The fatigue and creep interaction

A new approach has been proposed to the problem of interaction of fatigue and creep. That is, a constitutive equation for crack propagation was proposed which includes both the number of cycles $\text{N}$ and time $\text{t}$ based on the stochastic and kinetic theory approaches to time-dependent fracture. Using dhis equation the failure life rule was studied. Both the linear and the nonlinear life roles were shown to be derived by the present approach, the trends of which are in agreement with experimental data.     

Static fatigue of AlN in a glycol--water environment

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Static creep behaviour of Al-Zn-Mg and Al-Zn-Mg-Cu alloys

The creep behaviour of Al-Zn-Mg (7039) and Al-Zn-Mg-Cu (7075) alloys is evaluated at elevated temperatures (443T533 K and 483T563 K) under constant stresses between 49 and 123 MPa, respectively, in a custom-built creep testing facility. The measured activation energies of these alloys are 172–185 kJ mo^–1 and 248–272 kJ mol^–1. As the stress increases, the activation energy in both cases decreases due to the high density of dislocations. The average exponent values of these alloys are 7 and 9. The microstructure observation reveals that the dominant fracture mode of 7039 alloy is intergranular and that of 7075 alloy is transgranular.     

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained 90% -SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10×10^–10 s^–1) at temperatures 1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (0.8 to 3 × 10^–9 s^–1) at stresses below a threshold value of 190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n=17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature 1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.     

Static fatigue in glass

Experimental developments and theoretical treatments of static fatigue in glass are reviewed. Areas requiring further research are highlighted.     

Creep and creep fracture of commercial aluminium alloys

Using standard power law equations, creep rate and creep life measurements at 373–463 K are analysed for a series of aluminium alloys, namely, 2419, 2124, 8090 and 7010. The seemingly complex behaviour patterns are easily rationalized through a modified power law expression, which incorporates the activation energy for lattice diffusion in the alloy matrices (145 kJ mol⁻¹) and the value of the ultimate tensile stress at the creep temperature. By considering the changes in microstructure and creep curve shape as the test duration and temperature increase, all results are then interpreted straightforwardly in terms of the processes shown to govern strain accumulation and damage evolution. Moreover, the data rationalization procedures are also included in new relationships which superimpose the property sets onto sigmoidal ‘master curves’, allowing accurate prediction of the 100,000 h creep-rupture strengths of 2124 by extrapolation of creep lives determined from tests having a maximum duration of only around 1000 h.     

Static fatigue of thermoplastic elastomers

The failure behaviour in static fatigue of thermoplastic elastomers was estimated on the basis of the stochastic theory proposed for the failure of rubber vulcanizates. A statistical analysis of lifetime distributions for a styrene-butadiene-styrene triblock copolymer (SBS) in creep and stress relaxation experiments was made by using a Weibull distribution, and the failure behaviour of SBS was related to morphological changes of structure in stretched states, observed with a transmission electron microscope. Wear-out failure, in which the failure rate increases with time, occurs in the creep and stress-relaxation processes, and is similar to that in a carbon-reinforced rubber vulcanizate. These results suggest that in SBS, polystyrene (PS) domains dispersed in a continuous polybutadiene matrix serve as physical crosslinks and reinforcing fillers. In the stress relaxation process, however, the increased energy dissipation caused by plastic deformation and disruption of the PS domains leads the material to be more stabilized at a constant stretch ratio. This prolongs the lifetime of the material, due to multi-crack initiation at many portions of a specimen. The differences between the failure behaviour of SBS and that of the rubber vulcanizates are mainly caused by morphological changes of the structure in SBS on deformation.     

Some creep and fatigue properties of dispersed-oxide-strengthened lead

Creep and fatigue tests have been made on dispersed-oxide-strengthened lead. The presence of dispersed oxide in the lead resulted in a marked improvement in creep and fatigue resistance. Observations showed that the presence of oxide particles affected the incidence of slip and grain-boundary migration. It is suggested that the role of the dispersed oxide, in improving the creep and fatigue properties of lead, arises from the effect of oxide particles on the amount of slip, the amount of grain-boundary sliding and migration, and, in fatigue failure, the number of vacancies available for cavity nucleation.On the staff of the Broken Hill Associated Smelters Pty Ltd.     

Static and dynamic fatigue of polycrystalline alumina

The fatigue failure of polycrystalline alumina was measured in a moist air environment at 30° C as a function of constant applied tensile stress and stressing rate. The good correlation found between the fatigue test data and fracture mechanics theory indicates that fatigue is controlled by the slow crack growth of pre-existing flaws and that static and dynamic fatigue test techniques adequately define the fatigue parameters needed for failure predictions. Comparisons of proof-test predictions with experiment indicate that the proof test can be effective in eliminating weak samples from the population and in assuring against the delayed failure of polycrystalline alumina in a moist environment.     

