Local Stress-strain Behavior Research Articles

A Combined Numerical–Analytical Study for Notched Fatigue Crack Initiation Assessment in TRIP Steel: A Local Strain and a Fracture Mechanics Approach

In the fatigue design of metallic components using the safe-life approach, fatigue crack initiation as a development of slip systems at the nanoscale, followed by microstructurally short crack growth, is critical for the onset of structural failure. The development of reliable analytical tools for the prediction of crack initiation, although very complex due to the inherent multiscale fatigue damage processes involved, is important for promoting a more sophisticated design but, more importantly, enhancing the safety in regard to fatigue. The assessment of fatigue crack initiation life at the root of a V-shaped notch is performed by implementing a local strain and a fracture mechanics concept. In the low cycle fatigue analysis, the finite element method is used to determine the local stress–strain response at the notch root, which takes into account elastoplastic material behavior. Fatigue crack initiation is treated as the onset of a short corner crack by incremental damage accumulation and failure of a material element volume at the notch root. The finite element results are compared against established methodologies such as the Neuber and strain energy density methods. In the fracture mechanics approach, fatigue crack initiation is treated as the onset and propagation of a corner crack to a finite short crack. Fatigue experiments in two different transformation-induced plasticity (TRIP) steels were conducted to evaluate the analytical predictions and to determine the physical parameters for the definition of crack initiation. The analytical results show that the finite element method may be successfully implemented with existing fatigue models for a more accurate determination of the local stress–strain behavior at the notch tip in order to improve the assessment of fatigue crack initiation life compared to the established analytical methodologies.

Study on microstructural characterization and local stress–strain behavior of transition zone within K-TIG welded SAF2205/Q235 dissimilar joints

Dissimilar metal welded joints are increasingly and widely applied in any field of manufacturing. Meanwhile, a novel keyhole gas tungsten arc (K-TIG) welding technology presents the advantages of high productivity, high precision as well as low cost. This paper investigated microstructural characterization of K-TIG welded SAF2205/Q235 dissimilar joints with different welding torch deviations. Moreover, evolution of local microstructure and stress–strain behavior from transition zone was discussed by means of crystal plasticity finite element modeling. The results show that solid–solid phase transformation mode in the weld metal changes due to the change of constituent alloy elements. Noted that transition zone on the Q235 side includes compact martensite with high hardness, and the elements and mechanical properties present a severe gradient distribution. Transition zone with high strength has a greater resistance to plastic deformation. In addition, a large strain gradient takes place near the transition zone and on the side of an austenite grain with a high Schmid factor. Besides, the twin boundary is prone to deform at a lower stress level, which leads to a new strain partition. Therefore, stress–strain incompatibility caused by a difference in both mechanical properties and crystal structure is the root cause of K-TIG DMWJs failure.

Investigating the stress–strain of wheat in silos using triaxial tests based on local deformation measurements

To safely store wheat in silos considering filling and discharging process, the stress–strain behaviour of wheat plays a key role for silos design. A series of triaxial tests of wheat were performed using a digital imaging triaxial apparatus, which can capture local deformation across the entire surface of the specimen. By comparing the global and local stress–strain behaviour of wheat, the non-uniform deformation affected by the end restraints of triaxial apparatus were identified during the pre-failure and failure processes. The radial strain contour corresponding with non-uniformity coefficient values was presented. It was concluded that local stress–strain of the 40 mm-high specimens from the central regions of the specimen was less affected by the end restraints, indicating that they better characterise the stress–strain behaviour of wheat. A constitutive model of wheat was then deduced based on local measurements, from which strength and deformation characteristics were quantificationally analysed. It was concluded that the mechanical parameters from the local data of wheat specimen were greater than the global ones identified from the whole data.

A framework for predicting the local stress-strain behaviors of additively manufactured multiphase alloys in the sequential layers

The additive manufacturing (AM) process often results in non-uniform microstructure and different mechanical properties in sequential layers, impacting the overall performance of the AM-ed component. However, it is extremely challenging to evaluate the local stress-strain behavior of each individual layer, owing to the limited size of the AM-ed layered structure. To this end, a framework for characterizing and predicting the mechanical evolution of AM-ed multiphase alloys by combing nanoindentation and microstructure-based finite element method (FEM) was proposed. The sample used in this study was superduplex stainless steel (SDSS) manufactured by wire arc additive manufacturing (WAAM), and the microstructure varied from layer to layer. Firstly, the mechanical properties of the two constituent phases in each layer, including elastic modulus and hardness, were obtained by nanoindentation, and the indentation size effect (ISE) was also evaluated. The yield strength and hardening exponent of each phase were subsequently estimated by reverse analysis method, and therefore the constitutive behaviors of the individual phase, which served as input parameters for FEM, were acquired. By aid of real microstructure-based FEM under uniaxial tension, the overall stress-strain behaviors of each layer and the distributions of the stress and strain during the deformation process were investigated. This work provides a new avenue for the characterization of the multiphase alloys in AM industry, beneficial to the understanding of the mechanical evolution in AM-ed materials.

