Displacement-controlled Tensile Tests Research Articles

Characterization of the strain-rate–dependent mechanical response of single cell–cell junctions

Cell-cell adhesions are often subjected to mechanical strains of different rates and magnitudes in normal tissue function. However, the rate-dependent mechanical behavior of individual cell-cell adhesions has not been fully characterized due to the lack of proper experimental techniques and therefore remains elusive. This is particularly true under large strain conditions, which may potentially lead to cell-cell adhesion dissociation and ultimately tissue fracture. In this study, we designed and fabricated a single-cell adhesion micro tensile tester (SCAµTT) using two-photon polymerization and performed displacement-controlled tensile tests of individual pairs of adherent epithelial cells with a mature cell-cell adhesion. Straining the cytoskeleton-cell adhesion complex system reveals a passive shear-thinning viscoelastic behavior and a rate-dependent active stress-relaxation mechanism mediated by cytoskeleton growth. Under low strain rates, stress relaxation mediated by the cytoskeleton can effectively relax junctional stress buildup and prevent adhesion bond rupture. Cadherin bond dissociation also exhibits rate-dependent strengthening, in which increased strain rate results in elevated stress levels at which cadherin bonds fail. This bond dissociation becomes a synchronized catastrophic event that leads to junction fracture at high strain rates. Even at high strain rates, a single cell-cell junction displays a remarkable tensile strength to sustain a strain as much as 200% before complete junction rupture. Collectively, the platform and the biophysical understandings in this study are expected to build a foundation for the mechanistic investigation of the adaptive viscoelasticity of the cell-cell junction.

An Energy-Based Unified Approach to Predict the Low-Cycle Fatigue Life of Type 316L Stainless Steel under Various Temperatures and Strain-Rates.

An energy-based low-cycle fatigue model was proposed for applications at a range of temperatures. An existing model was extended to the integrated approach, incorporating the simultaneous effects of strain rate and temperature. A favored material at high temperature, type 316L stainless steel, was selected in this study and its material characteristics were investigated. Tensile tests and low-cycle fatigue tests were performed using several strain rates at a temperature ranging from room temperature to 650 °C. Material properties were obtained in terms of temperature using the displacement-controlled tensile tests and further material response were investigated using strain-controlled tensile tests. Consequently, no pronounced reduction in strengths occurred at temperatures between 300 and 550 °C, and a negative strain rate response was observed in the temperature range. Based on the low-cycle fatigue tests by varying strain rates and temperature, it was found that a normalized plastic strain energy density and a strain-rate modified cycle were successfully correlated. The accuracy of the model was discussed by comparing between predicted and experimental lives.

Quantification of strain induced damage in medial collateral ligaments.

In the past years, there have been several experimental studies that aimed at quantifying the material properties of articular ligaments such as tangent modulus, tensile strength, and ultimate strain. Little has been done to describe their response to mechanical stimuli that lead to damage. The purpose of this experimental study was to characterize strain-induced damage in medial collateral ligaments (MCLs). Displacement-controlled tensile tests were performed on 30 MCLs harvested from Sprague Dawley rats. Each ligament was monotonically pulled to several increasing levels of displacement until complete failure occurred. The stress-strain data collected from the mechanical tests were analyzed to determine the onset of damage and its evolution. Unrecoverable changes such as increase in ligament's elongation at preload and decrease in the tangent modulus of the linear region of the stress-strain curves indicated the occurrence of damage. Interestingly, these changes were found to appear at two significantly different threshold strains (P<0.05). The mean threshold strain that determined the increase in ligament's elongation at preload was found to be 2.84% (standard deviation (SD) = 1.29%) and the mean threshold strain that caused the decrease in the tangent modulus of the linear region was computed to be 5.51% (SD = 2.10%), respectively. The findings of this study suggest that the damage mechanisms associated with the increase in ligament's elongation at preload and decrease in the tangent modulus of the linear region in the stress-strain curves in MCLs are likely different.

Dynamic Strain Ageing in a Twin-Induced Plasticity Steel

This work was to clarify the characteristics of serrated flow in an austenitic FeMnC twin-induced plasticity (TWIP) steel at room temperature (RT) using both strain- and crosshead displacement-controlled tensile tests. Three types of serrations were observed in strain-controlled but not in displacement-controlled tests, indicating that strain-controlled tensile tests provide more deformation details. The occurrence of the different types of serrations depends on both strain rate and strain level. Type C serrations were observed in TWIP steels at RT for the first time. The critical strain for the onset of serrations exhibits a positive strain rate dependence at higher strain rates, whereas an “inverse” critical strain behavior was observed in the lower strain rate region.

