Lower Fatigue Crack Growth Rate Research Articles

中炭素鋼および二相ステンレス鋼における疲労き裂成長に及ぼす負の応力比の影響

Fatigue crack growth behaviors such as the crack growth rate and crack closure were studied using medium carbon steel (S45C) and two phase stainless steel (SUS329J1) under various combinations of the negative stress ratio, R and the maximum stress relative to the yield strength, σmax/σy. The main results obtained are as follows;(1) In the range of intermediate fatigue crack growth rate, the crack growth rate, da/dN increased with a decrease of R and an increase of σmax/σy for both the carbon steel and the two phase stainless steel. In this case, it becomes clear quantitavely that the compresion stress in the load cycle relative to the yield strength, Sc/σy is a very important factor which dominates the crack opening level and consequently da/dN in the both materials.(2) In the range of low fatigue crack growth rate, however, the compression stress in the cycle, Sc caused da/dN to decrease for the carbon steel but to increase slightly for the two phase stainless steel. This is closely related to the fact that the oxide deposite forms easily on the fracture surfaces of the carbon steel but does not form on the stainless steel.

Mechanical properties of carburized Cr-Ni-Mo steels with added case nitrogen

A new carbonitriding technique which gives a microalloyed case with nitrogen has been developed. The effect of nitrogen content on the case microstructures, the residual stress distribution and the fatigue initiation life,N, and the fatigue crack growth rate through the case,da/dn, were studied. An ideal combination of fine microstructure and high compressive residual stress distribution were obtained in the carburized Cr-Ni-Mo steels with added case nitrogen. The highest fatigue crack initiation life and the lowest fatigue crack growth rate were shown to appear in the nitrogen added case where the nitrogen content was kept as low as 0.1% N. Nickel was proven to be an effective alloying element in the steel for the refinement of microstructures and the increase in compressive residual stress in the case. The retained austenite proved to be beneficial to the decrease inda/dn and its most effective content in the case was found to be in the range of 25–35%.

高マンガン鋼の疲労強度特性に及ぼす介在物の影響

Fatigue experiments were conducted to elucidate the influence of inclusions on fatigue crack growth behavior of high manganese steel at low stress amplitude near the threshold region and in the region II for the temperature range of 297K to 113K. Two series of steel with different inclusion contents were used. Compact tension specimens were prepared in SL or ST orientation with respect to the rolling direction.The results showed that the fatigue crack growth rate da/dN of SL specimen was higher than that of ST specimen for both series at low stress levels near the threshold region. On their fracture surfaces, slightly larger roughness was measured in the case of ST specimen compared with the SL specimen and the roughness increased slightly with an increase of the content of sulphide inclusion regardless of experimental temperature. Based on these results, it is considered that da/dN in this region is affected by the crack closure due to the surface roughness which depends on the inclusion content and on the orientation. In the region II, the influence of orientation on da/dN was more pronounced than that of content at both 297K and 113K. However, ST specimen with a high inclusion content showed the lower fatigue crack growth rate than the base material at 113K. In order to make clear this reason, fracture surface were observed by a SEM. It is pointed out that microcracks produced in elongated MnS inclusions increase da/dN for SL specimen but reduce da/dN for ST specimen.

Some considerations on the occurrence of intergranular fracture during fatigue crack growth in steels

The occurrence of a maximum in the percentage of intergranular fracture on the fracture surface during the transition from intermediate to low fatigue crack growth rates has been observed for a high strength steel. It is suggested that transgranular planar slip leading to slip localization is essential in promoting intergranular fracture when the cyclic plastic zone size becomes equal to the prior austenite grain size.

A comparative study of monotonic fracture, environmentally assisted fracture and fatigue behaviour of cast and forged high strength steels

The microstructure and fracture characteristics of a high strength cast steel and a forged steel with the same chemical composition and strength and both subjected to the same heat treatment were studied. It was found that the cast steel has better fracture mechanics characteristics ( K Ic , K ISCC , dd a d t and d a d N although the conventional mechanical properties of the cast steel (tensile ductility, notch toughness, fatigue strength etc.) are poorer than those of the forged steel. The roles that the microstructure of the cast steel plays in the two kinds of tests are discussed. The microporosity causes the conventional properties to deteriorate and in particular leads to much more scatter in these properties but it does not appear to have any effect on the fracture mechanics parameters. The dendrite segregation exerts a very harmful influence on both the conventional and the fracture mechanics characteristics. The inclusion distribution state (the secondary dendrite arm spacing) and the weaker inclusion-matrix interfacial bonding are the main reasons why the cast steel has a higher fracture toughness; in particular, the weak interface between the inclusions and the matrix is to a large extent responsible for the lower hydrogen embrittlement, the susceptibility to stress corrosion cracking and the slightly lower fatigue crack growth rate.

高強度鋼の下限界近傍の疲労き裂伝ぱ挙動に及ぼす酸化物誘起き裂閉口と粒界破壊の影響

Near threshold fatigue crack growth behavior of high strength steels has been investigated by varying the yield strength (921∼1845MPa) and stress ratio (R=0.06, 0.4). Three kinds of Cr-Mo steels were used in this investigation and the fatigue tests were carried out at room temperature and in laboratory air.The threshold stress intensity factor range, ΔKth, generally decreased with increasing yield strength and/or stress ratio. Then, the influence of stress ratio decreased as the yield strength became high. Several of the materials used, however, showed different tendencies.These observations were rationalized in terms of the concept of oxide-induced crack closure. Then, it was found that the intergranular fracture occurring at low fatigue crack growth rates remarkably encouraged oxide-induced crack closure in thee case that the area percentage is more than 50%, and it played the important role to the behavior of ΔKth.

