Perhaps you should try some spelling variations:
"strengthen of materials" » "strengthening of materials", "strengthen of material", "strengthen of materias", "strengthen of materialls"
"strengthen of materia" » "strengthening of materia", "strengthen of material"
"strengthen of materiales" » "strengthening of materiales", "strengthen of materialles", "strengthen of material"

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

• Performing a fuzzy search may retrieve terms with similar spellings: (((("strengthen of materials") OR ("strengthen of materia"))) OR ("strengthen of materiales"))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"strengthen of materials\") OR (\"strengthen of materia\"))) OR (\"strengthen of materiales\"))*".
• Removing quotes may allow a broader search: (((( strengthen of materials ) OR ( strengthen of materia ))) OR ( strengthen of materiales ))*.