Biaxial testing of textile material - methodology and instrument assembly procedure

Membrane structures are becoming popular because of the potential usage in structures with higher aesthetic claims. For the roofing of these structures, different textile materials are used. These special materials offer an alternative to conventional roofing materials and allow, besides its lightweightness, also a possibility to roof a difficult floor plan and big span. When designing such a construction, there are some challenges related to the properties of the specific material. In order to exploit the full potential of textile membrane materials, it is necessary to choose an appropriate material model for numerical modelling, which takes into account its nonlinear behaviour. The two most important material characteristics needed for characterizing the behaviour are Young’s modulus of elasticity and Poisson’s ratio. Since the material is orthotropic in most cases, it is necessary to test the material in two directions; therefore, these characteristics need to be obtained from the biaxial test. This contribution is focused on the research of the methodology of biaxial tests and test instrument assembly procedure, which will be used for the following testing of textile materials.

Comparison of two techniques (in vivo and ex-vivo) for evaluating the elastic properties of the ascending aorta: Prospective cohort study

Introduction Aneurysms of the ascending aorta (AA) correspond to a dilatation of the ascending aorta that progressively evolves over several years. The main complication of aneurysms of the ascending aorta is type A aortic dissection, which is associated with very high rates of morbidity and mortality. Prophylactic ascending aorta replacement guidelines are currently based on maximal AA diameter. However, this criterion is imperfect. Stretching tests on the aorta carried out ex-vivo make it possible to determine the elastic properties of healthy and aneurysmal aortic fragments (tension test, resistance before rupture). For several years now, cardiac magnetic resonance imaging (MRI) has provided another means of evaluating the elastic properties of the aorta. This imaging technique has the advantage of being non-invasive and of establishing aortic compliance (local measurement of stiffness) without using contrast material by measuring the variation of the aortic surface area during the cardiac cycle, and pulse wave velocity (overall stiffness of the aorta). Materials and methods Prospective single-center study including 100 patients with ascending aortic aneurysm requiring surgery. We will perform preoperative cine-MRI and biomechanical laboratory stretching tests on aortic samples collected during the cardiac procedure. Images will be acquired with a 3T MRI with only four other acquisitions in addition to the conventional protocol. These additional sequences are a Fast Low Angle Shot (FLASH)-type sequence performed during a short breath-hold in the transverse plane at the level of the bifurcation of the pulmonary artery, and phase-contrast sequences that encodes velocity at the same localization, and also in planes perpendicular to the aorta at the levels of the sino-tubular junction and the diaphragm for the descending aorta. For ex-vivo tests, the experiments will be carried out by a biaxial tensile test machine (ElectroForce®). Each specimen will be stretched with 10 times of 10% preconditioning and at a rate of 10 mm/min until rupture. During the experiment, the tissue is treated under a 37°C saline bath. The maximum elastic modulus from each sample will be calculated. Results The aim of this study is to obtain local patient-specific elastic modulus distribution of the ascending aorta from biaxial tensile tests and to assess elastic properties of the aorta using MRI, then to evaluate the correlation between biaxial tests and MRI measurements. Discussion Our research hypothesis is that there is a correlation between the evaluation of the elastic properties of the aorta from cardiac MRI and from stretching tests performed ex-vivo on aorta samples collected during ascending aorta replacement.

Advanced Crystal Plasticity Modeling of Multi-Phase Steels: Work-Hardening, Strain Rate Sensitivity and Formability

This work presents an advanced crystal plasticity model for the simulation of the mechanical behavior of multiphase advanced high-strength steels. The model is based on the Visco-Plastic Self-Consistent (VPSC) model and uses information about the material’s crystallographic texture and grain morphology together with a grain constitutive law. The law used here, based on the work of Pantleon, considers how dislocations are created and annihilated, as well as how they interact with obstacles such as grain boundaries and inclusions (carbides). Additionally, strain rate sensitivity is implemented using a phenomenological expression derived from literature data that does not require any fitting parameter. The model is applied to the study of two bainitic steels obtained by applying different heat treatments. After fitting the required parameters using tensile experiments in different directions at quasi-static and high strain rates, formability properties are determined using the model for the performance of virtual experiments: uniaxial tests are used to determine r-values and stress levels and biaxial tests are used for the calculation of yield surfaces and forming limit curves.

