Full-field Stress Research Articles

Full-Field Stress Analysis of Perforated Members from Aligned Single-Element Strain Gages

Full-Field Stress Analysis of Perforated Members from Aligned Single-Element Strain Gages

Data processing of wave propagation in viscoelastic split Hopkinson pressure bar

In this study, the polymethyl methacrylate (PMMA) bar was taken as an example to study the data processing of the viscoelastic Split Hopkinson pressure bar (SHPB) during shock wave propagation. First, SHPB tests of the PMMA bar were conducted, and the strain data measured at the position of the strain gauges on the viscoelastic PMMA bar were processed by using the improved Lagrange analysis method (LAM) to obtain the full-field strain, particle velocity, and stress data. Then, the Zhu–Wang–Tang dynamic viscoelastic constitutive model was adopted, and the parameters were calibrated to determine the dynamic constitutive equation of the PMMA bar. By combining the characteristics method and the dynamic constitutive equation, numerical simulation was conducted to obtain the physical quantity data at each point on the PMMA bar, so as to realize the closed-loop test. By comparing the data obtained by the improved LAM with the data obtained by the characteristics method, it was found that the improved LAM can improve the calculation accuracy at the later loading stage and was more consistent with the actual situation, and the validity of data processing and the applicability of the dynamic constitutive equation at the early loading stage were verified as well. The improved LAM can be extended to the propagation calculation of the attenuation wave in SHPB tests of soft materials or low density materials.

Automatic Calibration of a Geomechanical Model from Sparse Data for Estimating Stress in Deep Geological Formations

Summary In this study, we demonstrate geomechanical modeling with fully automatic parameter calibration to estimate the full geomechanical stress fields of a prospective US carbon dioxide (CO2) storage site, based on sparse measurement data. The goal is to compute full stress tensor field estimates (principal stresses and orientations) that are maximally compatible with observations within the constraints of the model assumptions, thereby extending pointwise, incomplete partial stress measurement to a simulated full formation stress field, as well as a rough assessment of the associated error. We use the Perch site, located in Otsego County, Michigan, USA, as our case study. The input data consist of partial stress tensor information inferred from in-situ borehole tests, geophysical well logs, and processing of seismic data. A static earth model (SEM) of the site was developed, and geomechanical simulation functionality of the open-source MATLAB Reservoir Simulation Toolbox (MRST) was used to model the stress field. Adjoint-based nonlinear optimization was used to adjust boundary conditions and material properties to calibrate simulated results of observations. Results were interpreted through a Bayesian framework. The focus of this paper is to demonstrate how the fully automatic calibration procedure works and discuss the results obtained; it does not attempt a detailed analysis of the stress field in the context of the proposed CO2 storage initiatives. Our work is part of a larger effort to noninvasively determine in-situ stresses in deep formations considered for CO2 storage. Guided by previously published research on geomechanical model calibration, our work presents a novel calibration approach supporting a potentially large number of linear or nonlinear calibration parameters to produce results optimally agreeing with available measurements and thus extend partial pointwise estimates to full tensor fields compatible with the physics of the site.

Evaluation and Origin of Residual Stress in Hybrid Metal and Extrusion Bonding and Comparison with Friction Stir Welding

Hybrid metal and extrusion bonding (HYB) is an emerging solid-state welding technique that was developed about ten years ago. HYB exploits the fundamental idea of the well-established friction stir welding (FSW) technology, but a filler material is employed to enhance control of the microstructure and the mechanical properties of the joint. HYB and FSW allow joining to be performed at lower temperatures than classical fusion welding methods. Still, thermal gradient effects seem impossible to be entirely avoided, thus leading to residual stress within the weld region and neighbouring material. Although the FSW-induced residual stress evaluation has been extensively studied and understood, the evaluation and interpretation of HYB-induced residual stress have not been tackled so far. In the present paper, a quantitative investigation on residual stress and its origin in HYB was carried out for the first time. Specifically, a 4 mm thick AA6082-T6 HYB and a 4 mm thick AA6082-T6 FSW butt welds were considered. For the particular case of HYB, an AA6082-T4 was used as the filler material. In both cases, the full-field longitudinal residual stress was experimentally assessed using the Contour Method. The results showed that the HYB joint yields a higher magnitude of tensile residual stress compared to that of the FSW counterpart. A physical explanation for this difference in magnitude was attributed to the lower yield stress point exhibited by the filler material. Furthermore, the analysis revealed peak values of residual stress as high as 205±25 MPa and 165±15 MPa, for the HYB and FSW joint, respectively. Despite this, a similar distribution of residual stress across the weld was observed in both cases. An additional qualitative analysis on the transverse distortion of the welds outlined a pronounced undesired “V-like” deformation of the HYB joint of approximately 1.4°. By contrast, the FSW joint seemed not to show any perceptible bend.

