Cut Compliance Research Articles

Effect of the mutual position between weld seam and reinforcement on the residual stress distribution in Friction Stir Welding of AA6082 skin and stringer structures

In the paper, a numerical and experimental study was carried out to highlight the effect of the distance d between the weld seam and the reinforcement on the residual stress distribution in Friction Stir Welded AA6082-T6 structures. An L-shaped profile was welded to a sheet metal with varying tool rotation and distance d from the weld seam. The Cut Compliance method was used to determine the resulting longitudinal residual stress. A dedicated FE model for FSW was set up, validated and utilized to predict the longitudinal residual stress in the assembled part. The analysis allowed the identification of a few design guidelines in order to reduce the detrimental effects of the residual stresses.

Geometrical and Material Characterization of Quenched and Self-Tempered Steel Reinforcement Bars

Quenched and self-tempered (QST) steel reinforcement bars (rebars) are manufactured by Thermex or Tempcore processes. Characterization studies of QST rebars are mostly limited to reveal the hardened outer layer and some improved mechanical properties compared to hot-rolled (HR) and cold-worked (CW) rebars. However, investigations on residual stresses and imperfections originating from the manufacturing process as well as stress concentrations arising from the ribbed profile are rarely found in the literature. Surface residual stress may be beneficial or detrimental to the fatigue performance of rebars although they have been studied only on the subsurface of QST rebars. Surface imperfections are zones of stress concentration from where fatigue cracks may initiate. Imperfections are usually identified in the fractured cross section resultant from fatigue tests of rebars. Stress concentrations can also arise from the rib geometry. Although the rib geometric parameters affect the stress concentration factor Kt on rebars, stress concentration analysis are restricted to two-dimensional (2D) finite-element models (FEMs). The study presented herein is part of a further detailed investigation on the fatigue behavior of HR-CW and QST rebars in the very high cycle domain. In the present work, characterization analyses of QST rebars include (1) experimental investigation of surface and subsurface residual stresses on QST rebars by cut compliance and X-ray diffraction techniques (residual stresses from both techniques are discussed); (2) identification of surface imperfections by scanning electron microscopy (SEM) analysis; and (3) three-dimensional (3D) finite-element analysis of stress concentrations on the ribbed profile. The influence of the rib geometry such as radius, width, height, and inclination as well as the rebar diameter on Kt values are analyzed. The critical zones are determined along the ribs. (C) 2016 American Society of Civil Engineers.

Through thickness property variations in a thick plate AA7050 friction stir welded joint

In this study, moderately thick (32mm) AA7050 plates were joined by friction stir welding (FSW). Various methods were used to characterize the welded joints, including nugget grain size measurements at different locations through the thickness, micro-hardness indentation through nugget, thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) at different cross section heights, and residual stress measurement using the cut compliance method with full thickness and partial thickness specimens. All testing results are consistent with the presence of a strong gradient in peak temperature through the plate thickness during FSW.

Fatigue Assessment of Brazed T-Joints Based on Damage Tolerance Including Residual Stress Effects

Abstract The fatigue crack growth behavior of a brazed joint is characterized by a Paris-exponent that is much higher than the one of bulk metals, which means that brazed components have only a short residual fatigue life after the initiation of a fatigue crack. Therefore, the threshold of fatigue crack growth of brazed joints is of particular importance in the fatigue analysis of brazed components and needs to be understood well. Residual stresses have to be considered in crack growth analysis in the threshold regime. The corresponding effects are explored experimentally and theoretically. The stress intensity factors due to the residual stresses in a brazed T-joint of compressor impellers were measured by using the cut compliance method. It was found that residual stresses are present but relatively small. They do not affect the endurance stress significantly. A concept for an endurance analysis of a brazed T-joint that includes imperfections and residual stresses is suggested.

