Measurement Of Strain Fields Research Articles

Dynamic shear localization of a titanium alloy under high-rate tension characterized by x-ray digital image correlation

Dynamic and quasi-static tension experiments are conducted on Ti-6Al-4V alloys, with in situ, synchrotron-based, high-speed, x-ray phase contrast imaging implemented to characterize the dynamic deformation and fracture process of Ti alloys at the Advanced Photon Source. X-ray digital imaging correlation (XDIC) is applied for strain field mapping. The size distribution of x-ray speckles are quantified via a morphological analysis, with a mean of ∼20 μm. Systematic error analyses of displacement and strain field measurements are firstly conducted for XDIC, and demonstrate that the displacement and strain errors can be controlled below 0.01 pixel and 0.1%, respectively. Mesoscale strain characteristics measured via XDIC are consistent with and reveal mechanisms for the bulk-scale stress–strain responses. Under dynamic tension, a sharp transition to strain softening occurs when the bulk strain exceeds about 0.04, which leads to a lower dynamic fracture strain (0.08) than the quasi-static one (0.1). The corresponding strain filed mapping demonstrates that shear deformation localizations grow and coalesce rapidly into a narrow shear deformation band under dynamic loading, while tensile deformation and necking progresses gradually under quasi-static loading. Scanning electron microscopy shows that void nucleation occurs mainly at the interface of α and β phases for both quasi-static and dynamic tension. However, microvoids coalesce preferentially along colony boundaries or the boundary α phase under quasi-static tension, but along the maximum shear stress direction (across colonies) under dynamic tension. Fractography of recovered samples also shows consistent features.

Strength of SiCf-SiCm composite tube under uniaxial and multiaxial loading

The authors report mechanical strength of nuclear grade silicon carbide fiber reinforced silicon carbide matrix composite (SiCf-SiCm) tubing under several different stress states. The composite tubing was fabricated via a Chemical Vapor Infiltration (CVI) process, and is being evaluated for accident tolerant nuclear fuel cladding. Several experimental techniques were applied including uniaxial tension, elastomer insert burst test, open and closed end hydraulic bladder burst test, and torsion test. These tests provided critical stress and strain values at proportional limit and at ultimate failure points. Full field strain measurements using digital image correlation (DIC) were obtained in order to acquire quantitative information on localized deformation during application of stress. Based on the test results, a failure map was constructed for the SiCf-SiCm composites.

Strain field measurement around the crack tip in Ni-base superalloy using Digital Image Correlation

Strain field measurement around the crack tip in Ni-base superalloy using Digital Image Correlation

Experiment and finite element analysis of U-profile subjected to dynamic loading

The presented study deals with the FEM simulation of dynamic behaviour of U-profile crash under three point bent loading conditions verified by experimental investigations. The material ductile damage behaviour under wide strain rate region covering 0.001 – 1 000 s-1 was experimentally determined with the use of standard and micro tensile tests (M-TT). DIC systems were used for strain field measurements under quasi-static and dynamic loading conditions. Based on these experimental data, material model considering ductile damage was established in Abaqus/Explicit code. Additionally, also metallographic investigations were performed for the fracture behaviour description.

Strain determination of self-adhesive resin cement using 3D digital image correlation method

Introduction/Objective. In an attempt to simplify dental procedures, a new group of resin cements, self-adhesive resin cements (SARCs), have been introduced. Performance of SARCs can widely vary. One of the main reasons of adhesion failure is polymerization shrinkage. The aim of this study was to determine, evaluate, and measure strain field of self-adhesive dual cure resin cement during polymerization in self-cure mode using 3D digital image correlation (DIC) method. Methods. The self-adhesive Maxcem Elite (Kerr, Orange, CA, USA) cement was tested in five cylindrical samples (5 mm in diameter and 2 mm in thickness) prepared by filling plastic ring-type molds. Digital images were recorded immediately after sample preparation. Results. Non-uniform strain distribution was found in resin cement with higher strain values along the periphery (up to 15%) and lower strain values in central parts (around 4%) of each sample. Conclusion. It can be concluded that DIC is a powerful tool for full-field strain measurements in material characterization.

Four-point-bending device for bending moment controlled cyclic reverse loading on plate materials and its application on AZ31B magnesium sheets

Fatigue testing of materials under various loading conditions is an important basis for designing high performance structures. One common load state is bending that can be often found for thin walled components like sheet metal structures. For this purpose, a four-point-bending device for standard uniaxial test rigs is presented within this work. This device allows fatigue testing of plate materials like sheet metals and offers fatigue testing under pure bending about the sheet metal normal axis and reverse loading from the low- to the high-cycle fatigue regime. An analytic analysis offers the necessary kinematic and kinetic relations for bending moment control. A finite element analysis was performed to validate the performance of the device, showing high accuracy of the analytically predicted specimen’s bending moment and stress distribution. Furthermore, strain gauge measurements, optical strain field measurements, and fully reversed bending moment controlled fatigue tests on magnesium sheet metals (AZ31B) were performed to evaluate the four-point-bending device. The fatigue tests show low scattering in results.

