Transient Thermo-mechanical Loading Research Articles

Effect of Cr and Ni concentrations on resilience of cast Nb-alloyed heat resistant austenitic steels at extreme high temperatures

Austenitic Cr–Ni alloyed heat resistant steels with Nb additions are used for intensively thermo-mechanically loaded cast components working in extreme high temperature oxidizing environment. Their performance during static oxidation and transient thermo-mechanical loading was investigated to recommend an optimal cost-effective Cr/Ni composition of Nb-alloyed austenitic class steels. The static oxidation and transient thermo-mechanical behavior of three austenitic steels with different Cr/Ni alloying levels were investigated and compared for variety of working conditions. Static oxidation was performed between 900 °C and 1000 °C in air for 400 h. The critical temperature which increases spallation during static oxidation was determined for each of the steel alloying levels. In addition, thermal cycling of a constrained specimen was done with varying upper cycling temperatures between 850 °C and 1000 °C. SEM and TEM analyses were supported by thermodynamic simulation of the phases precipitated in the metal matrix and the structure of formed oxide layers. These studies were used to determine the mechanisms of degradation of thermo-mechanically loaded Cr/Ni austenitic steels at extreme high temperatures. A recommendation for a cost-effective Cr/Ni alloying level for different working conditions was determined.

Numerical investigation of transient thermo-mechanical loading effect on combined damaged (crack and delamination) curved shell structure: An experimental verification

The transient responses of the curved shell panels are examined under thermo-mechanical loading and verified experimentally. The damaged shell model is formulated mathematically considering polynomials of higher-order based on isoparametric finite element (FE) steps. The dynamic (time-dependent deflection) responses are evaluated numerically using developed computer code in MATLAB, utilizing Newmark's constant average acceleration method. The accuracy of the present model is proven by equating the consequences with the available data in the published literature. Additionally, a few experimentations have been accomplished and outcomes are equated with the numerical solution to show the effectiveness of the present developed numerical model. Further, the impact of damages (debond and/or crack), numerous structural input parameters (curvature ratio, shell geometry) and temperature on the curved shell panel is demonstrated and presented through a wide variety of numerical examples.

Effect of Micro-Structural Dispersity of SiMo Ductile Iron on High Temperature Performance during Static Oxidation

High silicon and molybdenum (SiMo) ductile iron is commonly used for car exhaust systems, and its micro-structural dispersity depends on intrinsic parameters, which include alloy composition and inoculation efficiency, as well as extrinsic factors, such as casting wall thickness and molding material, which define cooling rate during solidification. Micro-structural dispersity is referred to as the degree of heterogeneity of sizes of structural constituencies within the microstructure. A variation in the micro-structural dispersity could impact the high temperature performance of SiMo ductile iron during static oxidation and transient thermo-mechanical loading conditions. In this study, static high temperature tests were performed on SiMo ductile iron solidified in a casting with varying wall thicknesses from 5 mm to 100 mm. The faster solidified specimens (taken from near chilled casting surfaces) had extremely high micro-structural dispersity as compared to the thicker section samples. After thermal exposure, each of the samples were characterized using 2D sections and 3D µCT images, and the results indicated an order of magnitude difference in graphite phase dispersity. The surface degradation was quantified after static oxidation experiments were implemented at temperature intervals between 650 °C and 800 °C. Non-destructive µCT 3D analysis and SEM/EDS were performed on cross sections and used to quantify the scale topology and structure. Carbon analysis was used to decouple the scale formation and decarburization phenomena that occurred within the samples. These methods enabled the quantification of the oxidation of the SiMo cast iron with different micro-structural dispersity levels after being exposed to high temperature static oxidation. Additionally, the complex material behavior during oxidation-assisted transient thermo-mechanical loading will be presented in a separate article.

Control of High-Temperature Static and Transient Thermomechanical Behavior of SiMo Ductile Iron by Al Alloying

