Showing papers in "Experimental Mechanics in 2014"

Vibration Monitoring of Multiple Bridge Points by Means of a Unique Vision-Based Measuring System

Abstract: Bridge static and dynamic vibration monitoring is a key activity for both safety and maintenance purposes. The development of vision-based systems allows to use this type of devices for remote estimation of a bridge vibration, simplifying the measuring system installation. The uncertainty of this type of measurements is strongly related to the experimental conditions (mainly the pixel-to-millimeters conversion, the target texture, the camera characteristics and the image processing technique). In this paper two different types of cameras are used to monitor the response of a bridge to a train pass-by. The acquired images are analyzed using three different image processing techniques (Pattern Matching, Edge Detection and Digital Image Correlation) and the results are compared with a reference measurement, obtained by a laser interferometer providing single point measurements. Tests with different zoom levels are shown and the corresponding uncertainty values are estimated. As the zoom level decreases it is possible not only to measure the displacement of one point of the bridge, but also to grab images from a wide structure portion in order to recover displacements of a large number of points in the field of view. The extreme final solution would be having wide area measurements with no targets, to make measurements really easy, with clear advantages, but also with some drawbacks in terms of uncertainty to be fully comprehended.

In Situ Measurements of Strains in Composite Battery Electrodes during Electrochemical Cycling

Abstract: The cyclic stress in lithium-ion battery electrodes induced by repeated charge and discharge cycles causes electrode degradation and fracture, resulting in reduced battery performance and lifetime. To investigate electrode mechanics as a function of electrochemical cycling, we utilize digital image correlation (DIC) to measure the strains that develop in lithium-ion battery electrodes during lithiation and delithiation processes. A composite graphite electrode is cycled galvanostatically (with constant current) in a custom battery cell while optical images of the electrode surface are captured in situ. The strain in the electrode is computed using an in-house DIC code. On average, an unconstrained composite graphite electrode expands 1.41 % during lithiation and contracts 1.33 % during delithiation. These strain values compare favorably with predictions based on the elastic properties of the composite electrode and the expansion of graphite-lithium intercalation compounds (G-LICs). The establishment of this experimental protocol will enable future studies of the relationship between electrode mechanics and battery performance.

Identification of Post-Necking Hardening Phenomena in Ductile Sheet Metal

Abstract: A combined theoretical/experimental approach accurately quantifying post-necking hardening phenomena in ductile sheet materials that initially exhibit diffuse necking in tension is presented. The method is based on the minimization of the discrepancy between the internal and the external work in the necking zone during a quasi-static tensile test. The main focus of this paper is on the experimental validation of the method using an independent material test. For this purpose, the uniaxial tube expansion test is used to obtain uniaxial strain hardening behavior beyond the point of maximum uniform strain in a tensile test. The proposed method is used to identify the post-necking hardening behavior of a cold rolled interstitial-free steel sheet. It is demonstrated that commonly adopted phenomenological hardening laws cannot accurately describe all hardening stages. An alternative phenomenological hardening model is presented which enables to disentangle pre- and post-necking hardening behavior. Additionally, the influence of the yield surface on the identified post-necking hardening behavior is scrutinized. The results of the proposed method are compared with the hydraulic bulge test. Unlike the hydraulic bulge test, the proposed method predicts a decreased hardening rate in the post-necking regime which might be associated with probing stage IV hardening. While inconclusive, the discrepancy with the hydraulic bulge test suggests differential work hardening at large plastic strains.

Determination of All 21 Independent Elastic Coefficients of Generally Anisotropic Solids by Resonant Ultrasound Spectroscopy: Benchmark Examples

Abstract: We present an experimental methodology for determination of all 21 elastic constants of materials with general (triclinic) anisotropy. This methodology is based on contactless resonant ultrasound spectroscopy complemented by pulse-echo measurements and enables full characterization of elastic anisotropy of such materials from measurements on a single small specimen of a parallelogram shape. The methodology is applied to two benchmark examples: a material with generally rotated cubic anisotropy (single crystal of silicon) and an isotropic material (silicon-infiltrated silicon carbide ceramics). In both the proposed approach is able to provide a full triclinic tensor with relatively low experimental errors and to identify indubitably the anisotropy class of the material; for the cubic material also the orientations of the principal axes and the cubic elastic coefficients are reliably determined.

