Overcoming Limitations of the Conventional Strain-Life Fatigue Damage Model

1996 ◽  
Vol 118 (1) ◽  
pp. 103-108 ◽  
Author(s):  
T. E. Langlais ◽  
J. H. Vogel
Keyword(s):  
Damage Model ◽  
Strain Curve ◽  
Cyclic Stress ◽  
Material Behavior ◽  
Perfectly Plastic ◽  
Linear Relationships ◽  
Elastic Perfectly Plastic ◽  
Plastic Components ◽  
Log Linear ◽  
Very High

The strain-based approach to fatigue life prediction usually relies on the conventional strain-life equation which correlates the elastic and plastic strain to the life. The correlation is based on separate log-linear curve fits of the elastic and plastic components of the strain data versus the life. It is well known, however, that these linear relationships may be valid only within a specific interval of stress or strain. When material behavior approaches elastic-perfectly plastic, for instance, it is not uncommon for the test data to deviate from linearity at both very high and very low strains. For such materials a separate fit of each curve is likely to give material constants significantly inconsistent with the fit of the cyclic stress-strain curve, especially if a good local fit over a restricted interval is obtained. In this work, some of the errors that arise as a result of this inconsistency are described, and recommended methods are developed for treating these errors. Numerical concerns are also addressed, and sample results are included.

A General Autofrettage Model of a Thick-Walled Cylinder Based on Tensile-Compressive Stress-Strain Curve of a Material

2005 ◽  
Vol 40 (6) ◽  
pp. 599-607 ◽  
Author(s):  
X. P Huang
Keyword(s):  
Strain Hardening ◽  
Compressive Stress ◽  
Bauschinger Effect ◽  
Strain Curve ◽  
Accurate Prediction ◽  
Stress Strain Curve ◽  
Stress Strain ◽  
Perfectly Plastic ◽  
Hardening Models ◽  
Elastic Perfectly Plastic

The basic autofrettage theory assumes elastic-perfectly plastic behaviour. Because of the Bauschinger effect and strain-hardening, most materials do not display elastic-perfectly plastic properties and consequently various autofrettage models are based on different simplified material strain-hardening models, which assume linear strain-hardening or power strain-hardening or a combination of these strain-hardening models. This approach gives a more accurate prediction than the elastic-perfectly plastic model and is suitable for different strain-hardening materials. In this paper, a general autofrettage model that incorporates the material strain-hardening relationship and the Bauschinger effect, based upon the actual tensile-compressive stress-strain curve of a material is proposed. The model incorporates the von Mises yield criterion, an incompressible material, and the plane strain condition. Analytic expressions for the residual stress distribution have been derived. Experimental results show that the present model has a stronger curve-fitting ability and gives a more accurate prediction. Several other models are shown to be special cases of the general model presented in this paper. The parameters needed in the model are determined by fitting the actual tensile-compressive curve of the material, and the maximum strain of this curve should closely represent the maximum equivalent strain at the inner surface of the cylinder under maximum autofrettage pressure.

A General Solution for the Pressurized Elastoplastic Tube

1992 ◽  
Vol 59 (1) ◽  
pp. 20-26 ◽  
Author(s):  
David Durban ◽  
Michael Kubi
Keyword(s):  
General Solution ◽  
Finite Strain ◽  
Cylindrical Tube ◽  
Yield Function ◽  
Material Behavior ◽  
Deformation Pattern ◽  
Perfectly Plastic ◽  
Plastic Phase ◽  
Thin Walled ◽  
Elastic Perfectly Plastic

The problem of a thick-walled cylindrical tube subjected to internal pressure is investigated within the framework of continuum plasticity. Material behavior is modeled by a finite strain elastoplastic flow theory based on the Tresca yield function. The deformation pattern is restricted by the plane-strain condition but arbitrary hardening and elastic compressibility are accounted for. A general solution is given in terms of quadratures. The analysis also includes treatment of a second plastic phase, characterized by corner relations, that may develop at the inner boundary. It is shown that the interface between the two plastic regions moves initially outwards and then, beyond a certain strain level, it moves back inwards. Some useful and simple results are given for thin-walled tubes of hardening materials and for thick-walled elastic/perfectly plastic tubes.

scholarly journals Thermo-Mechanical Modeling of Distortions Promoted during Cooling of Ti-6Al-4V Part Produced by Superplastic Forming

2016 ◽  
Vol 838-839 ◽  
pp. 196-201
Author(s):  
Maxime Rollin ◽  
Vincent Velay ◽  
Luc Penazzi ◽  
Thomas Pottier ◽  
Thierry Sentenac ◽  
...  
Keyword(s):  
Finite Element ◽  
Thermal Stresses ◽  
Superplastic Forming ◽  
Model Material ◽  
Material Behavior ◽  
Mechanical Modeling ◽  
Temperature Dependent ◽  
Finite Element Software ◽  
Perfectly Plastic ◽  
Elastic Perfectly Plastic

In AIRBUS, most of the complex shaped titanium fairing parts of pylon and air inlets are produced by superplastic forming (SPF). These parts are cooled down after forming to ease their extraction and increase the production rate, but AIRBUS wastes a lot of time to go back over the geometric defects generated by the cooling step. This paper investigates the simulations of the SPF, cooling and clipping operations of a part on Abaqus® Finite element software. The different steps of the global process impact the final distortions. SPF impacts the thickness and the microstructure/behavior of material, cooling impacts also the microstructure/behavior of material and promotes distortions through thermal stresses and finally, clipping relaxes the residual stresses of the cut part. An elastic-viscoplastic power law is used to model material behavior during SPF and a temperature dependent elastic perfectly plastic model for the cooling and clipping operations.

