Comparison Between Surface and Near-Surface Residual Stress Measurement Techniques Using a Standard Four-Point-Bend Specimen

Background

Determination of near-surface residual stresses is challenging for the available measurement techniques due to their limitations. These are often either beyond reach or associated with significant uncertainties.

Objective

This study describes a critical comparison between three methods of surface and near-surface residual stress measurements, including x-ray diffraction (XRD) and two incremental central hole-drilling techniques one based on strain-gauge rosette and the other based on electronic speckle pattern interferometry (ESPI).

Methods

These measurements were performed on standard four-point-bend beams of steel loaded to known nominal stresses, according to the ASTM standard. These were to evaluate the sensitivity of different techniques to the variation in the nominal stress, and their associated uncertainties.

Results

The XRD data showed very good correlations with the surface nominal stress, and with superb repeatability and small uncertainties. The results of the ESPI based hole-drilling technique were also in a good agreement with the XRD data and the expected nominal stress. However, those obtained by the strain gauge rosette based hole-drilling technique were not matching well with the data obtained by the other techniques nor with the nominal stress. This was found to be due to the generation of extensive compressive residual stress during surface preparation for strain gauge installation.

Conclusion

The ESPI method is proven to be the most suitable hole-drilling technique for measuring near-surface residual stresses within distances close to the surface that are beyond the penetration depth of x-ray and below the resolution of the strain gauge rosette based hole-drilling method.

     

Precision of Hole-Drilling Residual Stress Depth Profile Measurements and an Updated Uncertainty Estimator

Background

Measurement precision and uncertainty estimation are important factors for all residual stress measurement techniques. The values of these quantities can help to determine whether a particular measurement technique would be viable option.

Objective

This paper determines the precision of hole-drilling residual stress measurement using repeatability studies and develops an updated uncertainty estimator.

Methods

Two repeatability studies were performed on test specimens extracted from aluminum and titanium shot peened plates. Each repeatability study included 12 hole-drilling measurements performed using a bespoke automated milling machine. Repeatability standard deviations were determined for each population. The repeatability studies were replicated using a commercially available manual hole-drilling milling machine (RS-200, Micro-Measurements). An updated uncertainty estimator was developed and was assessed using an acceptance criterion. The acceptance criterion compared an expected percentage of points (68%) to the fraction of points in the stress versus depth profile where the measured stresses ± its total uncertainty contained the mean stress of the repeatability studies.

Results

Both repeatability studies showed larger repeatability standard deviations at the surface that decay quickly (over about 0.3 mm). The repeatability standard deviation was significantly smaller in the aluminum plate (max ≈ 15 MPa, RMS?≈?6.4 MPa) than in the titanium plate (max ≈ 60 MPa, RMS?≈?21.0 MPa). The repeatability standard deviations were significantly larger when using the manual milling machine in the aluminum plate (RMS?≈?21.7 MPa), and for the titanium plate (RMS?≈?18.9 MPa).

Conclusions

The single measurement uncertainty estimate met a defined acceptance criterion based on the confidence interval of the uncertainty estimate.

     

Periodic solution and chaotic behavior of a class of nonautonomic pendulum systems with large damping

In this paper the existence and uniqueness of the periodic solution is studied for a class of second order nonautonomic pendulum aystems x + ax + ψ(t)sinx=F(t) anil the parameter regions tor which the system in chaos is myestigated when ψ(t)=1-ελcosωt, F(t)=β +εμ(cosωt-ωsinωt) and the tamping coefficient a>0 is large. The result obtained generalize the corresponding conclusions of papers [1-8].     

Mechanical Stimuli for Left Ventricular Growth During Pressure Overload

Background

The mechanical stimulus (i.e., stress or stretch) for growth occurring in the pressure-overloaded left ventricle (LV) is not exactly known.

Objective

To address this issue, we investigate the correlation between local ventricular growth (indexed by local wall thickness) and the local acute changes in mechanical stimuli after aortic banding.

