Initial Prestrain Research Articles

Resonant ultrasound spectroscopy for crystalline samples containing initial strain

Resonant ultrasound spectroscopy (RUS) is a mature and robust technique for the nondestructive characterization of the elastic properties of solids capable of providing the elastic constants of anisotropic crystalline solids. The traditional method is based on assuming that the solid is linear elastic and, therefore, obeys a linearized Hookean constitutive relationship (Hooke’s law). In this article, an alternative constitutive law is provided that allows for an initial strain or prestrain to be present stemming from residual stress. Then, the constitutive relationship is integrated into the RUS framework. The model is demonstrated using a realistic prestrain field obtained by simulating shot-peening processing of a polycrystalline Cu sample. The sensitivity of the resonances to the developed prestrain is established and discussed. This work allows researchers to consider the influence of initial strain or residual stress in their samples and the potential influence on accurate elastic constant estimates. The model also supports the potential of RUS for the nondestructive characterization of prestrain in materials.

Multi-strain path deformation behavior of AA6016-T4: Experiments and crystal plasticity modeling

The development of backstresses that occur during a strain path change when forming sheet metal renders final part geometry prediction, after springback, difficult using conventional models. Most deformation models do not explicitly account for the influence of these stresses. A more recently developed elasto-plastic self-consistent (EPSC) model incorporates the backstresses to influence the activation of slip systems for more accurate simulations of the complex material response. The current study assesses the performance of the EPSC model for deformation response of AA6016-T4 via multiple biaxial, plane-strain, and uniaxial tension strain paths. The response to complex strain paths was examined by first pre-straining under uniaxial, biaxial, and plane-strain tension, then by loading in uniaxial tension. The EPSC model predictions closely matched experimental results. The model correctly predicted the highest yield stress and sharpest transition from elastic to plastic deformation for uniaxial tension after an initial uniaxial pre-strain. Lower yield stress for uniaxial tension after first pre-straining in biaxial and plane-strain tension is also correctly predicted, along with a smoother transition from elastic to plastic behavior. A linear geometrically necessary dislocation (GND) development, with strain, was observed using high-resolution electron backscattered diffraction (HREBSD) while a quadratic statistically stored dislocation (SSD) development was predicted by the model. The comparison revealed an expected transition from kinematic to isotropic hardening at higher strains. Finally, at higher strain levels the backstress accounted for around 15% of the total subsequent flow stress in all pre-strain cases.

Numerical investigation of the clinched joint loadings considering the initial pre-strain in the joining area

The components of a body in white consist of many individual thin-walled sheet metal parts, which usually are manufactured in deep-drawing processes. In general, the conditions in a deep-drawing process change due to changing tribology conditions, varying degrees of spring back, or scattering material properties in the sheet blanks, which affects the resulting pre-strain. Mechanical joining processes, especially clinching, are influenced by these process-related pre-strains. The final geometric shape of a clinched joint is affected to a significant level by the prior material deformation when joining with constant process parameters. That leads to a change in the stiffness and force transmission in the clinched joint due to the different geometric dimensions, such as interlock, neck thickness and bottom thickness, which directly affect the load bearing capacity. Here, the influence of the pre-straining in the deep drawing process on the force distribution in clinch points in an automotive assembly is investigated by finite-element models numerically. In further studies, the results are implemented in an optimization tool for designing clinched components. The methodology starts with a pre-straining of metal sheets. This step is followed by 2D rotationally symmetric forming simulations of the joining process. The resulting mesh of each forming simulation is rotated and 3D models are obtained. The clinched joint solid model with pre-strains is used further to determine the joint stiffnesses. With the simulation of the same test set-up with an equivalent point-connector model, the equivalent stiffness for each pre-strain combination is determined. Simulations are performed on a clinched component to assess the influence of pre-strain and sheet thinning on the clinched joint loadings by using the equivalent stiffnesses. The investigations clearly show that for the selected component, the loadings at the clinch points are dependent on the sheet thinning and the stiffnesses due to pre-strain. The magnitude of the influence varies depending on the quantity considered. For example, the shear force is more sensitive to the joint stiffness than to the sheet thinning.

