H. Mayer

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.

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.

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.

Near threshold fatigue crack growth in aluminium alloys at low and ultrasonic frequency: Influences of specimen thickness, strain rate, slip behaviour and air humidity

Abstract Near threshold fatigue crack growth in ambient air and in vacuum has been studied with ultrasonic fatigue testing equipment (cycling frequency 20 kHz) and with servo-hydraulic equipment (20 Hz). Aluminium alloy 2024-T3 and 7075, heat treated to promote planar slip (7075-UA) and homogeneous slip (7075-OA), respectively, have been tested at load ratio R=-1. In vacuum, no strain rate influences were found and similar crack growth rates were observed at both frequencies. Specimen thickness does not influence the threshold stress intensity amplitude (Kmax,th) determined at 3.5×10–13 m/cycle in vacuum whereas crack growth resistance increases with decreasing specimen thickness at higher growth rates. Kmax,th determined at 10-10 m/cycle in ambient air are similar at 20 Hz and 20 kHz and are 45–55% of those measured in vacuum. In ambient air and at higher stress intensities, fatigue cracks may propagate by a factor of 5–50 faster at the lower frequency. In vacuum, 7075-UA shows better crack growth properties than 7075-OA, more pronounced at higher growth rates. In ambient air, the planar slip materials show a plateau like regime in the (Δa/ΔN vs Kmax)-curves, where growth rates are hardly affected by the stress intensity.

The influence of air humidity on near-threshold fatigue crack growth of 2024-T3 aluminum alloy

Abstract The environmental influence of humid air, dried air and vacuum on the fatigue crack growth behavior of the aluminum alloy 2024-T3 was measured in the very low threshold regime (down to approximately 10−13 m per cycle), using the high frequency (20 000 Hz) technique of ultrasonic resonance fatigue. The crack growth curves, (Δa/ΔNvs. Kmax relationship) obtained in humid air were characterized by a plateau-like regime (regime with reduced slope of crack growth curve) between 10−9 and 10−10 m per cycle and a threshold value of 2.1 MPa m 1 2 . The threshold in vacuum was 3.3. MPa m 1 2 and no plateau-like regime occurred. In dried air, the curve was very close to that in vacuum, when the Kmax values were high enough; at the very lowest Kmax values, however, the curve approached that for humid air, and an almost identical threshold stress intensity value (2.3 MPa m 1 2 ) was found. The fracture morphology reflected the changing cracking mechanisms: ductile fracture with more plastic deformation and some crystallographic and intercrystalline features were observed for “high” Kmax values, whereas less plastic deformation and no crystallographic or intercrystalline features were characteristic of the threshold regime. Hydrogen embrittlement is assumed to be the main mechanism responsible for the observed corrosion fatigue behavior of alloy 2024-T3 in humid air.

Fatigue properties of aluminium foams at high numbers of cycles

Abstract 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.

Lifetime measurements for random loading in the very high cycle fatigue range

Abstract The ultrasound technique (20 kHz loading freqeuncy) has been investigated as a time- and energy-saving method for measuring lifetimes under service loading conditions. AISI C1020 steel was tested between 3 × 106 and 3 × 109 cycles with two Gaussian-like random loading programs (Gauss distribution generated with a Markov matrix and straight-line distribution). The resulting lifetime curves can be approximated by straight lines in a log-log plot. If the maximum values are plotted, these lines lie above the S/N curves and have approximately the same slope as the S/N curve for finite lifetimes. The experimentally found lifetimes are compared with predictions according to Miner and the Miner-Haibach rule (with half the slope of the S/N curve in the endurance range). Good agreement is found for measured and calculated results according to the Miner-Haibach rule if the measured amplitude distributiom is introduced into the calculations. This agreement is especially good for Markov random loading. Predictions according to the original Miners rule give lifetimes that are too long in the very high cycle range. This result is explained by damaging effects of amplitudes below the endurance limit. For linear distribution random loading this effect has not been observed in the cycle range up to 3 × 109.

Fatigue and fatigue crack propagation in AlSi7Mg cast alloys under in-service loading conditions

Abstract In this study, the influence of microstructure and microporosity on fatigue crack initiation and propagation in AlSi7Mg cast alloy has been investigated at very high numbers of cycles (10 8 –10 9 ). Two alloys with similar chemical composition were used; one had finer microstructure, somewhat high microporosity and better static strength properties. The experiments were performed both at constant cyclic loads and with load sequences that simulated random in-service loading conditions of automobiles. The results show that casting pores predominantly influence the number of cycles to crack initiation. For in-service loading conditions, the detrimental effect of microporosity on crack formation cannot be adequately predicted on the basis of linear damage calculation, because of premature crack initiation during high loads of the random sequence. Comparison of the number of cycles to fracture as determined with smooth cylindrical specimens and with pre-cracked flat specimens shows the relatively high portion of the crack propagation time. The good fatigue crack growth properties of both AlSi7Mg alloys in the threshold regime (which are even better for the coarser alloy) are explained by roughness-induced crack closure.

High frequency method for torsion fatigue testing

Abstract A new experimental method to determine the fatigue properties of materials by torsional loading is presented in this paper. In principle, resonance vibrations are generated in fatigue specimens at a frequency of 21 kHz. A piezoelectric transformer produces mechanical torsional vibrations and their torsional amplitude is magnified mechanically. The mechanical and the electrical set-ups are described and first results, which have been obtained with this new testing system, are presented.