Perhaps you should try some spelling variations:
"mechanical engineering theories" » "electrical engineering theories", "mechanical engineering theses", "mechanical engineering textiles", "mechanical engineering tools"
"mechanical engineering theory" » "electrical engineering theory", "mechanical engineering tools"

Try widening your search to All Fields.

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• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"mechanical engineering theories\") OR (\"mechanical engineering theory\"))".
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