Hi Fredrik...
I can help you with this. Before I get to the technique, though I wanted to say a few words about sheet metal and Pro/E. When creating sheet metal parts in Pro/E, it's really helpful to understand the capabilities and limitations of the software tools before you begin. Often you can begin your design in several different ways. Which way to choose depends on what you're trying to do with the part. For instance, if you're interested in preserving "shown" dimensions, you'll choose certain design techniques over others. Many times one technique will hold an advantage such as making downstream work (like ripping and unbending) easier.
Having said this, I'd respectfull suggest you probably haven't chosen the best way to model this part. I don't mean that as a knock at you, just an observation based on experience. While I always start my sheet metal objects in standard part mode (for reasons beyond this email discussion), I avoid adding rounds and other features before converting. While the convenience of standard modeling is hard to ignore, sometimes it hamstrings you once you're in sheet metal mode. This is what's happened in your model.
The best way to create the rip you want using the model "as is", would be to use a SURFACE RIP along with a CONVERSION feature. The corner surface (circled in the attached graphic) is your big problem. Were it not for this piece, your rip would be a piece of cake.
In Sheet Metal Mode, go to Insert>Shape>Rip>Surface Rip and choose the trianglular corner surface to remove You can do more than one rip if there are other odd corners to modify. This is not a very pretty rip because the larger your bends, the larger the missing chunk of material you'll have. However, unless you remodel the part to take advantage of other rip/unbend techniques, you're stuck with it. See the graphic attached for before/after pictures.
Next, select Insert>Conversion and select Rip Connect from the "SMT Conversion" menu. Choose Add and then pick two points (shown in the graphic) to define the rip. You can do multiple rips at other locations if necessary. Once you've completed the conversion feature, you should be able to unbend the part.
As I mentioned, there are probably better ways to model the part such that these corners won't present big problems. The sheet metal conversion feature is very powerful. You can use it to add BENDS to sharp corners, point and corner relief, and rips all in one feature. Check the Pro/E help files for more information. By making your initial solid model less restrictive, you can utilitze conversions to add bends and rips without having to remove large chunks of surfaces with the surface rip. This can result in much tighter joints and a better looking finished sheet metal part.
Good luck!
-Brian