Evaluation of Residual Stresses During Fatigue Test in an FSW Joint

Abstract: This paper shows an application of the adjusted compliance ratio method (ACR), and the on‐line crack‐compliance technique for determination of the effects of the residual stress during a fatigue test. The fatigue crack growth tests were carried out on a friction stir welded (FSW) joint in Ti‐6Al‐4V titanium alloy. On‐line crack compliance enables the determination of the residual stress intensity factor in real time from a fatigue test. The ACR methodology was used to separate the residual stress effects from the crack growth rate data. Finally, the residual stress distribution of the FSW joint was found from the knowledge of the residual stress intensity factor through an integral solution. It would have to be noted that both methods are based on ratios of displacements; therefore, the practical application does not require the use of the influence functions needed for the cut‐compliance method. Moreover, a specific test, which determines residual stresses, can be avoided because the effect of the same residual stresses on the crack growth is evaluated during the fatigue test. This methodology is more accurate than cut compliance because it reflects the real crack growth behaviour.

Residual stress and microstructure effects on fatigue crack growth in AA2050 friction stir welds

In this work, results of a study conducted on fatigue crack propagation in friction stir welded AA2050 and the effects of FSW induced residual stresses, as well as changes in the microstructure, are presented. Longitudinal residual stress profiles across butt welded 2050 plates were determined using the cut compliance technique and fatigue crack growth testing was conducted on compact tension specimens machined from the friction stir welds. Tests were performed with the crack propagating nominally perpendicular to the weld and with a constant, applied, cyclic, stress intensity factor. Two different material tempers were investigated and in both cases residual stresses were found to have a major effect on the fatigue crack propagation. It was shown that compressive residual stresses are present in the vicinity of the weld, leading to crack closure and a decrease in the fatigue crack growth rate as the crack approaches the weld. Once in the weld nugget, the crack propagation rate increases. This increase is believed to be linked in part to tensile residual stresses but also to a microstructural effect present when the crack grows through the recrystallized nugget. It was also observed that other closure mechanisms, such as oxide-induced closure may affect the fatigue behavior of the 2050 FSW’s. Two methods to predict fatigue crack growth rates in the weld zones were attempted: using the residual stress profiles determined by cut compliance and using the effective stress intensity factor, Δ K eff, obtained during fatigue crack growth testing.

Residual stresses, defects and non-destructive evaluation of FSW joints

Summary This article describes some of the research activities in progress at the German Aerospace Centre, DLR, on the friction stir welding process. It shows in detail the principal defects that can be found in FSW joints, particularly those defects that are due to incomplete filling, incomplete penetration and root discontinuities. The non-destructive ultrasonic method has been used as a promising technique to detect and characterise any of the imperfections mentioned above. The results obtained are confirmed by both light and scanning electron microscopy (SEM). The second part of the analysis focuses on the quantitative evaluation of the residual stresses using the ‘cut compliance’ method, and the mathematical tools provided by fracture mechanics. Finally, two examples of fatigue life predictions for welded joints are presented that are considered to be significant in light of the above considerations. One concerns the propagation of defects in a specimen FSW weld in the presence of a residual stress field and variable amplitude loads, and the other concerns the prediction of the fatigue behaviour of defective FSW specimens without the influence of residual stresses. The results obtained showed good agreement with the experimental data.

Near-Surface Stress Measurement in 2D and 3D by the Cut Compliance Technique

The cut-compliance (CC-) method can be applied in a semi-destructive way by introducing only a shallow slot instead of a cut through the thickness. This requires the influence function to account for the U-shape of the shallow slot and the 3D-effects that affect the stress relaxation. In the present paper a simplified way to evaluate surface stresses from a slot is presented and discussed. By examples it is shown that a two-parameter approximation is often sufficient to characterize the near surface residual stresses in a narrow range of depth. Beyond this range, the procedure described herein can be matched with the common procedure of the CC-method. The analysis is explained first for the simpler case of a 2D-structure. Then the generalization to a 3D-case is shown and discussed. For both cases illustrative examples of measurements are given.

Residual Stress Measurement in Cracked Components: Capabilities and Limitations of the Cut Compliance Method

The cut compliance method is a powerful experimental tool to measure residual stress distributions. Based on fundamental theoretical relations of liner-elastic fracture mechanics, it is particularly well suited to be applied to components that contain cracks, since it delivers not only the residual stress-field but the stress intensity factor due to the latter as well. However, in the presence of cracks, there are special problems to be dealt with: Contact between crack faces due to crack closure, large stress gradients near the crack tip, residual stresses as high as the yield stress, problems of cutting and unsteady heat production. Most of these effects disturb the linearity of the problem, so the basic assumptions of the theory are violated to some degree. Some possibilities of how to deal with these problems are proposed and discussed.