Complex-phase (CP) steels are employed in applications that require high-strength and good edge formability. These steels derive their strength from a fine-grained bainite-ferrite microstructure, and alloying to provide solid-solution and precipitation strengthening. CP steels are produced industrially through a process of controlled rolling and cooling to produce desirable microstructures. Hole-expansion tests are typically used as a measure of edge formability for applications such as stretch-flanges. It has been shown that CP microstructures are susceptible to large fluctuations in hole-expansion performance with little change in processing or resulting tensile properties. The steel’s characteristics of damage evolution are critical to the hole-expansion performance. This study investigates the role of microstructure in the development of damage in CP microstructural variants. Two variant pairs of different thicknesses were produced from the leading and trailing edge of industrially produced hot-rolled sheet. Each pair consisted of a variant with poor hole-expansion performance, and a variant with good hole-expansion performance. Each variant was tested via interrupted double-notched uniaxial tension testing to induce damage. Damage evolution in each variant was quantified by X-ray micro-computed tomography (XµCT), and supplementary optical micrography. The damage results were correlated with microstructural characteristics. It was shown that poor hole-expansion variants failed by intergranular fracture. In these variants, void damage induced by hard martensite and retained austenite was not critical in producing failure. Purely void-damaged microstructures failed by ductile fracture, whereas cracked microstructures failed in a mixed brittle-ductile failure initiated by planar cracks. Microstructural banding of large elongated ferrite grains correlated with the existence of intergranular planar fractures. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-12-17 15:03:02.206

In this dissertation, experimental investigations and theoretical studies on the stochastic matrix cracking evolution under static and fatigue loading in composite laminates with defects are presented. The presented work demonstrates a methodology that accounts for the statistically distributed defects in damage mechanics models for the assessment of the integrity of composites and for the structural design of composites. The experimental study deals with the mechanisms of the formation of a single crack on a micro-scale and the stochastic process for the multiplication of cracks on a macro-scale. The defects introduced by the manufacturing processes are found to have significant effect on the matrix cracking evolution. Influenced by the distributed defects, the initiation and multiplication of cracks evolve in a stochastic way. The experimental study on the in-plane shear stress finds the detrimental effect of the shear stress on the fatigue performance of composite laminates. Combined with the transverse tensile stress, the in-plane shear stress induces multiple inclined microcracks in the matrix, which enhance the initiation and propagation of the major matrix cracks. Based on the experimental investigations, a statistical model for the stochastic matrix cracking evolution on the macro-scale is developed. Simulations based on the statistical model yield accurate predictions for both static and fatigue loading compared to the experimental data. The Weibull distribution of the static strength is estimated by the statistical model by comparing against the experimental crack density data. The estimated Weibull distribution of the static strength provides an efficient approach to characterize the manufacturing quality of composite laminates. Compared to deterministic approaches, the Weibull distribution of the static strength provides comprehensive information of the strength property of composite laminates.

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Parmi les technologies disponibles de production d’électricité décarbonée et compétitive, les centrales solaires à concentration ponctuelle permettent d’atteindre les meilleurs rendements. Le développement commercial est actuellement orienté vers les tours à récepteur tubulaire ou surfacique. L’enjeu majeur consiste à faire fonctionner ces récepteurs à des températures supérieures à 800°C de manière cyclique, ce qui nécessite l’emploi de matériaux à hautes performances stables dans le temps. Les alliages ou céramiques réfractaires présentent des propriétés adaptées, mais l’évolution de leur endommagement est encore mal connue. Afin d’étudier ce comportement, un dispositif expérimental basé sur la technique d’émission acoustique a été développé. Cette technique permet d’estimer in situ la sévérité, le type, et la position d’un endommagement. Une méthode de test originale a été élaborée de manière à identifier une stratégie expérimentale de vieillissement pertinente. / Among the available technologies for carbon-free and competitive electricity production, solar tower power plants can achieve the best efficiency. Commercial development is currently focused on tubular or surface receivers, as they allow a low-intricacy design. The major challenge is to cyclically operate these receivers at temperatures above 800°C, which involves the use of durable and high performance materials. Refractory alloys and ceramics have adequate properties, but their damage evolution under these specific conditions is still poorly understood. An innovative set up based on the acoustic emission technique has been designed and realized with a view to scrutinize these materials in situ behavior. Indeed, acoustic emission enables to estimate the severity, the type, and the position of a damage appearing under concentrated solar irradiation tests. Besides, an original numerical method has been developed to identify the suitable test conditions for relevant experimental aging.

