The fatigue life of notched magnesium sheet metals with emphasis on the effect of bands of twinned grains

Highlights

• Strain is localized in wedge-shaped bands of twinned grains in notched Mg-specimens.

• Due to bands of twinned grains, the notch sensitivity is reduced.

• The endurable load is only slightly reduced by a notch at 500–10³ cycles to fracture.

• Specimens are highly notch sensitive under loading without twin formation.

Abstract

Notched AM50 sheet metal specimens with four different stress concentration factors are tested under quasi-static and cyclic uniaxial loading, and the results are analyzed and compared with those of unnotched specimens. Special attention is given to the formation of bands of twinned grains and the inhomogeneous local strain field in the vicinity of notches, which is measured by an in situ optical strain measurement technique. It is shown that the notch sensitivity is reduced, if bands of twinned grains occur. In the range between 500 and 1000 load cycles to fracture, the endurable load is only slightly reduced by the presence of a notch for completely reversed fatigue tests. In contrast, specimens are notch sensitive under loading conditions, where no twin formation occurs.

Introduction

Magnesium alloys exhibit a hexagonal close packed crystal structure with the major deformation mechanisms at ambient temperature being basal 〈a〉 slip and {1 0 −1 2}〈1 0 −1 1〉 extension twinning [1]. Further slip mechanisms of magnesium alloys are non-basal prismatic {1 0 −1 0} and pyramidal {1 0 −1 1} slip, which have a higher critical resolved shear stress compared to that of basal 〈a〉 slip [2]. Extension twins can be formed at low stresses in polycrystalline magnesium alloys for specific load directions [1]. These are a tensile stress along the c-axis or a compressive stress perpendicular to the c-axis of the lattice system [1], [3], [4]. Consequently, the strong basal texture of wrought magnesium sheet metals with the c-axis lying almost normal to the sheet plane [3], [5], [6] results in an asymmetry of the tensile and compressive yield stress for the same load direction [7], [8]. In this case, twinning can be activated under tensile loading in the direction normal to the sheet metal surface or under compressive loading perpendicular to the normal direction.

The fatigue life of wrought magnesium sheet metals is strongly influenced by twinning [3], [7], [9], [10], [11], [12], [13]. When the compressive yield stress is exceeded (perpendicular to normal direction) {1 0 −1 2}〈1 0 −1 1〉 twinning occurs and strain hardening is nearly absent for the first load cycle and low for all further load cycles [14], [15]. Most of the twins are detwinned by reverse loading [16], [17]. Thus, in case of twinning during completely reversed fatigue tests, stress-strain hysteresis areas are significantly enlarged [15] and the fatigue life is distinctly reduced [7].

Once formed, twins are not randomly distributed along the gauge section of a specimen, but are rather grouped in bands of twinned grains. These bands are often called twin bands. However, the expression “twin band” is used in different contexts. Some authors refer to a single twin within a single grain when using the expression twin band [3], [4], [18], [19]. A chain of twins in twinned grains where the twins adjoin each other may be called a long twin band [20]. In [7], [21], [22], however, a twin band describes the accumulation of twinned grains within a macroscopic band that is referred to as a band of twinned grains (BTG) within the scope of this paper.

The compressive strain within a BTG is considerably larger compared to that of the area next to a BTG [23]. During three-point bending of 1 mm thick AZ31 magnesium sheet metals, localized BTGs occur within the compressive zone [21]. The observed BTGs are widest at the top surface of the specimen where the compressive bending stress is at its maximum, follow an angle of approximately 45° relative to the normal direction of the sheet metal, and are parallel to the transverse direction [21].

Owing to stress concentrations in the vicinity of notches, the yield stress can be exceeded locally by applying comparatively low external loads. Different results are published concerning the notch sensitivity according to Thum and Buchmann [24] of magnesium alloys within the high cycle fatigue regime. While the notch sensitivity of cast AZ91HP, AM50HP, and AM20HP has been observed to be about 30% in the long life fatigue [25], extruded AZ80 wrought alloys [26], [27] show a notch sensitivity of 100% in this cycle range.

To enable a broad use of wrought magnesium alloys, a detailed knowledge of the influence of notches on the fatigue life is necessary [25], [28], [29], [30], especially for designing structural components. There are a number of studies that examine the fatigue performance of wrought magnesium alloys within the low cycle fatigue regime where twinning plays a prominent role [3], [7], [19], [22], [31], [32], [33]. Nevertheless, there is no investigation on the low cycle fatigue behavior of notched magnesium sheet metals and there are a limited number of studies, which consider the influence of different stress concentration factors on the fatigue behavior of magnesium alloys. In addition, the influence of BTGs at notches on the fatigue life has not been examined to date as well. Hence, the present study is dedicated to these subjects. The magnesium alloy AM50 is used for all investigations within this study and is a commercially available alloy [34] that is applied in automotive industry [35].