Cyclic Fatigue Crack Growth Behavior Research Articles

An Abrupt Transition to an Intergranular Failure Mode in the Near-Threshold Fatigue Crack Growth Regime in Ni-Based Superalloys

Cyclic near-threshold fatigue crack growth (FCG) behavior of two disk superalloys was evaluated and was shown to exhibit an unexpected sudden failure mode transition from a mostly transgranular failure mode at higher stress intensity factor ranges to an almost completely intergranular failure mode in the threshold regime. The change in failure modes was associated with a crossover of FCG resistance curves in which the conditions that produced higher FCG rates in the Paris regime resulted in lower FCG rates and increased ∆Kth values in the threshold region. High-resolution scanning and transmission electron microscopy were used to carefully characterize the crack tips at these near-threshold conditions. Formation of stable Al-oxide followed by Cr-oxide and Ti-oxides was found to occur at the crack tip prior to formation of unstable oxides. To contrast with the threshold failure mode regime, a quantitative assessment of the role that the intergranular failure mode has on cyclic FCG behavior in the Paris regime was also performed. It was demonstrated that even a very limited intergranular failure content dominates the FCG response under mixed mode failure conditions.

Effect of Intermetallics on the Low Cyclic Fatigue Crack Growth Behavior of a Heavily Alloyed Al-Si Piston Alloy

This paper focuses on a typical automotive piston material to characterize its fractographic appearance after low cyclic fatigue test. The fatigue fracture of this heavily alloyed Al-Si alloy takes place in a brittle manner. The crack nucleated from a large intermetallic colony close to the specimen surface. When the crack encountered the intermetallics, it might progress along the interface among the intermetallics or the interface between the intermetallics and the eutectic. The debonded Al-Si eutectic region and intermetallics provided a weak material path for the crack propagation. A fatigue fracture mechanism in the heavily alloyed Al-Si alloy is elucidated.

Fatigue crack propagation and fracture characteristics of in-situ titanium-matrix composites

In this paper, the fracture and fatigue crack growth behavior of in-situ titanium-matrix composites reinforced with whiskers of titanium boride, are discussed. The influence of boron additions on the cyclic fatigue crack growth behavior of several titanium-base alloys is elucidated. The improved crack growth resistance of certain compositions is explained by considering the effects of crack–microstructure interactions and the limited effects of crack-tip shielding due to ligament bridging.

Fatigue Crack Growth Behavior in Silicon Nitride under Constant and Variable Amplitude Load Sequences

In order to investigate cyclic fatigue crack growth behavior of a gas-pressure-sintered silicon nitride under constant and variable amplitude load sequences, fatigue crack growth tests were carried out using compact type (CT) specimens. The crack length and macroscopic crack closure were measured using an unloading elastic compliance method. Grain interlocking was observed around crack wake in all fatigue test conditions, which implied fatigue crack growth of this material was associated with progressive degradation of the grain interlocking by cyclic loading. The fatigue crack growth rate, da/dn, under constant amplitude loading was controlled not only by the maximum stress intensity factor, Kmax, but also the stress amplitude, while Kmax was the most important factor in cyclic fatigue crack growth. The overload and high-level load excursion produced the acceleration of fatigue crack growth and the downward shift of crack closure point under low-level loading. This decrease was thought to result from much severer crash or frictional wear of grain interlocking and the crack closure could explain the acceleration behavior qualitatively.

Fatigue crack growth behaviour of gamma base titanium aluminides under different loading conditions

Static and cyclic fatigue crack growth behaviour of gamma base titanium aluminides with three different microstructures were investigated. Influence of cyclic test frequency on fatigue crack growth behaviour was also studied at room temperature under a controlled humidity condition. The crack growth behaviour both under static and cyclic loading was strongly influenced by the microstructure. The threshold stress intensity and crack growth behaviour under cyclic loading were much inferior than that under static loading indicating the ‘true-cyclic fatigue’ effect exhibited in gamma base titanium aluminides. No significant effect of test frequency on the crack growth behaviour was observed for the equiaxed and duplex microstructure materials.

CYCLIC FATIGUE CRACK GROWTH AND CLOSURE BEHAVIOUR IN ALUMINA CERAMICS

Cyclic fatigue crack growth behaviour in alumina ceramics is investigated and the effect of grain size discussed. Special attention is given to crack closure effects. Cyclic fatigue tests were carried out using four‐point bend specimens, and the load–strain and load–differential strain curves were monitored. These curves show hysteretic behaviour probably related to frictional sliding of bridging grains, and also include non‐linearities due to crack closure. The crack opening load is determined from the load–differential strain curve by using a method introduced in this study. Growth rates can be successfully described by the relationship da/dN = C[ΔKeff /(1 − Kmax /KIC )]m which is proposed in this study to account for the effects of crack closure and the maximum stress intensity factor. Irrespective of grain size, growth rates can be well represented by the above relationship, implying that the grain size exerts an influence on growth rates not only because of crack closure behaviour but also the material fracture toughness. The growth rate curve based on the proposed relationship shows a sigmoidal form for ceramic materials, which is similar to metals.

