O presente trabalho propõe o entendimento do comportamento no estado fresco de argamassas colantes, com base na caracterização reológica e físico-química de diferentes composições comerciais, servindo de base para analisar o fenômeno de deslizamento, a partir do ensaio estabelecido na norma brasileira. Para isso, foi utilizado o ensaio “Squeeze flow” (escoamento por compressão axial), empregado na caracterização de argamassas de revestimento no laboratório de microestrutura do CPqDCC da EPUSP, como ferramenta de análise do comportamento de argamassas colantes. Neste ensaio, o escoamento do material decorre da aplicação de uma carga de compressão sobre a amostra no estado fresco, a qual ocasiona deslocamentos no seu interior devido a esforços de cisalhamento radiais originados durante o fluxo. O critério de seleção das argamassas colantes comerciais (tipo AC-I) se baseou nos resultados do ensaio de deslizamento, escolhendo-se duas com resultado muito abaixo do limite especificado, duas com resultado próximo do limite e outras duas com resultado acima do mesmo. A composição química e física foi caracterizada com o objetivo de embasar a análise dos resultados obtidos no “Squeeze flow”. A separação da fração fina das argamassas na peneira no.200 contribuiu para o conhecimento da viscosidade da pasta e da sua influência no comportamento reológico das argamassas. Foi observado que as argamassas estudadas apresentam diferenças de composição físico-química e de comportamento reológico. As diferenças de comportamento reológico das argamassas decorrem, provavelmente, de ação sinérgica de alguns parâmetros da composição, com destaque para a distribuição granulométrica. O “Squeeze flow” mostrou-se uma ferramenta adequada na caracterização das argamassas colantes e contribuiu para explicar o deslizamento estabelecido na norma brasileira, pela proposição de modelos hipotéticos de comportamento. / Present thesis proposes the study of plastic-state behaviour of dry-set mortars based on the rheological and physicochemical characterization of different commercially available dry-set mortar compositions. Such characterization served as basis for the analysis of dry-set mortar slip phenomena using the tests recommended by brazilian standards (NBR). The Squeeze Flow test (slip by axial compression) originally used for coating mortars characterization by the EPUSP CPqDDC Microstructure Laboratory was adopted as a test tool for analysing the dry-set mortar behaviour. In the mentioned test the material slip is obtained by compressing the sample in its plastic state which caused internal displacements due to radial shearing tensions originated during the mentioned slip. The dry-se mortars (all of them AC-I type) used in the study were selected based in the slip tests results against brazilian standards specified limits resulting in the selection of two dry-set mortars below the specified limit, two dry-set mortars close to the specified limit and dry-set mortars above the specified limit. Chemical and physical compositions were characterized in order to serve as basis for Squeeze Flow results analysis. Fine fraction segregation, using number 200 sieve contributed to understanding of plastic-state mortar viscosity and its influence in mortar rheological behaviour. It was observed diverse physicochemical and rheological behaviour among the studied dry-set mortars. The rheological behaviour diversity of dry-set mortars were due to the synergy among some composition parameters, specially the granular distribution. The Squeeze Flow was considered a suitable tool for the characterization of dry-set mortars and contributed to develop hypotetical behaviour models that allowed to explain the slip as stated by brazilian standards.

