Mechanical Characterization of Damage and Failure in Polymeric Foams and Glass/Epoxy Composites

Citation

Kidd, Theresa Hiromi (2007) Mechanical Characterization of Damage and Failure in Polymeric Foams and Glass/Epoxy Composites. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/G25Y-KE07. https://resolver.caltech.edu/CaltechETD:etd-11102006-182329

Abstract

The mechanical characterization including evolution of damage and failure of foams and composites are becoming increasingly important, as they form the basic components of sandwich structures. Sandwich structures consist of two faceplates that surround a core material. In many modern applications, faceplates and cores are typically comprised of composite materials and polymeric foam, respectively. Knowledge of the failure behavior of these individual components is necessary for understanding the failure behavior and design of sandwich structures. A systematic investigation of the damage evolution and failure behavior of foams and composites was conducted using a variety of experimental techniques.

In-situ ultrasonic measurements were used to track the damage behavior in PVC polymeric foams with densities ranging from 130 to 250 kg/m³. The wave speeds were measured by two quartz piezoelectric shear transducers with a resonant frequency of 5 MHz in the transmission mode. A fixture was developed and constructed to protect the transducers during compression, while allowing them to take sound speed measurements of the sample along the axis of the load train. This fixture was placed in a servo-hydraulic MTS (Materials Testing System) machine, where the load-displacement response of the foam was recorded. A digital image correlation (DIC) method was used to capture the progression of failure under compression. Two dominant failure modes, elastic buckling and plastic collapse, were identified – and their onsets corresponded to the change in elastic wave speeds in the material, measured by the in-situ ultrasonic technique.

The transverse response of S-Glass/Epoxy unidirectional composites was investigated under varying degrees of confinement and strain rates. The experimental setup utilizes a fixture that allowed for independent measurement of the three principal stresses in a confined specimen. A servo-hydraulic materials testing system and a Kolsky (split Hopkinson) pressure bar generated strain rates between 10⁻³ to 10⁴ s⁻¹. Post-test scanning electron microscopy (SEM) observations suggest that under transverse loading at low-strain rates, confinement contributes to localized band formation. In addition, micrographs indicated that macroscopic transverse failure is dominated by shear stress, and occurs within these localized bands. These shear dominated failure bands were found inclined in a direction approximately 35° to the direction of loading. Implications of this orientation deviation of failure bands from maximum shear trajectories at 45° are discussed in reference to the state of confinement.

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