Compression wave structure under plane impact deformation of a molybdenum single crystal [100] with different initial density of dislocations

The features of the evolution of the elastic-plastic compression wave in Mo single crystals along the crystallographic plane [100], having a different initial dislocation structure formed by small deformations of varying degrees of static compression at room temperature, have been studied. The analysis of wave profiles recorded using the VISAR laser interferometer in samples of different thicknesses showed a non-monotonic change in the Hugoniot elastic limit depending on the initial dislocation density pre-strain of a single crystal by compression reduces the Hugoniot elastic limit by 3 times. Keywords: Single crystal, small deformations, compression wave evolution, wave profile, VISAR interferometer.

Carbon-based aerogel in three-dimensional polyurethane scaffold: The effect of in situ unidirectional aerogel growth on piezoresistive properties

Carbon-based aerogels (CA) with durable piezoresistive properties possess great promise for applications in wearable electronics. However, it is still a great challenge to fabricate high performance aerogel sensors due to the limits of mechanical properties. We recently developed a novel strategy to build aerogel/foam hybrid structure that can effectively enhance the mechanical properties of aerogel-based sensors. Herein, CA/polyurethane foam (PUF) composite was fabricated via in situ unidirectional freeze-drying process starting from a carbon nanotubes (CNTs)/graphene oxide (GO) aqueous dispersion stabilized with chitosan (CS) which was filled into the porosity of an open cell PUF. The resulting composite shows a microscopic anisotropy in the CA with electrical conductivity as high as 10.54 S m−1. The piezoresistive sensitivity of the resulting composite can be simply modulated over a wide range from 1.1 to 3.6 by adequately selecting its initial pre-strain. The CA/PUF composite exhibits excellent compressible resilience at the strain of 50% for at least 100 cycles. In addition, the sensor was successfully applied for detecting various human motions. These unique properties make the realized composite a promising candidate for effective flexible piezoresistive strain sensors in wearable applications.

Strain ratcheting failure of dented steel submarine pipes under combined internal pressure and asymmetric inelastic cycling

This paper presents the results of an experimental study in which for the first time the progressive inelastic deformation of high strength steel pressurized pipes with plain and gouged dents was analyzed. The effect of some key loading and geometrical parameters such as initial pre-strain (εmon), dent depth ratio (δ/D) and the presence of the gouge was studied. Local dents with a hemispherical shape and gouges with different dimensions were introduced on the surface of the specimens. Specimens with plain or gouged dents were first internally pressurized, axially compressed to an initial pre-strain then subjected to cyclic nonlinear stressing.The mechanical defects were found to drastically degrade the inelastic cyclic response of the pressurized steel pipes. The presence of a dent, even with small amounts of initial pre-strains, made the pressurized pipes vulnerable to ratcheting failure. Furthermore, the ratcheting responses of the gouged dented specimens significantly differed from those for the corresponding specimens with plain dents. In fact, the combination of a dent and gouge was the severest form of the defect and demonstrated the lowest number of cycles before failure. The degradation effects of gouge were strongly dependent upon the gouge geometry, dent depth and initial pre-strain level.

Self-heating of metastable 304L austenitic stainless steel under cyclic loading: Influence of initial martensite volume fraction, testing temperature and pre-strain

In this study, special attention is paid to the thermal response of the austenitic stainless steel 304L under cyclic loading tests (i.e., self-heating tests) in different conditions. The cyclic behavior of the 304L austenitic stainless steel can be influenced by martensitic transformation. Hence, the role of martensitic transformation in the self-heating behavior of this material is studied. For this purpose, self-heating tests are performed on pre-strained specimens at a wide range of temperatures from 100 °C to −130 °C, which leads to different initial volume fractions of martensite ranging from 0% to 45%, respectively. Furthermore, the influence of the testing temperature and a pre-strain at room temperature on the self-heating curves is investigated. The results demonstrate that the decrease in the test temperature from room temperature to - 30 °C as well as the increase in the pre-strain values from 0% to 30% at room temperature, result in a shift of the self-heating curves to the right. It could mean that the fatigue limit of the material is improved.

Modeling and Verification of Stress Relaxation Behavior of Ti-6Al-4V

The phenomenon of gradual decrease of internal stress with the deformation of material maintained under the precondition of certain temperature and initial stress or pre-strain is called stress relaxation. Due to that, the flow stress of the metal material falls rapidly when the hot forming process pauses, and then the required forming load. In this paper, the experiment was carried out to study the stress relaxation property of Ti-6Al-4V, in the temperature range of 1023K~1123K and with the pre-tension strain 0.7%, 4% and 10%. The quartic delay function was used to describe the stress relaxation behavior. The predicted value of stress relaxation equation is in good agreement with the experimental data, and the correlation coefficient is above 0.99. Arrhenius creep constitutive equation embedded in CAE software was derived. The finite element model of stress relaxation process of test bar was builded, and the tensile-relaxation experiment was performed under the loading condition of 1:1 part forming process. The forming force results agree well, and the validity and accuracy of the constitutive model are verified, laying a foundation for the subsequent process simulation and optimization.