The consolidation time is the time available for the application of adhesive mortar on the substrate. The NBR 14081-1 does not specify a method of test to determining the consolidation time, however, the standard of enforcement procedures of covering floors and walls with ceramic tiles (NBR 13755 and NBR 13754) establish 2 h and 30 min as the minimum time. By considering the significant differences between the formulations, the presence of polymeric additives, besides the evolution of cement and additions, a study about how and when the consolidation of adhesive mortar happens is needed. Then, it was looked for to evaluate and determine the time of consolidation of adhesive mortars of the types ACI, ACII and ACIII by Squeeze flow and Pull out flow tests in different time intervals (30, 60, 120, 180, 240 and 300 min) in fresh state. The test methods shown to be sensitive for the determination of consolidation time of adhesive mortars, indicating the increase of the viscosity and of the adhesion of the mortar over time. For mortars in which the consolidation occurred during the time period studied, this time made up to 180 minutes after mixing. From this period the adhesive mortars suffered losses in their rheological characteristics, which would cause difficulties in the settlement stages of ceramic plates and later problems in its performance and durability. On the other hand, the tensile bond strength tests showed great variability imparing a correlation with the rheological tests. / O tempo de consolidação é o período disponível para a aplicação da argamassa colante no substrato. A NBR 14081-1 não especifica um método de ensaio para a determinação do tempo de consolidação, já as normas de procedimento de execução de revestimento de pisos e paredes com placas cerâmicas (NBR 13755 e NBR 13754) estabelecem 2 h e 30 min como tempo mínimo. Considerando as diferenças significativas entre as formulações, a presença de aditivos poliméricos, além da evolução dos cimentos e das adições, faz-se necessário um estudo sobre como e quando se dá a consolidação da argamassa colante. Em virtude disso, procurou-se avaliar e determinar o tempo de consolidação das argamassas colantes dos tipos ACI, ACII e ACIII por meio dos ensaios de Squeeze flow e Pull out flow, em diferentes intervalos de tempo (30, 60, 120, 180, 240 e 300 min) no estado fresco. Os métodos de ensaio mostraram-se sensíveis para a determinação do tempo de consolidação das argamassas colantes, indicando o aumento da viscosidade e da adesão da argamassa com o passar do tempo. Para as argamassas em que o tempo de consolidação ocorreu durante o período estudado, este tempo deu-se aos 180 minutos após a mistura. A partir deste período as argamassas colantes sofreram perdas nas suas características reológicas o que poderá causar dificuldades nas etapas de assentamento das placas cerâmicas e posteriormente, problemas no seu desempenho e durabilidade. Já os ensaios de resistência de aderência à tração apresentaram grande variabilidade prejudicando uma correlação com os ensaios reológicos.

A conformação em torno elétrico é um dos métodos utilizados na fabricação de peças cerâmicas, principalmente utilitários e decorações. Porém nem todas as massas cerâmicas possuem plasticidade adequada para serem torneadas. A porcelana fosfática é um desses exemplos devido à composição de 50 % de cinza de ossos bovinos, 25 % de caulim e 25 % de feldspato. Uma massa de porcelana comercial de alta temperatura, branca e com boa plasticidade foi a referência de massa propícia para se trabalhar no torno elétrico e foi feito a caracterização desse material como distribuição granulométrica, picnometria a gás, composição química por fluorescência de raio X (FRX) e difração de raio X (DRX). Os mesmos métodos de caracterização foram feitos na porcelana fosfática. O limite de Atterberg foi utilizado como técnica para medir os teores de água das massas e a reometria por squeeze flow foi o método de análise para diferenciar massas cerâmicas plásticas e não-plásticas. Testes no torno elétrico foram feitos para concluir sobre melhoria na plasticidade da porcelana fosfática através da adição de aditivo como bentonita e polímero à base de éter celulose (MHEC). Adição de 4 % de bentonita na porcelana fosfática aumentou o índice de plasticidade de Atterberg em cerca de 100 % e as curvas de squeeze flow ficaram próximos das curvas do material de referência, apresentando assim plasticidade suficiente para fabricar peças no torno elétrico. / Throwing on electric wheel is one of techniques used to forming ceramic wares as tableware and decorative. However, ceramic body needs to have enough plasticity for hands working on throwing wheel. The phosphatic porcelain composition is 50 % of bone ash, 25 % of kaolin and 25 % of feldspar and generally has low plasticity. A commercial porcelain for throwing on the electric wheel was used as default and compared with the phosphatic porcelain. Raw material characterization as particle size distribution analysis, gas pycnometry, chemical composition by x-ray fluorescence (XRF), zeta potential and x-ray diffraction (XRD) was doing to compare both porcelains. Atterberg limits were used to measure moisture content of ceramic body and rheometry was evaluated by squeeze flow technique to determine the viscosity difference between porcelain and phosphatic porcelain. Test on the throwing wheel were made to verify plasticity improvement by addition of bentonite or a polymer based on ether cellulose (MHEC). The 4 % of bentonite addition increased about 100 % the Atterberg limit and the consequent change in the squeeze flow curves demonstrate to be similar with reference and with enough plasticity to throwing on the electric wheel. Keyword: Phosphatic porcelain. Plasticity. Throwing wheel. Bentonite. Squeeze flow.