Determination of the Effect of Stress State on the Onset of Ductile Fracture Through Tension-Torsion Experiments

Abstract: A tubular tension-torsion specimen is proposed to characterize the onset of ductile fracture in bulk materials at low stress triaxialities. The specimen features a stocky gage section of reduced thickness. The specimen geometry is optimized such that the stress and strain fields within the gage section are approximately uniform prior to necking. The stress state is plane stress while the circumferential strain is approximately zero. By applying different combinations of tension and torsion, the material response can be determined for stress triaxialities ranging from zero (pure shear) to about 0.58 (transverse plane strain tension), and Lode angle parameters ranging from 0 to 1. The relative displacement and rotation of the specimen shoulders as well as the surface strain fields within the gage section are determined through stereo digital image correlation. Multi-axial fracture experiments are performed on a 36NiCrMo16 high strength steel. A finite element model is built to determine the evolution of the local stress and strain fields all the way to fracture. Furthermore, the newly-proposed Hosford-Coulomb fracture initiation model is used to describe the effect of stress state on the onset of fracture.

Determination of Anisotropic Plastic Constitutive Parameters Using the Virtual Fields Method

Abstract: The aim of the present study is to retrieve all the anisotropic plastic constitutive parameters from uniaxial loading. A complex geometry which can provide very heterogeneous stress states in a uniaxial tensile test was chosen for steel sheet specimens. A digital image correlation technique was used for the full-field heterogeneous strain measurement. The orthotropic Hill1948 yield criterion with Swift isotropic hardening was adopted as an elasto-plastic constitutive model. The virtual fields method (VFM) was employed as an inverse analytical tool to determine the constitutive parameters. All the parameters were successfully identified using the VFM by combining two tensile test results obtained in rolling and transverse directions.

Direct Stress-Strain Measurements from Bulged Membranes Using Topography Image Correlation

Abstract: This paper discusses an experimental method to characterize thin films as they are encountered in micro-electronic devices. The method enables the measurement of the stress and strain of pressure deflected bulged membranes without using a priori defined bulge equations. An enrichment to the Global Digital Image Correlation method is detailed to capture the membrane strain and curvature while robustly dealing with acquisition noise. The accuracy of the method is analyzed and compared to the standard bulge test method. The method is applied to a proof of principle experiment to investigate its applicability and accuracy. Additionally, it is shown for two experimental cases that the method provides accurate results, although the bulge equations do not hold.

Identification of Cracking Mechanisms in Scaled FRP Reinforced Concrete Beams using Acoustic Emission

Abstract: Acoustic emission was used to monitor the cracking mechanisms leading to the failure of scaled concrete beams having Glass Fiber Reinforced Polymer (GFRP) longitudinal reinforcement and no shear reinforcement. Dimensional scaling included that of the effective depth of the cross section, which is a key parameter associated with the scaling of shear strength; and maximum aggregate size, which affects the shear-resisting mechanism of aggregate interlock along shear (inclined) cracks. Five GFRP reinforced concrete (RC) beams with effective depth up to 290 mm and constant shear span-to-effective depth ratio of 3.1 were load tested under four-point bending. Two types of failures were observed: flexural, due to rupture of the GFRP reinforcement in the constant moment region; and shear, due to inclined cracking in either constant shear region through the entire section depth. Acoustic emission (AE) analyses were performed to classify crack types occurring at different points in the load history. The results of this study indicate that appropriate AE parameters can be used to discriminate between developing flexural and shear cracks irrespective of scale, and provide warning of impending failure irrespective of the failure mode (flexural and shear). In addition, AE source location enabled to accurately map crack growth and identify areas of significant damage activity. These outcomes attest to the potential of AE as a viable technique for structural health monitoring and prognosis systems and techniques.

