Did you see the related literature? where you can find the answer for your query.

Can you give more details?

You say "fiber or filler material".  fiber is not filler? What is your sample?

You question is not quite clear.    

However, there must be an interface between the fiber and the filler materials , if the filler materials was the matrix which held the fiber together.  The SEM images of the failure of the fiber should be of that of a brittle fracture,i.e., a small reduction in the cross surface area of the sample & present of river patters.   In contrast, the SEM images of the filler  material should be of a ductile fracture, i.e., a large reduction in the cross surface area(necking phenomena) & present of the cup & cone feature. 

He might be asking, The reason for failure, means is the failure occurred due to debond  filler material or its due to debonding of fiber materials.  

OR - He might be asking, is that any role of filler materials for initiating  the failure ( crack formation) in composite materials..

@Vithal Rao Chauhan

Why have you asked question if you never come back to check answers?

@Vladimir Dusevich 

thank you for your valuable suggestion but still now what the answer i got it is upto mark by research review