The method described here provides an approach for carrying out micro-tensile tests on single wood fibers. Recording of load-strain curves, with a high resolution of fiber elongation, is achieved by ...combining a microtensile apparatus, a microscope and video extensometry unit. For mounting fibers easily onto the tensile device a foliar frame was developed, which allows measurements of short fibers like compression wood tracheids or libriform fibers. Measurements on other plant fibers are planned, in order to characterize cell mechanics for a wide range of structural and chemical features of cell walls.
Fatigue properties of aluminium foams at high numbers of cycles Zettl, B.; Mayer, H.; Stanzl-Tschegg, S.E. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
11/2000, Letnik:
292, Številka:
1
Journal Article
Recenzirano
Endurance fatigue experiments have been performed with two kinds of Al–Mg–Si foams and one Al–Si foam under fully reversed loading conditions using the ultrasound fatigue testing method. Young’s ...modulus of the foams is 3.9 GPa. Constant amplitude fatigue data show endurance limits on the basis of 10
9 cycles between 1.1 and 1.4 MPa which is 16–23% of the plateau stress. Lifetimes exhibit a pronounced scatter, which is caused by the inhomogeneous structure of the foams. Fatigue damage is governed by the formation of cracks, which preferentially initiate in the interior sections of cell walls at initial defects, like precracks or holes. No strain localization and formation of deformation bands was found. Fatigue crack growth preferentially follows areas of cell walls with a minimum wall thickness, and eventually may stop near cell-nodes. The cyclic properties of foams can be improved, if initial defects are small, if the mean cell sizes are reduced, and if a more homogeneous foam is obtained.
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.
The nanostructure of the S2 cell wall layer in tracheids of Picea abies (Norwegian spruce), in particular the cellulose microfibril angle, has been shown to control not only the stiffness but also ...the extensibility of wood within a wide range. In order to further elucidate this effect, the deformation of wood under tensile load parallel to the longitudinal cell axis was studied in a contact-free way using a video extensometer. The combination of these measurements with small-angle X-ray scattering on the same microtome sections allowed us to establish a direct relationship between the microfibril angle and deformation behaviour. The microfibril angle was shown to influence not only the extensibility in longitudinal direction but also the deformation perpendicular to the applied load. Moreover, the results showed that the energy absorption capacity is higher for specimens with larger microfibril angle. SEM pictures of the fractured samples indicated clearly the differences in the fracture process as the fracture zones of samples with low microfibril angle were smooth and the fracture zones of samples with high microfibril angle were heavily torn and deformed indicating a more ductile behaviour.
•High-frequent ultrasonic VA-blocks are superimposed to low-frequent square loads.•Life-times under specified combined loading conditions are shorter than predicted.•Predictions used completely ...reversed S–N data, FCG-rates and a modified Miner rule.•Correlation of life-times with fractographic results allows interpretation of results.•Small crack arrest and micro-plasticity seem to be main life-limiting mechanisms.
Aim of this study is an interpretation of the influence of variable-amplitude (VA) cycles superimposed to low-frequency loads on fatigue life of 7075-T651 Al-alloys. Constant-amplitude (CA) 20kHz stress/strain-life (S–N) and (ε–N)-curves with and without superimposed mean loads serve as basis. For combined fatigue loading, life-time measurements were performed. Life-time estimations based on the S–N results reveal a damaging effect of the superimposed ultrasonic vibrations in the high cycle fatigue (HCF) and the very high cycle fatigue (VHCF) regimes. The CA and VA-life time results are correlated with fractographic observations. An interpretation of fatigue lives under combined low and high-frequency VA-loading is proposed considering small/short-crack propagation and arrest mechanisms.
Fatigue crack growth near threshold has been studied in 2024-T3 and in over-aged 7075 (7075-OA) using servo-hydraulic equipment (cycling frequency 20 Hz) and ultrasonic equipment (20 kHz). ...Experiments have been performed at load ratio
R=0.05 and
R=0.5 in ambient air and in vacuum. Below 10
−9 m/cycle, fatigue crack growth in vacuum is not influenced by the cycling frequency and no strain rate effects are found. Air humidity accelerates near threshold fatigue crack growth, and threshold stress intensities at 10
−10 m/cycle in ambient air are approximately 50–60% of those measured in vacuum. Similar threshold stress intensities at 20 Hz and 20 kHz, however, higher crack growth rates at the lower frequency at stress intensities above threshold were found in ambient air, where the frequency influence was most pronounced for fully reversed loading. Near threshold 2024-T3 shows planar slip behaviour and crystallographic (stage I-like) fatigue crack growth in vacuum, whereas stage II fatigue crack growth is found in ambient air. 7075-OA shows homogeneous slip properties and stage II fatigue crack growth in both environments. No apparent influences of load ratio and cycling frequency on fracture surfaces produced at very low growth rates are visible.
