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.

This research program focused on the behaviour of low cycle fatigue (LCF) of wrinkled pipes, and was designed to develop the LCF life prediction models for the wrinkled pipes. It consisted of three phases of work, which are strip tests, full-scale pipe tests, and finite element analysis (FEA). In strip tests, 39 strip specimens were tested by a complete-reversed stroke-controlled method to investigate the effects of bend angle, bend radius, and stroke range on the low-cycle fatigue (LCF) life. Also, the LCF behaviour was explored by viewing the spectra of key variables and their corresponding hysteresis loops. The failure mechanism was discussed by examining the fracture surfaces. Two LCF life prediction models, life-based and deterioration rate-based, were developed and their prediction results were evaluated. In full-scale pipe tests, two specimens were tested according to a complicated loading procedure. The loading was a combination of axial load, bending moment, and internal pressure; and it consisted of monotonic loading stage and cyclic loading stage. Based on those two tests, the global and local behaviour were investigated, the failure mechanism was studied and the application of the developed LCF life prediction models was discussed. In FEA, three numerical models were developed and they were the strip model, the half-pipe model and the full-scale pipe model. In the strip model, the residual stresses and strains were analyzed and discussed. In the half-pipe model, the effects of pipe geometry, internal pressure, and global deformation on the wrinkle geometry were studied and discussed. In the full-scale pipe model, the full-scale pipe tests were simulated and both the global behaviour and local behaviour were discussed. From this research program, some important conclusions were obtained. The wrinkle geometry is found to be greatly related to the pipe geometry, internal pressure, and global deformation. The global deformation has become localized after the wrinkle is fully developed. The opening deformation cycle is more detrimental to wrinkled pipes than the closing deformation cycle. The test results also show that the seam weld governs the failure of wrinkled pipes if the pipes are subjected to cyclic axial deformation. The LCF life prediction models developed from this research program demonstrate good prediction capacity when they are applied to both strip tests and full-scale pipe tests. / Structural Engineering

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.

Accidental errors are inevitable in the detenninatiorr of the fatigue curves parameters and mechanical characteristics of welded joint materials. That's why the investigation results are accidental and are analysed by means of statistical method. The use of statistical methods includes analysis of literature about test methods and treatment of experimental data. The purpose of this work is to propose consequent methods of statistical treatment of investigation results by the means of the main computation methods.

Stulpinaitė A. Parameters of low cycle fatigue curves according to elastic and plastic strain: Master thesis of mechanical engineer / research advisor associate professor dr. R. Šniuolis; Šiauliai University, Technological Faculty, Mechanical Engineering Department. – Šiauliai, 2004. -60p. Parameters of fracture (parameters of low cycle fatigue curves m, C) are determined at homogeneous stress state and symmetric or asymmetric low cycle straining. Low cycle loading in structures appear near stress concentrators, cracks in little volumes of material which is limited of elastic strained zones. The elastic plastic loading may be in the welded joints, too. In the condition of variable loading the elastic strained metal impedes the accumulation of strain in these areas. Therefore the conditions of straining in these areas are similar to low cycle straining. At low cycle loading lifetime depends on reduction of areay. Coffin’s equation for calculation of constant C is not suitable for most materials and more complex equations must be used. Parameter C markedly differs from the experimental, therefore high errors occur in evaluating lifetime of the material. It is noticed in many works that parameter proposed by Coffin is close to experimental. Parameter m = 0,5, proposed by Coffin, is determined according the low cycle fatigue curves in coordinates . In some works the total strain e is used instead of plastic strain d . The difference of values of parameter m in this case is... [to full text]

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DIN 35NiCrMoV125 (equivalent to AISI 4340), which is a high strength low alloy steel (HSLA), is mainly used at military applications in the production of gun barrels. The main aim of this study was to determine the low cycle fatigue (LCF) behaviour and the influence of temperature on low cycle fatigue failure properties of that steel. Three different temperatures (room temperature, 2500C and 4000C) were used in the experiments in order to analyze the effect of temperature. For each temperature, five strain amplitudes (in the range of 0.2% offset yield point to 2% strain) were applied and the duplicates of each experiment were performed to obtain more accurate results. Strain amplitudes and the corresponding stresses were calculated from tension tests performed at each temperature. Strain amplitude versus fatigue life (e- N) curves for three different temperatures predicted that fatigue life at a given strain increases with increasing temperature. The transition lives of those three curves were observed at 1 % strain amplitude and no significant effect of temperature on transition lives was observed. For stress based analysis, stress versus fatigue life (S-N) curves were drawn. These curves pointed that fatigue strength at a given number of cycle decreases with increasing temperature. Fractographic analyses of the fracture surfaces were performed to examine the effects of load and temperature on the specimens. It was observed that the number of crack initiation sites increases with increasing strain.

Damage to steel moment frames in the 1994 Northridge and 1995 Hyogken-Nanbu earthquakes subsequently motivated intensive research and testing efforts in the US, Japan, and elsewhere on moment frames. Despite extensive past research efforts, one important problem remains unresolved: the degree of panel zone participation that should be permitted in the inelastic seismic response of a steel moment frame. To date, a fundamental computational model has yet to be developed to assess the cyclic rupture performance of moment frames. Without such a model, the aforementioned problem can never be resolved. This dissertation develops an innovative way of predicting cyclic rupture in steel moment frames by employing artificial neural networks. First, finite element analyses of 30 notched round bar models are conducted, and the analytical results in the vicinity of the notch root are extracted to form the inputs for either a single neural network or a competitive neural array. After training the neural networks, the element with the highest potential to initiate a fatigue crack is identified, and the time elapsed up to the crack initiation is predicted and compared with its true synthetic answer. Following similar procedures, a competitive neural array comprising dynamic neural networks is established. Two types of steel-like materials are created so that material identification information can be added to the input vectors for neural networks. The time elapsed by the end of every stage in the fracture progression is evaluated based on the synthetic allocation of the total initiation life assigned to each model. Then, experimental results of eight beam-to-column moment joint specimens tested by four different programs are collected. The history of local field variables in the vicinity of the beam flange - column flange weld is extracted from hierarchical finite element models. Using the dynamic competitive neural array that has been established and trained, the time elapsed to initiate a low cycle fatigue crack is predicted and compared with lab observations. Finally, finite element analyses of newly designed specimens are performed, the strength of their panel zone is identified, and the fatigue performance of the specimens with a weak panel zone is predicted.