Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Fretting wear results in the loss of fit and tolerance at contact interfaces. The aerospace and aircraft industry is severely impacted by fretting wear and fretting fatigue that frequently occurs in turbo machinery and riveted structural connections. There have been numerous studies, investigating the fretting phenomenon for these aerospace applications. Literature available in regard to fretting wear encountered in these aerospace applications is limited. This study is therefore aimed at investigating the fretting wear encountered in aerospace application. An in-house fretting test apparatus was specially designed and developed in order to perform the fretting wear experiments. Ti-6Al-4V and Al7075-T6 are the two aerospace materials that were tested using the fretting test apparatus. An extensive experimental study was conducted in order to investigate the effect of the normal force on the fretting wear and friction behaviour of the two aerospace materials. The most severe of these experiments were identified and then repeated for up to 106 fretting cycles. Additional fretting wear experiments were also conducted between the two aerospace materials and cemented carbides, since the carbides are currently being utilized as coatings in some aerospace contacts that are prone to fretting induced damage. The experimental study revealed that a decrease in the normal force resulted in an increase in the severity of the fretting wear of both aerospace materials. The additional fretting wear experiments involving carbide-metal contact couples found that Ti-6Al-4V and Al7075-T6 were prone to adhesive wear. / AFRIKAANSE OPSOMMING: Knaagslytasie veroorsaak materiaalverlies by die kontakoppervlakke. Die lugvaart industrie is erg geraak deur knaagslytasie en knaaguitputting wat dikwels voorkom in turbo-enjin toepassings en strukturele verbindings. Daar was al talle studies gedoen oor die effek van knaag op lugvaart toepassings. Literatuur met betrekking tot knaagslytasie in lugvaart toepassings is egter beperk. Hierdie studie was dus gemik daarop om knaagslytasie in sekere lugvaart toepassings te ondersoek. Tydens die studie is ʼn toetsopstelling ontwerp en ontwikkel om knaagslytasie eksperimente uit te voer. Ti-6Al-4V en Al7075-T6 is die twee lugvaartmateriale wat ondersoek is met behulp van die toetsopstelling. ʼn Omvattende eksperimentele studie is gedoen om die effek van die normaal krag op knaagslytasie en die wrywings gedrag van die lugvaartmateriale te ondersoek. Die eksperimente wat die ergste slytasie en hoogste wrywing getoon het, is herhaal vir 106 siklusse. Bykomende knaag eksperimente was ook tussen die twee lugvaartmateriale en sekere karbiede gedoen, aangesien karbiede tans as deklae in sommige lugvaart kontakte gebruik word. Die eksperimentele studie het getoon dat 'n afname in die normale krag gelei het tot 'n toename in wrywing vir beide lugvaartmateriale. Die bykomende knaagslytasie eksperimente op karbied metaal pare het getoon dat Ti-6Al-4V en Al7075-T6twee lugvaart materiale nie in staat was om enige van die karbide te beskadig nie. Die lug-en Ruimte-materiaal aan die ander kant ervaar het kwaadaardige dra.

Critical velocity is defined as projectile striking velocity that causes instantaneous rupture of the specimen under transverse impact. The main goal of this dissertation was to determine the critical velocities of a Twaron^® 2040 warp yarn impacted by different round indenters. Special attention was placed to develop models to predict the critical velocities when transversely impacted by the indenters. An MTS 810 load frame was utilized to perform quasi-static transverse and uniaxial tension experiments to examine the stress concentration and the constitutive mechanical properties of the yarn which were used as an input to the models. A gas/powder gun was utilized to perform ballistic experiments to evaluate the critical velocities of a Twaron^® 2040 warp yarn impacted by four different type of round projectiles. These projectiles possessed a radius of curvature of 2 μm, 20 μm, 200 μm and 2 mm. The results showed that as the projectile radius of curvature increased, the critical velocity also increased. However, these experimental critical velocities showed a demonstrated reduction as compared to the classical theory. Post-mortem analysis via scanning electron microscopy on the recovered specimens revealed that the fibers failure surfaces changed from shear to fibrillation as the radius of curvature of the projectile increased. To improve the prediction capability, two additional models, Euler-Bernoulli beam and Hertzian contact, were developed to predict the critical velocity. For the Euler–Bernoulli beam model, the critical velocity was obtained by assuming the specimen ruptured instantaneously when the maximum flexural strain reached the ultimate tensile strain of the yarn upon impact. On the other hand, for the Hertzian contact model, the yarn was assumed to fail when the indentation depth was equivalent to the diameter of the yarn. Unlike Smith theory, the Euler-Bernoulli beam model underestimated the critical velocity for all cases. The Hertzian model was capable of predicting the critical velocities of a Twaron^® 2040 yarn transversely impacted by 2 μm and 20 μm round projectiles.

No description available.

No description available.

No description available.

