When conventionally reinforced concrete elements are subjected to fire, their carbon-steel reinforcing bars can experience a relatively long duration of modest temperature increase (up to 600 degrees C) due to the insulation pro-vided by the concrete cover before the element fails or the fire decays. Tensile tests under constant applied stress were performed on 25 M (#8 US) reinforcing bars that conform to ASTM A615 Grades 420, 520, and 690 to measure time-dependent creep at constant temperatures from 400 to 600 degrees C (i.e., up to the critical temperature per ASTM E119). Creep rates at 400 degrees C were small for all grades regardless of the applied stress level. Specimens across all grades showed an increase in secondary creep rate when applied stress and/or temperature also increased. Tertiary creep was only observed once the temperature was increased to 600 degrees C with a higher level of applied stress (similar to 40% of ambient yield). The experimental data is used to develop predictive expressions based on both the Dorn-Harmathy model and a combined time-hardening model. Both can be used as input for numerical analysis of heated reinforced concrete structures. The models are applied in two simple examples to predict cumulative creep strains when the bars are under constant stress and transiently heated from 400 to 600 degrees C.

Life integrity assessment of industrial components often requires investigations of the cyclic inelastic response at a range of operating temperatures. Some high strength steels exhibit a well-known ambient temperature creep behaviour, which can also impact the cyclic behaviour, especially under long-term operation. In this study, a direct method known as the Linear Matching Method has been used to predict the cyclic shakedown and ratchet limits of high-strength steel (AISI 1144). The numerical predictions are compared with a recent testing campaign that was completed at room temperature to characterise the multiaxial behaviour of AISI 1144. Due to creep of the material, inelastic strain accumulation is also observed for loading conditions within the shakedown limit. The extended Direct Steady Cyclic Analysis (eDSCA) approach has been used to predict the cyclic behaviour in the presence of creep. In addition, for specific load cases of interest, a newly revised creep-ratcheting limit has been derived and compared with the experimental tests.

This study aims to determine the safe elastoplastic shakedown limit of type 316L stainless steel at room temperature and at an elevated temperature of 600°C. Components made of type 316L stainless steel are commonly subjected to combined thermomechanical loads in various engineering applications. Shakedown based designs can be used to expand the feasible design space under combined thermomechanical cyclic loading conditions compared to conventional yield-limited designs. Extensive experimental and numerical effort has been made to model ratchetting and cyclic hardening/softening of stainless steels under uniaxial cyclic loading but a very limited number of studies have focused on determining the shakedown limit. In this work, shakedown limits are determined experimentally and numerically. A numerical model that combines time-dependent creep deformation and time-independent hardening is used to develop Bree load-interaction diagrams. It is found that shakedown occurs at room temperature at stress levels up to two times the linear elastic limit of the material and is limited by the time-dependent creep deformation. At 600oC, an enhanced shakedown behavior is observed that expands the feasible design space up to 4 times the linear elastic limit.

Many industries rely on the Inconel 625 alloy to serve under thermomechanical operating conditions. Understanding the macroscopic cyclic inelastic behavior of this material is vital for accurate assessment of its load-carrying capacity at elevated temperatures. In this work, a uniaxial experimental program at 600∘C is conducted to demonstrate designing for cyclic elastoplastic behavior (shakedown) as opposed to more restrictive first-yield, while still avoiding ratchetting or alternating plasticity. In particular, a range of cyclic stress amplitudes are imposed at non-zero mean stresses while maintaining a constant maximum stress. In addition, the effect of dynamic strain aging (DSA) on the macroscopic shakedown behavior is established under load control. The inelastic work done per cycle is used as a measure of severity of the cyclic inelastic behavior, and is evaluated by monitoring the evolution of the hysteresis loop width. It is found that when the maximum stress is constant, larger mean stress tests approach shakedown behavior. Furthermore, for the range of stress amplitudes and mean stresses considered, the cyclic elastoplastic shakedown behavior is not affected by the DSA, and only depends on the mean stress and stress amplitude.

