Jefferson Cuadra

An integrated structural health monitoring approach for crack growth monitoring

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.

Energy dissipation via acoustic emission in ductile crack initiation

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.

Acoustic Emission and Digital Image Correlation as Complementary Techniques for Laboratory and Field Research

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.

Cross-validated detection of crack initiation in aerospace materials

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.

In-situ acousto-ultrasonic monitoring of crack propagation in Al2024 alloy

A data fusion technique implementing the principles of acoustic emission (AE), ultrasonic testing (UT) and digital image correlation (DIC) was employed to in situ monitor crack propagation in an Al 2024 alloy compact tension (CT) specimen. The specimen was designed according to ASTM E647-08 and was pre-cracked under fatigue loading to ensure stable crack growth. Tensile (Mode I) loads were applied according to ASTM E1290-08 while simultaneously recording AE activity, transmitting ultrasonic pulses and measuring full-field surface strains. Realtime 2D source location AE algorithms and visualization provided by the DIC system allowed the full quantification of the crack growth and the cross-validation of the recorded non-destructive testing data. In post mortem, waveform features sensitive to crack propagation were extracted and visible trends as a function of computed crack length were observed. In addition, following a data fusion approach, features from the three independent monitoring systems were combined to define damage sensitive correlations. Furthermore a novelty detector based on the Mahalanobis outlier analysis was implemented to quantify the extent of crack growth and to define a more robust sensing basis for the proposed system.

In-situ X-ray CT results of damage evolution in L6 ordinary chondrite meteorites

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).

Wave Propagation in Fluid Loaded Thin Walled Waveguides

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

Quantification of Microstructure‐Properties‐Behavior Relations in Magnesium Alloys Using a Hybrid Approach

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.

Data-Fusion NDE for Progressive Damage Quantification in Composites

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.

The Role of Multiscale Strain Localizations in Fatigue of Magnesium Alloys

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