Effect Of Multiaxial Stress State Research Articles

FEM simulation of thermo-mechanical stress and thermal fatigue life assessment of high-speed steel work rolls during hot strip rolling process

Work rolls used for hot strip rolling undergo the cyclic sequence of heating-cooling over the roll surface due to contact with the hot strip and the following cooling systems during every rolling revolution. Severe temperature gradient and thermal stress caused by cyclic heating-cooling loading are dominant factors for thermal fatigue failure of work rolls. Therefore, investigations on temperature and thermal stress-strain are essential for understanding the thermal fatigue of work roll. In this study, a thermo-mechanical analysis with elastic-plastic material parameters via a simplified finite element model is carried out to compute the stress-strain range experienced by the work roll during rolling and idling. Then, a model with the initial crack in the roll surface considering different depths and a model with the hot strip is conducted to better understand the thermal behaviors of the work roll during hot rolling respectively. Furthermore, the thermal fatigue life of work roll is assessed based on the modified Coffin-Manson relation, considering the effect of multi-axial stress state and temperature-dependent material properties caused by temperature fluctuations in work roll during hot rolling.

Effect of multiaxial stress state on creep damage in ASME T92 welded joint

AbstractThis study reports the type IV fracture process and the influence of multiaxial stress state in ASME T92 welded joints during creep. The type IV fracture occurs at the fine‐grained heat‐affected zone (ie, FGHAZ), involving void initiation, growth, and coalescence, microcrack occurrence, propagation and extension, and eventual macrocrack with consequent joint failure. The creep damage is not uniformly distributed along the thickness direction in the FGHAZ, and the central part of the welded joint is the most seriously damaged region. The equivalent creep strain is higher at the external surface, but the stress triaxiality is larger in the centre section. Large equivalent creep strain could promote creep void initiation, whereas high hydrostatic pressure and stress triaxiality factor accelerate void growth in the FGHAZ of T92 joints. Besides, reducing groove angle and HAZ width of the joints is recommended to delay the occurrence of type IV cracking because of lower equivalent creep strain and stress triaxiality factor.

Material, structural and modelling aspects of alkali aggregate reaction in concrete

Concrete nuclear structures have been identified as suffering from alkali aggregate reaction (AAR) in both Canada and the United States. Although much research effort has been directed toward understanding its effect on reinforced concrete structures, unresolved issues still remain. Among them are the effect of multiaxial stress states, commonly developed in nuclear structures, on the response of AAR-affected concrete; also of concern is the possible invalidity of conventional methods of analysis for structural assessment due to the induced anisotropy in the mechanical properties of concrete. The Canadian Nuclear Safety Commission initiated and funded an extensive study aimed to reveal the implications of concrete deterioration due to AAR on structural integrity, which included material and structural testing and modeling. This paper presents an overview of the entire research program and discusses the key results.

Anisotropic creep damage and fracture mechanism of nickel-base single crystal superalloy under multiaxial stress

The influence of orientation on the creep rupture properties of a single crystal superalloy DD6 under multiaxial stress was carried out at 980 °C and 400 MPa. A circumferential V-type notched specimen had been designed to investigate the effect of multiaxial stress state on the creep behavior. It was found that the creep lifetimes of the [111] oriented notched specimen were slightly longer than that of the [011] orientation and 1.36 times longer than that of the [001] orientation. The computational results showed that the strain and damage distribution revealed fourfold symmetry, double symmetry and threefold symmetry for [001], [011] and [111] orientation respectively, which was confirmed by experimental observation. At high temperature the creep anisotropy in three different orientations exhibited mainly in primary and secondary creep stages. Through the study of notched specimens by SEM, the morphology evolution of γ′-phase proved that the directional coarsening was strongly dependent on the local effective stress and the direction of the local max principal stress with respect to loading axis. Fracture morphology displayed uneven cleavage configuration with multi-level feature, and the cleavage planes parallels (001), (011) and (111) crystal plane for [001], [011] and [111] orientation, respectively. The cleavage plane, which is attributed to the cracks propagated along the interfaces of γ/γ′phases, displayed square-like, rhombus-like and hexagon-like feature for [001], [011] and [111] orientation, respectively. Due to the higher density in dislocations of {111} planes, it is more easily as the secondary crack when the crack reach the {111} planes, which is thought to be the main reason of forming different feature on primary cleavage plane.