Experimental Characterization and Simulation of the Mechanical Behavior of an Epoxy Adhesive

In this contribution an experimental procedure to characterize an epoxy-based adhesive is presented. Displacement controlled tensile tests at different rates of loading, relaxation experiments and deformation controlled loading and unloading processes with intermediate relaxations are used to classify the nonlinear, inelastic mechanical behavior. Based on these results parameter identification for a continuum based material model has been carried out. The proposed material model is a viscoelastic constitutive law based on an overstress formulation.

Serrated Yielding at Cryogenic Temperatures in Structural Components of Wendelstein 7-X

Structural materials like stainless steels are prone to serrated yielding at cryogenic temperatures. The serration effect is the unstable yielding resulting in a zigzag shape of the stress-strain curve observed in displacement-controlled tensile tests. Moreover, a local temperature rise in the plastic zone is observed. In the current paper, the theories explaining the serration effect are briefly discussed. With a coupled thermal-mechanical Finite element (FE) model of a tensile test set-up the influence of the test conditions is demonstrated on the observed shape of the stress-strain curve. The FE model is extended to quantify the temperature rise and significance of the serration effect in critical support structures of the magnet system of W7-X. For W7-X loading rates the temperature rise due to yielding is shown to be moderate and yielding does not localize. As a simple and safe approach, plastic collapse of critical components to be operated at 4 K can be judged with a pure mechanical analysis using yield curves obtained at 77 K. In doing so, the thermal-mechanical interaction may be neglected.

Inelastic material behavior of polymers – Experimental characterization, formulation and implementation of a material model

In this contribution an experimental procedure based on displacement controlled tensile tests at different rates of loading, relaxation experiments and deformation controlled loading and unloading processes with intermediate relaxations to experimentally characterize and classify the nonlinear, inelastic mechanical behavior of polymers is presented. These experiments provide data for a structured approach to parameter identification. In line with the experiments, a small strain uniaxial viscoplastic material model is derived, subsequently generalized to multiaxial loadings and implemented into a finite element program. The combination of the experimental procedure and the proposed material model is then used to characterize and model the mechanical behavior of the thermoplastic polypropylene. After the identification of the necessary material parameters, stress–strain curves have been computed for different uni- and multiaxial loadings and are compared with experimental results.

An atomistic investigation of elastic and plastic properties of Au nanowires

In this study, mechanical properties of cylindrical gold nanowires with diameters ranging from 5 nm to 17.5 nm were investigated using atomistic computer simulations. Displacement-controlled tensile tests were carried out to investigate the role of defects such as surface steps and twin boundaries commonly found in electrodeposited wires. The high surface-to-volume ratio of nanowires plays a critical role in the mechanical properties. Yield stress was found to be significantly affected by any disturbance in surface morphology. Twin boundaries were not more favorable as a source for dislocation nucleation but disturbed the surface in the neighborhood of the twin, resulting in lower yield strength. Twin boundaries are seen as obstacles for the propagation of dislocations leading to some hardening effects.

Tensile properties of thin Cu-wires with a bamboo microstructure

Displacement controlled tensile tests have been performed on high purity Cu wires of thickness 20, 50 and 125 μm in bamboo grain structure making use of a laser speckle extensometer. Scanning electron micrographs of the wire surface after fracture revealed large grains deformed in single slip, in double slip, undeformed ones, and sample parts which have remained fine grained being nearly undeformed. Stress–strain curves of the deformed material are obtained inserting the sample length being active in deformation. They are discussed with respect to single crystal deformation curves of bulk wires. The comparably smaller fracture strains can be explained from surface inhomogeneities by wire drawing and slip steps.

Discontinuous deformation during load- and displacement-controlled tensile tests and optical observation in liquid helium

Abstract The temperature rise accompanying discontinuous deformation during displacement-controlled and load-controlled tensile tests was measured for high manganese steel at liquid helium temperature. The deformation behaviour and heat transfer at the specimen surface in liquid helium were observed directly with a fibrescope. The relationship between the temperature rise and the boiling helium behaviour was examined. In spite of the large discontinuous deformation during the load-controlled test, the magnitude of the temperature rise during the deformation was not as great as that expected from the displacement-controlled test. It is evident that the change of helium flow and heat transfer at the specimen surface caused the abrupt and large deformation during the load-controlled test.