Crack Paths 2012

Overload experiments with SENB3specimens

Standard single-edge-notched three-point bending specimens (SENB3)with dimensions

10x20x100 m m 3 were used for the fatigue tests on a servo-hydraulic M T Stesting

machine. The specimens have a machined starter notch of 3 mm,from which the fatigue

crack was started by fatigue precracking. The experiments were performed under load

controlled conditions and the crack growth was measured by crack length foils on the

back side of the specimen. Furthermore, the P R Mwas used on the front of the specimen

in order to monitor the microstructural effects whenthe overloads were applied.

The overload experiment was performed in the following manner: The fatigue crack

was grown under constant baseline level loading with a stress ratio of

(1)

R = Fmin/ Fmax = 0.5

and a maximumload of Fmax = 3.5 kN. Then single spike tension overloads with

different magnitude were introduced every 2000 cycles. In order to study the influence

of the magnitude of the overload, the overload ratio Rol was increased from 1.1 to 2.5

for the different overloads.

(2)

Rol = Fol / Fmax

For the quantification of the observed overload effects, the stress intensity factor

concept from linear elastic fracture mechanics (LEFM)is employed. Within the L E F M

framework the modeI stress intensity factor KI for standard specimens is calculated by

(3)

KI ) ID : % ¥ :

where F is the applied force, a is the crack length, W is the specimen’s width and B is

the specimen’s thickness. Furthermore, the geometry function f(a/W) for a standard

SENB3specimen is given in fracture mechanics handbooks, see e.g. [9]. Using Eq. (3)

the cyclic stress intensity factor during constant amplitude loading can be calculated as

' K= 7.0 MPa¥P ZKDW LV VOLJKWO\ DERYH WKH WKUHVKROG YDOXH 'Kth of the investigated

material.

Overload experiments under biaxial loading

Overload experiments under biaxial loading were performed with cruciform slotted

specimens, which are similar in shape to those used in previous experiments with

aluminium [10]. The samples have a thinner inner measurement region with 2 m m

thickness resulting in a useable measurement area of approximately 120 x 120 mm2, see

Fig. 3. The samples were machined from a casted block of EN-GJS-400-18LT by C N C

milling. The starting notch (one hole with diameter 10 m m )was inserted by electrical

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