The effect of multiaxial stress state on creep behavior and fracture mechanism of P92 steel

The creep experiments on plain and double U-typed notched specimens were conducted on P92 steel at 650°C. The notch strengthening effect was found in the notched specimens. Fracture appearance observed by scanning electron microscopy revealed that dimpled fracture for relatively blunt notched specimen, and dimpled fracture doubled with intergranular brittle fracture for relatively sharp notched specimen, which meant that fracture mechanism of P92 steel altered due to the presence of the notch. Meanwhile, based on Norton–Bailey and Kachanov–Robotnov constitutive models, a modified model was proposed. Finite element simulations were carried out to investigate the effect of multiaxial stress state on the creep behavior, fracture mechanism and damage evolvement of P92 steel. The simulation results agreed well with the fracture behaviors observed experimentally.

Prediction of creep rupture life of a V-notched bar in DD6 Ni-based single crystal superalloy

The circumferential V-type notched round bar has been designed to investigate the effect of multi-axial stress state on creep rupture behavior of DD6 Ni-based single crystal superalloys at 1100°C. Creep test on both smooth and notched specimens were carried out under 160MPa and 200MPa on the minimum net-section. Time to creep fracture was found to be higher in notched specimens than in smooth specimens. Finite element (FE) analysis coupled with continuum damage mechanics (CDM) was carried out to understand the stress distribution and the creep damage evolution under uniaxial and multi-axial stress states. The creep rupture life of the smooth specimen was successfully predicted by the classical Kachanov–Rabotnov damage law. The creep rupture life of the notched specimen predicted by finite element calculation incorporating a multi-axial creep CDM model was in good agreement with the experimental results. Both the fracture morphology and FE damage analysis indicate that rupture initiates first at the notch root in the notched specimen and finally towards to the center location.

Numerical analysis of compressed concrete columns confined with CFRP: Microplane-based approach

This paper presents the results of numerical studies carried out on concrete columns confined by Carbon Fiber Reinforced Polymer (CFRP) and loaded in uni-axial compression. A numerical simulation of confined columns experimentally tested by [1] is performed for relatively small concrete specimens of constant size but with different cross-section corner radius. The experimental results, reported in terms of axial stress–strain relationships and failure modes, constitute a useful database for the calibration of numerical models. These results clearly demonstrate that CFRP confinement is much less effective in square than in circular cross-sections. Therefore, the influence of the corner radius on the non-uniform stress distribution due to confinement, is numerically investigated using the proposed microplane-based model that takes into account the effect of multi-axial stress states in concrete. The experimental results are used to calibrate and verify the prediction capability of a three-dimensional finite element code (3D FE) that is based on the microplane constitutive law for concrete [2]. In the finite element model carbon fibers are modelled using nonlinear truss elements, while epoxy resin as well as concrete are modelled using microplane-based constitutive law and 3D finite elements. Given that experimental results for unconfined and confined configuration are available for each specimen, the 3D FE concrete model has been preliminarily calibrated on unconfined concrete specimens and then used in the analysis of a confined specimen. It is demonstrated that numerical models can predict behavior of confined concrete columns from the experimental investigations, confirming the predictability of the numerical microplane-based approach used.

Creep damage mechanism and γ′-phase morphology of a V-notched round bar in Ni-based single crystal superalloys

The circumferential V-type notched specimen has been designed to investigate the effect of multiaxial stress state on creep rupture behavior of Ni-based single crystal superalloys at 980°C. The stress on the minimum net-section is chosen to be 400MPa. Tests on smooth specimens were also carried out under 400MPa for comparison. Time to creep fracture was found to be higher in the notched specimens. Scanning Electrical Microscope (SEM) analysis indicated that rupture initiates at the notch root in the notched specimen and at the inner in the smooth specimen. Finite element analysis was carried out to understand the stress distribution and the creep damage evolution under multiaxial stress state. The morphology the evolution of γ′-phase proved that directional coarsening was strongly dependent on the local principal stress field. These findings can be helpful to understand the effect of notch on the creep rupture behavior and mechanism, especially on the γ′-phase morphology evolution.

Effect of multi-axial stress state on creep behavior and stress rupture life of a Ni-based DS superalloy

Effect of multi-axial stress state on creep behavior and stress rupture life of a Ni-based directionally solidified (DS) superalloy has been systematically investigated. Emphasis is placed on the roles of the applied net section stress level, initial theoretical stress concentration factor (Kt), notch shape, plate thickness and sample shape. The results show that creep behavior is obviously affected by all the mentioned factors, which are concretely embodied in the redistribution of stresses, accumulation of creep strains and evolution of creep damage. The indicators for describing the constraint degree are further quantitatively discussed and the tri-axiality factor I1/σMises is exhibited as a good indicator. Based on the Monkman–Grant relationship, the stress rupture lives of notched specimens are predicted, which are related to the constraint degree and agree well with the existing experimental laws.

