FRICTIONAL EFFECTS ON FATIGUE CRACK GROWTH IN β ‐ANNEALED Ti‐6Al‐4V

A comparison of the growth of fatigue cracks under constant-amplitude straining and under strain histories having periodic compressive overstrains revealed that the fracture surface near the crack tip and the crack growth rate changed dramatically with the application of compressive overstrains. When the magnitude of compressive overstrains was increased, the height of the fracture surface irregularities was reduced as the increasing overstrain progressively flattened fracture surface asperities near the crack tip. The reduced asperity height was accompanied by a lower crack closure stress and a higher crack growth rate. The fatigue strength was reduced by a factor that ranged from 1.24 at short lives (104 cycles) to 3.37 at long lives (107 cycles) when periodic compressive overstrains of near yield point magnitude were applied in uniaxial tests. The corresponding reductions in the fatigue strength for shear tests varied from 1.40 to 1.70. A model of the plastic deformation of the fracture surface asperities at the crack tip (under periodic compressive overstrains) was developed to relate the crushing of the asperities to crack closure and crack growth rate. The model correlates the magnitude of the periodic compressive overstrain, the fracture surface asperity height, and the plastically flattened area to the fully effective strain intensity factor range (ΔKeff) and its ratio to the range of strain intensity factor (ΔK(ε)) obtained from constant-amplitude straining, U =ΔKeff/ΔK(ε). For both uniaxial and shear fatigue straining, the strain range at a fatigue life of 107 cycles for constant-amplitude straining (Δεfl) and the effective strain range, (Δεeff.fl) at 107 cycles in the strain-equivalent life curve, obtained from strain histories containing periodic compressive overstrains, were used to calculate the U ratio (U= Δεeff.fl /Δεfl). These values were close to those obtained from the model.

Fatigue fracture is one of the main failure modes of Ti-6Al-4V alloy, fracture toughness and crack closure have strong effects on the fatigue crack growth(FCG) rate of Ti-6Al-4V alloy. The FCG rate of Ti-6Al-4V is investigated by using experimental and analytical methods. The effects of stress ratio, crack closure and fracture toughness on the FCG rate are studied and discussed. A modified prediction model of the FCG rate is proposed, and the relationship between the fracture toughness and the stress intensity factor(SIF) range is redefined by introducing a correcting coefficient. Notched plate fatigue tests (including the fracture toughness test and the FCG rate test) are conducted to investigate the influence of affecting factors on the FCG rate. Comparisons between the predicted results of the proposed model, the Paris model, the Walker model, the Sadananda model, and the experimental data show that the proposed model gives the best agreement with the test data particularly in the near-threshold region and the Paris region, and the corresponding calculated fatigue life is also accurate in the same regions. By considering the effects of fracture toughness and crack closure, the novel FCG rate prediction model not only improves the estimating accuracy, but also extends the adaptability of the FCG rate prediction model in engineering.

The micro- and macroscopic fatigue crack growth (FCG) rates of a wide class of structural materials were analyzed and it was concluded that both rates coincide either during high-temperature tests or at high stress intensity factor (SIF) values. Their coincidence requires a high level of cyclic deformation of the metal along the entire crack front as a necessary condition for the formation of fatigue striations (FS). Based on the analysis of digital fractographic images of the fatigue fracture surfaces, a method for the quantitative assessment of the spacing of FS has been developed. The method includes the detection of FS by binarization of the image based on the principle of local minima, rotation of the highlighted fragments of the image using the Hough transform, and the calculation of the distances between continuous lines. The method was tested on 34KhN3M steel in the initial state and after long-term operation (~3 × 105 h) in the rotor disk of a steam turbine at a thermal power plant (TPP). Good agreement was confirmed between FCG rates (both macro and microscopic, determined manually or using digital imaging techniques) at high SIF ranges and their noticeable discrepancy at low SIF ranges. Possible reasons for the discrepancy between the micro- and macroscopic FCG rates at low values of the SIF are analyzed. It has also been noted that FS is easier to detect on the fracture surface of degraded steel. Hydrogen embrittlement of steel during operation promotes secondary cracking along the FS, making them easier to detect and quantify. It is shown that the invariable value of the microscopic FCG rate at a low SIF range in the operated steel is lower than observable for the steel in the initial state. Secondary cracking of the operated steel may have contributed to the formation of a typical FS pattern along the entire crack front at a lower FCG rate than in unoperated steel.

