Fracture mechanics of stainless steel foams
Introduction
Porous stainless steel foams are interesting for functional applications such as catalyst carrier, evaporator, acoustic and thermal insulation. The porous structure of the material makes it possible to achieve extreme low densities, high specific surfaces and in the case of open cell foams, the permeability towards fluids and gases. Porous materials have many voids and flaws because of their porous structure and it is essential to study the fracture mechanics of these materials. Up to this date, studies about fracture toughness and fatigue crack growth rate of these materials and the amount of stress needed to propagate a pre-existing flaw in such materials is limited. In contrast to limited mechanical studies particularly on fracture mechanics of titanium [1], [2] and stainless steel foams, there are various studies on titanium foam for biomedical applications [3], [4], [5] and also on aluminum foams [6], [7], [8], [9], [10], [11]. One example is the study carried out by Combaz et al. [6]. Their study shows that relative density is an important factor in the toughness testing. In porous materials, cracks grow by breaking discrete elements of solid materials [6], which in open cells are usually in the form of struts. The crack propagation in metal foams often occurs by the formation of a plastic yielding around the crack tip. Combaz et al. have carried out fracture toughness of open cell aluminum foams with uniformed cell. They have found that even though their foam fractures with similar physical characteristics to other metal foams, they have higher scaling relation exponent [6]. Motz et al. [7] have studied the fatigue crack propagation in closed cell aluminum and hollow sphere 316L structures. Besides finding high Paris exponent in the closed cell aluminum, they found a continuous fatigue crack growth in this material. However, in the hollow sphere structure, the fatigue crack growth shown to be concentrated in the vicinities of the sintering necks [7].
In general, the fracture toughness of metal foams is dominated by plastic deformation. Therefore, elastic plastic fracture toughness testing is usually conducted using a compact tension specimen [12]. When investigating the fracture mechanics of metal foams, it is important to note that the fracture response of brittle foams is different from ductile foams. In general, brittle foam materials shatter in response to an exterior force, while ductile foam materials only deform. In ductile foams, the coalescence of cavities causes crack nucleation. When ductile foam is loaded, the ductile matrix deforms and the cavities grow larger. Then, the cavities interact with each other, merge and form a crack. Ductile crack growth is much more stable than brittle fracture [13] due to the increasing resistance curve. In elastic plastic fracture mechanics, this is known as R-curve behavior showing that resistance to fracture increases as the crack size grows.
In the present work, our aim is to investigate the fracture toughness, fatigue crack growth and material properties of 45 ppi stainless steel foam and analyze the fractured compact tension samples by scanning electron microscopy (SEM) and 3D micro CT scanning.
Section snippets
Specimen preparation
Open cell steel foams have been manufactured by using a powder metallurgical replication technique [14]. The method essentially involves three steps: First, a reticulated polyurethane sponge is coated by a metal powder suspension (Atmix 316L, mean powder size 6 µm). Water based slurries with PVA-binder or carbon acid binder and solids content between 87 and 90% were used. In the next step the substrate and the binder are removed by heat treatment (maximum temperature 650 °C), and finally the
Inverted spherical model
In the study of open or closed cell foams, the explicit representation and modelling of the actual foam structure is quite complex and impractically expensive and time consuming. There have been many attempts to create simplified pore structure models that achieve reasonable correlation with experimental [16], [17]. Assuming that plastic zone size is small compared to the geometric dimensions of the specimen, small scale yielding (SSY) boundary layer models have been used to investigate the
Plane strain and initiation fracture toughness
The method is based on the principle of J-integral and characterises the material's toughness close to the commencement of slow-stable crack extension from a pre-existing fatigue crack. The J-integral characterizes an approach to estimate the strain energy release rate per fracture surface area [22]. It is developed to help the complexity involved in calculating the stress near a crack in an elastic–plastic material [22]. In this work, the load is measured during fracture toughness tests as
Crack growth rate
The da/dN–ΔK generally has three regions called region I, II and III. Regions I and III are the near-threshold and the rapid-crack propagation regions, respectively. The rates of fatigue crack growth for near threshold (ΔKth) are extremely slow and it takes a while to grow a small crack. In region III, the crack growth rate is extremely high and therefore is called unstable region and obtaining data is quite difficult. Therefore, in this work, region II or Paris region, the stable crack region
Conclusions
The mode I fatigue crack growth of stainless steel foam has been measured and explained in terms of microstructure. The compact tension specimens showed full plastic collapse along the ligament. It was concluded that the microstructure of the foam has a significant influence on the fatigue crack growth of stainless steel foam, and this was in agreement with previous studies on fracture behavior of porous aluminum and titanium foams. Stainless steel foam has a higher Paris exponent than solid
Acknowledgement
This work is financially supported by the Australian Research Council (Project No: DP0770021) and ARC grant from ARNAM. The authors also thank David Dick for his support in the E-CORE laboratory at the University of Toledo.
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