Study on Creep Behaviors of T92 Steel under Multiaxial Stress State

Creep tests for smooth specimens and notched specimens of T92 steel were carried out to study the effect of multiaxial stress state on creep rupture behaviors at 650°C. Creep rupture life was estimated by representative stress at multiaxial state of stress, the failure behavior of multiaxial creep was analyzed, and Kachanov creep damage formula was used to analyze the experimental data. The results show that the notch strengthens rupture life, multiaxial rupture behavior is controlled by mixed parameters, the creep ductility of the smooth and notched specimen decreases with rupture time, and damage factors of the smooth specimen and notched specimen are similar according to Kachanov formula.

Effect of Multiaxial Stress State on Morphology and Spatial Distribution of Voids in Deformed Semicrystalline Polymer Assessed by X-ray Tomography

Cavitation in the semicrystalline polymer polyamide 6 has been studied in terms of 3D void morphology and distribution in the notched region of axisymmetric specimens using synchrotron radiation tomography at submicrometer resolution. Ex-situ (interrupted and unloaded) tests at different stages of straining reveal damage initiation in form of penny-shaped crazes at maximum load. An in-situ (under load) test confirms the damage morphology at maximum load. When a neck appears and extends within the notch, the penny-shaped crazes extend in height, resulting in a volume change. Final failure is seen to occur from the specimen interior via coalescence of several voids resulting in large cavities. The multiaxial stress state generated by the axisymmetric notch causes crazes/cracks that are larger in diameter than those occurring during necking of an initially smooth specimen. The distribution void volume fraction as a function of the radius is measured via image analysis, showing a damage maximum at the specimen center that decreases toward the specimen border. This distribution was found to be consistent with that of the stress triaxiality ratio.

Creep elastic follow-up factors under multi-axial displacement-controlled loading

In this paper, the effect of multi-axial stress states on the elastic follow-up behaviour of creeping structures is investigated by deriving analytical elastic follow-up factor solutions for two examples; power-law creeping square and cruciform plates under bi-axial displacement-controlled loading. The validity of the analytical solutions is checked against results from finite element analyses. The agreement is found to be good even for large amounts of stress relaxation. Based on the solutions developed, the effects of stress biaxiality, geometry and creep exponent on elastic follow-up are discussed.

The Effect of Multiaxial Stress State on Formation of Rafts in CMSX-4 Superalloy during Creep

The effect of multiaxial stress state caused by a notch effect on microstructure degradation of single crystal nickel based superalloy CMSX-4 was studied during creep at a temperature of 950 °C, under constant nominal stress of 120 MPa for 500 and 1000 h. In order to minimize surface oxidation effects on microstructure degradation and notched geometry of specimens, all experiments were performed under protective atmosphere. The geometry of creep specimens was designed with the aim to obtain defined interval of principal stresses, which is sufficiently large to get reliable microstructure response during tensile creep experiments. Flat specimens with “U-type” notch were designed using elastic finite element method (FEM) calculations. Microstructure analysis of the crept specimens showed that notches affect degradation processes of the studied superalloy significantly. FEM elastic-plastic analysis was performed to simulate creep of the notched specimens. Material creep behaviour was integrated into calculations by a subroutine using equations derived from experimentally measured data for cylindrical creep specimens tested at various constant nominal stresses. Magnitude, distribution and orientation of principal stresses resulting from the specimen loading were calculated and their effect on formation of rafts was described.

Creep damage evaluation for HAZ of Mod. 9Cr-lMo steels under multi-axial stress conditions

This study is concerned with the creep damage evaluation for fine grained heat affected zone (HAZ) of Mod. 9Cr-lMo steel. Circumferentially notched bar creep rupture tests have been conducted in order to examine the effect of multiaxial stress state on creep rupture and creep damage of the fined grained HAZ. The simulated fine grained HAZ has been used. Finite element predictions based on a continuum damage mechanics model with ductility exhaustion approach has been used to predict the creep rupture and the creep damage in the notched specimens. It is found that a ductility exhaustion approach provides very large creep damage which leads to too conservative prediction of creep life for notched specimens. © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11

In-situ TEM investigation of deformation behavior of metallic glass pillars

AbstractWe show results of in situ TEM (Transmission electron microscope) quantitative investigations on the compression behaviors of amorphous micropillars fabricated by focused ion beam from Cu47Ti33Zr11Ni6Sn2Si1 metallic glass (MG) ribbon. Pillars with well defined gauge sections and tip diameter ranging from 100 nm to 640 nm are studied. Quantitative compression tests were performed by a recently developed Picoindenter TEM holder, with the evolution of individual shear bands monitored in real time in TEM. It is found that the deformation of the MG pillars at the present size domain is still dominated by discrete shear banding as demonstrated by intermittent events in the load-displacement curves. However, the frequency, amplitude and distribution of these shear banding events are clearly size dependent at submicrometer scale, leading to an apparently transition in deformation mode from highly localized inhomogeneous deformation to less localized and more distributed deformation with decreasing pillars diameter. Deformation of a 105 nm diameter pillar having rounded tips is characterized with fully homogeneous bulge at the initial stage of deformation, indicating prompting effect of multi-axial stress state on transition to fully homogeneous deformation.

