Acoustic emission multiplets as early warnings of fatigue failure in metallic materials.

Fatigue, i.e. the failure of mechanical structures under cycling loading, remains a considerable technological challenge as it occurs unexpectedly when the structure is operating apparently in a safe and steady state regime, without external signs of mechanical deterioration. Here we report for the first time, in different metallic materials, the detection of acoustic emissions specific of fatigue crack growth. These so-called acoustic multiplets are characterized by nearly identical waveforms, signature of a unique source, are repeatedly triggered over many successive loading cycles at the same stress level, and originate from a single location. They mark the slow, incremental propagation of a fatigue crack at each cycle, or the rubbing along its faces. Being specific to fatigue cracking, they can be used as early warnings of crack propagation, which will ultimately lead to structural failure. Their detection and characterization thus open the way towards a new, reliable monitoring of the onset of fatigue cracking during mechanical tests or within structures in service.

Sound and light from fractures in scintillators.

Prompted by intriguing events observed in certain particle-physics searches for rare events, we study light and acoustic emission simultaneously in some inorganic scintillators subject to mechanical stress. We observe mechanoluminescence in Bi4Ge3O12, CdWO4, and ZnWO4, in various mechanical configurations at room temperature and ambient pressure. We analyze the temporal and amplitude correlations between the light emission and the acoustic emission during fracture. A novel application of the precise energy calibration of Bi4Ge3O12 provided by radioactive sources allows us to deduce that the fraction of elastic energy converted to light is at least 3×10(-5).

Pattern transformation triggered by deformation.

Periodic elastomeric cellular solids are subjected to uniaxial compression and novel transformations of the patterned structures are found upon reaching a critical value of applied load. The results of a numerical investigation reveal that the pattern switch is triggered by a reversible elastic instability. Excellent quantitative agreement between numerical and experimental results is found and the transformations are found to be remarkably uniform across the samples. It is proposed that the mechanism will also operate at much smaller scales opening the possibility for imprinting complex patterns at the nanoscale or switching photonic and phononic crystals in a controlled way.