Applied Mechanics Commons^™

A Maximum Energy Release Rate Computational Framework For Fatigue Crack Trajectory And Life Prediction Under Mixed-Mode Proportional And Nonproportional Loading, Nahla Helmy Matar

Theses and Dissertations

The problem of mixed-mode fatigue crack growth has been a persistent research challenge. The determination of fatigue crack propagation direction directly impacts the safety and integrity of components and structures, making it crucial in various industrial applications, including welded drilling pipes, aerospace, and automotive engineering. Unlike uniaxial loading, mixed-mode cracks tend to change direction frequently (kinking), and nonproportional loading adds to the complexity of the problem. This research aims to estimate the crack propagation direction and fatigue crack life in linear elastic materials subjected to mixed-mode (I+II) loading conditions under proportional or non-proportional cyclic loading. Numerical software was developed based …

Interplay Between Phase Instability And Deformation Mechanisms In Beta Titanium Alloys In Aggressive Environments, Benjamin Elbrecht

All Dissertations

Modern designs, such as commercial jet turbines, are reliant on the high temperature performance of their constituent materials. Improvements to the strength-at-temperature of materials are critical to the continued advancement of efficiency and payload capacity in these advanced applications. This dissertation presents studies that progress towards this goal through the construction of a new experimental setup for complex temperature and force profiles that also accurately captures deformation behavior through non-contact measurements. Using this setup, studies into two specialty titanium alloys were executed: discovering the microscopic mechanisms responsible for the macroscale behavior and testing in conditions closely replicating aircraft turbine operating …

Application-Driven Materials Development Of Solid-State Conductive Composites Of Ultra-High Molecular Weight Polyethylene, Peder Solberg

Dartmouth College Ph.D Dissertations

Ultra-high molecular weight polyethylene (UHMWPE) is a linear long chain homopolymer valued for its toughness, wear resistance, and chemical inertness. In the medical field, these properties make it the material of choice for bearing surfaces despite recognized tradeoffs between wear resistance, toughness, and oxidation resistance. Electrically conductive composites of UHMWPE show promise to enable new types of smart and active load-bearing implants in a future of personalized medicine, if beneficial properties can be maintained with the addition of solid-state additives.

Clinical need-finding conducted through Dartmouth’s Training Program in Surgical Innovation revealed several potential applications for conductive, load-bearing polymers. These include: …

An In-Situ Method For Quantifying Entropy Generation In Fatigue Via A Novel Thermographic Approach, Peyton J. Wilson

LSU Master's Theses

This work features a new approach that is used to measure the rate of plastic work during a cyclic fatigue test, based on the first and second laws of thermodynamics. Using the one-dimensional Fourier heat equation coupled with the concept of fracture fatigue entropy, the rate of plastic work during fatigue can be measured without the need to interrupt a fatigue test, and the accumulated entropy to failure can be quantified. Experimental fatigue tests are conducted on Aluminum 6061-T6 specimens under cyclic axial loading and Carbon Steel 1018 specimens under fully reversed cyclic bending loading. The obtained results in addition …

Investigation Of Fatigue Response With Analytical And Machine Learning Models And Hygroscopic Analysis Of Asymmetric Bistable Cfrp Composites, Shoab Ahmed Chowdhury

All Dissertations

Asymmetric bistable carbon fibre reinforced plastic (CFRP) composites enable a broad range of applications as they can sustain multiple stable configurations and have small snap-through load requirements. These unique features, coupled with their light strength-to-weight and stiffness-to-weight ratios, have made them preferred options for multifunctional systems. This study investigates the fatigue and hygroscopic response of 2-ply, [0/90] bistable CFRP laminates and proposes predictive modeling approaches for improved performance.

While previous studies widely researched and documented the fatigue of general composites in axial loading, fatigue analysis of asymmetric bistable composites in the out-of-plane snap-through direction is inadequate. This study performs fatigue …

Metal Fatigue And Crack Propagation Using Thermodynamic/Thermoelectric Approach, Behnam Hajshirmohammadi

LSU Doctoral Dissertations

The present dissertation concentrates on the fatigue life of metals and fatigue crack growth (FCG). The basic concept of fatigue thermodynamics is used for this purpose. With the application of thermoelectricity, a method is developed on the basis of fracture fatigue entropy (FFE) for the rapid estimation of the fatigue life of components. To this end, thermoelectric signals captured from a specimen during cyclic loading are analyzed, and a procedure is proposed for the calculation of fatigue life based on the thermoelectric signals in the beginning and the steady-state zone of fatigue. Temperature gradient and electric potential gradients in metals …