A three-dimensional phenomenological model for martensite reorientation in shape memory alloys

In this work, we propose a macroscopic phenomenological model that is based on the classical framework of thermodynamics of irreversible processes and accounts for the effect of multiaxial stress states and non-proportional loading histories. The model is able to account for the evolution of both twinned and detwinned martensite. Moreover, reorientation of the product phase according to loading direction is specifically accounted for. Towards this purpose the inelastic strain is split into two contributions deriving, respectively, from creation of detwinned martensite and reorientation of previously existing martensite variants. Computational tests demonstrate the ability of the model to simulate the main aspects of the shape memory response in a one-dimensional setting and some of the features that have been experimentally found in the case of multiaxial non-proportional loading histories. Experimental non-proportional loading paths have also been simulated and a good qualitative agreement between numerical and experimental response is observed.

Effect of Multiaxial Stress State on High-Temperature Low-Cycle Fatigue Properties of SUS304 Steel

Fully reversed strain-controlled low-cycle fatigue tests, so called PP tests, were conducted on thin-walled tubular specimens of SUS304 austenitic stainless steel at 973K in air under proportional and nonproportinal loading conditions. Push-pull straining, cyclic torsion straining and in-phase straining of push-pull and cyclic torsion were selected as proportional loading, and 90deg out-of-phase straining of push-pull and cyclic torsion as nonproportional loading. Experimental data were analyzed by using both the Manson-Coffin equation and the critical plane model proposed by Brown and Miller, and the effects of the multiaxial stress state on the strain range versus life relationship were discussed. As the results, a new parameter was proposed that has a hopeful potential to estimate the effect of nonproportional loading on creep-fatigue life quantitatively based on the strain range partitioning method.

Fracture modelling of a high performance armour steel

The fracture characteristics of the high performance armour steel Armox 500T is investigated. Tensile mechanical experiments using samples with different notch geometries are used to investigate the effect of multi-axial stress states on the strain to fracture. The experiments are numerically simulated and from the simulation the stress at the point of fracture initiation is determined as a function of strain and these data are then used to extract parameters for fracture models. A fracture model based on quasi-static experiments is suggested and the model is tested against independent experiments done at both static and dynamic loading. The result show that the fracture model give reasonable good agreement between simulations and experiments at both static and dynamic loading condition. This indicates that multi-axial loading is more important to the strain to fracture than the deformation rate in the investigated loading range. However on-going work will further characterise the fracture behaviour of Armox 500T.

Monte-Carlo Simulation and the Chain-of-Bundles Model for Titanium-Matrix Composite Materials

A numerical simulation technique has been developed and is used to estimate the ultimate strength of several unidirectional metal-matrix composite systems. The results of multiple realizations of this technique, which explicitly accounts for the local nature of fiber load redistribution, are presented and show excellent agreement with experimental results. In addition to synthesizing some recent numerical modeling efforts for these types of composite systems, this simulation technique incorporates both the multi-axial stress state effects and the change in constituent residual stresses as the matrix plastically deforms. A thorough statistical analysis of the ultimate strength predictions is performed not only to assess the accuracy of the simulation model presented, but also to provide a measure of goodness of the various analytical models reviewed.

On the stress state dependence of creep rupture

The influence of the stress state on the creep rupture time is studied for polycrystalline metals at high temperatures. The study is based on a set of constitutive relations that account for the nucleation and growth of grain boundary cavities and for the effect of grain boundary sliding. Both diffusive growth and creep growth of cavities is incorporated and also the mechanism of creep constrained cavitation is described. The effect of multi-axial stress states on the lifetime is studied by computing isochronous rupture loci corresponding to a number of different combinations of material parameters. These rupture loci are compared with available experimental results for multi-axial stress states. Lifetime predictions for varying stresses or temperatures are also investigated, and compared with results of the linear cumulative damage rule and with experimental results. Finally, non-proportional loading is studied by analysing a tube subject to a constant tensile load and a torsional load which is reversed a number of times.