Methods

LV geometric data were extracted from 3D echo measurements at baseline and 2 weeks in the aortic banding swine model (n?=?4). We developed and calibrated animal-specific finite element (FE) model of LV mechanics against pressure and volume waveforms measured at baseline. After simulation of the acute effects of pressure-overload, the local changes of maximum, mean and minimum myocardial stretches and stresses in three orthogonal material directions (i.e., fiber, sheet and sheet-normal) over a cardiac cycle were quantified. Correlation between mechanical quantities and the corresponding measured local changes in wall thickness was quantified using the Pearson correlation number (PCN) and Spearman rank correlation number (SCN).

Results

At 2 weeks after banding, the average septum thickness decreased from 10.6?±?2.92 mm to 9.49?±?2.02 mm, whereas the LV free-wall thickness increased from 8.69?±?1.64 mm to 9.4?±?1.22 mm. The FE results show strong correlation of growth with the changes in maximum fiber stress (PCN?=?0.5471, SCN?=?0.5111) and changes in the mean sheet-normal stress (PCN?=?0.5266, SCN?=?0.5256). Myocardial stretches, however, do not have good correlation with growth.

Conclusion

These results suggest that fiber stress is the mechanical stimuli for LV growth in pressure-overload.

     

Adaptive Remodeling in the Elastase-Induced Rabbit Aneurysms

Background

Rupture of brain aneurysms is associated with high fatality and morbidity rates. Through remodeling of the collagen matrix, many aneurysms can remain unruptured for decades, despite an enlarging and evolving geometry.

Objective

Our objective was to explore this adaptive remodeling for the first time in an elastase induced aneurysm model in rabbits.

Methods

Saccular aneurysms were created in 22 New Zealand white rabbits and remodeling was assessed in tissue harvested 2, 4, 8 and 12 weeks after creation.

Results

The intramural principal stress ratio doubled after aneurysm creation due to increased longitudinal loads, triggering a remodeling response. A distinct wall layer with multi-directional collagen fibers developed between the media and adventitia as early as 2 weeks, and in all cases by 4 weeks with an average thickness of 50.6?±?14.3 μm. Collagen fibers in this layer were multi-directional (AI?=?0.56?±?0.15) with low tortuosity (1.08?±?0.02) compared with adjacent circumferentially aligned medial fibers (AI?=?0.78?±?0.12) and highly tortuous adventitial fibers (1.22?±?0.03). A second phase of remodeling replaced circumferentially aligned fibers in the inner media with longitudinal fibers. A structurally motivated constitutive model with both remodeling modes was introduced along with methodology for determining material parameters from mechanical testing and multiphoton imaging.

Conclusions

A new mechanism was identified by which aneurysm walls can rapidly adapt to changes in load, ensuring the structural integrity of the aneurysm until a slower process of medial reorganization occurs. The rabbit model can be used to evaluate therapies to increase aneurysm wall stability.

     

Overview of High-temperature Deformation Measurement Using Digital Image Correlation

Background

Developments in digital image correlation (DIC) in the last decade have made it a practical and effective optical technique for displacement and strain measurement at high temperatures.

Objective

This overview aims to review the research progress, summarize the experience and provide valuable references for the high-temperature deformation measurement using DIC.

Methods

We comprehensively summarize challenges and recent advances in high-temperature DIC techniques.

Results

Fundamental principles of high-temperature DIC and various approaches to generate thermal environment or apply thermal loading are briefly introduced first. Then, the three primary challenges presented in performing high-temperature DIC measurements, i.e., 1). image saturation caused by intensified thermal radiation of heated sample and surrounding heating elements, 2) image contrast reduction due to surface oxidation of the heated sample and speckle pattern debonding, and 3) image distortion due to heat haze between the sample and the heating source, and corresponding countermeasures (i.e., the suppression of thermal radiation, fabrication of high-temperature speckle pattern and mitigation of heat haze) are discussed in detail. Next, typical applications of high-temperature DIC at various spatial scales are briefly described. Finally, remaining unsolved problems and future goals in high-temperature deformation measurements using DIC are also provided. 