Cyclic fatigue crack growth behaviour of silicon nitride at 1400°C

Abstract Subcritical crack growth under sustained and cyclic loads at 1400°C has been investigated in a high strength Si 3 N 4 . Two different crack growth behaviours were found in this material depending on the high temperature testing history. During short-time testing, fatigue crack growth obeyed the Paris power-law expression, and under these conditions high frequency loading markedly reduced the crack propagation rate coMPared with that under sustained load or lower frequencies. Scanning electron microscopy observation revealed that this beneficial effect of cyclic loading was associated with viscous bridging by the glass phase at the crack wake, which effectively shielded the crack tip. As the fatigue test proceeded, however, crack growth rates reduced, eventually resulting in a lack of correlation between crack growth rate and stress intensity factor, and producing no discernible effect of loading frequency. Microstructural observations revealed that during this phase cavity nucleation and growth occurred and accumulated ahead of the crack tip owing to the combined action of three time-dependent events: creep, oxidation and crystallisation of the intergranular glass phase. The observed crack growth behaviour during this stage can be explained by crack tip blunting due to cavitation and glass flow and by a reducing bonding force of the glass phase across the crack surfaces due to the combined effects of oxidation, creep and crystallisation of the intergranular glass phase.

Cyclic fatigue crack growth behaviour in β-(Si-Al-O-N) at ambient and elevated temperatures

Four-point bending fatigue tests on a hot-pressed sintered Sm–β-(Si–Al–O–N) ceramic were conducted at room temperature, 900 °C and 1000 °C in air under different load ratios and cyclic frequencies. The growth of indentation cracks was measured during the fatigue tests. The results indicate that the cyclic fatigue crack growth threshold is lower and crack growth rates are higher, for given values of Kmax, at 1000 °C than those at room temperature. The cyclic fatigue crack growth behaviour at 900 °C is similar to that at room temperature. It was found that the crack growth retardation due to cyclic fatigue loading is much more pronounced at higher frequencies. An increase in cyclic frequency from 1 to 10 Hz cause a reduction of up to two orders of magnitude in crack propagation rates. High-temperature cyclic fatigue crack growth rates increased and threshold stress intensity factor ranges decreased with increasing load ratio. Possible mechanisms for cyclic crack growth are discussed.

Environmentally Affected Small Cyclic Fatigue Crack Growth Behavior of Silicon Nitride in Atmospheric Air and Toluene.

To investigate the effect of atmospheric humidity on cyclic fatigue crack growth behavior, fourpoint bending fatigue tests were performed on silicon nitride in laboratory air and toluene. Crack aspact ratios for surface cracks in laboratory air were clarified to be lower than in toluene, The relationship between crack growth rate and maximum stress intensity factor for small surface cracks was obtained using the above crack aspect ratios. This result indicated that crack growth rate was accelerated by atmospheric humidity in the low stress intensity factor range, but not in the high range. This acceleration was clarified to be causes by the mutual action of cyclic stressing and atmospheric humidity. Microcrack initiation ahead of the main crack, the cutting process of ligament between the micro and the main crack, and cyclic loosening of crack bridging at wake of the crack tip were ciarified to be dominant factors which influence the crack growth behavior in both environments.

窒化けい素における一定および非定常変動振幅荷重下の疲労き裂進展挙動

In order to investigate the effect of load variation on the cyclic fatigue crack growth behavior of gas-pressure-sintered silicon nitride, fatigue crack growth tests under constant amplitude loading, multiple peak overloading, Lo-Hi and Hi-Lo two-step loadings were carried out using compact type (CT) specimens. Crack length and macroscopic crack closure were measured using the unloading elastic compliance method. Grain interlocking was observed around crack wake in all fatigue test specimens by SEM. Fatigue crack growth rate, da/dn, under constant amplitude loading was controlled by not only maximum stress intensity factor, Kmax, but also load amplitude. Crack opening stress intensity factor, Kop, decreased as load amplitude increased, as was concerned with breaking of grain interlocking. Overload caused the acceleration of fatigue crack growth rate, which was in contrast to the retardation observed in metallic materials. The acceleration was due to the breaking of grain interlocking by overload. The crack growth rate recovered as crack grew in a relatively short distance after overload, because grain interlocking in crack wake was reconstructed during crack growth.

