Naturally occurring cellular materials are always optimized in terms of morphology, structural resistance, and functionality. The use of cellular materials is based on the application as well as the loading condition. Cellular materials are composed of an interconnected network of struts, plates, or repeating unit cells, forming edges or faces. The properties of these structures can be tailored according to the requirements by changing one or more of the parameters mentioned above. This makes cellular materials suitable for various applications ranging from aerospace to biomedical. In biomedical applications, these cellular materials can be used to manufacture porous implants to match the properties of the surrounding bone. They can also be used as coatings on solid implants to promote bone tissue ingrowth for better implant fixation. The production of these complex, porous implants using traditional manufacturing methods is a difficult task. However, the development of additive manufacturing processes such as Laser Powder Bed Fusion (LPBF) has made it possible to manufacture complex and intricate shaped cellular materials with minimum material wastage and considerable accuracy. Therefore, with the combination of the LPBF process and cellular materials design, it is possible to produce a wide range of cell topologies with customized mechanical properties depending on the implant location, material, and the needs of the patient. Titanium and its alloys such as Ti6Al4V have been used in biomedical applications due to their high strength to weight ratio, corrosion resistance, and good biocompatibility. Also, the LPBF process has been used to produce various Ti6Al4V components for various applications including cellular materials. The development of cellular materials for implants is dependent on the relative density, response of the unit cell to loading conditions, and the optimal pore size for bone ingrowth. Studies have been carried out to understand the behavior of the cellular materials under compressive loads since most of the implants experience compression loads during their operation. Nevertheless, the implants also undergo fatigue loading due to day-to-day activities and tensile loads when the implant is loose or when the host performs an extensive physical activity. Therefore, designing and studying the cellular materials for these loads is necessary to completely understand their behavior. Considering the pore size, studies have suggested that a pore size of ~ 800 μm is suitable to induce bone ingrowth after implantation. The cellular materials can be broadly classified into stretching and bending dominated. Stretching dominated cellular materials are characterized by high strength and stiffness while bending dominated structures are high compliant. This behavior of cellular materials is dependent solely on the unit cell topology. Therefore, the development of different types of cell topologies and their characterization is required to produce optimized fully porous implants. Also, the effect of the LPBF process on the designed parameters of the unit cell alters the obtained mechanical properties from the desired values. The present work aims at developing different Ti6Al4V cellular materials that can be potentially used for application in implants. A combination of different cellular materials can be used to develop completely porous implants or single cellular materials can be used as coatings for solid implants to induce osseointegration. A major portion of the work is focused on the mechanical properties of LPBF manufactured cellular materials characterized using static and fatigue tests. The study also investigates the discrepancy between the as-designed and as-built geometrical parameters of these cellular structures. Finite elements analysis and the Gibson-Ashby modeling has been employed to understand the difference between the as-designed and as-built properties. Another part of the study was focused on the effect of designed geometrical parameters on the as-built geometry of cellular materials. The aim was to develop a relationship between the as-designed and the as-built parameters. This thesis covers all the aspects mentioned in the above paragraph in detail. The research work has been provided in three different chapters (Chapter 2, 3, and 4) which are well connected to each other. Each chapter is composed of an abstract, introduction, materials and methodology, results and discussion, and conclusion. A conclusion on the complete research and the future scope is provided at the end. The first chapter introduces all the aspects concerned with the development of cellular materials for biomedical applications. Literature review on all aspects have been provided, ranging from the properties of the bone, cellular materials, manufacturing process for cellular materials, and the properties of bulk materials suitable for biomedical applications. In chapter 2, Ti6Al4V cellular materials with three different cell topologies namely cubic regular, cubic irregular, and trabecular have been investigated. The irregular specimens are obtained by skewing the junctions of the cubic regular configuration. Trabecular specimens are designed by randomly joining 4-6 struts at a node to mimic human trabecular bone. The three cell topologies were manufactured at three different porosity levels by changing their strut thickness and pore size. The cubic regular cells are considered due to their stable and simple configuration, while irregular and trabecular based specimens have shown promising results in the osseointegration according to the partner company. However, the mechanical properties of irregular and trabecular specimens play an important role in implant design. Therefore, all the specimens were subjected to compression test and as well as a novel tensile test under two different types of loading conditions, monotonic and cyclic to obtain their strength and stiffness. However, a misalignment in the struts with the loading direction in compression led to an asymmetric behavior between tensile and compression. Higher strength and stiffness values were observed under tensile loading, the results of which were in conjunction with the theoretical prediction from the Gibson-Ashby model. The experimental results indicated the irregularity tends to reduce the strength, stiffness and induce bending dominate behavior. Morphological analysis was carried out to obtain the discrepancy between the as-designed and the as-built thickness values. This led to the FE analysis of as-designed models to obtain the difference in the properties of as-designed and as-built cellular materials. Furthermore, as-built FE models were generated using morphological data to study the effect of strut defects and compare them with the experimental results. The next step involved comparing the experimental results with the FE analysis carried out tomography-based FE models. The last part of the study involved obtaining a relation between the as-designed and as-built Young’s modulus for cubic regular, cubic irregular, and trabecular specimens to create a reference database. The mechanical properties from the compression and tensile test of the highest porosity specimens were closer to the properties of human bone. The tensile tests were successful in predicting the mechanical properties accurately. These observations were the motivation to further study the effect of irregularity on various cell topologies, by subjecting them to static and fatigue loads. In chapter 3, seven different types of unit cells, three regular configurations, three irregular configurations, and one trabecular based unit cell. The unit cells used in the study consisted of regular and irregular configurations of the cubic-based, star-based, and cross-based specimens. These specimens were selected to have a comparison of properties from stretching dominate cubic specimen to bending dominated cross-shaped specimens and to study the effect of irregularity. Therefore, the specimens were subjected to and mechanical characterization using compression, tension, and compression – compression fatigue tests along with porosity and morphological analysis. The tensile specimens in this chapter were designed with a thicker transition at the ends, while compression specimens had uniform configuration throughout the specimen. FE analysis was carried on the as-designed configuration of these specimens to study the effect of transition and to compare the as-designed and tensile experimental results to understand the effect of decreased porosity on the failure mechanisms. Fatigue tests were carried under compression-compression load and failure mechanisms in different unit cells were captured. The results of the study indicated that the irregularity has a greater effect on the strength and stiffness of stretching-dominated cellular material and has a negligible effect on bending-dominated cross-based specimens. The trabecular specimens display excellent mechanical properties under static load with good strength, stiffness and sustain high strain values. The normalized S-N curves indicate a clear demarcation between the bending and stretching-dominated cellular materials. The FE analysis showed a similar failure location as compared to the experimental results despite the decrease in the porosity due to manufacturing. The morphological analysis showed the effect of strut orientation of the as-built thickness. The morphological analysis and the difference between the as-designed and as-built geometrical parameters show that an in-depth study on the geometrical deviation due to the LPBF process is necessary. The next chapter focuses on the geometrical deviation in LPBF manufactured cellular specimens and the parameters influencing this deviation. In chapter 4, cubic regular cellular materials with filleted junctions are studied for geometrical deviation and to obtain a relationship between the as-designed and as-built geometric parameters. Initially, nine different specimens with different strut thickness, fillet radius, and unit cell size were manufactured at three different orientations with respect to the printing plane. The main aim of this study was to devise a compensation strategy to reduce the geometrical deviation due to the LPBF process. A linear relation between the as-designed and as-built geometrical values is obtained, which is used for compensation modeling. Struts perpendicular to the building plane were uniform in cross-sections while horizontal and inclined struts had an elliptical cross-section. The internal porosity analysis has been carried out which indicates that the porosity at the junctions is lesser than the porosity at the junctions. The compensation strategy worked well for the second set of specimens produced using the same parameters, thereby reducing the geometrical deviation between the as-designed and the as-built parameters. Finally, the effect of filleted junctions, building directions, and compensation modeling on fatigue properties have been studied. Specimens with load-bearing struts printed parallel to the building plane had the lowest mechanical properties, while the specimens with struts inclined to the loading direction and building plane displayed excellent static and fatigue properties. The fillets at the junctions improve the fatigue resistance of the specimen by reducing the stress concentration. The printing direction and the presence of fillets influence the fatigue failure locations as well. Therefore, filleted junctions that can be reproduced well by the LPBF process can be used to reduce the stress concentration in cellular materials.

