Adopting lock-in infrared thermography technique for rapid determination of fatigue limit of aluminum alloy riveted component and affection to determined result caused by initial stress

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Abstract

The lock-in infrared thermography technique was adopted to rapidly determine the fatigue limits of two kinds of riveted components, which were both made of 2A12 aluminum alloy and had different magnitudes of interference. The results obtained by the lock-in infrared thermography technique were compared with the existing fatigue limits of 2A12 aluminum alloy, and experimental research based on the staircase method was also carried out. It is shown that the magnitudes of interference and the corresponding initial stresses have important and apparent affections to the results determined by the lock-in infrared thermography technique.

Highlights

► Fatigue limits of two kinds of riveted components are rapidly determined by lock-in infrared thermography technique. ► The rapidly determined results of conventional riveted components are proved to be accurate. ► The rapidly determined results of large interference riveted components are proved to be incorrect. ► It is proved that the initial stresses around riveted holes affect the rapidly determined results obviously.

Introduction

The application of infrared thermography as a non-destructive method to detect the occurrence of damage and to investigate the fatigue process of materials has become popular and has been widely investigated in literature in last 15 years. From the experimental point of view, thermographic techniques allow to measure the surface temperature of a specimen by means of an infrared thermal scanner and to monitor its growth during the test. On the basis of thermal experimental data, some analysis methods have been developed over the years in order to correlate the growing temperature to the physical processes of damage and failure in materials.

Existing achievements [1] show that the amplitude of the surface temperature change of metal material, ΔT, varies with the loading cycle N, and presents three different stages under cycled tensile loading Δσ. In the first stage, ΔT increases linearly with N. In the second stage, ΔT stays stable and constant. In the third stage, ΔT increases rapidly with N, and the material is then failed immediately.

La and Risitano [1] defined the fatigue limit as the stress value for which the temperature ΔT increased, or the mean rate of temperature at the first stage, ΔTN, increased. As a consequence, it was proposed that the fatigue limit could be determined by plotting curves of the increase in temperature ΔT, or the initial thermal gradient ΔTN, against the applied load and finding the value of the fatigue limit as the intercept of the curve on the x-axis (ΔT = 0 or ΔTN = 0).

Luong [2], [3] noticed the increase in surface temperature was very low when applied stresses were below the fatigue limit, but it was not negligible. Therefore, Luong utilized two curves to interpolate experimental data, one for stresses below and the other for stresses above the fatigue limit, and the corresponding intersection was indicated as the fatigue limit.

Based on the creative achievements presented by Risitano and Luong, scientists and engineers started to try to apply this rapid determination method of fatigue limit to some structures and components, where the stress concentration coefficients kT  1 [4], [5], including some welding components [6], which widened the applicable range of this method.

It should be emphasized that in all the above literatures the “fatigue limit” actually means the “fatigue strength” in the megacycle range. The presented method is not applied to low cycle fatigue or very high cycle fatigue.

According to authors’ knowledge, the stress states of the fatigue dangerous positions of the referred structures and components in the published literatures are all uniaxial, no matter the fatigue dangerous positions are notches or blind holes. Therefore it is still a question to be discussed that if this rapid determination method could be applied to some structures or components (e.g. the riveted components), of which the stress states at the fatigue dangerous positions are multiaxial.

In this paper, with the lock-in infrared thermography technique, the rapid determination method of fatigue limit presented by Luong was applied to aluminum alloy riveted components which were really familiar in aero industry. The fatigue limits of two kinds of aluminum alloy riveted components which had different magnitudes of interference were rapidly determined by the lock-in infrared thermography technique. Experiments based on the conventional staircase method were carried out to prove the credibility and accuracy of the results given by the lock-in infrared thermography technique. Affections to the determined results caused by different magnitudes of inference and initial stresses in local riveted zones were also discussed.

In order to compare with the existing achievement [4], in this paper, the fatigue limit is defined as the fatigue strength at 2 × 106 cycles.

Section snippets

Basic theory

In the year 1985, Chrysochoos et al. [7] presented the complete theory of thermal-mechanics coupling based on all the achievements obtained before, which becomes the theory foundation of the lock-in infrared thermography technique. As shown in Eq. (1), the theory presented by Chrysochoos is a thermal-mechanics coupling equation described in reference configuration:ρ0CTθ˙-K02θ=ρ0h+wctm+wc,where ρ0 is the density in reference configuration, K is the coefficient of heat conduction, CT is the

Aluminum alloy riveted components and their initial stresses

Two kinds of dog-bone shaped riveted components with two different magnitudes of interference were made of 2A12 aluminum alloy. As shown in Fig. 1, two rivets were placed in the single row along the central line, which was parallel to the loading direction.

The riveted component with relative small interference magnitude is defined as “the conventional riveted component”. Its interference magnitude is +0.05 mm, and the parallel length is L = 100 mm. The diameter of the rivet is d = 6.71 mm. The

Rapid determination of fatigue limit

Cycled sinusoidal loadings were applied to the two kinds of riveted components by the MTS 810 fatigue testing machine. The stress ratio was R = 0. The loading frequency was f = 10 Hz. The amplitudes of the loadings varied from 5 kN to 45 kN, and the interval was 5 kN. The JADE-III550 lock-in infrared thermography system supplied by the former CEDIP company (in France) was used to determine the surface temperature distributions of the specimens, after they had been loaded 5000 cycles at each amplitude of

Analysis and validation

The special loading amplitudes of the two kinds of riveted components were obtained by the lock-in infrared thermography technique, which were 26.7 kN for the conventional riveted component and 31.5 kN for the large interference component. According to Risitano and Luong’s theory and definition [1], [2], [3], when the conventional riveted component made of the 2A12 aluminum alloy is under the special loading of 26.7 kN, the stress at the fatigue dangerous position in the local riveted zone will

Summary

The lock-in infrared thermography technique was adopted to rapidly determine the fatigue limits of two kinds of riveted components, the conventional riveted component and the large interference riveted component, which were both made of 2A12 aluminum alloy and had different magnitudes of interference. The results obtained by the lock-in infrared thermography technique were compared with the results obtained by interpolating the existing fatigue limits of 2A12 aluminum alloy under different

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Supported by the Defense Industry Technical Foundation Research Project of China under Grant No. Z052009T002.

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