Severe marginal breakdown is seen with a tooth restored with low copper amalgam filling. Which of the following could be responsible for this?
High creep
Stress relaxation
High tensile strength
Low fracture toughness
Creep is defined as the time dependent plastic strain of a material under a static load or constant stress. 
For a given load at a given time, the low-copper amalgam has a greater strain. The implications and clinical importance of this are that the greater creep in the low-copper amalgam makes it more susceptible to strain accumulation and fracture, and also marginal breakdown, which can lead to secondary decay. The high creep behavior of low-copper amalgam contributed to its decline in popularity.
Stress relaxation is the reduction in stress in a material subjected to constant strain, whereas creep is the increase in strain in a material under constant stress.

Craig’s Restorative dental materials. 14th edition page Pg-41