E. K. Tschegg

Mode III and Mode I fatigue crack propagation behaviour under torsional loading

Fatigue crack growth rates have been measured under cyclic torsional loading (R = −1, 1 Hz loading frequency) in AISI 4340 steel tempered at 650° C, with circumferentiallynotched specimens (12.7 mm specimen diameter). The Mode III fatigue crack growth curve for macroscopically flat fracture surface — being obtained by an extrapolation procedure which eliminates the “Mode III crack closure” influence — has higher crack growth rates and shows a greater slope than Mode I results in the stress intensity range of ΔKIIIeff = 18 to 50 MPa m1/2. In the range of ΔKIIIeff < 18 MPa m1/2, the fracture surface has “factory roof” morphology (Mode I). The difference between fatigue crack growth behaviour in Mode III and Mode I as well as mechanisms that can lead to a fracture mode change are discussed. A comparison of the Mode III crack closure influence for specimen diameters of 12.7 mm (this study) and of 24.5 mm (an earlier study) shows that this influence is not only dependent on the depth of the crack and applied torque but also on the specimen diameter. The extrapolated crack growth rates show good agreement with measurements for various specimen diameters.

Implant failure of the gamma nail

The gamma nail is a temporary implant characterised by a limited life expectancy under continuous dynamic stress. We reviewed a series of 839 patients with gamma nail stabilisation and found two fatigue fractures (0.2%) at the aperture of the distal locking holes. This complication has not been described in the literature. Metallurgic and scanning electron microscopic examinations proved that the fatigue zones occurred at the clover-leaf grooves, which is where the diameter of the gamma nail is reduced. The clover-leaf diameter is of no biomechanical use in gamma nail stabilisation. We suggested product modification of the gamma nail to produce implants with a round diameter instead of a clover-leaf shape. A modified implant is already in use at our institution.

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.

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.

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.

Mode I fracture and acoustic emission of softwood and hardwood

Abstractu2002The Mode I fracture behaviour of two softwoods (spruce and pine) and three hardwoods (alder, oak and ash) was studied in the RL crack propagation system using the splitting test in combination with monitoring acoustic emission (AE) activity. Test parameters measured included notch tensile strength, specific fracture energy, characteristic length and AE cumulative counts, AE amplitudes as well as parameters characterizing the frequency spectra of the emitted acoustic emission events. The notch tensile strength was found to correlate with density. The specific fracture energy and characteristic length showed the different crack propagation process between the softwoods and hardwoods. The softwoods fractured in a more ductile way and the hardwoods showed a more linear elastic behaviour. This finding was supported by the AE measurements showing much less cumulative counts for the hardwoods indicating that less microcracks were formed and that processes like fiber bridging were not so effective. Differences in the frequency domain of the AE signals between softwoods and hardwoods could not be detected.

In-service loading of AlSill aluminium cast alloy in the very high cycle regime

Abstract The fatigue properties of two AlSi11 cast alloysas used for car wheels have been tested in the regime of 10 8 −10 9 cycles. An AlSi11 aluminium alloy with 0.12–0.20% Mn and one with 0.05% Mn and increased H 2 content during the pouring procedure are compared. It is shown that mainly cast voids determine the fatigue properties. These material inhomogeneities reduce the fatigue strength and lead to early fracturing. The number of cycles to failure decreases with increasing void cross-section. With a statistical analysis of the fatigue data, it is shown that the mean number of cycles to failure is higher for the alloy with fewer large cast voids and a higher Mn content. This alloy is less susceptible to cast voids, and the threshold stress intensity value is approximately 10% higher.

The effect of microstructure and environment on fatigue crack growth in 7049 aluminium alloy at negative stress ratios

Abstract The influence of environment and microstructure on fatigue crack growth has been investigated on a high strength 7049 aluminium alloy. This aluminium alloy was artificially aged to underaged (UA) and overaged (OA) microstructures. The heat treatment procedure was performed in order to obtain an UA and OA microstructure having the same yield strength properties, but differing in the mode of slip deformation: the UA alloy deforms by planar slip and that of the OA alloy by wavy slip. The crack growth measurements were performed in MT specimens at constant load ratios for R =0, −1, −2, −3 near-threshold and Paris regime in ambient air and vacuum conditions. Crack closure loads were measured in order to determine the P open for each R ratio. Micromechanisms of near-threshold crack growth are briefly discussed for several concurrent processes involving environmentally assisted cracking with intrinsic microstructural effects. The results showed that the presence of humid air leads to a larger reduction in Δ K th for both the ageing conditions, but the UA specimens were superior probably because of crack branching. The role of environmental effect and microstructures near-threshold regime seems to be more significant than any mechanical contributions to the crack closure, such as plasticity, roughness, oxide, etc.

Influence of microstructure and load ratio on fatigue threshold behavior in 7075 aluminum alloy

Abstract The influence of microstructure and load ratio on fatigue crack propagation in the near-threshold regime has been investigated on the 7075-aluminum alloy. The alloy heat treatment was varied to give underaged and overaged microstructures with approximately the same yield strength properties. Crack growth measurements were performed at constant load ratios between R =−1 and +0.8 in ambient air. The influence of load ratio is discussed in terms of deformation slip, crack branching and fracture mode. Vacuum data from the literature are compared with the current air results. For both environmental conditions, it was observed that the crack growth and threshold properties of the underaged alloy were better than the overaged alloy. The overall fatigue behavior is discussed in terms of the two parameter concept namely Δ K and K max .

Models of wood fracture in Mode I and Mode II

The fracture mechanisms of wood depend on the mode of loading (Mode I and Mode II) and on the orientation of load application (e.g. TL and RL) within a specimen. Experiments were performed in TL and RL orientation and the critical stress intensity factorsKIC andKIIC were determined for different orientations. The micromechanisms of wood fracture were investigated by SEM observations. The reasons why (1) theKIIC are higher than theKIC values, (2) the fracture process of mode II is unstable, (3) the fracture surface is rough and full of warped wood fibers are attempted to answer by considering the micromechanical fracture features of wood.ZusammenfassungDie Bruchmechanismen von Holz hängen von der Beanspruchungsart (Modus I oder Modus II Beanspruchung) und von deren Orientierung im Holz ab (z.B. TL und RL). In dieser Arbeit wurden Experimente in TL and RL Orientierung durchgeführt und dabei die Bruchzähigkeitswerte,KIC andKIIC für diese Orientierungen bestimmt. Für die Untersuchung der Bruchmechanismen stand ein Rasterelektronenmikroskop zur Verfügung. Unter Einbeziehung der beobachteten verschiedenen mikromechanischen Brucherscheinungen wird versucht, folgende experimentellen Ergebnisse zu erklären: 1) DieKIIC Werte sind wesentlich höher als dieKIC Werte, 2) Der Modus II Bruch erfolgt instabil, 3) Die Modus II Bruchflächen sind rauh und bedeckt von gebrochenen und gekrümmten Fasern.