Numerical study on full-field stress evolution and acoustic emission characteristics of rocks containing three parallel pre-existing flaws under uniaxial compression

A linear parallel bond model combined with Weibull distribution to take into account the mechanical heterogeneity of brittle rocks is used to investigate the cracking behavior, acoustic emission (AE) characteristics and full-field stress evolution of rocks containing three parallel pre-existing flaws under uniaxial compression. The intact model is calibrated to the macro mechanical properties of rocks obtained from laboratory tests. The numerical results indicate that the presence of three parallel pre-existing flaws weakens the mechanical properties of rocks. As the flaw inclination increases, uniaxial compression strength shows a trend of first decreasing and then increasing, and the elastic modulus and Poisson's ratio are roughly increasing. The displacement localization zones chronologically formed around pre-existing flaws can interact with each other, and the damage or fracture processes of pre-cracked samples are complicated. And the tensile stress in x direction concentrates around pre-existing flaws. As the flaw inclination increases, the tensile stress concentration area at the end of the pre-existing flaws in the x direction also changes, at the same time, the area of the tensile stress concentration area in the y direction gradually decreases. And AE events with high fracture intensity are located in the macroscopic shear zone or split zone.

Residual stress characterization during laser-based Directed Energy Deposition using in-situ digital image correlation based on specular light reflection on as-built surfaces of thin walls

Residual stresses present one of the major challenges within additive manufacturing as they lead to part distortion and cracking, production issues, scrapped parts and mechanically underperforming components. The current work presents a method to determine full-field residual stress information by deploying in-situ Digital Image Correlation during the Directed Energy Deposition process of thin walls. In an attempt not to disturb the process to apply the typical ‘speckle pattern’, the correlation analysis is based on the specular light reflection of active lighting on the as-built surface of thin walls under construction. This approach allows the continuous monitoring of the thermally induced stresses, both during the actual deposition process as well as during the subsequent cooldown phase. The method presents significant time-savings while providing maximized insight in the formation process of residual stresses. The manuscript presents the used test configuration, initial proof-of-concept results along with an accuracy estimation of the used method.

Super-Resolution Stress Imaging for Terahertz-Elastic Based on SRCNN

Limited by diffraction limit, low spatial resolution is one of the shortcomings of terahertz imaging. Low spatial resolution is also one of the reasons limiting the development of stress measurement using terahertz imaging. In this paper, the full-field stress measurement using Terahertz Time Domain Spectroscopy (THz-TDS) is combined with Super-Resolution Convolutional Neural Network (SRCNN) algorithm to obtain stress fields with high spatial resolution. A modulation model from a plane stress state to a THz-TDS signal is constructed. A large number of simulated sets are obtained to train the SRCNN model. By applying the trained SRCNN model to imaging the numerical and physical stress fields, the improved spatial resolution of stress field calculated from the captured THz-TDS signal is obtained.

Construction of Nonsingular Stress Fields for Non-Euclidean Model in Planar Deformations

A material with a microstructure is considered. A material is described on the basis of a non-Euclidean model of a continuous medium. In equilibrium, the total stress field is represented as the sum of elastic and self-balanced stresses, the parameterization of which is given through the scalar curvature of the Ricci tensor. It is proposed to use the spectral biharmonic equation to calculate the scalar curvature. Using the example of a plane strain state of a material, it is shown that the amplitude coefficients of elastic and self-balanced fields can be chosen so that singularities of the same type compensate each other in the full stress field

Full-field stress measuring method based on terahertz time-domain spectroscopy.

Terahertz wave has been used to obtain the internal information of materials due to its penetrability to optically opaque materials. In this paper, we propose a full-field stress measuring method using terahertz time-domain spectroscopy. The theoretical model of the method was established based on the stress-optical law, and the method was then validated experimentally. A four-point bending experiment was conducted to determine the stress-optical coefficient of the specimen material, after which the proposed method was used to measure the stress distribution of a diametrically loaded disk. The experimental results present a reasonable agreement with the theoretical solution.