The objectives of this thesis comprise of overcoming the challenges in designing lightweight welded structures such as material selection, choice of fatigue design methods, and increased performance by using improvement techniques. Material selection of welded joints is dependent on the filler and base material strengths. Partially and fully penetrated cruciform and butt welded joints were designed in under-matching, matching, and over-matching filler materials. Base material steel grades were S600MC, S700MC, and S960. Current design rules are developed for welds in steel up to yield strength of 700MPa. Therefore, design rules in Eurocode3, AWS d1.1, and BSK 07 were verified and recommendations for developing design rules for designing welded joints in S960 were concluded. Numerical methodology for estimating static strength of welded joints by simulating heat affected zone was also developed. Another objective of the thesis work was to overcome the challenges in selection of fatigue design methods. The available design curves in standards are developed for uniaxial stress states, however, in real life the welds in mechanical structures are subjected to complex multiaxial stress states. Furthermore; weld toe failures are frequently investigated, weld root failures are seldom investigated. Therefore, in this work the multiaxial fatigue strength of welded joints failing at the weld root was assessed using experiments and various nominal and local stress based approaches. Butt welded joints with different weld seam inclinations with respect to applied uniaxial loading were designed to assess the root fatigue strength in higher multiaxial stress ratio regime. The fatigue strength of multi-pass tube-to-plate welded joints subjected to internal pressure only and combined internal pressure and torsion in and 90° out of phase loading was also investigated. Test data generated in this thesis was evaluated together with the test data collected from literature. Last objective of the thesis included investigation of the increased performance in fatigue strength by post weld treatment methods such as HFMI. The behavior of residual stresses induced due to HFMI treatment during fatigue loading is studied. Numerical residual stress estimations and residual stress relaxation models are developed and the effect of various HFMI treatment process parameters and steel grade on the induced residual stress state is investigated. Specimens studied were non load carrying longitudinal attachments and simple plates. Residual stresses in both test specimens were measured using X-ray diffraction technique. / <p>QC 20170206</p>

