Biaxial Mechanical Properties Research Articles

Research on mechanical properties of HTPB propellants

Abstract Slat propellants with different sizes and shapes are designed to investigate the strength criteria of HTPB propellants. According to the calculation and simulation results, with the increase of multiple, the designed size slat specimens can achieve equivalent biaxial tensile stress ratios of 1:2, 1:2.5, 1:3, 1:3.5, and 1:4 under uniaxial tensile conditions. The biaxial mechanical properties of the slab with a stress ratio of 1:2 were tested at low temperatures (25°C, −30°C, −50°C, and 0.4 s−1, 4.0 s−1, 14.29 s−1, 42.86 s−1). It was found that as the temperature and the strain rate rise, an increase is observed in the maximum tensile strength and the initial modulus, a decrease is observed in the maximum elongation of the propellant, and the strength envelope is constructed according to the typical mechanical parameters: the strength envelope of HTPB propellant increases as the temperature decreases. The research findings are expected to provide statistical support for the analysis of the structural integrity of HTPB propellants under low-temperature ignition.

A coupled thermo-mechanical model for investigating cracking and failure of composite interbedded rock

Composite interbedded rock is a special geological phenomenon that occurs when layers of rock (beds) of a given lithology lie between or alternate with other layers of a different lithology. The thermal and mechanical properties of the composite interbedded rocks exhibit strong anisotropy. Studying the thermomechanical coupling properties of natural composite interbedded rocks using existing experimental techniques is challenging. A coupled thermo-mechanical composite interbedded rock model was developed based on Fourier's heat-conduction law and Newton's second law. The thermodynamic responses and biaxial mechanical properties of composite interbedded rocks with different interlayer angles under the coupling of initial stress and thermal loading were studied for the first time. The results indicate that the thermal stresses within the granite interlayers are higher than those within the sandstone interlayers. Throughout the thermal loading process, thermal cracking occurred primarily within the (stiffer) granite interlayers. The number of thermal cracks generated during the cooling process far exceeds that during heating; at 400 °C, the number of thermal cracks during cooling is more than 35 times that during heating. When the interlayer dip angle approaches 90° and is aligned with the major principal stress direction, the phenomenon of the “double peaks” and layered failure becomes more pronounced. Within the range of variables considered, the relative impact of the three variables on peak stress in composite interbedded rock is as follows, from greatest to least: interlayer angle, thermal shock temperature, and model size. The results and developed modeling methods will contribute to the management and optimization of future engineering geology projects.

Investigation on the biaxial tensile testing method for metal foil using cruciform specimen

The assessment of mechanical properties of metal foil under complicated stress states poses a considerable challenge, requiring the development of new testing methods and the optimization of new specimen geometry. In this research, the biaxial tensile method and specimen shape were designed and optimized by using simulation and experiment. It is determined that the strain measurement area can be suggested as the central area of 0.6 to 0.8 times the arm width. The influence of different parameters on the tensile results of cruciform specimens was comprehensively evaluated by finite element method and multi-factor analysis. The results indicate that the number of slits markedly impacts the stress–strain uniformity and shear stress in the central region, followed by the slit length. The effect of the arm width and the slit width is comparable, more influential than the corner radius. According to the analyzed results and application requirements, a four-axis independently driven biaxial tensile test machine for small scale samples was fabricated. During the biaxial tension, the center point offset of the specimen remains within ±0.01 mm. The displacement and load synchronization errors do not exceed 0.01 mm and 50 N, respectively, facilitating the precise control of four-axis synchronization. Conclusively, cruciform specimens featuring diverse geometric characteristics were designed for industrial pure titanium, 304 stainless steel, and Inconel 718 superalloy with thickness of less than 0.3 mm. The biaxial tensile tests were performed to delineate the yield loci, revealing notable anisotropy and size effect. The validation confirms the suitability of the testing method and optimized cruciform specimens for conducting biaxial tensile mechanical properties testing on metal foils characterized by diverse types and thicknesses.