Calibration of the Discrete Element Method and Modeling of Shortening Experiments

The discrete element method (DEM) is becoming widely accepted as an effective method for addressing tectonic problems in granular materials. It is capable of reproducing structures observed in the analog model (AM). However, the previous experiments also pointed to variability among DEM models and AMs in the number of fault zones, their dip angle and spacing, and the evolution of the surface slope of a thrust wedge. The accuracy of the DEM depends on the input parameter values, so the calibration of the discrete element method is very important. Microscopic properties of particles and macroscopic properties of loose quartz sand were calibrated through a series of repose angle and biaxial tests. Furthermore, an AM was constructed to simulate the evolution of the thrust wedge to compare with DEM results. DEM and AM results indicate an encouraging overall agreement in model evolution. Based on a new automated wedge quantification method, DEM results were systematically compared with AM results on the number of fault zones, their dip angle and spacing, the evolution of the surface slope of a thrust wedge, and other parameters. Our study provides a necessary comparison between commonly applied modeling approaches, which is important for more confidently applying these methods to understand real fold and thrust belt systems.

Study of biaxial mechanical properties of the passive pig heart: material characterisation and categorisation of regional differences

Regional mechanics of the heart is vital in the development of accurate computational models for the pursuit of relevant therapies. Challenges related to heart dysfunctioning are the most important sources of mortality in the world. For example, myocardial infarction (MI) is the foremost killer in sub-Saharan African countries. Mechanical characterisation plays an important role in achieving accurate material behaviour. Material behaviour and constitutive modelling are essential for accurate development of computational models. The biaxial test data was utilised to generated Fung constitutive model material parameters of specific region of the pig myocardium. Also, Choi-Vito constitutive model material parameters were also determined in various myocardia regions. In most cases previously, the mechanical properties of the heart myocardium were assumed to be homogeneous. Most of the computational models developed have assumed that the all three heart regions exhibit similar mechanical properties. Hence, the main objective of this paper is to determine the mechanical material properties of healthy porcine myocardium in three regions, namely left ventricle (LV), mid-wall/interventricular septum (MDW) and right ventricle (RV). The biomechanical properties of the pig heart RV, LV and MDW were characterised using biaxial testing. The biaxial tests show the pig heart myocardium behaves non-linearly, heterogeneously and anisotropically. In this study, it was shown that RV, LV and MDW may exhibit slightly different mechanical properties. Material parameters of two selected constitutive models here may be helpful in regional tissue mechanics, especially for the understanding of various heart diseases and development of new therapies.

Tear strength of a laminated fabric for stratospheric airship under uniaxial and biaxial tests

Envelopes are main structures of stratospheric airships. They are usually made of laminated fabrics and prone to tearing, so it is significant to study their tear strengths. This paper aims to investigate the tear strength of an envelope for stratospheric airships by uniaxial and biaxial tear tests. Three uniaxial tear specimens and 15 biaxial tear specimens under five different stress ratios (warp stress vs weft stress) were tested, and their tear strengths were measured. Two-dimensional digital speckle correlation method was used to obtain specimens’ strain contours. The test results show that the average tear strength of uniaxial specimens is 32.99 N/mm, over 20% higher than those of biaxial specimens, ranging from 25.50 N/mm to 27.25 N/mm. It reveals that weft stress reduces the tear strength; nevertheless, the stress ratios slightly affect the tear strength. Besides, the strain contours clearly show three zones in each specimen – the low-strain zone, the high-strain zone, and the medium-strain zone. Depending on the strain contours and previous research on imperfect composite materials, we inferred that the crack-tip stress concentration factor of a uniaxial specimen is lower than that of a biaxial specimen. It explains the difference in tear strength between uniaxial and biaxial specimens. These findings suggest using biaxial tear test to measure the tear strength of an SSA’s envelope.

Feasibility Study of a Combined Tension-Torsion Hopkinson Bar

The increasing interest to improve the description of the plastic behavior and the fracture prediction for ductile materials under complex loading conditions conducted the researchers to overcome the J2 plasticity theory. To do this more sophisticated plasticity model base on ductile damage have been implemented. Material model parameters must be identified by means of proper testing and calibration procedures requiring different loading conditions. Different types of tests must be performed, imposing multiaxial stress paths to the specimens. Tensile tests on smooth and round notched bars, plane strain tests, torsion tests, compression and combined tension–torsion tests on hollow and solid cylindrical bars must be executed. While multiple works are present in literature for model assessment and validation in quasi-static conditions, nothing can be found at high strain rate in biaxial conditions (tension-torsion). Biaxial tests in dynamic conditions are very difficult to carry out especially if you are interested to register the entire story of stress and strain. In this work, analytical and numerical study to evaluate the feasibility to carry out dynamic tension, dynamic torsion, dynamic torsion-static tension/compression and dynamic tension–dynamic torsion tests is discussed. The tests will be performed using a properly designed Split Hopkinson Bar.