ABSTRACT PEEK matrix reinforced by carbon fibers as one thermoplastic composite material is studied. Thermoplastic composites have the advantages of high specific stiffness and strength, longer fatigue life, good resistance to moisture absorption and high temperature condition. The thesis is aimed to investigate the mechanical properties and fracture mechanism of the centrally notched AS-4/PEEK composite laminates subjected to tension-tension fatigue loading at elevated temperature. We use three common types of laminates, such as cross-ply , quasi-isotropic and angle-ply . After centrally notched, we first obtain the base-line data of mechanical properties by tensile tests at five different temperatures, such as 25¢J¡B75¢J¡B100¢J¡B125¢J¡B150¢J. Then, the fatigue tests are conducted, we receive the fatigue strength and life and establish the stress-life curves. The fatigue characteristics and fracture mechanism of a centrally notched composite laminate at elevated temperature are also recorded and observed. The empirical results can be concluded as follows. At the same temperature, the laminate of cross-ply possesses the largest ultimate strength and fatigue strength, quasi-isotropic the second angle-ply the smallest. As for the elastic modulus, the laminate of cross-ply is larger than that of quasi-isotropic. However, the large strain of angle-ply is within the plastic range that is out of the limit of the study. Thus, a further investigation is needed for angle-ply laminates alone. After centrally notched, the net area is reduced of the specimen, and then the elastic modulus is raised and the ultimate strength and fatigue strength of composite materials are lower. As the temperature increasing, the ultimate strength, fatigue strength and elastic modulus are all decreasing.

Specimens of steel 42CrMoS4 were quenched from the austenite (γ) and the ferrite (α) + austenite + cementite phase fields to produce fully martensitic matrices with 0 – 14 % ferrite dispersed in the matrix. After tempering at 300°C or 600°C mechanical and fatigue properties were determined. As expected yield strength, tensile strength and hardness decreased with increased tempering temperature and ferrite content. Quite unexpected, the fatigue properties were mildly affected. A small amount of ferrite, approximately 3% even appears to improve the fatigue strength. Then, at even higher ferrite amounts, slightly below 20% the fatigue strength appears to decrease again.

No description available.

A compliant mechanism gains its motion from the deflection of flexible members or the deformation of one portion of materials with respect to other portions. Design and operation of compliant mechanisms are very important, as most of the natural objects are made of compliant materials mixed with rigid materials, such as the bird wings. The most serious problem with compliant mechanisms is their fatigue problem due to repeating deformation of materials in compliant mechanisms. This thesis presents a study on the computational framework for designing a compliant mechanism under fatigue strength control. The framework is based on the topology optimization technique especially ground structure approach (GSA) together with the Genetic Algorithm (GA) technique. The study presented in this thesis has led to the following conclusions: (1) It is feasible to incorporate fatigue strength control especially the stress-life method in the computational framework based on the GSA for designing compliant mechanisms and (2) The computer program can well implement the computational framework along with the general optimization model and the GA to solve the model. There are two main contributions resulting from this thesis: First one is provision of a computational model to design compliant mechanisms under fatigue strength control. This model also results in a minimum number of elements of the compliant mechanism in design, which means the least weight of mechanisms and least amount of materials. Second one is an experiment for the feasibility of implementing the model in the MATLAB environment which is widely used for engineering computation, which implies a wide applicability of the design system developed in this thesis.

