This dissertation describes the development and application of various mechanical characterization techniques to four types of polymer composite materials. The composite nature of these materials ranges from molecular to macro-scale, as do the size scales probed by the techniques chosen. The two main goals of this work are to evaluate the suitability of existing characterization methods to new composite materials (and augment the methods as needed), and to use these methods to determine optimal composite system parameters to maximize the desired mechanical response. Chapter 2 employs nondestructive ultrasonic spectroscopy for characterizing the stiffness response of both micron-scale woven composites and macro-scale glass-polymer-glass laminates. Both traditional wavespeed measurement as well as aspects of resonant ultrasonic spectroscopy are applied to determine material parameters. The laminates are also examined in Chapter 3, which utilizes both large-scale and small-scale quasi-static and dynamic puncture tests to elucidate the size- and rate-dependence of dynamic behavior. Because of limitations encountered with these methods, a smaller-scale, more fundamental test is developed and applied which focuses solely on the deformation and delamination of the polymer. These two processes, which account for the vast majority of energy absorbed during a puncture event, can be evaluated in terms of self-similar process zone propagation process models. Identifying and optimizing the relevant model parameters can promote the design of systems with maximum energy absorption. Exploratory work on nanocomposite systems is presented in Chapter 4. The polymer matrix from the laminated systems of the previous chapter is used to produce nanosilica composites. A range of techniques are employed to determine the level of dispersion and the mechanical reinforcement provided. The final project presented investigates copolycarbonates, or molecular composites, that have been developed to lessen the detrimental effect of aging on mechanical properties. Mechanical and thermal measurements can elucidate the effect of structure, specifically molecular mobility, on susceptibility to physical aging. The differences in molecular mobility contribute to differences in energy absorption by plastic deformation and damage, which is required for material toughness. Thus, understanding the molecular structure allows for determination of an optimal structure or copolymer concentration to maximize fracture toughness.

The object of this study was to investigate the micro-structural response of DP 600 subjected to high strain rate, ballistic impact tests. The ballistic tests were conducted using normal impact of a hardened steel penetrator into a 2 mm thick sheet of DP 600. The average strain rates produced from this test method are on the order of 10^5 s-1. Multiple methods were used to investigate the micro-structure before and after high strain rate deformation including optical microscopy, electron microscopy, and X-ray diffraction. A large variation in material response was observed between tests conducted at 0.8 x 10^5 and 2.5 x 10^5 s-1.

The split Hopkinson bar test is the most commonly used method for determining material properties at high rates of strain. The theory governing the specifics of Hopkinson bar testing has been around for decades. It has only been the last decade or so, however, that significant data processing advancements have been made. It is the intent of this thesis to offer the insight of its author towards new advancements. The split Hopkinson bar apparatus consists of two long slender bars that sandwich a short cylindrical specimen between them. By striking the end of a bar, a compressive stress wave is generated that immediately begins to traverse towards the specimen. Upon arrival at the specimen, the wave partially reflects back towards the impact end. The remainder of the wave transmits through the specimen and into the second bar, causing irreversible plastic deformation in the specimen. It is shown that the reflected and transmitted waves are proportional to the specimen's strain rate and stress, respectively. Specimen strain can be determined by integrating the strain rate. By monitoring the strains in the two bars, specimen stress-strain properties can be calculated. Several factors influence the accuracy of the results, including longitudinal wave dispersion, impedance mismatch of the bars with the specimens, and transducer properties, among others. A particular area of advancement is a new technique to determine the bars dispersive nature, and hence reducing the distorting effects. By implementing numerical procedures, precise alignment of the strain pulses is facilitated. It is shown that by choosing specimen dimensions based on their impedance, the transmitted stress signal-to-noise ratio can be improved by as much as 25dB. An in depth discussion of realistic expectations of strain gages is presented, along with closed form solutions validating any claims. The effect of windowing on the actual strains is developed by analyzing the convolution of a rectangular window with the impact pulse. The thesis concludes with a statistical evaluation of test results. Several recommendations are then made for pursuing new areas of continual research. / Master of Science