Experimental analysis of control technology and deformation failure mechanism of inclined coal seam roadway using non-contact DIC technique

The deformation and failure forms of inclined coal seam roadway under the joint action of dip angle and various geological conditions are complex, and there is a lack of targeted support measures, which brings great problems to the stability control of roadway surrounding rock. In order to safely and economically mine inclined coal seams, taking the engineering geology of Shitanjing No. 2 mining area as the background, and the physical similarity model of right-angle trapezoidal roadway in inclined coal seam, in which the non-contact digital image correlation (DIC) technology and the stress sensor is employed to provide full-field displacement and stress measurements. The deformation control technology of the roadway surrounding rock was proposed, verified by numerical simulation and applied to engineering practice. The research results show that the stress and deformation failure of surrounding rock in low sidewall of roadway are greater than those in high sidewall, showing asymmetric characteristics, and the maximum stress concentration coefficients of roadway sidewall, roof and floor are 4.1, 3.4 and 2.8, respectively. A concept of roadway "cyclic failure" mechanism is proposed that is, the cyclic interaction of the two sidewalls, the sharp angles and roof aggravated the failure of roadway, resulting in the overall instability of roadway. The roadway sidewall is serious rib spalling, the roof is asymmetric "Beret" type caving arch failure, and the floor is slightly bulging. On this basis, the principle of roadway deformation control is revealed and asymmetric support design is adopted, and the deformation of roadway is controlled, which support scheme is effective.

Optical method for identification and quantification of full-field stress distributions and evolution in assembled lining structures based on additively printed models and phase-shifting methods

The accurate knowledge and characterization of the full-field stress distributions and evolution in assembled lining structures underlie the design, construction, and maintenance of tunnel linings. However, it is challenging for traditional field monitoring and laboratory experiments to identify directly and quantify accurately such distributions and evolution. Numerical simulations, as alternative solutions, are limited, as their results are significantly dependent on the accuracy of mechanical parameters, constitutive laws, failure criteria, mesh optimization, and boundary conditions, which are difficult to determine experimentally. In this study, a plane-stress-model-based photoelastic testing method was developed for the direct observation and quantification of the full-field stress distribution and evolution in an assembled lining structure based on additive manufacturing or three-dimensional printing technology and phase shifting methods. Three-dimensional printing techniques and stress- sensitive photopolymers were employed to fabricate transparent assembled models based on the information of real lining structures. The model split-merge technique, phase-shifting method, and unwrapping algorithm were combined to determine the full-field distribution of the principal stress difference and shear stress within the assembled segments. The proposed method was verified based on a comparison of the calculated and measured fringe orders at different loading stages. The experimental results indicated that the largest stress concentration was around the joint that connected the adjacent Segment L1 and basic Segment B1, which formed the potential critical failure zone of the assembled structure. An approximate X-shaped shear band was also identified. Comparisons of the results verified that the boundary and contact conditions, which are difficult to simulate, can lead to significantly different simulation results with respect to the distribution and evolution of the principal stress difference and shear stress. This study shows that the proposed method can effectively and quantitatively characterize the full-field stress distributions and evolution within complex assembled lining structures.

Analyze the influence of explosion stress wave on the dynamic fracture of PMMA plates with different curvature radius defects

Defects in brittle solid materials affect the transmission of stress waves, thereby influencing the fracture process of the material. In this paper, the effect of defect curvature radius on the propagation of explosion stress wave and crack propagation is studied. Polymethyl methacrylate (PMMA) is used as the experimental material. The explosive loading and transmission caustics systems are combined to observe and record the explosive fracture process of a PMMA plate with different curvature radius defects. First, the interaction process between the explosion stress wave and defects with different radii of curvature is analyzed via caustics. Then the propagation process of prefabricated defects under the action of the explosion stress wave is studied. Moreover, crack tip propagation speed and the change of the stress intensity factor are simultaneously calculated. Numerical simulation is used to restore the full-field stress change under the action of the explosion stress wave and the defect. Lastly, stress time history curve of the point in the proximity of the defect is analyzed.

