Here are some seamless 2048x2048 black and white layers you can add to your textures to 'crack' them using blend modes or masks. The sample images on the right above were created using them.
I made two quick PSD files to show how you might use them with a mask or layer with blend modes.
The PSD images can be downloaded on my site by selecting the link below then the 'Textures' link. [link]