Stress Relaxation Aging Behavior and Constitutive Modelling of AA7150-T7751 under Different Temperatures, Initial Stress Levels and Pre-Strains

Age forming is an advanced manufacture technology for forming large aluminum panels. Temperature, initial stress level and pre-strains have a great effect on the formability and performance. The stress relaxation aging behavior of AA7150-T7751 under different temperatures, initial stress levels and pre-strains was studied through stress relaxation tests, tensile tests and TEM observations. The results show that the formability can be improved with the increase of temperature, initial stress levels and pre-strains. Deformation mechanisms during stress relaxation of the material were analyzed on the basis of creep stress exponent and apparent activation energy. The aging precipitates of the studied alloy were not sensitive to the age forming conditions, but drastically coarsened at over aging temperature, which decreased the mechanical properties. In addition, the relationship between stress relaxation behavior and aging strengthening is discussed. Based on the dislocation theory and the modified Arrhenius equation, a stress relaxation constitutive equation considering the initial mobile dislocation density and temperature dependent activation energy was established. This model can predict very well the stress relaxation behavior under various temperature, stress level and pre-strain conditions, with an average error of 2%.

High-performance self-centering steel columns with shape memory alloy bolts: Design procedure and experimental evaluation

Steel moment-resisting frames are popular structural systems used extensively around the world. However, conventional column base connections are vulnerable to large residual deformation after strong earthquakes. By contrast, shape memory alloys (SMAs), which are high-performance metallic materials, can experience large strains and still recover their initial shape through either heating (shape memory effect) or unloading (superelastic effect). The superelastic behavior of SMAs is appealing to the earthquake engineering community because of the material’s excellent self-centering (SC) and energy dissipation capabilities. In this paper, a novel type of steel columns equipped with NiTi SMA bolts was introduced and its potential for achieving earthquake resilience were investigated. Structural details of the column base and mechanical properties of the SMA bolts were described first. Subsequently, an analytical model of the SC column for different limit states and the corresponding design procedure were presented. The seismic behaviors of two steel column specimens were experimentally tested to investigate the effects of the initial prestrain in the SMA bolts and the axial compressive force in the column under cyclic loading. Results showed that the steel columns equipped with SMA bolts exhibited satisfactory and stable flag-shaped hysteresis loops with excellent SC and moderate energy dissipation capabilities. More importantly, SMA bolts with prestrain could still be tightened after removal of lateral force. Therefore, the proposed SC column could achieve seismic resilience design that requires no (or minimal) repair even after strong earthquakes and remains highly functional for aftershocks or future earthquakes. In addition, the analytical model was verified through a comparison with test results obtained at key limit states.

Modified-Weibull-stress-based evaluation of brittle fracture occurrence during earthquakes in steel members

Brittle fractures can originate from shallow surface cracks on welded parts of steel structures during earthquakes. Since the occurrence of such fractures is significantly influenced by the shape of the welded parts, the existing methods based on material testing cannot predict fractures. This study focused on a modified Weibull stress, which considers the processes of nucleation and propagation of microcracks at different depths, and demonstrated its applicability to the evaluation of brittle fracture initiation during earthquakes. The demonstration was implemented on three types of structural steel using low-temperature experiments and numerical stress analyses on crack tips. Additionally, the independency of the obtained critical modified Weibull stress from the initial crack depth, strain rate and prestrain was examined in the prediction of the critical crack-tip opening displacement. Finally, the relationship between the critical deformation of the beam-to-column connection of a steel bridge pier at the fracture time (in the case of a fracture originating from a shallow crack) and its weld toe radius was quantitatively evaluated as an example of actual use of the modified-Weibull-stress-based evaluation.

