Simple Uniaxial Tests Research Articles

A predictive micromechanically-based model for damage and permanent deformations in copolymer sutures

An effective description of the mechanical behavior of biodegradable copolymers suture threads requires the analysis of their response under cyclic loading and the prediction of the fundamental damage and residual stretches effects. In this paper we propose a micromechanically-based model adopting a new form of Worm Like Chain free energy for the copolymer chains, which takes care of the insurgence of residual stretches on the basis of a rigorous statistical mechanics result. Under the affinity hypothesis we subsequently derive the macroscopic response of the material. The obtained model has a clear physical interpretation and depends on a small number of parameters, which can be fitted by a simple uniaxial test. The effectiveness of the theoretical results has then been verified by performing cyclic tests on Monocryl® monofilament sutures and showing the ability of the model in predicting with high accuracy the history dependence, the damage and permanent deformations in the obtained response.

Fatigue Strength Reduction Factors Based on Strain Energy Density Applied to Sharp and Blunt Notches under Multiaxial Loading

Real mechanical assemblies favor the initiation and propagation of fatigue cracks due to stress concentration phenomena arising from the geometrical features such as notches, corners, holes, welding toes, etc. Classical fatigue analysis of notched specimens is done using an empirical formula and a fitted fatigue strength reduction factor, which is experimentally expensive and lacks physical scene. In the present paper, a simple and meaningful methodology is proposed to assess notched components against multiaxial fatigue. In this method, by precisely defining a finite-size volume surrounding the fatigue crack initiation site (notch tip), over which the strain energy is averaged, the morphological effect on the process zone is fully addressed. Such a method takes into account the effect of combination of different modes (I, II, III) and the load ratio. In order to implement it for components with a sharp or blunt notch, it is enough to analyze a linear elastic finite element model and to know only the properties of materials obtained from simple uniaxial tests. New relationships for determining an effective (tensile-type) stress and fatigue strength reduction factors are derived for notched specimens. The accuracy of the proposed model is validated by experimental data available in the literature, related to tubular specimens weakened with sharp/blunt notches under combined bending-torsion loading. Such a situation widely appears in equipment used for various branches of industry such as piping, automotive, power plant, drilling, etc.

Anisotropic elastic-plastic deformation of paper: In-plane model

Laminated paperboard and paper is widely used in packaging products. It generally exhibits highly anisotropic and nonlinear mechanical behavior. The aim of this study is to describe the in-plane material behavior with an orthotropic elastic-plastic model based on the observed experimental behavior. A structural tensor-based approach was applied to model the elastic deformation, while a multi-surface based yield criterion was adopted to describe the yield behavior. The model incorporated nonlinear kinematic and isotropic hardening to capture the anisotropic hardening effect. In the experiment, the compressive yield stress was found to be insensitive to the previous tensile deformation. The proposed model could capture this compression yield stress preserving effect under reverse loading, which in turn reduced the required material parameters as expected. With the material parameters calibrated from a set of simple uniaxial tests in various directions, the model was shown to predict the stress-strain behavior for other orientations satisfactorily. The model was further validated with experiments under complex loading conditions and found to capture the highly anisotropic, elastic-plastic behavior accurately.

Identifying Polymeric Constitutive Equations for Incremental Sheet Forming Modelling

Recent publications have revealed an increasing interest in forming polymer materials using Incremental Sheet Forming. Therefore, several constitutive material models are being developed in an attempt to predict the physical response of polymeric materials during the process. This paper discuss several material models that could be used to predict experimental data collected on samples of PVC and PC subjected to simple uniaxial test performed at various temperatures and testing speeds. The results have shown that the Marlow and the rule of mixture material models could be used to describe viscoelastic and softening and permanent set effects, respectively, to predict the behaviour of a part formed by Incremental Sheet Forming.

A Study of Shear Stress Intensity Factor of PP and HDPE by a Modified Experimental Method together with FEM

Shear testing is one of the most complex testing areas where available methods and specimen geometries are different from each other. Therefore, a modified shear test specimen (MSTS) combining the simple uniaxial test with a zone of interest (ZOI) is tested which gives almost the pure shear. In this study, material parameters of polypropylene (PP) and high density polyethylene (HDPE) are first measured by tensile tests with a dogbone shaped specimen. These parameters are then used as an input for the finite element analysis. Secondly, a specially designed specimen (MSTS) is used to perform the shear stress tests in a tensile testing machine to get the results in terms of forces and extension, crack initiation etc. Scanning Electron Microscopy (SEM) is also performed on the shear fracture surface to find material behavior. These experiments are then simulated by finite element method and compared with the experimental results in order to confirm the simulation model. Shear stress state is inspected to find the usability of the proposed shear specimen. Finally, a geometry correction factor can be established for these two materials in this specific loading and geometry with notch using Linear Elastic Fracture Mechanics (LEFM). By these results, strain energy of shear failure and stress intensity factor (SIF) of shear of these two polymers are discussed in the special application of the screw cap opening of the medical or food packages with a temper evidence safety solution.

Evaluating Material Constitutive Equations for the Simulation of Incremental Sheet Forming Applied to Form Thermoplastic Materials

Incremental Sheet Forming (ISF) is able to produce highly customized products at a reasonable manufacturing cost and it has gain importance in the last years, becoming the focus of interest for many researchers and institutions. Some recent publications have revealed an increasing interest in forming thermoplastic materials. There is a tremendous amount of effort put in developing a model that may describe the equilibrium hysteresis and rate-dependence of thermoplastic materials in ISF. This paper will present a brief review of the most common constitutive equations that are able to model the behaviour of glassy polymers. It will be shown that by using a small number of material parameters defined in the Marlow model, it is possible to accurately predict experimental data collected on samples of PVC subjected to simple uniaxial test performed at room temperature. Additionally, some parts have been formed with ISF in order to verify whether the material is incompressible or not. It can be concluded that Marlow model might be used in future work to model the ISF manufacturing process.

