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Indiana University-Purdue University Indianapolis (IUPUI) / PURPOSE: To evaluate the fatigue failure load of distinct lithium disilicate restoration designs cemented on a chairside titanium-base (VariobaseTM for CEREC®, Straumann® LLC, USA) for restoring anterior implant restoration. MATERIALS AND METHODS: Left maxillary incisor restoration was virtually designed in 3 groups (n=10; CTD: lithium disilicate crowns cemented on custom-milled titanium abutments; VMLD: monolithic full-contour lithium disilicate crowns cemented on titanium-base; and VCLD: lithium disilicate crowns cemented on lithium disilicate customized anatomic structures then cemented on titanium-base). The titanium-base was air-abraded with aluminum oxide particles, 50 µm at 2 bars. Subsequently the titanium-base was steamed, air-dried and a thin coat of silane (Monobond Plus, Ivoclar Vivadent®, USA). All ceramic components were surface treated with hydrofluoric acid etching gel, follow by silanized, and bonded with resin cement (Multilink Automix, Ivoclar Vivadent®, USA). Specimens were fatigued at 20 Hz, starting with a load of 100 N (×5000 cycles), followed by stepwise loading up to 1400 N at a maximum of 30,000 cycles each. The failure loads, number of cycles, and fracture analysis were recorded. Data were statistically analyzed using one-way ANOVA followed by pair-wise comparisons (p < 0.05). Kaplan-Meier survival plots and Weibull survival analyses were reported. RESULT: For catastrophic fatigue failure load and total number of cycles for failure, VMLD (1260 N, 175231 cycles) was significantly higher than VCLD (1080 N, 139965 cycles) and CDT (1000 N, 133185 cycles). VMLD had higher Weibull modulus (11.6), demonstrating higher structural reliability. CONCLUSIONS: VMLD performed the best fatigue behavior when compared with the two other groups.

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Fiber reinforced polymer composite laminates make up more than 50% of modern aircrafts. Such composite laminates are exposed to various environmental and in-service thermo-mechanical load conditions. Transverse/intralaminar cracking is usually the first form of damage appears in a composite laminate and they tend to increase in number during the service life. The growth in number of these cracks significantly degrades the thermo-elastic properties of the composite laminate and eventually leads to final failure. Thus, it is important to predict the crack density (number of cracks per unit length) growth in both non-interactive crack density region and interactive crack density region and its effect in thermo-elastic properties degradation. Non-interactive crack density region is the region where the cracks are far apart and stress perturbation between cracks do not overlap. Interactive crack density region is where the cracks are close to each other and stress perturbation between cracks overlaps and affects the formation of new cracks. In this study, transverse cracks in thick Glass Fiber Epoxy (GF/EP) cross-ply composite laminates under quasi-static tensile loading and tension-tension fatigue loading have been analyzed and predicted. In the first paper attached here, increase in number of transverse cracks in GF/EP cross-ply laminates under quasi-static tensile loading at room temperature (RT) are analyzed using 2 material systems. The failure stress distribution in 90° plies of the laminates is defined by Weibull distribution and the Weibull parameters are determined from crack density versus applied thermo-mechanical transverse stress in 90° layer (σTCLT) data points within the non-interactive crack density region. The crack density growth is then predicted versus the σTCLT and applied mechanical strain in the laminate from the determined Weibull parameters using Monte Carlo method and the stress distribution models between adjacent cracks. The predicted results using the novel stress distribution model introduced here were in good agreement with the non-interactive and interactive crack density regions of test results. The importance of using the Monte Carlo method and novel stress distribution model to predict the whole crack density region have been emphasized in the article, in addition to that it also redefined the interval of non-interactive crack density region.  The second paper expands the concept from the first paper, to address the tension-tension fatigue loading at RT. It deals with the crack density analysis and prediction in [0/90]s GF/EP laminate under fatigue loading at RT. The fatigue tests were performed at 3 maximum stress levels. Here the Weibull parameters were determined from the data points within the non-interactive crack density region in quasi-static and fatigue loading. From the determined Weibull parameters of each stress level and using Monte Carlo method and the novel stress distribution model, the crack density versus the number of fatigue cycles were predicted and in good agreement with the fatigue test results at the respective stress level. The intention here was to use Weibull parameters of one stress level to predict crack density at arbitrary stress levels. Based on it, the predicted results were not sufficiently good and suggested to revisit the Weibull parameter determination by performing fatigue tests at two stress levels.  In the attached paper 3, new methodology on crack density growth simulation and Weibull parameter determination in tension-tension fatigue loading has been developed. In the newly developed methodology, in detailed fatigue tests are performed at one maximum stress level to obtain all data points and at higher stress level to obtain one data point that is a crack density data point at certain number of cycles to determine Weibull parameters. Using the determined Weibull parameters from non-interactive crack density region, the whole crack density region was successfully predicted for other stress levels.