Determination of the stress and strain fields in porous structures by photoelasticity and digital image correlation techniques

Quantitative visualization of the full-field stress and strain in porous structures is of significance to reveal the mechanism of the damage and failure of engineering materials. To quantitatively characterize the full-field stresses and strains, optical measurement methods, such as photoelasticity for determining the stress fields, moiré and digital image correlation (DIC) methods for measuring the strain fields, have been developed in previous works. However, it is challenging to combine these methods to measure stress and strain fields simultaneously because the speckles and gratings for strain measurement will disturb the interference fringes in the photoelasticity. In this study, a method incorporating photoelasticity and DIC techniques was developed to determine the stress and strain fields simultaneously in different porous structures which were fabricated by three-dimensional (3D) printed technique. The tested models were printed with a highly transparent, colourless material and the speckles on the front surface of the tested models were printed with a translucent, magenta material. Under compressive loads, the stress fields in these models were obtained based on the photoelastic patterns and the strain fields were determined by DIC method. The results indicate the proposed method can simultaneously determine the full-field stresses and strains and the comparison of the distribution of the principal stress and strain difference and the distribution of the shear stress and strain verified its good accuracy. In addition, the stress and strain distribution in different pore structures were compared and found that the optimal design of the pore distribution can greatly change the concentration of the stress and strain fields.

Similar biomechanical properties of four tripled tendon graft models for ACL reconstruction

PurposeThe present study tested and compared the biomechanical properties of four different triplicate graft tendon techniques.Methods32 tripled tendons from the common extensor muscle of bovine fingers were tested on a material testing machine, passing the end loop over a metal rod of a clevis connected to the load cell on the upper side, and fixing the lower end to a clamp. The samples were divided into four groups: (A) tripled with a free end sutured only to one of the two fixed bundles (B) tripled with a free end positioned between the two fixed strands and sutured to both (C) tripled with an S-shape and all the three strands sutured together at the upper and lower extremities of the graft (D) partially quadrupled with the free end sutured together with the other three bundles at the upper extremity. Each sample was pretensioned at 50 N for 10 min and then subjected to 1000 load control cycles between 50 and 250 N. Finally, each sample was subjected to a load to failure test.Authors also present some preliminary results on the feasibility of a non-contact and full-field Thermoelastic Stress Analysis technique, based on Infrared Thermography, to evaluate the level of stress on the whole graft, and hence on each strand, during fatigue loading.ResultsEighty five percent of the samples failed at the level of the clamp. The cyclical elongation progressively decreased in all the samples and there was a simultaneous increase in stiffness. An increased stiffness was noted between Group 2 vs Group 3 and Group 2 vs Group 4 at the 500th and 1000th cycle. The failure loads were as follows: (a) 569.10 N, (b) 632.28 N, (c) 571.68 N, (d) 616.95 N. None of the parameters showed a statistically significant difference between the four groups.ConclusionThis study reported similar biomechanical behavior of four different models of tripled grafts suitable for ACL reconstruction. In addition, the biomechanics of overall tripled tendon grafts seems more affected by the viscoelastic property of the tendon itself rather than the preparation method.

Bioinspired nacre-like GO-based fiber with improved strength and toughness by staggered layer structure regulation and interface modification

High-performance fiber materials are widely used in aerospace, automobile and other hi-tech fields due to their excellent mechanical properties. Here, inspired by natural organisms, a kind of bionic composite fiber with excellent mechanical properties, consisting of graphene oxide (GO) and thermoplastic polyurethane (TPU), was prepared by wet spinning method. In the composite fiber, the hard GO sheets and flexible TPU were arranged staggered to form a nacre-like "brick-mortar" layered structure. It shows that the strength and toughness of the composite fiber could be effectively improved by regulating the mass ratio of TPU to GO and the modification of their interfacial properties. Further study indicated that the full-field and local stress distribution can be optimized by regulating the proportion between TPU and GO, and the load transfer efficiency can be further improved by modifying the interface between them, then achieve excellent strength and toughness of the composite fiber. These results will provide useful guidance for the microstructure optimization and performance improvement of GO-based composite fiber.

On the crack-tip stress field due to the presence of isotropic dilatational inclusion: theoretical and numerical analysis

In this paper, the stress field at the crack-tip of a semi-infinite crack interacting with an isotropic dilatational inclusion is studied. The fundamental solution for a semi-infinite crack interacting with an eigenstrained point is presented first. By employing it as Green’s function, a crack enclosed by a homogenous inclusion is formulated and its Kolosov–Muskhelishvili complex potentials are obtained. Then the approximate solutions for the full-field stress in the vicinity of the crack-tip are derived. Finally, numerical method is performed, which not only reveals the distribution of the stress field, but also validates the theoretical analysis. As expected, theoretical and numerical results show that the presence of dilatational inclusion will produce a significant shielding effect on crack-tip. Importantly, it is found that a stress jump occurs at the interface between inclusion and matrix, which may unexpectedly trigger a micro-damage prior to the semi-infinite crack-tip. This mechanism is capable of leading to the crack growth fundamentally, when it is exposed to an oxidizing or corrosive environment.