Mechanical Testing of Thin Sheet Magnesium Alloys in Biaxial Tension and Uniaxial Compression

Abstract: Tension and compression experiments on magnesium rolled sheets and extruded products of AZ31 (Mg + 3%Al + 1%Zn) and ZE10 (Mg + 1%Zn + 0.3%Ce based misc metal) were performed at room temperature. The tests were conducted along the longitudinal and the transverse direction to quantify the in-plane anisotropy. Samples built from adhesively-bonded layers of sheets were used for in-plane as well as through-thickness compression testing. It was verified that this simple testing method leads to identical results as using comb-like dies and equi-biaxial bulge testing, respectively. In the case of uniaxial loading, the longitudinal and transverse strain components were measured using independent extensometers. R-values were calculated from these signals. The mechanical responses were correlated to the microstructure and the texture. The recorded differences between tensile and compressive response reveal the strength differential effect of the materials. The distortional character of the plastic behaviour is evidenced through their responses to equi-biaxial tensile loading. Significant differences in the compressive responses of the two alloys were identified by comparing the respective hardening rates.

Residual Stress Measurement Using Hole Drilling and Integrated Digital Image Correlation Techniques

Abstract: The effectiveness of optical (mostly interferometric) methods for the measurement of residual stresses is largely demonstrated in literature. Nevertheless, these techniques are still confined to optical laboratories due to their high sensitivity to vibrations which makes it very difficult to perform the measurement in an industrial environment. Digital Image Correlation (DIC) has recently been proposed as a possible solution to this problem: this non-interferometric technique is much less affected by vibrations, but its sensitivity is relatively low, thus negatively affecting the accuracy of results. This work proposes to use a variant of Digital Image Correlation, known as Integrated DIC (iDIC), in combination with the hole drilling technique. Since iDIC directly incorporates in its formulation the displacement field related to hole drilling, it overcomes most of the problems of standard DIC; in this way it is possible to obtain accurate results without using interferometric techniques.

CAD-based calibration and shape measurement with stereoDIC

Abstract: A new calibration procedure is proposed for a stereovision setup It uses the object of interest as the calibration target, provided the observed surface has a known definition (eg, its CAD model) In a first step, the transformation matrices needed for the calibration of the setup are determined assuming that the object conforms to its CAD model Then the 3D shape of the surface of interest is evaluated by deforming the a priori given freeform surface These two steps are performed via an integrated approach to stereoDIC The measured 3D shape of a machined Bezier patch is validated against data obtained by a coordinate measuring machine The feasibility of the calibration method’s application to large surfaces is shown with the analysis of a 2-m2 automotive roof panel

Experimental Study on the Perforation Process of 5754-H111 and 6082-T6 Aluminium Plates Subjected to Normal Impact by Conical, Hemispherical and Blunt Projectiles

Abstract: This paper presents an experimental investigation on the perforation behaviour of 5754-H111 and 6082-T6 aluminium alloys. The mechanical response of these materials has been characterized in compression with strain rates in the range of $10^{-3}~s^{-1} < \dot {\varepsilon } < 5 \cdot 10^{3}~s^{-1}$ . Moreover, penetration tests have been conducted on 5754-H111 and 6082-T6 plates of $4~mm$ thickness using conical, hemispherical and blunt projectiles. The perforation experiments covered impact velocities in the range of $50~m/s < V_{0} < 200~m/s$ . The initial and residual velocities of the projectile were measured and the ballistic limit velocity obtained for the two aluminium alloys for the different nose shapes. Failure mode and post-mortem deflection of the plates have been examined and the perforation mechanisms associated to each projectile/target configuration investigated. It has been shown that the energy absorption capacity of the impacted plates is the result of the collective role played by target material behaviour, projectile nose shape and impact velocity in the penetration mechanisms.