A conformação em torno elétrico é um dos métodos utilizados na fabricação de peças cerâmicas, principalmente utilitários e decorações. Porém nem todas as massas cerâmicas possuem plasticidade adequada para serem torneadas. A porcelana fosfática é um desses exemplos devido à composição de 50 % de cinza de ossos bovinos, 25 % de caulim e 25 % de feldspato. Uma massa de porcelana comercial de alta temperatura, branca e com boa plasticidade foi a referência de massa propícia para se trabalhar no torno elétrico e foi feito a caracterização desse material como distribuição granulométrica, picnometria a gás, composição química por fluorescência de raio X (FRX) e difração de raio X (DRX). Os mesmos métodos de caracterização foram feitos na porcelana fosfática. O limite de Atterberg foi utilizado como técnica para medir os teores de água das massas e a reometria por squeeze flow foi o método de análise para diferenciar massas cerâmicas plásticas e não-plásticas. Testes no torno elétrico foram feitos para concluir sobre melhoria na plasticidade da porcelana fosfática através da adição de aditivo como bentonita e polímero à base de éter celulose (MHEC). Adição de 4 % de bentonita na porcelana fosfática aumentou o índice de plasticidade de Atterberg em cerca de 100 % e as curvas de squeeze flow ficaram próximos das curvas do material de referência, apresentando assim plasticidade suficiente para fabricar peças no torno elétrico. / Throwing on electric wheel is one of techniques used to forming ceramic wares as tableware and decorative. However, ceramic body needs to have enough plasticity for hands working on throwing wheel. The phosphatic porcelain composition is 50 % of bone ash, 25 % of kaolin and 25 % of feldspar and generally has low plasticity. A commercial porcelain for throwing on the electric wheel was used as default and compared with the phosphatic porcelain. Raw material characterization as particle size distribution analysis, gas pycnometry, chemical composition by x-ray fluorescence (XRF), zeta potential and x-ray diffraction (XRD) was doing to compare both porcelains. Atterberg limits were used to measure moisture content of ceramic body and rheometry was evaluated by squeeze flow technique to determine the viscosity difference between porcelain and phosphatic porcelain. Test on the throwing wheel were made to verify plasticity improvement by addition of bentonite or a polymer based on ether cellulose (MHEC). The 4 % of bentonite addition increased about 100 % the Atterberg limit and the consequent change in the squeeze flow curves demonstrate to be similar with reference and with enough plasticity to throwing on the electric wheel. Keyword: Phosphatic porcelain. Plasticity. Throwing wheel. Bentonite. Squeeze flow.

Carbon mat thermoplastics (CMT) consisting of 12.7 mm or 25.4 mm long, 7.2 micrometer diameter, chopped carbon fibers in a polypropylene (PP) or poly(ethylene terephthalate) (PET) thermoplastic matrix were manufactured using the wetlay technique. This produces a porous mat with the carbon fibers well dispersed and randomly oriented in a plane. CMT composites offer substantial cost and weight savings over typical steel construction in new automotive applications. In production vehicles, automotive manufacturers have already begun to use glass mat thermoplastic (GMT) materials that use glass fiber as the reinforcement and polypropylene as the matrix. GMT parts have limitations due to the maximum achievable strength and stiffness of the material. In this study the glass fibers of traditional GMT are replaced with higher strength and higher stiffness carbon fibers. The tensile strength and modulus and the flexural strength and modulus of the CMT materials were calculated for fiber volume fractions of 10-25%. Additionally, the length of the fiber (12.7 mm or 25.4 mm) was varied and four different fiber treatments designed to improve the bond between the fiber and the matrix were tested. It was found that the fiber length had no effect on the mechanical properties of the material since these lengths are above the critical fiber length. The tensile and the flexural moduli of the CMTs were found to increase linearly with the FVF up to 25% FVF for some treatments of the fibers. For the other treatments the linearly increasing trend was valid up to 20% FVF, then stiffness either stayed constant or decreased as the FVF was increased from 20% to 25% . The strength versus FVF curves showed trends similar to those of the modulus versus FVF curves. It is shown that choosing an appropriate sizing can extend the usable FVF range of the CMT by at least 5%. Published micromechanical relations over-predicted the tensile modulus of the composite by 20-60%. An empirical fiber efficiency relation was fit to the experimental data for the tensile modulus and the tensile strength giving excellent agreement with the experimental results. Flow tests simulating the compression molding process were conducted on the CMT to determine what factors affect the flow viscosity of the CMT. The melt viscosity of the neat PP was measured using cone and plate rheometry at temperatures between 180°C–210°C and was fit with the Carreau relation. The through thickness packing stress of the CMT mat was measured for FVFs of 8-40% and was found to follow a power law behavior based on the local bending of fibers up to a FVF of 20.9%. Above this FVF the power law exponent decreases, and this is attributed to fracture of some of the fibers. Heated platens were used to isothermally squeeze the CMT at axial strain rates of 0.02-6 s^-1. The plot of the load-displacement behavior for the 10% FVF CMT was similar in shape to that for a fluid with a yield stress. For FVFs of 15-25% the load-displacement curves showed a load spike at the beginning of the flow, then followed the curve for a fluid with a yield stress. The matrix was burned off the squeezed samples, and the remaining carbon mat was dissected and visually inspected. It was found that fiber breakage increased and fiber length decreased as the FVF of the sample was increased. / Ph. D.

