Damage Analysis on Marine Vessels due to Impact and Grounding

Abstract

Composite material technology application within the general engineering field is rapidly increasing. Composite structures are only limited by the designer’s imagination. This is even more so in the marine craft manufacturing industry. Reinforced plastic composites are now the choice material of pleasure craft, commercial fishing vessels, work boats high speed crafts and naval vessels, only to mention a few applications. Fibre reinforced plastic (FRP) offers many advantages over more traditional materials such as resistance to corrosion and rot, high specific material properties which can be engineered to one’s own requirements and the ease of forming composite complex shapes. FRP in addition to a marine grade core material, produces sandwich panels. The variety of sandwich panels produced and method of fabrication ultimately depends upon its location and application on the marine craft. The sandwich panel construction is closely controlled by maritime classification society rules, enabling the marine craft to obtain certification.

Laminated sandwich panels are susceptible to impact damage events. Such events on the hull are not limited to the normal hydrostatic loadings normally associated with high speed craft (HSC) and the associated out-of-plane events, but impact events of collisions with foreign objects, which can occur at varying speeds of impact, such as floating debris, other craft, docks, falling objects and grounding.

Damage on a composite panel is a dynamic event and is complex, due to the many constituents making up the part. The large global deflections induce membrane and shear deflections. Damage modes in the laminate skin may induce internal delamination, matrix cracking, fibre fracture and ply shear. Damage which may occur is not always clearly visible, however, it can cause a significant reduction in strength which can lead to premature failure of the laminate of the overall sandwich panel. Understanding the damage characteristics of a composite sandwich laminate is therefore essential in order to optimise their design against such failure.

Research into the damage mechanics, both experimental and numerical is increasing. Many researchers have investigated single skin laminates with varying constituents, however there are very few reports on marine laminates and barely any mention of impact on marine grade sandwich panels.

Many authors have studied impact behaviour on composite structures. Most literature focuses and discusses slow rate impact of single composite laminates and fast non/instrumented drop weight testing mainly on composites fabricated from carbon fibre and epoxy resin. Few discuss marine grade laminates or even sandwich constructions.

The work of damage analysis has had a good start. Ongoing projects deal with design, build and test for marine sandwich structures, using thin steel face skins and marine grade foam core. This project established the various tests that sandwich panels have to undergo to obtain the important characteristics necessary for further analysis. Following this an MSc by research project using fibre glass skins and same marine grade foam using an innovative (for Malta) vacuum bagging process, that produces enhanced hybrid panels of superior mechanical and impact properties. A quasi static impact apparatus has been designed, built and test quasi static impact of these marine sandwich panels according to the current and latest ASTM standards. These tests shown the damage that can be caused by a standard indentor, the energy absorbed both when rigidly backed and simply supported. These tests and standard has already been further extended to include other shaped indentors simulating rocks as the sandwich panel hull grounds on these rocks with either bluff, round, sharp or pointed rocks. These panels can then be tested in another rig (not yet built) to assess the residual energy of such panels, for subsequent further damage. Another project has designed built and tested an instrumented dynamic drop weight testing machine. This apparatus allows the same marine grade panels to be impacted at higher strain rates, whereby the damage can be contrasted with the quasi static tests. There are many further projects that can enhance and follow from these projects, such as carbon fibre for additional strength, Kevlar for abrasion resistance, and many such other materials with properties specific to their purpose. Multi-layered panels can be designed and tested for specific applications. There is also the potential of ballistic testing and sports protection, on just to name two applications. The projects that can be undertaken in this field is very extensive and innovative. This work has resulted in a peer reviewed journal publication.

Impact Damage: Impacts of objects can be intentional or accidental; leading to either a controlled event or in the extreme cases can result in catastrophic failure. The impact of a structure, normally comprising of simple structural elements, such as beams, plates and shells is to be studied with the view of applying the impact scenarios to various applications.

The impact velocity determines the domain of the analysis, which can range from low, resulting in a quasi-static response; to medium speed resulting in a dynamic loading event; up to very fast velocities, e.g. in ballistic applications. The variation of impact velocity may significantly change the properties and resulting deformation of the target material and consequently affect the impact behaviour of the structure. Besides the velocity of impact, the shape of the impactor can influence the resulting damage significantly; from blunt contact to sharp stabbing conditions.

The materials to be considered are manufactured out of Fibre Reinforced Plastic (FRP), a composite manufactured with orthophthalic polyester resin and E-glass fibres. This composite laminate is also to be combined with a closed cell linear Poly Vinyl Chloride (PVC) foam to form a sandwich panel. This panel is itself manufactured using a vacuum packing fabrication process, which is innovative for Malta. The resulting hybrid sandwich panels offer an improved, high strength and lightweight structural solution.

Results can be of benefit to many applications such as marine, sports, protection etc. The field of application is large, active and growing rapidly. The results of such a study are to be used to guide development of rational design procedures, to improve safety in collision protection, to develop and improve energy-absorbing capabilities and to help impact prevention for safety improvement applications, amongst many other applications