Silicon and molybdenum (SiMo) ductile iron is commonly used for exhaust manifolds because these components experience thermal cycling in oxidizing environment, which requires resistance to fatigue during transient thermomechanical loads. Previous studies have demonstrated that alloying elements, such as Al, to SiMo ductile iron reduces the amount of surface degradation during static high-temperature exposure. However, deterioration of sphericity of the graphite nodules and a decrease in ductility could affect the tendency of cracking during thermal cycling. In this article, the effect of Al alloying on static and transient thermomechanical behavior of SiMo ductile iron was investigated to optimize the amount of Al alloying. A thermodynamic approach was used to confirm the effect of the Al alloying on the phase transformations in two SiMo cast irons, alloyed by 1.8% Al and 3% Al. These two alloys were cast in a laboratory along with the baseline SiMo ductile iron. Several experimental methods were used to evaluate the dimensional stability, physical properties, static oxidation, and failure resistance during constrained thermal cycling testing to compare their high-temperature capability. Experimental results verified that Al alloying increases the temperature range and decreases volume change during eutectoid transformation, which together with enhancement of oxidation protection improved the dimensional stability. Thermocycling tests showed that the number of cycles to failure depends on the amount of Al alloying and the applied high-temperature exposure during each cycle. SEM/EDX, high-resolution TEM and µCT analysis were used to verify the mechanism resulting from the Al alloying protection. It was shown that an optimal level of Al alloying for balancing oxidation and thermal cracking resistance depends on thermomechanical conditions of application.

Structural analysis of gas turbine blades made of Mo-Si-B under transient thermo-mechanical loads

Experimental investigations of turbine blade materials under real operating conditions are very demanding. Therefore, the thermal and mechanical analysis should be carried out based on the finite element method. The purpose of the present analysis is to estimate the stress-strain condition of a turbine blade model by performing two sequential calculations: thermal analysis and structural mechanics analysis. In this study, two molybdenum-based alloys Mo–17.5Si–8B and Mo–9Si–8B are compared to the state-of-the-art nickel-based superalloy CMSX-4 as a blade material during turbine start-up and shut-down processes. The time-dependent thermo-mechanical stresses are taken into account, so that the influence of temperature gradient on stress-strain behavior of the blade could be represented.

Viscoplastic analysis method of an aeroengine turbine blade subjected to transient thermo-mechanical loading

Viscoplastic analysis method of a turbine blade manufactured with directionally solidified (DS) superalloy under transient thermo-mechanical loading was investigated in depth. The second deviatoric stress tensor invariant was modified by a fourth-order tensor to simulate the anisotropic behavior of the DS superalloy. In order to capture the monotonic and cyclic behaviors of the alloy under non-isothermal conditions, temperature interpolation of a set of material parameters and the modification of internal state variables were performed. The model was implemented as a user subroutine (UMAT) in Abaqus for complex structure analysis. With the improved Chaboche constitutive model, the tensile and cyclic responses of the DS material GTD-111 under isothermal and non-isothermal conditions were carefully modeled and predicted with high accuracy. Finally, the model was implemented to simulate the transient thermo-mechanical behavior of a hollow turbine blade with a typical flight profile. The location-dependent thermo-mechanical fatigue response of the blade were determined and analyzed.

Shape and operation optimization of a thick-walled power boiler component

The aim of this article is the proposition of a two-stage optimization of power boiler components subjected to transient thermo-mechanical loads. As the basis of transient load regime, the TRD 301 regulations are used. In the first stage of an investigation, the genetic algorithms are used to determine the optimal shape of the structure. As optimization constraints, the maximum total stresses in the whole cycle are assumed. In the next stage, the method for the start-up and shut-down operation optimization is shown. The problem is defined to minimize the time of start-up and shut-down processes with allowable stress constraints according to TRD regulations. These new methods are of high practical significance and can be implemented throughout the industry, wherever heating and cooling processes take place. The obtained optimization results translate directly into the power unit energy effectiveness because the shut-down and start-up losses are reduced.

Functionally Graded Hollow Sphere With Piezoelectric Internal and External Layers Under Asymmetric Transient Thermomechanical Loads

Abstract In this paper, an analytical method is developed to obtain the solution for the two-dimensional (2D) (r,θ) transient thermal and mechanical stresses in a hollow sphere made of functionally graded (FG) material and piezoelectric layers. The FGM properties vary continuously across the thickness, according to the power functions of radial direction. The temperature distribution as a function of radial and circumferential directions and time is obtained solving the energy equation, using the method of separation of variables and Legendre series. The Navier equations are solved analytically using the Legendre polynomials and the system of Euler differential equations.

