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Improved surgical procedures and implant developments for ligament or tendon repair require an in-depth understanding of tissue load-deformation and fatigue properties. Cyclic testing will provide crucial information on the behavior of these materials under reoccurring loads and on fatigue strength. Sparse data are available describing soft tissue behavior under cyclic loading. To examine fatigue strength, a new technology was trialed deploying 3D-printing to facilitate and standardize cyclic tests aiming to determine tendon fatigue behavior. Cadaveric flexor digitorum tendons were harvested and mounted for tensile testing with no tapering being made, using 3Dprinted clamps and holder arms, while ensuring a consistent testing length. Loads ranging between 200 to 510 N were applied at a frequency of 4 Hz, and cycles to failure ranged between 8 and >260,000. S–N curves (Woehler curves) were generated based on the peak stresses and cycles to failure. Power regression yielded a combined coefficient of determination of stress and cycles to failure of R 2 = 0.65, while the individual coefficients for tissues of single donors ranged between R 2 = 0.54 and R 2 = 0.88. The here-presented results demonstrate that S–N curves of human tendons can be obtained using a standardized setting deploying 3D-printing technology

Utmattning av stålmaterial är ett fenomen som varit känt i närmare 190 år men trots det är det fortfarande långt ifrån helt utforskat. Utmattningsegenskaper hos grundmaterial och svetsar är idag relativt utforskat, medan områden som utmattningsegenskaper hos kallformade stålprofiler fortfarande mer eller mindre är outforskat. Med utforskningen av utmattningsegenskaper hos kallformade stålprofiler som huvudsyfte delades examensarbetet upp i två delar. En överliggande kranbomsoptimering med huvudmålet att sänka vikten så mycket som möjligt med hjälp av materialbyte till ett mer höghållfast stål och optimal användning av material med avseende på utmattning. Den andra delen av examensarbetet är en framtagning av en provkropp med huvudmålet att kunna testa utmattningsegenskaper för kallformade stålprofiler samt att ge dimensionerande värden till kranbomsoptimeringen. Provstavsframtagningen resulterade i tre versioner av provkroppar som i teorin ska ge utmattningsegenskaper i longitudinell riktning för kallformade stålprofiler. Eftersom provstavarna inte kunde prövas på grund av tidsbrist baserades kranbomsoptimeringen på teoretiska värden. Resultatet av kranbomsoptimeringen var att vikten kunde sänkas från 1027 kg till 435 kg utan att påverka andra beaktade kranegenskaper. / Even though fatigue in steel structures is a phenomenon that was discovered almost 190 years ago it is still far from fully explored. The fatigue properties of base materials and welded materials are currently relatively well known, but the fatigue properties of cold-formed steel profiles are still more or less unknown. With the investigation of fatigue properties of cold-formed steel profiles as the main purpose, this master thesis was divided into two parts. An overhead optimization of a crane with the main goal of reducing weight as much as possible with the help of upgrading materials to higher strength steels and using the materials in an optimal way with respect to fatigue. The second part of the master thesis is the development of a test specimen with the main goal of testing the fatigue properties of cold-formed steel profiles and to provide dimensioning values for the optimization of the crane. The development of test specimen resulted in three versions of specimens which in theory will provide fatigue properties in the longitudinal direction for cold-formed steel profiles. Since the test specimens could not be tested because of shortage of time, the optimization of the crane had to be based on theoretical values. The results of the optimization were that the weight could be lowered from 1027 kg to 435 kg without affecting any other consideration characteristics of the crane.

