Hard external armors have to defend against a lifetime of threats yet are traditionally understood by their ability to withstand a single attack. Survival of bivalve mollusks thus can depend on the ability to repair shell damage between encounters. We studied the capacity for repair in the intertidal mussel Mytilus californianus by compressing live mussels for 15 cycles at ∼79% of their predicted strength (critically fracturing 46% of shells), then allowing the survivors 0, 1, 2 or 4 weeks to repair. Immediately after fatigue loading, mussel shells were 20% weaker than control shells that had not experienced repetitive loading. However, mussels restored full shell strength within 1 week, and after 4 weeks shells that had experienced greater fatiguing forces were stronger than those repetitively loaded at lower forces. Microscopy supported the hypothesis that crack propagation is a mechanism of fatigue-caused weakening. However, the mechanism of repair was only partially explained, as epifluorescence microscopy of calcein staining for shell deposition showed that only half of the mussels that experienced repetitive loading had initiated direct repair via shell growth around fractures. Our findings document repair weeks to months faster than demonstrated in other mollusks. This rapid repair may be important for the mussels’ success contending with predatory and environmental threats in the harsh environment of wave-swept rocky coasts, allowing them to address non-critical but weakening damage and to initiate plastic changes to shell strength. We highlight the significant insight gained by studying biological armors not as static structures but, instead, as dynamic systems that accumulate, repair and respond to damage.

As an increasing number of thermal wells are drilled in arctic and subarctic regions, such as the north slope of Alaska and northern Canada, there is an urgent need for lightweight cement systems with thermally insulating properties. Significant temperature changes resulting from activities such as shut-in, steam injection, and production can lead to increased temperatures in the wellbore. As the wellbore temperature rises, there is an increased risk of melting permafrost, which can allow the formation to move and result in costly damage to the well. Lightweight and thermally insulating cement would contribute to the life of a well by maintaining low thermal conductivity while providing structural support for the casing strings. This study compares the thermal and mechanical properties of water-extended, foam, and microsphere cements with densities of 1.32, 1.50, and 1.68 specific gravity (SG) (11, 12.5, and 14 lbm/gal). To simulate several different conditions in a well, thermal conductivity of the foam system was measured for dried, as-poured, and saturated conditions. While the amount of air or fluid in the foam samples influenced the measured thermal conductivity, both microsphere and foamed systems appeared to be comparable. Initial findings from mechanical properties testing demonstrated foamed slurries have higher tensile and compressive strengths. Under confining pressure, the foam cement system had a larger failure envelope and would be able to withstand greater downhole pressure increases compared to the microsphere design at the same density. When designing wells in areas with permafrost, including a cement system with low thermal conductivity would help minimize the risk of melting the permafrost and maximizing the longevity of the well. This paper reviews several possible lightweight solutions and presents the thermal and mechanical properties of various foam and microsphere cement designs.