Mechanical and Thermodynamic Considerations of an Assemblage of Homogeneous Elastic-Plastic States

1968 ◽  
Vol 35 (3) ◽  
pp. 596-603 ◽  
Author(s):  
David Rubin
Keyword(s):  
Plastic Deformation ◽  
Mechanical Behavior ◽  
Strain Energy ◽  
Material Behavior ◽  
Free State ◽  
Elastic Plastic ◽  
Elastic Range ◽  
Perfectly Plastic ◽  
Stress Free State ◽  
Elastic Perfectly Plastic

The mechanics and the thermodynamics of plastic deformation are considered in terms of a general assemblage or continuum of elastic, perfectly plastic elements or states. Such models not only match the external mechanical behavior of real materials structures and continua, but they also afford a simple thermodynamic definability. A consideration of the internal behavior shows that the stress-free state has the maximum elastic range. Hardening in the sense of an increasing macroscopic elastic range is accompanied by a release of stored strain energy; the stress-free state always is restorable. This behavior is appropriate for real structures and continua. However, it is precisely these continuum characteristics which make the assemblages inappropriate models of material behavior. Barriers to continuing plastic deformation are required which do exist on the microscale, but lie outside of the scope of the most complex of these thermodynamically well-defined assemblages.

Numerical and Experimental Study on Ultimate Strength of Stiffened Column Under Axial Compression

2020 ◽  
Author(s):  
Hongyuan Mei ◽  
Deyu Wang ◽  
Qi Wan
Keyword(s):  
Ultimate Strength ◽  
Axial Compression ◽  
Strain Curve ◽  
High Strength ◽  
Tensile Tests ◽  
True Stress ◽  
Stress Strain ◽  
Test Fixture ◽  
Perfectly Plastic ◽  
Elastic Perfectly Plastic

Abstract Six specimens with one Tee-bar stiffener and its attached plating were tested under axial compression to investigate the ultimate strength. The specimens have one longitudinal span and the simply supported boundary conditions at the end edge of loading were produced based on a horizontal test fixture. The initial geometrical imperfections were measured and tensile tests of high tensile steel used in the specimens with different thickness were conducted. The results calculated by FE analysis with true stress-strain curves, average measured thickness and measured initial geometrical deformation could reach a good agreement with experimental results. The ultimate strength calculated with elastic/perfectly plastic curve is approximately 10% larger than that with true stress-strain curve. The reason is that the proportional limit stress of material is significantly lower than 0.2% proof stress for the high strength steel used in specimens. And the occurrence of buckling is earlier than the time that the material enters into plastic stage. As a result, the ultimate strength assessed with elastic/perfectly plastic curve doesn’t always the lowest result and it should be adopted carefully.

Nonlinear Analysis of Anchored Tanks Subject to Equivalent Seismic Loading

2002 ◽  
Author(s):  
D. Redekop ◽  
P. Mirfakhraei ◽  
T. Muhammad
Keyword(s):  
Nonlinear Behavior ◽  
Seismic Loading ◽  
Material Behavior ◽  
Seismic Action ◽  
The Finite Element Method ◽  
Perfectly Plastic ◽  
Liquid Storage ◽  
Failure Loads ◽  
Elastic Perfectly Plastic ◽  
Hydrodynamic Effects

The finite element method is applied to the problem of the nonlinear behavior of anchored cylindrical liquid-storage tanks subject to horizontal seismic loading. The tank alone is modelled with assumptions of fixed conditions at the base and free conditions at the top. Geometric nonlinearity is considered and the material behavior is taken as elastic-perfectly plastic. The loading consists of a constant hydrostatic pressure to which is added an equivalent static pressure representing hydrodynamic effects arising from seismic action. The latter loading is increased until failure occurs. As an indication of the validity of the approach a comparison with a test result is given. A parametric study is then conducted. Nonlinear failure loads are calculated in each case, and these are compared with previously determined elastic buckling loads.

