Academic literature on the topic 'Strain Rate Sensitivity (SRS)'

Doutoramento em Ciência e Engenharia de Materiais<br>Nesta tese são apresentados estudos experimentais e microestruturais para a análise e controlo da sensibilidade à velocidade de deformação (SRS) da liga AA5182 e do aço TWIP com o objetivo de melhorar o comportamento mecânico destes materiais. Os aços TWIP são materiais com elevada resistência mecânica e excecional capacidade de encruamento, parâmetros que conduzem à absorção de uma quantidade significativa de energia antes de rotura. As ligas de AlMg são materiais leves, com boa resistência à corrosão e boas propriedades mecânicas. A larga variedade de aplicações, como por exemplo na indústria automóvel, permitirá melhorar a performance dos produtos e economizar energia. O maior problema destes materiais prende-se com a baixa ou negativa sensibilidade à velocidade de deformação que conduz a uma deformação heterogénea e limita a deformação após estricção. Neste trabalho são estudados métodos para melhorar a SRS das ligas de AlMg através de combinação de deformação plástica severa e tratamentos térmicos, e é investigada a origem física da baixa ou até negativa SRS do aço TWIP através de ensaios à escala macro, micro e nano. Estes estudos são complementados e sustentados por um amplo programa de observações microestructurais através de técnicas de microscopia TEM, SEM e EBSD. A deformação plástica severa na liga de AlMg foi aplicada através de laminagem. Foi demonstrado que o tipo de laminagem (simétrica versus assimétrica), o grau de redução de laminagem e o tratamento térmico realizado após a laminagem são os principais fatores que afetam a evolução da SRS. Especificamente, o aumento do grau de laminagem (de 50% para 90%) resulta num aumento da SRS. A técnica de laminagem assimétrica inversa (ASRR) revelou ser a mais eficiente no aumento do SRS, sendo que esta produz a maior deformação equivalente no material. Adicionalmente, para este tipo de laminagem e uma redução da espessura de 90%, verificou-se que a tensão de cedência aumenta para um tratamento térmico mais longo (de 30min a 120min). Conjetura-se que o processo físico associado ao comportamento observado está relacionado com a movimentação de ida e volta de solutos de Mg da solução sólida para precipitados/cachos durante o processo de laminagem e posterior tratamento térmico. A investigação à sensibilidade da velocidade de deformação de aço TWIP com base em testes mecânicos e caracterização microestrutural foi outro objetivo desta tese. Demonstrou-se que as amostras testadas com uma velocidade de deformação reduzida apresentam uma densidade de maclas maior do que as amostras testadas a uma velocidade de deformação maior. À escala macroscópica este traduz-se numa taxa de encruamento maior para velocidades reduzidas, conduzindo a um coeficiente de sensibilidade à velocidade de deformação em termos de taxa de encruamento negativo. Foi observada uma diminuição da SRS com o aumento da deformação, passando de valores positivos a negativos. O presente estudo demonstrou a importância da medida de escala utilizada na investigação do SRS através de uma combinação de testes de micro- e nano-indentações. Nomeadamente, quando o material é testado a uma escala nanométrica, através de nano-indentação, as amostras pré-deformadas em tração com taxas de deformação menores apresentam sistematicamente uma dureza menor do que as amostras pré-deformadas com taxas mais elevadas. À medida que o volume de material testado aumenta, a dureza relativa das duas amostras passa gradualmente da tendência observada à escala nano para aquela observada à escala macroscópica. O efeito está ligado ao mecanismo de interação entre as estruturas de deslocações e maclas.<br>In this thesis are presented experimental and microstructural studies for strain rate sensitivity (SRS) control and analysis of AA5182 and Twinning Induced Plasticity steel for improved mechanical performance. TWIP steels are materials with very high strength and exceptional strain hardening capability, parameters leading to large energy absorption before failure. Al-Mg alloys are lightweight materials with good corrosion resistance and adequate material properties. The broader use of these materials, for example in the automotive industry, would allow improved product performance and energy savings. The formability of these materials is strongly affected by their negative strain rate sensitivity (SRS) which leads to early failure and limits the post necking deformation. In this work we study ways to improve the strain rate sensitivity of Al-Mg alloys through a combination of severe plastic deformation and annealing, and we investigate the physical origins of the low and potentially negative strain rate sensitivity of TWIP steel through macro, micro and nanoscale testing. These studies are supported by extensive microstructural observations. The severe plastic deformation applied to Al-Mg alloys is applied by rolling. It is shown that the type of rolling (symmetric versus asymmetric), the rolling reduction degree and the applied heat treatment performed after rolling are the main factors affecting the evolution of SRS. Specifically, SRS increases with increasing the degree of rolling for given post-rolling heat treatment. The reversed asymmetric rolling technique appears to be the most efficient in increasing SRS since it produces the largest equivalent plastic strain in the sample. Furthermore, the evolution of tensile flow stresses depends on the chosen thermal treatment; it was observed that the yield stress increases with increasing the annealing time for rolling reduction of 85%. It is conjectured that the physical process responsible for the observed behavior is related to the movement of Mg from solid solution to precipitates/clusters and back during rolling and subsequent annealing. The investigation of the strain rate sensitivity of TWIP steel based on mechanical tests and microstructural characterization is another objective of this thesis. It was demonstrated that slower-deformed samples have a higher twin density, which leads to larger flow stress measured in a macroscopic uniaxial test and results in negative strain hardening rate sensitivity. The SRS is observed to decrease with strain, becoming negative for larger strains. The correlation between SRS and the probing scale was revealed by a combination of micro- and nano-indentation experiments. When probed at the nanoscale by nano-indentation, samples pre-deformed in tension at smaller strain rates exhibit systematically smaller hardness than samples pre-deformed at higher rates. As the volume of material probed increases, the relative hardness of the two types of samples gradually shifts from the trend observed at the nanoscale to that observed macroscopically. The effect is linked to the dislocation-twin interaction mechanism.

