Larry Lawson

The effect of fatigue microcracks on rapid catastrophic failure in AlSiC composites

The fatigue resistance of discontinuous reinforced aluminum metal matrix composites is greatly influenced by the rapid nucleation and growth of a very large number of microcracks. These microcracks, which result from an abundance of potential crack initiation sites, are often retarded in growth by the presence of reinforcement through crack trapping. With continued fatigue, microcracking becomes so extensive that it induces widespread coalescence leading to increasingly larger microcracks. Inevitably, some of these large microcracks link together to form the fatal crack and instability takes place very shortly afterwards at unusually small critical crack sizes. The present study examines the form of catastrophic failure in smooth specimens of a 2124 aluminum alloy reinforced with silicon carbide whiskers. Experimental observations of microcrack initiation and stage-by-stage growth through to final failure are reported. Effort is directed at characterizing the distribution and orientation of microcracks present, particularly when linkage(s) result in the formation of the fatal crack, and developing a geometric probability method for predicting coalescence. Results show that microcracks initiate and grow preferentially in arrays which maximize the energy release rate. Near the end of life, the interaction of some microcracks brings about large increases in their stress intensity factor leading to coalescence and fatal crack formation.

The distribution of fatigue microcracks in an aluminum-matrix silicon carbide whisker composite

Discontinuously-reinforced metal matrix composites (DRMMCs) have been widely studied in the past decade due to their enormous potential in applications requiring high stiffness, strength, and low weight. DRMMCs generally exhibit superior stress-controlled high cycle fatigue properties with higher volume fractions of reinforcement typically leading to longer fatigue lives. The density of fatigue microcracks in DRMMCs, however, is much higher as compared with monolithic alloys. This study reports actual histograms for the distribution of surface microcracks in un-notched specimens of an AL-SiC[sub w] composite and how it changes with fatigue. The histograms were obtained by measuring the length of each microcrack seen on replicas of the specimen surface. The observed MLD is fitted with various commonly-used distribution functions to find the best fitting one and to test the goodness of fit of each.

Flux Maximization Techniques for Compton Backscatter Depth Profilometry

Resolution in x-ray backscatter imaging has often been hampered by low fluxes. But, for a given set of resolution requirements and geometric constraints, it is possible to define a maximization problem in the geometric parameters for which the solution is the maximum flux possible in those circumstances. In this way, resolution in noncritical directions can be traded for improved resolution in a desired direction. Making this the thickness, or surface normal direction, makes practicable the depth profiling of layered structures. Such techniques were applied to the problem of imaging the layered structure of corroding aircraft sheet metal joints using Compton backscatter.

Measurement of Layered Corrosion with Compton Backscatter

Compton backscatter has intrigued NDT researchers for a number of years because of its capability for making x-ray pictures without requiring access to both sides of the piece being examined[l]. The major obstacles to commercial development have been its slowness and the high cost of the equipment. Additionally, the resolution obtained has been circumscribed; the best reported resolution appears to be less than 1.5 lp/mm.[2]. As a result, little commercial application has emerged for Compton backscatter as an imaging tool. Recently, interest in aircraft corrosion has renewed interest in Compton backscatter for NDT. This interest appears to be justified partly because aircraft corrosion often takes a layered morphology; and, the needed information is the thickness of the layers. Consequently, it is possible to trade resolution in the directions whose axes lie in the plane of the layers for better resolution along the thickness axis. Furthermore, because the layers usually have a lateral extension of several inches or more, the measurement problem can be reduced to a one dimensional scan in the thickness direction. These characteristics allow for a great reduction in the complexity of the apparatus, a substantial improvement in resolution and an increase in the speed of measurement.

A model of thermomechanical fatigue in a lead-base alloy

A model for the growth of a grain boundary crack in thermomechanical fatigue is derived for a single-phase or low alloy fcc metal at homologous temperatures near 0.5. Crack growth is hypothesized to proceed through vacancies binding in pairs at the crack tip to impurities or an oxide layer. This model is applied to 97Pb−3Sn, and the results compared with experiment. Good agreement is shown between the model and experiment, especially in predicting the effects of frequency and thermal-mechanical phasing. These effects do not appear to have been previously modeled successfully.

Eddy Current Detection of Fatigue Microcrack Distributions in Al-SiC Composites

The modern philosopy of materials reliability hinges upon the ability to detect flaws before they reach a critical size that might lead to the failure of a component before the next inspection opportunity. It is the role of nondestructive evaluation (NDE) to provide this capability. In advanced materials such as Al-SiC composites, however, fatigue failure under high stress is governed by the nucleation and growth of a large number of distributed microcracks 1, often much smaller than the detection threshold of conventional NDE techniques (i.e. 200 03BCm). The “fatal” crack forms through the linking of several microcracks. A given fatal crack can spend as little as 5 percent of the fatigue life as a crack detectable by NDE in these materials since some ductility is often sacrificed to achieve their higher strength (i.e. crack instability occurs for crack sizes much smaller than would be expected in conventional alloys). The difficulties that arise for the NDE of advanced materials are that at one end of the fatigue spectrum the microcracks are too small to detect for most of the component’s life. At the other end of the spectrum, by the time a crack long enough to detect appears, failure is imminent.

Coherent X-Ray Imaging for Corrosion Evaluation: A Preliminary Assessment

Corrosion products sometimes complicate the task of measuring metal loss. For example, in layered joints in aircraft, the products of corrosion usually remain trapped between layers. Their presence interferes with ultrasonic and x-ray measurements. Recently, Compton backscatter tomography has been suggested as a means of measuring the amount of metal loss resulting from corrosion. This x-ray method is appealing in that it is one-sided. However, no successful results seem to be yet reported. Suggested reasons for this have included long counting times [1] and poor contrast between metal and corrosion products [2]. Both these problems are related to the high resolution required.