Thermal Fatigue Damage Assessment in an Isotropic Pipe Using Nonlinear Ultrasonic Guided Waves

Abstract: The use of nonlinear ultrasonic waves has been accepted as a potential technique to characterize the state of material micro-structure in solids. The typical nonlinear phenomenon is generation of second harmonics. Second harmonic generation of ultrasonic waves propagation has been vigorously studied for tracking material micro-damages in unbounded media and plate-like waveguides. However, there are few studies of launching second harmonic guided wave propagation in tube-like structures. Considering that second harmonics could provide useful information sensitive for material degradation condition, this research aims at developing a procedure for detecting second harmonics of ultrasonic guided wave in an isotropic pipe. The second harmonics generation of guided wave propagation in an isotropic and stress-free elastic pipe is investigated. Flexible polyvinylidene fluoride (PVDF) comb transducers are used to measure fundamental wave and second harmonic one. Experimental results show that nonlinear parameters increase monotonically with propagation distance. This work experimentally verifies that the second harmonics of guided waves in pipe have the cumulative effect with propagation distance. The proposed procedure is applied to assessing thermal fatigue damage indicated by nonlinearity in an aluminum pipe. The experimental observation verifies that nonlinear guided waves can be used to assess damage levels in early thermal fatigue state by correlating them with the acoustic nonlinearity.

Impact Behaviour of GLAREs with MWCNT Modified Epoxy Resins

Abstract: In this study, GLAREs (Glass Reinforced Aluminum Laminates) with 3/2 configuration were fabricated in-house with multi-walled carbon nanotubes (MWCNTs) modified epoxy resins. Uniform dispersion of MWCNTs in epoxy resin was achieved via a two-step dispersion method with concentration up to 2.0 wt%. The influence of MWCNTs on the flexural property and the impact performance of GLAREs was investigated through the three-point bending and drop weight Dynatup impact testings, respectively. The incorporation of MWCNTs into epoxy evidently improved the flexural strength and modulus. In comparison with pure epoxy bonded GLARE, the modified GLAREs generally showed improvement of impact resistance. The improvement was more obvious at low concentration of MWCNT due to better dispersion of nanotubes in the resin and reasonable wettability of glass fibres and aluminium to the modified resin. Diversified failure mechanisms including plastic deformation and rupture of metal layers, breakage of fibres and matrix, delamination between composite and metal layers, and delamination between composite plies were observed. In addition, debonding, pull-out, and bridging effects of carbon nanotubes were observed, which proved the contribution of MWCNTs to the improved impact resistance of the modified GLAREs.

Annular Pulse Shaping Technique for Large-Diameter Kolsky Bar Experiments on Concrete

Abstract: The goal of this study is to design a novel annular pulse shaping technique for large-diameter Kolsky bars for investigating the dynamic compressive response of concretes. The purpose of implementing an annular pulse shaper design is to alleviate inertia-induced stresses in the pulse shaper material that would otherwise superpose unwanted oscillations on the incident wave. This newly developed pulse shaping technique led to well-controlled testing conditions enabling dynamic stress equilibrium, uniform deformation, and constant strain-rate in the testing of a chosen concrete material. The observed dynamic deformation rate of the concrete is highly consistent (8 % variation) with the stress in the specimen well equilibrated confirming the validity of this new technique. Experimental results at both quasi-static (10−4 s−1) and dynamic (100 s−1, 240 s−1) strain rates showed that the failure strength of this concrete is rate-sensitive.