Fatigue analysis is a critical stage in the design of any structural component. Typically fatigue is analysed during post-processing, but as the size of the analysed component increases, the amount of data stored for the analysis increases simultaneously. This increases the computational and memory requirements of the system, intensifying the work load on the engineer. A continuum mechanics approach namely ’Continuous time fatigue model’, for fatigue analysis is available in a prior study which reduces the computational requirements by simultaneously computing fatigue along with the stress. This model implements a moving endurance surface in the stress space along with the damage evolution equation to compute high-cycle fatigue. In this thesis the continuous time fatigue model is compared with conventional model (ie.Cycle counting) to study its feasibility. The thesis also aims to investigate the continuous time fatigue model and an evolved version of the model is developed for non-proportional load cases to identify its limitations and benefits.

This work deals with fatigue properties and stability of microstructure of ultrafine-grained (UFG) copper prepared by severe plastic deformation by means of equal channel angular pressing (ECAP) method. The effect of different fatigue loading regimes and thermal exposition on microstructural changes was investigated and the fatigue lifetime curves were experimentally determined. The research attention was focussed on localization of cyclic plastic deformation and fatigue crack initiation in UFG structure. Experimental results indicate that after stress-controlled fatigue loading (both symmetrical and asymmetrical) the microstructure remains ultrafine; no grain coarsening was observed. Contrary to this, strain-controlled fatigue loading results in formation of bimodal structure. Grain coarsening was observed also after thermal exposition at 250 °C for 30 minutes. Annealing at lower temperatures does not result in grain coarsening or development of bimodal structure. Fatigue loading results in development of surface relief in form of cyclic slip markings. Their density, distribution and shape differ for particular fatigue loading regimes. Differences in crack initiation mechanism in low- and high-cycle fatigue region were found. Nevertheless, the characteristic feature for all loading regimes was stability of UFG microstructure in the region of cyclic slip bands and fatigue cracks.

