The Design of Fibrous Composites Having Improved Mechanical Properties

J. G. Morley
1970 Proceedings of the Royal Society A  
P re s e n t-d a y fibrous com posites h av e v e ry a ttra c tiv e lo n g itu d in a l stre n g th a n d stiffness c h aracteristics, b u t th e ir m ech an ical p ro p e rtie s m ea su re d in o th e r directio n s are m u ch less a ttra c tiv e . B esides th e pro b lem of a n iso tro p y th e re is a conflict in th e re q u ire m e n ts o f th e p ro p ertie s of th e in terface since for m a x im u m in te rla m in a r sh ear a n d tra n sv e rs e tensile stre n g th s a stro n g fibre m a
more » ... stro n g fibre m a trix b o n d is re q u ire d w hile for to u g h n ess a n d h ig h v alu es o f w o rk o f fra c tu re a w eak fibre m a trix b o n d is req u ired . T his p ro b lem is discussed a n d m ore com plex com posites c o n tain in g tw o co m p o n en t fibres are considered. T his a p p ro a c h enables a n e x tra set of interfaces to be in tro d u c e d as a m ean s o f m ax im izin g com posite p erfo rm an ce over a ran g e o f failure m odes. G e n e r a l m e c h a n i c a l p r o p e r t i e s o f f i b r e REINFORCED POLYMER COMPOSITES The specific strength and specific stiffness values of unidirectional boron and carbon fibre reinforced synthetic resin composites, measured in the direction of fibre orientation, are about five times greater than orthodox structural metal alloys and further increases in the tensile strengths and Young modulus values of com mercially available carbon fibres can be expected. However, the transverse and shear properties of unidirectional composites are very poor compared with the properties measured in the direction of the fibre alinement. Transverse tensile and shear strengths are typically about 10 000 lbf in-2 (69 MN m -2), while longitudinal tensile strengths of about 300 000 lbf in-2 (2 GN m-2) are attainable. Transverse stiffness and longitudinal shear stiffness values are controlled primarily by the m atrix properties and again show a similar level of anisotropy being of the order of 10 % of the longitudinal Young modulus of the composite (Adams & Doner 1967a, h). The relatively low shear strengths present problems where it is necessary to join one structural member to another and indeed some care has to be taken with the design of a simple unidirectional test specimen if premature shear failure near the grips where the load is being fed into the material is to be avoided. The low failing strain of fibre composite materials, compared with metals, limits the am ount of energy which can be absorbed and the anisotropy in strength values is a dis advantage where there are complex stress distributions such as are generated by impact loading. Fibrous composite materials can be made more isotropic, in two dimensions, by distributing the fibres in a number of thin laminations and arranging for the [ 117 ]
doi:10.1098/rspa.1970.0169 fatcat:cbtovo4c7jftrhidh6utt65jxm