Stefanie E. Stanzl-Tschegg

Influence of porosity on the fatigue limit of die cast magnesium and aluminium alloys

Abstract High cycle fatigue properties of high-pressure die-cast magnesium alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and of similarly produced cast aluminium alloy AlSi9Cu3 have been investigated. Ultrasonic fatigue tests up to 10 9 cycles show mean fatigue limits of approx. 38–50 MPa (magnesium alloys) and 75 MPa (AlSi9Cu3) in the tested casting condition. Fatigue cracks initiated at porosity in 98.5% of the samples. Considering porosity as initial cracks, specimens fail, if critical stress intensity amplitude, K cr is exceeded. K cr of the magnesium alloys range from 0.85±0.05 to 1.05±0.05 MPa√m, and 1.85±0.10 MPa√m was found for AlSi9Cu3. Below K cr , fatigue cracks may initiate at porosity, however, do not propagate until failure. Using K cr , the statistical distribution of defects is linked to the fracture probability at different stress amplitudes.

Bone-implant interface strength and osseointegration: Biodegradable magnesium alloy versus standard titanium control.

Previous research on the feasibility of using biodegradable magnesium alloys for bone implant applications mainly focused on biocompatibility and corrosion resistance. However, successful clinical employment of endosseous implants is largely dependent on biological fixation and anchorage in host bone to withstand functional loading. In the present study, we therefore aimed to investigate whether bone-implant interface strength and osseointegration of a novel biodegradable magnesium alloy (Mg-Y-Nd-HRE, based on WE43) is comparable to that of a titanium control (Ti-6Al-7Nb) currently in clinical use. Biomechanical push-out testing, microfocus computed tomography and scanning electron microscopy were performed in 72 Sprague-Dawley rats 4, 12 and 24 weeks after implantation to address this question. Additionally, blood smears were obtained from each rat at sacrifice to detect potential systemic inflammatory reactions. Push-out testing revealed highly significantly greater maximum push-out force, ultimate shear strength and energy absorption to failure in magnesium alloy rods than in titanium controls after each implantation period. Microfocus computed tomography showed significantly higher bone-implant contact and bone volume per tissue volume in magnesium alloy implants as well. Direct bone-implant contact was verified by histological examination. In addition, no systemic inflammatory reactions were observed in any of the animals. We conclude that the tested biodegradable implant is superior to the titanium control with respect to both bone-implant interface strength and osseointegration. These results suggest that the investigated biodegradable magnesium alloy not only achieves enhanced bone response but also excellent interfacial strength and thus fulfils two critical requirements for bone implant applications.

New splitting method for wood fracture characterization

SummaryA testing procedure with a new and simple specimen shape is presented which is appropriate to characterize fracturing of inhomogeneous and complex materials like wood. With this, the fracture energy of spruce wood is determined in the TL and RL direction. The “size effect”, i.e. influences of specimen dimensions on KIC and Gf (specific fracture energy) are investigated. Stress and deformation distribution in the newly developed specimens are analysed with FE methods. The measured load-displacement curves are approximated by bilinear softening diagrams and FE analysis. Based on these results, it is tried to interpret typical deviations from LEFMs behaviour by mechanisms like microcracking, crack branching or crack tip bridging.

Effects of surface ageing on wettability, surface chemistry, and adhesion of wood

The effects of ageing on the surface chemistry, wettability and the surface free energy were determined according to the acid base theory using contact angle measurements and x-ray photoelectron spectroscopy. Time is a significant variable affecting wettability and adhesion. All measured surface parameters of the wood surface were storage time dependent. It could be demonstrated that the effect of ageing on the adhesion of a coating on the wood surface depends not only on the properties of the wood surface, but also on the chemical nature of the coating. Therefore, measurements on wettability without considering the time dependence of surface energy and, above all, its components, are not sufficient to assess effects of ageing on adhesion.ZusammenfassungDie Wirkung der Alterung auf die Oberflächenchemie, die Benetzbarkeit und die freie Oberflächenenergie nach der Säure/Base Theorie wurden mittels Kontaktwinkelmessungen und Photoelektronenspektroskopie untersucht. Die Zeit beeinflusste die Benetzbarkeit und Adhäsion des Holzes deutlich. Mit der Lagerungszeit veränderten sich alle gemessenen Parameter der Holzoberfläche. Es konnte gezeigt werden, daß der Alterungseffekt auf die Adhäsion einer Beschichtung nicht nur von den Eigenschaften der Holzoberfläche, sondern auch von der Chemie der Beschichtung abhängt. Benetzungsmessungen ohne Berücksichtigung der freien Oberflächenenergie und ihrer Komponenten reichen daher nicht aus, um Alterungseffekte auf die Adhäsion abzuschätzen.

Effects of surface treatments on high cycle corrosion fatigue of metallic implant materials

Long term corrosion fatigue properties of two materials which are candidates for skeletal implants — cold worked c.p. Niobium and c.p. Tantalum — have been investigated. Fatigue properties have been compared to two implant materials in clinical use — c.p. Titanium and Ti‐6Al‐7Nb alloy. Constant amplitude fatigue experiments (S‐N curves) were performed at ultrasonic frequency (20 kHz) with two different surface structures (ground surface and blasted and shot peened surface) in ambient air and in a corrosive fluid similar to the body fluid in the oral cavity. The endurance limit at 2 〈10 8 cycles of all materials decreased by 5‐20% if they were cycled in corrosive fluid instead of ambient air. The loss of fatigue strength is more pronounced for ground Ti‐6Al‐7Nb alloy and c.p. Ti than for ground c.p. Nb cw and c.p. Ta cw. Fracture surfaces show a more pronounced embrittlement of ground Ti‐ 6Al‐7Nb alloy and c.p. Titanium after cycling in corrosive fluid than ground c.p. Tantalum and c.p. Niobium. A beneficial influence of surface structuring by blasting and shot peening on the fatigue properties was found for all materials in both environments. Fatigue loading using ultrasonic frequency allows one to select appropriate implant materials and to determine their very-high cycle corrosion-fatigue behaviour within reasonable testing times. Though the obtained high-frequency values may not be fully representative of actual in vivo behaviour, they are regarded as useful material characterizing values.