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<p>With the emergence of additive manufacturing (AM), there is a need to understand the role of microstructures resulting from AM on the mechanical performance of the material. Ti-6Al-4V alloys are widely used within the aerospace industry as well as other industries to achieve high strength, low weight premium performance parts. There is a desire to utilize AM to produce Ti-6Al-4V, although these materials need to be qualified prior to their use in safety critical applications. Within the qualification of AM Ti-6Al-4V in aeronautics, fatigue loading is a crucial aspect to. It has been seen that within AM Ti-6Al-4V, prior β boundaries can be locations of microscopic localization of plastic strain which often lead to fatigue crack initiation. This thesis aims to further understand and predict the role of AM Ti-6Al-4V microstructures in dictating fatigue behavior. Specifically, the goal was to gauge the contributions of two microstructural features resulting from AM, prior β boundaries and α lathe-shaped grains, to the localization behavior. With the need to understand and predict the emergent behavior of the material system, crystal plasticity finite element (CPFE) methods were used in this thesis as the main method. </p> <p><br></p> <p>Within the context of CPFE, there is an existing gap in the current literature of realistic synthetic microstructures of Ti-6Al-4V that capture both the prior β boundaries and α lathes. With the ability to generate realistic FE models, the effects of the microstructural features can be better studied and characterized. The first portion of this thesis focuses on the generation of such synthetic microstructures which are simulated within the CPFE framework. An emphasis is placed on modeling the prior β boundaries and α grains. As these generated models are statistically equivalent to actual microstructures, material characterization via EBSD was performed on specimen that were used in the experimental fatigue testing. With the framework’s ability to generate synthetic microstructures that consider one prior β grain or multiple β grains (and thus prior β boundaries), simulations were conducted on both conditions of microstructures. </p> <p><br></p> <p>In the second portion of this thesis, simulations are conducted on two conditions of synthetic microstructures: models which contain 𝛼 lathes associated with one prior 𝛽 grain and models which contain multiple prior 𝛽 boundaries and the respective 𝛼 lathes. The goals of the simulations included: (1) lifing the different synthetic microstructures using a fatigue lifing model by way of the accumulated plastic strain energy density (APSED), (2) analyzing the microscopic localization of APSED at the prior β boundaries, and (3) analyzing the effects of the α lathes on the microscopic localization. This investigation aimed to further shed light on the effects of the additive manufacturing process and the implications of the resulting microstructure on the fatigue properties of AM Ti-6Al-4V. Furthermore, physics-based prognosis strategies similar to what is employed here will enable the rapid qualification of materials/structures and the ability to tailor component design on fatigue performance. </p>

This thesis has two parts. In the first part, we use the results of new fatigue experiments conducted with variable load levels as well as variable stress ratios to critically assess three (two old and one relatively new) cumulative fatigue damage models. These models are deterministic. Such models are usually tested using multiple blocks of periodic loading with differing amplitudes. However, available data pertains to zero-mean loading, and does not investigate the role of variable stress ratio (Smin/Smax). Here, we present experimental results for variable stress ratios. Two specimen geometries and two materials (Al 2014and Al 2024)are tested. Manson’s double linear damage rule (DLDR)gives the highest accuracy in predicting the experimental outcome, even in the presence of variable stress ratios, whereas predictions of the newer model (“A constructive empirical theory for metal fatigue under block cyclic loading,” Proceedings of the Royal Society A, 464 (2008), 1161-1179) are slightly inferior. The widely used Miner’s rule is least accurate in terms of prediction. The merits and drawbacks of these models, in light of the experimental results, are as follows. The DLDR, though accurate, has minor scientific inconsistencies and no clear generalization. The constructive model has possible generalizability and more appealing scientific consistency, but presently has poorer accuracy. Miner’s rule, least accurate, lies within the constructive approach for special parameter values. The DLDR can guide the new (constructive)approach through new parameter fitting criteria. In the second part of this thesis, we consider the scatter in fatigue life and use the Weibull distribution to describe ‘S-N-P’ curves. We first assume homoscedasticity (load-independent or constant variance) and present a way to draw a p-percentile line on a log-log load-life plot. Then heteroscedasticity (load-dependent variance) in fatigue life is incorporated and a simple statistical model is proposed, to obtain a straight line percentile plot at a pre-specified probability of survival ps. The proposed method is illustrated for Al 2014-T6 and Al 2024-T4 data sets (extracted manually) from MMPDS-01 (a data handbook).

Performance of static analysis of the London Eye structure is the subject of this diploma thesis. First chapters of the thesis analyses overview of realized structures of the Ferris wheel. The main part of the thesis analyses the London Eye structure itself. Specifically are described response analyses, sensitivity analyses, analyses of ultimate limit state and serviceability, stress analyses, analyses of fatigue stress and lifetime analyses. The thesis describes among static analysis also a design of dynamic vibration absorbers. The results of each analysis are evaluated in the final chapters.