Thermal-mechanical Loading Conditions Research Articles

A unified model for isothermal and non-isothermal phase transformation in hot stamping of 22MnB5 steel

The hot stamping of tailored properties is widely established in the automotive industry. However, unlike conventional press hardening, this technique is designed to generate varying mechanical properties by controlling the cooling rate locally. Current material models cannot accurately represent the locally very different thermomechanical paths. This research aims to address this problem by developing a new phase transformation model capable of handling complicated thermal histories and the effects of deformation. A ’unified JMAK‘ model is proposed to capture various thermal-mechanical loading conditions. These conditions include the deformation effect and cooling paths where isothermal conditions range from 450° to 725°C and non-isothermal conditions range from 5° to 40°C/s. Furthermore, for the extension to linear and non-linear cooling paths, correction factors for the phase transformations are introduced into the model. Finally, the model's performance is compared to well-known transformation models, such as the non-isothermal JMAK and the K-V (Kirkaldy and Venugopalan), by comparing the results obtained from isothermal and non-isothermal dilatometer tests and a scaled-down b-pillar experiment. The results from the comparison are classified into 3 cases, isothermal, continuous cooling and non-linear cooling (b-pillar). Case 1 shows that the non-isothermal and K-V models provide low accuracy, while the proposed model yields an error below 2 %. For Case 2, the proposed models and non-iso JMAK are agreeable with the experiment, and the K-V model shows a maximum error of 15 %. Finally, in the b-pillar case study mean absolute errors of hardness predictions are 33.06 HV (mean relative error is 9 %), 43.92 HV (13 %), and 67.70 HV (20 %) for the unified JMAK, non-iso JMAK, and K-V model, respectively.

Failure analysis of orthotropic composite material under thermo-elastic loading by XFEA

Present work describes the crack growth characteristics in orthotropic composite materials by utilizing extended finite element analysis under combined thermal–mechanical loading conditions. In combined system, temperature and displacement approximations are extrinsically governed by additional functions to define discontinuity profile across crack surface. In present work, two step simulation methodologies are adopted where thermal discrete equations are solved for thermo-elastic problems to obtain temperature distribution in domain. Thereafter, these results acts as load input criterion for elastic discrete equations. Post process of simulation, SIFs at crack tips are calculated using interaction integral approach for few orthotropic composites with preexisted crack.

Augmented finite-element method for arbitrary cracking and crack interaction in solids under thermo-mechanical loadings.

In this paper, a thermal-mechanical augmented finite-element method (TM-AFEM) has been proposed, implemented and validated for steady-state and transient, coupled thermal-mechanical analyses of complex materials with explicit consideration of arbitrary evolving cracks. The method permits the derivation of explicit, fully condensed thermal-mechanical equilibrium equations which are of mathematical exactness in the piece-wise linear sense. The method has been implemented with a 4-node quadrilateral two-dimensional (2D) element and a 4-node tetrahedron three-dimensional (3D) element. It has been demonstrated, through several numerical examples that the new TM-AFEM can provide significantly improved numerical accuracy and efficiency when dealing with crack propagation problems in 2D and 3D solids under coupled thermal-mechanical loading conditions. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Adhering failure and optimization of cemented carbide inserts for the heavy-duty cutting of high-strength steel forgings

In this study, the adhering failure of cemented carbide inserts during the heavy-duty cutting of large-scale, high-strength steel forgings is investigated. First, the heavy-duty cutting of high-strength steel forgings is simulated. According to the results, the maximum cutting temperature and force were approximately 950 ℃ and 42 KN, respectively. Next, the effects of these thermal-mechanical loading conditions on the material performance of the inserts are discussed. In addition, the adhering failure of the inserts is analyzed. Then, an insert-chip adhering model and the high-temperature strength of the insert material are used to illustrate the critical condition of the insert-chip adhering process via MATLAB simulations. Furthermore, the anti-adhering performance of the inserts is improved and an optimized insert design for the heavy-duty cutting process is constructed from the aspects of insert material, structure and coating. According to the results, the service lift of the heavy-duty cutting inserts XF8 was two times greater than that of conventional welded cemented carbide inserts. The cutting parameters of the large-scale forging process are also optimized using the orthogonal experimental method. The results of this study could be used to improve the anti-adhering performance, service life, and production efficiency of cemented carbide inserts intended for the cutting of large-scale forgings.

