Perhaps you should try some spelling variations:
"nonlinear material theory" » "nonlinear materials theory", "nonlinear matrix theory", "nonlinear partial theory"
"nonlinear spatial theory" » "nonlinear partial theory", "nonlinear spectral theory", "nonlinear static theory", "nonlinear special theory", "nonlinear social theory", "nonlinear spatial memory"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("nonlinear material theory") OR ("nonlinear spatial theory")).
• Removing quotes may allow a broader search: (( nonlinear material theory ) or ( nonlinear spatial theory )).
• Adding a wildcard symbol may retrieve word variants: (("nonlinear material theory") or ("nonlinear spatial theory"))*.