Conclusions

We expect this review can guide to build a suitable DIC system for kinematic field measurements at high temperatures and solve the challenging problems that may be encountered during real tests.

     

Hard-material Adhesion: Which Scales of Roughness Matter?

Background

Surface topography strongly modifies adhesion of hard-material contacts, yet roughness of real surfaces typically exists over many length scales, and it is not clear which of these scales has the strongest effect. Objective: This investigation aims to determine which scales of topography have the strongest effect on macroscopic adhesion.

Methods

Adhesion measurements were performed on technology-relevant diamond coatings of varying roughness using spherical ruby probes that are large enough (0.5-mm-diameter) to sample all length scales of topography. For each material, more than 2000 measurements of pull-off force were performed in order to investigate the magnitude and statistical distribution of adhesion. Using sphere-contact models, the roughness-dependent effective values of work of adhesion were measured, ranging from 0.08 to 7.15 mJ/m² across the four surfaces. The data was more accurately fit using numerical analysis, where an interaction potential was integrated over the AFM-measured topography of all contacting surfaces.

Results

These calculations revealed that consideration of nanometer-scale plasticity in the materials was crucial for a good quantitative fit of the measurements, and the presence of such plasticity was confirmed with AFM measurements of the probe after testing. This analysis enabled the extraction of geometry-independent material parameters; the intrinsic work of adhesion between ruby and diamond was determined to be 46.3 mJ/m². The range of adhesion was 5.6 nm, which is longer than is typically assumed for atomic interactions, but is in agreement with other recent investigations. Finally, the numerical analysis was repeated for the same surfaces but this time with different length-scales of roughness included or filtered out.

Conclusions

The results demonstrate a critical band of length-scales—between 43 nm and 1.8 µm in lateral size—that has the strongest effect on the total adhesive force for these hard, rough contacts.

     

Residual stress analysis near a cold expanded hole in a textured alclad sheet using X-ray diffraction

In this paper we present the methods of determination and the stress obtained at the periphery of a cold expanded hole in a 2024-T3 Alclad aluminum alloy sheet. The measurements in the aluminum clad were performed by the sin²Ψ method, taking experimental precautions to deal with the texture effects. In the core aluminum a special method had to be implemented to determine the stress values in a direction not accessible to the X-ray diffraction. The strains were measured in sample orientations selected according to the texture characteristics and stress factorsF _ij were used to calculate the stress tensor. TheF _ij values were determined assuming a quasi-isotropic material behavior, after concluding that the stress results were not significantly affected by factors calculated for textured material. The residual stress profile, both in the clad and in the sheet, shows a nearly axisymmetric stress state. Compressive stresses were observed near the periphery of the hole, with values that are higher on the exit than on the entrance face. Residual stresses were also higher in the hoop direction than in the radial direction. They decreased with the radial distance to the hole and affected the previous stress state over a distance of 6 mm. The plastic deformation induced by the cold expansion is well evidenced by the FWHM values, which in the affected zone decrease with increasing distance from the hole edge.     

Arterial Wall Stiffening in Caveolin-1 Deficiency-Induced Pulmonary Artery Hypertension in Mice

Background

Pulmonary artery hypertension (PAH) is a complex disorder that can lead to right heart failure. The generation of caveolin-1 deficient mice (CAV-1^?/?) has provided an alternative genetic model to study the mechanisms of pulmonary hypertension. However, the vascular adaptations in these mice have not been characterized.

Objective

To determine the histological and functional changes in the pulmonary and carotid arteries in CAV-1^?/? induced PAH.

Methods

Pulmonary and carotid arteries of young (4–6 months old) and mature (9–12 months old) CAV-1^?/? mice were tested and compared to normal wild type mice.

