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Indiana University-Purdue University Indianapolis (IUPUI) / High-temperature alloys have been extensively used in many applications, such as furnace muffles, fuel nozzles, heat-treating fixtures, and fuel nozzles. Due to such conditions, these materials should have resistance to cyclic loading, oxidation, and high heat. Although there are numerous prior experimental and theoretical studies, there is insufficient understanding of application of the unified viscoplasticity theory to finite element software for fatigue life prediction. Therefore, the goal of this research is to develop a procedure to implement unified viscoplasticity theory in finite element (FE) model to model the complex material deformation pertaining to thermomechanical load and implement an incremental damage lifetime rule to predict the thermomechanical fatigue life of high-temperature alloys. The objectives of the thesis are: 1. Develop a simplified integrated approach to model the fatigue creep deformation under the framework of ‘unified viscoplasticity theory’ 2. Implement a physics - based crack growth damage model into the framework 3. Predict the deformation using the unified viscoplastic material model for ferritic cast iron (Fe-3.2C-4.0Si-0.6Mo) SiMo4.06 4. Predict the isothermal low cycle fatigue (LCF) and LCF-Creep life using the damage model In this work, a unified viscoplastic material model is applied in a FE model with a combination of Chaboche non-linear kinematic hardening, Perzyna rate model, and static recovery model to model rate-dependent plasticity, stress relaxation, and creep-fatigue interaction. Also, an incremental damage rule has been successfully implemented in a FE model. The calibrated viscoplastic model is able to correlate deformations pertaining to isothermal LCF, LCF-Creep, and thermal-mechanical fatigue (TMF) experimental deformations. The life predictions from the FE model have been fairly good at room temperature (20°C), 400°C, and 550°C under Isothermal LCF (0.00001/s and 0.003/s) and LCF-Creep tests. The material calibration techniques proposed for calibrating the model parameters resulted in a fairly good correlation of FE model derived hysteresis loops with experimental hysteresis, pertaining to Isothermal LCF (ranging from 0.00001/s to 0.003/s), Isothermal LCF-Creep tests (withhold time) and TMF responses. In summary, the method and models developed in this work are capable of simulating material deformation dependency on temperature, strain rates, hold time, therefore, they are capable of modeling creep-stress relaxation and fatigue interaction in high-temperature alloy design.

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The largest single cause of failure in fan and compressor components in the cold frontal sections of commercial and military gas turbine engines has been attributed to high cycle fatigue (HCF). Additionally, both high-cycle fatigue (HCF) and lowcycle fatigue (LCF) loadings are widely recognized as unavoidable during operation of these components and because the classic Linear Damage Rule (LDR) neglects to account for the synergistic interaction between these damage contributors, dangerous over predictions of lifetime can result. Combined low-cycle fatigue / high-cycle fatigue (HCF/LCF) loadings were investigated in smooth Ti-6Al-4V. The specimens were subjected to a variable amplitude block loading history comprised of completely-reversed (R = -1) tensioncompression overloads followed by constant-amplitude zero-tension (R = 0) minor cycles. Axial specimens were excised from forgings representative of turbine engine fan blade forgings, and consisted of approximately 60% primary α in a matrix of lamellar α + β. Data are reported for smooth specimens of Ti-6Al-4V subjected to both constant amplitude and variable amplitude loadings. The axial specimens were prepared according to two distinct specimen conditions: low stress ground and longitudinallypolished (LSG+LP) and stress-relieved and chemically milled (SR+CM) conditions. Significantly longer lives were observed for the LSG+LP specimen condition under both constant and variable amplitude loading, due to the presence of a beneficial compressive surface residual stress. The presence of this residual stress was confirmed by x-ray diffraction, and its magnitude was of the order of 180 MPa (~20% of the yield stress). In either specimen condition, no appreciable effect of periodic overloads on the life of subsequent minor cycles was observed. / Master of Science

This doctoral thesis deals with the fatigue behaviour of AZ31 and AZ61 magnesium alloy casted by advanced methods of casting squeeze casting. Based on the regression functions and measured fatigue data were determined fatigue characteristics of these alloys. The work also extends into the area of fatigue cracks in the AZ31 alloy. Were measured fatigue crack propagation rate on modified cylindrical specimens. The paper also outlines the possibilities for describing the fatigue behaviour of AZ61 alloy in extremely low-cycle fatigue.