Experimental investigation on biaxial mechanical properties of laminated rock and floor heave mechanism of tunnel based on DIC

Floor heave at the bottom of a tunnel is a common defect during tunnel service. To study the deformation behavior and failure characteristics of the floor, laminated sandstone specimens with precracked U-shaped grooves were prepared, and a series of biaxial compression tests were carried out in the laboratory. Two typical floor structures of a tunnel (with and without inverts) and four lateral stress levels (σ2 = 0, 5, 10 and 15 MPa) were considered. The effects of lateral stress and inverted structure on the deformation and failure characteristics of the floor were investigated. The displacement, strain evolution and failure extent of the floor were analyzed based on the digital image correlation (DIC) technique. The testing results show that the stress concentration at the U-shaped grooves of the specimens can be effectively reduced by the inverted structure. The specimen with an invert can bear more axial deformation and axial stress than that without an invert. The V-shaped notch failure behavior occurred near the tunnel sidewalls for the specimen without an invert, and it occurred in the middle of the tunnel floor for the specimen with an invert. The failure intensity of the V-shaped notch was positively related to the lateral stresses. With the increasing of axial stress, the heave displacement at the tunnel floor gradually increased, while the settlement displacement of the sidewall showed a trend of first increasing and then decreasing. The tensile strain concentration area of the floor undergoes a process of increasing and then decreasing and finally transforms to compressive strain. The failure characteristics of the sandstone specimen observed in the laboratory tests are consistent with the phenomena in real tunnel engineering. In addition, the deformation and failure mechanism of the tunnel floor was analyzed. The key points for the treatment of tunnel floor heave are put forward based on the testing results.

Experimental measurement and simulation of the biaxial mechanical properties of solid propellant HTPB

AbstractHydroxyl‐terminated polybutadiene (HTPB) solid propellant was widely used in carrier rocket and guided missiles, owing to its excellent mechanical properties. The material of HTPB subjected to complex load such as in the process of manufacturing, storage and transportation. Therefore, the mechanical properties of HTPB had important implications for the safety of structure and service life. The purpose of this work is to study the effects of different loading ratios and loading speeds on the biaxial tension of hydroxyl‐terminated polybutadiene (HTPB) solid propellant with a proper kind of biaxial tensile specimen and a high volume fraction three dimensional particle filling model of the material. The proper kind of biaxial tensile specimen with which the stresses in its central part could be obtained with the loads acted on each loading direction was designed and used in the study, and the strains in its central parts were obtained with digital image correlation (DIC) method. The stress‐strain relationship at each direction could be obtained by experiments. Experimental results indicated that the loading speed remarkably influenced the biaxial tensile behaviors of HTPB propellant at room temperature and at different loading ratios. The finite element simulation results demonstrated the accuracy of the biaxial experiment. Based on the experimental results, the particle debonding in biaxial experiments were analyzed and an equivalent Poisson's ratio parameter was proposed to evaluate the degree of debonding. The highly volume fraction three dimensional particle filling model provides a good simulation with experimental results and the equivalent Poisson's ratio parameter can conveniently and accurately determine the debonding of particles which is meaningful to the structure integrity assessment of HTPB solid propellant.

Determining the hot forming limits of titanium alloy sheet under different strain paths by constant equivalent strain rate hot gas bulging tests

This study aims to determine the hot forming limit diagram (FLD) of titanium alloy sheet using constant equivalent strain rate hot gas bulging tests. A novel method was proposed to calculate pressure-time curves under different strain paths, ensuring that the equivalent strain rate during hot deformation remains constant. The key of this method is the development of a VUMAT material subroutine that accurately reflects anisotropic hardening and r-value evolution, and the pressure loading curves under different strain paths were determined by coupling with the VUAMP amplitude subroutine embedded with a PID control algorithm. Based on dies with different cavity structures and unbonded double-layer samples, the FLDs of TA32 titanium alloy sheet under linear and nonlinear strain paths were obtained by carrying out hot gas bulging tests at 800 ℃ with a strain rate of 0.001 s-1. The results demonstrate that the proposed approach is applicable for determining the pressure-time curves under various complex strain paths, and it is experimentally verified that the equivalent strain rate of the sample apex is maintained near the target value during the bulging process. Furthermore, the significant effects of anisotropy and strain path change on the forming limits were quantitatively analyzed. The developed method is not only beneficial for testing the hot biaxial mechanical properties of materials, but also useful for controlling the strain rate during the hot forming of complex thin-walled components to improve the formability of materials.