Results

Artery stiffness increases in CAV-1^?/? mice, especially the circumferential stiffness of the pulmonary arteries. Increases in stiffness were quantified by a decrease in circumferential stretch and transition strain, increases in elastic moduli, and an increase in total strain energy at physiologic strains. Changes in mechanical properties for the pulmonary artery correlated with increased collagen content while changes in the carotid artery correlated with decreased elastin content.

Conclusions

We demonstrated that an increase in artery stiffness is associated with CAV-1 deficiency-induced pulmonary hypertension. These results improve our understanding of arterial remodeling in PAH.

     

The Association Between Curvature and Rupture in a Murine Model of Abdominal Aortic Aneurysm and Dissection

Background

Mouse models of abdominal aortic aneurysm (AAA) and dissection have proven to be invaluable in the advancement of diagnostics and therapeutics by providing a platform to decipher response variables that are elusive in human populations. One such model involves systemic Angiotensin II (Ang-II) infusion into low density-lipoprotein receptor-deficient (LDLr?/?) mice leading to intramural thrombus formation, inflammation, matrix degradation, dilation, and dissection. Despite its effectiveness, considerable experimental variability has been observed in AAAs taken from our Ang-II infused LDLr?/? mice (n?=?12) with obvious dissection occurring in 3 samples, outer bulge radii ranging from 0.73 to 2.12 mm, burst pressures ranging from 155 to 540 mmHg, and rupture location occurring 0.05 to 2.53 mm from the peak bulge location.

Objective

We hypothesized that surface curvature, a fundamental measure of shape, could serve as a useful predictor of AAA failure at supra-physiological inflation pressures.

Methods

To test this hypothesis, we fit well-known biquadratic surface patches to 360^o micro-mechanical test data and used Spearman’s rank correlation (rho) to identify relationships between failure metrics and curvature indices.

Results

We found the strongest associations between burst pressure and the maximum value of the first principal curvature (rho?=??0.591, p-val?=?0.061), the maximum value of Mean curvature (rho?=??0.545, p-val?=?0.087), and local values of Mean curvature at the burst location (rho?=??0.864, p-val?=?0.001) with only the latter significant after Bonferroni correction. Additionally, the surface profile at failure was predominantly convex and hyperbolic (saddle-shaped) as indicated by a negative sign in the Gaussian curvature. Findings reiterate the importance of shape in experimental models of AAA.

     

Scanning-Digital Image Correlation for Moving and Temporally Deformed Surfaces in Scanning Imaging Mode

Background

Images from scanning electron microscopes, transmission electron microscopes and atomic force microscopes have been widely used in digital image correlation methods to obtain accurate full-field deformation profiles of tested objects and investigate the object’s deformation mechanism. However, because of the raster-scanning imaging mode used in microscopic observation equipment, the images obtained from these instruments can only be used for quasi-static displacement measurements; otherwise, spurious displacements and strains may be introduced into the deformation results if these scanning microscopic images are used directly in general digital image correlation calculations for moving and temporally deformed surfaces.

Objective

Realizing kinematic parameter and dynamic deformation measurements on a scanning electron microscope platform.

Methods

Establishing a scanning imaging model of moving and temporally deformed objects that contains motion and deformation equations, a scanning equation and an intensity invariance assumption for small deformations. Then proposing a scanning-digital image correlation (S-DIC) method based on combing the characteristics of the scanning imaging mode with digital image correlation.

Results

Quantitatively investigating the effects of the spurious displacements and strains introduced when using scanning images to represent moving and temporally deformed surfaces in the measurement results. Numerical simulations verify that the accuracy of the S-DIC method is 10^?2pix for the displacement, 10^?4 for the strain, 10^?4pix/s for the velocity and 10^?6s^?1 for the strain rate. Experiments also show that the proposed S-DIC method is effective. Conclusions: The results of this work demonstrate the utility of S-DIC on the field of microscopic dynamic measurement.

