Micromechanistic analysis of fatigue in aluminium silicon casting alloys

Due to increasingly stringent environmental legislation, there is a requirement for lower emissions and greater overall efficiency of light vehicle diesel (LVD) engines. This continues to be achieved through the optimisation of design and careful selection of the materials used in key LVD engine components, for example pistons, so that they are lighter and can operate at higher temperatures. Pistons are non-serviceable parts and so must be able to withstand the fatigue and high temperature environment of the car engine. It is therefore important to understand the mechanisms of fatigue in these alloys to help inform alloy development for the next generation of pistons. Pistons are typically produced from multi-component Al-Si casting alloys. These alloys exhibit a complex, multiphase microstructure comprising α-aluminium as the matrix with silicon particles and several intermetallic phases. Previous research on Al-Si casting alloys has demonstrated that porosity is detrimental to fatigue life as cracks initiate freely at pores. However, with improved casting techniques porosity can be greatly reduced and other microstructural features influence fatigue life. In particular, Si particles have been shown to play an important role in the initiation and subsequent propagation of fatigue cracks. This study assesses the role of Si content and other microstructural features on fatigue behaviour by testing a set of well-characterised multi-component, Al-Si casting alloys with varying Si content. Fatigue initiation behaviour was investigated at room temperature using S-N and short fatigue crack growth experiments. Pores, Si particles and intermetallic phases were shown to cause fatigue crack initiation. In a 0.67wt% Si containing alloy, large-scale porosity was observed and was the foremost reason for fatigue initiation. In two alloys the Al9FeNi phase was observed to be the most detrimental hard particle causing fatigue crack initiation. Nanoindentation results showed that Al9FeNi had a lower hardness and higher modulus than Si and so Al9FeNi may be expected to fracture preferentially, consistent with the fatigue results. X-ray computed tomography demonstrated that all the alloys investigated contained a complex, interconnected, intermetallic sub-structure. As a result, the micromechanisms of fatigue are different to those in conventional particulate Al-Si alloys because particle fracture is required to ensure a level of crack continuity. At room temperature and 350˚C, and at low and high crack growth rates, the crack tip may be described as a diffuse region of micro-damage and intact ligaments. It is the extent of this damage in the alloys that controls the crack growth rates exhibited and simple trends between the Si content and roughness, reported for particulate systems, do not hold true in the alloys investigated in this study. The balance of the micromechanisms of fatigue was shown to be dependent on temperature. This highlights the importance of fatigue studies at temperatures that are characteristic of those experienced in service.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Advances of Mathematical Morphology and Its Applications in Signal Processing

This thesis describes some advances of Mathematical Morphology (MM), in order to improve the performance of MM filters in I-D signal processing, . especially in the application to power system protection. MM methodologies are founded on set-theoretic concepts and nonlinear superpositions of signals and images. The morphological operations possess outstanding geometrical properties which make it undoubted that they are efficient image processing methods. However in I-D signal processing, MM filters are not widely employed. To explore the applications of MM for I-D signal processing, our contributions in this area can be summarized in the following two aspects. Firstly, the fram.ework of the traditional signal processing methods is based on the frequency domain representation of the signal and the analysis of the operators' transfer function in the frequency domain. But to the morphological operations, their representations in the frequency domain are uncertain. In order to tackle this problem, this thesis presents our attempt to describe the weighted morphological dilation in the frequency domain. Under certain restrictions to the signal and the structuring element, weighted dilation is transformed to a mathematical expression in the frequency domain. Secondly, although the frequency domain analysis plays an important role in signal processing, the geometrical properties of a signal such as the shape of the signal cannot be ignored. MM is an effective method in dealing with such problems. In this thesis, based on the theory of Morphological Wavelet (MW), three multi-resolution signal decomposition schemes are presented. They are Multiresolution Morphological Top-Hat scheme (MMTH), Multi-resolution Morphov logical Gradient scheme (MMG) and Multi-resolution Noise Tolerant Morphological Gradient scheme (MNTMG). The MMTH scheme shows its significant effect in distinguishing symmetrical features from asymmetrical features on the waveform, which owes to its signal analysis operator: morphological Top-Hat transformation, an effective morphological technique. In this thesis, the MMTH scheme is employed in the identification of transformer magnetizing inrush curr~nt from internal fault. Decomposing the signal by MMTH, the asymmetrical features of the inrush waveform are exposed, and the other irrelevant components are attenuated. The MMG scheme adopts morphological gradient, a commonly used operator for edge detection in image and signal processing, as its signal analysis / operator. The MMG scheme bears significant property in characterizing and recognizing the sudden changes with sharp peaks and valleys on the waveform. Furthermore, to the MMG scheme, by decomposing the signal into different levels, the higher the level is processed, the more details of the sudden changes are revealed. In this thesis, the MMG scheme is applied for the design of fault locator of power transmission lines, by extracting the transient features directly from fault-generated transient signals. The MNTMG decomposition scheme can effectively reduce the noise and extract transient features at the same time. In this thesis, the MNTMG scheme is applied to extract the fault generated transient wavefronts from noise imposed signals in the application of fault location of power transmission lines. The proposed contributions focus on the effect of weighted dilation in the frequency domain, constructions of morphological multi-resolution decomposition schemes and their applications in power systems.EThOS - Electronic Theses Online ServiceGBUnited Kingdo