Nickel-base single crystal superalloy components are designed to experience extreme and often cyclic multiaxial loads. While much of the literature has provided the foundation to use these materials near critical yield conditions, their use under inelastic design, exploiting safe elastoplastic shakedown behaviors is largely unexplored. In particular, this paper focuses on establishing the conditions for the macroscopic shakedown response of a 2nd generation nickel-base superalloy under tension-torsion loadings from numerical and experimental perspectives. A numerical model is developed to more broadly evaluate shakedown limit loads and it is found that by allowing shakedown to occur, an 18% increase in feasible design load space is expected compared to traditional first-yield design criteria. Experiments on hollow cylindrical specimens are also conducted and used to assess the prediction capabilities of the numerical finite element model.

Thermal fatigue tests on AISI 316L(N) austenitic stainless steel samples are performed through pulsed laser on specimen that can be subjected to an additional static mechanical load. These tests are carried out in Helium environment with a dedicated and heavily instrumented set-up. The fatigued surface is monitored by a hybrid multiview system composed of two visible light and one infrared cameras that, through 3D-registration, provides in-situ access to the 3D surface displacement fields and 2D temperature fields. At a fatigue frequency of 1 Hz, the surface temperature range covered per cycle can be varied from 150 degrees C to 250 degrees C, conditions that allow surface damage to be reproduced. The multiview system reveals the time-resolved mechanisms of surface damage, from significant cyclic plasticity with persistent slip bands to microcrack initiation and growth, leading to their quantitative characterization (microcrack density, length of major crack, orientation) all along the test. These observations are confirmed at a few check points where the test is interrupted for optical microscopy inspection of the surface. Finally, the thermal fatigue data are compared to purely mechanical isothermal uniaxial fatigue data through the use of an equivalent strain, and an excellent (and conservative) agreement is obtained.

In this work, shakedown design strategies are demonstrated and tailored for concrete structural systems with reinforcement in extreme thermomechanical environments. It is found that allowing for shakedown to occur doubles the structure's mechanical load bearing capacity under constant thermal load. The proposed model also allows for proposing design parameters for shakedown purposes. Two additional test cases are illustrated in which two different rebar diameters are used, namely 6 and 12 mm. In the first case an inadmissible behavior is depicted while in the second case, for the same loading and boundary conditions, the shakedown behavior is observed.

A new experimental setup called FLASH (THErmal Fatigue by LASer or Helium pulses) has been developed to perform thermal fatigue tests on materials used in structures of Sodium-cooled Fast Reactors (SFRs). A set of thermal loadings ranging from 150°C to 200°C prescribed by a high energy laser has been applied to A316L(N) stainless steel samples. Thermal fatigue tests are performed until a macrocrack is formed. The 3D displacement fields of the surface impacted by the laser beam are measured by a hybrid multiview system. A thermomechanical model is used to compute displacement fields that are compared with correlation measurements. These results are compared in terms of strain levels to the fatigue curve determined from standardized isothermal uniaxial mechanical fatigue tests.

Thermal shocks are applied to a 304L austenitic stainless steel plate with a pulsed laser. A stroboscopic reconstruction is used for infrared (IR) and visible light camera measurements. The displacement fields are measured with a digital image correlation (DIC) technique. Different IR devices are used to measure the temperature variations (i.e. medium wave camera and short wave pyrometry). Several ways of determining the emissivity or absorptivity are discussed. The complete 3D thermal loading is numerically determined by minimising the difference between experimental measurements and finite element analyses of thermal fields. An elastoplastic model is then used to compute mechanical fields that are compared with DIC measurements.

The acquisition of images with different modalities may involve different alterations with respect to an ideal model. Inhomogeneous brightness and contrast, blur due to non-ideal focusing, distortions are common. It is proposed herein to account for such effects for instance by registering a calibration target image with an actual optical image to measure lens distortions. An Integrated Digital Image Correlation (I-DIC) algorithm is proposed to account for the above artifacts and the algorithm is detailed. The resolution and uncertainty of the technique are first investigated on synthetic images, and then applied to the measurement of distortions for infrared (IR) images. The procedure is shown to reduce drastically the residual level assessing the validity of the image formation model, but more importantly allowing for a much improved registration of images.