PSI - Issue 17

Meike Funk et al. / Procedia Structural Integrity 17 (2019) 183–189 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

In technical aluminum alloys fatigue cracks are often initiated at broken inclusions or precipitates (Payne et. al. (2010)). The initiated cracks keep on propagating until they coalescent forming a through-thickness crack (Zhixue (2006)). In flat notched specimens (i.e. SEN), they usually are initiated in the edges of the notch root and propagate in the shape of quarter-ellipse until they coalescence. In some cases, middle cracks also appear in the form of semi ellipses as shown by Bär and Tiedemann (2017). Funk and Bär (2018) showed that the location of the individual cracks and the microstructure have a strong influence on the time of coalescence and with that on the total lifetime. The period up to the formation of a through-thickness crack covers the main part of the total lifetime, up to 80 or 90%, and crack length of more than 1mm (Funk and Bär (2018)). For the investigation of this period, not only an accurate measurement of the crack length is essential, but also a detection method to define the time of coalescence is needed. There are different methods of detection, among which the electrical potential method stands out due to the easy usability and automation. The Direct Current Potential Drop Method (DCPD) is an integral method, therefore a conventional use gives no information about the location and shape of a crack. In this work the capability of the measurement with two potential probes will be shown in crack propagation experiments performed on single edge notched (SEN) specimens.

Nomenclature a

crack length on the specimen surface

a k

depth of the notch

mean crack length including the notch depth a k

a mean

b c

thickness of the specimen

crack length in the notch root DCPD Direct Current Potential Drop Method I current conducted through the specimen N cycle number N f lifetime of the specimen SEN single edge notched specimen t time U

voltage drop between the welding points of the DCPD voltage drop between the welding points of an uncracked specimen

U 0

w y 0

width of the specimen

contact distance of the DCPD

1.1. Experimental setup

Single edge notched specimens (SEN) were milled out of AA 7475-T761 clad sheet material. The specimens have the dimensions of width w = 12 mm, thickness b = 2.75 mm, notch depth a k = 1 mm and notch radius r = 0.5 mm (compare with Fig. 1a). Additionally, for a defined crack initiation a secondary notch has been introduced by a small laser cut with the length of 0.2 mm and a depth of 3 µm in one edge of the notch root. The specimens were tested under uniaxial loading conditions using a servo-hydraulic testing machine with parallel specimen guidance (Funk and Bär (2018)). An integrated DCPD not only detects the crack length time-synchronously, but also enables direct the test control via changes in the crack length using the control electronics of DOLI EDC 580. For contacting the DCPD on the specimen, copper wires with a diameter d = 0.2 mm are welded directly onto the surface using a laser welding device (Fig. 1b). Two pairs of wires symmetrical to the notch with a distance of 2 y 0 = 4 mm were welded on each specimen, as shown in Fig. 1. The chosen distance was found to be the best compromise between high sensitivity and good signal to noise ratio. The contacts were placed near the edges of the specimen. The DCPD named “ U 1 ” measures the crack propagation of the edge with the secondary notch.