Aramids compressive strength

According to the data in Table 25.5 and to Eq. (25.6) the compressive strength of filaments of refractory materials such as carbon and silicon carbide have compressive strengths about 10 times as large as those of organic fibres. This would seem to be a serious restriction to the use of organic polymers such as aramids in their application in composites. For most of the applications this restriction is of minor importance, however, since long before ac max is reached, instability in the construction will occur. The resistance of a column or a panel under pressure is proportional to the product of a load coefficient and a material efficiency criterion ... 

Whereas the compressive strengths of carbon and aramid differ by a factor 10, the material efficiency criterion of the composite differs less than 20%. 

The main disadvantages of the aramid fibers include high moisture absorptivity, low compressive strength, and difficulty in cutting and machining of the resulting composites. 

Aramid belongs to the family of polyamides, but the bonding of matrix (resin) appears to be more difficult than on PA6 (nylon). In order to improve the behavior of the fiber, it is treated with finishing substances. Nevertheless, the compressive strength reaches only 25 % of the tensile strength. 

It appears that the increase in compressive strength of plain HP-PE composites incorporating treated fibres is caused by the change in failure mode. It seems likely that compressive failure processes in the fibre itself are strongly determined by the low shear modulus and shear strength. These failure processes are comparable with those found in more detailed studies for aramid fibres (6). 

Although the uses of ceramic fibres in composite structures lie mainly in ceramic-matrix and metal-matrix composites, where their outstanding chemical and thermal resistance are important, there are a few applications in organic polymers. Their relevant properties are low thermal expansion, low electrical conductivity, low dielectric constant, high stiffness, good compressive strength, and in most cases complete resistance to combustion. On the other hand they are very brittle, hard to process, and mostly considerably more expensive than carbon and para-aramid fibres. They have, for example, been used in hybrid structures with carbon and para-aramid and in electronic circuit boards. The fibres available or potentially available include alumina, combinations of alumina with... 

These fibres are representative of the high performance aramid products currently available. The anisotropy of unidirectional composites is apparent as is the poor compression strength of aramid composites. The composite densities are extremely low. 

The integration of various functional fillers is a traditional route to achieve highly wear-resistant polymer composites. In order to reduce the adhesion of polymers to metallic counterparts, internal lubricants, such as PXra powders and graphite flakes, are frequently incorporated. Short aramid (AF), glass (GF) or carbon (CF) fibers are used to increase the creep resistance and the compressive strength of the polymer matrix. Not so many efforts have been undertaken so far in the development of such composites by integrating inorganic particles with conventional fillers. 

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