Stephanie C. TerMaath

Joint Strike Fighter Airframe Durability and Damage Tolerance Certification

The F-35 Joint Strike Fighter is a stealthy combat aircraft that will serve as the next generation strike fighter. Certification of the airworthiness of this aircraft will be achieved through the demonstration, both by analysis and by test, that a number of structural capability requirements have been met. The testing required for certification is planned and executed according to a building block approach, a time phased test program that parallels and supports the aircraft development. This paper will describe the durability and damage tolerance (DaDT) aspects of this testing and the subsequent application of the results for development and verification of metallic DaDT analytical and computational tools as well as demonstrating overall aircraft structural integrity.

Ventricular catheter development: Past, present, and future

Cerebrospinal fluid diversion via ventricular shunting is the prevailing contemporary treatment for hydrocephalus. The CSF shunt appeared in its current form in the 1950s, and modern CSF shunts are the result of 6 decades of significant progress in neurosurgery and biomedical engineering. However, despite revolutionary advances in material science, computational design optimization, manufacturing, and sensors, the ventricular catheter (VC) component of CSF shunts today remains largely unchanged in its functionality and capabilities from its original design, even though VC obstruction remains a primary cause of shunt failure. The objective of this paper is to investigate the history of VCs, including successful and failed alterations in mechanical design and material composition, to better understand the challenges that hinder development of a more effective design.

A computational fracture mechanics approach for the analysis of facesheet-from-core disbond of honeycomb core sandwich panels

The modified crack closure method is combined with finite element analysis to study the damage mode of facesheet-from-core disbond of honeycomb core sandwich structure. Explicit panel models containing an initial disbond region are analyzed to better understand this type of failure. Calculation of energy release rates is used to determine loading conditions that will cause disbond propagation. The edge peel-off case is used as an example, and parameter studies are performed to determine the importance of key properties with regard to damage tolerance of the panels.

Multiple Crack Analysis in Finite Plates

In this study, interactions of multiple cracks in finite rectangular plates are analyzed. A finite plate is treated as a polygon-shaped plate which can be created by contiguous crack segments embedded in an infinite plate forming an enclosed region so there are no true crack tips remaining around the plate. The problem is formulated in terms of integral equations expressed by edge dislocation distributions representing opening displacement profiles for the cracks with both normal and tangential components. To solve this boundary value problem we employ the superposition approach based on the dislocation distributions requiring the determination of crack opening displacement profiles that satisfy the traction- free condition on interior crack faces and the given boundary tractions on four exterior cracks defining the rectangular plate. Applying the method with a new point allocation scheme reducing the number of evaluation points leads to a set of linear algebraic equations in terms of unknown weighting coefficients of opening displacement profiles. These opening displacement profile components are categorized in terms of polynomial terms which are integers and wedge terms which are rational numbers approximated from irrational wedge eigenvalues to evaluate integrals in closed forms. Once the coefficients of opening displacement profile components are obtained, stress and displacement fields on the entire body, as well as stress intensity factors at the crack tips and kinks are calculated. After the accuracy of the method is shown by comparing the results for different sample problems, the strong interactions of multiple cracks in a rectangular plate under various loading conditions are demonstrated with figures and tables.

Sensitivity Analysis of Out-of-Plane Composite Lamina Properties Relative to Configuration and Constitutive Properties

A probabilistic framework and the supporting tools are developed and demonstrated to investigate the out-of-plane properties of a composite lamina. The framework methodology consists of a sensitivity analysis to determine the most influential input parameters followed by a probabilistic analysis to quantify the effects of the uncertainty in these parameters on lamina properties. To demonstrate the framework, sensitivity analysis is performed through finite element analysis of unit cell models to identify the most influential constituent and configuration parameters on lamina behavior, specifically on out-of-plane lamina properties. The effects of uncertainty in these input parameters are then quantified through Monte Carlo simulation using probabilistic distributions to represent each input under consideration. One of three distribution types (Normal, Lognormal, and Weibull) can be assigned, and a different distribution type can represent each input parameter. To study configuration effects, a representative volume element (RVE) consisting of two different fiber materials is investigated. The location and percent composition of the two different fiber materials are varied to analyze their effects on out-of-plane lamina properties.

Investigation of a new analytical method for treating kinked cracks in a plate

A new analytical method for treating kinked and branched cracks is investigated to understand its subtleties and their effects on accuracy and efficiency. Based on a weighted superposition approach, this method uses analytical functions built from opening displacement profiles to determine full stress and displacement fields in a linearly elastic two-dimensional infinite plate containing interacting kinked and branched cracks under far field loading. In addition, results include stress intensity factors at tip and kink locations. Rapid convergence is achieved for few degrees of freedom yielding crack face tractions consistent with those prescribed. Moreover, this method involves no numerical integration, eliminating unnecessary computational errors. The features of this method are demonstrated by the solution of a V-shaped crack. The method yields stress intensity factor results that are in agreement with a particular case found in a well-known handbook.

A computational fluid dynamics simulation framework for ventricular catheter design optimization

In this research an optimization methodology and 3D computational fluid dynamics algorithm were coupled to reach an important design objective for ventricular catheters: uniform inlet flow distribution. The optimized catheter design presented significantly improves on previous designs explored in the literature and on standard catheter designs used clinically. The automated, iterative fluid simulation framework described in this work can be used to rapidly explore design parameter influence on other flow-related objectives in the future.

Sensitivity Analysis of Composite Patch Design Parameters under Low Velocity Impact Loading Conditions

Composite patches are bonded to damaged or undamaged aerospace structure as a means to improve or restore damage tolerance and load-carrying capacity. While this repair and reinforcement method is currently demonstrated on structures around the globe, design and analysis methodologies that account for the high variability in design parameters, material properties, and loading conditions are lagging behind its implementation. In order to achieve safe and optimized patch design, a comprehensive understanding of the effects of uncertainty on patch performance is essential. One aspect of patch performance of particular concern is internal damage that is not visible, but could propagate and cause patch and consequentially structural failure. To investigate the damage tolerance of patched structure subjected to low-velocity impact loading and to quantify the effects of uncertainty on performance, a 3D high fidelity finite element model of a patched structure was developed and validated. This model of a metal plate with a composite patch that is adhesively bonded captures multiple damage mechanisms including composite damage, delamination between composite layers, adhesive disbond, and plastic deformation of the metal. Computational simulation with this model is performed to study damage tolerance under low-velocity impact with respect to uncertain input parameters to determine the sensitivity of patch performance relative to these parameters.

A Solution for Interacting Kinked and Branched Cracks

To study realistic fracture problems that include many interacting cracks of complex shapes, a technique has been developed based on superposition and dislocation theory. This method is valid for brittle fracture conditions in an infinite plate under general far field loading. This technique can be used to determine the full stress and displacement fields in a cracked body containing numerous straight, kinked, and branched cracks. The solution also includes the stress intensity factors at both crack tips and crack kinks, both necessary components for an evaluation of potential crack propagation. This method is successfully applied to a multiply kinked and branched crack, in addition to an array of interacting cracks of complex shapes.