Programmable Melt Electrowriting to Engineer Soft Connective Tissues with Prescribed, Biomimetic, Biaxial Mechanical Properties

AbstractAppropriate load‐bearing function of soft connective tissues is provided by their nonlinear and often anisotropic mechanics. Recapitulating such complex mechanical behavior in tissue‐engineered structures is particularly crucial, as deviation from native tissue mechanics can trigger pathological biomechanical pathways, causing adverse tissue remodeling and dysfunction. Here, a novel method combining computational modeling, melt electrowriting (MEW), and design of experiments (DOE) is reported to generate scaffolds composed of sinusoidal fibers with prescribed biaxial mechanical properties, recapitulating the distinct nonlinear, anisotropic stress–strain behavior of three model tissues: adult aortic valve, pediatric pulmonary valve, and pediatric pericardium. Finite element analysis is used to efficiently optimize scaffold architecture over a broad parameter space, representing up to 65 conditions, to define MEW print parameters to achieve polycaprolactone scaffolds with target mechanical properties. Architectural parameters are further optimized experimentally using DOE and regression to account for uncertainties involved in the simulation, yielding functional scaffolds with accurate, prescribed mechanics. The prescribed architecture also primarily governs the mechanics of hybrid structures generated by casting cell‐laden fibrin hydrogel within the scaffolds. This high‐fidelity approach recapitulates biaxial mechanical properties over a broad range of mechanical nonlinearity and anisotropy and is generalizable for programmed biofabrication in a variety of tissue engineering applications.

2023 TERMIS – AMERICAS Conference & Exhibition Boston Marriott Copley Place April 11–14, 2023

2023 TERMIS – AMERICAS Conference & Exhibition Boston Marriott Copley Place April 11–14, 2023

Mechanical properties of CFSST with steel reinforcement cage under biaxial eccentric compression

To study the biaxial eccentric compression mechanical properties of high-strength concrete-filled square steel tube (CFSST) columns (high-strength materials are used for both steel tube and concrete) with steel reinforcement cage, twelve high-strength CFSST columns with the same cross-sectional dimensions were designed and fabricated with the form of the steel reinforcement cage, eccentricity, and the steel tube width-to-thickness ratio as the main parameters. The constraint enhancement mechanism of the concrete by the steel reinforcement cage and the effect of different parameters on the biaxial eccentric ultimate bearing capacity of column specimens was further analyzed by combining with ABAQUS simulation. The results show that the bearing capacity of the specimens decreases sharply with the increase of eccentricity and steel tube width-to-thickness ratio; the steel reinforcement cage can improve the bearing capacity and ductility of the specimen but has little effect on the stiffness; the arrangement of double orthogonal horizontal steel reinforcement can enhance the constraint effect of the steel tube in the middle of the cross-section edge length on the concrete of the specimens within a certain height range and weaken the constraint effect of the corner within the same height range; the enhancement effect of the steel reinforcement cage on the biaxial eccentric ultimate bearing capacity of the column specimens gradually decreases with the increase of eccentricity; when the eccentricity is constant, the effect of the steel reinforcement cage on the improvement of the biaxial eccentric ultimate bearing capacity of the column specimens with different width-to-thickness ratios is basically the same.

Biaxial mechanical properties of the bronchial tree: Characterization of elasticity, extensibility, and energetics, including the effect of strain rate and preconditioning

