Is the sheet thin enough that you can cut it with scissors tin snips? If the sheet is thin enough, you can combine a partial cut with a pull-apart of the rest. This will definitely damage both AAO and Al foil, but this simple and quick methods might still work for some purposes like getting an overview of the structure. You'll have damage, but on a long cut and pulling-off edge you still have a good chance to get places without severe compression or elongation damage.For thicker sheets, you might consider pre-cutting before you do the anodization. Of course there are also more sophisticated and more elaborate solutions, e.g. cutting with a FIB. Some materials can be frozen in liquid N2 which can enable an brittle fracture afterwards. As far as I know, Al does not really get brittle, but maybe it helps anyway.

Good luck with your research.

For specimens like yours a "sandwich" method may work. Glue with an epoxy two pieces of your sheet together, brittle (oxide) layer to brittle layer to create a "sandwich". Once  a glue is settled, embed specimen in plastic (usual metallographic procedure). Polish, observe.

Dear Dominik Kramer thank you sincerely for your time. the thickness of my Al sheet is nearly 4mm and I don't think I can employ the first mentioned method. and also I don't have easy access to FIB instrument right now. but about the liquid N2 idea, don't you think that the brittle and sensitive Al oxide part of the sheet may be damaged in such a freezing medium because of this thermal shock? have you had any exprience with that?

Dear Vladimir Dusevich thank you for you time. this was good idea, but I am afraid that polishing will damage the sensitive oxide part of the sheet.

I think that the brittle nature of the AAO is an advantage here: You’ll see in the micrographs whether they are damaged (cracked, crushed, crashed), in contrast to ductile metallic structures where deformation can result in changes of the geometry which are not obvious in the images. I would like to support the idea of that polishing can be a good approach. Your point of view that there is a risk of damage is correct, but I would suggest a try. Embedding the material is a good way to minimize the damage. However, I would also try a quick test using a rough grinding without embedding. This will cause severe damage, but maybe you’ll can find an area where the AAO is cracked in a way which exposes an area useful for imaging (similar argument like that above, why not just try it, good if it works, if not, you’ll recognize that easily). If you need a perfect image, e.g. to demonstrate the uniformity or size distribution of the pores in detail for a large cross section, embedding and careful step-by-step grinding and polishing is the best. If you just want to measure the porous layer thickness, this is not be necessary probably. I don’t have done the N2 freezing yet on my own, but colleagues confirmed that this can be a good way to get a useful cracking mode. However, I just read that Al (like lead and silver) belongs to the metals that remain ductile at extreme subzero temperatures. Therefore, this might not help much here. Anyway, you might try the breaking your thick Al sheet, which seems to require a notch made beforehand.

Dear colleagues, I finally take a good SEM cross sectional view from my AAO samples. actually I change my sheet thickness to 0.5 mm and manage to bend my 1.3*1.3 cm sample using my hand pressure!! the brittle part of the sample will crack and you can see both top and cross sectional view easilly.

Thanks and good luck to all.