Computationally efficient finite difference method for metal additive manufacturing: A reduced-order DFAM tool applied to SLM

Metal Additive Manufacturing (MAM) allows the production of complex lattice structures that exceed the manufacturing capability of traditional processes. However, the MAM process is highly complex, including: transient thermo-mechanical loading, spatially and temporally transient boundary conditions and multi-scale affects. Furthermore, MAM is subject to significant experimental and statistical uncertainties. Consequently, the MAM process is poorly understood and build-process simulations are computationally demanding; this limits the availability of Design for Additive manufacturing (DFAM) tools, and necessitates experimental validation for commercial MAM applications. This research develops a novel finite difference method (FDM) simulation of the MAM temperature field that is based on a computationally efficient 1D transient tri-diagonal system of equations, and reduces data generation effort by the reuse of existing MAM production data. The 1D simulation is particularly suited to lattice geometries due to the slenderness of lattice strut elements. The reduced-order simulation method developed in this research provides a timely and useful DFAM tool that enables qualitative design insight early in the design phase and pre-production validation. The proposed method is validated by comparison to existing analytical results, numerical results and by application to a titanium and aluminium lattice structures manufactured by a commercial Selective Laser Melting (SLM) process.

A generalized J-integral for thermal shock analyses of 3D surface cracks in spatially and temperature dependent materials

Thanks to their favorable mechanical properties and modern manufacturing technologies, functionally graded materials (FGM) and structures have gained increasing utilization in engineering. One major applications of functionally graded ceramics, refractories or heat protecting layers is to improve the resistance against thermal shock impact. Here, fracture mechanical methods are needed to evaluate the strength and durability of such structures exposed to transient thermomechanical loading. Despite a lot of theoretical work has been done for two-dimensional crack configurations in thermoelastic FGM, real defects and structural components are of three-dimensional (3D) nature. In most cases the real gradation of the thermoelastic material properties does not obey simple mathematical functions, but shows a complex physical dependency on location due to manufacturing. Moreover, the elastic and thermodynamic properties depend on temperature itself.In this paper, we present the derivation of the 3D J-integral for arbitrary location and temperature dependent anisotropic material behavior. The functionally varying material properties are implemented in the finite element method (FEM) by means of »graded elements«. The J-integral is calculated in post-processing using the equivalent domain integral technique. The method is applied to surface cracks in a functionally graded plate under mechanical and thermal shock loading. Hereby, the real measured variation of all thermal and mechanical properties with porosity and temperature is exploited for a CaAl ceramic. The influence of the material gradation on the fracture parameters J and KI is investigated in various examples in order to find optimal gradation functions. It could be shown that the temperature dependency of thermoelastic material properties has an important effect on the results and must not be neglected.

Cleavage fracture assessment of transient thermo-mechanical loading situations by local approach

Brittle failure of any nuclear component due to instable propagation of a postulated crack needs to be excluded, and assessed reliably. Currently, cleavage fracture for complex transient loading situations with decreasing temperature and increasing load is analyzed based on isothermal fracture toughness curves with large conservatisms. In this study these transient loading situations were studied experimentally by testing different specimen types (stress conditions) with varying slopes of load versus temperature, and different pre-loads. A probabilistic local approach concept which considers local stress and strain developments, as well as load history, was used to numerically assess cleavage fracture. In all cases the model was able to quantify the fracture probability excellently in a slightly conservative manner. The strong over-conservatism of the Master Curve concept for transient loading was reduced to a tolerable minimum.

Weight function method for transient thermomechanical fracture analysis of a functionally graded hollow cylinder possessing a circumferential crack

ABSTRACTThis article introduces a weight function method for fracture analysis of a circumferentially cracked functionally graded hollow cylinder subjected to transient thermomechanical loading. Analytical solutions for transient temperature and stress distributions in the uncracked cylinder are derived by applying finite Hankel transformation. These solutions are utilized to determine stress acting on the faces of the circumferential crack in the local perturbation problem. Thermomechanical material properties are assumed to be power functions of the radial coordinate in the derivations. Coefficients of the weight function are found using reference stress intensity factors computed through the finite element method. Domain form of the J-integral is used in the finite element calculations. Comparisons of the numerical results calculated by the proposed weight function method to those generated by finite element analysis demonstrate the high level of accuracy attained by the application of the developed procedures. Further parametric analyses are presented to illustrate the influences of dimensionless time, crack depth to thickness ratio, power law index, and convection coefficient upon transient mode I thermomechanical stress intensity factors.