In-situ DIC investigation on local stress-strain behavior in creep-fatigue test of dissimilar steel welded joint

The digital image correlation technique was employed to investigate the variations of local stress–strain behavior in tensile and creep-fatigue tests for 9Cr/CrMoV welded joint. The strain concentration shifts from fine grain HAZ to over-tempered HAZ of CrMoV in tensile test. For strain-controlled creep-fatigue test, different ratcheting of sub-regions is observed, which is attributed to mismatched yielding and negative mean stress caused by stress relaxation. In addition, crack initiates and propagates at fine grain HAZ with characteristics of fatigue-dominated damage, where the actual strain amplitude and hysteresis energy density are about 4 and 10 times of base steel respectively.

Voronoi Diagram-Based Microstructure Modeling and Micromechanical Analysis of Quenched C35 Steel

In this work, austenite–martensite transformation in a low alloy steel is modeled using Voronoi algorithm. The evolved fraction of martensite is predicted from the geometry of the Voronoi diagram constructed, incorporating nucleation kinetics of martensite and Voronoi algorithm. The obtained results are validated against the Koistinen–Marburger equation. The geometrical properties of evolved martensite phase are identified using the characteristics of Poisson–Voronoi diagram. Finally, the microstructure developed is used to analyze local stress–strain behavior of different phases in the microscale. Results show that higher stress values are distributed in martensite phase and higher strain values are distributed in austenite phase.

The Impact of Temperature Oscillations and Heat Transfer Conditions in Thick-Walled Elbows and Tubes on the Local Stress-Strain Behaviour During the Fast Start-Up of Power Boilers

The paper presents results of tests designed for predicting the behaviour of components subjected to variable temperature and mechanical loading conditions. Elbows and tubes, as examples of components widely used in power plant pipelines, have been examined. This analysis includes a description of model characteristics, the operating parameters of devices under industrial conditions, and the results of computational modelling (FEM).

Stress-strain calculation and fatigue life assessment of V-shaped notches of turbine disk alloys

For local stress-strain behavior modeling at the notch root, both the Neuber's rule and equivalent strain energy density (ESED) method are commonly utilized in engineering practice, in which the former overestimates notch elasto-plastic responses while the latter gives conservative predictions. In this work, a correction coefficient related to the effective Poisson's ratio veff is proposed to quantify the weight of dissipated heat energy term with the modified ESED model. Notch-root strains predicted by the proposed model are compared with nonlinear FE calculations of GH4169 and TC4 V-shaped notched-bars, in which the proposed solution provides good agreement with FE results for both cases. Then, consulting the idea of local stress-strain approaches, a fatigue lifing model is derived assuming that the notched specimens fail once reach the critical total strain energy as same as that of smooth ones. Results indicated that prediction accuracy of local equivalent strains at notch roots is improved, and yields better life predictions than the Ellyin's model for the two alloys.

Fatigue behaviour of corrosion pits in X65 steel pipelines

Corrosion pits are a form of geometrical discontinuity that lead to stress and strain concentration in engineering components, resulting in crack initiation under service loading conditions and ultimately fracture and failure. Initiation and propagation of cracks in offshore pipelines can lead to loss of containment and environmental and commercial impacts. In order to prevent such failures, tools to predict the structural integrity of pipelines need to be improved. This work investigates the fatigue behaviour of corrosion pits in API-5L X65 grade steel pipeline utilising numerical and analytical methods. Firstly, load-controlled fatigue tests were carried out on smooth X65 steel samples to establish S–N data. Secondly, local stress–strain behaviour at corrosion pits and its effect on fatigue crack initiation were investigated using elastic-plastic finite element analysis of samples containing a single corrosion pit under cyclic loading. Analysis of stabilised stress–strain hysteresis loops at corrosion pits showed that the local stress ratio at the pit changes from 0.1 to −0.4 while the applied stress amplitude increases with the same stress ratio of 0.1. Analytical methods were also used to predict the local maximum stress and strain at the pit, which showed a similar local stress ratio to the finite element analysis result but lower stress and strain ranges. Finally, fatigue crack initiation life was predicted using the combination of finite element stress and strain analysis and the Smith–Watson–Topper strain–life approach. An advantage of this method for life estimation is that this approach considers the local stress and strains at corrosion pits rather than applied stress.