Predicting fatigue crack growth rates and thresholds at low temperatures

Modeling of temperature influence on the fatigue crack growth behavior of superalloys

Biaxial fatigue tests were performed on thin‐walled tubular 1045 steel specimens in a test fixture that applied internal and external pressure and axial load. There were two test series, one in which constant amplitude fully reversed strains (CAS) were applied and another in which large periodic compressive overstrain (PCO) cycles causing strains normal to the crack plane were inserted in a constant amplitude history of smaller strain cycles. Ratios of hoop strain to axial strain ofλ = −1, −0.625, −νand +1 were used in each test series. Fatigue crack growth behaviours under CAS and PCO histories were compared, and revealed that the morphology of the fracture surface near the crack tip and the crack growth rate changed dramatically with the application of the compressive overstrains. When the magnitude of the compressive overstrains was increased, the height of the fracture surface irregularities was reduced as the increasing overstrain progressively flattened the fracture surface asperities near the crack tip. The reduced asperity height was accompanied by drastic increases in crack growth rate and decreases in fatigue life.Using a pressurizing device attached to the confocal scanning laser microscope (CSLM), crack opening measurements were obtained. Crack opening measurements showed that the biaxial cracks were fully open at zero internal pressure for block strain histories containing in‐phase PCO cycles of yield stress magnitude. Therefore, for the shear‐strained samples, there was no crack face interference and the strain intensity range was fully effective. For PCO tests (with biaxial strain ratios of −0.625 and +1), effective strain intensity data were obtained from tests with positive stress ratios for which cracks did not close. A number of strain intensity parameters derived from well‐known fatigue life parameters were used to correlate fatigue crack growth rates for the various strain ratios investigated. Predicted fatigue lifetimes based on a fatigue crack growth rate prediction program using critical shear plane parameters showed good agreement with the experimental fatigue life data.

Corrosion fatigue behaviour of magnetic metallic glasses : Tandon, K.N. Collins, L. and Cahoon, J.R.Can. Metall. Q. Apr.–June 1990 29, (2), 167–170

Understanding the effect of crack tip deformation on fatigue crack growth behavior in 300-series austenitic stainless steel

The paper presents a technique for estimation of minimal fatigue crack growth (FCG) rate after the single overloads by tension in D16chT aluminum alloy. The function of FCG rate decreasing depending on overload factor and stress ratio was proposed. This function is based on Walker equation for determining of FCG rate under constant amplitude cyclic loading and retarding part. Proposed phenomenological equation enables the minimal FCG rate to be predicted after a single overload taking into account overload factor and stress ratio in D16chT alloy. Good enough agreement of calculation results and testing data of minimal FCG rates after a single overload was shown graphically.

Fatigue behavior of notched steel beams reinforced with bonded CFRP plates: Determination of prestressing level for crack arrest

In order to investigate whether hydrogen enhance the fatigue crack growth (FCG) rate of stable austenitic stainless steels or not, FCG tests of a type 310 steel were performed in a hydrogen gas environment and in a nitrogen gas environment. The FCG rate tested at 5.0Hz in hydrogen is higher than that in nitrogen. It seems that the acceleration of FCG rate in hydrogen does not necessarily need the formation of strain-induced α' martensite, though it increases the degree of the acceleration. The cause of the acceleration of type 310 seems to be hydrogen enhanced concentration of slip at a crack tip. The degree of the acceleration became smaller in a frequency of 0.1Hz. It seems that the generation of heat at the crack tip concern. Further investigation is necessary.

Crack closure of Ni-based superalloy 718 at high negative stress ratios

In order to investigate the hydrogen effect on fatigue crack growth (FCG) behavior in a few kinds of practical alloys; austenitic stainless steels (solution-treated metastable type 304 and stable type 316L), an aluminum alloy (age-hardened 6061) and a low carbon steel (annealed 0.13%C-Fe), FCG tests were carried out in hydrogen gas and in nitrogen gas. The FCG rates of these materials are enhanced by hydrogen, though the acceleration degrees are different. A crack grows across grains by slip-off in 316L stainless steel and in age-hardened 6061 aluminum alloys even in hydrogen. Faceted area increases in 304 stainless steel and in low carbon steel in hydrogen. In 304 stainless steel, the ratio of facets to the entire fracture surface was not so large. Thus, the FCG rate is not significantly affected through the facets in 304 stainless steel. In low carbon steel, facets were increased considerably, though a crack grows step by step or after a large number of loading cycles even along grain boundaries. Anyhow hydrogen enhances the FCG rate of these materials through the influence on slip behavior. Based on above-mentioned results, the effect of loading frequency on FCG rate in hydrogen of the age-hardened 6061 aluminum alloy was also investigated. The FCG rate increases as the testing frequency decreases, though the FCG rate in hydrogen shows the tendency to saturate.

Overload-induced anisotropy of fatigue crack growth in laser deposited Ti–6Al–4V alloy

Fatigue crack growth from plain and cold expanded holes in aluminium alloys