Fatigue Damage Evaluation Research of Aluminum Alloy Based on Energy Degradation

Experimental investigations of fatigue cracking behavior of LC9 aluminum alloy (AA LC9) subjected to elevated temperature were conducted with scanning electron microscope (SEM). Results indicate elevated temperature is important for the fatigue crack growth of AA LC9. Based on small crack growth, crack growth rate for AA LC9 is characterized.

SEM In Situ Study on Fatigue Crack Growth of LC9 Aluminum Alloy Subjected to Elevated Temperature

Experimental investigations of fatigue cracking behavior of LC9 aluminum alloy (AA LC9) subjected to elevated temperature were conducted with scanning electron microscope (SEM). Results indicate elevated temperature is important for the fatigue crack growth of AA LC9. Based on small crack growth, crack growth rate for AA LC9 is characterized.

A Meso-Mechanical Constitutive Model of Particle Reinforced Titanium Matrix Composites Subjected to Impact Loading

A meso-mechanical constitutive model of TiC particle reinforced titanium matrix composites (TiC/TMCs) under impact loading is established to investigate the mechanical behavior of TiC/TMCs. Based on Eshelbys equivalent inclusion theory and Mori-Tanakas concept of average stress in the matrix, the compliance tensor is formulated. By adding nucleation and growth crack models, the influences of micro-cracks on compliance tensor and damage evolution are examined. Finally, a one-dimensional dynamic constitutive model subjected to impact loading is presented to explore the mechanical behavior of TiC/TMCs.

TEXTURAL FRACTOGRAPHY

The reconstitution of the history of a fatigue process is based on the knowledge of any correspondences between the morphology of the crack surface and the velocity of the crack growth (crack growth rate - CGR). The textural fractography is oriented to mezoscopic SEM magnifications (30 to 500x). Images contain complicated textures without distinct borders. The aim is to find any characteristics of this texture, which correlate with CGR. Pre-processing of images is necessary to obtain a homogeneous texture. Three methods of textural analysis have been developed and realized as computational programs: the method based on the spectral structure of the image, the method based on a Gibbs random field (GRF) model, and the method based on the idealization of light objects into a fibre process. In order to extract and analyze the fibre process, special methods - tracing fibres and a database-oriented analysis of a fibre process - have been developed.

AUTO-SHAPE ANALYSIS OF IMAGE TEXTURES IN FRACTOGRAPHY

The aim of this study is to estimate the velocity of fatigue crack growth (crack growth rate - CGR) from the texture in SEM images of crack surfaces. A simple and quick method is based on fitting training images as a linear combination of several small subimages selected from the images themselves. The size of basic subimages is derived from autocorrelation functions of the image in row and column direction. The selection of basic subimages is based on two indicators: "appeal" evaluating their shape content, and mutual coefficient of correlation. The method is easy to implement and quick in computations, while results of testing application are fully comparable with best ones obtained within textural fractography of fatigue failures.

SEM Study of High-Chromium Martensitic Steel LCF Fracture

Low cycle fatigue of high-chromium 13Х11Н2В2МФ stainless steel has been studied after cyclic tests at room temperature with the frequency of loading, 0.45Hz and amplitude, ± 1mm. The samples were v-notched with the dimensions x2x50, where =3mm. The peculiarities of fatigue crack propagation and influence of heat treatment, sizes of grains and laths, and disposition of microcrack and microstructure elements of the steel were studied. Next, the main effect on propagation direction is caused by the shape of grains and laths. It turned apparent that main microcrack is composed of individual micro-components with the lengths correlating with the dimensions of grains and martensitic laths. During growth crack propagation direction changes from lath to lath; however, general trend remains unchanged. The results of tests indicate that speed of fatigue failure rises when the frequency and amplitude of loading increases. The work includes x-ray characterization of the steel, statistical distribution curve for angles between the main direction of macrocrack propagation and micro-components, and explanation of micro- and macrocrack propagation alteration is given.

Growth of Surface-Type Stress Corrosion Cracks in Near-Neutral pH Environments Under Disbonded Coatings

Near-neutral pH stress corrosion cracking (NNPHSCC), which occurs when ground water penetrates under the pipe coating, causes longitudinal cracks to develop on the surface of pipelines. Such cracks grow over time and can ultimately lead to pipeline failure. NNPHSCC is currently managed by in-line inspection or hydrostatic testing for oil and gas pipelines respectively. These procedures are enormously expensive and have to be repeated at predetermined intervals. Re-inspection intervals are currently determined by empirical models, which have been found rather imprecise. A major flaw in currently applied models is that they assume that once a NNPHSCC crack is formed, it grows at a constant rate that is independent of pipeline operating variables and both pre- and in-service history of the pipeline material. This is not necessarily true as pipeline history, the nature of the service environment and operating factors, among several other factors, have a strong influence on the rate of NNPHSCC crack propagation. Most existing models also treat NNPHSCC cracks as long through thickness cracks rather than surface type cracks typically observed in the field. This research proposes to provide an empirical model that more accurately predicts the growth rate of near-neutral pH SCC cracks in near-neutral pH environments by studying the growth rate of surface type flaws while also accounting for the influence of operating factors, environmental factors, coating disbondment and cathodic protection on the rate of crack propagation. This paper reports some preliminary test results obtained using a long specimen with three semi elliptical surface flaws located in three reduced sections to simulate field observed NNPHSCC cracks. Preliminary results suggest that: 1) crack grows much faster at the open mouth, which was attributed to hydrogen effects; 2) crack dormancy can occur under certain combined mechanical factors; 3) although the benign mechanical loading cannot lead to a direct crack growth (crack dormancy), it causes damage to the crack tip, which makes the crack more susceptible to crack growth upon a more aggressive condition is encountered.