     

Determination of stress-strain characteristics at very high strain rates

A modified version of the Kolsky thin-wafer technique is described. The method permits one to obtain the dynamic plastic properties of materials at strain rates as high as 10⁵ sec^?1. Data obtained from compression tests on high-purity aluminum are presented for strain rates ranging from 4000 to 120,000 sec^?1 at room temperature. Specimen-size effects and the effect of lateral inertia are taken into account in analyzing the data. The results plotted as stress vs. strain rate at constant strains (5 to 20 percent) show that, at the highest strain rates, the stress rises very rapidly with strain rate suggesting that a limiting strain rate is being reached. At the lower strain rates (10³ to 10⁴ sec^?1), the stress is linearly proportional to the strain rate indicating that the material is deforming in a viscous manner.     

Physical Evidence of Stress-Induced Conformational Changes in Polymers

Background

Polymer mechanics and characterization is an active area of research where a keen effort is directed towards gaining a predictive and correlative relationship between the applied loads and the specific conformational motions of the macromolecule chains.

Objective

Therefore, the objective of this research is to introduce the preliminary results based on a novel technique to in situ probe the mechanical properties of polymers using non-invasive, non-destructive, and non-contact terahertz spectroscopy.

Methods

A dielectric elastomer actuator (DEA) structure is used as the loading mechanism to avoid obscuring the beam path of transmission terahertz time-domain spectroscopy. In DEAs, the applied voltage results in mechanical stresses under the active electrode area with far-reaching stretching in the passive area. Finite element analysis is used to model and simulate the DEA to quantify the induced stresses at the observation site over a voltage range spanning from 0 V to 3000 V. Additionally, a novel analysis technique is introduced based on the Hilbert-Huang transform to exploit the time-domain signals of the ultrathin elastomeric film and to defy the limits set forth by the current state-of-the-art analysis techniques.

Results

The computational result shows a nonlinear relationship between the effective stresses and the applied voltage. Analysis of the terahertz time-domain signals shows a shift in the delay times and a decrease in signal peak amplitudes, whereas these characteristics are implicitly related to the change in the index of refraction.

Conclusions

In all, the results evidentially signify the interrelationship between the conformational changes and applied mechanical stress.

     

Thermal Stress Measurement in Continuous Welded Rails Using the Hole-Drilling Method

The absence of expansion joints in Continuous Welded Rail has created the need for the railroad industry to determine the in-situ thermal stress levels for rail buckling and breakage prevention. This paper explores the hole-drilling method as a possible solution to this problem. A new set of calibration coefficients to compute the stress field relieved by fine hole depth increments required by the high strength steel was determined. The new calibration coefficients were experimentally validated on an aluminum plate subjected to a known uniaxial load. The thermal stress levels of constrained rails were estimated after compensation for the residual stress components, based on statistical relationships developed experimentally between the longitudinal and the vertical residual stresses. The results showed that the hole-drilling procedure, with appropriate calibration coefficients and residual stress compensation, can estimate the in-situ rail thermal stresses with an expected accuracy that is within the industry acceptable levels.     

Full-field Thermal Deformation Measurements in a Scanning Electron Microscope by 2D Digital Image Correlation

Using recently developed methods for application of a nano-scale random pattern having high contrast during SEM imaging, baseline full-field thermal deformation experiments have been performed successfully in an FEI Quanta SEM using 2D-DIC methods. Employing a specially redesigned commercial heating plate and control system, with modified specimen attachment procedures to minimize unwanted image motions, recently developed distortion correction procedures were shown to be effective in removing both drift and spatial distortion fields under thermal heating. 2D-DIC results from heating experiments up to 125°C on an aluminum specimen indicate that (a) the fully corrected displacement components have nearly random variability and a standard deviation of 0.02 pixels (≈25 nm at 200× and ≈0.5 nm at 10,000×) in each displacement component and (b) the unbiased measured strain fields have a standard deviation ≈150 × 10⁻⁶ and a mean value that is in good agreement with independent measurements, confirming that the SEM-DIC based method can be used for both micro-scale and nano-scale thermal strain measurements.

