarily the acoustic emission, combined with the infrared thermography and digital image correlation methods. Both tensile loads were applied and the specimen was continuously monitored using the nondestructive approach. Crack initiation was witnessed visually and was confirmed by the characteristic load drop accompanying the ductile fracture process. The full field deformation map provided by the nondestructive approach validated the formation of a pronounced plasticity zone near the crack tip. At the time of crack initiation, a burst in the temperature field ahead of the crack tip as well as a sudden increase of the acoustic recordings were observed. Although such experiments have been attempted and reported before in the literature, the presented approach provides for the first time a cross-validated nondestructive dataset that can be used for quantitative analyses of the crack initiation information content. It further allows future development of automated procedures for real-time identification of damage precursors including the rarely explored crack incubation stage in fatigue conditions.

The objective of this article is to present a progressive damage quantification framework for fiber reinforced polymer composites (FRPC) that have widespread use in aerospace and wind-turbine applications. To this aim, a novel optico-acoustic nondestructive evaluation (NDE) setup is presented based on integration of Digital Image Correlation (DIC), Acoustic Emission (AE), and Infrared Thermography (IRT). DIC and IRT full-field strain and temperature maps reveal early development of structural hot spots, associated with locations where inelastic strains accumulate, damage initiates, and final fracture occurs in both tensile and fatigue experiments. Damage quantification is further related to: (i) energy dissipation, (ii) residual stiffness, (iii) average both temporal and spatial temperature variations, and (iv) AE features in time and frequency domains. The extracted NDE parameters suggest three characteristic stages of fatigue life that can be used to construct appropriate models for reliable remaining life-predictions.

This study presents a hybrid experimental mechanics approach combining multi-scale mechanical testing, in situ nondestructive evaluation and targeted microscopic quantification to identify and quantify critical micro structural parameters that affect properties and overall plasticity of Mg alloys. Room temperature monotonic and cyclic experiments monitored by Digital Image Correlation (DIC) coupled with Acoustic Emission (AE) of Mg Alloys of the AZ series were used for this investigation. Data obtained using the optico-acoustic nondestructive system revealed for the first time the direct connection between surface strain localization effects similar to Luder’s bands and pronounced twin activity. Electron Back Scatter Diffraction (EBSD) measurements showed the profuse and spatially inhomogeneous nature of twinning at early stages of plasticity which is related with the onset of yielding and the macroscopic plateau region in the stress-strain curve. Furthermore, twinning/detwinning activity was identified in several grains of tested specimens and during characteristic points of fatigue cycles.

This research demonstrates the use of Digital Image Correlation (DIC) as a non-contact, non-destructive testing and evaluation (NDT and E) technique by presenting experimental results pertinent to damage monitoring and quantification in several material systems at different length scales of interest. At the microstructural level compact tension aluminum alloy specimens were tested under Mode I loading conditions using an appropriate field of view to track grain scale crack initiation and growth. The results permitted the quantification of the strain accumulation near the tip of the fatigue pre-crack, as well as the computation of the relevant crack opening displacement as a function of crack length. At the mesoscale level, damage quantification in fiber reinforced composites subject to both tensile and fatigue loading conditions was achieved by using the DIC as part of a novel integrated NDT approach combining both acoustic and thermal methods. DIC in these experiments provided spatially resolved and high accuracy strain measurements capable to track the formation of damage "hot spots" that corresponded to the sites of the ultimately visible fracture pattern, while it further allowed the correlation of mechanical parameters to thermal and acoustic features. Finally, at the macrostructural level DIC measurements were also performed and compared to traditional displacement gauges mounted on a steel deck model subject to both static and dynamic loads, as well as on masonry structures including hollow and grouted concrete walls.

SUMMARY A novel optico-acoustic sensing system based on digital image correlation (DIC), guided ultrasonic waves (GUW), and acoustic emission (AE) and its application in detecting breaks on seven-wire steel strands is presented. The implementation of the emerging optical nondestructive testing (NDT) method of DIC in parallel with established acoustic NDT techniques enables the cross-correlation/validation of in situ recorded information related to progressive damage accumulation, which is the focal point of this research. To the authors' best knowledge, it is the first time that full field strain accumulations have been obtained on the surface of the seven-wire strands. Furthermore, acoustic waveforms and their extracted features are found complementary to full field strain measurements and prove capable to detect damage initiation in critical structural sites (‘hot spots’). To demonstrate the potential of the novel NDT system, pristine and prenotched strands were loaded using a mechanical testing frame while simultaneously recording DIC, GUW, and AE data. The DIC and GUW were acquired and excited at specific load intervals to avoid overlapping of GUW with AE activity. The reported DIC results directly reveal strain accumulations at the notched areas prior to breaking, whereas AE waveforms and related features show sudden changes at time instances that correspond to wire breaks. In addition, the GUW signals show a decrease in their amplitude upon progressive load of the strands and wave speed variations. Detailed post-processing of the acoustic results was performed to cross-correlate recorded information from novel optico-acoustic sensing system and create methodologies for effective data filtering, alignment, synchronization, and fusion (through unsupervised pattern recognition techniques) that could lead to robust damage identification in structural health monitoring applications. The results demonstrate for the first time that the use of full field strains in correlation with acoustic techniques visually validates the damage source location and, in turn, enhances the damage predictive capabilities of each NDT method for prestressed and post-tensioned cables found in stay cable bridges. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a methodology to make coefficient of thermal expansion measurements through the combined use of two non-contact and full field optical metrology methods including digital image correlation and infrared thermography. In this context, active Infrared Thermography techniques combined with contact and non-contact deformation measurement methods have already been reported to measure materials' thermal expansion. In addition, such techniques have been reported to be capable to detect surface and subsurface defects from changes in homogenous heat diffusion due to damage. Based on this knowledge, it is hypothesized in this article that the material response induced by thermal loading and quantified by coefficient of thermal expansion measurements could be further used as an indicator of damage. To validate the hypothesis three measurements were performed. The first established the effectiveness of using deformation and thermal full field data for coefficient of thermal expansion measurements. The second intended to demonstrate the advantage of using such full field data in order to provide site-specific measurements of thermal expansion. Finally damage was a priori induced to a metallic specimen, and the measured variations of local CTE confirmed the potential of using the described approach as a means of damage quantification in materials and structures.

Two-photon lithography is a direct laser write process that enables fabrication of millimeter scale 3D structures with nanoscale building blocks. In this technique, writing is achieved via a nonlinear two-photon absorption process during which two photons are near-simultaneously absorbed at high laser intensities. Due to the high laser intensities, it is essential to carefully select the incident power so that two-photon polymerization (TPP) occurs without any laser damage of the resist. Currently, the feasible range of laser power is identified by writing small test patterns at varying power levels. Herein, we demonstrate that the results of these tests cannot be generalized because the damage threshold power is dependent on the proximity of features and reduces by as much 37.5% for overlapping features. We have identified that this reduction occurs due to a combination of reduced TPP for overlapping features and increased single-photon absorption of the resin after curing. We have captured the damage arising out of this proximity effect via 3D computed tomography images of a non-homogenous part that has varying feature density. Part damage manifests in the form of internal spherical voids that arise due to boiling of the resist at high laser intensities. Herein, we have empirically quantified this proximity effect by identifying the damage threshold power at different writing speeds and feature overlap spacings. In addition, we present a first-order analytical model that captures the scaling of this proximity effect. The scaling laws and the empirical data generated here can be used to select the appropriate writing process parameters so as to correct for proximity effects and prevent part damage during sub-micron additive manufacturing of parts with closely spaced features.