Distal airways commonly obstruct in lung disease and despite their importance, their mechanical properties are vastly underexplored. The lack of bronchial experiments restricts current airway models to either assume rigid structures, or extrapolate the material properties of the trachea to represent the small airways. Furthermore, past works are exclusively limited to uniaxial testing; investigating the multidirectional tensile loads of both the proximal and distal pulmonary airways is long overdue. Here we present comprehensive mechanical and viscoelastic properties of the porcine airway tree, including the trachea, trachealis muscle, large bronchi, and small bronchi, via measures of elasticity, extensibility, and energetics to explore regional and directional dependencies, cross-examining strain rate and preconditioning effects using planar equibiaxial tensile tests for the first time. We find bronchial regions are notably heterogeneous, where the trachea exhibits greater stiffness, energy loss, and preconditioning sensitivity than the smaller airways. Interestingly, the trachealis muscle is similar to the distal bronchi, despite being anatomically located adjacent to the proximal ring. Tissues are anisotropic and axially stiffer under initial loading, losing more energy with greater stress relaxation circumferentially. Strain rate dependency is also noted, where tissues are more energetically efficient at the faster strain rate, likely attributable to the microstructure. Findings highlight assumptions of homogeneity and isotropy are inadequate, and enable the improvement of aerosol flow and dynamic airway deformation computational predictive models. These results provide much needed fundamental material properties for future explorations contrasting healthy versus diseased pulmonary airway mechanics to better understand the relationship between structure and lung function. Statement of significanceWe present comprehensive multiaxial mechanical tensile experiments of the proximal and distal airways via measures of maximum stress, initial and ultimate moduli, strain and stress transitions, hysteresis, energy loss, and stress relaxation, and further assess preconditioning and strain rate dependencies to examine the relationship between lung function and structure. The mechanical response of the bronchial tree demonstrates significant anisotropy and heterogeneity, even within the tracheal ring, and emphasizes that contrary to past studies, the behavior of the proximal airways cannot be extended to distal bronchial tree analyses. Establishing these material properties is critical to advancing our understanding of airway function and in developing accurate computational simulations to help diagnose and monitor pulmonary diseases.

Pressure-bearing performance of sliding poly-p-phenylene benzobisoxazole-rope-reinforced spherical composite envelope structures

Stratospheric airships require a lightweight envelope to contain lighter-than-air (LTA) buoyancy gas, making the lightweight design and pressure-bearing performance of the envelope structure a key research issue. This paper proposes a structural design scheme for a sliding poly-p-phenylene benzobisoxazole (PBO) fiber-rope-reinforced spherical envelope. The aims were to reduce envelope weight and improve pressure-bearing performance while solving the challenges of envelope structural strength. Two types of spherical envelope structures were constructed from Vectran fabric with PBO rope placed in various positions for reinforcement, and subjected to inflatable burst tests. Strain in the seam tape and envelope gores were measured by digital image correlation. A refined envelope structure model was established to simulate the bursting process with consideration of the cutting pattern effect and the mechanical properties of the weld seams. Comparison of the simulation and test results, combined with observations of the tear textures of damaged envelopes, enabled the determination of failure positions. The envelope material at the failure point was disassembled and the micromorphology and microstructure of the bearing layer were observed by scanning electron microscopy (SEM). The process and characteristics of damage origination, propagation, and collection in the envelope material are explained, and the stress behavior and state of the damage positions are analyzed. Based on the study of the mechanical behavior at the failure position, design suggestions for PBO-rope-reinforced spherical envelopes are provided. A strength criterion to incorporate tensile–shear coupling is proposed, which bridges the gap between the uniaxial tensile strength (UTS) and biaxial tensile mechanical properties of the envelope structure.

Finite element analysis of biaxial tensile mechanical properties and burst properties of woven fabric for layflat hose

AbstractThis paper explores the effects of woven fabric structures on the biaxial tensile mechanical properties and burst properties of woven fabric for layflat hose. A repeat unit cell finite element modeling for woven fabrics for layfalt hose is adopted. To investigate the effects of fabric structure, two parameters are studied: warp yarn spacing and weft yarn linear density. The mechanical response outcomes such as stress distribution, intensity, breaking elongation, and burst properties such as burst pressure, burst direction, are defined and the effects of structure parameters studied on these outcomes are studied. The effects of the warp yarn spacing and the weft yarn linear density on the biaxial tensile mechanical properties and the burst properties of woven fabric for layflat hose are determined. Moreover, the results suggest finite element method exhibits high accuracy in predicting the burst pressure of layflat hose.

Linking the region-specific tissue microstructure to the biaxial mechanical properties of the porcine left anterior descending artery