Includes bibliographical references (leaves 210-219)<br>An investigation into high strain rate behaviour of polymer composites was performed by developing a finite element model for a fibre reinforced polymer (FRP) plates impacted at varying strain rates. The work was divided into three facets, firstly to characterize the FRP material at varying strain rates, to develop a constitutive model to elucidate the relationship between strain rate and ultimate stress and lastly to use the experimental data to develop a finite element model. Experimental work performed in support of this model includes material characterization of unidirectional carbon and glass fibre reinforced epoxy at varying impact strain rates. The data is then used to develop a suite of constitutive equations that relate the strain rate, ultimate stress and material loading type. The model is of a linear and non-linear viscoelastic type, depending on the type of loading and is applicable to a FRP plate undergoing out-of-plane stresses. This model incorporates techniques for approximating the quasi-static and dynamic response to general time-varying loads. The model also accounts for the effects of damage, the linear and non-linear viscoelastic constitutive laws reporting failure by instantaneously reducing the relevant elastic modulus to zero. An explicit solver is therefore utilised in order to ensure stability of the numerical procedure. Glass fibre reinforced plastics (GFRP) was found to be more strain rate sensitive in all directions when compared to carbon fibre reinforced plastics (CFRP). The validation process therefore involves plate impact experimental testing on GFRP plates. The data from these experiments compare to within 8% of the finite element model that incorporates both damage and the developed strain rate sensitivity constitutive equations. For the first time a model that includes progressive damage with built-in strain rate sensitivity is developed for these particular FRP systems. Furthermore, the ultimate stress has been related to strain rate using an empirical technique. This technique allows for the prediction of dynamic ultimate stresses given the quasi-static ultimate stresses, again for this particular material systems.