Micromechanical Modeling of the Ratcheting Behavior of 304 Stainless Steel

Numerical simulations of 304 austenitic stainless steel (SS304) cyclic and ratcheting responses are performed using polycrystalline plasticity models. On the basis of the polycrystalline model of Cailletaud and Pilvin (1994, “Utilisation de modèles polycristallins pour le calcul par éléments finis,” Rev. Eur. Élém. Finis, 3, pp. 515–541), a modification of the β rule that operates the transition between the macroscopic level and the grain level is proposed. The improvement of the transition rule is obtained by introducing a “memory variable” at the grain level, so that a better description of the local stress–strain behavior is provided. This new feature is calibrated by means of previous simulations using finite element (FE) aggregate models. The results of the updated polycrystalline plasticity model are in good agreement with the macroscopic responses.

Characterization of Reactor Pressure Vessel Steel by ABI Testing

Microstructure of the base metal, the multilayer welding seam and the two-layer cladding was characterized through the thickness of the WWER 440 reactor pressure vessel wall. Mechanical properties were determined by performing a series of instrumented indentations across the weld at room temperature. The results were treated by so-called automated ball indentation technique. Mechanical properties obtained by instrumented indentation from the local stress-strain behavior were compared with minimum values required by the standard.

Micromechanical modelling of phase transformation behaviour of a transitional soil

Experiments show that silts and silty soils exhibit contraction followed by dilation during shearing and the slope of failure line decreases at large strains, termed as phase transformation behaviour. This paper is to develop a new micromechanical stress-strain model that accounts for the phase transformation behaviour by explicitly employing the phase transformation line and its related friction angles. The overall strain includes plastic sliding and plastic compression among grains. The internal-friction angle at the phase transformation state and the void state variable are employed to describe the phase transformation behaviour. The model is examined by simulating undrained and drained triaxial compression tests performed on Pitea silts. The local stress-strain behaviour for contact planes is also investigated.

Characterization of Local Stress-Strain Behavior in WWER 440 Weld and Base Metal by Instrumented Indentation Technique

15Ch2MFA (base metal) as well as 10ChMFT (weld) steels used for WWER 440 nuclear reactor pressure vessel manufacturing present a gradient in mechanical properties through the wall thickness, which can hardly be assessed by conventional testing such as tensile or Charpy tests. Mechanical properties in the weld and base metal were therefore determined by performing a series of instrumented indentations across the weld at room temperature. The results were treated by so-called automated ball indentation technique. Local stress-strain behavior obtained by instrumented indentation was correlated to the tensile test data and microstructure characterized by metallographic analysis.

Damage Assessment of Threaded Connections based on an Advanced Material Model and Local Concepts

In the actual guidelines, for example the German VDI guideline 2230 [1] or the EUROCODE 3 [2], the fatigue assessment of bolted connections and bolts is based on the nominal stress approach. The FKM guideline for analytical strength assessment [3] does not include concepts for a threaded connection, it refers to the existing guidelines as [1]. At present there does not exist any guideline, which provides a damage assessment for general threaded connection by applying local concepts. In the present paper the basics of such a concept are outlined. The investigated metal is 34CrNiMo6, which is generally used for screw threads. The experimental determination of the local stress-strain behaviour in mechanically threaded connections is practically not feasible and needs a numerical approach. This numerical attempt consists of two main tasks, namely the description of the material properties and finite element (FE) analysis. The first one is concerned with the identification of material parameters, which are necessary for the FE analysis, especially for the advanced material model. The experimental work is given by determining the initial monotonic stress- strain curve, the initial unloading curve, the cyclic stress relaxation and cyclic creep. The parameter identification is accomplished using the software MATLAB, which includes the essential nonlinear optimization algorithms. The obtained parameters are entered in the FE software, which is used for the further FE analysis. The distinctive features of the FE analysis are the nonlinearity, especially nonlinear material behaviour, nonlinear contact algorithms and the consideration of large deformations, the high number of load steps and the various load ratio values (R = 0.06 and R = 0.71). The results of the FE analysis are used for the assessment of threaded connections. The damage assessment is realized with the local strain approach based on the damage parameters PSWT and PJ. Finally the result of the simplified method according to Schneider is applied on a high preloaded bolt connection (R = 0.71) and compared with the result achieved with the advanced material model.

Local Stress-Strain Behavior of a High-Temperature Steam Valve Under Transient Mechanical and Thermal Loading

The problem of determining stress-strain characteristics in a selected component operating in a power plant is discussed. Steam valve in transient working conditions during a power unit start-up is taken as an example. The aim of the paper is to prove the possibility to use measurements of steam temperature and the temperature of selected points of the investigated elements for defining boundary conditions of the heat transfer problem as well as to verify operative correctness of the elaborated computer models. The essential importance of time-variable and non-uniform temperature field for the course of local stress changes that results in metal fatigue processes is shown. The method of the computer model validation is discussed as the one of important problems which should be solved if we wanted to predict properly the variable temperature field. The problem of materials behavior under mechanical and thermal loading in the selected component has been discussed.