On fatigue crack propagation of titanium alloys under dwell time conditions

The effect of dwell-time at peak load on the fatigue crack propagation of a near-α alloy (Ti-11) and on α + β alloy (Ti-6A1-4V) was investigated. Several composition, microstructure and texture conditions were studied under fatigue cycling with a 5 min dwelltime. Three different product forms of Ti-6A1-4V were used: cross-rolled plate, highly textured plate and highly textured low interstitial plate. No deleterious effect on fatigue crack growth rate was observed for the 5 min swell cycling when compared to a 6 Hz baseline data for any of the material variables studied. In fact, for some of the microstructures studied, the dwell cycling resulted in a significantly lower fatigue crack growth rate. This deceleration can be explained on the basis of the observed increase in crack path tortuosity associated with the dwell cycling. This increased tortuosity may be the result of a crack tip blunting process which occurs during the dwell period of the load cycle.

Fatigue of annealed and cold worked stable and unstable stainless steels

Fatigue crack growth rates (FCGR) in AISI 301 and 302 austenitic stainless steel alloys have been measured in controlled load cycles withR = 0.05. Both annealed and cold rolled conditions were examined. The austenite phase of the AISI 301 alloy was unstable under stress and transformed martensitically to α′ to a much greater extent than the AISI 302 alloy. At low values of mean stress the unstable alloy had a lower FCGR than the more stable 302 alloy. The FCGR increased with mean stress until values of mean stress ⪞70 MPa, where the FCGR was independent of mean stress and was the same for both alloys. Various metallographic and macroscopic measurements were made to try to understand this behavior. It was concluded that residual compressive stress due to transformation at the crack tip was responsible for the lower crack growth rates of the unstable 301 alloy. Cold worked specimens had significantly lower crack growth rates than the annealed specimens, and both alloys behaved identically.

The Effect of ITMT’s and P/M Processing on the Microstructure and Mechanical Properties of the X7091 Alloy

The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔKth, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation (FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution, a weak texture, and a low concentration of oxides at grain boundaries.

The fracture behaviour of a PXE/HIPS polyblend

As part of an overall examination of the fatigue crack propagation (FCP) behaviour of impact-modified polymers, a study of the fracture morphology of a PXE/HIPS polyblend polymer subjected to monotonic and cyclic loading conditions is reported. The HIPS rubbery-phase particles are found to fail by particle rupture in both fatigue and fast fracture. Another impact modifying addition, PE, is found to fail by a combination of interfacial rupture and tearing, the balance depending on the prevailing stress intensity value and the strain rate. Matrix failure is via multiple crazing at low fatigue crack growth rates, but shear yielding is believed to become a major fracture mechanism with increasing ▵ K. The degree of plastic deformation of the matrix increases with increasing strain rate. This fact is manifested by the increasing void size associated with the interfacial separation of the PE particles.

The influence of heat treatment on low frequency fatigue crack growth rates for a marine steel tested in air and seawater

A quenched and tempered marine steel has been cooled at different rates after tempering and low frequency fatigue crack growth rates in air and seawater compared. Water quenching was shown to result in significantly lower fatigue crack growth rates compared to furnace cooling. The likely causes and the implications of the results are discussed.

Influence of environment on fatigue crack growth in the threshold region

Low fatigue crack growth rates (down to 4 × 10 −14 m/ cycle) were produced using a high frequency 20 kHz ultrasonic fatigue testing machine. The influence of non corrosive (silicone oil) and corrosive (3.5% sodium chloride solution) environments was compared. Down to crack propagation rates of some 10 −10 m/cycle which corresponds to a crack growth rate of one lattice space per cycle no difference of crack growth rates was found. However, below this rate there seems to exist for non corrosive environments a threshold cyclic stress intensity, below which crack growth becomes diminishingly small, whereas no threshold was found for the corrosive environment. In the first case crack propagation is controlled by plastic deformation processes, in the second case these processes are markedly restricted. For this region, a transition in fracture mode from ductile transcrystalline to intergranular cracking was found.

Relationships Between Low-Cycle Fatigue and Fatigue Crack Growth Rate Properties

AbstractA fracture mechanics model is developed to describe crack growth in a low cycle fatigue test specimen. The model involves a J integral analysis and a growth rate hypothesis in terms of ΔJ. A relationship for low cycle fatigue is derived that has strain energy density as the controlling variable. This relationship reduces to well-known low cycle fatigue equations in terms of elastic and plastic strains for the limiting conditions of fully elastic and fully plastic strain fields. These equations in turn define relationships between the material properties commonly employed to describe low cycle fatigue and fatigue crack growth rate data. The latter are used to demonstrate the facility of predicting fatigue crack growth rate curves from standard low cycle fatigue properties.

Fatigue Crack Growth Rate in Base Plate and Welded Joints of High Tensile Strength Steel

The fatigue crack growth rate in HT80 base plate and welded joints has been measured using compact tension specimens, and examined by means of fracture mechanics and electron fractography.The lowest fatigue crack growth rate was obtained in the heat-affected zone and the highest rate in the stress-relieved weld metal. In the weld metal, the compressive residual stress affected the crack growth rate and reduced the effective stress intensity factor range ΔKeff.The fatigue crack growth rate coincided almost completely with the microscopic growth rate calculated from the striation spacing for base metal and heat-affected zone. The fatigue crack growth rate for both weld metal and stress-relieved weld metal was, however, higher than the microscopic growth rate in the present stress intensity factor range. It is supposed that this discrepancy is caused by the fracture accompanied with dimples and striations. The relation between the dimple size, ds(mm), and the stress intensity factor range, ΔK(kg/mm3/2), has been obtained as follows, ds=1.4×10-5·(ΔK)