The split Hopkinson pressure bar (SHPB) continues to be one of the most common methods of testing materials at medium rates of strain. Elevated rates of strain, such as those found in impact and explosive applications, have been shown to induce phenomena such as strain hardening and phase transitions that can significantly affect the strength of most materials [14]. Due to its relative simplicity and robustness, the SHPB remains one of the preferred platforms for evaluating mechanical properties of materials at rates of strain up to approximately 104 in/in-s (s-1). At the Naval Surface Warfare Center Dahlgren Division (NSWCDD), research has been conducted in which a semiconductor laser diode has been used to measure the radial strain of a plastically deforming cylindrical test specimen in the SHPB. The SHPB consists of two long, slender cylindrical bars, denoted input and output bars, that "sandwich" a cylindrical test specimen. Utilizing a high-pressure gas gun, a third cylindrical steel bar, known as the striker bar, is fired at the input bar, causing a compressive stress wave to travel through the input bar to the input bar - test specimen interface. At this interface, a portion of the stress wave propagates through the test specimen while the remainder of the pulse reflects back through the input bar as a tensile stress wave. The non-reflected portion of the stress pulse transmits through the test specimen and into the output bar causing the specimen to deform both elastically and plastically. Strain gages mounted to the input and output pressure bars measure both the incident, transmitted and reflected pulses. Specimen stress can be calculated using the transmitted strain signal while specimen strain and strain rate can be computed using the reflected strain pulse. In order to measure the specimen strain directly, a 670-nm wavelength semiconductor laser diode was affixed to the SHPB such that a vertical line of light approximately 250 micrometer (µm) wide was generated across the diameter of the test specimen. A collector lens located aft of the specimen was positioned to collate the light not occluded by the diameter of the specimen and refocus the light to be collected by a 25 MHz photodetector. Thus, changes in specimen diameter due to the impact event would result in more light being occluded by the specimen and less spectral energy being collected by the photodetector. The light collected by the photodetector is then converted to a voltage output before being recorded by a digital storage oscilloscope. With a known voltage-to-diameter calibration relationship, medium strain rate compressive tests were conducted to compare the optically measured strain results with the data gathered with the existing strain gages. It was found that the optical measurement system provided increased bandwidth and greater resolution than the conventional strain gage instrumentation while generating strain and strain rate results within 6.7% of corresponding strain gage data. This increased bandwidth and resolution allows the identification of both the elastic and plastic behavior of the specimen. In addition, the loading and unloading of the specimen can be clearly seen in the optical strain signal. These phenomena are evident in the peak diameter and strain achieved by the specimen, data not previously available with strain gage instrumentation. The plastic modulus, the theoretical relationship between the stress and strain in the plastic regime, also exhibits a significant increase in magnitude due to this ability to measure peak rather than average strain. Finally, by ridding the experiment of the input bar strain gage, input bar dispersion and the electrical and mechanical errors associated with the input bar strain gage were nullified. These conclusions will be validated through the presentation of several sets of experimental data correlated to data gathered previously. / Master of Science

