Millions of free NASA technical reports, papers and design data — structures, materials, fatigue, propulsion.
Best for: deep, citable engineering data and real test results. Skip if: you want a quick consumer-level answer.
ntrs.nasa.gov
Free materials-science teaching packages with interactive diagrams and a micrograph library.
Best for: truly understanding phase diagrams, deformation, failure. Skip if: you just need a property number — use MatWeb.
doitpoms.ac.uk
Searchable database of material property data — metals, polymers, ceramics, composites.
Best for: fast property lookups (yield, modulus, density). Skip if: you need certified values — always confirm against the supplier datasheet.
matweb.com
Authoritative thermophysical and chemical property data, free from the US standards body.
Best for: fluid properties, thermodynamic data you can trust. Skip if: you need mechanical/structural data — wrong domain.
webbook.nist.gov
A vast free reference of formulas, tables and data — fluids, thermo, materials, HVAC, fasteners.
Best for: the quick formula or constant you half-remember. Skip if: the application is safety-critical — it's reference, not a certified standard.
engineeringtoolbox.com
Engineering fundamentals reference — formulas, material data, unit conversions, design calculators.
Best for: a second source to cross-check a formula or material value. Skip if: you hit a paywalled premium section — much is free, some isn't.
efunda.com