Ductile plastic hinge zones in beams of reinforced concrete frames are known to incur extensive damage and elongate. This ‘beam elongation’ can inflict serious damage to adjacent floor diaphragms, raising concerns of life safety. In light of this, the slotted reinforced concrete beam was investigated as a promising non-tearing floor substitute for conventional design. It consists of a conventional reinforced concrete beam, modified with a narrow vertical slot adjacent to the column face, running approximately three-quarters of the beam depth. Seismic rotations occur about the remaining concrete “top-hinge”, such that deformations are concentrated in the bottom bars of the beam, away from the floor slab, and beam elongation is minimised. The inclusion of the slot raised several design issues which needed to be addressed. These were the shear transfer across the top-hinge, buckling of bottom longitudinal reinforcement, low cycle fatigue, bond anchorage of reinforcement in interior joints, interior joint design, detailing with floor units and beam torsion resulting from eccentric floor gravity loads. These issues were conceptually investigated in this project. It was found that most issues could be resolved by providing additional reinforcement and/or specifying alternative detailing. As part of the experimental investigation, quasi-static cyclic tests were performed on in-plane beam-column joint subassemblies. Specimens tested included exterior and interior joint subassemblies with slotted-beams and a conventional exterior joint as a benchmark. This was followed by a test on a slotted-beam interior joint subassembly with precast floor units and imposed gravity load. Experimental tests revealed significant reductions in damage to both the beam and floor when compared to conventional beams. Issues of bar buckling, bond-slip and altered joint behaviour were also highlighted, but were resolved in the final test. A simple analytical procedure to predict the moment-rotation response of slotted-beams was developed and verified with experimental results. This was used to perform sensitivity studies to determine appropriate limits for the concrete top-hinge depth, top-to-bottom reinforcement ratio and depth of diagonal shear reinforcement. For the numerical investigation, a multi-spring model was developed to represent the flexural response of slotted-beams. This was verified with experimental tests and implemented into a five-storey, three-bay frame for earthquake time history analyses. To provide a benchmark, a conventional frame was also setup using the plastic hinge element developed by Peng (2009). Time history analyses showed that the slotted-beam frame response was very similar to the response of a conventional frame. Due to greater hysteretic damping, there was a slight reduction in the average interstorey drift and lateral displacement envelopes. The slotted-beam frame also exhibited 40% smaller residual drifts than the conventional frame. The research carried out in this thesis showed slotted reinforced concrete beams to be an effective non-tearing floor solution, which could provide a simple and practical substitute for conventional reinforced concrete design.

New stricter environmental legislation requires lower emissions and fuel consumption of automotive engines. Therefore the fuel efficiency must be increased but this leads to higher loads in the engine. As for the exhaust system it is affected by higher thermomechanical loads. Until today the turbo manifold has been nitrocarburized in order to increase the wear resistance in slip joints with other exhaust components. The problem is that there is no knowledge of how the nitrocarburizing affects the thermomechanical properties of the material. The purpose of this thesis work is to examine the difference in thermomechanical properties with and without nitrocarburizing on the three different ductile irons High Silicon, SiMo51 and SiMo1000 intended for exhaust components. Thermo-mechanical fatigue (TMF) experiments were performed on test rods to evaluate difference in number of cycles to failure. In each cycle the test-rod was affected by a combination of mechanical loads and thermal loads resembling those found on exhaust components. Light optical microscopy, scanning electron microscopy and x-ray radiography were used to examine microcracks and damage mechanisms of the materials. It was found that the nitrocarburizing did not affect the number of cycles to failure in any large extent. Further, it was also found that SiMo1000 on average has the longest lifetime followed by SiMo51 and High Silicon. Although, the difference is small for many loadings and taking a 95% confidence band into account the curves overlap for many loading cases.

