JEC COMPOSITES MAGAZINE - Issue #48 - May 2009 - 32

FEATURE “Aeronautics” Pushing the envelope in composites design For years, composites design has been based on black aluminium. This approach has not done justice to what composites can really do. As we continue to gain a better understanding of these materials, we should challenge the old rules in order to free designers, so they can apply more imagination and learn to build lighter and stronger composites. triangle), thus reducing the number of suitable laminate combinations from 66 to 36 – a loss of 20 possible laminate combinations to choose from. In addition to this shortcoming, imposing this rule increases the minimum gauge thickness of the laminate, hence posing a considerable challenge in the design and manufacture of composites structures; if the rule of symmetric laminate is imposed, this penalty gets doubled. PRANAV DHOJ SHAH, PHD, RESEARCH ASSOCIATE, DEPARTMENT OF AERONAUTICS AND ASTRONAUTICS, STANFORD UNIVERSITY ue to the high specific strength, corrosion resistance and other superior properties of composites, we have seen a tremendous increase in the use of these materials over metals. This trend has been bolstered by recent advances in analytical, numerical and experimental methods, furthering our understanding of their behaviour and performance. But composite design practices are still confined to the realm of black aluminium (quasi-isotropic laminate). An often cited excuse for such practice has been the directional uniformity of black aluminium, which yields better control over its behaviour. This perception, and the practices stemming from it, significantly underestimate the potential of composites. As a result, the weight and cost savings achieved are considerably lower than would be possible with a smarter design using composites. Now that we understand composites and their performance better, it is time to break through this “black aluminium” constraint and use the directional behaviour of composites to best advantage. For example, in Figure 1, a carpet plot is shown for the effective longitudinal modulus of a carbon/epoxy laminate with percentages of 0°, 90° and ±45° angled plies in the layup. It is apparent that focusing only on black aluminium will severely limit the ability of attaining better performance – in our example here, in terms of the modulus. D Fig. 1: Use of black aluminium limits the full potential of composites [1]. Challenging traditional design rules In conjunction with the use of 0°, 90° and ±45° angled plies, many organizations follow a design practice called the 10% rule, whereby each ply orientation used in a laminate must account for at least 10% of the total plies. As shown in Figure 2, the use of this rule restricts the range of the possible laminates (red Thus, to fully utilize the capabilities of composites, these design rules should be challenged. One example is the use of laminates with two-angle or three-angle ply definition, which opens up a possibility to attain a whole new range of laminate properties. This is illustrated in Figure 3 by carpet plots for the longitudinal modulus [(E)0] and shear modulus (E0) for a three-angle-ply laminate, [0/±φ]φ. Here, it can be noted that a three-angle-ply laminate of [0/±50] has about 1.5 times higher longitudinal modulus while maintaining the same shear modulus, as in the case of black aluminium. Similarly, the strength of a four-angleply laminate can be challenged with a three-angle-ply laminate as depicted in Figure 4. Here, the failure envelopes for a threeangle-ply [π/3] and four-angle-ply [π/4] laminates are shown. It is apparent that the strength plot for the three-angle ply laminate is comparable with that for the four-angle ply laminate. As a matter of fact, at some points the three-angle ply laminate exhibits better strength. Further, it should be noted that the use 32 jec composites magazine / No48 May 2009

JEC COMPOSITES MAGAZINE - Issue #48 - May 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #48 - May 2009