James J. Stone

Improvement of Interfacial Adhesion between UHMWPE Fiber and Epoxy Matrix Using Functionalized Graphitic Nanofibers

Ultra high molecular weight polyethylene (UHMWPE) fiber has advanced mechanical properties and excellent physical properties that include high effectiveness for cosmic radiation shielding, which is valuable for outer space missions. Unfortunately, poor interface in UHMWPE fiber—polymer composites hinders the structural integrity and safety, and restricts its effectiveness for the radiation shielding. In this study, we focus on the improvement of the adhesion property between the UHMWPE fiber and resin matrix by developing a nano-epoxy matrix through making a modification on the matrix with reactive graphitic nanofibers (r-GNFs). The interfacial adhesion between UHMWPE fiber and the nano-epoxy matrix with low concentrations of r-GNFs (0.15, 0.30, and 0.50 wt% ) is characterized by analyzing load—displacement curves from microbond tests. Study results show that crack initiation force, debonding force, debonding energy, and friction energy from the microbond tests increase considerably, due to the effective improvement of interfacial adhesion property by the nano-epoxy matrix. One-dimensional shear-lag analysis is applied for theoretical calculations. Calculated results on ultimate adhesive strength and external stress show agreement with the experimental results. Study results reveal that the nano-matrix with 0.30 wt% of r-GNFs is the most effective to improve adhesion properties between UHMWPE fiber and the epoxy.

Effect of residual stresses on fatigue crack growth behavior of aluminum substrate repaired with a bonded composite patch

Composite patches bonded to cracked metallic aircraft structures have been shown to be a highly cost-effective method for extending the service life of the structures. The fatigue crack growth behavior of pre-cracked 7075-T6 aluminum substrate with the 12.7-mm V-notch crack repaired with boron/epoxy composite patches was investigated. 1-ply, 2-ply, 3-ply and 4-ply composite patches were studied. The residual stresses due to mismatch of the coefficients of thermal expansion between the aluminum plate and boron/epoxy composite patch were calculated based on the classical equation. The effects of the residual stresses and patch layers on fatigue lifetime, fatigue crack growth rate, and fatigue failure mode of the repaired plates were examined experimentally. A modified analytical model, based on Roses analytical solution and Paris power law, was developed for this research. This model considered the residual stress effect and successfully predicted the fatigue lifetime of the patched plates. Results showed that the composite patch had two competing impacts on the structure. The composite patch could cause residual tensile stress in the aluminum substrate, which could consequently increase the crack growth rate. Moreover, reinforcement with the composite patch could also retard the crack propagation in the aluminum plate. If a 4-ply composite patch was used, it resulted in high residual stresses and effectively would not extend the fatigue lifetime of cracked aluminum plates.

Drop testing and finite element simulation of stacked chip scale packages with and without underfill

Drop testing is performed on stacked chip scale packages in eight configurations, including the use of two types of commercially available underfills. Full failure analysis using techniques such as dye penetrant and scanning electron microscopy (SEM) is performed. Corresponding explicit finite element simulations are performed using ANSYSreg LS-DYNA. These simulations are used to determine a suitable damage parameter and consequently, drop test life correlations are constructed. Considerable differences in drop impact reliability between Sn63Pb37 and SAC305 solder are observed.

The effect of patch geometry on the static and fatigue behaviors of defective aluminum panels repaired with a composite patch

In this study, 6.125-mm cracked Al 7075-T6 plates unpatched and repaired with 4-ply boron/epoxy composite patches of several geometries have been investigated under both static and fatigue loads. The stress distributions around the crack tip for these specimens were calculated using the finite element method. It was shown that Roses model was not adequate to calculate the stress intensity factor for different patch geometries where patch dimensions were on the order of those of the cracked structure. A new definition based on the stress near the crack tip was introduced. Also, based on the experimental data, a new definition for relative repair efficiency was introduced, and the effects of patch geometry on the static tensile and fatigue behaviors of the repaired structures were examined experimentally. Combining the results of static tensile and fatigue tests, it was concluded that the geometry of the patch had large effects on the properties of the repaired structures, the effects not included in Roses model.

Stress Analysis of Pin-Loaded Composite Plates

The frictional contact of the pin-loaded joint in composite plates was studied. This included the effects of pin clearance and variations in material and geometry. Full-filed displacements were measured by high sensitivity moire interferometry. Considerable effort was expended to develop a loading frame, relevant fixtures and monitoring capability to ensure that the plate was loaded uniformly through its thickness, particularly at the pin-loaded hole. Numerical techniques were prepared for processing the optical fringe data. A reliable finite element model for a bolted joint was also formulated. The efficient finite element program, which is capable of handling friction and/or clearance at the loaded hole, has been validated analytically, experimentally and numerically.Copyright

Thermo-mechanical simulation of stacked chip scale packages with moire interferometry vaildation

Moire interferometry and finite element (FE) analysis are used to quantify the deformation of stacked chip scale packages under thermal and accelerated thermal cycling loads. Basic thermo-elastic material property measurements are made of the constituent materials and found to be in good agreement with published values. Viscoplastic FE-based solder joint fatigue simulations indicate good reliability for several common design configurations of stacked packages

Characterization of Tissue Scaffolds Fabricated by Rapid Prototyping Techniques Aided by Finite Element Analysis

Numerous techniques for fabricating tissue engineering scaffolds have been proposed by researchers covering many disciplines. While literature regarding properties and efficacy of scaffolds having a single set of design parameters is abundant, characterization studies of scaffold structures encompassing a wide range of design parameters are limited. A Precision Extrusion Deposition (PED) system was developed for fabricating poly-e-caprolactone (PCL) tissue scaffolds having interconnected pores suitable for cartilage regeneration. Scaffold structures fabricated with three-dimensional printing methods are periodic and are readily modeled using Computer Aided Design (CAD) software. Design parameters of periodic scaffold architectures were identified and incorporated into CAD models with design parameters over the practical processing range represented. Solid models were imported into a finite element model simulating compression loading. Model deformation results were used to identify apparent modulus of elasticity of the structure. PCL scaffold specimens with design parameters within the modeled range were fabricated and subjected to compression testing to physically characterize scaffold modulus. Results of physical testing and finite element models were compared to determine effectiveness of the method.Copyright

Analysis of finger pip implants

Total joint arthroplasty (TJA) is implemented primarily for the relief of pain, and secondarily for achieving better function by increasing the joint’s strength and motion. In order to keep health costs low, it is desirable that the TJA achieves and maintains a long-term and secure fixation of the implanted components. Unfortunately, clinical follow-up shows that the prosthetic finger implant components have long-term complications including bone resorption, wear, loosening, and failure of the implant components. Although the mechanism of complications is not fully understood, it is well known that the wear and failure of prostheses are highly related to the mechanical forces or stresses of implant components. It is therefore desirable that reliable 3-D computational finite element analysis (FEA) models can be developed and used for the stress analysis of implants. In this study, the finger proximal interphalangeal (PIP) prosthetic components were analyzed using a nonlinear finite element method. Implant components under different joint flexion angles as well as different forces were studied. The stress distribution on the contact surface of the implant component was obtained. The developed FEA models can be used to examine the contact situations (contact stress, contact region, and stress distribution), which are critical to the wear and potential failure of the implant components. Based on FEA results, the design of the current finger PIP implants can be improved for optimum performance and a long-term fixation.Copyright