Investigation of thermomechanical failures in metal housing of catalytic converter used for automotive exhaust system

In automotive exhaust systems are exposed to different vibrations and thermal loads throughout in working life. Vibrational loads range from simple sinusoidal vibrations to randomly generated maximum amplitude loads and sudden shock loads. Thermal loads originated from suddenly fallen high temperature to low temperature. Heating in parts in the automotive is caused by the energy resulting from the explosion of the fuel-air mixture in the engine cylinder. This energy is distributed to the engine block, mainly to the exhaust manifold and to the other components on the engine, including the turbo, and to the parts on the exhaust. Extraneously, close to these systems are affected by the heat generated. The heat loads above the catalytic converter, which is one of the components positioned to develop the emission on the exhaust and exhaust line, which is considered to be the primary heated zone, will be mainly studied in this study. Apart from the thermal loads, the forces coming from the vibrations in the vehicle will be measured and it is aimed to develop an existing test methodogy to verify its durability. Life tests are the basis tests required to demonstrate the durability of catalyst system. Life tests show the effect of damage, caused by accelerated thermal and vibration loads on the component. This ensures an error-free operation time during the use of the part in the field. In order to carry out life tests, it is necessary to know the distribution of heat and force on the part under operating conditions. Based on the measured force and temperature values, the part will be subjected to an accelerated test-specific test laboratory. In the first part of the thesis, exhaust emission, catalytic converter and usage conditions of catalytic converter are explained. And mentioned about the importance of exhaust emission in diesel engines and why catalytic converters are developed. In addition, it is given information about catalytic converter types, internal structure and materials used in its structure. The formulations developed for thermal fatigue and life cycle models, which are seen in the literature research, are explained in the second part of the thesis. Life calculation models with finite element analysis are mentioned. Thermal fatigue studies with different materials were analyzed. After the third part, the test work was explained and the test devices and the measurement devices were informed with working princible explanations. Afterwards, the damage analysis study is explained with the visuals. In the last section, the results of the tests and the results of the damage analysis are given. In the present study, life simulation tests were performed to show the results of life tests which are the most critical stage of the design and development stages of exhaust systems. In order to disclose the damage mechanisms, thermal and vibrational loads which the most significant effect in this type of systems, were tested on the sample. Thermal expansion and deformations are measured with DIC Camera system. In addition, the strain gauge measurements were verified for the DIC method. At the beginning of the study, expansion test was performed with DIC camera on a stainless steel plate, which has a similar material with catalytic converter, which was chosen as a separate test sample. With this analysis, it is confirmed that the DIC camera provides reliable measurement results regardless of the size of the selected area and the measuring range. Tests are rare examples of thermal stress analysis with the DIC camera. Thansk to this method, stress measurement can be taken without gluing any measuring device on the test piece and eliminating the effect of the measurement method on the test piece. In addition, it is possible to obtain more accurate results since there is no notch effect stress on the piece as seen in other stress measurement methods. Thanks to the DIC camera measurement method, it can be measured regardless of the temperature on the test piece and contains a measurement principle that will not be affected by the change in temperature. Considering these advantages, it is deemed appropriate to use the DIC Camera system as a method of measurement for the stress-strain analysis of a part in the exhaust system. In order to measure the forces due to vibration on the vehicle, data have been taken in the exhaust system with accelerometers. With the accelerometer data, the forces on the part were evaluated and transformed into a vibration map which would create the desired life effect with the help of the transfer function. With this study, the tests on the catalyst were divided into three groups. The results of thermal stress tests, cold vibration tests and vibration tests in hot conditions were assessed by the leakage measurement taken at the end of each section. In leak measurements made by compressed air, it is determined that the most amount of leakage is seen as a result of vibration tests performed under thermal stresses. However, when the cold vibration is applied, the leakage on the part of the piece is negligible. With this analysis, it was concluded that thermal stresses were more effective on the catalyst than on vibration. At the end of the test, damage analysis was performed to determine if cracks on the exhaust catalyst were formed due to fatigue stress amplitude. The crack surfaces were separated from each other and cracked surface screen were taken under the electron microscope. The findings were examined in order to demonstrate the test methodology causes termomechanical fatigue and it proved. Fractographic SEM images of thermomechanical fatigue are shown in thesis. Fatigue lines (fatique striations) and secondary fractures in the images clearly indicate that the damage is caused by fatigue. In addition, the porous structure, called dimple on the ductile fracture zone, was observed on the SEM images in accordance with the fracture direction and the material characteristic of the part. Crack formation is a result of penetrating examination. To examine the cracked surface, the cracked area was first cut away from the catalytic converter and then separated from each other so that the cracked surface was not damaged. During SEM examination, the image of the fracture surface was taken from the macroscopic images. Furthermore, a resource improvement proposal was given for the cracks seen at the end of the test and it was revealed that the welding method should be reviewed again.