Creep-Cyclic Plasticity and Damage Assessment of an SS304 Weldolet

Creep-fatigue and creep-ratcheting life assessment of an SS304 weldolet considering full creep-cyclic plasticity interaction is investigated using the extended Direct Steady Cycle Analysis (eDSCA) within the Linear Matching Method Framework (LMMF). The creep behaviour is modelled using the Norton relationship, which is modified by the Arrhenius rule to account for the temperature variation within the weldolet. The introduction of a creep dwell increases the reverse plasticity resulting from the creep relaxation. This leads to both creep-fatigue and creep-ratcheting damage mechanisms at different regions within the weldment. For thermal load dominated loading combinations, creep ratcheting due to both cyclically enhanced creep and creep enhanced plasticity are observed based on the dwell period. The effect of dwell period, load and temperature on the creep-fatigue and creep-ratcheting interaction of a weldolet are presented. The simultaneous presence of various damage mechanisms at different locations within the weldment highlights the importance and requirement of the proposed creep-cyclic plasticity investigations at weld locations.

Mitigation of Emissions through Injection Strategies for C I Engine

Fuel conversion efficiency is high with diesel engines compared to petrol engines. However high emissions from diesel is a matter of concern and its mitigation paves way for scope of research. Exhaust gas recirculation is one of the method widely accepted to curb NOx emissions. Recently, split or multiple-injection strategy has been explored by researchers to precisely control the fuel injected per cycle and also to mitigate emissions. Present work reflects technical review of effect of injection strategies on performance, emissions and combustion on C.I. engine with diesel and biodiesel as fuel. Injection strategies like duration of injection, number of injections, the dwell period between two injections, quantity of injection, and multiple injections are analyzed for their influence on engine output and brake specific fuel consumption. Also their effect on emissions especially soot and NOx emission are reviewed. First the effect of injection strategies with diesel fuel is discussed followed by biodiesel.

Experimental analysis on friction stir welding using flat-faced pins in AA2024-T3 plate

In friction stir welding, lesser tool life restricts the usage of non-circular pin in friction stir welding tool eventhough it delivers comparatively better weld joints than circular pin. Process peak temperature during the process affects the shear strength of the flowing material around the tool pin. Maintaining the process peak temperature as low as possible improves the properties in heat affected zone but on the other hand it increases the stress on the tool pin.Especially on the usage of non-circular pin, the pin surface experiences uneven stress distribution and causes premature tool failure. In this paper, optimum thermal environment through proper selection of process parameters and dwell period with respect to the pin geometry are analysed. A comparative analysis is also made to understand the impact of increase in flat surfaces in the pin surface on weld quality in the view of developing a suitable thermal environment that can improve tool life without compromising joint strength. Apart from this, optimum dwell period for the chosen tool pin geometry is analysed based on the empirical softening temperature of the material.

Improved Atmospheric Corrosion Testing for Aluminum Alloys, Part II: Developing Improved Testing Protocol

A modified version of ASTM G85-A2 was developed in this work with the intention of targeting a relative humidity (RH) of 75% during the dwell period. The outcome was two different RH profiles, one that averaged 74% RH during the dwell period and another that averaged 61.5% RH during the dwell period. Both tests produced moderate exfoliation in AA2060-T3 after just 12 days of exposure. Other high-strength aluminum alloys (AA7075, AA2024) were exposed to the modified RH profiles, and both tests could correctly differentiate exfoliation resistance for these alloys. An average RH between 74% and 61.5% during the dwell period was found to produce consistent exfoliation ratings after a short exposure time. Electrochemical measurements made during salt spray testing were used to propose electrochemical mechanisms that occur during wetting and drying in atmospheric corrosion testing.