An In-Situ X-Ray Microtomography Study of Split Cylinder Fracture in Cement-Based Materials

Abstract: In an effort to quantify microstructure-property relationships, three dimensional imaging experiments were conducted on small cylinder specimens subjected to split cylinder fracture. 3-D images were made using synchrotron-based x-ray microtomography, and the experiments were conducted with an in-situ frame such that a specimen could be examined while under load at varying degrees of damage. The specimens were made of fine-grained portland cement mortar and 0.5 mm glass beads, which served as aggregates. The diameter of the specimens was 5 mm. 3-D image analysis routines were developed or adapted to characterize microstructure and internal damage, which could then be related to bulk splitting strength and fracture energy. For fracture energy calculation, crack surface area could be measured in a way that accounted for roughness, branching, and fragmentation. Results showed that, for the specimens tested, aggregate surface roughness had little effect on strength but significant effect on fracture energy. Split cylinder strength showed correlation with specimen porosity, although there was considerable scatter. Strength did not correlate with maximum flaw size, although flaw location was not evaluated.

Localized Tissue Surrogate Deformation due to Controlled Single Bubble Cavitation

Abstract: Cavitation-induced shock wave, as might occur in the head during exposure to blast waves, was investigated as a possible damage mechanism for soft brain tissues A novel experimental technique was developed to visualize and control single bubble cavitation and its collapse, and the influence of this process on a nearby tissue surrogate was investigated The experiment utilized a Hopkinson pressure bar system which transmits a simulated blast pressure wave (with over-pressure and under-pressure components) to a fluid-filled test chamber implanted with a seed gas bubble Growth and collapse of this bubble was recorded during passage of the blast wave with a high speed camera To investigate the potential for cavitation damage to a tissue surrogate, local changes in strain were measured in hydrogel slices placed in various configurations next to the bubble The strain measurements were made using digital image correlation (DIC) technique by monitoring the motion of material points on the tissue surrogate In one configuration, bubble contact dynamics resulted in compression contact (>60 μs) followed by inertially-driven tension (>140 μs) In another configuration, the influence of local shock waves emanating from collapsed bubbles was captured Large compressive strains (025 to 05) that were highly localized (018 mm2) were measured over a short time period (<24 μs) after bubble collapse High bubble collapse pressures 29 to 125 times that of peak blast overpressure are predicted to be the source of these large strains Consistent with theoretical predictions, these cavitation-based strains are far larger than the strains imposed by passage of the simulated blast wave alone Finally, the value of this experimental platform to investigate the single bubble cavitation-induced damage in a biological tissue is illustrated with an example test on rat brain slices

Strain-Rate-Dependent Failure of a Toughened Matrix Composite

Abstract: The strain-rate-dependent behavior of a toughened matrix composite (IM7/8552) was characterized under quasi-static and dynamic loading conditions. Unidirectional and off-axis composite specimens were tested at strain rates ranging from 10−4 to 103 s−1 using a servo-hydraulic testing machine and split Hopkinson pressure bar apparatus. The nonlinear response and failure were analyzed and evaluated based on classical failure criteria and the Northwestern (NU) failure theory. The predictive NU theory was shown to be in excellent agreement with experimental results and to accurately predict the strain-rate-dependent failure of the composite system based on measured average lamina properties.

The Combined Effects of Moisture and Temperature on the Mechanical Response of Paper

Abstract: To model advanced 3-D forming strategies for paper materials, the effects of environmental conditions on the mechanical behavior must be quantitatively and qualitatively understood. A tensile test method has been created, verified, and implemented to test paper at various moisture content and temperature levels. Testing results for one type of paper for moisture contents from 6.9 to 13.8 percent and temperatures from 23 to 168 degrees Celsius are presented and discussed. Coupled moisture and temperature effects have been discovered for maximum stress. Uncoupled effects have been discovered for elastic modulus, tangent modulus, hardening modulus, strain at break, tensile energy absorption (TEA), and approximate plastic strain. A hyperbolic tangent function is also utilized which captures the entire one-dimensional stress-strain response of paper. The effects of moisture and temperature on the three coefficients in the hyperbolic tangent function may be assumed to be uncoupled, which may simplify the development of moisture- and temperature-dependent constitutive models. All parameters were affected by both moisture and temperature with the exception of TEA, which was found to only be significantly dependent on temperature.

