Structural testing of an ultralight UAV composite wing and fuselage

The details of an experimental investigation focusing on obtaining the static and vibration characteristics of a full-scale carbon composite wing and fuselage structural assemblies of an ultralight unmanned aerial vehicle (UAV) are presented. The UAV has a total empty weight of 155-lb and an overall length of approximately 20.6t. A three-tier whiffletree system and the tail fixture were designed and used to load the wing and the fuselage in a manner consistent with a high-g flight condition. A shaker-table approach was used for the wing vibration testing, whereas the modal characteristics of the fuselage structure were determined for a freeree configuration. The static responses of the both structures under simulated loading conditions as well as their dynamic properties such as the natural frequency, damping coefficient and associated mode shapes were obtained. The design and implementation of the static and vibration tests along with the experimental results are presented in this thesis

3D-Printed Energy-Absorbing Polymer Structures for Reducing Injury Risk from Overhead Impacts to Hard Hats

Project of Merit Winner Struck-by accidents (i.e., being hit by a falling object) are a leading cause of traumatic brain injuries (TBIs) in the construction industry. While hardhats are the conventional means of head protection in this setting, their overall design has not appreciably changed in decades. In this project, a commercially available hardhat was augmented by creating a compliant cantilever on the superior surface supported by a 3D-printed block (10 x 10 x 50mm, with hexagonal lattice infill) to serve as a sacrificial energy-absorbing structure. The lattices were created using three polymer materials (PLA, ABS, and High-Impact Polystyrene (HIPS)) at four levels of porosity (0%, 32.5%, 56%, and 69.3%). A Hybrid III head/neck assembly was fitted with each hardhat design, and a vertical impact test was conducted using a 1.8-kg steel impactor dropped from 1.83 m. The maximum acceleration and head injury criterion (HIC) were calculated from the impact data for each test. Analysis of variance (ANOVA) revealed that HIC was significantly reduced for all lattices with 56% porosity (

Effects on Fatigue of Applying Titanium Nitride Coating to 17-4 PH Stainless Steel

Honorable Mention Winner Surface coatings when applied to metals have many useful applications such as improving the biocompatibility and resistance to corrosion and wear. In this study, the effect of thickness of titanium nitride (TiN) on 17-4 precipitation hardened (PH) stainless steel under cyclic (i.e. fatigue) loading is investigated. TiN are applied to wrought 17-4 PH stainless steel using physical vapor deposition (PVD) technique in a low-pressure vacuum chamber filled with a mixture of inert argon and nitrogen gases with a ratio of 10:1. In addition, these specimens are subjected to two different vapor durations, but a constant voltage and pressure, to obtain different coating thicknesses. The fatigue resistance of 17-4 PH stainless steel specimens with TiN layer is obtained from rotating bending fatigue tests with constant stress levels which correlated to 75%, 50%, and 25% of the ultimate tensile strength. Failure analysis of the specimens were conducted using a scanning electron microscope (SEM) and an optical microscope to view the topography of the fracture surface and the thickness of the deposited TiN layer. The study expects that an increase in fatigue life of the specimens will result from increasing the thickness of the TiN layer due to an improvement in the hardness of the surface

Fracture characteristics of PEEK at various stress triaxialities

Polyether-ether-ketone (PEEK) is an alternative to metal alloys in orthopaedic applications. It gives significant advantages including excellent mechanical properties and non-toxicity. In this work, a set of specimens with different notched radii were selected to examine the effect of triaxial state of stress on the fracture behavior of PEEK. Fractographic analysis via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) further elucidated the fracture micromechanisms. Distinct fracture patterns were identified under different stress triaxialities. In addition, the microstructural inclusion properties in PEEK specimen such as inclusion size and chemical composition were analysed and determined. Finite element simulations were carried out to evaluate the correlation of observed fracture characteristics with different stress triaxialities

Hot tensile fracture characteristics and constitutive modelling of polyether-ether-ketone (PEEK)

The effects of strain rate and deformation temperature on the deformation behaviors of polyether-ether-ketone (PEEK) were studied by uniaxial tensile tests with the temperature range of 23–150 °C and strain rate of 0.01–1 s−1. The effects of deformation temperature and strain rate on the hot tensile deformation behavior and fracture characteristics were investigated by scanning electron microscope (SEM) and discussed in detail. SEM experimental results suggest that fracture morphology is not strain rate sensitive but temperature sensitive. Based on the tensile results, the Johnson-Cook and modified Johnson-Cook constitutive models were established for PEEK. Furthermore, a comparative study has been made on the accuracy and effectiveness of the developed models to predict the flow stress. The results show that the original Johnson-Cook model reflects the deformation behavior more accurately throughout the entire test temperature and strain rate range under uniaxial tensile conditions

Statistical Characterization of Viscoelastic Creep Compliances of a Vinyl Ester Polymer