Experimental study on mechanical behaviors of Al-alloys under transient aerodynamic heating

Determination of mechanical parameters such as the ultimate strength of aerospace materials under the condition of transient heating is critical for reliability evaluation, fatigue life prediction and safety design of high-speed aircrafts including missiles. Al-alloys are widely used as aeronautical materials due to their advantages such as light weight, easy manufacturability and relatively low cost. However, their mechanical properties under transient thermal conditions are not available in the handbooks on material strength, mainly due to the difficulty of simulating the transient thermal conditions for thermal strength test. To solve this problem, a joint thermal–mechanical loading system has been developed in the present investigation to simulate the aerodynamic thermal conditions. The mechanical behaviors of some Al-alloys have been examined with this system. The ultimate strengths and durations of two different types of Al-alloys, i.e. Al7178 and Al2024, under joint thermal–mechanical loading conditions have been tested. Furthermore, their different failure modes and mechanisms under various temperatures have been investigated and compared. These data provide a reliable basis for the study of load bearing capacity and weight reduction of aerospace materials and structures under transient aerodynamic thermal conditions.

Behavior and failure of uniformly hydrided Zircaloy-4 fuel claddings between 25 °C and 480 °C under various stress states, including RIA loading conditions

The anisotropic plastic behavior and the fracture of as-received and hydrided Cold-Worked Stress Relieved Zircaloy-4 cladding tubes are investigated under thermal–mechanical loading conditions representative of Pellet–Clad Mechanical Interaction during Reactivity Initiated Accidents in Pressurized Water Reactors. In order to study the combined effects of temperature, hydrogen content, loading direction and stress state, Axial Tensile, Hoop Tensile, Expansion Due to Compression and hoop Plane Strain Tensile tests are performed at room temperature, 350 °C and 480 °C on the material containing various hydrogen contents up to 1200 wt. ppm (hydrides are circumferential and homogeneously distributed). These tests are combined with digital image correlation and metallographic and fractographic observations at different scales. The flow stress of the material decreases with increasing temperature. The material is either strengthened or softened by hydrogen depending on temperature and hydrogen content. Plastic anisotropy depends on temperature but not on hydrogen content. The ductility of the material decreases with increasing hydrogen content at room temperature due to damage nucleation by hydride cracking. The plastic strain that leads to hydride fracture at room temperature decreases with increasing hydrogen content. The influence of stress triaxiality on hydride cracking is negligible in the studied range. The influence of hydrogen on material ductility is negligible at 350 °C and 480 °C since hydrides do not crack at these temperatures. The ductility of the material increases with increasing temperature. The evolution of material ductility is associated with a change in both the macroscopic fracture mode of the specimens and the microscopic failure mechanisms.

A numerical analysis of cyclic creep fracture

A new continuum damage mechanics (CDM) based model for describing cyclic creep behavior of selected engineering materials under complex thermal-mechanical loading conditions is incorporated into a finite element computer code for cyclic creep fracture analysis. A mixed explicit-implicit (EI) algorithm was used to deal with the evolution of creep damage in the material. Three case studies were conducted on a cracked thin panel made of 316L stainless steel. Loading conditions for these case studies correspond to static, cyclic without dwell time and cyclic with dwell time, respectively. Mechanical load and temperature changes applied in phase with each other are considered in this investigation. Numerical results indicate that both the load dwell time and the loading/unloading cycling process have profound influence on subsequent creep behavior.

A THERMAL-MECHANICAL FATIGUE CRACK-GROWTH TESTING SYSTEM

An automated system for crack growth studies under thermal-mechanical loading conditions utilizing a compact tension (CT) specimen has been developed. Since the materials to be tested are for high temperature machinery, they are of high cost and low availability, making the CT testing capability advantageous. The system described uses quartz lamps and a multiple-zone control system to accurately control the specimen temperature according to a predetermined profile. A cyclic mechanical loading can be applied along with cyclic temperature profile in a prescribed manner. Test results are discussed for fatigue crack growth behavior in a titanium aluminide intermetallic. 7 references.