Direct Evaluation of Cohesive Law in Mode I of Pinus pinaster by Digital Image Correlation

Abstract: The direct identification of the cohesive law in pure mode I of Pinus pinaster is addressed in this work. The approach couples the double cantilever beam (DCB) test with digital image correlation (DIC). Wooden beam specimens loaded in the radial-longitudinal (RL) fracture propagation system are used. The strain energy release rate in mode I (G I) is uniquely determined from the load–displacement curve by means of the compliance-based beam method (CBBM). This method relies on the concept of equivalent elastic crack length (a eq) and therefore does not require the monitoring of crack propagation during test. DIC measurements are processed with two different purposes. Firstly, the physical evidence of a eq is discussed with regard to actual estimation of the crack length based on post-processing full-field displacement measurements. Secondly, the crack tip opening displacement in mode I (w I) is determined from the displacements near the initial crack tip. The cohesive law in mode I (σ I − w I) is then identified by numerical differentiation of the G I − w I relationship. The methodology and accuracy on this reconstruction are addressed. Moreover, the proposed procedure is validated by finite element analyses including cohesive zone modelling. It is concluded that the proposed data reduction scheme is adequate for assessing the cohesive law in pure mode I of P. pinaster.

On the Testing of the Dynamic Mechanical Properties of Soft Gelatins

Abstract: This research focuses on the measurement of the static and dynamic mechanical properties of ballistic gelatin. We present a simple, novel experimental setup for measuring the dynamic material properties of ballistic gelatin that includes the classic metallic incident and transmission bars as opposed to the polymeric Kolsky bars used by additional research groups. This method is mathematically validated, while providing sought out for stress–strain curves for three different ballistic gelatin concentrations. The results are then compared to two additional research groups, while being consistent with one and contradictory to the other. Finally, an empirical constitutive law is presented that is consistent with the results obtained through the experimental setup.

Internal Deformation Measurement and Force Chain Characterization of Mason Sand under Confined Compression using Incremental Digital Volume Correlation

Abstract: The mechanical behavior of granular materials such as sand is not well understood due to its complex solid/fluid-like behavior. In this paper, Mason sand was investigated to determine the grain-level Young’s modulus and hardness by nanoindentation, and the mesoscale behavior through X-ray tomography of a sample in compression. Mason sand specimen was confined in a polycarbonate tube and compressed in the axial direction at ten axial compressive strains up to -21.8 % while its microstructures were observed. The mesoscale deformations were determined by incremental digital volume correlation of reconstructed volumetric images. A procedure for characterization of internal force chains is developed. The minor principal strains and their principal directions were obtained and used to determine the formation and evolution of force chains.

Evaluation of Large Amplitude Deceleration Data from Projectile Penetration into Concrete Targets

Abstract: We examined data from two sets of penetration experiments that recorded deceleration during penetration into concrete targets with compressive strengths of 23 and 39 MPa. The 76.2-mm-diameter, 3.0 caliber-radius-head (CRH), 13 kg projectiles were machined from 4340 Rc 45 steel and contained a single-channel, 15 kHz acceleration data recorder. The data recorder was fitted into a circular hole in the solid nose of the projectile, so during penetration the accelerometer mounted in the data recorder measured structural responses as well as rigid-body projectile deceleration. Since the deceleration data were limited to 15 kHz, higher frequency responses were not measured. Furthermore, there are no available internationally accepted calibration procedures for accelerometers. Because of these complications, we present a method to correct the deceleration data so that an integration of the deceleration data agreed with the measured striking velocity. These corrections were small, and a double integration of the corrected deceleration was in good agreement with the measured depth of penetration. In addition, we developed two empirical penetration models that described deceleration versus displacement and specific kinetic energy (kinetic energy divided by projectile mass) versus displacement for the rigid-body response of the projectile. Data and model predictions showed that the deceleration-displacement response could be closely approximated by a linear rise with a depth of two projectile diameters followed by a region with constant deceleration until the projectile came to rest. Specific kinetic energy-displacement data and model predictions showed a nearly constant slope after the entry region, and this slope is the magnitude of the constant deceleration. Predictions from both methods closely agree with each other and the deceleration data.