The stress-strain behavior and failure of composite materials are strain rate sensitive, and influenced by the dimensions of the structure. To elucidate the combined effects of scaling and strain rate on the strength of unnotched continuous fiber reinforced composites, an experimental investigation has been conducted on Newport NB321/7781 fiberglass/epoxy and Toray T800/3900-2B unitape/epoxy materials. The experimental results have been characterized in terms of failure strength, failure modes and the Weibull modulus m. A 2D-scaling approach has been followed and composite coupons were fabricated with [0]4 and [±45]s stacking sequences. The experimentation has been conducted at strain rates ranging from quasi-static (0.0002 s^-1) to high strain rate (50 s^-1), to study the mechanical responses and associated failure modes. Subsequently, the Weibull statistical model was utilized to characterize the scaling behavior at different strain rates. The average failure stress of [0]4 carbon, [0]4 fiberglass and [±45]s fiberglass specimens were observed to decrease with increasing specimen size at each strain rate. However, at high strain rate, the percentage of strength reduction was observed to be lower in comparison to the quasi-static strain rate. Owing to the free edge effects, the scaling effect was maximum for [+45/-45]s carbon unitape specimens. But unlike the other stacking sequences, the percentage of strength reduction at higher strain rates was higher compared to quasi-static strain rate, indicating increased scaling effects with strain rate. Weibull modulus m for the specimens tended to increase with increasing strain rate indicating diminishing scaling effects, while [+45/-45]s carbon specimens exhibited opposite trend. Failure at multiple locations was observed in larger coupons at high strain rate, which results in size and strain rate dependent fracture behavior.<br>Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.

Plasticity in crystalline solids is controlled by the microscopic line defects known as “dislocations”. Decisive role of dislocations in crystal plasticity in addition to fundamentals of plastic deformation are presented in the current thesis work. Moreover, major features of numerical modeling method “Discrete Dislocation Dynamics (DDD)” technique are described to elucidate a powerful computational method used in simulation of crystal plasticity. First part of the work is focused on the investigation of strain rate effect on the dynamic deformation of crystalline solids. Single crystal copper is chosen as a model crystal and discrete dislocation dynamics method is used to perform numerical uniaxial tensile test on the single crystal at various high strain rates. Twenty four straight dislocations of mixed character are randomly distributed inside a model crystal with an edge length of 1 µm subjected to periodic boundary conditions. Loading of the model crystal with the considered initial dislocation microstructure at constant strain rates ranging from 103 to 105s1 leads to a significant strain rate sensitivity of the plastic flow. In addition to the flow stress, microstructure evolution of the sample crystal demonstrates a considerable strain rate dependency. Furthermore, strain rate affects the strain induce microstructure heterogeneity such that more heterogeneous microstructure emerges as strain rate increases. Anisotropic characteristic of plasticity in single crystals is investigated in the second part of the study. Copper single crystal is selected to perform numerical tensile tests on the model crystal along two different loading directions of [001] and [111] at two high strain rates. Effect of loading orientation on the macroscopic behavior along with microstructure evolution of the model crystal is examined using DDD method. Investigation of dynamic response of single crystal to the mechanical loading demonstrates a substantial effect of loading orientation on the flow stress. Furthermore, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Likewise, strain induced microstructure heterogeneity displays the effect of loading direction such that more heterogeneous microstructure evolve as single crystal is loaded along [111] direction. Formation of slip bands and consequently localization of plastic deformation are detected as model crystal is loaded along both directions.<br><p>QC 20151015</p>

High-entropy alloys (HEAs) that demonstrate excellent mechanical properties over steel-based alloys are not exempt from the common dilemma of strength–ductility trade-off, which limits their potential applications. One way to improve the property of CrMnFeCoNi HEA is by using the rotationally accelerated shot peening technique to introduce a gradient structure. Two gradient profiles—a thin gradient layer with an undeformed core and a fully deformed structure—are introduced by adjusting the processing parameters. The effects of these gradient profiles on mechanical properties and microstructural evolution at various loading conditions and temperatures are systematically explored. In this thesis, various mechanical tests are performed to investigate the effect of the gradient structure on mechanical properties such as tensile properties at room and cryogenic temperatures, compression at different strain rates and dynamic compression at high strain rates. Material characterisations are performed using various electron microscopic techniques to build a structure–property relationship and investigate microstructural evolution.