Fatigue testing of stretch sensors often focuses on high amplitude, low-cycle fatigue (LCF) behavior; however, when used for orthopaedic, athletic, or ergonomic assessments, stretch sensors are subjected to low amplitude, high-cycle fatigue (HCF) conditions. As an added layer of complexity, the fatigue testing of stretch sensors is not only focused on the life of the material comprising the sensor, but also on the reliability of the signal produced during the extension and relaxation of the sensor. Research into the development of a smart sock that can be used to measure the range of motion (ROM) of the ankle joint during athletic practices and competitions using stretch sensors is ongoing at Mississippi State University. The current smart sock prototype utilizes StretchSense™ StretchFABRIC capacitive dielectric elastomer sensors. These sensors are no longer manufactured, and FlexSense stretch sensors are being investigated as a potential replacement. To assess the reliability of the signal of the StretchFABRIC sensors currently used in the prototype, two sensors were subjected to 25,000 cycles of fatigue, under with simultaneous capture of the capacitance. The capacitances of the fatigued sensors were then compared to the capacitance of an unfatigued StretchFABRIC sensor during participant trials. Participants completed four static movements and six dynamic gait trials using either the fatigued or unfatigued sensor. Following completion of the initial static and dynamic movements, the movements were repeated using the opposite sensor. Comparison of the fatigued sensor to the unfatigued sensor revealed an upward drift in the capacitance of the fatigued sensor for all trials. Two FlexSense sensors were then subjected to either 450,000 or 250,000 cycles of fatigue with simultaneous capture of the signal from the sensor. To assess the signal, the peak capacitance recorded during the fatigue test was compared to the peak stretch percentage produced by the sensor. The peak displacement remained tight about the mean, while the peak stretch percentage exhibited a high level of scatter. From a materials standpoint, the sensors conformed to the Rabinowitz-Beardmore model of polymer fatigue where an initial monotonic overload of the material is followed by a transition to cyclic stability of the material.

[pt] O presente trabalho se baseia na teoria dos defeitos equivalentes, também conhecida como teoria de Murakami, que permite a predição da vida-fadiga de materiais estruturais adotando o parâmetro (raiz quadrada de area), responsável por uma equivalência quantitativa entre heterogeneidades microestruturais (inclusões metalúrgicas) existentes na região de análise do material e descontinuidades mecânicas (furos) usinados nos corpos de prova. Neste contexto, a tese propõe novas equações, a partir da teoria de Murakami, para prever falhas por fadiga em carregamentos ultralongos (fadiga de altíssimo ciclo, VHCF) do aço DIN42CrMo4, de larga aplicação na fabricação de eixos virabrequins para unidades geradoras de usinas termoelétricas. Corpos de prova do aço DIN42CrMo4 foram usinados com furos de diâmetros variando entre 0,18 mm e 0,70 mm e ensaiados num regime de fadiga de altíssimo ciclo, variando o valor da amplitude de tensão do ensaio, para assim determinar em quais condições o material falha e obter a curva experimental S-N do aço DIN42CrMo4 na presença de diferentes tamanhos de defeitos (furos). Os resultados experimentais permitiram o desenvolvimento de equações em função do parâmetro e curvas de Wohler do material, em conformidade com diferentes descontinuidades mecânicas nos corpos de prova e resistências à fadiga do material. Consequentemente, se estabeleceu uma metodologia que permite uma correlação entre inclusões metalúrgicas, tensão aplicada e vida superlonga em fadiga de eixos virabrequins em serviço em usinas termoelétricas. / [en] The present work is based on the theory of equivalent defects, also known as Murakami s theory, which allows the prediction of the fatigue life of structural materials by adopting the parameter (square root of area), responsible for a quantitative equivalence between microstructural heterogeneities (metallurgical inclusions) existing in the region analysis of the material and mechanical discontinuities (holes) machined in the specimens. In this context, the thesis proposes new equations, based on Murakami s theory, to predict fatigue failures in ultralong loads (very high cycle fatigue, VHCF) of DIN42CrMo4 steel, widely used in the manufacture of crankshafts for generating units of thermoelectric power plants. DIN42CrMo4 steel specimens were machined with holes with diameters varying between 018 mm and 0.70 mm and tested in a very high cycle fatigue regime, varying the value of the test stress amplitude, to determine under what conditions the material failure and obtain the experimental S-N curve of DIN42CrMo4 steel in the presence of different sizes of defects (holes). The experimental results allowed the development of equations as a function of the parameter and Wohler curves of the material, in accordance with different mechanical discontinuities in the test specimens and resistance to fatigue of the material. Consequently, a methodology was established that allows a correlation between metallurgical inclusions, applied stress and super-long fatigue life of crankshafts in service in thermoelectric power plants.