Elastocaloric effect vs fatigue life: Exploring the durability limits of Ni-Ti plates under pre-strain conditions for elastocaloric cooling

Structural fatigue is the major obstacle that prevents practical applications of the elastocaloric effect (eCE) in cooling or heat-pumping devices. Here, the eCE and fatigue behaviour of Ni-Ti plates are systematically investigated in order to define the fatigue strain limit and the associated eCE. Initially, the eCE was evaluated by measuring adiabatic temperature changes at different strain amplitudes and different mean strains along the loading and unloading transformation plateaus. By comparing the eCE with and without pre-strain conditions, the advantages of cycling an elastocaloric material at the mean strain around the middle of the transformation plateau were demonstrated. In the second part of this work, we evaluated the fatigue life at the mean strain of 2.25% at the loading plateau and at the unloading plateau after initial pre-straining up to 6% and 10%, respectively. It is shown that on polished samples, durable operation of 105 cycles can be reached with a strain amplitude of 0.50% at the loading plateau, which corresponds to adiabatic temperature changes of approximately 5 K. At the unloading plateau (after initial pre-strain of 10%), durable operation was reached at a strain amplitude of 1.00%, corresponding to adiabatic temperature changes of approximately 8 K. The functional fatigue was analysed after the cycling and it is shown that once the sample has been stabilized there is no further degradation of the eCE, even after 105 cycles. These results present guidelines for the design and operation of efficient and durable elastocaloric devices in the future.

Analysis of the Mechanical Properties of the Human Tympanic Membrane and Its Influence on the Dynamic Behaviour of the Human Hearing System.

The difficulty to estimate the mechanical properties of the tympanic membrane (TM) is a limitation to understand the sound transmission mechanism. In this paper, based on finite element calculations, the sensitivity of the human hearing system to these properties is evaluated. The parameters that define the bending stiffness properties of the membrane have been studied, specifically two key parameters: Young's modulus of the tympanic membrane and the thickness of the eardrum. Additionally, it has been completed with the evaluation of the presence of an initial prestrain inside the TM. Modal analysis is used to study the qualitative characteristics of the TM comparing with vibration patterns obtained by holography. Higher-order modes are shown as a tool to identify these properties. The results show that different combinations of elastic properties and prestrain provide similar responses. The presence of prestrain at the membrane adds more uncertainty, and it is pointed out as a source for the lack of agreement of some previous TM elastic modulus estimations.

Numerical Study of Laser Shock Peening Effects on Alloy 600 Nozzles With Initial Residual Stresses

This paper investigates the effect of initial residual stress and prestrain on residual stresses due to laser shock peening for Alloy 600 using numerical simulation. For simulation, the strain rate dependent Johnson–Cook hardening model with a Mie–Grüneisen equation of state is used. Simulation results are compared with published experimental data, showing good agreement. It is found that the laser shock peening (LSP) process is more effective for higher initial tensile residual stress and for larger initial prestrain in terms of compressive stress at the near surface. However, the effective depth decreases with increasing initial tensile residual stress and initial prestrain.

Simulation of the response of shrunk reinforced concrete elements subjected to short-term loading: a bi-dimensional numerical approach

This work deals with the extension of a smeared crack constitutive model for reinforced concrete elements subjected to a general biaxial stress state, in order to include the effects of shrinkage occurring before the application of any external load. Indeed, shrinkage markedly influences the structural behavior even under short-term loading, in terms of both member deformations and cracking load. A new set of equilibrium and compatibility equations is formulated to include concrete initial prestrains, which are determined on the basis of the moisture field within the structural element. The numerical strategy is validated through comparisons with data from the literature.

Static Strain Aging Behavior of a Manganese-Silicon Steel After Single and Multi-stage Straining

In this work, static strain aging behavior of an alloy steel containing high amounts of silicon and manganese was examined while the influences of initial microstructure and pre-strain on the aging kinetics were evaluated as well. The rate of strain aging in a low carbon steel was also determined and compared with that occurred in the alloy steel. The rates of static strain aging in the steels were defined at room temperature and at 95 °C by means of double-hit tensile testing and hardness measurements. In addition, three-stage aging experiments at 80 °C were carried out to estimate aging behavior under multi-pass deformation processing. The results showed that in-solution manganese and silicon atoms could significantly affect the aging behavior of the steel and reduce the kinetics of static strain aging as compared to the low carbon steel. The initial microstructure also played an important role on the aging behavior. The rapidly cooled steel having mean ferrite grain size of 9.7 μm showed the least aging susceptibility index during the aging experiments. Accordingly, the activation energies for static strain aging were calculated as 93.2 and 85.7 kJ/mole for the alloy steel having fine and coarse ferrite-pearlite structures, respectively while it was computed as 79.1 kJ/mole for the low carbon steel with ferrite mean grain size of about 16.2 μm.

