determined. All investigations were accompanied with local surface strain measurements, which show the development of the fracture process zone during crack initiation and crack propagation. EXPERIMENTAL SETUP Specimen preparation All investigations were carried out on ALPORAS® aluminium foams with two densities, 0.25 g/cm3 and 0.40 g/cm3. Chemical composition, production route and material properties of these foams are described in [8]. Standard compact tension (CT) specimens with a size range of W=50 mm to W=300 mm and a thickness of B=30 mm were used. The specimens were machined with a diamond wire saw to avoid damage of the foam. All specimens had an open surface, whereby the average cell size of the foam was about 3.5 mm. For the pre-crack a diamond wire saw cut with a notch tip radius of about 150 µm was used, which gave undistinguishable experimental results compared to specimens pre-cracked in fatigue (see also [9]). Standard fracture mechanic tests Standard fracture mechanic tests were carried out on a displacement controlled universal mechanic testing machine at room temperature and at a cross-head speed of 0.2 mm/min for specimen sizes W≤100 mm and 0.5 mm/min for W>100 mm. The load and the load line displacement were measured with a standard load cell and a clip gauge, respectively. Additionally, the crack or notch opening displacement (COD) was determined with a videoextensiometer 5 mm behind the initial crack tip. The crack extension was monitored by a potential drop technique and was verified by optical observations. Images from the foam surface were taken at different load line displacements with a CCD-camera at a resolution of 1528x1146 pixels and were used for local surface deformation measurements and for documentation of the crack extension, subsequently. The procedure for local deformation measurements is described in detail in [10]. In-situ experiments In order to investigate crack initiation, crack propagation and local deformations during crack growth, in-situ fracture tests in the scanning electron microscope (SEM) were performed. Due to the restricted space inside the SEM the specimen size is limited to W=50 mm and B=25 mm. A small in-situ loading device, with a displacement rate of 0.15 mm/min, was used to fracture the CT-specimens. In order to assign the different stages of the fracture process to the load versus load line displacement curve, the load and the displacement were measured too. Images from the foam surface, containing 1 to 6 cells, with a resolution up to 4000x3200 pixels were taken at different load line displacements. Subsequently, these images were analysed to measure crack extension, local deformations and to identify the fracture processes. RESULTS AND DISCUSSION Standard fracture mechanic tests Due to the somewhat unusual mechanical behaviour of ductile metallic foams, e.g. a marked stress plateau in compression, a deformation dependence of the Young’s modulus or strain localisation during deformation, also in the fracture mechanics tests some atypical effects can be expected. Fig. 1 shows a typical load F and crack extension ∆a versus load line displacement vLLD curve and a corresponding surface deformation map for a foam with a density of 0.40 g/cm3. It is evident from the load curve that these foams reveal only a very small linear elastic stage, followed by an extended plastic regime. The plastic deformation is strongly localised in a fracture process zone (FPZ), which can bee seen in the surface strain maps and can result in micro cracking of some high strained cell walls. In the region of the peak load a main crack starts from the notch root and propagates in a relatively large FPZ through the specimen, whereby the load decreases. This is usually accompanied with a characteristic kink in the crack extension curve, followed by a regime of larger crack growth rate. Both, the load and the crack extension curve, show in the region of stable crack growth a certain waviness, which is related to the inhomogeneous structure of the foam and results in a variation in the local crack growth resistance.
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