Concrete has a tendency to ‘creep,’ or deform progressively under mechanical stress, which leads to many of the crumbling bridges and cracked roads we're now seeing across the US.
Creep, also known as cold flow, occurs when a solid material moves slowly or permanently deforms under the influence of long-term mechanical stress that is below the yield strength. This process can increase if the material is exposed to heat, and the effect becomes more severe as the material nears the melting point. Creep rate is a function of material properties, length of exposure, temperature, and applied stress. Since material properties and exposure time are difficult to control and cannot cause creep on their own, creep rate is generally only a concern in high temperature or high stress situations. Because materials undergo increasing strain under long-term stress, creep is known as a “time-dependent” process. However, creep can be beneficial under the right circumstances. For example, when concrete undergoes moderate creep, the material’s movement eases tensile stress which could lead to cracking if left alone.
As a general guideline, creep effects do not become noticeable until the operating temperature reaches 35% of the melting point for metals and 45% of the melting point for ceramics. An example of creep in ice is glacier flow.
Three Stages of Creep
Creep takes different forms in three separate stages. In the first stage, known as primary creep, the strain rate begins high, and then slows over time due to work hardening. In the section stage, secondary creep, the strain rate is nearly constant and at a minimum due to a balance of work hardening and annealing (also known as thermal softening). Secondary creep is also known as steady-state creep because of the near-constant strain rate and is the most understood and simplest to analyze stage of creep for that same reason. In the final stage, known as tertiary creep, the strain rate increases exponentially due to necking until the material fractures.
Polymers are viscoelastic materials and when subjected to a step constant stress, undergo fully reversible viscoelastic creep. Polymers are able to undergo reversible creep because their physical crosslink or mechanical entanglement points allow the material a reference to keep track of the original position of the material. Therefore, once the molecules slide past each other during creep and the stress is subsequently removed, these crosslink points allow the molecules to easily return to their original position.
A polymer’s molecular weight affects its creep behavior because increasing a polymer’s molecular weight tends to increase secondary bonding between the chains, thereby making the material stiffer and more creep resistant. For the same reason, aromatic polymers tend to be more resistant to creep due to the stiffness from the aromatic rings. Both an increased molecular weight and the presence of aromatic rings increase a polymer’s thermal stability, which also increases the material’s creep resistance.
Metals, on the other hand, undergo irreversible, viscoplastic creep. This is because creep in metals is caused by the irreversible movement of vacancies, dislocations, or grain boundaries. None of those movements keep track of the original position of the material. Without a memory of the original position, the material cannot return to its original form once the stress is removed.
While creep is not a phenomenon that causes significant damage in all situations, it must be accounted for in high temperature or high stress situations, or when using materials with a low melting point. Due to its status as a time-dependent phenomenon, creep can generally be visually identified before it enters the final stage and approaches failure.