Aramid fractures

Brown, J.R., P.J.C. Chappell, Z. Mathys (1991), Plasma surface modification of advanced organic fibers part I. Effects on the mechanical, fracture and ballistic properties of aramid/aramid composites. J. Mater. Sci. 26. 4172 178,... 

Types 1 and 4 tensile failures, which are of less interest to conservationists at this time, are caused by brittle crack propagation and by long axial splits, respectively. Such fractures can occur in ceramic and elastomeric fibers (both Type 1) and highly oriented aramids (Type 4). 

Kevlar 29 and versions thereof (K 129 and Kl j.) are also used extensively in lightweight body armor as well as composite liners (with vinylester, polyester or epoxy as the matrix). A quick look at the properties of different Kevlar aramid fibers in Table 4.2 shows why K29 is better than K49 for lightweight body armor applications. K29 has a higher strain to failure than K49. That means that the total work of fracture, i.e. the area under the stress-strain curve, is larger for K29 than K49. Hence, the energy absorbed in the fracture process is higher for K29... 

Fracture Fatigue Axial compression Aramids HMPE Shear splitting High modulus High tenacity... 

In view of a comment on fracture to be mentioned later, I will contrast in Fig. 1 two chemical types the para-aramid of Kevlar and Twaron and one of the experimental polyamide-hydrazide X500 fibres made by Monsanto, whose lack of commercial utility led to the disclosure of extensive technical detail in Black and Preston (1973). A critical difference is the greater number of -CO.NH- groups in the Monsanto polymer, which will give stronger intermolecular bonding. 

An early view of fracture of para-aramid fibres was given by Yang (1993, p. 97), who refers to three basic forms. The caption to his fig. 3.28 describes fracture morphology of Kevlar aramid fibre in tensile breaks as Type (a), pointed break type (b) fibrillated break type (c) kink-band break. The kink-band breaks, which extend over a length approximately equal to a fibre diameter can be attributed to fibres that have been weakened by axial compression and will be discussed in a later section. 

Konopasek, L. and Hearle, J.W.S. (1977) The tensile fatigue behaviour of para-oriented aramid fibres and their fracture morphology. J. Appl. Polym. Sci., 21 2791-2815. 

Usually aramid composites yield somewhat in compression in a bending test before they finally fracture in shear. This also has a more pronounced negative effect on the SBS values of composites with treated than on those with untreated fibres. Application of the compression correction as mentioned in [7] reduces the intercept of eq. (5) further and leads to an even closer agreement with eqs. (1) and (3). Compressive yielding, and hence the correction for this effect is less for beams with... 

The 3p-SBS of untreated aramid composites decreases with increasing V-. The absolute shear strength is much lower than for carbon or glass. This must be caused by the weak aramid-epoxy interface, since the SCF is very low for aramid in epoxy. On the assumption of a weak interface this decrease may also be explained by the rule of mixtures, with the exception of the strong fall at =80 %. This sharp decrease is a good indication that fracture occurs at the interface. 

Fracture surface analysis shows that the transverse crack runs straight through the adhesion treated filaments while it circumvents the untreated filaments. Further improvement in the off-axis strength of aramid-epoxy composites seems unlikely since it is limited by the shear and transverse tensile strength of the aramid fibre itself. 

Figure 5.6 Scanning electron micrograph of aramid fibre-reinforced polyamide 6,6 fracture surface. Compound was prepared in a co-rotating twin-screw extruder.

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