Creep Behavior Of Steel Research Articles

Analysis of Monkman-Grant relationships and damage tolerance factor for creep of 25Cr35NiNb micro-alloyed steel

Creep behavior of high Cr austenitic stainless steel was investigated at temperatures ranging from 650 to 1050 °C and stress levels in the range 47–120 MPa. Primary, secondary and tertiary creep strains were analyzed in terms of minimum creep rate, constant true stress and temperature. Minimum creep rates, strain to rupture and rupture time were analyzed in terms of Monkman-Grant and modified Monkman-Grant relation. Good correlation between minimum creep rate and rupture time using both Monkman-Grant relationships is observed for the creep data on the steel. Validity of the relationship between time to reach Monkman-Grant ductility tMGD and rupture life tr in terms of damage tolerance factor λ was established for the steel. Effect of temperature on microstructure and carbide content of the steel is presented in this paper. Microstructural studies reveal failure mechanisms by nucleation and growth of cavities around carbide particles at grain boundary regions. Based on the critical continuum damage mechanics approach, the ratio of tMGD to tr express by constant fCDM separated safe and unsafe regimes of creep behavior of the steel.

Creep Mechanisms in Precracked Polypropylene and Steel Fiber–Reinforced Concrete

Recently, fiber-reinforced concrete (FRC) creep behavior has become a much addressed topic, and the main gap found in the literature is whether cracked FRC is stable in the serviceability limit state. Therefore, this work aims to investigate the creep behavior of steel and polypropylene (PP) FRC by analyzing the growth of the crack opening displacement over time in prismatic specimens subjected to sustained bending. Sustained load tests were also performed on FRC constituents and on the fiber matrix interface in order to evaluate individually their long-term response, and to understand their contribution in bending. Based on a model proposed, the rotation in a plastic hinge located in the crack area could be calculated considering the three mechanisms evaluated—fiber pullout, concrete compression, and shrinkage. It could be concluded that the fiber pullout mechanism plays a primary role in creep deformation in precracked elements subjected to sustained bending loads.

Creep behavior of steel bonded reinforced concrete members under small eccentric compression

In this paper, the creep behavior of steel bonded reinforced concrete members under small eccentric compression is studied. Firstly, based on the existing mechanical characteristics of concrete under multi axial stress state, the stress-strain relationship of steel bonded small eccentric concrete is theoretically deduced and analyzed by considering the reduction coefficient of hoop force. Secondly, according to the creep characteristics of confined core concrete, combined with the existing concrete creep model and the analysis of its influencing factors, the influence of sticking steel on the creep of small eccentric reinforced concrete compression members is discussed. The corresponding creep calculation formula is established. Finally, the existing experimental data are used to verify the proposed formula, and its applicability is analyzed.

Creep behavior and model of high-strength steels over 500 MPa at elevated temperatures

In fire events, the high-temperature creep behavior of steels may critically affect the fire resistance of steel structures. Most studies related to high-temperature creep behavior have been typically performed for normal structural steels but seldom conducted for high-strength structural steels. In this study, a series of high-temperature steady-state creep tests is conducted on Q550, Q690, and Q890 high-strength structural steels at various stress levels within the 400–800 °C temperature range. The test data obtained demonstrate that the total amount of creep deformation is relatively significant, and the increase in the creep strain rate is considerably rapid at high-temperatures and high-stress levels. The three-stage creep model, together with creep parameters, is proposed to express the creep behaviors of three high-strength steels at various stress levels within the 400–800 °C temperature range. The creep behavior and calibrated creep models obtained in this research can be utilized as a theoretical foundation for accurately predicting the response of high-strength steel structures exposed to fire considering the effect of creep.

High-temperature creep behaviour of 15CrMoG heat-resistant steel

Rapid increase in power plant efficiency requires development of heat-resistant steels for use in the boiler and piping systems. In this study, the creep activation energy of 15CrMoG steel was first investigated on the basis of the relationship between creep rate and temperature subjected to different stresses. Long-time creep tests at selected temperatures were performed for the 15CrMoG steel. Manson-Haferd model was utilized to predict the creep behavior of the steel, the results of which indicate that the experimental creep data agreed well with the data from the Manson-Haferd model prediction. Besides, the strain vs. creep time curves obtained from the creep tests were in good accordance with theoretical calculation based on the CDM model over a long creep time. Both Manson-Haferd model and CDM model are powerful tools in studying the high-temperature creep behaviour of the 15CrMoG steel.

Creep behaviour of steel bonded joints under hygrothermal conditions

The aim of this research is to study the influence of moisture absorption at low moisture contents on the creep behaviour of an epoxy adhesive in steel bonded joints. Single lap joints were manufactured using high strength steel adherends and a two-component epoxy adhesive. The single lap joints were tested at load levels corresponding to average lap shear stresses of ± 5%, 15%, 30% and 45% of the dry lap shear strength in both 40 °C air and 40 °C distilled water. Specimens were not pre-aged to be able to analyse the coupled effect of moisture and loading. The test results show that an increase in the load level resulted in an increase in the instantaneous strain and in the creep strain rate. The creep strain of single lap joints loaded in water was generally larger than for the ones loaded in air. For joints loaded in water the creep behaviour was found to be dependent on the moisture concentration in the adhesive. At low moisture percentages creep was suppressed, resulting in a lower instantaneous strain. At higher moisture percentages creep was promoted, resulting in a larger strain rate. The suppression of creep at low moisture percentages is attributed to water molecules bonding to the epoxy macromolecules, resulting in a reduction in molecular mobility and a smaller creep strain. At higher moisture percentages the plasticizing effect of the water dominates, resulting in a larger creep strain. The Maxwell three-element solid model and Kelvin-Voigt three-element solid model were used to simulate the creep behaviour of the single lap joints loaded in air and water. The models gave good representations of the creep response across the different load levels in both water and air, they were however unable to give a correct representation of the instantaneous strain of the single lap joints loaded in water. This is attributed to the models being unable to account for the present short-term relaxation process that is dependent on the moisture concentration.

Modeling creep of steel under transient temperature conditions of fire

This paper presents a methodology for explicit modeling of the time-dependent behavior or creep of structural steel in the Abaqus finite element (FE) models under transient temperature conditions representative of building fires. In this methodology, available creep models based on conventional creep tests under constant or steady-state temperature conditions were used to predict the time-dependent or creep behavior of steel under variable or transient-state temperature conditions. Specifically, user-defined CREEP subroutines were created in Abaqus to define time-dependent or viscoplastic deformation of structural steel under changing temperatures. To demonstrate the robustness of the proposed methodology, the time-dependent behavior of structural steel at elevated temperatures was modeled under three distinct conditions. These were: steady-state temperature conditions, stepwise steady-state temperature conditions, and transient-state temperature conditions. This paper further presents and discusses the application of the methodology in studying the creep behavior of a shear tab connection assembly under the transient temperature conditions of a fire. Through this application, the importance of explicit consideration of creep in predicting the response of connection assemblies at different stages in the evolution of a structural-fire is also emphasized.

Effect of Dislocations Substructure on Creep Behavior of Steels Strengthened by Fine Carbides

Effect of Dislocations Substructure on Creep Behavior of Steels Strengthened by Fine Carbides