Nonlinear mechanics of non-dilute viscoelastic layered composites

This paper investigates the nonlinear mechanics of layered composites that include a stiff elastic constituent and a soft viscoelastic constituent. Layered composites buckle with an infinite wavelength at small compressive strains in the case of a high volume fraction of the stiff constituent (the non-dilute case). An iterative algorithm is derived for the finite deformation of viscoelastic non-dilute layered composites with neo-Hookean phases. After validation by comparison to nonlinear finite element simulations, we analyze the effect of initial layer direction, strain rate, and prestrain on the response to time-dependent prescribed compressive strains. Interestingly, these composites have both a very high stiffness prior to buckling and a large energy dissipation capacity in the postbuckling regime. When these composites are subjected to cyclic strains of small amplitude, the effective stiffness and damping properties can be tuned by orders of magnitude by adjusting the prestrain.

Dynamic response of a high-rise chemical tower controlled by pseudo-elastic shape memory alloy cables

The wind-induced vibration of a high-rise chemical tower controlled by pseudo-elastic shape memory alloy cables was studied in this work. Based on taking into account phase transformations of shape memory alloy, the motion equation of the shape memory alloy–controlled chemical tower was established by the finite element method, which was coupled with phase transformation equations of shape memory alloy. The incremental finite-element-based Newmark integration method and the iterative method were utilized to solve this motion equation, and the dynamic response of the shape memory alloy–controlled chemical tower was analyzed. The results were compared with those of the chemical tower without any cables and under the control of common cables. It is found that pseudo-elastic shape memory alloy cables have good vibration suppression effect on the chemical tower. In addition, the vibration suppression effect of shape memory alloy cables is affected by the operating temperature, initial pre-strain of shape memory alloy, cross-sectional size of shape memory alloy, and the angle between each shape memory alloy cable and the ground.

Uniaxial strain ratcheting behavior of dented steel tubular: An experimental study

Changes in the internal pressure and the temperature in a steel pipeline generates cycles of high axial compression/relaxing in the line. This makes the line vulnerable to cyclic creep and progressive plastic straining (ratcheting). On the other hand, offshore pipelines are usually laid unburied, so they might become disposed to heavy accidental transverse loads, such as those caused by impacts from trawl gears, anchors or other dropped objects. This paper deals with the ratcheting behavior of dented steel offshore pipelines under uniaxial cyclic loading. The experimental study were conducted on small scale specimens from X80 steel pipes with an outer diameter of D=44mm and D/t of 22. Effects of some key parameters such as the initial pre-strain, the cyclic stress amplitude and the dent depth magnitude were investigated. It was found that while the initial pre-strain and the cyclic stress amplitude had strong effects on the ratcheting response of both dented and intact tubes, the strain ratcheting rate was radically increasing by the presence of a dent. The dent was also significantly affecting the number of stress cycles which could be tolerated by the pipe. When the initial pre-strain was low the changes in the ratcheting behavior of the dented pipes and the intact pipe were less significant. With higher initial pre-strains, even very small dents were resulting in remarkable degradation of the pipe ratcheting response.

Determinants of Fibrinolysis in Single Fibrin Fibers

To prevent local hemorrhage upon injury, a blood vessel will constrict to prevent blood loss; platelets will aggregate at the injury site and trigger the coagulation cascade, forming a platelet plug held together by fibrin fibers. When the hemostatic role of a thrombus is fulfilled, it is effectively removed from the circulatory system via the fibrinolytic system, which is mediated by the serine protease plasmin. Throughout the fibrinolytic process, fibrin fibers undergo significant morphological and biochemical changes as thrombi are digested by plasmin. Understanding the digestion of whole blood clots requires fundamental knowledge of how various structural and physical properties affect the rate of fibrinolysis at the level of single fibrin fibers. We have identified and characterized a novel pathway in plasmin-induced fibrin degradation where fibrin fibers remain intact and exhibit an increase in length and Brownian motion during fibrinolysis. Other members of our lab have identified the αC-domains as the dominant source of low strain (<100%) fiber extension and relaxation. Since elongated fibers exhibit a strain of <20%, our results indicate that elongation occurs when plasmin predominately cleaves the αC-domains of fibrin molecules, causing the fiber to relax. These results also suggest a fiber's initial diameter and prestrain determine whether a fiber will lyse or elongate.