Journal of Composite Materials

This article studied a possible mechanism of a failure from a thick hoop layer of a filament-wound rocket motor case with long cylindrical portion analytically and numerically. The hoop-wound fibers at the cylindrical portion was slightly inclined from the circumferential direction as a finite width prepreg tape was wound without overlapping or gap by alternately feeding in forward and backward direction. Fiber stress concentration in the hoop layer due to possible damage states was assessed through analytical studies and a rough estimate of the stress concentration was given by explicit functions. The possible damage consisted of transverse cracks slightly inclined from each other and debonding of the interface between the hoop layer and the base layer. The approximate analytical solution agreed well with the numerical results in spite of simplification. The results could show the possibility of stress enhancement due to damage in the thick hoop layer made of two slight inclined fibers and important guideline for the design of filament winding pressure vessels.

Many years ago, free edge effects were studied extensively, perhaps excessively, for ordinary tape laminate composites. Various techniques were used to show that the free edge effects extended only a short distance from the free edge and that stacking sequence had a significant impact on interlaminar stresses, which could cause premature failure. In contrast, the analytical study of woven composites has focused on periodic analysis of unit cells. Since such analyses assume an infinite array of unit cells, free edge effects were not considered. This article investigates the significance of traction free surfaces on stresses in woven composites. Effects due to both free edge and free lateral surfaces (due to finite thickness) were investigated. The significance of the free edge effect was found to be sensitive to the waviness of the plain weave mat. Also, the stress distribution is quite different for finite and infinitely thick stacks of woven mats.

The strength of unidirectional composites is essentially dependent on the mechanical behavior of their reinforcement. The variability in mechanical properties of glass fibers has been studied using bundles of about 1400 filaments. This article proposes an experimental study on the fiber fracture strength distributions during tensile test by using acoustic emission (AE) in order to verify individual filament failures. AE monitoring of a bundle of E-glass fibers provides a convenient and relatively quick method to obtain the Weibull parameters of strength distribution. Results show that AE can be used as an effective tool to recognize the failure strength distribution as well as flaw population of fibers.

Composite material usage is necessary on NASA’s future launch vehicles in order to obtain a low mass vehicle. While aircraft and launch vehicles that utilize load-bearing composite components have many similar damage tolerance requirements, the distinct differences between a part that has a lifetime of ∼500 s (one launch) and can be inspected in detail before use and one that has a lifetime of many tens of thousands of flight hours and can only undergo a ‘walk around’ inspection before each flight (commercial transport) needs to be taken into account. This article presents these differences and uses data from the ARES I composite interstage as an example of how to arrive at preliminary compression after impact strength values for the sandwich structure in the acerage of this part using residual strength curves. Results show that if severity of damage can be quantified by a nondestructive method (other than dent depth), the mass of the structure can be reduced due to better characterization of the damage.

In the present work, the elastic behavior of different hypoeutectic and hypereutectic Al-Si alloys and different Al₂O₃ short fiber and SiC particle reinforced materials (SFRM and PRM, respectively) is studied. The effective Young’s modulus (E) of materials was experimentally measured and compared with the different theoretical predictions of Hashin-Strikman, Tuchiniskii, shear lag, and the rule of mixtures (ROM). The unreinforced alloys present an interconnected lamellar Si structure after fast solidification, which increases the Young’s modulus up to that of the Tuchiniskii prediction for interpenetrating skeletal structures. On the other hand, alloys presenting isolated and coarse Si particles (after spheroidization treatment at 540°C) are well described by the lower bound of the ROMs. Similarly, the interconnected Si-SiC structure observed in 10 and 70 vol% SiC reinforced AlSi7Mg and AlSi7 matrices in the as cast condition is responsible for the higher stiffness of the composite, if compared with that of Al99.5 or spheroidized AlSi7 matrices. An analogous behavior is observed in the SFRMs in the as cast condition, where the Si lamellae bridge the Al₂O₃ fibers, increasing the Young’s modulus of the composites, if compared with the conditions of spheroidized Si. Furthermore, the primary Si particles produce an improvement in the Young’s modulus by connecting several fibers in the case of a short fiber-reinforced hypereutectic AlSi18 matrix.

Hydroxyapatite (HA) impregnated poly-(acrylamide) nanocomposites were synthesized using γ-radiations and the prepared composites were characterized by different instrumental techniques. Fourier transform infrared spectroscopy studies revealed the presence of carbonated HA, while the X-ray diffraction analysis showed the mean particle size of HA to be nearly 6.53 nm and width 1.278 nm. The nanosize of the hydroxyapatite crystals were also confirmed by Transmission electron microscopy analysis. The crystallinity of material was found to be 76.10% with interchain separation between 0.1998 and 0.1933 nm for native HA and HA nanocomposites, respectively. Thermogravimetric analysis studies resulted in the thermogram that resembled with that of the natural bone. Scanning electron microscope results revealed the pore size of prepared composite ranging between 50 and 100 µm. The modulus and compressive strength of prepared materials were found to be 726.40 and 39.55 MPa, respectively. Water uptake capacity of the prepared nanocomposites was quantified in terms of swelling ratio and the data were used to calculate network parameters of HA-PAm composites. The in vitro - blood compatibility results confirmed a fair level of blood compatibility of the prepared composites.

This article discusses a simplified approach to analyze mechanical properties of randomly distributed short fiber composites. Mechanical properties of three different randomly oriented short fiber composites, cotton, nylon, and aluminium with vinylester resins, were experimentally investigated. The analytical results were compared with experimental results and a very good correlation was found. Further, the experimental results and the predictions showed that the strength of the composites is less than the strength of the matrix material, for all three composites tested.