Size-dependent vibration in two-directional functionally graded porous nanobeams under hygro-thermo-mechanical loadingSalman Ebrahimi-Nejad, Gholam Reza Shaghaghi, Fatemeh Miraskari and Majid KheybariEur. Phys. J. Plus, 134 9 (2019) 465DOI: https://doi.org/10.1140/epjp/i2019-12795-6