Select Mesh
Edit from the main menu bar to combine two adjacent triangular elements into a single quadrilateral element, as shown in the following figure:
To combine triangular elements:
Enter the Mesh module and do one of the following:
From the Object field in the context bar, select Assembly.
From the Object field in the context bar, select Part and select a part from the list.
From the main menu bar, select MeshEdit.
Abaqus/CAE displays the Edit Mesh dialog box.
Tip:
You can also display the Edit Mesh dialog box using the 
tool, located at the bottom of the Mesh module toolbox.
In the dialog box, do the following:
In the Category field, select Element.
From the Method list, select Combine (tri to quad).
Select the first triangular element to combine.
Select the second triangular element to combine. The second element must be adjacent to the first, and the second element must be the same order as the first.
Abaqus/CAE indicates how it will combine two adjacent triangular elements into a single quadrilateral element and prompts you to continue.
Click Yes to combine the elements.
Abaqus/CAE prompts you to select the next elements to combine.
Tip: If you make a mistake while combining triangular elements, click Undo in the Edit Mesh dialog box to undo the change. Abaqus/CAE restores the two triangular elements.
When you have finished combining elements, click mouse button 2 or the cancel button 
in the prompt area to exit the procedure.