Investigation of surface/bulk stresses of nanoparticles with diffusive interfaces using the phase field crystal model

We investigate the surface stress of solid-liquid diffusive interfaces at equilibrium using the phase field crystal model in two dimensions. To analytically study the surface energy dependence on elastic strains, we employ the amplitude equation formalism and recast the free energy functional in terms of strains and amplitudes of density waves to examine the intricate coupling between them. For planar interfaces, the surface stress and its anisotropy are explored using the phase field crystal model and its amplitude equations. The anisotropy of surface stress is shown to be closely related to the anisotropic density waves across the interface, and a stronger anisotropy is observed in the surface stress than that in the surface energy. In addition, to investigate the curvature effect, we examine resultant strain fields at equilibrium within nanoparticles of various sizes subjected to their surface stresses. The measured strains are compared with the classical sharp interface model. The discrepancy in strain fields arises as the size of nanoparticles becomes smaller which suggests a curvature dependent surface stress for diffusive interfaces. We construct the effective surface stress using the measured strain fields and classical linear elasticity theory. The overall magnitude of the effective surface stress decreases with the radius of the nanoparticle, and the effective surface stress is shown to be more isotropic for small nanoparticles.

Hybrid MEFPI/FBG sensor for simultaneous measurement of strain and magnetic field

Abstract A hybrid fiber-optic sensor consisting of a micro extrinsic Fabry-Perot Interferometer (MEFPI) and an etched fiber Bragg grating (FBG) is proposed, which can measure strain and magnetic field simultaneously. The etched FBG is sealed in a capillary with ferrofluids to detect the surrounding magnetic field. FBG with small diameter will be more sensitive to magnetic field is confirmed by simulation results. The MEFPI sensor that is prepared through welding a short section of hollow-core fiber (HCF) with single-mode fiber (SMF) is effective for strain detection. The experiment shows that strain and magnetic field can be successfully simultaneously detected based on hybrid MEFPI/FBG sensor. The sensitivities of the strain and magnetic field intensity are measured to be up to 1.41 pm/μe and 5.11 pm/mT respectively. There is a negligible effect on each other, hence simultaneously measuring strain and magnetic field is feasible. It is anticipated that such easy preparation, compact and low-cost fiber-optic sensors for simultaneous measurement of strain and magnetic field could find important applications in practice.

Measuring strain fields in FRP strengthened RC shear walls using a distributed fiber optic sensor

This paper presents results on the applications of a truly distributed fiber-optic sensor (DFOS) in the cyclic testing of a reinforced concrete (RC) shear wall. To improve the performance of the wall, the RC shear wall is strengthened using externally bonded fiber-reinforced polymer (FRP) sheets. The DFOS is used as a replacement for conventional strain gauges to measure the strain distribution in the FRP sheet. Conventional electrical strain measurement techniques require a priori knowledge of measurement location and do not provide spatial information over a large area of a structural component, such as a wall. In recent years, the development of fiber-optic sensing technologies offers clear advantages compared to the conventional strain measurement approach. In particular, the DFOS has the unique capability of capturing strain at any location along the length of a fiber, without pre-determining the measurement locations before the test. The DFOS provides high precision strain measurement by optical frequency domain reflectometry. This allows full two-dimensional spatial strain measurement over the entire area of the shear wall. The DFOS can capture detailed response and failure mechanisms of the concrete and FRP as well as their contributions to the resistance of the shear wall. Experimental results are used to develop a better understanding of the behavior of RC shear walls retrofitted with FRP sheets, and assist in improving design guidelines for the application of FRP in RC structures.

Operation load estimation of chain-like structures using fiber optic strain sensors

The recent advancements in sensing technologies allow us to record measurements from target structures at multiple locations and with relatively high spatial resolution. Such measurements can be used to develop data-driven methodologies for condition assessment, control, and health monitoring of target structures. One of the state-of-the-art technologies, Fiber Optic Strain Sensors (FOSS), is developed at NASA Armstrong Flight Research Center, and is based on Fiber Bragg Grating (FBG) sensors. These strain sensors are accurate, lightweight, and can provide almost continuous strain-field measurements along the length of the fiber. The strain measurements can then be used for real-time shape-sensing and operational load-estimation of complex structural systems. While several works have demonstrated the successful implementation of FOSS on large-scale real-life aerospace structures (i.e., airplane wings), there is paucity of studies in the literature that have investigated the potential of extending the application of FOSS into civil structures (e.g., tall buildings, bridges, etc.). This work assesses the feasibility of using FOSS to predict operational loads (e.g., wind loads) on chain-like structures. A thorough investigation is performed using analytical, computational, and experimental models of a 4-story steel building test specimen, developed at the University of Southern California. This study provides guidelines on the implementation of the FOSS technology on building-like structures, addresses the associated technical challenges, and suggests potential modifications to a load-estimation algorithm, to achieve a robust methodology for predicting operational loads using strain-field measurements.