The objective of this study was to develop a model to predict the viscoelastic material functions of a vinyl ester (VE) polymer (Derakane 441-400, Ashland Co.,) with variations in its material properties. Short-term tensile creep/creep recovery experiments were conducted at two stress levels and at four temperatures below the glass transition temperature of the VE polymer, with 10 replicates for each test configuration. Experimental strains in both the longitudinal and transverse directions were measured using a digital image correlation technique. The measured creep strain versus time responses were subsequently used to determine the creep compliances using the generalized viscoelastic constitutive equation with a Prony series representation. The variation in the creep compliances of Derakane 441-400 was described by formulating the probability density functions (PDFs) and the corresponding cumulative distribution functions (CDFs) of the creep compliances using the two-parameter Weibull and log-normal distributions. The maximum likelihood estimation technique was used to obtain the Weibull shape and its scale parameters and the log-normal location and its scale parameters. The goodness-ofit of the distributions was determined by performing Kolmogorov-Smirnov (K-S) hypothesis tests. Based on the K-S test results, the Weibull distribution is a better representation of the creep compliances of Derakane 441-400 when compared to the log-normal distribution. Additionally, the Weibull scale and shape parameters of the creep compliance distributions were shown to be time and temperature dependent. Therefore, two-dimensional quadratic Lagrange interpolation functions were used to characterize the Weibull parameters to obtain the PDFs and subsequently the CDFs of the creep compliances for the complete design temperature range during steady state creep. At each test temperature, creep compliance curves were obtained for CDF values of 0.05, 0.50 and 0.95 and compared with the experimentally obtained lowest, mean and highest creep compliances, respectively. The predicted creep compliances of Derakane 441-400 in the design space are in good agreement with the experimental data

An investigation into the effects of cyclic strain rate on the high cycle and very high cycle fatigue behaviors of wrought and additively manufactured Inconel 718

Additive Manufacturing (AM) has increasingly been used to fabricate parts in aerospace applications, which may require service lives beyond ten-million cycles due to the imposed high loading frequencies. Understanding the very high cycle fatigue (VHCF) behavior of these additive manufactured (AM) parts is an important step towards their design and qualification processes. This study focuses on the high cycle fatigue (HCF) and VHCF behaviors of both wrought and laser beam-powder bed fusion (LB-PBF) fabricated Inconel 718 in machined/polished surface condition, emphasizing on the influence of test frequency (i.e., cyclic strain rate). Uniaxial, fully-reversed force- and stress-controlled fatigue tests were conducted utilizing a servo-hydraulic and an ultrasonic test system operating at 5 Hz and 20 kHz, respectively, on wrought as well as LB-PBF vertically and diagonally built specimens. Fatigue cracks in the majority of the specimens were found to initiate from intra-granular slip bands near or at the surface, which gives rise to strong anisotropy in fatigue resistance in LB-PBF specimens due to the presence of columnar grains along the build directions. Longer fatigue lives were obtained at 20 kHz, which was ascribed to possibly lower-than-intended stresses applied in the ultrasonic tests. The corrected stress-life fatigue data at 20 kHz were found to converge to the one obtained from conventional testing at 5 Hz, implying no effect of cyclic strain rate on the fatigue behavior of Inconel 718 regardless of the fabrication process. The findings of this work confirm the use of ultrasonic fatigue testing to expedite generation of AM materials data to keep up with the current demand; however, the applied stress may need to be corrected

Comparison of Rotating-Bending and Axial Fatigue Behaviors of LB-PBF 316L Stainless Steel

Additive manufactured (AM) materials are prone to internal defects such as entrapped gas pores and lack of fusions along with having a rough surface. There are different types of fatigue tests that are used to characterize the effects of such defects on the structural integrity of AM parts. The present study aims to investigate the effect of stress gradient on the fatigue behavior of 316L stainless steel (SS), fabricated using a laser beam powder bed fusion (LB-PBF) process. Axial fatigue tests are performed on as-built (non-machined) LB-PBF 316LSS round specimens with uniform gage section, while rotating bending fatigue tests are conducted on hourglass specimens (i.e. reduced gage section). Fatigue tests revealed that the specimens subjected to the axial loading exhibited lower fatigue resistance compared to the specimens failed under rotating bending test. Such differences in the fatigue life was attributed to the variation in the stress distribution resulting from different loading types and its effect on the fatigue crack propagation. Fractography analysis conducted to determine the failure mechanism showed that all of the cracks initiated from the surface of the specimen irrespective to the loading conditions. Furthermore, fracture surface observed for LB-PBF 316L SS specimens resembled a typical fracture surface of notched specimens, which supports the fact that for the as-built specimens cracks initiates from the micro-notches as a result of layer wise fabrication in AM process.Mechanical Engineerin