Long fiber reinforced thermoplastic composites are used for creating lightweight, but mechanically sound, automotive components. Injection molding is a manufacturing technique commonly used for traditional thermoplastics due to its efficiency and ability to create complex geometries. Injection molding feedstock is often in the form of pellets. Therefore, fiber composites must be chopped for use in this manufacturing method. The fibers are cut to a length of 13 mm and then fiber attrition occurs during processing. The combination of chopping the fibers into pellets and fiber breakage creates a distribution of mostly short fiber lengths, with some longer fibers remaining. Discontinuous fiber reinforcements are classified as long for aspect ratios greater than 100. For glass fibers, that distinction occurs at a length of 1 mm, and for carbon fibers 0.5 mm. Traditional composite materials and manufacturing processes utilize continuous fibers with a controlled orientation and length. The use of chopped discontinuous fibers requires a method to predict the orientation of the fibers in the final molded piece because mechanical properties are dependent on fiber length and orientation. The properties and behavior of the flow of a fiber reinforced polymer composite during molding are directly related to the mechanical properties of the completed part. Flow affects the orientation of the fibers within the polymer matrix and at locations within the mold cavity. The ability to predict, and ultimately control, flow properties allows for the efficient design of safe parts for industrial uses, such as vehicle parts in the automotive industry. The goal of this work is to test material characterization techniques developed for measuring and predicting the orientation of fiber reinforced injection molded thermoplastics using commercial grade long carbon fiber (LCF) reinforced nylon 6,6 (PA 6,6). Forty weight percent LCF/PA 6,6 with a weight averaged fiber length of 1.242 mm was injection molded into center gated disks and the orientation was measured experimentally. A Linux based Numlab flow simulation process that utilizes the finite element method to model the flow and orientation of fiber reinforced materials was tested and modified to accurately predict the orientation for this composite and geometry. Fiber orientation models used for prediction require the use of empirical parameters. A method of using non-lubricated squeeze flow as an efficient way to determine the strain reduction factor, , and Brownian motion like factor, CI, parameters for short glass fiber polypropylene orientation predictions using the strain reduction factor (SRF) model was extended to use with the LCF/PA 6,6 composite. The 40 weight percent LCF/PA 6,6 material was compression molded and underwent non-lubricated squeeze flow testing. The flow was simulated using finite element analysis to predict the fiber orientation using the SRF model. The empirical parameters were fit by comparing the simulated orientation to experimentally measured orientation. This is a successful method for predicting orientation parameters that is significantly more efficient than optimizing the parameters based on fitting orientation generated in injection molded pieces. The determined orientation parameters were then used to reasonably predict the fiber orientation for the injection molded parts. The authors proved that the experimental and simulation techniques developed for the glass fiber reinforced polypropylene material are valid for use with a different, more complex material. / Doctor of Philosophy / Fibers reinforce thermoplastic polymers to create lightweight, but mechanically sound, automotive parts. Thermoplastics flow when heated and harden when cooled. This work compares two of the commonly used thermoplastics, polypropylene (plastic grocery bags, food storage containers) with a glass fiber reinforcement and a form of nylon called PA 6,6 with a carbon fiber reinforcement. Injection molding is a manufacturing technique commonly used for un-reinforced thermoplastics due to its efficiency and ability to create complicated shapes. Injection molding feedstock is often in the form of pellets. Therefore, fiber composites must be chopped for use in this manufacturing method. The fibers are cut to a length of 13 mm and then fiber breakage occurs in the injection molder. The combination of chopping the fibers into pellets and fiber breakage creates a range of lengths. This distribution consists of mostly short fiber lengths, with some longer fibers remaining. Discontinuous fiber reinforcements are classified as long for aspect ratios (the ratio of length over diameter) greater than 100. For glass fibers, that distinction occurs at a length of 1 mm, and for carbon fibers 0.5 mm. Traditional composite materials and manufacturing processes utilize continuous fibers with a controlled orientation and length, such as the weave pattern one might see in a carbon fiber hood. The use of chopped fibers requires a method to predict the orientation of the fibers in the final molded piece because mechanical properties are dependent on fiber length and orientation. The way that the plastic flows during molding is directly