Joel S. Fenner

A Constitutive Model for Fiber-reinforced Polymer Composites

A constitutive model for fibrous polymer composites was established based on an elastic—plastic approach. The proposed potential function is a linear combination of functions related to deviatoric and dilatational deformations. A unidirectional carbon/epoxy composite, AS4/3501-6, and a woven-glass/vinylester composite were fabricated and tested under quasi-static off-axis tension and compression and used in developing and verifying the model. Model parameters were determined from the off-axis test results. Stress—strain curves predicted by the model were in good agreement with experimental results. The constitutive model established can describe the nonlinear orthotropic mechanical behavior of the composites, and it accounts as well for their tension—compression behavior.

Characterization and Constitutive Modeling of Composite Materials under Static and Dynamic Loading

Composite materials were characterized under quasi-static and dynamic loading and a constitutive model was adapted to describe the nonlinear multi-axial behavior of the materials under varying strain rate. The materials investigated were unidirectional glass fiber/vinylester, and carbon fiber/epoxy composites. Multiaxial static and dynamic experiments were conducted using off-axis specimens to produce stress states combining transverse normal and in-plane shear stresses. Stress-strain curves were obtained for various loading orientations with respect to the fiber direction at three strain rates, quasi-static, intermediate and high strain rate. A nonlinear constitutive model is proposed to describe the rate-dependent behavior under states of stress including tensile and compressive loading. Experimental results were in good agreement with predictions of the proposed constitutive model.

Fracture Toughness and Fatigue Behavior of Nanoreinforced Carbon/Epoxy Composites

In this study, the objective was to develop, manufacture, and test hybrid nano/microcomposites with a nanoparticle reinforced matrix and demonstrate enhancements to damage tolerance properties in the form of fracture toughness and fatigue life. The material employed was a woven carbon fiber/epoxy composite, with multi-wall carbon nanotubes (CNT) as a nano-scale reinforcement to the epoxy matrix. A direct-mixing process, aided by a block copolymer dispersant and sonication, was employed to produce the nanoparticle-filled epoxy matrix. Initial tests were performed on cast epoxy sheets (neat and with nanotubes) to determine effects of nanotubes on the matrix alone. Specimens were tested in Mode I three point bend, showing a 20 % increase in critical stress intensity factor K for nanotube-filled epoxy over neat resin. Woven carbon fiber performs were then infused with epoxy (neat and with nanotubes) by a wet layup process to produce flat composite plates. Composite specimens cut from these plates were subjected to Mode I double cantilever beam (DCB) tests (straight and tapered) showing nearly a 200 % increase in Mode-I fracture toughness G for nano-reinforced composite over reference composite. Fatigue tests were then performed on the woven carbon fiber composite in the form of cyclic short-beam three point bend to produce interlaminar shear fatigue. Stress-life curves obtained from cyclic short-bearm three point bend showed an increase of more than an order of magnitude in cyclic life at a given cyclic load between reference and nano-reinforced composite. Fatigue-fracture tests were performed on interlaminar Mode-I tapered double cantilever beams to produce Mode-I interlaminar fatigue-crack growth. The results of cyclic interlaminar Mode-I testing showed a much lower crack growth rate for nano-reinforced composite than for reference material. SEM micrographs of failed specimens also showed significant differences in fracture surface morphology between nano-reinforced and reference composite.

Mechanical and failure behavior of composite materials under static and dynamic loading

Composite materials were characterized under quasi-static and dynamic loading and failure theories were developed/expanded to describe static and dynamic failure under multi-axial states of stress. The materials investigated were unidirectional glass fiber/vinylester, and carbon fiber/epoxy composites. Multi-axial static and dynamic experiments were conducted using off-axis specimens to produce stress states combining transverse normal and in-plane shear stresses. A Hopkinson bar apparatus was used for multi-axial characterization of the above materials at high strain rates. Quasi-static and dynamic failure envelopes were obtained by the various available failure theories including the recently introduced Northwestern (NU) theory and compared with experimental results. The NU theory was extended to the dynamic loading regime and was shown to be in excellent agreement with experimental results.

Testing the 2-3 Shear Strength of Unidirectional Composite

In this study, the objective was to measure the “out-of-plane” 2-3 shear strength of unidirectional composite, working within constraints in supplied material geometry. Unidirectional carbon/epoxy composite material was tested using a sandwich-type beam specimen under 3-point bending with a low span-to-thickness ratio to achieve failure under 2-3 shear. Specimens were carefully designed to deliberately cause shear failure near the midplane, avoiding other possible failure mechanisms. A photoelastic coating and post-mortem microscopy were used to verify failures. Results were compared with a simple analytical description of failure and found to have good agreement. Notably, this approach was able to accommodate the limitations of the supplied material (thin sheets) while still providing an accurate means of obtaining the F23 shear strength of the material. The results also imply the possibility of testing the transverse tensile strength (F2t) in lieu of performing a shear test, which is far simpler, and inferring the out-of-plane shear strength F23.

Mixed-Mode and Mode-II Fatigue Crack Growth in Woven Composites

A woven carbon/epoxy composite was subjected to fatigue crack growth under mixed Mode-I/Mode-II loading to obtain crack growth behavior at different cyclic strain energies. Owing to the woven structure of the material, pure Mode-II fracture is usually a difficult proposition because of friction, interference, and interlock of woven tows in adjacent plies at an interlaminar crack. These limitations were overcome by the use of a novel form of mixed Mode-I/Mode-II specimen, which imposes sufficient crack surface opening (Mode-I) to alleviate ply–ply interactions, but not so much as to obscure the sliding (Mode-II) response. Comparison with pure Mode-I fatigue crack growth data, in conjunction with a fracture interaction criterion, provided a means to extract the Mode-II behavior.

Mechanical Characterization and Modeling of Ceramic Foam Materials

The materials investigated were silicon carbide foams of various densities ranging from 7.7 % to 12.3 % of the bulk material density. They were characterized under pure shear and uniaxial compression. Special test procedures were developed for this testing. For shear characterization two pairs of prismatic strips were used in a three-rail fixture. Stress–strain curves to failure were obtained from which the shear modulus, shear strength and ultimate shear strain were determined. A statistical analysis based on the Weibull distribution function was conducted to determine expected differences in results obtained by different test methods, specifically differences between three-rail shear and torsion test results. A power law model was proposed to describe the variation of shear modulus with relative density. It was also shown that the parameters of this model depend on the porosity structure of the foam for the same density. Similar tests were conducted under uniaxial compression. It was found that the Young’s modulus varies linearly with the relative density of the foam.