On the true extrema of Young's modulus in hexagonal materials

In a paper, which has been recently published in Applied Mathematics and Computations, Khan and Ahmad (2012) [1] deal with the detection of the extrema of Young’s modulus, E, in hexagonal materials. A few issues presented in that paper, which deserve being outlined and thoroughly discussed, are tackled. Moreover, in the case of hexagonal materials, a suitable classification is suggested, an exhaustive panoramic view of the possible shape of the surface E(n) generated by Young’s modulus for all possible orientations n is illustrated, and some meaningful numerical examples are proposed