Ermüdungsanalyse mit Pro/Mechanica am Beispiel einer Förderschnecke

Abstract Fretting is present where the contacting surfaces of mechanical parts are subjected to rubbing and an alternating stress, resulting in fatigue in the material. This is the situation between the hub and shaft in a shrink-fitted assembly. In practice, fretting is found in the same assembly with normal fatigue, and it occurs in the axle in a shrink-fit between an axle and a hub, while normal fatigue cracks are found outside the assembly. Fretting phenomena have been investigated by many authors in laboratories, but the dimensioning criteria for shrink-fit assemblies are insufficient for the construction engineer. Fretting causes a considerable reduction in the fatigue strength of a shrink-fit assembly, and failures caused by fretting are as numerous as failures resulting from normal fatigue. The purpose of this investigation was to examine the effect of hub material, contact pressure, slip amplitude and shaft geometry on fatigue strength. The goal of this investigation was to determine an optimal contact pressure and a favourable fillet radius and axle diameter ratio at which fretting failure can be avoided and maximum normal fatigue strength will be obtained. The torsional fatigue strength of shrink-fitted shaft couplings was estimated using tests that varied the material of the hub, the contact pressure, the geometry of the shaft and the torsional stress amplitude of the shrink-fitted assembly. Based on the information obtained from the test, aluminium as a hub material appears to produce little damage to a steel shaft, whereas bronze appears to cause damage and fatigue. Cast iron and steel caused a medium amount of damage. To increase contact pressure at the shoulder, hub overhang past the shoulder was used. These experiments showed that increasing the contact pressure decreased the slip amplitude, thereby reducing fretting. With low contact pressure, shaft fracturing began from fretting fatigue inside the hub, but with high contact pressure the shaft fractured at the fillet due to normal fatigue. Selecting the fillet radius according to the contact pressure makes it possible to dimension the shrink-fit shaft assembly to prevent fretting. The shrink-fitted shaft should be designed according to the normal fatigue limit, because fretting fatigue may occur when the number of load cycles exceeds 2×107. Fretting can be prevented by using a sufficiently high contact pressure and by choosing optimal ratios for the fillet radius and the diameter. To avoid fretting, the slip amplitude should be under 3 μm. This is achieved by using a contact pressure of over 100 N/mm2, calculated according to Lamé's theory. A method for dimensioning a shrink-fitted shaft with respect to fretting fatigue was presented based on a specific geometry (Ø 50 mm) and the materials used in the tests. The method was applied in verifying the fretting fatigue of a shrink-fitted shaft with a diameter of 300 mm. / Tiivistelmä Kitkaväsyminen esiintyy yleisesti, kun koneenosien kontaktipintoihin kohdistuu edestakainen jännitysamplitudi hankaavan liikkeen lisäksi johtaen materiaalin väsymiseen. Tällainen tilanne vallitsee navan ja akselin välissä kutistusliitoksessa. Kitkaväsyminen esiintyy käytännössä samassa kutistusliitoksessa kuin normaali väsyminenkin ja se ilmenee navan ja akselin välissä yleensä akselissa, kun taas tavanomainen väsyminen tapahtuu akselissa liitoksen ulkopuolella. Kitkaväsymistä on tutkittu paljonkin, mutta kutistusliitoksen mitoituskriteerit kitkaväsymisen osalta ovat puutteelliset. Kitkaväsyminen aiheuttaa merkittävän väsymislujuuden heikkenemisen sekä täten väsymisiän alenemisen kutistusliitoksessa ja sen aiheuttamat vauriot ovat määrältään samaa luokkaa tavanomaisen väsymisen kanssa. Tämän tutkimuksen tarkoituksena oli selvittää napamateriaalin, liitospaineen, liukuma-amplitudin ja geometrian vaikutus väsymislujuuteen. Tavoitteena oli määrittää optimaalinen liitospaine sekä sopivat olakkeen pyöristyssäde ja akselisuhde, jotta kitkaväsyminen voidaan välttää ja saavuttaa maksimaalinen normaali väsymislujuus. Kitkaväsymislujuutta väännön suhteen testattiin varioimalla napamateriaalia, liitospainetta, geometriaa ja jännitysamplitudia. Testien perusteella alumiininen napamateriaali sopi hyvin teräsakselin kanssa, kun taas pronssi aiheutti akseliin pintavaurion ja sitä kautta väsymisilmiön. Teräs- ja valurautanapa olivat näiden kahden materiaalin välissä. Liitospaineen nostamiseksi olakkeen reunalla käytettiin navan ylitystä olakkeen yli. Kokeet osoittivat kitkaväsymisen vähenevän korkeammilla liitospaineilla liukuman aletessa. Alhaisella liitospaineella akselin vaurioituminen alkoi kitkaväsymisenä navan sisältä, mutta hyvin korkealla liitospaineella murtuminen tapahtui olakkeesta tavanomaisena väsymisenä. Valitsemalla pyöristyssäde liitospaineen perusteella kutistusliitos on mitoitettavissa kitkaväsymistä vastaan. Kutistusliitos tulisi mitoittaa tavanomaisen väsymisen mukaan, koska kitkaväsymismurtuma voi tapahtua kuormanvaihtoluvulla yli 2×07. Kitkaväsyminen on ehkäistävissä käyttämällä riittävän korkeata liitospainetta sekä sopivaa olakkeen pyöristyssädettä yhdistyneenä oikeaan akselisuhteeseen. Kitkaväsyminen estyy kun liukuma-amplitudi on alle 3 μm. Tämä on saavutettavissa liitospaineella yli 100 N/mm2 laskettuna Lamén teorian mukaan. Tutkimuksessa on esitetty mitoitusmenetelmä kitkaväsymisen suhteen perustuen Ø 50 mm akselilla sekä käytetyillä materiaaleilla tehtyihin testeihin. Menetelmää on sovellettu kitkaväsymisen tarkasteluun kutistusliitokseen, jonka akselin halkaisija on 300 mm.