[pt] Uma relação direta onde a vida em fadiga Número de ciclos por falha pode ser descrita como função das amplitudes macroscópicas de tensão normal e cisalhante, Amplitude de tensão normal macroscópica e Amplitude macroscópica da tensão de cisalhamento, é obtida. Utilizando o critério de Carpinteri e Spagnoli (CeS) como uma ferramenta de inspeção, foram obtidas curvas de nível elípticas sobre um domínio Amplitude de tensão normal macroscópica x Amplitude macroscópica da tensão de cisalhamento. A expressão das curvas de nível obtidas foi generalizada, proporcionando uma ferramenta capaz de prever o número de ciclos para falha associado a qualquer combinação (Amplitude de tensão normal macroscópica, Amplitude macroscópica da tensão de cisalhamento). As previsões obtidas através do método da curva elíptica foram comparadas às observações experimentais, bem como a previsões obtidas a partir de versões adaptadas de modelos consagrados na literatura, a saber: Findley (F), Matake (M), McDiarmid (McD), Susmel e Lazzarin (SeL), Carpinteri e Spagnoli (CeS) e Papadopoulos (P). O modelo proposto forneceu previsões em boa concordância com as observações experimentais, e sua capacidade de avaliar o comportamento em fadiga se revelou o melhor dentro todos os critérios considerados. Por fim, uma leve tendência conservadora do modelo foi atenuada através da introdução de um parâmetro de ajuste, melhorando ainda mais sua capacidade de avaliação de comportamento em fadiga. / [en] A direct relation where fatigue life Number of cycles to failure can be determined as function of macroscopic normal and shear stress amplitudes Macroscopic normal stress amplitude and Macroscopic shear stress amplitude is established. Using the Carpinteri and Spagnoli (CandS) criterion as a survey tool, elliptical level curves in the Macroscopic normal stress amplitude x Macroscopic shear stress amplitude domain were revealed and further generalised, providing means to determine the number of cycles to failure for any given (Macroscopic normal stress amplitude, Macroscopic shear stress amplitude ) combination. Predictions obtained through the elliptical curve method (E) were compared to experimental observations, as well as to predictions obtained from adapted versions of popular fatigue criteria, namely Findley (F), Matake (M), McDiarmid (McD), Susmel and Lazzarin (SandL), Carpinteri and Spagnoli (CandS) and Papadopoulos (P). The proposed model delivered predictions in fair agreement with experimental observations and its predictive capability was seen to be the best among all the considered criteria. Finally, a slight bias towards conservativeness was attenuated with the introduction of an adjusting parameter, further improving the predictive capability of the model.

Es wird ein einfacher Berechnungsansatz für ein Baukastensystem aus Kastenprofilen verschiedener Querschnittsabmessungen erarbeitet. Diese Profile bestehen aus WVC (Wood Veneer Composites, Holzfurnierlagenverbundwerkstoffen). Der Ansatz bildet den statischen Lastfall und das Ermüdungsverhalten unter schwellende Dreipunktbiegung ab. Am Beispiel eines ausgewählten Strukturbauteils aus handelsüblichen Birkensperrholz wird der Berechnungsansatz konkretisiert und durch Versuche evaluiert. Aufbauend auf dem Kraft-Verformungsverhalten der analysierten Einzelbauteile und der kapillarporösen Struktur des Holzes wird bei dem Berechnungsansatz auf eine Analyse der Spannungen verzichtet. Stattdessen wird als Berechnungskriterium die kritische Normaldehnung in der Randfaser der Strukturbauteile genutzt. Weiterhin wird eine Methode vorgestellt um mittels niederzyklischen Ermüdungsversuchs (LCF, ca. 1e+03 Lastwechsel) den „Knickpunkt“ der Zeitfestigkeitslinie eines einstufigen Ermüdungsversuchs bei hohen Lastspielzahlen (HCF, 1e+06 bis 1e+07 Lastwechsel) für diese Strukturbauteile zu bestimmen. / It was developed a simple approach for the calculation of a modular construction system for box sections (profile structures) of different cross-sectional dimensions. These profile structures consists of WVC (Wood Veneer Composites). The approach maps the static load case and the fatigue behavior under pulsating three-point bending. By using a structural component made from commercial birch plywood, the calculation approach is specified and verified. Based on the force-deformation behavior of the analyzed single components in connection with the capillary-porous structure of the wood, the calculation approach dispense on an analysis of the tensions. Instead, a criterion, which calculates the critical normal strain in the outer fibers of the structural components, is used. Furthermore, a method of a low-cycle fatigue test (LCF, abbr. 1e+03 cycles) is presented. This method detects the “knee point" of the fatigue limit line for the profiles. That point is usually determined by the use of a high-cycle fatigue tests (HCF, 1e+06 until 1e+07 cycles).

