Introduction to Creep | Creep Abaqus modeling
One of the important issues in studying mechanical systems is determining the service life of their components. Engineers need to be aware of the time to failure of system components to prevent extensive damages. As you may know, one of the causes of mechanical component failure is the creep phenomenon.
Creep is a phenomenon in materials science and engineering where gradual deformation or strain occurs over time under a constant load or stress, particularly at elevated temperatures. Understanding and analyzing creep behavior is of significant importance in various industries, including power generation, aerospace, and manufacturing. Creep analysis allows engineers to predict the long-term deformation and potential failure of materials, enabling the design of structures and components that can withstand these conditions. Therefore, determining the life of components subjected to creep is of great importance.
Lesson 1: What is Creep?
In this lesson, first, the creep phenomenon is defined: Creep is time-dependent deformation of a material, and it typically occurs at high temperatures and under constant stress. These stresses are usually lower than the yield stress. Therefore, we can consider three influential factors on creep: time, temperature, and stress level. Since many industrial components operate under constant stress lower than the yield stress and in high-temperature environments, numerous industrial components experience failure due to creep phenomenon. Therefore, the investigation and simulation of the creep phenomenon are of great importance.
Moreover, you will learn more about the temperature factor: The first temperature effect is Vacancy Concentration. In crystalline materials, some atoms are missing from their intended positions in the lattice. These defects in the crystal lattice are called vacancy Concentration. Increasing temperature leads to an increase in the concentration of vacancies, and an increased concentration of vacancies aids the creep process. Also, the creep curve will be discussed, which helps you to better understand the stages of creep. Usually, the strain-time diagram is used to study the creep process. To examine this diagram, you should know the standard creep test methods. The standard creep test is generally performed in two ways: constant load and constant stress. In first method, only a constant load is applied to the sample and the amount of load does not change over time. In the second method, which requires advanced equipment, a constant stress is applied. In this method, considering that the cross-sectional area of the sample is constantly decreasing during the test, the force must also decrease in order to keep the stress constant.
Lesson 2: What are creep life estimation models?
Engineers need to be aware of the time to failure of system components to prevent extensive damages. As you may know, one of the causes of mechanical component failure is the creep phenomenon. In this section, you will study methods for determining the life of components under creep: Larson-Miller, Orr-Sherby-Dorn, and Polynomials estimation models.
Lesson 3: What are creep models?
There are several models to simulate the creep behavior with and in this lesson, you will learn all the models in the Abaqus. Simulation of creep behavior in Abaqus, provides valuable insights into material response under sustained loads and high temperatures. Abaqus offers sophisticated creep modeling capabilities that enable engineers to accurately predict creep deformation and assess the structural integrity of components over time. In Abaqus, creep can be simulated using various creep models:
- Time-Hardening law: When dealing with loading conditions where the stress or temperature is changing, it is necessary to have a proper hardening relation to accurately models the material behavior. However, in general, it is not recommended to use the time hardening relation when the stress is changing.
- Time-Power law: The time-power law is equivalent to the time hardening law but with modifications to avoid computational problems.
- Strain-Hardening law: This law is suitable for modeling the creep behavior under changing stress conditions. However, similar to the time hardening law, it is more appropriate for low stress levels.
- Power law: The power law is equivalent to the strain hardening law with some slight differences.
- Hyperbolic-Sine law: The hyperbolic sine law, along with the subsequent laws, effectively models creep due to their consideration of the effects of temperature, stress, and time.
- Anand law
- Darveaux law
- Double Power law
These models capture the time-dependent behavior of materials by incorporating parameters like creep exponent, activation energy, and reference stress. Also, if you need another model or a user-defined creep model, you can use the Creep subroutine, which will be explained in workshop 2. Moreover, according to the Abaqus documentation, none of the above models are suitable for modeling the creep cyclic loading, which leaves us with only one option and that is the Creep subroutine.
Workshop 1: Creep process on a standard specimen using Strain-Hardening law
In this workshop, the creep process is simulated to a standard specimen using the Strain-Hardening law. One side of the model is fixed and the other is subjected to the tensile stress. Two steps are used for this modeling; one for the initial loading and the other for the creep. The first step is “Static, General” and the second step is “Visco”. The “Visco” step is for quasi-static problems and the ones whose answers are time-dependent, such as creep and viscoelasticity. In the end, we monitor the strain results due to the creep process.
Workshop 2: Applying creep process on a standard specimen using Creep subroutine
The problem description of this workshop is just like the previous one, but this time the Creep subroutine is used to apply the Strain-Hardening law to the model. The subroutine and all its variables are introduced; the subroutine is explained line by line in this workshop so you can understand how to work with it. In the end, the results of this modeling will be compared with the previous workshop to validate the subroutine results.