Dear Holly Au,

My experimental investigations on the RC structures strengthened by FRP composite materials, being tested under reversal load, did not show any dips in the stress-strain curve. I have tested my specimens up until there failure stage. I think the problem is in the model you are considering in your work. Please check the model you have built for carrying out your investigations. How correctly you were able to describe in the model the non-linear behavior of your structure?

Best regards, Mikayel Melkumyan Doctor of Sciences (Engineering), Professor Academician of the Saint-Petersburg Arctic Academy of Sciences Academician of the Athens Institute for Education and Research President of the Armenian Association for Earthquake Engineering Vice-President of the International Association of CIS Countries on Base Isolation Member of the USA Association for Science and Technology Foreign member of the Research Center of Seismic Resistant Structures of the Institute of Industrial Science, University of Tokyo Eminent Expert of the Committee of Eminent Experts in International Research Base of Seismic Mitigation and Isolation of Gansu Province in China Founder of the "Save the Yerevan Schools From Earthquakes" foundation CEO of the "Melkumyan Seismic Technologies" LLC +374 (91) 94-54-02 www.melkumyanseismic.com

Mikayel Grigor Melkumyan

Hello Dr Melkumyan,

Thank you for your response. Yes - I know that my ABAQUS model is not working as expected, but I have run out of ideas of what the problem is. Exporting the data shows that the problem is coming from the S values in the axial direction. The stress values in the two lateral directions are fine. Could you suggest some possible reasons of why a stress dip would appear? From that, I could then narrow down the issues.

Your issue likely stems from incorrect implementation of the confining pressure in your USDFLD subroutine. The post-peak behavior of FRP-confined concrete depends on accurately simulating the interaction between the concrete core and the FRP jacket. Here are some possible causes and solutions:

1. Delayed Activation of Confinement: If the lateral confining pressure is not applied immediately after the peak stress, a dip may occur. Ensure your subroutine correctly calculates and applies the confinement as soon as the concrete reaches its peak stress.

2. Inaccurate Confining Pressure Calculation: The lateral pressure should increase with the dilation of concrete in the post-peak region. Check your formulation for calculating the field variable and ensure it is based on the actual lateral strain of the concrete.

3. Material Model Mismatch: The concrete material model you are using may not account for the confinement effect properly. Verify that the model includes the necessary parameters to represent FRP-confined behavior accurately.

4. Mesh Sensitivity: A coarse mesh can result in numerical inaccuracies, especially in the post-peak region. Refine your mesh and ensure proper element selection for better accuracy.

5. Inconsistent Parameters: Ensure that all parameters in your USDFLD subroutine, including FRP stiffness and ultimate strain, are consistent with the experimental or theoretical model you are using.

6. Boundary Conditions: Improper boundary conditions can lead to unrealistic stress-strain behavior. Double-check the loading and confinement conditions in your model.

Suggested Rectification:

Revisit the stress-strain relationship for FRP-confined concrete and ensure it is correctly implemented in your subroutine.

Validate your model by comparing it with experimental results or analytical predictions for FRP-confined concrete.

Use a stepwise debugging approach to isolate and address the issue in your USDFLD subroutine.

Dear Dr Altawil,

Thank you very much for your detailed response. I will investigate the issues you mentioned.