Severe marginal breakdown is seen with a tooth restored with low copper amalgam filling. Which of the following could be responsible for this?
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
High creep
Stress relaxation
High tensile strength
Low fracture toughness