Coronary atherosclerosis is the main cause of death worldwide. Advancing the understanding of coronary microstructure-based mechanics is fundamental for the development of therapeutic tools and surgical procedures. Although the passive biaxial properties of the coronary arteries have been extensively explored, their regional differences and the relationship between tissue microstructure and mechanics have not been fully characterized. In this study, we characterized the passive biaxial mechanical properties and microstructural properties of the proximal, medial, and distal regions of the porcine left anterior descending artery (LADA). We also attempted to relate the biaxial stress-stretch response of the LADA and its respective birefringent responses to the polarized light for obtaining information about the load-dependent microstructural variations. We found that the LADA extensibility is reduced in the proximal-to-distal direction and that the medial region exhibits more heterogeneous mechanical behavior than the other two regions. We have also observed highly dynamic microstructural behavior where fiber families realign themselves depending on loading. In addition, we found that the microstructure of the distal region exhibited highly aligned fibers along the longitudinal axis of the artery. To verify this microstructural feature, we imaged the LADA specimens with multi-photon microscopy and observed that the adventitia microstructure transitioned from a random fiber network in the proximal region to highly aligned fibers in the distal region. Our findings could offer new perspectives for understanding coronary mechanics and aid in the development of tissue-engineered vascular grafts, which are currently limited due to their mismatch with native tissue in terms of mechanical properties and microstructural features. Statement of significanceThe tissue biomechanics of coronary arteries is fundamental for the development of revascularization techniques such as coronary artery bypass. These therapeutics require a deep understanding of arterial mechanics, microstructure, and mechanobiology to prevent graft failure and reoperation. The present study characterizes the unique regional mechanical and microstructural properties of the porcine left anterior descending artery using biaxial testing, polarized-light imaging, and confocal microscopy. This comprehensive characterization provides an improved understanding of the collagen/elastin architecture in response to mechanical loads using a region-specific approach. The unique tissue properties obtained from this study will provide guidance for the selection of anastomotic sites in coronary artery bypass grafting and for the design of tissue-engineered vascular grafts.

Designing a Cruciform Specimen via Topology and Shape Optimisations under Equal Biaxial Tension Using Elastic Simulations.

Stress uniformity within the gauge zone of a cruciform specimen significantly affects materials’ in-plane biaxial mechanical properties in material testing. The stress uniformity depends on the load transmission of the cruciform specimen from the fixtures to the gauge zone. Previous studies failed to alter the nature of the load transmission of the geometric features using parametric optimisations. To improve stress uniformity in the gauge zone, we optimised the cross-arms to design a centre-reduced cruciform specimen with topology and shape optimisations. The simulations show that the optimised specimen obtains significantly less stress variation and range in the gauge zone than the optimised specimen under different observed areas, directions, and load ratios of von Mises, S11, S22, and S12. In the quantified gauge zone, a more uniform stress distribution could be generated by optimizing specimen geometry, whose value should be estimated indirectly each time through simulations. We found that topology and shape optimisations could markedly improve stress uniformity in the gauge zone, and stress concentration at the cross-arms intersection. We first optimised the cruciform specimen structure by combining topology and shape optimisations, which provided a cost-effective way to improve stress uniformity in the gauge zone and reduce stress concentration at the cross-arms intersection, helping obtain reliable data to perform large strains in the in-plane biaxial tensile test.

Mechanical characterization and material modeling of ascending aortic aneurysm with different bicuspid aortic cusp fusion morphologies

Bicuspid aortic valve (BAV) is frequently associated with ascending thoracic aortic aneurysm (ATAA). Impact of cusp fusion patterns on biomechanics and microstructure of the ATAA remains unknown. This study aims to investigate biaxial mechanical properties of the ATAAs with right-left (RL) and right-noncoronary (RN) cusp fusion patterns. Fresh ATAA samples (n = 26) were obtained from patients who underwent surgical aneurysm repair. Biaxial extension tests were performed to characterize mechanical behaviors of the RL and RN BAV-ATAAs. A material model was fitted to biaxial experimental data to obtain model parameters. Histological and mass fraction analyses were employed to investigate the underlying microstructure and dry weight percentages of elastin and collagen in the ATAA tissue. The RL and RN BAV-ATAAs exhibited nonlinear and anisotropic mechanical responses to biaxial loading. Tissue stiffness of the RN BAV-ATAAs was significantly higher than that of the RL BAV-ATAAs in the circumferential (2679 ± 755 vs 1942 ± 578 kPa, mean ± SD, p = 0.04) and longitudinal (2535 ± 630 vs 1709 ± 512 kPa, mean ± SD, p = 0.02) directions under the equibiaxial stresses. Laminar structure of elastic fibers was disrupted in both RL and RN BAV-ATAAs. Notably, interstitial fibrosis and thinner elastic fibers were identified in the RN BAV-ATAAs. Mass fraction of collagen was significantly higher for the RN BAV-ATAAs than that of the RL BAV-ATAAs. The tissue stiffness in the circumferential direction was significantly increased and strongly correlated with the mass fractions of collagen for both RL and RN BAV-ATAAs. Our results suggest that elastic properties of the RN BAV-ATAAs are more deteriorated than those of the RL BAV-ATAAs. Changes in biomechanical properties may have great impact on ascending aortic dilation.

