Stress marks in polycarbonate or acrylic at bends are typically caused by too low of a temperature at the bend during bending, or a fast/wide temperature swing during cooling after the bend is completed. Sometimes these show up right away, sometimes later.
By using a strip heater, <disclaimer>and letting it get the plastic to full bending temperature</disclaimer>, You have very little chance of fractures. If you play with bending plastics at all, a strip heater is worth every penny. Just be sure to follow the simple instructions if you get one.