Transient thermo-mechanical analysis of dynamic curving cracks in functionally graded materials

Mixed-mode dynamic crack growth behavior along an arbitrarily smoothly varying path in functionally graded materials (FGMs) under transient thermo-mechanical loading is studied. An asymptotic analysis in conjunction with displacement potentials is used to develop transient thermo-mechanical stress fields around the propagating crack-tip. Asymptotic temperature field equations are derived for exponentially varying thermal properties, and later, these equations are used to derive transient thermo-mechanical stress fields for a curving crack in FGMs. The effect of the transient parameters (loading rate, crack-tip acceleration, and temperature change) and temperature gradient on the maximum principal stress and circumferential stress associated with the propagating crack-tip is discussed. Finally, using the minimum strain energy density criterion, the effect of temperature gradient, crack-tip speeds, and T-stress on crack growth directions is determined and discussed.

FEM-based prediction of workpiece transient temperature distribution and deformations during milling

In high-speed dry milling of thin-walled parts, the cutter-workpiece temperature rises asymptotically with cutting speed, causing excessive cutter tooth wear and workpiece thermal expansion, which in turn reduces the cutter life and produces dimensional and geometrical variabilities in the machined part. Therefore, a basic understanding of the thermal aspect of machining and the effecting parameters is essential for achieving better part quality with improved productivity. This paper presents a transient milling simulation model to assist manufacturing engineers in gaining in-depth understanding of the thermomechanical aspects of machining and their influence on resulted part quality. Based on the finite-element method approach, the model can predict transient temperature distributions and resulted elastic-plastic deformations induced during the milling of 2.5D prismatic parts comprising features like slots, steps, pockets, etc. The advantages of the proposed model over previous works are that it (1) performs feature-based machining simulation considering transient thermomechanical loading conditions; (2) allows modeling the effects of coolant on convective heat transfer rate; and (3) considers the nonlinear behavior of the workpiece due to its changing geometry, inelastic material properties, and flexible fixture-workpiece contacts. The prediction accuracy of the model was validated with experimental results obtained during the course of the research work. A good agreement between the numerical and experimental results was found for different test cases with varying part geometries and machining conditions.

Cleavage fracture modeling of pressure vessels under transient thermo-mechanical loading

The next generation of fracture assessment procedures for nuclear reactor pressure vessels (RPVs) will combine non-linear analyses of crack front response with stochastic treatments of crack size, shape, orientation, location, material properties and thermal-pressure transients. The projected computational demands needed to support stochastic approaches with detailed 3-D, non-linear stress analyses of vessels containing defects appear well beyond current and near-term capabilities. In the interim, 2-D models become appealing to approximate certain classes of critical flaws in RPVs, and have computational demands within reach for stochastic frameworks. The present work focuses on the capability of 2-D models to provide values for the Weibull stress fracture parameter with accuracy comparable to those from very detailed 3-D models. Weibull stress approaches provide one route to connect non-linear vessel response with fracture toughness values measured using small laboratory specimens. The embedded axial flaw located in the RPV wall near the cladding-vessel interface emerges from current linear-elastic, stochastic investigations as a critical contributor to the conditional probability of initiation. Three different types of 2-D models reflecting this configuration are subjected to a thermal-pressure transient characteristic of a critical pressurized thermal shock event. The plane-strain, 2-D models include: the modified boundary layer (MBL) model, the middle tension (M(T)) model, and the 2-D RPV model. The 2-D MBL model provides a high quality estimate for the Weibull stress but only in crack front regions with a positive T-stress. For crack front locations with low constraint ( T-stress < 0), the M(T) specimen provides very accurate Weibull stress values but only for pressure load acting alone on the RPV. For RPVs under a combined thermal-pressure transient, Weibull stresses computed from the 2-D RPV model demonstrate close agreement with those computed from the corresponding crack front locations in the 3-D RPV model having large negative T-stresses. Applications of this family of 2-D models provide Weibull stress values in excellent agreement with very detailed 3-D models while retaining practical levels of computational effort.

Prediction of grinding residual stresses

Residual stresses due to machining are the results of the thermo-mechanical history of the piece/tool interface. The magnitude and the gradient of stress play a key role for the surface integrity. A thermo-mechanical model has been developed. It allows simulating the rolling/sliding contact between an elastic tool in rotation along its own axis and an elastic-plastic flat in translation. The analysis includes the effects of both the normal and tangential loading. Frictional heating is also considered. The model is based on a semi-analytical method and the transient 3D contact problem is fully solved. Compared to the finite element method the computing time is reduced by several orders of magnitude. This technique has already been successfully applied to the simulation of running-in and wear, and to fretting wear, and a first attempt to simulate residual stress and strain due to the contact between a grinding tool and a work piece is made here. First results are presented for various stationary and transient thermo-mechanical loading histories.