Evaluation of hot hardness and creep of a 350 grade commercial maraging steel

The hardening response and the indentation creep of a 350 grade commercial maraging steel were evaluated using a hot hardness tester. The hardness versus temperature plot exhibited three distinct regions. Hardness response was noted between 500–800 K. The unusually high values of activation energy and stress exponent obtained during the creep experiment could be rationalized by a novel concept of introducing a back stress term in the indentation creep relation. The corrected value of the activation energy was found to be reasonably in agreement with the activation energy for diffusion of Ni in iron. Results are supplemented with microstructural observation.     

Interfaces between fatigue,creep, and fracture

A discussion is presented of the fatigue process as one of initiating a crack and propagating it to failure, and a formula is presented for estimating the effective point of crack initiation. This formula is speculatively applied to three interface problems in fatigue: (a) Life of a quasi-brittle material which can sustain only a relatively small crack before failure takes place according to the laws of fracture mechanics. An example is presented to illustrate the procedure and to indicate the probable validity of the approach. (b) Estimation of the fatigue characteristics at high temperatures within the creep range of materials. By assuming that intercrystalline cracking has the effect of by-passing much of the crack initiation process, the number of cycles to failure becomes related more importantly to the crack propagation period. A numerical procedure for estimating life in terms of applied strain range is described, and its validity investigated by application to a number of materials for which data have been presented in the literature. (c) Application of a linear damage rule individually to crack initiation and crack propagation. It is possible to predict the effect of order of application of loads in a two-step cumulative fatigue test. The method is checked by using literature data.     

Static, dynamic and fatigue analysis of a semi-automaticgun locking block

Reduction of the recoil forces on shotgun parts and even effects on the human body are a considerable importance during design of the semi-automatic shotgun parts. These forces are strongly affected by the dynamics of motion of rifle parts upon firing. Therefore, managing of these recoil forces would be crucial issue to produce functional, ergonomic, safe, reliable, and robust designs. In the literature, many researchers have investigated static, dynamic, and fatigue behaviors of most mechanical parts which especially take a role under the dynamic loads. However, shotgun parts have not been investigated formally yet. Therefore, in this study we particularly focused on investigating static, dynamic, and fatigue behaviors of a semi-automatic shotgun’s locking block, which is an integral part of the shotgun mechanism during firing. In this study, techniques such as hardness measurements, analysis of the recoil forces of a semi-automatic shotgun, and finite element analysis were performed. Pro/Engineer Wildfire 3.0 series software was used to model the locking block and the other parts of the gun. Moreover, the finite element code ANSYS/LS-DYNA, and ANSYS Workbench were used to determine the stress distribution, and fatigue behaviors of the locking block, based on the Morrow Theorem.     

Static and fatigue load performance of a gfrp honeycomb bridge deck

This paper investigates the performance of a glass fiber-reinforced polymer (GFRP) bridge deck under static and fatigue load cycles. The bridge deck has a sandwich panel configuration, consisting of two stiff face shells separated by a light-weight honeycomb core. The deck was manufactured using a hand lay-up technique. In this study, a full-size panel that had the same design as an actual bridge deck was tested. The experimental data are analyzed and compared to the results of finite element analysis. The data obtained have indicated that the failure of the system is governed by the delamination of the face shells from the honeycomb core, and the failure behavior is pseudo-ductile even though the material itself is brittle. Hence, the design of such a deck panel should be based on the shear strength of the face–core interface. However, the shear strength can depend significantly on the workmanship in the fabrication process. For design, if the interface shear strength can be reliably identified, the maximum shear stress should be no greater than 15% of the shear strength to avoid fatigue damage under the service load condition.     

Silicon nitride and sialon ceramics

The most intriguing recent development in the field of silicon nitride ceramics has undoubtedly been the discovery of a cubic form of silicon nitride. Major advances were made in α-SiAlON ceramics, including the development of thermally stable, in situ reinforced grades. Significant achievements were reported in tailoring the mechanical properties of silicon nitride ceramics through control of secondary phase chemistry and grain morphology.     

Static and impact fatigue behaviour of borosilicate glass

Failure of a borosilicate glass as a result of repeated impact has been studied. Impact fatigue study was conducted in an improved pendulum type repeated impact apparatus specially designed and fabricated for determining single and repeated impact strength. For elimination of the effect of humidity, repeated impact tests were carried out under liquid nitrogen. Quasi-static measurements were determined under four-point bending. Using a square waveform as applicable to the present impact tests and fracture mechanics interpretation, the number of cycles to failure during impact fatigue tests were predicted from quasi-static fatigue measurements. It has been shown that repeated impact loading has a deleterious effect on the failure cycles compared to slow stressing. The role of an added mechanical effect during repeated impacts has been suggested in controlling the cyclic fatigue behaviour. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995