Don't forget that although different materials have different physical strengths and as such they also need to be used in way that best utilizes them, just swapping perspex for Ali in an design that was intended for Ali is likely to fail.

Metal has to be bolted (large parts to allow this) as welding can distort the shape but the advantage of acrylic is that it can be glued, (therefore design a monocoque structure) If you can machine small rebates for thin plates to sit in then and really triangulate the beam I would have thought that help reduce the problem with localized stress points that Irving pointed out. Hope this makes sense, I'm not very good at explaining things .

there is also the option of creating a composite beam. acrylic structure with thin sheet metal like a flitch beam or just adding steel rods to the tension side like a concrete lintel.

I need to brush up on plastics, not really used them, is there a difference between Acrylic and perspex?

Of course This is in the context of a small desktop machine