Thesis (MScEng)--Stellenbosch University, 2013. / AFRIKAANSE OPSOMMING: Saamgestelde materiale het baie gewilde materiale in die lugvaart- en motor industrië geword as gevolg van die gewigsbesparende voordele wat dit inhou. Kostes en ander verwerkingsprobleme het tradisioneel die wydverspreide gebruik van spesifiek termoplasties-versterkte vesels in hierdie areas verhinder. Baie van die vervaardigingsprobleme (spesifiek lang siklusse) is aangespreek met die aanvang van termoplastiese matriks materiaal soos Polyphenolien Sulfied (PPS). Hierdie materiaal voldoen ook aan die lugvaart-industrie se brand-, rook- en giftigheidstandaarde. Termoplastiese saamgestelde materiale kan byvoorbeeld gevind word op komponente in vliegtuie se binneruimtes en ook die voorste rand van die vlerke. Hierdie komponente is hoogs vatbaar vir impakskade. Die hoë sterkte en styfheid tot gewig verhoudings van saamgestelde materiale laat toe vir dun materiaal dwarssnitte. Komponente is dus kwesbaar vir uit-vlakkige impak beladings. Saamgestelde materiale kan ook intern deur hierdie beladings beskadig word en kan nie met die blote oog waargeneem kan word nie. Dit is dus nodig om die skade weens hierdie beladings tydens normale gebruik akkuraat te voorspel. Verder sal dit nuttig wees om die struktuur se gedrag te bepaal in toepassings waar byvoorbeeld passasier veiligheid krities is, soos op vliegtuig ruglenings tydens noodlandings. In hierdie studie is die potensiële vervaardigingsvoordele van termoplastiese saamgestelde materiale gedemonstreer. Daarbenewens is 'n uit-vlakkige impak deur 'n sagte liggaam herbou in 'n laboratorium omgewing. Die primêre doelwit van hierdie studie was om die impak numeries te modelleer. Vervaardigingsvoordele van `n vesel versterkte termoplastiese laminaat is gedemonstreer deur die vervaardiging van 'n konkawe, agt laag laminaat uit 'n vooraf gekonsolideerde geweefde doek. Die totale verwerkingstyd van die plat laminaat na 'n konkawe laminaat was minder as vyf minute. 'n Eenvoudige plat laminaat en 'n konkawe laminaat is onderwerp aan 'n lae snelheid impak deur 'n sagte projektiel. Die impak is gemodelleer deur die evaluering van drie modelleringsmetodes vir die saamgestelde paneel. Die evalueringskriteria het o.a. ingesluit of laminaat se volle gedrag suksesvol gemodelleer kon word met behulp van slegs 2D dop elemente. Die reaksie van die saamgestelde paneel en gepaardgaande faling is met wisselende vlakke van sukses deur die drie geëvalueerde modelle voorspel. Die faling van tussen-laminêre bindings (verwys na as delaminasie) kon nie deur enige van die modelle voorspel word nie. Twee van die modelle het egter in-vlak faling met redelike akkuraatheid voorspel. / ENGLISH ABSTRACT: Due to weight saving advantages composite materials have become a highly popular material in the aerospace and automotive industries. Traditionally processing difficulties and costs have been a barrier to widespread composite material use in these industries. With the advent of thermoplastic matrix materials such as Polyphenoline Sulphide (PPS) the processing difficulties (especially long cycle times) experienced with traditional thermosetting resins can be addressed while maintaining aerospace Fire-Smoke and Toxicity (FST) approval. Thermoplastic composites can for example be found on aircraft interior components and leading edges of the wings. These areas are highly susceptible to impact damage. The high strength- and stiffness to weight ratios of composites allows for thin material cross sections. This leaves the components vulnerable to out-of-plane impact loads. Composite materials may also be damaged internally by these loads, leaving the damage undetectable through visual inspections. It may therefore be necessary to predict the amount of damage a component would sustain during normal operation. Additionally, it would be useful to predict structural response of these materials in applications where passenger safety is crucial, such as aircraft seat backrests during emergency landings. In this study the potential processing benefits of thermoplastic composite materials were demonstrated. Additionally an out-of-plane impact from a soft bodied projectile was reconstructed in a laboratory environment. The primary objective was to numerically model the impact event. Processing benefits of thermoplastics were demonstrated by producing a single curvature eight layered laminate from a pre-consolidated woven sheet. The total processing time from flat panel to a single curvature panel was below five minutes. A simple flat laminate and a single curvature laminate were subjected to a low velocity drop weight impact load from a soft bodied projectile. These impact events were modelled by evaluating three modelling methods for the composite panel structural response and damage evolution. Part of the evaluation criteria included whether laminate failure could be modelled successfully using only 2D shell elements. The response of the composite panel and accompanying failure were predicted with varying levels of success by the three evaluated models. The failure of interlaminar bonds (referred to as delamination) could not be predicted by either model. However two of the models predicted in-plane failure with reasonable accuracy.

