What does A,n,m indicates in the MATVP?

Abhishek Thakur_21110

Respected Sir/Mam 

I am solving a case for creep analysis but have some doubts regarding the values used in the MATVP.

I have also referred to the Altair but things are not clear to me.

Please let me know if you can help, to setup creep analysis conditions in the Optistruct module.

Regards

Abhishek Thakur

Rajashri_Saha

Hi Abhishek,

A,n,m are the material paramters.

See the below syntax.

Thanks

Rajashri

Abhishek Thakur_21110

Hello Rajashri 

Thank you for your reply.

But I want to know what these material parameter indicates & how can I get values for these parameters.

Regards

Abhishek 

Rajashri_Saha

Abhishek Thakur_21110 said:

Hello Rajashri 

Thank you for your reply.

But I want to know what these material parameter indicates & how can I get values for these parameters.

Regards

Abhishek 

Hi Abhishek,

Below are few more information regarding creep analysis and the material parameters.

Creep data in its raw form consists of periodic strain measurements taken from a number of constant load tests. Andrade gave the first widely accepted description of a single creep test. He suggested that a tensile creep test could, after an initial elastic strain, be divided into creep regions called primary, secondary and tertiary. Figure 1 shows an idealized creep test on a tensile specimen with a constant applied stress over time. The curve shows the total strain (=elastic  + creep strain) plotted against time.

The format used most frequently to describe the primary and secondary regions of the Creep behavior is the equation below :

                                                                      Ɛ = A σⁿ t^m exp (-Q /k T deg K)  ---------------------------------------------------------------- (1)

 

                                                 Where,

                                                  A is constant

                                                  n is the creep stress index (ranging from 1 to 10)

                                                  m is the time index (taking values 0<m<=1)

                                                  Q is the activation energy

                                                  k is the Boltzmann constant

 

This is the hyperbolic sine form of stress dependency which will be discussed further in the subsequent sections

Primary Creep (0 < m <1)

 Primary creep is the initial creep strain occurring whilst work hardening dominates the internal processes in the material. Figure (1) shows primary creep developing very rapidly and then slowing to give further elastic strain. Material scientists often record some limited recovery of strain when the load is removed after a period of primary creep

Secondary Creep ( m = 1)

For times greater than one hour in Figure (1), the material deforms at a constant secondary creep rate that accounts for most of the creep observed. The existence of a constant secondary creep rate for constant stresses over long period of time is crucial to most creep calculations. In general majority of creep testing is aimed at establishing the best values for the parameters A, n and Q of equation (1) to use in a secondary creep rule (with  m = 1).

 The Creep stress index – n

When the creep rate is controlled by the movement of dislocations it responds to stress σ in a non-linear manner i.e 

                                                      Ɛ = σⁿ 

The value of n is known as the creep stress index. When the creep stress index n = 1, the associated structural problems are linear (Diffusion creep). Other examples where n = 1 are

1. When the material is subject to intense irradiation in which each atom is displaced from its site many times ex : in fast reactors.
2. When the material is continually stressed internally, the creep rate varies linearly with applied stresses much less than Yield stress.

More generally, however, the structure will be designed for the high stress, high temperature where the secondary creep processes operate with a stress index in the range of 2 to 10.

Hope this helps you to understand more. You can make the answer correct to make the query resolved.

Thanks

Rajashri

Abhishek Thakur_21110

Hello Rajashri 

Thank you for the data provided by you. I have read it all but still my doubt is the same.

For time index, I have understood the conditions but for the creep index & A (constant) how to calculate the values.

I have also looked one or two models provided in the Altair community but still I am finding it hard to co-relate.

If you have calculation related to steel in which constant A & creep index is being calculated then please share or if you want to connect over the meeting we can also do the same.

Regards

Abhishek

 

Rajashri_Saha

Abhishek Thakur_21110 said:

Hello Rajashri 

Thank you for the data provided by you. I have read it all but still my doubt is the same.

For time index, I have understood the conditions but for the creep index & A (constant) how to calculate the values.

I have also looked one or two models provided in the Altair community but still I am finding it hard to co-relate.

If you have calculation related to steel in which constant A & creep index is being calculated then please share or if you want to connect over the meeting we can also do the same.

Regards

Abhishek

 

Hi abhishekh,

May be you can check the below information on creep power law.

Creep Power Law

In this model, an equivalent creep strain rate is generally expressed as a power of applied stress and time as

Under conditions of constant applied stress, this can be integrated to give creep strain as a function of time. The expression can be used to replace time, in the rate equation, with creep strain. This gives what is known as the strain hardening form of the rate equation i.e.

                                                                  

The two approaches give the same result for a constant stress. However, when there is a change in the stress the subsequent strain time responses will be different. The time-hardening version of the power-law creep model is most suitable when the stress state remains essentially constant. The strain-hardening version of power-law creep should be used when the stress state varies during an analysis.

Thanks

Rajashri

Abhishek Thakur_21110

Hello Rajashri 

Thank you for sharing this information.

But I have some more questions regarding this type of analysis.

It would be more helpful if you can spare some time to discuss.

Regards

Abhishek