Identification of the elastic properties of isotropic and orthotropic thin-plate materials with the pulsed ultrasonic polar scan

Abstract: Already in the early 1980's, it has been conjectured that the pulsed ultrasonic polar scan (P-UPS) provides a unique fingerprint of the underlying mechanical elasticity tensor at the insonified material spot. Until now, that premise has not been thoroughly investigated, nor validated, despite the opportunities this would create for NDT and materials science in general. In this paper, we report on the first-ever implementation of an inverse modeling technique on the basis of a genetic optimization scheme in order to extract quantita- tive information from a P-UPS. We validate the optimization approach for synthetic data, and apply it to experimentally obtained polar scans for annealed aluminum, cold rolled DC-06 steel as well as for carbon fiber reinforced plastics. The investigated samples are plate-like and do not require specific preparation. The inverted material characteristics show good agreement with literature, micro-mechanical models as well as with results obtained through conventional testing procedures.

Sub-Millimeter Measurement of Finite Strains at Cutting Tool Tip Vicinity

Abstract: The present paper details a simple and effective experimental procedure dedicated to strain measurement during orthogonal cutting operations. It relies on the use of high frame-rate camera and optical microscopy. A numerical post-procedure is also proposed in order to allow particle tracking from Digital Image Correlation (DIC). Therefore strain accumulation within finite strains framework is achieved. The significant magnitude of the calculated strains is partially due to a singular side effect that leads to local material disjunction. The strain localization in the Adiabatic Shear Band (ASB) exhibits different strain paths at various locations along this band and a non-linear evolution of the strain accumulation. A focus is made on the formation mechanisms of serrated chips obtained from Ti6Al4V titanium alloy. The side observation performed during this work allow to proposed three possible scenarios to explain this very phenomenon.

Influence of Drilling Parameters on the Accuracy of Hole-drilling Residual Stress Measurements

Abstract: The objective of this research is to define drill speeds that produce acceptable results when using the hole-drilling technique for measuring residual stress. For this study, three common engineering materials; alloy 6061-T651 aluminum, 304 stainless steel, and A36 carbon steel were used. This was achieved by performing ESPI/hole-drilling stress measurement of a known applied stress in discrete rpm intervals ranging from 2-40K rpm for each material. To produce a known state of stress specimens were bent elastically in a four-point bend fixture. Stress measurements were taken using single-axis electronic speckle-pattern interferometry (ESPI). It was found that for 6061-T6 aluminum, accurate and repeatable results can be achieved between speeds of 2-40K rpm. For 304 stainless steel, result accuracy diminishes when drill speeds go below 6K rpm. The A36 steel had a large as-received stress gradient across the longitudinal dimension and was therefore removed from this study.

Repeatability of the Contour Method for Residual Stress Measurement

Abstract: This paper describes the results of a residual stress measurement repeatability study using the contour method. The test specimen is an aluminum bar (cut from plate), with cross sectional dimensions of 50.8 × 76.2 mm (2″ × 3″) with a length of 609.6 mm (24″). There are two bars, one bar with high residual stresses and one bar with low residual stresses. The high residual stress configuration (±150 MPa) is in a quenched and over-aged condition (Al 7050-T74) and the low residual stress configuration (±20 MPa) is stress relieved by stretching (Al 7050-T7451). Five contour measurements were performed on each aluminum bar at the mid-length of successively smaller pieces. Typical contour method procedures are employed with careful clamping of the specimen, wire electric discharge machining (EDM) for the cut, laser surface profiling of the cut faces, surface profile fitting, and linear elastic stress analysis. The measurement results provide repeatability data for the contour method, and the difference in repeatability when measuring high or low magnitude stresses. The results show similar repeatability standard deviation for both samples, being less than 10 MPa over most of the cross section and somewhat larger, around 20 MPa, near the cross section edges. A comparison with published repeatability data for other residual stress measurement techniques (x-ray diffraction, incremental hole drilling, and slitting) shows that the contour method has a level of repeatability that is similar to, or better than, other techniques.

