Hi, I'm new to adobe illustrator. I'm trying to create this exact diagram with the grey axis plane. I can make the T shape by making the T using two rectangle and extrude it but I cannot change the borderline of the shape like the diagram. Also how do I make the plane look like it is going through the shape. I would be grateful if anyone helps!!
Personally, I wouldn't use Illustrator's 3D extrude functionality for something like this.
A simpler approach would be to create the pieces of the design manually, using the original as a guide to trace the shapes, drawing them with the pen tool. In the example below there are just 4 shapes, i.e. the T, the top horizontal rectangle, and two vertical side rectangles. Then add some gradient fills to the shapes, then draw separate strokes on top. It's not that complicated.
Then you could do the same for the grey part, and in the appearance panel set its blending mode to multiply.
And for a little more attention to detail, using the Add Anchor Point tool, add some anchors to the strokes, and then select and delete some segments to make a dotted line. Do the same for the blue stroke at the other side.
If you are going to make a collection of elasticity theory examples and problems you should think a few things
Making drawings of 3D scenes with 2D tools needs much work. Your time can become short. Using a 3D CAD program can be much more effective (after learning to use it). You may easily recycle parts in a 3D program - no need to draw new images for different views and you may collect a good library of components to be used later. Using Illustrator's 3D effects like extrude is an useful, but only a minor step to that direction.
Making copies of existing drawings is copying - no matter you draw the copy manually. Someone may want to say something of it. Any physical constellation of parts, forces, bending moments etc...probably will not be the source of harms. Something may arise when the illustration of that constellation is copied.
You can partially beat both problems by presenting the physical constellation with a simpler drawing. The coordinate axles and other annotations are a must. They cannot be simplified, but the base drawing which contains the T-bar and the plane of the bending moment can be simpler without sacrificing the readability. An example without annotations:
The blue plane is transparent and the bar profile (T) is split to 3 parts before extruding it. The T must have no stroke!
The job (sorry for using legacy Illustrator) starts by dividing the T with a line (Object > Path > Divide objects below). The three pieces of the T are grouped and extruded in the right. The right effect is the Classic 3D Extrude & Bevel. Illustrator CC has also modern 3D effects which create bitmap images. Use the Classic one to get vectors!
The view is Isometric Right. No perspective is used because it would make inserting annotations and the moment plane more difficult.
In the next image the extrusion is expanded (Object > Expand Appearance) to get 2D shapes. The result must be ungrouped several times to release the surfaces for coloring. Before ungrouping check if there's a clipping mask. Illustrator CC wants to insert them without asking. Use Object > Clipping Mask > Release. Delete the useless clipping mask shape after ungrouping.
The surfaces are recolored manually with greys. Everyone got a thin black stroke. The line tool is used to draw a couple of lines on the image to get the edges for the moment plane. They are moved apart. You can make in the same way the lines for the coordinate axles when you insert the annotations.
In the middle the lines are duplicated and joined together to get the right shape for the moment plane. In the right the shape is scaled to bigger size and filled with blue.
In the next image the blue plane is sent to back and moved to the approximately right place. The place is not by any means critical:
In the right parts of the T-bar are sent to back and the opacity of the plane is dropped to 25% in the Appearance panel.
The opacity of the plane can be much lower than 25%. In the next image it's only 10%. It's still well distiquishable, because an 100% opaque duplicate is inserted. The duplicate has no fill, but it has the same stroke as the T.
As you see, there's also inserted some annotations which resemble your originals. The M-curves on the plane can be tricky to get right if one for some reason cannot draw them right in the fly with the Pen. An easy trick to make them fit to the plane is to make a rectangle and draw the needed items inside. Group them, insert 3D effect rotate and search a good view. The rectangle can be compared to the edges of the plane and that helps substantially to find the right view angle without making calculations. The items can be released for individual placing by expanding the appearance, removing the clipping mask and ungrouping.