related to the mechanical properties of the completed part because flow affects the way that the fibers arrange. The ability to predict, and ultimately control, flow properties allows for the efficient design of safe parts for industrial uses, such as vehicle parts in the automotive industry. The goal of this work is to test the techniques developed for measuring and predicting the orientation of fiber reinforced injection molded thermoplastics using commercial grade long carbon fiber (LCF) reinforced nylon 6,6 (PA 6,6). LCF/PA 6,6 with an average fiber length of 1.242 mm was injection molded into a disk and the orientation was measured experimentally. A computer flow simulation process that utilizes the finite element method to model the flow and orientation of fiber reinforced materials was tested and modified to accurately predict the orientation for this composite and geometry. Fiber orientation models used for prediction require the use of parameters. There is no universal method for determining these parameters. A method of using non-lubricated squeeze flow as an efficient way to determine the parameters for short glass fiber polypropylene orientation predictions was extended to use with the LCF/PA 6,6 composite. The LCF/PA 6,6 material was compression molded and underwent non-lubricated squeeze flow testing. The flow was modeled to predict the fiber orientation. The empirical parameters were fit by comparing the simulated orientation to experimentally measured orientation. This is a successful method for predicting orientation parameters. The determined orientation parameters were then used to reasonably predict the fiber orientation for the injection molded parts. The authors proved that the experimental and simulation techniques developed for the glass fiber reinforced polypropylene material are valid for use with a different, more complex material.

<p dir="ltr">Non-Newtonian fluid models such as the Bingham and Herschel-Bulkley models are used to characterize the flow behavior of many complex fluids and soft solids. The three parameter Herschel-Bulkley model captures the yield stress behavior and the nonlinear power law behavior. In this thesis, the semi-analytical solution of Herschel-Bulkley fluids provided by Covey and Stanmore is used to experimentally characterize the squeeze flow behavior. A ‘Squeeze Flow and Thermal Resistance Tester’ was custom designed to perform velocity controlled squeeze flow experiments. The tester has an additional capability of performing thermal resistance characterization adhering to the ASTM-D5470 standard. A novel framework is described for characterizing the three Herschel-Bulkley parameters (τy, n and ηHB) using the developed tester. </p><p dir="ltr">Thermal Interface Materials (TIMs) are used to efficiently dissipate heat from a heat generating component to a heat sink in an electronic package. Thermal grease is a type of TIM comprising of a base material (e.g. polymer) loaded with highly conducting filler particles (e.g, boron nitride, alumina or sometimes conducting metals such as aluminum or silver). These greases are expected to exhibit Herschel-Bulkley flow behavior. Hence, thermal greases are used as candidate materials for squeeze flow characterization. In addition to the flow characterization, the thermal resistance across these thermal greases are also characterized using the custom designed tester. Characterization of mechanical and thermal behavior of TIMs is crucial to predicting their long-term reliability. </p><p dir="ltr">The effect of in-situ isothermal baking duration and test temperature on flow behavior is studied. The increase in duration of isothermal baking at test temperature of 55◦C showed that the material tends to stiffen with baking duration. The increase in test temperature from 5◦C to 100◦C resulted in a decrease in the power law index n and viscosity ηHB. </p><p dir="ltr">Finally, a numerical simulation strategy for performing squeeze flow simulations is described. The characterized flow parameters from the squeeze flow experiments were used as input material parameters for a dynamic mesh-based numerical simulation of squeeze flow between parallel surfaces. The results of the experimental force response and numerical simulation results were compared and found to be in close agreement. In order to simulate flow of thermal greases in a package undergoing deformation, a non-flat test setup was fabricated and squeeze experiments were performed. Numerical simulations were subsequently performed for the non-flat surface using material parameters extracted from previous experiments and the results were compared. The results from both experiments and numerical simulations showed that the force response of thermal greases under non-flat surfaces was significantly higher than the planar case.</p>