Fatigueâ oxidation interaction models for life prediction of hot-forming tools steels under transient thermomechanical loadings

This contribution deals with the life prediction of the virgin and nitrided hot-working tool steels X38CrMoV5 (AISI Hll, 47 HRC) under Transient Thermomechanical (TMF) loading. Isothermal Low Cycle Fatigue (LCF) and thermomechanical (TMF) behaviour are investigated at different temperatures and various frequencies. It is shown that nitruration can enhance the fatigue life only when the strain amplitude is lower than a critical value. Two Manson-Coffin laws govern the fatigue life of the virgin and nitrided steels for all temperatures and strain rates investigated. However, these models have shortcomings for predicting TMF life. Predictive fatigue life models take into account the interaction with oxidation. Regarding the second tempering temperature (550°C) and the steel surface (virgin or nitrided), a Paris-type (T > 550°C) or a Tomkins-type (T < 550°C) law is used. A modified Tomkins model considering the mean residual stress measured by X-Ray at high temperature in the nitrided layer is used. The model is applied for isothermal and thermal mechanical fatigue experiments performed and reported in literature.

Predicting the Reliability of Ceramics Under Transient Loads and Temperatures with CARES/Life

Abstract A methodology is shown for predicting the time-dependent reliability of ceramic components against catastrophic rupture when subjected to transient thermomechanical loads (including cyclic loads). The methodology takes into account the changes in material response that can occur with temperature or time (i.e., changing fatigue and Weibull parameters with temperature or time). This capability has been added to the NASA CARES/Life (Ceramic Analysis and Reliability Evaluation of Structures/Life) code. The code has been modified to have the ability to interface with commercially available finite element analysis (FEA) codes executed for transient load histories. Examples are provided to demonstrate the features of the methodology as implemented in the CARES/Life program.

Surface hardening of AISI 4340 steel by machining: a preliminary investigation

An attempt has been made to investigate feasibility of applying machining, a material removal process, to surface harden steel parts. The idea is to utilize transient thermomechanical loading induced during machining to purposely generate a hardened layer while simultaneously removing excess materials. This approach, if capable, will be a cost-effective alternative to conventional surface hardening, giving possible benefits such as low production cost, short cycle time, and waste reduction, etc. Feasibility has been examined from thermal and mechanical perspectives. A preliminary experiment using cutting tools with large wear land (1.2 mm) to turn annealed AISI 4340 steel shows distinctly 30 μm deep phase-transformed structures. The machined surface has about 7 vol.% austenite, an evidence of phase transformation. The hardness of the phase-transformed structure increases to a maximum of about 49 HRC versus 28 HRC of the bulk. Residual stresses, measured by X-ray diffraction, were compressive down over 25 μm below the surface. These results suggest the possibility of using machining to surface harden steel parts. However, the challenge is to extensively increase wear land size to achieve hardened depth comparable to that in conventional surface hardening. In addition, surface properties and performance remain keys to justify the usage of machining hardening processes.

Transient reliability of ceramic structures

Present capabilities of the NASA Ceramic Analysis and Reliability Evaluation of Structures/Life (CARES/Life) code include probabilistic life prediction of ceramic components subjected to fast fracture, slow crack growth (SCG) (stress corrosion), and cyclic fatigue failure modes. Currently, this code has the capability to compute the time‐dependent reliability of ceramic structures subjected to simple time‐dependent loading. For example, in SCG type failure conditions CARES/Life can handle the cases of sustained and linearly increasing time‐dependent loads, whereas for cyclic fatigue applications, it can account for various types of repetitive constant amplitude loads. In real applications applied loads are rarely that simple, but rather vary with time in more complex ways such as engine start up and shut down and dynamic and vibrational loads. In addition, when a given component is subjected to transient environmental and/or thermal conditions, the material properties also vary with time. The objective of this paper is to demonstrate a methodology capable of predicting the time‐dependent reliability of components subjected to transient thermomechanical loads that take into account the change in material response with time. In this article, the dominant delayed failure mechanism is assumed to be SCG. This capability has been added to the NASA CARES/Life code, which has also been modified to have the ability of interfacing with commercially available finite element analysis codes executed for transient load histories. An example involving a ceramic exhaust valve subjected to combustion cycle loads is presented to demonstrate the viability of this methodology and the CARES/Life program.