窒化けい素の２段繰返し変動荷重下における疲労き裂進展挙動

In order to investigate the cyclic fatigue crack growth behavior of a gas-pressure-sintered silicon nitride under repeated two-step loadings, fatigue crack growth tests were carried out using compact type (CT) specimens. Crack length and macroscopic crack closure were measured using the unloading elastic compliance method. Regardless of repeated two-step loading patterns, the acceleration of fatigue crack growth rate was observed. High-level load execution was found to crash grain interlocking more severely and result in decrease of crack opening stress intensity factor, Kop, under low-level loadings. The fatigue crack growth acceleration behavior under repeated two-step loadings, where maximum load was kept constant, could be explained by crack closure behavior measured by the macroscopic method. However, under repeated two-step loadings where minimum load was kept constant, macroscopic crack closure behavior could account for the acceleration behavior qualitatively but not quantitatively.

Cyclic fatigue crack growth of SiCw/Y-TZP composites: long- and short-crack behaviour

Cyclic fatigue crack growth behaviour has been investigated for 10 vol % SiCw/Y-TZP composites with grain sizes varying from 1.50–2.00 μm in a residual stress field. To investigate the effect of precracking procedure, cyclic fatigue tests were performed on unannealed specimens using the four-point bending method under two conditions: (a) with a sharp crack and precracking procedure, and (b) with a natural sharp flaw. For all specimens in both conditions, an overall V-shaped da/dN versus K app relation was obtained. However, for the specimens without precracking, the da/dN had an unusual dependence on the applied stress intensity, giving a zigzag V-shaped curve. Explanations for these different results for the two conditions are discussed in terms of crack-tip shielding effects and residual stress field.

Cyclic fatigue crack growth resistance of Al 2O 3Al 2TiO 5 composites

The role of the aluminium titanate and the grain size of alumina in the flaw tolerance of Al 2O 3Al 2TiO 5 composites has been studied. The fabrication of these in situ formed composites with different grain sizes of alumina with homogeneous and heterogeneous microstructure, is described. An investigation was carried out regarding whether a correlation exists between the cyclic fatigue crack growth behaviour in composites with different distribution and quantity of aluminium titanate and different average grain size of the alumina matrix, and the magnitude of the bridging forces developed when the cracks propagate at similar rates.

Cyclic Fatigue Crack Propagation Behavior of 9Ce‐TZP Ceramics with Different Grain Size

Cyclic fatigue crack growth behavior has been investigated in 9 mol% Ce‐TZP ceramics with grain sizes varying from 1.1 to 3.0 μm. To ascertain the interaction between crack resistance curve behavior and cyclic fatigue crack growth, cyclic fatigue tests were conducted with short double‐cantilever‐beam specimens in two conditions: (a) with a sharp precrack without preexisting t‐m transformation and (b) with a sharp crack after R‐curve measurements, i.e., with preformed t‐m transformation in the crack region. Fatigue crack propagation occurs at applied stress intensity factor values as low as about 40% of the KI,∞ values measured in the R‐curves. The size and shape of the t‐m transformation zones are found to be different for specimens obtained in monotonic loading R‐curve measurements and in cyclic fatigue tests. For the specimens without preexisting t‐m transformation the overall crack growth behavior can be described by the Paris power law relation: da/dN=AδKmI with m values of 15 for the 1.1‐μm grain size and between 8 and 9 for the material with larger grain sizes. For the specimens with the preformed transformation zone, a “V”shape da/dN versus ΔKI relation is obtained. Explanations for these different results in the two conditions are discussed in terms of crack tip shielding effects.

Role of Crack‐Bridging Ligaments in the Cyclic Fatigue Behavior of Alumina

The cyclic fatigue crack growth behavior in two aluminas with different grain sizes and purities has been investigated using tapered cantilever beam specimens. The curves of load–crack opening displacement revealed a hysteretic loss behavior, in both aluminas, produced by frictional forces generated in the crack wake. From measurements of the specimen complinces it is concluded that the fatigue cracks in these materials were strongly bridged by both “frictional” and “elastic” load‐bearing ligaments. It is shown that the “elastic” contribution to the total crack‐bridging force is far greater than that produced by fristional sliding ligaments. The results support the view that cyclic loads produce degradation of the strength of the bridging ligaments. The effect of grain size and compressive loads on the magnitude of the bridging forces is discussed.

Cyclic fatigue-crack growth behaviour of short cracks in SiC-reinforced lithium aluminosilicate glass-ceramic composite

Etude du comportement de courtes fissures (20 a 300 μm) dans des plaques (epaisseur= 3 mm) de vitroceramique renforce, en continu, avec 50% en volume de fibres unidirectionnelles