A neural network approach to describing the fretting fatigue in aluminium-steel couplings

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Abstract

Fatigue data for specimens from two aluminium alloys, two surface-finishing conditions, and different surface pressures, subjected to constant dynamic loading with different stress amplitudes, were used to evaluate possible artificial neural network (ANN) architectures for a description of the fretting-fatigue phenomena. A neural network approach shows that ANNs can be trained to model fretting-fatigue phenomena. The ANN we used provided an accurate prediction of the occurrence of fretting fatigue. The main benefit of the trained ANN was that it precisely described the effects of different factors on the occurrence of fretting fatigue. After the ANN has been trained it represents a robust tool for a description of fretting-fatigue phenomenon in aluminium–steel couplings. The ANN was able to acquire the same knowledge that it took many researchers to acquire. The robustness of the field applications is only restricted by the range of known (measured) data used for the ANN training.

Introduction

The problems that occur with mechanical joints which are subjected to dynamic loading are very complex and heterogeneous. One of the most important of these problems is fretting fatigue, which occurs when a minute tangential surface motion arises between components pressed together by normal forces. A significant decrease in fatigue life—by a factor of 10 or more—occurs as a result of fretting, and fretting is one of the main causes of failure in a wide range of engineering applications, including the following: blades and disks in turbine engines; clutches; riveted compounds; bolts and couplings; drive shafts; bearings; wheel hubs; steel ropes; etc. [1], [2], [3].

There have been numerous studies of fretting phenomenon [4], [5], [6], [7] and several key factors—surface pressure; relative motion; loading frequency; stress ratio and amplitude; loading waveform; specimen size; metallurgical composition; surface finish, and many others—have been investigated to determine their influence on fretting fatigue. The influence of these factors on the fretting involves complicated synergistic interactions [3], [4] and accurate databases. In order to establish a functional dependence in a conventional way it would be necessary to vary one factor while all the others are kept constant. Unfortunately, despite remarkable progress, the existing fatigue databases are incomplete for most factors, and the fretting fatigue of mechanical joints made up of steel and aluminium alloys is particularly difficult to describe [2], [5], [6].

Artificial neural networks (ANNs) have evolved over the past few years as a relatively new area of artificial intelligence [8], [9], and they are suitable for applications in a wide range of fields. ANNs are especially useful for simulations of correlations that are difficult to describe with physical models because of their ability to learn by example and to recognise patterns in a series of input and output values from example cases.

ANNs are very capable when it comes to describing fretting-fatigue phenomena. The main advantages of ANNs over other, more conventional, types of description are that the description is based purely on data from the database, not on preconceptions. ANNs can generalise and learn trends and patterns and even though factors affecting the fretting fatigue exert synergistic effects on one another, ANNs can detect and utilise these effects in their descriptions [10].

The aim of the research was to show, using examples of fretting fatigue with steel and aluminium-alloy couplings, that ANNs can acquire the same knowledge that it took the combined efforts of many researchers to acquire, but in a shorter time and with the use of a smaller database.

Section snippets

Experimental details

The fretting-fatigue database for training and testing the ANNs contained 114 experiments. The database included studies of the effects of surface pressure and surface treatment on the fatigue life of aluminium alloys in contact with steel that were subjected to dynamic reverse bending. It covered a modest range of factors: two aluminium alloys (AlSi7Mg-T6 and AlSi11MgSr), two surface-finishing conditions (machined and machined plus shot-peened), different stress amplitudes (ρΔ = 48–138MPa) and

Neural network approach

ANNs are computing systems composed of highly interconnected, but very simple, processing elements (or neurons) that process data using the ANN's response to external inputs. The primary characteristics of ANN models are: massive parallelism, non-linearity, processing by multiple layers of neurons, and, finally, dynamic feedback among neurons [16], [17]. Due to their parallel structure, which is inspired by the parallelism of the human brain, ANNs can be used for a range of multivariable

Neural network estimates

To describe the fretting fatigue of aluminium alloys in contact with steel that were subjected to dynamic reverse bending, a model based on ANNs was created. The ANN was trained and used to predict the occurrence of fretting fatigue for various sets of input parameters. A confusion matrix (Table 5) shows the prediction of fretting fatigue for 28 tests from the database never seen by the ANN. The ANN incorrectly classified three fretting and one no-fretting tests and the overall classification

Results and discussion

A small database was the major handicap that prevented good ANN training and, as a consequence, a good fretting-fatigue description. In order to prevent overtraining of the ANN a cross-correlation subset was used. The use of the cross-correlation subset, at the expense of 15% of the entire database, as an indicator for the best training solution, turned out to be the optimum solution. The 84% accuracy level of the fretting-fatigue prediction in the training and cross-correlation subsets (Table 6

Conclusions

The neural network approach shows the tremendous potential that ANNs have for describing the fretting-fatigue phenomena of aluminium–steel couplings that are subjected to constant dynamic reverse bending. The ANN we used gave us an accurate description of the influence of different parameters on fretting fatigue and also good predictions for the occurrence of fretting fatigue based on input parameters. With the trained ANN it is possible to predict the occurrence of fretting fatigue for

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