Aplicações dos laminados híbridos metal-fibra incluem, além da indústria aeronáutica, as indústrias naval e automobilística. Diferentemente do setor aeronáutico, pesquisas sobre impactos mecânicos nas duas outras áreas da mobilidade são freqüentemente limitadas pela disponibilidade de equipamentos laboratoriais, de modo que é desejável o desenvolvimento de procedimentos de baixo custo para a determinação da resistência e tolerância a danos por impacto dos materiais de construção. Neste trabalho, a resistência a danos por impacto transversal do laminado híbrido metal-fibra Glare-5® foi determinada via três diferentes metodologias. A primeira utiliza um aparato sofisticado aparato Laser-Doppler para monitorar a aceleração e desaceleração de um impactador esférico de aço com 5 mm de diâmetro durante o evento do choque mecânico. O segundo método se baseia apenas nos valores de carga (força aplicada) vs. o tempo de impacto para a obtenção da energia absorvida pelo material. O terceiro considera somente os dados da velocidade do impactador, ou projétil, imediatamente antes e após o impacto. Concluiu-se que os valores de energia obtidos segundo as duas primeiras metodologias são similares, com o Laser-Doppler gerando resultados levemente não-conservadores, comprovando assim a possibilidade da derivação da resistência ao impacto do laminado através de um experimento simples e rápido, que utiliza apenas uma célula de carga digital para a monitoração da força aplicada em função do tempo. O terceiro método apresentou resultados substancialmente superiores às duas primeiras metodologias, sendo classificado como inadequado aos propósitos do projeto. Determinou-se que o laminado híbrido Glare-5®; absorve entre 60% e 80% da energia disponibilizada em impactos ditos leves, no intervalo de 1 a 6 Joules. Por fim, comprovou-se que a rigidez do material (módulo de elasticidade) é a propriedade residual (numa base de tolerância a danos) mais clara e consistentemente degradada pelo impacto previamente aplicado ao material. / Applications of hybrid fiber-metal laminates include, besides aeronautical industry, the automotive and naval industries. Unlike aeronautical field, impact research activities in the former areas of mobility industry are frequently limited by available laboratory equipment, so that it would be desirable to develop low-cost procedures to determine impact resistance and tolerance properties of construction materials. In this work, the transversal (trans-thickness) impact resistance and tolerance of hybrid fiber-metal laminate Glare-5® have been determined via three different methodologies. The first one utilizes sophisticated apparatus comprising a Laser-Doppler device to monitor deceleration/re-acceleration of 5 mm-diameter steel-ball impactor during the mechanical shock event. The second approach merely relies on the force (applied load) vs. impact time for determining the absorbed energy during the dynamic process. The third methodology requires only impactor velocity data points, immediately before and after the impact. It has been concluded that the energy values obtained from Laser- Doppler and load cell methods are very similar, with the former method producing slightly non-conservative results, allowing one to rapidly derive the impact resistance of hybrid laminate materials through very simple experimental set-ups employing digital load cells only. The third method presented somewhat higher results as compared to the concurrent techniques, so that it has been considered as inadequate for the research purposes. This study has shown that the fibre-metal laminate Glare absorbs between 60% and 80% of the apported impact energy during light impact events (ranging from 1 to Joules). Last, but not the least, materials stiffness was the most clearly and consistently imparted residual mechanical property (in a damage tolerance basis) due to the previous applied impact loading.

