Figueroa-Figueroa, Evaristo

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    Damage assessment under repeated slamming and creep of foam core sandwich composites
    (2013) Figueroa-Figueroa, Evaristo; Shafiq, Basir; College of Engineering; Serrano, David; Just-Agosto, Frederick; Department of Mechanical Engineering; Rodríguez, Manuel
    Sandwich composites are increasingly being used in the ship hull design due mainly to (i) their high strength to weight ratio, and (ii) the ability to functionally grade them for a broad range of commercial and naval applications. Sandwich composite are composed of widely differing constituent materials that can lead to substantial instabilities along the interfaces when subject to external loading and/or environmental effects. However, due to the layered nature of the material, damage in sandwich composites generally remains hidden along the interfaces and thus out of sight until reaching catastrophic dimensions – which make damage detection a very challenging task. Ship hulls routinely undergo a wide range of loading conditions that can compromise their safe operational life. With the main objective of understanding damage progression mechanisms, this thesis takes two of the most detrimental but least understood loading scenarios relevant to service life of the ship hull, namely, repeated slamming and cyclic creep in seawater of foam core sandwich composites. A test program designed and carried out to mimic the repeated slamming of the bow section of fast moving small vessel on the ocean surface provided some unique observations in terms of failure mode transition and associated changes in the lifetime. Testing was performed on flat rectangular specimens that contained symmetric semi-elliptical edge flaws produced near the end of the specimen held by the rotating cam. Damage progression and modes of failure were evaluated for two types of sandwich composites with comparable global strength and stiffness but different foam density and facesheet strength. In-spite of comparable mechanical properties, lifetime of Type 2 specimens was found to be over two orders of magnitude greater than Type 1 specimens – indicating that the lifetime is highly dependent upon the constituent materials. Type 1 specimens (softer core/stronger facesheet) consistently failed by interface and through the thickness core shear, independent of the flaw size. On the other hand, a gradual decrease in the flaw size (i.e., decreasing stress intensity) in Type 2 specimens (denser core/weaker facesheet) produced a striking transition in the mode of failure, from local buckling in the vicinity of the flaw site along with exponentially increasing lifetime, to interface shear failure at the free end accompanied by a dramatic drop in lifetime. These results are quite unique as a decrease in stress intensity is expected to lead to increasing lifetime and not to a decrease as currently observed. This curious phenomenon is attributed to a complex transition in the mode of failure from local buckling to interface delamination as a function of flaw site stress intensity. Foam core sandwich composites were also subjected to creep to failure and cyclic creep in seawater which has never been reported in the literature. The instantaneous and secondary responses varied dramatically depending on the environment and loading type. Compared with creep to failure tests performed in air, about 15% higher deflection and over 50% reduction in lifetime was witnessed in specimens subjected to seawater, which is quite a dramatic loss of life especially in light of the a maximum of 2.3% water gain observed. However, seawater with high salt content has the propensity to breakdown the cell walls due to plasticization and thus deteriorates the interfaces. Cyclic creep was performed in order to mimic an actual ship hull service lifetime scenario whereby cargo and passengers are loaded for extended periods of time and subsequently unloaded. The specimens were loaded for 24 hours while the unloading times varied from 24 to 6 hours. Significantly reduced life and extensive damage were observed under cyclic creep as compared with creep to failure specimens. Counter intuitively, lifetime and number of cycles to failure were found to decrease as a function of increasing unloading periods, which is explained in terms of stress relaxation and cyclic behavior of the sandwich composite. Modes of failure were predominantly indentation and core compression.