DLT-Lines Based Camera Calibration with Lens Radial and Tangential Distortion

Background

Camera calibration is an essential step for the optical measurement method used in the experimental mechanics. Most plumb line methods focus on solving lens distortions without considering camera intrinsic and extrinsic parameters.

Objective

In this paper, we propose a full camera calibration method to estimate the camera parameters, including camera intrinsic parameters, extrinsic parameters and lens distortion parameters, from a single image with six or more non-coplanar lines.

Methods

We parameterize the 3D lines with the intersection of two planes that allow the direct linear transformation of the lines(DLT-Lines). Based on the DLT-Lines, the projection matrix is estimated linearly, and then the camera intrinsic and extrinsic parameters are extracted from the matrix. The relationship between the distorted 2D lines and the distortion coefficients is derived, based on which the distortion coefficients can be solved linearly. In the last step, a non-linear optimization algorithm is used to jointly refine all the camera parameters, including the distortion coefficients.

Results

Both synthetic and real data are used to evaluate the performance of our method, which demonstrates that the proposed method can calibrate the cameras with radial and tangential distortions accurately.

Conclusions

We propose a DLT-lines based camera calibration method for experimental mechanics. The proposed method can calibrate all the camera parameters from a single image.

     

Analysis of Damage and Failure in Anisotropic Ductile Metals Based on Biaxial Experiments with the H-Specimen

Background

Dependence of strength and failure behavior of anisotropic ductile metals on loading direction and on stress state has been indicated by many experiments. To realistically predict safety and lifetime of structures these effects must be taken into account in material models and numerical analysis.

Objective

The influence of stress state and loading direction on damage and failure behavior of the anisotropic aluminum alloy EN AW-2017A is investigated.

Methods

New biaxial experiments and numerical simulations have been performed with the H-specimen under different load ratios. Digital image correlation shows evolution of strain fields and scanning electron microscopy is used to visualize failure modes on fracture surfaces. Corresponding numerical studies predict stress states to explain damage and fracture processes on the micro-scale.

Results

The stress state, the load ratio and the loading direction with respect to the principal axes of anisotropy affect the width and orientation of localized strain fields and the formation of damage mechanisms and fracture modes at the micro-level.

Conclusions

The enhanced experimental program with biaxial tests considering different loading directions and load ratios is suggested for characterization of anisotropic metals.

     

Dynamic Subsurface Deformation and Strain of Soft Hydrogel Interfaces Using an Embedded Speckle Pattern With 2D Digital Image Correlation

Background

Subsurface mechanisms can greatly affect the mechanical behavior of biological materials, but observation of these mechanisms has remained elusive primarily due to unfavorable optical characteristics. Researchers attempt to overcome these limitations by performing experiments in biological mimics like hydrogels, but measurements are generally restricted due to the spatio-temporal limitations of current methods.

Objective

Utilization of contemporary 3D printing techniques into soft, transparent, aqueous yield-stress materials have opened new avenues of approach to overcoming these roadblocks. By incorporating digital image correlation with such 3D printing techniques, a method is shown here that can acquire full-field deformation of a hydrogel subsurface in real-time.

Methods

Briefly, the method replaces the solvent of a transparent and low polymer concentration yield-stress material with an aqueous hydrogel precursor solution, then a DIC speckle plane is 3D printed into it. This complex is then polymerized using photoinitiation thereby locking the speckle plane in place.

Results

Full-field deformation measurements are made in real-time as the embedded speckle plane (ESP) responds with the bulk to the applied load. Example results of deformation and strain fields associated with indentation, relaxation, and sliding contact experiments are shown.

Conclusions

This method has successfully observed the subsurface mechanical response in the bulk of a hydrogel and has the potential to answer fundamental questions regarding biological material mechanical behaviors.