1. Project Gutenberg (www.gutenberg.org): Project Gutenberg is a library of over 60,000 free eBooks. You can find several classic books on strength materials, such as "Strength of Materials" by William John Macquorn Rankine and "A Text-Book of Strength of Materials" by Arthur Morley.
2. Internet Archive (www.archive.org): The Internet Archive is a non-profit library of millions of free books, movies, and other digital content. You can find many textbooks and reference books on strength materials in their collection.
3. OpenStax (openstax.org): OpenStax is a non-profit organization that provides free textbooks for college students. They offer a free textbook on strength of materials, which includes detailed explanations and practice problems.
4. University Websites: Some universities offer free online versions of their textbooks. For example, MIT OpenCourseWare (ocw.mit.edu) provides free access to lecture notes, readings, and assignments for various courses, including some on strength materials.
5. Google Books (books.google.com): Google Books offers a limited preview of many books, including some on strength materials. You can read parts of the books online or download a PDF of the preview.
Remember that while these sources offer free books on strength materials, they may not be as comprehensive or up-to-date as paid textbooks or resources. It is always a good idea to consult multiple sources and verify the information provided.