Experimental Uniaxial Tensile Tests Research Articles

Numerical homogenization of a linearly elastic honeycomb lattice structure and comparison with analytical and experimental results

This paper presents a verification and validation analysis of Finite Element (FE) models predicting the mechanical response of linearly elastic honeycomb structures. We have studied three main models, namely: analytical models based on beam theories, explicit FE models where the cell geometry is explicitly meshed with 3D elements, and numerical homogenized FE models where a plate made of a honeycomb structure is meshed with 2D elements whose mechanical properties were predicted from numerical homogenization. We compared the predictions of these simulations against experimental uni-axial tensile tests where we mechanically tested 3D printed honeycomb specimens having a relative density of 40% and made of 13 and 37 cells, respectively. Comparison of the experimentally measured axial stiffness to the numerical predictions revealed that the numerical homogenization models can predict the apparent in-plane stiffness in structures made of 37 cells within 4%, while the discrepancy increases to 70% when 13 cells are considered. To quantify this discrepancy, we also provided a relationship between the number of represented cells and the discrepancy of the numerical homogenized model against the explicit FE models to predict the in-plane stiffness. We believe that these results could be important for the application of the homogenized models in optimization of honeycomb lattice structures whose relative density varies with space.

Virtual parameter identification of the forming behaviour of discontinuous fibre reinforced thermoplastic composite sheets

Fibre-reinforced thermoplastics (FRTP), such as organo sheets or laminates, are increasingly being used in large-scale automotive production. The high weight-saving potential, high specific strengths and stiffnesses as well as processing times suitable for large-scale production are some of the reasons for using these materials. However, the formability of such semi-finished products is severely limited by the fibre reinforcement, which can lead to fibre breakage, fibre displacement or wrinkling in complex-shaped components. In order to increase the formability, an FRTP semi-finished product is developed, which consists of discontinuous tapes. Due to the local sliding of the tape sections, a pseudo-plastic material behaviour is achieved. Experimental uniaxial tensile tests at elevated temperatures are used to investigate the forming behaviour of the material for different tape lengths and overlap lengths. Subsequently, this tensile test is numerically modelled in order to fit the pseudo-plasticity to the experimental data by a virtual parameter identification. With the help of the parameters determined from the numerical tensile test, the sliding behaviour of the tape sections can be used for forming simulations in order to achieve a higher prediction quality.

4414 Collagen Dermal Replacement Scaffold Mechanobiology in Treatment of Difficult-to-Heal Wounds

OBJECTIVES/GOALS: Difficult-to-heal wounds of the skin are a common and costly medical problem. Dermal replacement strategies have emerged as a solution, but a challenge is identification of optimal scaffold parameters. We present a model for assessment of clinical potential of collagen scaffolds for wound healing. METHODS/STUDY POPULATION: In previous animal experiments, we evaluated dermal replacement scaffolds custom-fabricated from fibril-forming collagen oligomer with controlled fibril density (4, 20, 40mg/cm3) and spatial gradients in rat excisional wounds. Wound contraction and cellularization were monitored by gross and histological image analysis for comparison with model outcomes. We now parameterize the scaffold parameters for use in the mathematical model of wound healing with nonlinear curve fitting. A preliminary chemo-bio-mechanical finite element model including collagen, cells, and an inflammatory signal was adapted to simulate wound healing results. RESULTS/ANTICIPATED RESULTS: Collagen oligomer microstructure was quantified from scanning electron micrographs. A constitutive law for collagen mechanics was fit to experimental uniaxial tensile tests. We have conducted preliminary three-dimensional finite element model simulations to be validated against experimental wound contraction, recellularization, and collagen remodeling data collected from each experimental group. We show the effects of collagen density and stiffness on wound contraction by altering early wound mechanical properties. We anticipate future work to further improve the model of mechanotransduction, inflammation, and recellularization. DISCUSSION/SIGNIFICANCE OF IMPACT: This work represents the first step towards a computational model of wounds treated with collagen scaffold dermal replacements. In turn, the model will be used to explore cell-scaffold interactions for purposes of prediction and optimization of tissue regeneration outcomes.

Quantifying Mechanical Properties of Automotive Steels with Deep Learning Based Computer Vision Algorithms

This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone. Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials’ plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve; (2) profile of the indented surface; (3) analyzing of the 3D-measurement image; and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface.