A Nanoindentation Study Of The Fatigue Properties Of Al/A-Si Core-Shell Nanostructures, Jason Steck

Mechanical Engineering Undergraduate Honors Theses

Nanostructure-textured surfaces can reduce friction and increase the reliability of micro- and nanoelectromechanical systems (NEMS/MEMS). For MEMS incorporating moving parts, the fatigue properties of nanostructures pose a challenge to their reliability in long-term applications. In this study, the fatigue behavior of hemispherical Al/a-Si core-shell nanostructures (CSNs), bare hemispherical Al nanodots, and a flat Al/a-Si layered thin film have been studied using nanoindentation and nano-scale dynamic mechanical analysis (nano-DMA) techniques. Fatigue testing with nano-DMA shows that the deformation resistance of CSNs persists through 5.0 × 10⁴ loading cycles at estimated contact pressures greater than 15 GPa. When the a-Si shell …

Determination Of Chemical Notch, KChem On Aluminum And Steel When Subjected Under Slow Strain Rate Test In Corrosive Environment, Joshua Teo Lee Kuok

Masters Theses

When designing for any mechanical components or system, the question would arise as to how the material would react to the loads subjected on it? Would the component survive its service load? How would it react to environmental corrosion? To answer these questions, the technique used in this thesis paper is the Slow Strain Rate Test (SSRT) method. Aluminum and steel were chosen as the material to be tested in this paper. Al 7075-T651, and Al 6061-T651 was chosen due to its wide range of application, high strength to weight ratio and ease of machinability. It is highly used in …

Peridynamic Models For Fatigue And Fracture In Isotropic And In Polycrystalline Materials, Guanfeng Zhang

Department of Mechanical and Materials Engineering: Dissertations, Theses, and Student Research

To improve design and reliability, extensive efforts has been devoted to understanding damage and failure of materials and structures using numerical simulation, as a complement of theory and experiment. In this thesis, peridynamics is adopted to study fatigue and dynamic failure problems.

Fatigue is a major failure mode in engineering structures. Predicting fracture/failure under cyclic loading is a challenging problem. Classical model cannot directly be applied to problems with discontinuities. A peridynamic model is adopted in this work because of important advantages of peridynamics in allowing autonomous crack initiation and propagation. A recently proposed peridynamic fatigue crack model is considered …

Final Design Report: Polymer Fatigue Characterization Test Method, Wyatt Ayling, Tyler Price, Brandon Stell, Michela Upson

Mechanical Engineering

Polylogix is a team dedicated to the design, build, and testing of a fatigue machine to simulate cyclic loading on a biomedical polymer. This project is sponsored by Endologix, Inc. to provide test data characterizing mechanical material properties of various formulations of polymer used in abdominal aortic aneurysm surgeries. With this project goal, the machine must be able to test the polymer at body conditions; these include a testing temperature of 37°C and a cycling frequency ranging from 1 Hz to 10 Hz. This report proposes the following solution to this design challenge: an AC motor-driven mechanism utilizing a planetary …

Rebounder Fatigue Test Machine, Caroline Reeves, Will Robertson, Ethan Flory

Mechanical Engineering

JumpSport, a company that designs and sells trampolines and trampoline accessories, has sponsored this senior project team to design, build, and test a trampoline fatigue test machine. The machine must simulate a person jumping on the trampoline to test the life of JumpSport’s fitness trampolines and kids’ trampolines. Partway through the design process, the objectives were altered and this senior project team was tasked with merging with another Cal Poly senior project group to create an all-inclusive test machine to accommodate both full-trampoline testing and individual bungee cord testing.

The final design is centered on a slider crank linkage driven …

Finite Element Frequency Domain Solution Of Nonlinear Panel Flutter With Temperature Effects And Fatigue Life Analysis, David Yongxiang Xue

Mechanical & Aerospace Engineering Theses & Dissertations

A frequency domain solution method for nonlinear panel flutter with thermal effects using a consistent finite element formulation has been developed. The von Karman nonlinear strain-displacement relation is used to account for large deflections, the quasi-steady first-order piston theory is employed for aerodynamic loading and the quasi-steady thermal stress theory is applied for the thermal stresses with a given change of the temperature distribution, ΔΤ (x, y, z). The equation of motion under a combined thermal-aerodynamic loading can be mathematically separated into two equations and then solved in sequence: (1) thermal-aerodynamic postbuckling and (2) limit-cycle oscillation. The Newton-Raphson iteration technique …