Es wird ein einfacher Berechnungsansatz für ein Baukastensystem aus Kastenprofilen verschiedener Querschnittsabmessungen erarbeitet. Diese Profile bestehen aus WVC (Wood Veneer Composites, Holzfurnierlagenverbundwerkstoffen). Der Ansatz bildet den statischen Lastfall und das Ermüdungsverhalten unter schwellende Dreipunktbiegung ab. Am Beispiel eines ausgewählten Strukturbauteils aus handelsüblichen Birkensperrholz wird der Berechnungsansatz konkretisiert und durch Versuche evaluiert. Aufbauend auf dem Kraft-Verformungsverhalten der analysierten Einzelbauteile und der kapillarporösen Struktur des Holzes wird bei dem Berechnungsansatz auf eine Analyse der Spannungen verzichtet. Stattdessen wird als Berechnungskriterium die kritische Normaldehnung in der Randfaser der Strukturbauteile genutzt. Weiterhin wird eine Methode vorgestellt um mittels niederzyklischen Ermüdungsversuchs (LCF, ca. 1e+03 Lastwechsel) den „Knickpunkt“ der Zeitfestigkeitslinie eines einstufigen Ermüdungsversuchs bei hohen Lastspielzahlen (HCF, 1e+06 bis 1e+07 Lastwechsel) für diese Strukturbauteile zu bestimmen. / It was developed a simple approach for the calculation of a modular construction system for box sections (profile structures) of different cross-sectional dimensions. These profile structures consists of WVC (Wood Veneer Composites). The approach maps the static load case and the fatigue behavior under pulsating three-point bending. By using a structural component made from commercial birch plywood, the calculation approach is specified and verified. Based on the force-deformation behavior of the analyzed single components in connection with the capillary-porous structure of the wood, the calculation approach dispense on an analysis of the tensions. Instead, a criterion, which calculates the critical normal strain in the outer fibers of the structural components, is used. Furthermore, a method of a low-cycle fatigue test (LCF, abbr. 1e+03 cycles) is presented. This method detects the “knee point" of the fatigue limit line for the profiles. That point is usually determined by the use of a high-cycle fatigue tests (HCF, 1e+06 until 1e+07 cycles).

<p>This study is concerned with the predictive modeling of the machining and the mechanical behaviors of additive manufactured (AMed) Ti6AlV/TiC composites and Ti6Al4V, respectively, using microstructure-based hierarchical multiscale modeling. The predicted results could constitute as a basis for optimizing the parameters of machining and AM of the current materials.</p> <p>Through hierarchical flow of material behaviors from the atomistic, to the microscopic and the macroscopic scales, multiscale heterogeneous models (MHMs) coupled to the finite element method (FEM) are employed to simulate the conventional and the laser assisted machining (LAM) of Ti6AlV/TiC composites. In the atomistic level, molecular dynamics (MD) simulations are used to determine the traction-separation relationship for the cohesive zone model (CZM) describing the Ti6AlV/TiC interface. Bridging the microstructures across the scales in MHMs is achieved by representing the workpiece by macroscopic model with the microscopic heterogeneous structure including the Ti6Al4V matrix, the TiC particles, and their interfaces represented by the parameterized CZM. As a result, MHMs are capable of revealing the possible reasons of the peculiar high thrust forces behavior during conventional machining of Ti6Al4V/TiC composites, and how laser assisted machining can improve this behavior, which has not been conducted before.</p> <p>Extending MHMs to predict the mechanical behaviors of AMed Ti6Al4V would require including the heterogeneous microstructure at the grain level, which could be computational expensive. To solve this issue, the extended mechanics of structure genome (XMSG) is introduced as a novel multiscale homogenization approach to predict the mechanical behavior of AMed Ti6Al4V in a computationally efficient manner. This is realized by embedding the effects of microstructure heterogeneity, porosity growth, and crack propagation in the multiscale calculations of the mechanical behavior of the AMed Ti6Al4V using FEM. In addition, the XMSG can predict the asymmetry in the Young’s modulus of the AMed Ti6Al4V under tensile and compression loading as well as the anisotropy in the mechanical behaviors. The applicability of XMSG to fatigue life prediction with valid results is conducted by including the energy dissipations associated with cyclic loading/unloading in the calculations of the cyclic response of the material.</p>