This research focuses on the characterization of damage accumulation in concrete specimens. Specifically, a nonlinear vibration technique is used to characterize the damage introduced by ongoing alkali-silica reactions (ASR). The nonlinear resonance testing consists of an analysis of the frequency response of concrete specimens subjected to impact loading. ASR introduces a third gel like phase, which can be expansive in the presence of moisture. The result of ASR is the formation of microcracks and debonding between aggregate and cement phases. Collectively, these changes act to increase the specimens' nonlinearity. As a result, it is found that the concrete samples exhibit nonlinear behavior; mainly a decrease in resonance frequency with an increasing level of excitation strain. The relationship between the amplitude of the response and the amount of frequency shift is used as a parameter to describe the nonlinearity of the specimen. The specimens used in this research are of varying reactivity with respect to ASR, which is induced in accordance with ASTM C 1293. The level of nonlinearity is used as a measure of damage caused by the progress of ASR throughout the one year test duration. These nonlinear resonance results are compared to the traditional measures of expansion described in the standard. The robustness and repeatability of the proposed technique is also investigated by repeated testing of samples assumed to be at a specific damage state. Finally, a petrographic staining technique is used to complement nonlinearity measurements and to further gain understanding of ASR. The results of this study show that the proposed nonlinear resonance methods are very sensitive to microstructural changes and have great potential for quantitative damage assessment in concrete.

Orientador: Itamar Ferreira / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-15T00:28:03Z (GMT). No. of bitstreams: 1 Kruger_EduardoLeira_D.pdf: 8976154 bytes, checksum: 7ee2f8d46393e64f855a07891a200250 (MD5) Previous issue date: 2009 / Resumo: O ensaio Charpy clássico, utilizado desde o início do século XX, permite determinar a energia global de fratura, propriedade mecânica conhecida como tenacidade. Esta energia tem caráter qualitativo e na aplicação no projeto estrutural, reduz-se a comparações entre as curvas de transição dúctil-frágil dos materiais. Com o surgimento da instrumentação do ensaio Charpy clássico é possível determinar a propriedade mecânica, tenacidade à fratura dinâmica, KId, muito utilizada na Mecânica de Fratura. O objetivo deste trabalho é analisar o sistema de instrumentação desenvolvido de acordo com a normatização ISO 14556(2000) para uma máquina de ensaio Charpy clássico, determinar e avaliar a tenacidade à fratura dinâmica, a partir do ensaio Charpy instrumentado, do aço AISI 4340 sob várias condições metalúrgicas obtidas através de tratamentos térmicos de têmpera e revenimento nas temperaturas de 473K, 573K, 673K e 773K. Foi observado que o sistema de instrumentação da máquina, a interpretação dos resultados dos esforços envolvidos no processo de fratura e o tempo para fratura são parâmetros de influência na determinação da tenacidade à fratura dinâmica. / Abstract: The classical Charpy test is used since 1900 and it is possible to determine from it the global energy for deformation and fracturing the specimen and this energy is named toughness which is important for selection steels considering ductile-brittle transition temperature. The Instrumented Charpy test is very important because it produces more material parameters, for example, the dynamic fracture toughness (KId), that can be used in Fracture Mechanics. However, there are several difficulties associated with the instrumentation of the impact machine ant processing digital signal, mainly in terms of the interpretation instrumented impact test results. The purpose of this work is to analyze the instrumentation system developed in accordance with normalization ISO 14556 (2000) to an Charpy machine and to determine and analyze the dynamic fracture toughness of the AISI 4340 steel in some metallurgical conditions obtained from treatment of quenching and tempering at 473K, 573K, 673K, and 773K. It was observed that the instrumentation system, the interpretation of the loads involved in the process of fracture and time to fracture are significant parameters in determining the dynamic fracture toughness. / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