This thesis work is concerned with effect of material grade on fatigue strength of welded joints. Fatigue strength evaluation of welded joints in as welded and post weld treated condition was carried out with effective notch method. Results of peak stress method have also been compared with those of effective notch method for as welded joints. In addition, using the results of effective notch method, the effect of important weld and global geometry factors on notch stress concentration factor has been studied with 2-level design of experiment and a mathematical relation among stress concentration factor and the geometric factors has been proposed. Overall, thickness of the base plate and toe radius is found to be the most important factors determining fatigue strength of the joint. Welding induced residual stresses have also been predicted using 2D and 3D FEM analysis to see their effect on fatigue strength of the joints. Also, transversal residual stresses were measured using X-ray diffraction method to assess the accuracy of predicted results. Based on simulation results, effect of geometric factors on maximum value of transversal residual stress was also investigated.

Nowadays there is a strong demand for lighter vehicles in order to increase the pay load. Through this the specific fuel consumption is decreased, the amount of greenhouse gases is lowered and the transport economy improved. One possibility to optimize the weight is to make the components from high strength steels and join them by welding. Welding is the main joining method for fabrication of a large proportion of all engineering structures. Many components experience fatigue loading during all or part of their life time and welded connections are often the prime location of fatigue failure.Fatigue fracture in welded structures often initiates at the weld toe as aconsequence of large residual stresses and changes in geometry acting as stress concentrators. The objective of this research is to increase the understanding of the factors that control fatigue life in welded components made from very high strength steels with a yield strength of more than 800 MPa. In particular the influences of the local weld toe geometry (weld toe radius and angle) and residual stress on fatigue life have been studied. Residual stresses have been varied by welding with conventional as well as Low Transformation Temperature (LTT) filler materials. The three non-destructive techniques Weld Impression Analysis (WIA), Laser Scanning Profiling (LSP) and Structured Light Projection (SLP) have been applied to evaluate the weld toe geometry.Results suggest that all three methods could be used successfully to measure the weld toe radius and angle, but the obtained data are dependent on the evaluation procedure. WIA seems to be a suitable and economical choice when the aim is just finding the radius. However, SLP is a good method to fast obtain a threedimensional image of the weld profile, which also makes it more suitable for quality control in production. It was also found that the use of LTTconsumables increased fatigue life and that residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welded parts.