ABSTRACT The deleterious influence of atmospheric moisture on the fatigue properties of an aluminium wrought alloy AlZnMgCu1.5‐T6, an aluminium cast alloy AlSi9Cu3 and magnesium cast alloys AM60 hp, ...AZ91 hp and AS21 hp has been studied at a cycling frequency of 20 kHz. Atmospheric moisture accelerates fatigue crack growth and decreases the threshold stress intensities to 55–75% of the respective values in vacuum. In ambient air, fatigue crack growth rates were up to two decades higher than those in vacuum. Accelerated crack growth was found at propagation rates below about 2 × 10−9 m cycle−1 in aluminium alloys and below about 3 × 10−8 m cycle−1 in magnesium alloys. As the threshold regime is approached, fatigue cracks in ambient air either propagate at a minimum mean growth rate on average of approximately one lattice spacing per cycle or they stop propagating, whereas mean growth rates of 10−12 m cycle−1 were found in vacuum. Crack initiation and slow fatigue crack growth mainly determine lifetimes in the high cycle regime, and endurance data obtained at ultrasonic frequency in ambient air of 40–60% relative humidity are similar to lifetimes measured at conventional frequencies.
Fatigue properties in the very high cycle regime of normalized carbon steel with carbon content 0.61% C (Ck60) and 0.15% C (Ck15) have been investigated with the ultrasonic fatigue testing technique ...at load ratio
R
=
−1 and cycling frequency 20
kHz. Both steels show a distinct change of slope in the
S–
N curves at approximately 10
7
cycles. No specimen failed above 2.2
×
10
8
cycles although 25 specimens were stressed to over 10
9
cycles. Calorimetric measurements were used to evaluate the cyclic plastic strain amplitude at constant stress amplitude. Ck60 shows initial cyclic softening followed by cyclic hardening at stresses above and below the mean fatigue limit at 10
9
cycles. The lower the stress amplitude, the lower was the maximum plastic strain amplitude and the higher was the number of cycles when cyclic softening started. Cyclic hardening in Ck60 is seen to continue until 10
9
cycles. Ck15 undergoes cyclic softening mainly at stresses above the mean fatigue limit and specimens can fail at very low values (10
−6) of the cyclic plastic strain. No sub-surface crack initiation was detected.
Spruce and beech wood specimens were sanded with four different grain sizes. Surface morphological and chemical changes were determined by roughness measurement, surface free energy determination ...according to acid-base theory and X-ray photoelectron spectroscopy. An analysis model was proposed to describe chemical changes due to sanding and the data were analysed according to it. It could be shown that spruce and beech behave similarly over a wide range and that chemical changes are non-linear functions of grain size and thus surface roughness. A maximum of surface free energy and a maximum of carbon C1s sub-peak ratio (ratio of carbon bonded to a single non-carbonyl oxygen to carbon bonded only to hydrogen or other carbon atoms) was found for middle grain size.
Fatigue crack growth in 2024-T3 has been studied in ambient air and in vacuum at load ratios
= –1,
= 0.05 and
= 0.5 using ultrasonic equipment (cycling frequency 20 kHz) and servo-hydraulic equipment ...(20 Hz). In vacuum, no strain rate influences were found and similar growth rates and threshold stress intensities were measured at both frequencies. In ambient air, threshold stress intensities were similar at 20 Hz and 20 kHz and were 53 –62% of the respective values measured in vacuum. Above threshold, fatigue crack growth rates at ultrasonic frequency are slower (at
= –1) or similar (at
= 0.5) to growth rates at 20 Hz. Ultrasonic fracture mechanics tests in Ti-6Al-4V at load ratios
= 0.1,
= 0.5 and
= 0.8 in ambient air delivered threshold values similar to cycling frequency 50 Hz, whereas growth rates above threshold are approximately a factor of 3 higher at 20 kHz. The compressive part of a load cycle under fully reversed loading condition causes additional fatigue damage, and the maximum stress intensity factor at threshold is lower at
= –1 than at
= 0.05 or
= 0.1.