Measurement and evaluation of strain fields in T23 steel based on digital image correlation method

Surface strain fields of the designed compact tension (CT) specimens were investigated by digital image correlation (DIC) method. An integrative computer program was developed based on DIC algorithms to characterize the strain fields accurately and graphically. Strain distribution of the CT specimen was predicted by finite element method (FEM). Good agreement is observed between the surface strain fields measured by DIC and predicted by FEM, which reveals that the proposed method is practical and effective to determine the strain fields of CT specimens. Moreover, strain fields of the CT specimens with various compressive loads and notch diameters were studied by DIC. The experimental results can provide effective reference to usage of CT specimens in triaxial creep test by appropriately selecting specimen and experiment parameters.

The development of high strength brazing technique for Ti-6Al-4V using TiZrCuNi amorphous filler

The brazing joint of the Ti-6Al-4V alloy was produced with a designed brazing filler alloy and the optimized brazing temperature which is lower than the β-phase transformation of the matrix. The strength and the ductility of brazing joined Ti-6Al-4V samples were evaluated by conventional tensile tests with a DIC 2D–strain field measurement. The Widmanstätten microstructure with no voids or cracks or intermetallic compounds was found throughout the joint with a width of β-lamellar as ~1μm. Due to the fine acicular α-Widmanstätten and β-lamellar, and the uniformly diffused filler elements throughout the entire joint, the strength of the joint was as much as the matrix. In addition, the hardness test results agreed well with the tensile strength tests. All fractures occurred in the matrix rather than the brazing joints. Furthermore, the maximum local tensile strain was measured as 20% in the matrix, while under the same stress, the brazing joint only reached 6.3% tensile plastic strain. Thus, the mechanical properties of the joint with the associated microstructure demonstrated that a successful brazing filler alloy has been developed for the Ti-6Al-4V alloy.

The Measurement of Strain, Chemistry and Electric Fields by STEM based Techniques

The Measurement of Strain, Chemistry and Electric Fields by STEM based Techniques

Assessment of Fatigue Resistance of Aluminide Layers on MAR 247 Nickel Super Alloy with Full-Field Optical Strain Measurements

This work presents the results of fatigue tests of MAR 247 alloy flat specimens with aluminides layers of 20 or 40 µm thickness obtained in CVD process. Fatigue test was conducted at amplitude equal to half of maximum load and ranging between 300 and 650 MPa (stress asymmetry ratio R = 0, frequency f = 20 Hz). Additionally, 4 of the tests, characterized by the highest amplitude, were accompanied with non-contact strain field measurements by means of electronic speckle pattern interferometry and digital image correlation. Results of these measurements allowed to localize the areas of deformation concentration identified as the damage points of the surface layer or advanced crack presence in core material. Identification and observation of the development of deformation in localization areas allowed to assess fatigue-related phenomena in both layer and substrate materials.

Functional fatigue of Ni50.3Ti25Hf24.7 – Heterogeneities and evolution of local transformation strains

Shape memory alloys achieve their unique and desirable property of large recoverable strains through phase transformation. The attained magnitudes of transformation strains is strongly affected by the level of deformation heterogeneity during the transformation process and is impacted by plastic deformation and the accumulation of retained martensite/austenite following repeated cycling. This paper is dedicated to study the heterogeneity in the total and transformation strains of the high temperature shape memory alloy Ni50.3Ti25Hf24.7 subjected to fatigue loading and aims to provide further insight to the source of transformation strain instability. Under isobaric loading conditions, full field strain measurements were collected during thermal cycling and utilized to assess the local changes in the deformation field. Transformation strains increased globally in the first few cycles of loading followed by a relatively stable response and eventually started to exhibit drop in their magnitudes with continued loading. No homogenization of the transformation strain field was observed as a result of either stress increase or thermal cycling. The transformation strains were localized and the global evolution in their magnitudes was associated with local changes, either increasing or decreasing local strains, in spatially the same regions. The experimental results are discussed with the aim to provide a deeper understanding of the stability of transformation strains, their evolution under cyclic loading, the heterogeneities developing in the deformation field, and the relation between local and global response due to the accumulation of irrecoverable strains.