The initiation of high explosives (HEs) under shock loading lacks a comprehensive understanding: particularly at the particle scale. One common explanation is hot spot theory, which suggests that energy in the material resulting from the impact event is localized in a small area causing an increase in temperature that can lead to ignition. This study focuses on the response of HMX particles (a common HE) within a polymer matrix (Sylgard-184^®), a simplified example of a polymer bonded explosive (PBX). A light gas gun was used to load the samples at impact velocities ranging from 370 to 520 m/s. The impact events were visualized using X-ray phase contrast imaging (PCI) allowing real-time observation of the impact event. The experiments used three subsets of PBX samples: multiple particle (production grade and single crystal), drilled hole, and milled slot. Evidence of damage and deformation occurred in all of the sample types. While the necessary impact velocity for consistent hot spot formation leading to reactions was not reached, the damage (particularly cracking) that occurred provides a useful indication of where hot spots may occur when higher velocities are reached. With the multiple particle samples, evidence of cracking and debonding occurred throughout. One sample showed significant volume expansion due to possible reaction. The samples containing drilled holes demonstrated the expected pore collapse behavior at these velocities, as well as damage downstream from the holes under various two-hole arrangements. Milled slot samples were tested to simulate existing cracks in the HMX. These samples showed increased damage at the site of the milled slot, as well as unique cracking behavior in one of the samples.

<p>In this dissertation, we first used high-speed X-ray phase contrast imaging and infrared thermal imaging techniques to study the formation processes of adiabatic shear bands in aluminum 7075-T6 and 6061-T6 alloys. A modified compression Kolsky bar setup was developed to apply the dynamic loading. A flat hat-shaped specimen design was adopted for generating the shear bands at the designated locations. Experimental results show that 7075-T6 exhibits less ductility and a narrower shear band than 6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within the shear band zones for 7075-T6 and 6061-T6 respectively. This local high temperature zone and the resulting thermal instability were found to relate to the shear band formation in these aluminum alloys. Secondly, a high-speed laser phosphorescence thermal imaging technique is developed and integrated with the compression Kolsky bar setup. The temperature field measurement during dynamic loading are performed at 100 – 200 kHz frame rate with a spatial resolution of 13 µm/pixel. The dynamic compression of copper shows 312 K temperature rise among the material surface. Experiments with thermocouple are also conducted and the results verifies the laser measurement. In the dynamic shear of aluminums, the temperature evolution during adiabatic shear band formation was observed and the results are compared with infrared measurements. The shear band was found forming at approximately 400 K and 440 K for 7075-T6 and 6061-T6, respectively, while the maximum temperature is measured as 650 K for 7075-T6 and 800 K for 6061-T6. Although the maximum temperature agrees with the infrared results, thermal softening is not considered as the main cause of the ASB formation due to the low temperature when the shear band forms.</p>

<div>Sensing of mechanical state is critical in diverse fields including biomedical implants, intelligent robotics, consumer technology interfaces, and integrated structural health monitoring among many others. Recently, materials that are self-sensing via the piezoresistive effect (i.e. having deformation-dependent electrical conductivity) have received much attention due to their potential to enable intrinsic, material-level strain sensing with lesser dependence on external/ad hoc sensor arrays. In order to effectively use piezoresistive materials for strain-sensing, however, it is necessary to understand the deformation-resistivity change relationship. To that end, many studies have been conducted to model the piezoresistive effect, particularly in nanocomposites which have been modified with high aspect-ratio carbonaceous fillers such as carbon nanotubes or carbon nanofibers. However, prevailing piezoresistivity models have important limitations such as being limited to microscales and therefore being computationally prohibitive for macroscale analyses, considering only simple deformations, and having limited accuracy. These are important issues because small errors or delays due to these challenges can substantially mitigate the effectiveness of strain-sensing via piezoresistivity. Therefore, the first objective of this thesis is to develop a conceptual framework for a piezoresistive tensorial relation that is amenable to arbitrary deformation, macroscale analyses, and a wide range of piezoresistive material systems. This was achieved by postulating a general higher-order resistivity-strain relation and fitting the general model to experimental data for carbon nanofiber-modified epoxy (as a representative piezoresistive material with non-linear resistivity-strain relations) through the determination of piezoresistive constants. Lastly, the proposed relation was validated experimentally against discrete resistance changes collected over a complex shape and spatially distributed resistivity changes imaged via electrical impedance tomography (EIT) with very good correspondence. Because of the generality of the proposed higher-order tensorial relation, it can be applied to a wide variety of material systems (e.g. piezoresistive polymers, cementitious, and ceramic composites) thereby lending significant potential for broader impacts to this work. </div><div><br></div><div>Despite the expansive body of work on direct current (DC) transport, DC-based methods have important limitations which can be overcome via alternating current (AC)-based self-sensing. Unfortunately, comparatively little work has been done on AC transport-deformation modeling in self-sensing materials. Therefore, the second objective of this thesis is to establish a conceptual framework for the macroscale modeling of AC conductivity-strain coupling in piezoresistive materials. For this, the universal dielectric response (UDR) as described by Joncsher's power law for AC conductivity was fit to AC conductivity versus strain data for CNF/epoxy (again serving as a representative self-sensing material). It was found that this power law does indeed accurately describe deformation-dependent AC conductivity and power-law fitting constants are non-linear in both normal and shear strain. Curiously, a piezoresistive switching behavior was also observed during this testing. That is, positive piezoresistivity (i.e. decreasing AC conductivity with increasing tensile strain) was observed at low frequencies and negative piezoresistivity (i.e. increasing AC conductivity with increasing tensile strain) was observed at high frequencies. Consequently, there exists a point of zero piezoresistivity (i.e. frequency at which AC conductivity does not change with deformation) between these behaviors. Via microscale computational modeling, it was discovered that changing inter-filler tunneling resistance acting in parallel with inter-filler capacitance is the physical mechanism of this switching behavior.</div>

No description available.

No description available.