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This paper aims to statistically estimate the dynamic fatigue strength in brittle materials under a wide range of stress rates. First, two probabilistic models were derived on the basis of the slow crack growth (SCG) concept in conjunction with two-parameter Weibull distribution. The first model, Model I, is a conventional probabilistic delayed-fracture model based on a concept wherein the length of the critical crack growth due to SCG is enough larger than the initial crack length. For the second model, Model II, a new probabilistic model is derived on the basis of a concept wherein the critical cracks have widely ranging lengths. Next, a four-point bending test using a wide range of stress rates was performed for soda glass and alumina ceramics. We constructed fracture probability-strength-time diagrams (F-S-T diagrams) with the experimental results of both materials using both models. The F-S-T diagrams described using Model II were in good agreement with plots of the fracture strength and the fracture time of both materials more so than Model I. [ABSTRACT FROM AUTHOR]

In this paper, yield and fatigue behavior of a polypropylene nanocomposite and a polyamide-6 nanocomposite has been studied. The Eyring equation was used to model the temperature and strain rate sensitivity of the yield strengths of these two nanocomposites. Both activation volume and activation energy of the polypropylene nanocomposite were higher than those of the polyamide-6 nanocomposite. The fatigue strength of the polyamide-6 nanocomposite was higher than that of the polypropylene nanocomposite. However, the ratio of maximum fatigue stress to yield strength of the polypropylene nanocomposite was higher than that of the polyamide nanocomposite. The fatigue failure in both composites was initiated at agglomerated nanoparticles. [ABSTRACT FROM AUTHOR]

This paper presents an overview of fatigue fracture modes in selected structural alloys employed in gas turbine engines. These include the mechanisms of fatigue crack growth in the near-threshold, Paris and high-δK regimes obtained from Ti-6A1-4V, Inconel 718 and PWA 1472 (a single crystal nickel-based superalloy of similar chemical composition to Inconel 718). Fatigue fracture modes in these materials are shown to be strong functions of the stress intensity factor range, AK, and the maximum stress intensity factor, Kmax. Fatigue mechanism maps are also presented to show the parametric ranges of ΔK and Kmax corresponding to the different fatigue fracture modes. [ABSTRACT FROM AUTHOR]

An investigation has been carried out on the adiabatic shear band (ASB) in a Ti-3Al-5Mo-4.5V (TC16) alloy deformed at high strain rate by a split Hopkinson pressure bar (SHPB). ASB in TC16 alloy is a “white” band with a width of about 13 μm. Microhardness of the ASB is larger than that of the matrix. The elongated cell structures of width about 0.2–0.5 μm with thick dislocation exist in the boundary of the shear band. Results suggest that the fine equiaxed grains with α-phase and α″-phase coexist in the shear band. The “white” band is a transformation band. Calculation of the adiabatic temperature rise indicates that the maximum temperature within ASB is about 1,069 K that is above the phase transformation temperature. Finally, formation of an ASB in the TC16 alloy and its microstructure evolution are described. [ABSTRACT FROM AUTHOR]

Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the specimens from the load train. Positrons were generated by a 72Se/72 As source with a maximum activity of 0.9 MBq (25 μCi). The average positron lifetime has been determined by a β+ − γ-coincidence applying a simplified data evaluation procedure. Under constant stress or plastic strain amplitudes early stages of fatigue damage could be detected. The strong increase of the average positron lifetime already during the first 10% of fatigue life could be related to the fatigue life of the specimens. Issues of lifetime prediction by positron annihilation measurements are discussed. [ABSTRACT FROM AUTHOR]

Low cycle fatigue tests for a hot extruded Nickel base alloy tube material have been performed at room temperature and at 204°C. The alloy shows a normal hardening and softening cyclic stress-strain response at room temperature. At 204°C, however, the cyclic stress-strain response shows a strain hardening first followed by a relatively stable stress and finally a secondary cyclic strain hardening. This stable stress disappears with increasing strain amplitude. The mechanisms of the secondary cyclic strain hardening have also been investigated by transmission electron microscopy (TEM). Besides dislocation multiplication, interactions between stacking faults and moving dislocations and between interstitial atoms and moving dislocations could also contributed to this secondary cyclic strain hardening. The formation of micro-twins during cyclic loading at 204°C and its influence on the cyclic stress-strain response were also discussed. [ABSTRACT FROM AUTHOR]

The bipolar electric fatigue behaviour of antiferroelectric ceramics with the composition of Pb0.88Ba0.10La0.02 (Zr0.55Sn0.35Ti0.10)O3 was investigated under various cycling fields. The material exhibits a degradation in the maximum field induced strain, a diffuse AFE-FE phase transition and an enhancement in the diffusion character of the FE-AFE phase transition due to electric cycling. Those variations increase with cycle number, indicating a logarithmic fatigue up to 108 cycles. There is no indication for the variations to be recovered, and the symmetry of the negative and positive parts of the strain hysteresis loops still remains. Higher cycling field results in a stronger deterioration of the maximum field induced strain and a larger extent of diffusion in AFE-FE and FE-AFE phase transitions. The normalized maximum strain shows nearly the same scale of degradation when the materials are cycled at various electric fields. After a heat treatment at 500°C for 1 h, the variations in the AFE-FE and FE-AFE phase transition due to cycling disappear, whereas the maximum strain resumes almost to its original value. Electrochemical variations are considered to contribute to the main fatigue mechanism for the material under investigation. [ABSTRACT FROM AUTHOR]

Induced damage in brittle materials due to two interacting Vickers indentations at various orientations was investigated using a three-dimensional finite element model. The model considers `tensile cracking and compressive yielding' behavior of ceramics. Damage evolution due to both the simultaneous and sequential double indentations was studied. The simulation results indicated that the induced damage zone patterns are strongly a function of the relative orientation of the two indenters. The existence of another nearby indentation reduces the crack size on the side closer to the first indentation but increases the overall median damage zone size. These results were validated by sequential Vickers indentation experiments on borosilicate glass. The evolved damage patterns were further rationalized based on Bousinesq and blister field stress fields. Finally, the implication of these results on material removal mechanisms due to simultaneous interaction of several grits in a ceramic grinding process is discussed. [ABSTRACT FROM AUTHOR]

Presents an interpretation of failure under cyclic contact loading. Claim that the fatigue effect is influenced by periodical friction between the grain-interlock bridges existing in such materials; Suggestion that delayed failure in contact fatigue tests is caused by a reduction of the friction term due to cyclic loading and by a related increase of the effective stress intensity factor until fracture toughness is reached.