A Comparative Study of Hyperelastic Constitutive Models to Characterize the Behaviour of a Biopolymer Material for Diaphragm of Blood Pump Manufacturing

This article presents the results of the stress-strain state modelling of a blood pump diaphragm manufacturing from polyurethane ‘Vitur’ T-1413-85 for pulsatile type Ventricle Assist Device systems. Based on the analysis of the most distributed models of hyperelasticity, a model which most accurately describes the behaviour of the specified material has been determined (R2 = 0.9999 between numerical simulation and experimental uniaxial tensile tests data). Experimental studies have been carried out to determine the displacement of the diaphragm with a thickness of 0.3 mm and 0.5mm under the influence of control pressure. A comparison of the modelling results of the blood pump diaphragm stress-strain state and the conducted experimental studies on the determination of displacements confirmed the adequacy of the chosen model (R2 = 0.96 between numerical simulation and experiment) for both of the studied diaphragm thickness.

A Modified Hockett-Sherby Law Enabling the Description of the Thermomechanical Behaviour of the AA6061-T6

The use of warm temperatures in the forming process enhances the aluminium alloys formability and reduces the springback effect, when compared with room temperature conditions. However, in order to be able to design warm forming processes it is necessary to describe the influence of temperature and strain rate on the mechanical behaviour of the material. This work presents the procedure adopted to identify the parameters of a thermomechanical Hockett-Sherby type law, for the EN AW 6061-T6 aluminium alloy, based on previously reported results from experimental uniaxial tensile tests performed on a Gleeble machine [1]. The experimental data were analysed, in order to obtain the true stress-plastic strain curves, for a temperature range between room temperature and 200°C, at three different strain rates. A classical identification procedure was applied to identify the parameters of a modified Hockett-Sherby type law, which describes the dependence of the initial yield stress on temperature. A multi-step procedure is proposed, in combination with a gradient-based method, in order to enable the selection of an initial solution prone to lead to a feasible set of parameters. The identified parameters were used to perform the numerical analysis of the uniaxial tensile tests, taking into account the non-isothermal conditions, which occur in the Gleeble device. The analysis of these tests highlights that the non-isothermal conditions promote the increase of the strain rate in the centre of the specimen. However, the presence of the strain rate gradient along the specimen length seems to have a small impact in the classical analysis of the experimental data, as long as it is based on an effective gauge length that presents a uniform deformation.

Damage identification parameters of dual-phase 600–800 steels based on experimental void analysis and finite element simulations

In this work, the damage behavior of cold-rolled zinc-coated dual-phase steel sheets 600 and 800 grades was evaluated by means of standard uniaxial tensile testing, microstructural characterization and finite element modeling. The void formation in both steels was investigated as a function of the plastic strain level from digital image processing of scanning electron micrographs to quantify the average void size and to determine the corresponding measures of void density, void area fraction and void aspect ratio. Based on the void analysis and the experimental uniaxial tensile test data, a four-step procedure is proposed to identify the parameters of the Gurson–Tvergaard–Needleman (GTN) damage model. Bearing in mind the non-uniqueness of the GTN model parameters, 3D finite element simulations using a single element and an 1/8 symmetry uniaxial tensile specimen model were performed in a systematic manner to investigate the role of both nucleation and failure parameters on the uniaxial behavior. At low straining levels, most of the void initiated by debonding of globular aluminum-oxide inclusions identified by EDS in both dual-phase steels. At higher straining levels, the void density evolution is found to be more pronounced in DP600 steel owing to the faster void nucleation rate at the ferrite grain boundaries. A good agreement between predictions and experimental load-elongation is achieved allowing for the identification of the full set of GTN damage model parameters of both DP600 and DP800 steels. The proposed damage calibration of dual-phase steels can be very helpful in simulations of practical sheet metal forming processes.

Experimental and numerical study of crack behaviour for capsule-based self-healing cementitious materials

A 2D lattice model was constructed to simulate crack process of in shell-interlayer-cement paste zone. The simulated tensile strength was validated by an experimental uniaxial tensile test and used as the input for a 3D lattice model, which was constructed to perform mechanical analysis based on a series of 2D X-ray microtomography images. The fracture behaviour of the 3D lattice model with assigned mechanical properties gave a similar crack pattern and tensile strength as the real structure. This study is expected to provide a feasible approach for investigating the fracture and trigger behaviour miccrocapsules embedded in a self-healing cementitious system.

