Steven J. DeTeresa

Enhancement of strength and stiffness of Nylon 6 filaments through carbon nanotubes reinforcement

We report a method to fabricate carbon nanotube reinforced Nylon filaments through an extrusion process. In this process, Nylon 6 and multiwalled carbon nanotubes (MWCNT) are first dry mixed and then extruded in the form of continuous filaments by a single screw extrusion method. Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies have indicated that there is a moderate increase in Tg without a discernible shift in the melting endotherm. Tensile tests on single filaments have demonstrated that Young’s modulus and strength of the nanophased filaments have increased by 220% and 164%, respectively with the addition of only 1wt.% MWCNTs. SEM studies and micromechanics based calculations have shown that the alignment of MWCNTs in the filaments, and high interfacial shear strength between the matrix and the nanotube reinforcement was responsible for such a dramatic improvement in properties.

Delamination Failure Investigation for Out-Of-Plane Loading in Laminates

In contrast to failure approaches at the lamina level or the micromechanics level, the present work concerns failure characterization at the laminate level. Specifically, attention is given to the ultimate failure characterization for quasi-isotropic laminates. This is in further contrast to the commonly used approaches for initial damage or progressive damage. It is shown that the analytical failure forms decompose into two modes, one for out-of-plane, delamination-type failure and one for in-plane, fiber-controlled-type failure. The work here is mainly given over to the delamination mode of failure. Experimental results are presented for laminates in this mode of failure. These results are then integrated with the analytical forms to give a simple criterion for delamination failure.

The Effects of Through-thickness Compression on the Interlaminar Shear Response of Laminated Fiber Composites

The effects of through-thickness compression on the interlaminar shear response of laminated fiber composites were studied. The combined stresses were generated using a hollow cylindrical specimen that was subjected to axial compression and torsion. For both glass- and carbon-fiber composites, through-thickness compression resulted in a significant enhancement in the interlaminar shear stress and strain at failure. Under moderate compression levels, the failure mode changed from elastic to plastic. An attempt was made to predict the observed increase in shear strength for carbon fiber epoxy laminates using three-dimensional lamina failure criteria. Although all the failure theories correctly predicted the trend of increasing shear strength with compression, none were able to predict the full extent of the observed strength increase. These results indicate that improved models are needed for determining failure under a combined state of interlaminar stress. The experimental results demonstrate that there are significant gains to be made in improving interlaminar strengths of composite structures by applying through-thickness compression. This effect could be exploited for improved strength and possibly improved fatigue life of composite joints and other regions in structures where interlaminar stress states are critical.

Piezoresistivity and failure of carbon filaments in axial compression

Abstract A novel technique for axial compression of single carbon filaments is introduced. Single filaments were bonded to the surface of polymer compression specimens in a manner analogous to the mounting of metal-foil strain gages. By measuring the fiber piezoresistive behavior, filament compression could be monitored. In compression, the resistance of the filaments decreased nonlinearly and reached a minimum value just prior to failure. This nonlinear decrease in resistance with compression was found to be reversible up to strains of −3%. Filament failure was marked by an abrupt increase in resistance, and average ultimate compressive strains were −3% or greater for all fiber types. Because these failure strains are approximately double those measured for high-fiber-content composites in longitudinal compression, it was concluded that the compressive strength of composites fabricated from the fiber ekamined is not limited by the strength of the fiber and that a large potential exists to improve composite strength.

Failure criteria for isotropic materials, applications to low-density types☆

Isotropic failure criteria are derived in a general form that applies to both homogeneous materials and to porous, low-density materials. Specific applications are made to closed-cell foam materials, with the properties type parameters determined from experimental data. It is found that in addition to the usual yielding type of failure behavior, a fracture type behavior can arise under certain tensile stress conditions and must be taken into explicit account in forming the failure criteria for low-density materials.

Reduction in the number of independent parameters for the Tsai-Wu tensor polynomial theory of strength for composite materials

It is shown that the number of required parameters for the Tsai-Wu tensor polynomial strength criterion for fiber composites can be reduced from seven to five for composite materials that do not fail under practical levels of either hydrostatic or transverse pressure. For these materials, the interactive strength parameters can be defined in terms of their commonly measured uniaxial or noninteracting strength parameters, thereby eliminating the need to conduct combined stress tests. The derived parameters are given by F 12 = − F 11/4 F 23 = − F 22 These parameters fall within the stability limits of the theory, yet they lead to open failure surfaces in the compressive stress quadrant. The assumptions used to derive the interactive parameters were supported by measurements that showed typical carbon fiber composites did not fail under significant levels of hydrostatic pressure or unequal transverse compression. Comparison with previous work where the interactive parameters had been determined from combined stress experiments showed that predictions based on the derived parameters were generally in good agreement with test results. In evaluating the tensor polynomial failure theory, the behaviors under hydrostatic and transverse pressure were shown to be more discerning than other multiaxial stress states. It is therefore proposed that these two stress states be used in assessing other proposed three-dimensional failure models for fiber composites.

The Kink Band Mechanism for the Compressive Failure of Fiber Composite Materials

A simple strain-based yield/failure criterion for fiber composite materials is incorporated into a kink band analysis of compressive failure. Under realistic conditions of fiber misalignment the analysis predicts compressive failure at load levels about one-fifth of the ideal value, and with kink band inclinations of about 20 deg. Some parametric variations of the relevant physical variables are given in simple graphical forms, and comparisons are made with newly obtained micrographs of kink bands.

Failure plane orientations for transverse loading of a unidirectional fiber composite

Using a recently developed failure theory for transversely isotropic fiber composites, it is shown how the orientation of the failure surface can be determined for transverse tension and compression. It is also shown that failure surface orientations decompose into those of ductile type versus those of brittle type. Experimental data on failure surface orientations have been obtained for carbon fiber composite systems based on both thermoplastic and thermosetting matrix materials. Average compression failure planes for the different composite materials were measured to range from 31° to 38° from the load axis. Reasonable agreement was obtained between these measured angles and those predicted from application of the new failure theory.

Elimination/minimization of edge-induced stress singularities in fiber composite laminates†

Abstract The stress singularities along the edges of a linear elastic laminate consisting of 0° and off-axis plies in uniaxial stress are shown to either vanish or be minimized by a special fiber orientation for the off-axis plies. The particular orientation is obtained analytically using both a recently proposed specific three-dimensional constitutive equation and the more general constitutive theory, but for fiber-dominated lamina. The prescribed angle is defined by the simple relationship: 0 * = tan - 1 ( 1 v L ) , where v1 is the longitudinal Poissons ratio of the lamina. Tensile test data arc consistent with the theoretical result.

Characterization of Static- and Fatigue-Loaded Carbon Composites by X-Ray CT

The development and improvement of advanced materials is strictly connected to the understanding of the properties and behavior of such materials as a function of both their macro and micro-structures. The application of X-ray computed tomography (CT) to these materials allows for a better understanding of the materials properties and behavior on either macro or micro-structure scales. The authors applied CT to study a set of aerospace grade carbon fiber/thermoplastic matrix composites. Samples of APC-2 (PEEK/AS4) were subjected to either static or high-stress fatigue loading in tension. Both notched (central circular hole) and unnotched specimens were examined. They are investigating a high-temperature thermoplastic polyimide composite sample by acquiring CT data sets before, during (at set intervals), and after full-reversal (tension-compression), low-stress fatigue loading at the upper use temperature. The CT scanner employed and the results obtained in the analysis of 3D CT data sets to study the defects and other features within the different composites are presented in this report.