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Many energy absorption applications utilize flexible polymeric foams for their viscoelastic properties. It is desired that the material will perform consistently across repeated compression cycles. This study examines the effect of fatigue at low strain rates on the viscoelasticity of open‐cell polyurethane foam. Six polyurethanes of the same base composition with two porosities (70% and 80%) and three chemical indexes (79i, 100i, and 121i) are tested. Large deformation cyclic compression of the foams is conducted on a universal testing system (UTS). These data are then post‐processed leveraging dynamic mechanical analysis Fourier transform rheology to characterize changes in the viscoelasticity of the materials over fatigue cycles. Results show that foams can increase or decrease in stiffness up to 10% over 104 cycles. Specifically, higher chemical index, higher excitation frequency, and larger excitation amplitude correlate with a more pronounced decrease in stiffness. Damping can also change by 15% and correlates with chemical index and excitation frequency. Consequently, the findings suggest that internal foam structure and bulk material properties as well as applied loading parameters affect the viscoelastic fatigue response of flexible polymeric foams. [ABSTRACT FROM AUTHOR]

Composite structures used in aerospace, automotive, and military applications are often subjected to aggressive marine or water environments during their service life that can cause material degradation over time. Many composites absorb moisture when exposed to wet environments which may affect their microstructure, consequently influencing their bulk material properties and performance. Additionally, certain applications may require them to be subjected to seawater environments where the addition of saline can further degrade the material over time. This experimental investigation examines the effects of moisture absorption and saline on the dynamic fracture behavior of unidirectional carbon epoxy composites. To accelerate mass absorption, samples were soaked in elevated temperature baths (70°C) of either ASTM standard seawater or distilled water. To explore the role of absorption duration on the dynamic fracture behavior, soaking time varied between 3.5 hours and longer than 4 months. Pre-cracked specimens were impacted using a long-bar striker device at 4 m/s, to instigate a dominantly Mode-I (opening) fracture response. Digital image correlation was used in conjunction with ultra-high-speed imaging to track the crack tip surface displacements and an elastodynamic solution for an orthotropic material leveraged to extract the stress intensity factor (SIF) up to and at initiation. The critical SIF of the soaked samples is compared with results from a group of ambient condition, unsoaked samples. Results suggest that moisture absorption causes the matrix to expand and introduce microcracks or delamination between plies as the composite swells to accommodate the additional water, inversely affecting the material resistance to fracture. [ABSTRACT FROM AUTHOR]

This paper presents a novel, tunable impact fatigue device that can be configured to apply compressive strikes on a variety of materials and geometries at low to intermediate strain rates, and the ability to compliment the investigations using full-field quantitative visualization of material deformation and failure properties. The device ranges in velocities between 0.5 and 4.6 (±0.1) m/s at frequencies between 0.05 and 0.25 Hz using a modified-Hopkinson pressure bar design. The corresponding control and actuation system runs autonomously and can be programmed for 105 cycles at either a constant velocity, pre-entered velocity profiles with changing parameters, or a completely random velocity profile during testing. The device capabilities are demonstrated on 2.38 mm thick polycarbonate plates under low cycle fatigue at velocities of 1 and 2 m/s and impact energies of 0.15 and 0.60 J, highlighting different visible damage mechanisms present under different loading profiles. [ABSTRACT FROM AUTHOR]