Full-field, high-spatial-resolution detection of local structural damage from low-resolution random strain field measurements

Structural damage is typically a local phenomenon that initiates and propagates within a limited area. As such high spatial resolution measurement and monitoring is often needed for accurate damage detection. This requires either significantly increased costs from denser sensor deployment in the case of global simultaneous/parallel measurements, or increased measurement time and labor in the case of global sequential measurements. This study explores the feasibility of an alternative approach to this problem: a computational solution in which a limited set of randomly positioned, low-resolution global strain measurements are used to reconstruct the full-field, high-spatial-resolution, two-dimensional (2D) strain field and rapidly detect local damage. The proposed approach exploits the implicit low-rank and sparse data structure of the 2D strain field: it is highly correlated without many edges and hence has a low-rank structure, unless damage—manifesting itself as sparse local irregularity—is present and alters such a low-rank structure slightly. Therefore, reconstruction of the full-field, high-spatial-resolution strain field from a limited set of randomly positioned low-resolution global measurements is modeled as a low-rank matrix completion framework and damage detection as a sparse decomposition formulation, enabled by emerging convex optimization techniques. Numerical simulations on a plate structure are conducted for validation. The results are discussed and a practical iterative global/local procedure is recommended. This new computational approach should enable the efficient detection of local damage using limited sets of strain measurements.

A method for correcting field strain measurements to account for temperature effects

Measurements of deformation and strain are of critical importance for the structural health monitoring of geosynthetic reinforced soil (GRS) structures. For some commonly used wiring configurations and sensors, field strain measurements can exhibit apparent strains that are caused by changes in wire temperature or changes in gauge temperature, which do not correspond to changes in actual strain in the geosynthetic. Correcting for these effects is important for separating out real structural behavior from wiring and sensor issues, and is imperative for avoiding false trigger warnings in automated instrumentation reporting systems. The current paper consequently presents a method for correcting field strain measurements to account for temperature effects. Data collected from sensors embedded in a heavily-instrumented GRS abutment over a two-year monitoring period is presented. The distributed thermistor array embedded in the GRS abutment is shown to be particularly useful for understanding temperature effects on the measured foil strain gauge data. The presented approach and associated framework for data correction are useful for practicing engineers and other researchers, as the general concepts from this study can be applied to data collected from many instrumented geosynthetic field projects.

Strain and stress conditions at crack initiation during shearing of medium- and high-strength steel sheet

BackgroundStrain and stress conditions in sheet metal shearing are of interest for calibration of various fracture criteria. Most fracture criteria are governed by effective strain and stress triaxiality.MethodsThis work is an attempt to extend previous measurements of strain fields in shearing of steel sheets with the stress state calculated from the measured displacement fields. Results are presented in terms of von Mises stress and stress triaxiality fields, and a comparison was made with finite element simulations. Also, an evaluation of the similarities of the stress conditions on the sheet surface and inside the bulk material was presented.ResultsStrains and von Mises stresses were similar to the surface and the bulk material, but the stress triaxiality was not comparable. There were large gradients in strain and stress around the curved tool profiles that made the result resolution dependent and comparisons of maximum strain and stress values difficult.ConclusionsThe stress state on the sheet surface calculated from displacement field measurements is useful for validation of a three-dimensional finite element model.

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

Abstract. The macroscopic description of deformation and fluid flow in mudrocks can be improved by a better understanding of microphysical deformation mechanisms. Here we use a combination of scanning electron microscopy (SEM) and broad ion beam (BIB) polishing to study the evolution of microstructure in samples of triaxially deformed Callovo–Oxfordian Clay. Digital image correlation (DIC) was used to measure strain field in the samples and as a guide to select regions of interest in the sample for BIB–SEM analysis. Microstructures show evidence for dominantly cataclastic and minor crystal plastic mechanisms (intergranular, transgranular, intragranular cracking, grain rotation, clay particle bending) down to the nanometre scale. At low strain, the dilatant fabric contains individually recognisable open fractures, while at high strain the reworked clay gouge also contains broken non-clay grains and smaller pores than the undeformed material, resealing the initial fracture porosity.

Influence of off-axis in-plane yarns on the mechanical properties of 3D composites

In the current paper, investigations have been carried out on in-plane and out-of-plane mechanical properties of 3D multiaxial woven composites keeping in view three different architectures. The first two architectures constitute of additional bias yarns oriented at +θ° and −θ° along with 0° and 90°, whereas the third comprises of yarns directed at 0° and 90° only. Composites in which the bias yarns accompany the 0° and 90° yarns have improved mechanical properties in comparison to classical 3D orthogonal woven textile composites. In order to demonstrate the influence of off-axis yarns on the mechanical properties, uniaxial tensile (loading-unloading) and short beam shear (SBS) tests were conducted. To measure strain field and record the visual analysis of the cracking processes and de-bonding, all mechanical tests were equipped with Digital Image Correlation (DIC) system. It was concluded based on the experimental results that 3D multiaxial woven composites exhibit enhanced interlaminar shear strength compare to classical 3D-orthogonal composites.