The surface effects on solid and hollow nanowires under diametral loading

The size effects of energetic surfaces on the elastic behavior of solid and hollow nanowires are investigated under diametral loading for the first time. In order to model the diametral compression test on nanowires, the structural version of the locally-exact theory is developed for solid and hollow nanowires (LETNW) by considering energetic surfaces attached at the boundaries of bulk materials and modeled using the Gurtin-Murdoch model. The developed analytical solutions employ Fourier series representations of displacement fields in the cylindrical coordinate system that exactly satisfy the governing differential equations, facilitating rapid convergence of full-field stress distributions. The solution's accuracy is validated against the in-house finite-element (FE) program which is currently the computational standard, where excellent agreement is obtained. The surface effects on the local stresses are then illustrated by varying the surface parameters, loading angle, dimension of nanowires, as well as residual surface stress. The efficiency of the LETNW is attributed to its analytical nature and thus consumes much less execution time relative to the FE implementation. What's more, instabilities associated with the solution of stress fields are observed with surface properties in certain crystallographic orientations for both LETNW and FE approaches. Contrary to the LETNW technique, the bifurcation phenomenon is shown to depend on the finite-element mesh refinement. The locally-exact theory provides an alternative and independent way of identifying surface bifurcations in solid and hollow nanowires and hence is a good tool in assessing the finite-element method as well as other approaches’ predictive capability for this class of materials.

Deformation mechanisms and plasticity of ultrafine-grained Al under complex stress state revealed by digital image correlation technique

Abstract Conventional engineering stress–strain curve could not accurately describe the local deformability of the tensile necking part because the strain is calculated by assuming that the tensile specimen was deformed uniformly. In this study, we used 3D optical measuring digital image correlation to systematically measure the full strain field and actual flow stress in the necking region of ultrafine-grained (UFG) Al. The post-necking elongation and strain hardening exponent of the UFG Al were measured as 80% and 0.10, slightly smaller than those of the coarse-grained Al (117% and 0.28), suggesting the high plastic deformability of the UFG Al under complex stress state. Microstructural studies revealed the shear and ductile fracture, numerous micro-shear bands, and elongated UFG grains in the UFG Al, which are controlled by cooperative grain boundary sliding and multiple dislocation slips.

Degradation of Transformation Stress in a Bicrystal Cu-Al-Mn Shape-Memory Alloy Using Full-Field Stress and Strain Measurements

The functional fatigue behavior of the superelasticity of a bicrystal Cu-Al-Mn shape-memory alloy during compressive cycles is investigated using the combined techniques of digital image correlation and data-driven identification. The evolution of the inhomogeneous distribution of the transformation stress and strain fields with the number of cycles within two grains with different orientations is investigated. It is found that the transformation stress of the bottom grain is higher than that of the top grain. Furthermore, the transformation stress degrades faster in the bottom grain during cyclic deformation. The decrease in transformation stress in the bottom grain first occurs near the grain boundary and then propagates towards the lower part of the bottom grain. The correlations between the martensitic transformation, accumulation of irrecoverable strain, and degradation of transformation stress are discussed, giving the functional fatigue of bicrystal Cu-Al-Mn shape-memory alloys.

Experimental and computational modeling of bulk residual stress for aeronautical components with distinct geometries

To predict and minimize machining distortion in the manufacturing process, bulk residual stresses in aeronautical components with distinct geometries were investigated via experimental mechanics and numerical simulation. The residual stress state was appropriately simplified according to geometric/processing feathers and deformation patterns of the investigated parts. In each case study, an optimal experimental method was selected to reconstruct the concerned stress tensor. Thereafter, qualitative comparison and validation were performed using cross-method verification and/or numerical simulation. Additionally, the spatial resolution and distribution characteristics of the residual stress were analyzed and discussed in detail. The results revealed that thermal and mechanical nonuniformity caused by material processing is the main source of bulk residual stress in the investigated components. Furthermore, the effectiveness of the contour method on the measurement of different geometric components was verified by numerical simulation. Combining the accurate measurement of the characteristic plane and the appropriate numerical simulation of the global stress field, an engineering-oriented approach for full-field stress evaluation was proposed. This research can provide valuable engineering guidance and suggestions for stress evaluation and distortion analysis prior to manufacturing of integral structures.