Donald Klosterman

Interfacial characteristics of composites fabricated by laminated object manufacturing

Abstract This paper examines interfacial issues that arise when fabricating ceramic (SiC/SiC) and polymer matrix (glass/epoxy) composites using a novel, fully automated rapid prototyping method called Laminated Object Manufacturing (LOM). Discussed in this report are the three types of interfaces that are of concern in composites fabricated with the LOM system: interlaminar, particle–matrix in particulate composites, and fiber–matrix in fiber-reinforced composites. In order to obtain good interlaminar adhesion, it was necessary to address three issues: material preforms, LOM processing parameters, and high-temperature post-processing techniques. Developments in each of these areas led to the successful production of high-performance composites using the LOM process.

Development of a curved layer LOM process for monolithic ceramics and ceramic matrix composites

A novel rapid prototyping technology incorporating a curved layer building style was developed. The new process, based on laminated object manufacturing (LOM), was designed for efficient fabrication of curved layer structures made from ceramics and fiber reinforced composites. A new LOM machine was created, referred to as curved layer LOM. This new machine uses ceramic tapes and fiber prepregs as feedstocks and fabricates curved structures on a curved‐layer by curved‐layer basis. The output of the process is a three‐dimensional “green” ceramic that is capable of being processed to a seamless, fully dense ceramic using traditional techniques. A detailed description is made of the necessary software and hardware for this new process. Also reviewed is the development of ceramic preforms and accompanying process technology for net shape ceramic fabrication. Monolithic ceramic (SiC) and ceramic matrix composite (SiC/SiC) articles were fabricated using both the flat layer and curved layer LOM processes. For making curved layer objects, the curved process afforded the advantages of eliminated stair step effect, increased build speed, reduced waste, reduced need for decubing, and maintenance of continuous fibers in the direction of curvature.

Effect of filler dispersion on the electromechanical response of epoxy/vapor-grown carbon nanofiber composites

The piezoresistive response of epoxy/vapor-grown carbon nanofiber composites prepared by four different dispersion methods achieving different dispersion levels has been investigated. The composite response was measured as a function of carbon nanofiber loading for the different dispersion methods. Strain sensing by variation of the electrical resistance was tested through four-point bending experiments, and the dependence of the gauge factor as a function of the deformation and velocity of deformation was calculated as well as the stability of the electrical response. The composites demonstrated an appropriate response for being used as a piezoresistive sensor. Specific findings were that the intrinsic piezoresistive response was only effective around the percolation threshold and that good cluster dispersion was more appropriate for a good piezoresistive response than a uniform dispersion of individual nanofibers. The application limits of these materials for sensor applications are also addressed.

The influence of the dispersion method on the electrical properties of vapor-grown carbon nanofiber/epoxy composites

The influence of the dispersion of vapor-grown carbon nanofibers (VGCNF) on the electrical properties of VGCNF/Epoxy composites has been studied. A homogenous dispersion of the VGCNF does not imply better electrical properties. In fact, it is demonstrated that the most simple of the tested dispersion methods results in higher conductivity, since the presence of well-distributed nanofiber clusters appears to be a key factor for increasing composite conductivity.PACS: 72.80.Tm; 73.63.Fg; 81.05.Qk

Development of an On-Line, In-Situ, Fiber-Optic Void Sensor

Porosity is a common defect introduced during the molding of composite laminates, particularly with thick, complex, or high-production-rate parts. The deleterious effect of voids on the mechanical properties of composites has been well documented. Clearly, improved process control is needed to minimize voids. To this end, work has been initiated to develop an in-situ sensor for void detection that is nonintrusive, nondestructive, simple, reliable, of reasonable cost and able to withstand harsh curing en vironments. Initial development of an intensity-based fiber optic sensor using refractive in dex changes is summarized. The many factors that affect the sensor response are dis cussed, but the refractive index of the resin is found to be the only non-extraneous factor that affects light guidance in the sensor. Further, it is concluded that changes in the resin degree of cure and temperature can account for changes in the resin refractive index. An experimental approach for sensor calibration and operation is proposed, including description of an image analysis study to relate sensor signal to void size and distribution. A theoretical approach is also proposed that is based on a model for the sensor response given the state of the resin.

Degradation mechanisms of balsa wood and PVC foam sandwich core composites due to freeze/thaw exposure in saline solution

Structural engineers commonly use balsa wood and PVC foam as core materials for sandwich composite structures. These structures are frequently exposed to thermal cycling in sea water. The long-term performance and damage mechanism of these composite sandwich structures under such environmental conditions are still unclear. To simulate these effects, sandwich panels using balsa wood (SB100) and foam core (Airex C70.55) with fiber glass/vinyl ester face sheets were exposed to 100 days of freeze/thaw exposure (−20℃ to 20℃). The freezing and thawing occurred in presence of a saline solution. A total of 150 samples were tested for core shear, core compression, and peel tests. Results confirmed that exposure reduced the balsa wood core shear strength by 14%, compression strength by 36%, and compression modulus by 33%. Interestingly, the PVC foam core shear modulus increased by 25% after exposure, whereas the compression modulus reduced by 12%. Simulated lifetime core shear fatigue curves were developed and evaluated. Additional testing techniques such as scanning electron microscopy, optical microscopy, dynamical mechanical analysis, and X-ray computed tomography were used to rationalize the static and fatigue behavior of the core materials.

Failure mechanism of woven roving fabric/vinyl ester composites in freeze–thaw saline environment

This experimental study investigates the degradation mechanisms of a glass fiber-reinforced plastic material commonly used in civil engineering applications. A substantial reduction in tensile, shear, and compression properties was observed after 100 days of freeze–thaw cycling in saline environment (−20℃ to 20℃). Non-destructive inspection techniques were progressively conducted on unexposed (ambient condition) and exposed (conditioned) specimens. The dynamic mechanical analysis showed permanent decrease in storage modulus that was attributed to physical degradation of the polymer and/or fiber–matrix interface. This indicated the formation of internal cracks inside the exposed glass fiber-reinforced plastic laminate. The 3D X-ray tomography identified preferred damage sites related to intralaminar and interlaminar cracks. The ultrasonic C-scan and optical microscopy showed the nature of the damage and fibers fracture. The thermal cycling events degraded the matrix binding the warp and fill fibers, thus impairing the structural integrity of the cross-ply laminate. The result of this work could benefit a multi-scale durability and damage tolerance model to predict the material state of composite structures under typical service environments.