Inverse identification of creep of concrete from in situ load–displacement monitoring

Conventional algorithms for damage identification are effective in detecting local damage such as cracks. On the other hand, they are not directly applicable in identifying overall change in structural behavior. Considering that many structures show overall degradation without local cracks, alternative methods of identifying change in structural behavior are of great need. In this paper, we present a method that characterizes the overall time-dependent behavior of concrete using in situ load–displacement measurements. The method is based on a neural network constitutive model, finite element method, and numerical integration. Only by using the global load–displacement measurements, the method can compute approximate local stress–strain behavior at any location, enabling long-term monitoring of overall behavior of a structure with a small number of sensors. Application of the method to the identification of creep of a concrete beam shows a good match between the model prediction and the experimental measurement.

Determination of Weld Metal Mechanical Properties Utilising Novel Tensile Testing Methods

The aim of this study was to develop a method of extracting local mechanical properties from weld metal by strain mapping using the digital image correlation (DIC) technique. The feasibility of determining local stress-strain behaviour in the weld zone of a 316H stainless steel pipe with a girth weld was investigated by tensile tests of specimens machined from the pipe so that it contained the weld at its centre. The tensile test was recorded using a high resolution digital camera and the DIC technique was used to obtain the complete set of full field displacement maps during the tensile test. The local strain was calculated at every sub-region of 32×32 pixels, which enabled the local stress-strain behaviour for this region to be determined. Results from these tests show the variability of the elastic modulus, yield stress and UTS across the weld. To check the reliability of the technique, a set of micro tensile samples, with gauge length of 3.7mm and crosssectional area of 0.7×0.7 mm2, were machined from the various locations in and around the weld zone. The comparison of stress-strain curves determined from micro-samples to stress-strain curves from the corresponding locations within a larger more conventional tensile specimen shows reasonably good agreement.

Calculation of Local Stress-Strain Behavior at Notches for Non-Masing Material under Cyclic Loading

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, South Korea

Investigation on lap-joint behaviour of GFRP plates bonded to silica fume and rice husk ash concrete

Fibre-reinforced plastic (FRP) materials are increasingly being used for the rehabilitation of concrete structures. Various adhesives are being employed for attaching FRP materials to concrete and for creating force-transmitting joints between structural elements. Although various existing models focus on predicting the behaviour of FRP–concrete composites, a fundamental understanding of the lap-joint behaviour of FRP plates bonded to concrete still needs to be gained. In particular, there is a lack of a simple and reliable method that allows defining the stiffness of the adhesive in the FRP–concrete lap-joint area. In this study, GFRP plates were bonded to concrete blocks incorporating various proportions of silica fume and rice husk ash. A test set-up was developed to induce shear stresses in the GFRP–concrete lap-joint area. Specimens were instrumented with strain gauges at key locations to establish local stress–strain behaviour. Three-dimensional solid finite elements were used to model the GFRP plates and concrete block. The adhesive was modelled as a spring system with stiffness k v and k p simulating the shear and peel rigidity of the adhesive, respectively. Although it still needs future developments, the finite element model thus developed coupled with the simple test procedure provide a simple means for calculating the stiffness of the adhesive between the GFRP and concrete. Knowledge of this stiffness is expected to allow a more realistic modelling of the lap-joint behaviour of GFRP–concrete composites.

Inelastic Analysis and Thermal Fatigue Life Prediction of a Perforated Cylinder Based on the Effective Stress Concept

In order to examine the applicability of the effective-stress-based analysis method of perforated plates that is proposed by the authors, inelastic behavior and creep-fatigue life of a perforated cylinder subjected to cyclic thermal stress were predicted. Firstly, basic creep behavior of a perforated cylinder under axial tension at uniform temperature was examined by means of three-dimensional FEM creep analysis, and the results showed that macroscopic and local creep behavior of a perforated cylinder is predictable using the proposed method on perforated plates. Secondly, thermal fatigue testing was carried out on a perforated cylinder, and the elastic-plastic-creep behavior of the cylinder was numerically analyzed by modeling the perforated cylinder to the equivalent solid cylinder based on the effective stress concept. Predicted local stress-strain behavior around circular holes was used for creep-fatigue life prediction based on the linear damage rule. Comparison of numerical results and experimentally observed damage clarified that the difference in damage mode at the inner and outer surfaces of a perforated cylinder could be successfully reproduced.