Interfacial Residual Stress Analysis of Thermal Spray Coatings by Miniature Ring-Core Cutting Combined with DIC Method

Abstract: The residual stress in thermal barrier coatings (TBCs) fabricated from coating deposition plays a vital role in the coating design and processing parameters optimization. The main objective of the present work is to determine the interfacial residual stress in TBCs by means of miniature ring-core cutting and the digital image correlation (DIC) method. Both the ring-core cutting and the dot pattern used for DIC deformation measurement are implemented by the focused ion beam (FIB) milling on the cross-section of a coating. A finite element model (FEM) is developed to simulate the ring-core cutting process. From the FEM, the calibration coefficients are determined for general applications. The surface of the ring-core containing dot patterns is recorded before and after the FIB milling process. DIC technique is then performed to calculate the surface displacement caused by the release of residual stresses due to the cutting. Results demonstrate that the interfacial residual stress in TBCs is nearly in a uniaxial stress state and has a tendency to be compressive toward the interface. Finally, essential aspects of the technique are discussed.

Influence of Specimen Geometry on the Estimation of the Planar Biaxial Mechanical Properties of Cruciform Specimens

Abstract: It is in general challenging to characterize planar mechanical properties of extremely soft tissues such as cell-seeded collagen gels. One of the difficulties is related to premature failure of specimens. This issue may be resolved by employing fillets on stress-concentrated spots of the specimen. The existence of fillets, however, complicates the estimation of stress at the center of the specimen where stiffness data are collected. In this study, cruciform rubber specimens with two types of fillets (general vs. cut-in fillets) at the intersections of perpendicular arms were prepared and subjected to planar biaxial mechanical testing, aiming at investigating how the fillets affect the estimation of mechanical properties of cruciform specimens. Digital image correlation was used to analyze full-field deformation in the central region of the specimens. Finite element analysis with a Neo-Hookean model was performed to simulate the full-field deformation under the same experimental boundary conditions. The strain distribution for each specimen geometry obtained by finite element analysis was found to be in good agreement with that analyzed by digital image correlation, validating the finite element models. Finite element simulation showed that the greatest value of the maximum principal strain decreased with increasing the fillet radius regardless of the fillet type. Increasing the fillet radius, in general, also reduced the strain field uniformity in the central region. Compared with general fillets, however, the use of cut-in fillets provided greater strain field uniformity given the same fillet radius. Finite element analysis was also used to estimate effective transverse length required to convert tensile force at the boundary to local stress at the center. It was found that the effective transverse length for each specimen geometry remained relatively constant if the specimen was not excessively deformed (i.e., global equibiaxial stretch ≤ 1.2). We suggest using cut-in fillets at the intersections of perpendicular arms when preparing cruciform specimens for testing extremely soft tissues.

Quantification of three-dimensional surface derormation using global digital image correlation

Abstract: A novel method is presented to experimentally quantify evolving surface profiles. The evolution of a surface profile is quantified in terms of in-plane and out-of-plane surface displacements, using a Finite Element based Global Digital Image Correlation procedure. The presented method is applied to a case study, i.e. deformation-induced surface roughening during metal sheet stretching. The surface roughness was captured in-situ using a confocal optical profiler. The Global Digital Image Correlation method with linear triangular finite elements is applied to track the three-dimensional material movement from the measured height profiles. The extracted displacement fields reveal the full-field kinematics accompanying the roughening mechanism. Local deviations from the (average) global displacements are the result of the formation, growth, and stretching of hills and valleys on the surface. The presented method enables a full-field quantitative study of the surface height evolution, i.e. in terms of tracked surface displacements rather than average height values such as Root-Mean-Square or height-height correlation techniques. However, the technique does require that an initial surface profile, i.e. contrast, is present and that the contrast change between two measurements is minimal.