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Yes / Super-fine stainless wires (SSWs) with micron diameter and large specific surface area can simultaneously strengthen and toughen reactive powder concrete (RPC) at low volume fraction, so SSW reinforced RPC composites have potential for developing infrastructures bearing fatigue load or with aseismic requirements. In this paper, the uniaxial compressive fatigue characteristics of such composites under high stress levels were investigated, and the modification mechanisms of SSWs to RPC were revealed through failure state and microstructure analyses. The results showed that incorporating only 0.5 vol.% SSWs into RPC enables the fatigue life and energy dissipation capacity to increase by 252.0% and 262.3%, meanwhile, the fatigue limit strength of composites at the failure probability of 50% reaches up to 76.6% of static uniaxial compressive strength, due to the improvement effect on microstructure compactness, inhibiting effect on flaw initiation, and the ability to convert single main crack into radial multiple micro cracks centered on SSWs. Furthermore, the average maximum fatigue strain and residual strain of composites are improved by 73.7% and 87.2%, respectively, which can be ascribed to the bridging, debonding and being pulled-off effect of SSWs. It can be therefore concluded that the incorporation of SSWs endows RPC with excellent fatigue performance, thus further enlarging the application of composites. / The authors would like to thank the National Science Foundation of China (51908103 and 51978127), and the China Postdoctoral Science Foundation (2019M651116) for providing funding to carry out this investigation.

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In this study, dynamic crack propagation in brittle materials has been studied using a regularized phase field approach.The phase field model used has been validated using specific experimental results of a dynamic in-plane fracture. The crack branching phenomena and existence of a limiting crack tip velocity has been validated using a mode I simulation set-up. A parametric study has also been performed so as to normalize the various numerical parameters that affect the velocity at the crack tip. Following the validation of the phase field model a stochastic analysis of a PBX microstructure has been performed. The microstructure has a high HMX volume fraction of 79\%. The energetic material is HMX and the binder used is Sylgard. Artificial defects are introduced in the system using phase field cracks. The analysis uses a finite element framework that accounts for various thermal-mechanical processes like deformation, heat generation, conduction, fracture and frictional heating at the crack surfaces. The effect on the temperature and damage field due to varying parameters like loading velocities and critical energy release rates is studied. Critical hotspot formation due to localized frictional heating is also studied. A concept of dirty binder is introduced to increase the grain volume fraction of the energetic in the composite. This amounts to a homogenized binder that accounts for the influence of the subsume particles that do not contribute to fracture but affect material properties of the binder.

The non-reactive Dreiding and the reactive ReaxFF atomic potentials were applied within a family of atom molecular dynamics (MD) simulations to investigate and understand interfacial adhesion in graphene/vinyl ester composites. First, a liquid vinyl ester (VE) resin was equilibrated in the presence of graphene surfaces and then cured, resulting in a gradient in the monomer distribution as a function of distance from the surfaces. Then the chemically realistic relative reactivity volume (RRV) curing algorithm was applied that mimics the known radical addition regiochemistry and monomer reactivity ratios of the VE monomers during three-dimensional chain-growth polymerization. Surface adhesion between the cured VE resin and the graphene reinforcement surfaces was obtained at a series of VE resin “crosslink densities.” Both pristine and oxidized graphite sheets were employed separately in these simulations using a Dreiding potential. The pristine sheets serve as a surrogate for pure carbon fibers while oxidizing the outer graphene sheets serve as a model for oxidized carbon fibers. Hence, the effects of local monomer distribution and temperature on the interphase region formation and surface adhesion can be investigated. Surface adhesion was studied at various curing conversions and as a function of temperature. Uniaxial loading simulations were performed at different curing conversions for both models to predict the composites’ modulus of elasticity, Poisson’s ratio, and yield strength. The same analysis was performed for the neat cured matrix. The glass transition temperature (Tg) for the homogenized composite and neat VE matrix was determined at different degrees of curing. Subsequent MD simulations were performed to predict structural damage evolution and fracture in the neat VE matrix. The ReaxFF potential was used to quantify irreversible damage due to bond breakage in the neat VE matrix for different degrees of cure, stress states, temperatures, and strain rates. The predicted damage mechanisms in the bulk VE thermosetting polymer were directly compared to those for an amorphous polyethylene (PE) thermoplastic polymer.