Aplicações dos laminados híbridos metal-fibra incluem, além da indústria aeronáutica, as indústrias naval e automobilística. Diferentemente do setor aeronáutico, pesquisas sobre impactos mecânicos nas duas outras áreas da mobilidade são freqüentemente limitadas pela disponibilidade de equipamentos laboratoriais, de modo que é desejável o desenvolvimento de procedimentos de baixo custo para a determinação da resistência e tolerância a danos por impacto dos materiais de construção. Neste trabalho, a resistência a danos por impacto transversal do laminado híbrido metal-fibra Glare-5® foi determinada via três diferentes metodologias. A primeira utiliza um aparato sofisticado aparato Laser-Doppler para monitorar a aceleração e desaceleração de um impactador esférico de aço com 5 mm de diâmetro durante o evento do choque mecânico. O segundo método se baseia apenas nos valores de carga (força aplicada) vs. o tempo de impacto para a obtenção da energia absorvida pelo material. O terceiro considera somente os dados da velocidade do impactador, ou projétil, imediatamente antes e após o impacto. Concluiu-se que os valores de energia obtidos segundo as duas primeiras metodologias são similares, com o Laser-Doppler gerando resultados levemente não-conservadores, comprovando assim a possibilidade da derivação da resistência ao impacto do laminado através de um experimento simples e rápido, que utiliza apenas uma célula de carga digital para a monitoração da força aplicada em função do tempo. O terceiro método apresentou resultados substancialmente superiores às duas primeiras metodologias, sendo classificado como inadequado aos propósitos do projeto. Determinou-se que o laminado híbrido Glare-5®; absorve entre 60% e 80% da energia disponibilizada em impactos ditos leves, no intervalo de 1 a 6 Joules. Por fim, comprovou-se que a rigidez do material (módulo de elasticidade) é a propriedade residual (numa base de tolerância a danos) mais clara e consistentemente degradada pelo impacto previamente aplicado ao material. / Applications of hybrid fiber-metal laminates include, besides aeronautical industry, the automotive and naval industries. Unlike aeronautical field, impact research activities in the former areas of mobility industry are frequently limited by available laboratory equipment, so that it would be desirable to develop low-cost procedures to determine impact resistance and tolerance properties of construction materials. In this work, the transversal (trans-thickness) impact resistance and tolerance of hybrid fiber-metal laminate Glare-5® have been determined via three different methodologies. The first one utilizes sophisticated apparatus comprising a Laser-Doppler device to monitor deceleration/re-acceleration of 5 mm-diameter steel-ball impactor during the mechanical shock event. The second approach merely relies on the force (applied load) vs. impact time for determining the absorbed energy during the dynamic process. The third methodology requires only impactor velocity data points, immediately before and after the impact. It has been concluded that the energy values obtained from Laser- Doppler and load cell methods are very similar, with the former method producing slightly non-conservative results, allowing one to rapidly derive the impact resistance of hybrid laminate materials through very simple experimental set-ups employing digital load cells only. The third method presented somewhat higher results as compared to the concurrent techniques, so that it has been considered as inadequate for the research purposes. This study has shown that the fibre-metal laminate Glare absorbs between 60% and 80% of the apported impact energy during light impact events (ranging from 1 to Joules). Last, but not the least, materials stiffness was the most clearly and consistently imparted residual mechanical property (in a damage tolerance basis) due to the previous applied impact loading.

The thesis focuses on comparisons of usability of distinct types of glues to attach specimens. The specimens were glued using 4 types of glue: BISON Power Adhesive, BISON Epoxy Universal, PATTEX Repair Epoxy 5 min and a mixture of acetone with diluted ABS plastic as the last. The specimens underwent tensile and impact tests. For the tensile test, the specimens were made in 3 variants: blunt frontal joint, bevelled joint and gradually folded joint. For the impact test, the specimens were made in the form of a rod with a V-shaped notch. All variants of the joints consisted of 5 specimens with 3 unglued specimens for reference. The testing was conducted on the ZD 10/90 tensile strength machine and a Charpy impact test machine from the WPM company. The thesis contains technical-economic evaluation of the results of the tests with comparisons of the individual glues and forms.