Improved Atmospheric Corrosion Testing for Aluminum Alloys, Part I: Deconstructing ASTM G85-A2

In this work, the testing environment generated during ASTM G85-A2 exposure was deconstructed for two different commercial salt spray chambers. It was found that relative humidity (RH) control was critical to obtaining consistent results among different salt spray chamber designs, and seemingly small differences in chamber operation could have a significant impact on RH and associated corrosion damage. When RH was too high during the dwell period of the wet-dry cycle, rinsing of the sample prevented the accumulation of corrosion products along grain boundaries, which was necessary for exfoliation formation. When RH during the dwell period was too low, local anodes stifled during the dry air purge and corrosion rate was very low for most of the repeating 6 hour cycle. This work was the first in a two part study.

A Constitutive Model for Predicting Modified Creep Strain Rates due to Cyclic Loading of Gas Turbine Components

The traditional approach to the design of industrial Gas Turbines considers base load operation. This assumption is no longer applicable as owner operators require more operational flexibility and increased availability and reliability. Flexibility in operation manifests as increased cyclic loading and variations in on and off load dwell periods and thermal loads. These complex loading profiles inevitably lead to damage from both creep and fatigue, and interaction of these two damage mechanisms over the duration of the service interval. These interactions can result in higher average creep rates and more damage than expected. Robust, path dependent modeling approaches are required to better understand the effects of flexible operation on material response and subsequent damage. Moreover, a unified approach to creep-fatigue is significantly more effective at capturing this behavior. There are several types of interactions that can drive additional damage. These include relatively well understood mechanisms, such as the effect of plasticity on primary creep and the effect of creep dwells on cyclic material properties. Other interactions that are less well understood include interruptions to the load during creep dwells and the effects of off-load periods on the overall creep rate. This paper considers a constitutive approach to predict the modified creep rate due to load interruptions and off-load dwells using a backstress model. The backstress model is included in the calculation of inelastic strain rate equations, using a Chaboche type formulation. The model has been fitted to conventional material test data for typical superalloys used in gas turbine applications. To validate the approach, forward creep tests were conducted with varying interruptions to the load during the creep dwell period. These tests show a reduction in creep life and an increase in overall creep rate, when compared with the results for a constant stress and temperature condition. Previous work, presented by the authors [1], outlined a hypothesis that attributes the increase in overall creep rate to the influence of a recovery potential stress. This paper presents the subsequent work which demonstrates that the recovery potential stress can be defined by the difference between the applied stress and the backstress. It is shown that the dwell period between reload cycles is critical for calculating the recovery potential and overall creep rate.

Cyclic Viscoplasticity Testing and Modeling of a Service-Aged P91 Steel

A service-aged P91 steel was used to perform an experimental program of cyclic mechanical testing in the temperature range of 400 °C–600 °C, under isothermal conditions, using both saw-tooth and dwell (inclusion of a constant strain dwell period at the maximum (tensile) strain within the cycle) waveforms. The results of this testing were used to identify the material constants for a modified Chaboche, unified viscoplasticity model, which can deal with rate-dependant cyclic effects, such as combined isotropic and kinematic hardening, and time-dependent effects, such as creep, associated with viscoplasticity. The model has been modified in order that the two-stage (nonlinear primary and linear secondary) softening which occurs within the cyclic response of the service-aged P91 material is accounted for and accurately predicted. The characterization of the cyclic viscoplasticity behavior of the service-aged P91 material at 500 °C is presented and compared to experimental stress–strain loops, cyclic softening and creep relaxation, obtained from the cyclic isothermal tests.

Estimation of the Time Constant of Precision Plunge Grinding in Dwell Period Based on Acoustic Emission Measurement

Our previous work presented a method for estimating the time constant of grinding system in infeed period. To monitor the precision cylindrical plunge grinding process in dwell period, an estimation of the time constant of grinding system in the dwell period, using acoustic emission signal, is presented in this paper. Acoustic emission signals generated during precision grinding are sensitivity and present challenges for accurate and reliable process monitoring. Experiments demonstrate the results of the acoustic emission sensing approach in estimating the time constant in the dwell period of cylindrical plunge grinding.