Engineering

"A technical report (also scientific report) is a document that describes the process, progress, or results of technical or scientific research or the state of a technical or scientific research problem. It might also include recommendations and conclusions of the research. Unlike other scientific literature, such as scientific journals and the proceedings of some academic conferences, technical reports rarely undergo comprehensive independent peer review before publication. They may be considered grey literature. Where there is a review process, it is often limited to within the originating organization. Similarly, there are no formal publishing procedures for such reports, except where established locally." - from Wikipedia

Science.gov
Includes 200 million pages of science information and R&D results for 36+ U.S. government agencies

TRAIL (Technical Report Archive and Image Library)
Has historical government agency technical reports.. TRAIL currently contains the following report series: https://trailguides.crl.edu/series