1. Abstract:
In this work, a hybrid fiber reinforced composite rod fixed from one end, while the other end is left
free and subjected to a static torsional load, and left to vibrate freely after load releasing. Fiber
volume fraction of 40% is considered to rod of interest, the matrix is composed of 20% short fibers
and 40% pure matrix (this type of matrix is called composite matrix, which is well known and
widely used in fiber reinforced hybrid composites). Such a ratio is adopted since it gives enough
strength and reinforcement and meet the economic requirements as well, as mentioned in
Mechanics of Composite Materials by Jones 1999. Four various long and short fibers of the same
type and four matrices are taken to construct the rod in order to introduce the different properties to
investigate their effects on natural frequency under torsional excitation. The problem is manipulated
using software of AnsysV.14. The elastic properties of materials are determined using software of
Matlab v.7. The results show that natural frequency is mainly highly affected by matrix longitudinal
elastic modulus than that of fibers, and displays a regular pattern of behavior. The fibers show an
irregular behavior towards the natural frequency variation. This can be attributed to the interaction
effects between three types of constituents, matrix, long and short fibers composing the whole
structure of the rod. In addition, there is the effect of anisotropy of material effective elastic
properties, which plays an important role in results irregularities.