Modal and strain experimental analysis to an improved axial-axial cruciform specimen for ultrasonic fatigue testing.

Abstract

Fatigue behavior for any material or composite is an ever-evolving complex subject of big engineering importance. Within the very high 106 to 109 cycle regime the use of ultrasonic fatigue has been a big research topic in recent years, ranging from all metals and composite studies, new experimental methods and induced stress states, to the frequency effect and associated fracture mechanics. Following the associated cruciform ultrasonic fatigue testing research in development, new and improved cruciform specimens were acquired and subjected to experimental analysis. The conducted fatigue cruciform testing method induces an axial-axial in phase stress state through the use of a piezoelectric ultrasonic testing machine built at Instituto Superior Técnico laboratories. A modal response analysis was performed within the frequency working range of the used piezoelectric transducer using a Polytec laser vibrometer. A modal calculation method termed Frequency Domain Decomposition (FDD) was adapted to the measured cruciform vibration displacements. The points of measure were in correlation to a previous published numerical research. The results were then compared to previous cruciform specimens’ behavior and numerical results. Strain gauges were also used for the strain and stress study at the specimen’s center. With both conducted experiments showing promising results, the new and improved specimens were led to fatigue failure and a preliminary fracture analysis made.