Strain-rate correlation of biaxial tension and compression mechanical properties of HTPB and NEPE propellants

An effective biaxial tension and compression test method is proposed based on the shortcomings of current research for the mechanical properties of solid propellants under complex stress states. The equal proportion biaxial tension and compression test of HTPB (Hydroxyl-terminated polybutadiene) and NEPE (NitrateEster Plasticized Polyether) solid propellants is performed at different rates while at room temperature, and the damage morphology of the tension–compression zone is analyzed using micro-CT. The results show that the failure mode of the solid propellant under biaxial tension and compression loading is similar to that under uniaxial tension. Meanwhile, the compressive strength is much greater than the tensile strength, which will eventually cause tensile failure. With an increased loading rate, the growth trend of the initial modulus, ultimate strength, and maximum elongation of the propellant is gradually flattened, and the damage degree is gradually reduced. Additionally, damage that forms in the HTPB propellant is from dewetting and particle fracture while that for the NEPE propellant is from matrix tearing. The porosity can be used as the meso-damage parameter of the propellant.

Biomechanical analysis of sheep oesophagus subjected to biaxial testing including hyperelastic constitutive model fitting

High quality computational model of soft tissues is a function of accurate and reliable mechanical properties. Hyperelastic constitutive models are normally utilised in developing reliable computational models. Therefore, section of proper and reliable constitutive models for soft tissue is critical. This work presents the biomechanical properties of oesophagus subjected to biaxial mechanical tensile test. Additionally, six hyperelastic constitutive models commonly used for modelling behaviour of soft tissues were selected. The experimental data were then fitted on Fung, Choi-Vito, Holzapfel (2000), Holzapfel (2005), Polynomial (Anisotropic) and Four-Fiber Family hyperelastic constitutive models. The sheep oesophagus subjected to equi-biaxial tension has exhibited different stress magnitude in both longitudinal and circumferential directions. There is significant difference between circumferential and longitudinal stresses (p = 0.0034). The average circumferential and longitudinal stresses are recorded to be 82.87 ± 30.36 kPa and 41.42 ± 32.02 kPa, respectively (p = 0.0034). Between six hyperelastic constitutive models, it was observed that Four-Fiber model has produced better fit when compared to others. After fitting biaxial mechanical properties of oesophagus, it was found that the Four-fiber family hyperelastic constitutive model would best fit.

Cryogenic deformation behavior of 6061 aluminum alloy tube under biaxial tension condition

Cryogenic forming has been proposed for the fabrication of complex aluminum alloys tubular components to overcome the shortcomings of traditional forming processes. It is difficult to evaluate the cryogenic biaxial deformation behavior. In this study, a novel cryogenic hydro-bulging device was established to evaluate cryogenic deformation behavior and verify the feasibility of the new forming process. The cryogenic biaxial stress-strain relations were determined using a new analytical model to quantitatively characterize the hardening behavior. The cryogenic deformation mechanism was revealed by microstructure characterization. It is found that the maximum expansion rate of 6061 aluminum alloy tube at − 196 ℃ increases to 34.0% ± 0.6%, being 99.8% higher than that at room temperature. The obtained cryogenic biaxial flow curve exhibits the significantly improved plasticity and hardening ability with a strain hardening exponent of 0.43 at − 196 ℃. The high cryogenic strain hardening rate contributes to the stable uniform deformation. The improved cryogenic plasticity is associated with the diminished dislocations accumulation at grain boundaries. The enhanced cryogenic hardening ability is attributed to the impeded movable dislocations slip. The research demonstrates that cryogenic medium pressure forming has great potential for the fabrication of complex aluminum alloy tubular components. • A novel cryogenic hydro-bulging device was established for evaluating the cryogenic biaxial deformation behavior oftubes. • The cryogenic biaxial mechanical properties and hardening behavior were quantitatively evaluated by a new analytical model. • Excellent cryogenic biaxial formability can be obtained, contributing to the fabrication of complex tubular parts. • Microstructure was characterized to reveal the cryogenic deformation mechanism.