A comparison of concrete containing recycled concrete coarse aggregates and natural coarse aggregates subjected to high strain, low cycle compressive fatigue is presented. Using a strain based feedback control loop, concrete cylinders are compressed at 15µε/s to a specified strain then unloaded to zero stress for 10 cycles. After cycling, all samples are loaded to a strain of 0.008. Direct concrete material variables are the water to cement (w/c) ratio, taken as 0.60, 0.45, and 0.39, and percent coarse recycled concrete aggregate content, varied from zero to 100 percent. The primary testing variable is the specified unloading strain. Unloading strains include 60, 75, 90, 100, and 120 percent of the strain at peak stress. Ten batches of concrete were made, generating a total of 224 samples for testing. Findings confirm previous research showing a reduction in strength with increasing recycled concrete coarse aggregate content, an equivalent concrete with only 25 percent replacement of natural coarse aggregates and an equivalent strength concrete with a decrease in the w/c ratio and 100 percent recycled concrete coarse aggregates. Fatigue testing indicates that each cycle’s maximum stress remains unchanged, but the stiffness degrades more rapidly with increasing recycled aggregate content and a constant w/c ratio.

The aircraft engine industry is constantly striving to increase the operating temperature and stresses in hot section engine components, a goal that can only be achieved by accurately modeling and predicting damage mechanisms of potential engine materials. The objective of this work is to develop physically-based models that are able to accurately predict the high temperature crack initiation behavior of Rene 88DT, a commonly used aircraft engine disk material, under low cycle fatigue (LCF) conditions. Two different microstructural conditions were produced by subjecting the material to two separate heat treatments; the heat treatments were selected so that grain size remains the same but the size distribution of the strengthening gamma prime precipitate is different between the two conditions. LCF experiments were performed on specimens from each condition at 650C and R = -1 under strain ranges of 0.66%, 0.75%, and 1.5%. A third microstructural condition with a similar grain size but different gamma prime size distribution was tested by another source at 650C and R = 0 under strain ranges of 0.66%, 0.79%, 0.94%, and 1.14%. The results indicate that there are two competing crack initiation mechanisms: initiation from a microstructural defect such as an inclusion and initiation from slip band cracking. A physically based model, in the form of a modified Fatemie-Socie parameter, is utilized to predict the crack initiation mechanism and approximate cycles to failure based on the microstructure of the material and applied strain. Long crack growth models are also developed to model crack growth from subsurface inclusions and surface semi-elliptical cracks. These models predict that long crack growth is a small portion of the total fatigue life in these conditions, which suggests that the majority of the fatigue life is spent initiating a dominant fatigue crack.

Bridge design should take into account not only safety and functionality, but also the cost effectiveness of investments throughout a bridge life-cycle. This work presents a probabilistic approach to compute the life-cycle cost (LCC) of corroding reinforced concrete (RC) bridges in earthquake prone regions. The approach is developed by combining cumulative seismic damage and damage associated to corrosion due to environmental conditions. Cumulative seismic damage is obtained from a low-cycle fatigue analysis. Chloride-induced corrosion of steel reinforcement is computed based on Fick’s second law of diffusion. The proposed methodology accounts for the uncertainties in the ground motion parameters, the distance from source, the seismic demand on the bridge, and the corrosion initiation time. The statistics of the accumulated damage and the cost of repairs throughout the bridge life-cycle are obtained by Monte-Carlo simulation. As an illustration of the proposed approach, the effect of design parameters on the life-cycle cost of an example RC bridge is studied. The results are shown to be valuable in better estimating the condition of existing bridges (i.e., total accumulated damage at any given time) and, therefore, can help schedule inspection and maintenance programs. In addition, by taking into consideration the deterioration process over a bridge life-cycle, it is possible to make an estimate of the optimum design parameters by minimizing, for example, the expected cost throughout the life of the structure.