Helio Matos

Experimental Investigation on Underwater Buckling of Thin-Walled Composite and Metallic Structures

An experimental study on the underwater buckling of composite and metallic tubes is conducted to evaluate and compare their collapse mechanics. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure through the collapse. Filament-wound carbon-fiber/epoxy, glass/polyester (PE) tubes, and aluminum tubes are studied to explore the effect of material type on the structural failure. Three-dimensional digital image correlation (DIC) technique is used to capture the full-field deformation and velocities during the implosion event. Local pressure fields generated by the implosion event are measured using dynamic pressure transducers to evaluate the strength of the emitted pressure pulse. The results show that glass/PE tubes release the weakest pressure pulse and carbon/epoxy tubes release the strongest upon collapse. In each case, the dominating mechanisms of failure control the amount of flow energy released.

Pressure signature and evaluation of hammer pulses during underwater implosion in confining environments

The fluid structure interaction phenomenon occurring in confined implosions is investigated using high-speed three-dimensional digital image correlation (DIC) experiments. Aluminum tubular specimens are placed inside a confining cylindrical structure that is partially open to a pressurized environment. These specimens are hydrostatically loaded until they naturally implode. The implosion event is viewed, and recorded, through an acrylic window on the confining structure. The velocities captured through DIC are synchronized with the pressure histories to understand the effects of confining environment on the implosion process. Experiments show that collapse of the implodable volume inside the confining tube leads to strong oscillating water hammer waves. The study also reveals that the increasing collapse pressure leads to faster implosions. Both peak and average structural velocities increase linearly with increasing collapse pressure. The effects of the confining environment are better seen in relatively lower collapse pressure implosion experiments in which a long deceleration phase is observed following the peak velocity until wall contact initiates. Additionally, the behavior of the confining environment can be viewed and understood through classical water hammer theory. A one-degree-of-freedom theoretical model was created to predict the impulse pressure history for the particular problem studied.

Mitigation of Energy Emanating from Imploding Metallic and Composite Underwater Structures

An experimental study was conducted to investigate the energy mitigation of hydrostatically initiated implosions of composite and metallic structures. Energy mitigation is achieved by placing polyurea coatings of different thicknesses at different locations onto imploding structures. The implodable structures are made of AL 6061-T6 and carbon-fiber/epoxy composite tubes with seven layers of unidirectional carbon fabric reinforcement arranged in a [±15/0/±45/±15] layup. The implodable specimens are sealed from the water with end caps and suspended inside a large pressure vessel that simulates a free-field marine environment. The hydrostatic pressure inside the pressure vessel was gradually increased until the specimens became unstable and collapsed. The collapse velocities and localized pressures of the imploding structures were captured during the experiments. Two high-speed cameras recorded the implosion phenomenon while dynamic pressure transducers measured the emitted pressure pulses. The results of these experiments show that polyurea can strongly reduce the pressure pulses emitted during the implosion; in turn, mitigating the emanating implosion energy. Moreover, the overall increase in performance, regarding energy reduction, is due to the polyurea coating and not the higher mass of the implodable structures. Placing polyurea coatings in the interior of the implodable structure leads to a higher performance regarding mitigating released energy than exterior placed coatings. Lastly, considerable increase in exterior coating volumes may result in adverse effects such as increasing structural velocity and energy emitted.

Underwater Blast Response of Weathered Carbon Composite Plates

An experimental study was conducted to evaluate the response of weathered unidirectional composite plates subjected to near-field blast loading. The composite materials in this study are carbon-epoxy composite plates with a [45, −45]s layup and was subjected to simulated marine aging through submersion in seawater baths for 70 days at 65 degrees Celsius in order to simulate approximately 20 years of real life operating conditions. Experiments were performed by fully clamping the specimen plates to an air-backed enclosure in a water tank. An RP-503 explosive was placed underwater behind the composite structure to be loaded. During the experiments, transducers measured the pressure emitted by the explosive, and three high-speed cameras captured the entire event. Two of the cameras were placed apart facing the specimen to measure full-field displacements through 3-D Digital Image Correlation analysis. Results show that the diffusion of water into the composite material leads to degradation of the blast response behavior as well as a loss of flexural strength and modulus.

Confined Underwater Implosions Using 3D Digital Image Correlation

This study experimentally investigates fluid structure interactions occurring during confined implosions using high-speed digital image correlation (DIC). Aluminum tubular specimens are placed inside a confining cylindrical structure with one end open to a pressurized environment. These specimens are exposed to hydrostatic pressure, which is slowly increased until they collapse onto themselves. The implosion event is viewed through an acrylic window on the confining structure. Full field deformation and velocities are captured with DIC and are synchronized with the pressure history. Experiments show that implosion inside a confining structure leads to extremely high oscillating water hammer effects. Both peak structural velocities and hammer impulses increase linearly with increasing collapse pressure.