Laminate stiffeners

Work on analysis of the common structural shell element made of composite materials is very extensive. Contributions will be mentioned that parallel the developments in Chapter 5 on plates. Some of the first analyses of laminated shells are by Dong, Pister, and Taylor [6-44] and the monograph by Ambartsumyan [6-36]. Further efforts include the buckling work on laminated shells by Cheng and Ho [6-45] and on eccentrically stiffened laminated shells by Jones [6-46]. 

What kinds of configurations are possible for composite structures The most obvious is that of a fiber-reinforced laminate. With a laminate, we can change laminae orientations, stacking sequence, and laminae materials to arrive at a suitable structure. We can stiffen the laminate, or we can put a sandwich core in the middle of those laminae. We can do all of those possibilities, but recognize that we will also have, in vir-tuaiiy any structure, some kind of hoie or a cutout for some reason. Thus, we must have a procedure to place an appropriate amount of reinforcement around those cutouts so that ioad can be transferred around them. Without that reinforcement, the structure cannot do the job it is required to do. These various possibie configurations are shown in Figure 7-38. 

The simulation of the one-ply composite shows that the post-critical assumptions and the simple modelling procedure work fairly well — Figme 2.9. Hence, this approach can be used further for simulation of the four-ply laminate. A slight stiffening response of the material at the end of the test is probably caused by straightening of the yams in the loading direction, which has not been accounted in the model. 

Validation of the use of cohesive elements has been shown by [44] and Davies and Zhang [14] on stiffened compression panels, between and over stiffeners. Very sophisticated models of composite laminates may be modelled. Woven composites are attractive in that they can be draped and have a modest capability for absorbing impact energy [45]. 

Warship builder Vosper Thornycroft, initially a SCRIMP licensee, subsequently developed its own method for single-shot infusion of up to 30 laminate plies, including heavy shipbuilding fabrics, and complex structures with inserts, cores, stiffeners and festeners. Since vacuum bags are used for consolidation rather than precision matched tooling, set-up costs are modest and labor is said to be halved compared with hand lay-up. 

Calculations indicate that a cylinder with a W/D ratio of 0.44 and made of carbon-filament-RPs with an effective RP modulus of 0.1 MPa (15 X 10 psi) would not require a stiffening system. With this W/D ratio an unstiffened cylinder of semi-infinite length would have an elastic buckling depth exceeding 21,300 m (70,000 ft). At a depth of 12,200 m (40,000 ft) a RP stress of 690 MPa (100,000 psi) would be developed. Present problems with high-modulus, carbon-fiber RPs have been the inability of laminates to take high-compressive stresses. 

Form-Rite Plastics, Australia Form-Core for stiffening laminates... 

Out-of-plane joints have been reviewed by Junhou and Shenoi, mainly in the context of marine structures [47]. The most common marine topologies are the monocoque or top-hat stiffened structures, using single skin glass/ polyester or vinyl ester laminates, and the sandwich or double wall structures with a foam core. The authors illustrate six different kinds of out-of-plane joints used in this context with FRP. 

Fiber reinforced thermosetting polymer composites have been used in the form of laminate, sandwich, grid stiffened, stitched, Z-pinned, three-dimensional (3-D) woven fabric, and hybrid structures. In addition to carrying the designed static/dynamic loads, most composite stmcmres experience some kind of low or high velocity impact incidents during their life cycle. A low velocity impact is not uncommon. For example, dropping a tool on to a composite stmcmre... 

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