A Preliminary Material Model to Predict Stress Softening and Permanent Set Effects of Human Vaginal Tissue

In this paper the authors modified an available material model to account the stress softening and permanent set effects exhibit by human vaginal tissue during simple uniaxial tests. The preliminary model is based on four material parameter values i.e., the stress-like material constants, the stress softening and the permanent set effect material parameter constants. To assess the accuracy of the proposed model, the authors compared theoretical predictions with available simple uniaxial experimental data. It is shown that the proposed modifications describe well experimental data with 99% of accuracy.

Identification of mechanical properties of biopolymer composites sensitive to interface effect using hybrid approach

The mechanical behaviour of biopolymer composites is intimately dependent on the phase properties and microstructure features, especially interfacial properties. In this introductive work, a numerical method is proposed to assess the role of the interface on the elasticity behaviour of a biopolymer composite characterised by randomly structured phases. A hybrid approach is developed based on finite element calculation and inverse/genetic algorithm to allow the correlation between the interface, intrinsic properties and the effective properties of the composite to be identified. Idealised 2D microstructures are generated in order to represent the main features of real starch-based microstructures in the full phase content range. Monte Carlo scheme is used for such purpose allowing the control of both phase ratio and interface content. Microstructures are meshed using a regular scheme with plane and interface elements. Imperfect interface behaviour is handled using a cohesive zone model. The effective elasticity modulus is computed based on the simulation of a simple uniaxial test. Based on the finite element results, the identification of the effective law is obtained using a hybrid artificial neural network/genetic algorithm approach.

The uniaxial strain test - a simple method for the characterization of porous materials

The application of cellular materials in load-carrying and security-relevant structures requires the exact prediction of their mechanical behavior, which necessitates the development of robust simulation models and techniques based on appropriate experimental procedures. The determination of the yield surface requires experiments under multi-axial stress states because the yield behavior is sensitive to the hydrostatic stress and simple uniaxial tests aim only to determine one single point of the yield surface. Therefore, an experimental technique based on a uniaxial strain test for the description of the influence of the hydrostatic stress on the yield condition in the elastic-plastic transition zone at small strains is proposed and numerically investigated. Furthermore, this experimental technique enables the determination of a second elastic constant, e.g., Poisson\'s ratio.

Investigation of Cellular Solids Under Biaxial Stress States

The determination of the yield surface of cellular and porous materials requires experimental procedures under multi-axial stress states as the yield behavior is sensitive to the hydrostatic stress and as simple uniaxial tests aim only to determine one single point of the yield surface. Therefore, an experimental technique based on a plane strain test for the description of the elastic-plastic transition zone at small strain is proposed and achieved in a biaxial testing machine. A simple cubic model of a periodic cellular metal is used to demonstrate the method for materials of relatively high relative density. In contrast to common analytical and numerical models, experimental samples of the same geometry are easy to manufacture and to simulate. Furthermore, this experimental technique enables the determination of Poisson's ratio in the elastic range without any strain measurement. The experimental procedure is verified by the finite element method.

Assessing the compaction susceptibility of South African forestry soils. II. Soil properties affecting compactibility and compressibility

Factors affecting the compaction susceptibility of South African forestry soils were assessed. Two traditional measures of compaction susceptibility were used: maximum bulk density ( ρ mbd) determined by the standard Proctor test, defined compactibility, and the compression index using a simple uni-axial test, defined compressibility. Soils were chosen from a broad range of geological and climatic regions and they varied greatly in texture (8 to 66 g 100 g −1 clay) and organic matter content (0.26 to 5.77 g 100 g −1 organic carbon). Soils showed a wide range in ρ mbd values, from 1.24 to 2.00 Mg m −3, and this reflected the wide range of particle size distributions and organic matter contents of the soils. Very good correlations were achieved between measures of particle size distribution, particularly clay plus silt and both compactibility and compressibility. Both compactibility and compressibility were significantly correlated with loss-on-ignition ( L OI) which is a measure reflecting the combined effects of soil texture and organic matter on soil physical properties. Indices of compaction susceptibility were influenced more by particle size distribution than by organic carbon content. Clear effects of organic carbon on compaction behaviour were only evident for soils with low clay contents (< 25 g 100 g −1. No clear relationship between compactibility and compressibility was found. Compactibility generally increased with decreasing clay plus silt content, whereas compressibility increased up to about 70 g 100 g −1 clay plus silt before decreasing again. It is difficult to define compaction susceptibility solely in terms of indices of compactibility or compressibility particularly as there is no clear relationship between these two properties. A classification system for compaction risk assessment is presented, based on the relationship between compactibility ( ρ mbd) and L OI, and between clay plus silt content and compressibility.

Creep, relaxation and cyclic behavior of a beam using a state-variable constitutive model

Numerical solutions are presented for the time-dependent inelastic behavior of a Bernoulli-Euler beam under a variety of time-dependent moment loadings. The inelastic constitutive representation employed is the recently developed state-variable constitutive law of Robinson. Robinson's equations have several novel features, and these are examined within the context of the behavior of a beam. Despite the fact that the beam may contain relatively complex inhomogeneous stress fields, the overall behavior is shown to be analogous to that observed in simple uniaxial tests.