Fracture characteristics of different wood species under mode I loading perpendicular to the grain

Abstract Mode I fracture characteristics of different wood species (one softwood and three hardwoods) in two crack propagation systems were investigated using the wedge splitting test under loading perpendicular to the grain. From the obtained load–displacement curves the initial slope, the critical stress intensity factor and the specific fracture energy were determined. The initial slope and the critical stress intensity factor were shown to depend strongly on density within all species whereas for the specific fracture energy differences between the softwood and the hardwoods were found. Differences between the crack propagation systems could be explained by the higher volume fraction of radial oriented tissue (rays) of the hardwoods.

Compressive behaviour of softwood under uniaxial loading at different orientations to the grain

Abstract The compressive behaviour of spruce wood under uniaxial loading is studied at different orientations with regard to the longitudinal and radial direction. The dependence of the Young modulus, Poisson ratio and crushing strength on the loading angle with respect to the longitudinal direction is shown and described by a simple theory of orthotropic elasticity and a Tsai–Hill like strength criterion. The deformation and failure behaviour was also strongly influenced by the loading orientation. The different deformation and failure mechanisms found ranged from buckling of the elongated cells at loading in the longitudinal direction followed by final failure due to longitudinal cracks to shear deformation and failure at annual ring borders at loading angles of 20° and 45° due to the abrupt density change at the ring border and to the plastic yielding and collapsing followed by densification for loading in the radial direction.

Influence of loading frequency on high cycle fatigue properties of b.c.c. and h.c.p. metals

Abstract The influence of the loading frequency on the high cycle fatigue properties of two b.c.c. metals, commercially pure (c.p.) niobium and c.p. tantalum in annealed and cold worked condition, and of two annealed h.c.p. titanium and of Ti–6Al–7Nb alloy were investigated. Endurance data in the regime of 105 to 2×108 cycles to failure obtained with rotating bending and ultrasonic fatigue testing equipment (loading frequencies 100 Hz and 20 kHz, respectively) coincide within the ranges of scatter for niobium and Ti–6Al–7Nb alloy. The mean endurance limits at 2×108 cycles of these metals are ≈60% of the respective yield stress of the as produced material. The high loading frequency leads to prolonged lifetimes and increased mean endurance limits for tantalum and (less pronounced) for titanium. Fatigue crack initiation in tantalum changes from a preferentially ductile and transgranular mode at 100 Hz to a more brittle, crystallographic and intergranular mode at 20 kHz. The mean endurance limit of tantalum is above the yield stress of the as produced material, and high initial rates of plastic deformation therefore result. Cold working of tantalum and niobium improves their static strength properties, but is of only minor importance for the high cycle fatigue behaviour.

Influence of loading frequency on the high cycle fatigue properties of AlZnMgCu1.5 aluminium alloy

Abstract Fatigue investigations of AlZnMgCu1.5 aluminium alloy have been performed with conventional testing equipment (cyclic frequency 100 Hz) and with ultrasonic equipment (20 kHz). No statistically significant influence of cyclic frequency on lifetimes was found in the investigated regime, i.e. cycles to failure above 10 5 . Different heat treating influenced lifetimes and near threshold crack growth properties, and fatigue properties of AlZnMgCu1.5-T6 were superior to AlZnMgCu1.5-T66 and AlZnMgCu1.5-T64. Fatigue crack propagation in the range of approximately 10 −9 m/cycle and below measured at ultrasonic frequency is affected by air humidity, since growth rates are below the mean diffusion distance of hydrogen during one cycle. In ambient air, a minimum growth rate of a propagating crack of approximately 10 −10 m/cycle was found, whereas crack propagation rates may be as low as 10 −12 m/cycle in a vacuum. Threshold values of AlZnMgCu1.5-T6 in ambient air and in a vacuum are 1.5–1.55 MPa√m and 2.7–2.95 MPa√m, respectively.

Spiral angle of elementary cellulose fibrils in cell walls of Picea abies determined by small-angle X-ray scattering

SummaryThe spiral angle of the elementary cellulose fibril in the wood cell wall, often called microfibril angle, is of primary importance for the mechanical properties of wood. While there are a number of methods to estimate this angle, x-ray diffraction (XRD) techniques have recently obtained a lot of attention because of their ability to provide information averaged over a significantly large specimen volume. Here, we present results from a related method, small-angle x-ray scattering (SAXS). The advantage of SAXS is that, unlike XRD, it does not require any assumption on the orientation of the cellulose crystal axis with respect to the fibril axis. Full three-dimensional scattering patterns were collected using an area detector by rotating the sample around one axis. The distribution of fibrillar orientations was seen to reflect the typical cross-sectional shape of the tracheids (square or circular). In the stem, the spiral angle was found <5° in earlywood and ≈20° in latewood. In branches the angle was ≈30° in the upper part and ≈40° in the lower part, which strongly supports the idea that the spiral angle has primarily mechanical function.