SHAPE FORMING
Confidence in COMPOSITES
The rise in composites use on aircraft has not been without its challenges, as Dr Hao Cui from the Centre for Aeronautics at Cranfi eld, explains
performance and fuel effi ciency of aerospace products over the past few decades. Carbon fi bre reinforced polymer (CFRP) composites have been extensively used in the new generation of commercial airliners including Boeing 787 and Airbus A350, thanks to their higher strength and stiff ness per unit weight when compared to aluminium that was the primary aerospace material in the past. However, the nature of how CFRP was designed and manufactured has brought in many challenges in practice, and the legacy of metallic design experience is just not applicable due to the diff erence in its mechanical properties. The deformation and failure
A
processes of CFRP materials are largely dependent on the way they are loaded. The fi rst type of the failure is dominated by the polymer matrix, where intra-laminar cracks aligned with fi bres may split laminates transversely, or inter-fi bre cracks, also called delamination, may separate
dvanced lightweight materials and structures have largely contributed to the improved fl ight
the laminates. This damage can be caused by static loads that exceed the allowable design value, while in many scenarios this is caused by impact from some foreign object. For example, delamination cracks can be initiated on the upper skin of wings by impact from falling tools during maintenance work, which is very challenging to visually detect from the surface. Such inter- laminar cracks propagate under cyclic loading conditions, reducing the critical buckling load that potentially leads to catastrophic failure.
The second type of failure is fracture in the fi bres,
including fi bre rupture when loaded in tension and fi bre kinking under compressive stress, which usually results in considerable loss of structural loading capacity. Although carbon fi bres technically feature high tension/ compression strength, their measured strength on practical structures can be much lower, and the material is very sensitive to defects that inevitably occur during the manufacturing process.
CURRENT PRACTICE The aforementioned failure modes of CFRP need to be properly studied for its industrial application. A building
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