Bamboo fibers owe their hardness to the presence of cobble like polygonal cellulose nano grains with a diameter of 21-198 nm in their cell walls. These nano grains are basic building blocks of bamboo fibers. It is observed that nano grain structured fibers are not brittle (Zou et al. 2009). A continuous increase in hardness from center to outer surface is observed (Chand et al. 2006). Hardness of bamboo fibers is same in longitudinal as well as transverse directions. Measured hardness for parenchyma cell wall is 0.23 GPa. Hardness shows a decrease when moving from outer layer to inner layer (Yu et al. 2007). Research has also exposed that young bamboo culms are harder as compared to old culms and have high fracture toughness. The hardness of bamboo culms can be judged by crack deflection and crack bridging (Low et al.

2006) . The hardness of bamboo reduces due to steaming treatment (Lin et al. 2006). Tangled micro-fibrillated cellulose fibers when added to poly lactic acid/bamboo fiber composites, increase the hardness and prevent crack development (Naoya et al. 2004). Bamboo is 23 % harder than oak and 13 % harder than rock maple. Fracture toughness of bamboo is measured to be 56.8 MPa m1/2 (Amada and Untao 2001).

2.4.3 Impact Strength

Impact strength of bamboo fiber concretes is distinctly higher (Ramaswamy et al. 1983). The impact strength of steam exploded bamboo fiber filaments is very high (Tokoro et al. 2008). Bamboo fiber reinforced epoxy resins have impact strength of 63.54 KJ m-2 (Jain et al. 1992). The impact strength of poly lactic acid/bamboo fiber composites increases after addition of micro-fibrillated cellulose (MFC) (Naoya et al. 2004). The high percentage of alkali content in bamboo fibers reduces their impact strength (Kushwaha and Kumar 2009).