Biaxial Murine Vaginal Remodeling With Reproductive Aging.

Higher reproductive age is associated with an increased risk of gestational diabetes, pre-eclampsia, and severe vaginal tearing during delivery. Further, menopause is associated with vaginal stiffening. However, the mechanical properties of the vagina during reproductive aging before the onset of menopause are unknown. Therefore, the first objective of this study was to quantify the biaxial mechanical properties of the nulliparous murine vagina with reproductive aging. Menopause is further associated with a decrease in elastic fiber content, which may contribute to vaginal stiffening. Hence, our second objective was to determine the effect of elastic fiber disruption on the biaxial vaginal mechanical properties. To accomplish this, vaginal samples from CD-1 mice aged 2-14 months underwent extension-inflation testing protocols (n = 64 total; n = 16/age group). Then, half of the samples were randomly allocated to undergo elastic fiber fragmentation via elastase digestion (n = 32 total; 8/age group) to evaluate the role of elastic fibers. The material stiffness increased with reproductive age in both the circumferential and axial directions within the control and elastase-treated vaginas. The vagina demonstrated anisotropic mechanical behavior, and anisotropy increased with age. In summary, vaginal remodeling with reproductive age included increased direction-dependent material stiffness, which further increased following elastic fiber disruption. Further work is needed to quantify vaginal remodeling during pregnancy and postpartum with reproductive aging to better understand how age-related vaginal remodeling may contribute to an increased risk of vaginal tearing.

Evolution of yield behavior for AA6016-T4 and DP490—Towards a systematic evaluation strategy for material models

Uniaxial and cruciform biaxial tensile tests are performed on a face-centered cubic material AA6016-T4 and a body-centered cubic material DP490 under 19 different loading paths, i.e., uniaxial tension in seven directions, simple shear along the rolling direction, and biaxial tension in rolling/transverse and 45°/135° directions, with seven and four stress ratios, respectively. The ability of several yield criteria to describe the plastic anisotropy for the tested materials, under the associated and non-associated flow rules, is evaluated systematically. A new evaluation strategy to check the validity and applicability of the material models is proposed in this study. This strategy is different from the traditional evaluation strategy in that the biaxial tensile mechanical properties measured in the 45°/135° sampling direction are included in the investigation scope, and it is not limited to the rolling/transverse sampling direction, i.e., the yield stress functions are evaluated by comparing the experimental and predicted uniaxial yield stresses and plastic work contours on the normal and diagonal planes. Further, the plastic potential functions are confirmed by the description accuracy of uniaxial rα-values and the directions of plastic strain rate in the rolling/transverse and 45°/135° directions. A new analytical calibration program is developed for the material parameters of the Poly6 yield criterion related to normal (a1,a2,...,a7) and shear stress components (a8,a9,...,a16), which can further introduce the yield stresses under the near plane strain states (σPS0, σPS45, and σPS90) and pure shear yield stress along the rolling direction (τ0). The results indicate that the new calibration strategy of Analytical Poly6-II&2 can accurately describe the anisotropic yield behavior of the tested materials compared with other advanced yield criteria, especially for plastic work contours on the diagonal plane. Significant anisotropic yield behavior was observed under biaxial tensile stress states with the same loading ratio in different sampling directions. Therefore, investigating the prediction accuracy of the biaxial tensile mechanical properties in only the rolling/transverse sampling direction is insufficient for evaluating the effectiveness of the material models; the mechanical properties at the initial yield point cannot fully reflect the plastic anisotropy of the materials. The material parameters of the Analytical Poly6-II&2 yield criterion are expressed as functions of equivalent plastic strain, which can continuously capture the evolution of the anisotropic yield behavior for AA6016-T4 and DP490. Three different calibration strategies of the material parameter a16 are evaluated in this study. These strategies do not affect the description ability of the uniaxial yield stresses, rα-values, and plastic work contours on the normal plane; however, they influence the calculation results of plastic work contours on the diagonal plane. For attaining a balanced prediction accuracy, it is recommended to provide the values of both τ0 and σPS45.