Efficient fast Fourier transform-based numerical implementation to simulate large strain behavior of polycrystalline materials

In this paper, a new full-field numerical framework is proposed to model large strain phenomena in polycrystals. The proposed framework is based on the elasto-viscoplastic (EVP) fast Fourier transform (FFT) formulation presented by Lebensohn et al. (2012) and the rate dependent crystal plasticity framework developed by Asaro and Needleman (1985). In this implementation, the full-field solutions of micromechanical fields are computed on a regular, voxelized representative volume element (RVE) in which either a single or multiple grid points represent a single grain. The Asaro and Needleman (1985) formulation coupled with a semi-explicit, forward gradient time-integration scheme (Peirce et al., 1983) is used to compute local stresses and the FFT-based method is used to find local strain fluctuations at each grid point. The proposed model is calibrated using experimental uniaxial tensile test results of aluminum alloy (AA) 5754 sheet and then used to predict texture evolution and stress-strain response for balanced biaxial tension and plane-strain tension along rolling (RD) and transverse (TD) directions. The predicted stress-strain and texture results show a good agreement with experimental measurements. The CPU time required by the proposed model is compared with the original EVP-FFT model for two separate cases and the proposed model showed significant improvement in computation time (approximately 100 times faster).

On twinning and anisotropy in rolled Mg alloy AZ31 under uniaxial tension

The Elastic Visco-Plastic Self-Consistent (EVPSC) model, with the recently developed Twinning and De-Twinning (TDT) description, is applied to study the mechanical behaviour of hot-rolled Mg alloy AZ31 under uniaxial tension. Numerical results are compared to the experimental uniaxial tensile tests reported earlier by Chapuis et al. [29] for the out-of-plane directions of a thick plate along angles of α = 0°, 30°, 45°, 60° and 90° between the normal direction and longitudinal specimen axis. It is shown that accounting for the initial texture and calibrating the EVPSC-TDT model by using uniaxial tension tests along the rolling direction and normal direction permits prediction of the strength anisotropy and strain hardening behavior along all five tensile directions, i.e. for cases in which the contribution of twinning is dominating, negligible or intermediate.

Numerical analysis and experimental verification of elastomer bending process with different material models

Abstract The article presents the results of tests in order to verifying the effectiveness of the nine selected elastomeric material models (Neo-Hookean, Mooney with two and three constants, Signorini, Yeoh, Ogden, Arruda-Boyce, Gent and Marlow), which the material constants were determined in one material test - the uniaxial tension testing. The convergence assessment of nine analyzed models were made on the basis of their performance from an experimental bending test of the elastomer samples from the results of numerical calculations FEM for each material models. To calculate the material constants for the analyzed materials, a model has been generated by the stressstrain characteristics created as a result of experimental uniaxial tensile test with elastomeric dumbbell samples, taking into account the parameters received in its 18th cycle. Using such a calculated material constants numerical simulation of the bending process of a elastomeric, parallelepipedic sampleswere carried out using MARC / Mentat program.

Tensile stress–crack width law for steel fibre reinforced self-compacting concrete obtained from indirect (splitting) tensile tests

In this work, mode I fracture parameters of steel fibre reinforced self-compacting concrete (SFRSCC) were derived from the numerical simulation of indirect splitting tensile tests. The combined experimental and numerical research allowed a comparison between the stress–crack width (σ–w) relationship acquired straightforwardly from direct tensile tests, and the σ–w response derived from inverse analysis of the splitting tensile tests results. For this purpose a comprehensive nonlinear 3D finite element (FE) modeling strategy was developed. A comparison between the experimental results obtained from splitting tensile tests and the corresponding FE simulations confirmed the good accuracy of the proposed strategy to derive the σ–w law for these composites. It is concluded that the post-cracking tensile laws obtained from inverse analysis provided a close relationship with the ones obtained from the experimental uniaxial tensile tests.

Mechanical Behavior of Annulus Fibrosus: A Microstructural Model of Fibers Reorientation

Experimental uniaxial tensile tests have been carried out on annulus tissue samples harvested on pig and lamb lumbar intervertebral discs. When subjecting the samples to loading cycles, the stress-strain curves exhibit strong nonlinearities and hysteresis. This particular behavior results from the anisotropic microstructure of annulus tissue composed of woven oriented collagen fibers embedded in the extracellular matrix. During uniaxial tension, the collagen fibers reorient toward the loading direction increasing its global stiffness. To describe this behavior, we propose a heuristic two-dimensional rheological model based on three mechanical and one geometrical characteristics. The latter one is the fibers orientation angle becoming the key parameter that govern the macroscopic mechanical behavior. The experimental results are used to identify the physical properties associated with the rheological model, leading to an accurate representation of the stress-strain curve over a complete loading cycle. In this framework, the fibers reorientation can solely account for the rigidity increase while the hysteresis is associated with liquid viscous flows through the matrix. Based on this representation, unusual coupling effects between strains and fluid flows can be observed, that would significantly affect the cell nutrients transport mechanisms.