John F. Fellers

Energy Absorption in Polymer Composites for Automotive Crashworthiness

The energy absorption capability of a composite material is critical to developing improved human safety in an automotive crash. Energy absorption is dependent on many parameters like fiber type, matrix type, fiber architecture, specimen geometry, processing conditions, fiber volume fraction, and testing speed. Changes in these parameters can cause subsequent changes in the specific energy absorption (ES) of composite materials up to a factor of 2. This paper is a detailed review of the energy absorption characteristics in polymer composite materials. An attempt is made to draw together and categorize the work done in the field of composite energy absorption that has been published in the literature in order to better understand the effect of a particular parameter on the energy absorption capability of composite materials. A description of the various test methodologies and crushing modes in composite tubes is also presented. Finally, this paper raises certain design issues by examining the work rate decay necessary to keep the deceleration below 20g during an impact crash.

Rheo-optics of shear and elongational flow of liquid cystalline polymer solutions: hydroxypropyl cellulose/water and poly-p-phenylene terephthalamide/sulfuric acid

Abstract An experimental study is reported of the quiescent and flow birefringent characteristics of two liquid crystalline polymer solution systems, poly-p-phenylene terephthalamide (PPD-T) in sulfuric acid and hydroxypropyl cellulose (HPC) in water over a range of concentrations. It is shown that for the quiescent state the dilute solutions are optically isotropic while the concentrated solutions consists of negatively birefringent domains. During flow at low deformation rates, moving domains are still seen. At higher deformation rates, a homogeneous highly birefringet fluid is obtained. The birefringence increases with concentration at constant deformation rate and exhibits a major increase as the liquid crystalline state is formed. The source of the birefringence is due to (1) anisotropy of polarizability of oriented macromolecules, and (2) difference in refractive index of solvent and oriented macromolecules (form birefringence). The results are interpreted in terms of the level of polymer orientation which may be developed in flow for liquid crystalline polymer solutions as compared to solutions of flexible macromolecules.

Phase behavior and structure of liquid crystalline solutions of cellulose derivatives

SummaryStructural and thermodynamic characteristics of liquid-crystalline solutions of four cellulose derivatives in a range of solvents were studied. Basic observations were made on these systems using polarized light microscopy, small angle light scattering, dilute solution and concentrated solution viscosities. The polymers studied include hydroxypropyl cellulose (HPC), cellulose acetate butyrate (CAB), ethyl cellulose (EC), and cellulose triacetate (CT). The formation of the liquid crystalline phase was shown to strongly depend on polymer concentration, solvent type and temperature. The critical volume fraction of polymer required to form the liquid crystal phase varied significantly as the solvent changed. The critical volume fraction decreased with increasing solvent acidity and polymer intrinsic viscosity in a given solvent. The breadth of the two phase region seems to decrease with increasing acidity. The liquid crystalline phase was in most cases determined to be cholesteric. In all cases positively birefringent cellulose derivatives form negative spherulitic domains. In one case, the negativity birefringent system (cellulose triacetate) formed positively birefringent spherulitic liquid crystalline domains. This is interpreted to mean the structure organizes itself by a tangential alignment of polymer chains within the domain. SALS measurements appear to detect domains and in some cases cholesteristic pitch.

X‐ray scattering studies of graphite fibers

The structural features of three different graphite fibers were studied via small‐ and wide‐angle x‐ray techniques. The experimental evidence is consistent with a sheath/core fiber morphology. Graphitization, degree of orientation, crystallite size, and microporosity were analyzed. Samples included low (AS4) and high (HMS) modulus poly(acrylonitrile) (PAN) and melt‐spun pitch‐based (VSB‐16) fibers. By wide‐angle x‐ray diffraction (WAXD) VSB‐16 was found to have the highest degree of graphitization, the highest degree of orientation, and the largest crystallite regions, and AS4 the poorest graphitized structure. The void system in these graphite fibers was investigated by small‐angle x‐ray scattering (SAXS). SAXS from glycerin‐soaked fibers indicates the scattering at very small angles (2θ<10 mrad) is dominated by total reflection of x rays at the fiber surface. The pores in HMS and VSB‐16 fibers are inaccessible to glycerin and the pores in AS4 fiber are partially accessible. The pores in PAN‐based HMS an...

A Dynamic Small Angle X-Ray Scattering Study of Stressed Kevlar® 49/Epoxy Composites

The failure mechanisms of Kevlar® 49 fibers, epoxy resin and uniaxial Kevlar® 49 fiber reinforced epoxy composites under tensile loads have been studied using small angle x-ray scattering (SAXS) and scanning electron microscopy (SEM). The behavior of specimens of uniaxial Kevlar® 49 fiber reinforced epoxy with various angles between the fiber direction and the tensile load axis was examined using an Instron Mechanical Tester. SAXS showed the Kevlar® 49 fibers fail due to increases in the volume fraction of microvoids and enlargement of larger microvoids along the fiber axis direction. These changes in microvoids arise from the effect of Poissons ratio being less than 0.5. Hydrolytic degradation of Kevlar® 49 fibers leads to roughening of the surfaces and decreases in the mass densities. The epoxies and the composites failed in a catastrophic dynamic process, the crack originating from surface flaws or air bubbles entrapped in the epoxies during the curing process. The SEM investigations on the failed composites revealed fiber-splits and fiber-pullouts. The results of mechanical tests showed the composite moduli, the composite ultimate strengths and the elongations at break decrease as the angle between the fibers and loading axis increase. The maximum work theory provides a good fit with the experimental ultimate strength results of the com posites.

Time-dependent nonlinear deformation behaviour of glassy polymers

Abstract This paper develops a generalized nonlinear Maxwell model that is capable of treating glassy polymer deformation under diverse kinematics, including creep. The nonlinearization of the constitutive equation is accomplished by incorporation of a Williams-Watts relaxation element for the viscous dissipation. This equation describes the relaxation behaviour in polymer glasses over four orders of magnitude in time with the addition of only one new rheological parameter β. The parameter β is an exponent to the time variable and typically ranges between 0·02 and 0·2 for these materials. Decreasing molecular weight and/or increasing temperatures tends to increase the magnitude of β. The time constant τ ranges over many orders of magnitude and is generally inversely related to β. The physical significance of β is linked to the relative magnitude of short- versus long-term relaxation behaviour and can therefore be correlated to the creep and impact behaviour of materials. Finally the generalized Maxwell mo...

Processing and Characterization of Cellulose Triacetate Films from Isotropic and Liquid Crystalline Solutions

Cellulose triacetate (CTA) films have been prepared by extrusion of various liquid crystalline and isotropic CTA solutions in a mixture of dichloroacetic acid (DCA) and formic acid (FA) through an annular die. The extrudate was either given a uniaxial drawdown into a coagulation bath or biaxially deformed via expansion with recirculating coagulant to the inside of the extrudate. Thus solutions with CTA concentrations ranging from 23 to 30% were extruded as uniaxially‐ and biaxially‐oriented films. A survey of solvent systems to produce liquid crystalline CTA showed DCA or DCA/FA to be the only reasonable choices from among the solvents tried. Rheological characterization showed that CTA/DCA/FA could be processed in the 23 to 30% concentration range. A maximum in the viscosity vs. concentration curve, plastic flow stresses, and viscosity‐shear rate behavior typical of liquid crystalline systems were observed. At 60°C, the system reverts to isotropic and provides for some comparisons between isotropic and a...