Which one of the following statements is true? Explain why.
a) When a material is subjected to a cyclic stress, large cracks grow more slowly than small cracks.
b) The concept of a critical stress for crack propagation is only applicable to brittle materials.
c) Materials will not break if the applied stress is kept below the critical value predicted by the Griffith energy balance for crack propagation.
d) If a material is undergoing secondary creep, then doubling the stress will double the strain rate.
e) The toughness of a material is the energy required to break a unit volume of the material.