Molding is a simple manufacturing process whereby fluid fills a master tool and then solidifies in the shape of the tool cavity. The precise nature of material flow during molding has long allowed fabrication of plastic components with sizes 1 mm 1 m. Polymer molding with precise critical dimension control could enable scalable, inexpensive production of micro- and nanostructures for functional or lithographic use. This dissertation reports experiments and simulations on molding of polymer micro- and nanostructures at length scales 1 nm 1 mm. The research investigates two main areas: 1) mass transport during micromolding and 2) polymer mechanical properties during nanomolding at length scales 100 nm. Measurements and simulations of molding features of size 100 nm 1 mm show local mold geometry modulates location and rate of polymer shear and determines fill time. Dimensionless ratios of mold geometry, polymer thickness, and bulk material and process properties can predict flow by viscous or capillary forces, shape of polymer deformation, and mold fill time. Measurements and simulations of molding at length scales 100 nm show the importance of nanoscale physical processes distinct from bulk during mechanical processing. Continuum simulations of atomic force microscope nanoindentation accurately model sub-continuum polymer mechanical response but highlight the need for nanoscale material property measurements to accurately model deformation shape. The development of temperature-controlled nanoindentation enables characterization of nanoscale material properties. Nanoscale uniaxial compression and squeeze flow measurements of glassy and viscoelastic polymer show film thickness determines polymer entanglement with cooperative polymer motions distinct from those observed in bulk. This research allows predictive design of molding processes and highlights the importance of nanoscale mechanical properties that could aid understanding of polymer physics.

[pt] É uma prática usual em reologia medir o tensão limite de escoamento. Nessas medidas, a tensão limite de escoamento é definida como o máximo valor absoluto de tensão ao qual abaixo não ocorrem escoamentos irreversíveis. Sendo assim, tensão limite de escoamento aparente estimada é usada em conjunto com o critério de von Mises em qualquer escoamento complexo. Este critério compara esta medida a intensidade do segundo invariante do tensor deviatórico das tensões. Acontece que, para escoamento simples de cisalhamento, o mesmo é composto por tensões cisalhantes e diferenças de tensão normais, mas a contribuição do último nunca foi considerada na determinação experimental da tensão limite de escoamento. Em vista de avaliar a importância da contribuição das diferenças de tensões normais na tensão limite de escoamento aparente, foram realizadas uma sequência de testes de creep para cada material, estimando a tensão crítica que representa o valor médio obtido entre os valores das curvas de tensão nas quais o material escoa e não escoa com uma tolerância considerável. Depois disso, foram propostos testes para avaliar os valores de N1 − N2 e apenas N1 no nível de tensão crítica. E em seguida avaliando-se adequadamente a tensão limite de escoamento. Observou-se que, para alguns materiais, a contribuição das diferenças de tensões normais é muito maior do que a contribuiçõ da tensão cisalhante. Por fim, a validade do critério de von Mises para materiais elasto-viscoplásticos foi avaliada. Para este fim, com o intuito de generalizar o estudo, ensaios de compressão a volume constante e de tração foram realizados avaliando-se as correspondentes tensões limites de escoamento. Como conclusão mais importante, o critério de von Mises não foi considerado adequado como critério de falha para os materiais elasto-viscoplásticos analisados. / [en] It is usual practice in rheology to measure the yield stress in a simple shear flow. In these measurements, the yield stress is identified as the maximum value of the shear stress below which no irreversible flow occurs. Then, the thus determined yield stress is used in conjunction with the von Mises criterion in any complex flow. The latter compares it with the intensity of the deviatoric stress tensor. It happens that for simple shear flow the intensity of the deviatoric stress is composed of both the shear stress and the normal stress differences, but the contribution of the latter is never considered in the experimental determination of the yield stress. In view of assess the importance of the contribution of the normal stresses to the yield stress, a sequence o standard constant shear stress tests were performed for each material, estimating the critical stress which represents the mean value obtained between the stress values of the curves in which the material flows and does not flow with an accurate tolerance. After that, proposed tests were performed in order to obtain the values of N1 − N2 and solely N1 at the critical stresses. Following the appropriate yield stress evaluation. It was observed that for some materials the normal stress contribution is much larger than the shear stress contribution. Furthermore, the validity of the von Mises yielding criterion for elasto-viscoplastic materials was evaluated. For this purpose, in order to generalize the study for different flow conditions, constant volume squeeze flow and traction tests were performed evaluating the corresponding yield stresses. As the most important conclusion, the von Mises yielding criterion was considered not to be accurate representing yielding for the elastoviscoplastic materials analyzed.