Continuum Damage Mechanics Studies on the Dynamic Fracture of Concrete

1985 ◽  
Vol 64 ◽  
Author(s):  
E. P. Chen
Keyword(s):  
Dynamic Fracture ◽  
Damage Mechanics ◽  
Damage Model ◽  
Continuum Damage Mechanics ◽  
Damage Mechanism ◽  
Continuum Damage ◽  
Strain Rate Effects ◽  
Perfectly Plastic ◽  
Material Stiffness ◽  
Elastic Perfectly Plastic

ABSTRACTThe dynamic fracture of concrete in tension is studied by applying a continuum damage model developed by the author and his coworkers [1–3]. In this model, the degree of damage in concrete corresponds to the fraction of concrete volume that has been tension relieved, and tensile microcracking has been taken as the damage mechanism. In compression, the concrete is assumed to respond in an elastic/perfectly plastic manner. Strain-rate effects have been explicitly included in the model. Accumulation of damage in the material is reflected by the progressive weakening of the material stiffness. Examples involving center- and edge-cracked plate specimens subjected to the action of step and ramp loads are used to demonstrate the material responses predicted by the model. The bulk pressure versus strain relationships at locations close to the crack tip clearly show strainsoftening behavior. The damage tends to localize around the crack and its extent in the specimen is dependent upon both the crack geometry and the loading type. These results are presented and their implications are discussed.

On the Material Modeling of the Autofrettaged Pressure Vessel Steels

2008 ◽  
Author(s):  
G. H. Farrahi ◽  
E. Hosseinian ◽  
A. Assempour
Keyword(s):  
Uniaxial Tension ◽  
High Strength Steels ◽  
High Strength ◽  
Test Methods ◽  
Material Behavior ◽  
Material Modeling ◽  
Chemical Compositions ◽  
Accurate Analysis ◽  
Perfectly Plastic ◽  
Elastic Perfectly Plastic

Material modeling of the high strength steels plays an important role in accurate analysis of autofrettaged tubes. Although, the loading behavior of such materials is nearly elastic-perfectly plastic, their unloading behavior due to Bauschinger effect is very complicated. DIN1.6959 steel is frequently used for construction of autofrettaged tubes in some countries such as Germany and Switzerland. In spite of similarity between chemical compositions of this steel with A723 steel, due to different material processing, two steels have unlikely behavior. In this paper material behavior of DIN1.6959 has been accurately modeled by uniaxial tension-compression test results. Both 6 mm and 12.5 mm diameter specimens were used and compared. Also various functions for modeling of autofrettaged steels were investigated and new function was introduced for accurate modeling. Moreover, two test methods, i.e. uniaxial tension-compression and torsion tests, which used for modeling of autofrettage steels, were analyzed. As well, material models of three important autofrettage steels, i.e. A723, HB7 and Din1.6959 were compared.

Analytical Modeling of Hydraulically Expanded Tube-To-Tubesheet Joints

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
Nor Eddine Laghzale ◽  
Abdel-Hakim Bouzid
Keyword(s):  
Contact Pressure ◽  
Analytical Model ◽  
Plastic Material ◽  
Material Behavior ◽  
Initial Contact ◽  
Expansion Process ◽  
Element Analysis ◽  
Elastoplastic Behavior ◽  
Perfectly Plastic ◽  
Elastic Perfectly Plastic

The loss of the initial tightness during service is one of the major causes of failure of tube-to-tubesheet joints. The initial residual contact pressure and its variation during the lifetime of the joint are among the parameters to blame. A reliable assessment of the initial contact pressure value requires accurate and rigorous modeling of the elastoplastic behavior of the tube and the tubesheet during the expansion process. This paper deals with the development of a new analytical model used to accurately predict the residual contact pressure resulting from a hydraulic expansion process. The analytical model is based on the elastic perfectly plastic material behavior of the tube and the tubesheet and the interaction between these two elements of the expanded joint. The model results have been compared and validated with those of the more accurate finite element analysis models. Additional comparisons have been made with existing methods.

Theoretical Analysis of Hydraulically Expanded Tube-to-Tubesheet Joints With Linear Strain Hardening Material Behavior

2009 ◽  
Vol 131 (6) ◽  
Author(s):  
Nor Eddine Laghzale ◽  
Abdel-Hakim Bouzid
Keyword(s):  
Strain Hardening ◽  
Material Behavior ◽  
Plastic Behavior ◽  
Element Analysis ◽  
Hardening Material ◽  
Perfectly Plastic ◽  
Expansion Pressure ◽  
Proper Material ◽  
Elastic Perfectly Plastic ◽  
Design Calculations

The mechanism of failure of tube-to-tubesheet joints is related to the level of stresses produced in the tube expansion and transition zones during the expansion process. Maintaining a lower bound limit of the initial residual contact pressure over the lifetime of the expanded joint is a key solution to a leak free joint. An accurate model that estimates these stresses can be a useful tool to the design engineer to select the proper material geometry combination in conjunction with the required expansion pressure. Most existing design calculations are based on an elastic perfectly plastic behavior of the expansion joint materials. The proposed model is based on a strain hardening with a bilinear material behavior of the tube and the tubesheet. The interaction of these two components is simulated during the whole process of the application of the expansion pressure. The effects of the gap and the material strain hardening are to be emphasized. The model results are validated and confronted against the more accurate numerical finite element analysis models. Additional comparisons have been made to existing methods.

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