Macroscopic Crack Growth Rate Research Articles

Frequency effects on fatigue crack growth behavior in a near-α titanium alloy

The loading frequency effects on elevated temperature fatigue crack growth behavior in a new near-α, β-processed titanium alloy were examined in both air and vacuum environments. The individual influences of environment and creep are identified in terms of both the macroscopic crack growth rates and detailed fractographic observations as a function of the applied ΔK. Results indicate the existence of two regions of transgranular crack growth: microstructurally sensitive and insensitive. The transition between these two regions was found to depend on the testing temperature. Lower frequencies promoted an increase in cleavage fracture while an addition of hold time at maximum load resulted in prior β grain boundary fracture attributed to creep damage. In addition, the transition to intergranular fracture at high ΔK was found to take place at higher frequencies in vacuum, yielding a higher crack growth rate than in air which is indicative of a faster, more widespread viscoplastic flow without the presence of oxidation. Closure measurements indicated a dominant influence of asperity-induced closure in non-oxidizing environments and at low values of applied ΔK.

Influence of simultaneously occurring brittle cracking on ductile fatigue crack growth rates—Part II. Comparison of experimental results with model predictions

Experiments were carried out on an AISI-SAE 1005 mild steel at 298, 233 and 173 K with R-ratios of 0.1, 0.3 and 0.5 to verify the influence of cleavage or cyclic cleavage on the macroscopic fatigue crack growth rate, da dN . Cleavage during fatigue occurred at 233 and 173 K as quite uniformly distributed patches within a ductile (striated) fracture surface. The amount of cleavage during fatigue propagation increased as the temperature and as the R-ratio decreased and, at a given temperature, was determined primarily by the ΔK value. The influence of this cleavage on the macroscopic value of da dN was analysed through its influence on the crack tip opening displacement, employing the model of the semi-cohesive plastic zone developed in part I. This model was shown to explain satisfactorily the behavior observed.

ＳＵＳ３０４鋼のクリープ・疲労条件下の高温低サイクル疲労における微小表面き裂の進展挙動

The surface small crack growth behavior of SUS304 stainless steel under fast-fast and slow-fast strain controlled low-cycle fatigue conditions was investigated at 873K with the specimens having different grain sizes. It was shown that the cumulative probability of crack length distribution in both fast-fast and slow-fast cyclings could be represented by a Weibull distribution as a function of crack length in the early stage of fatigue life. In the stage after the half fatigue life, the cumulative probability of crack length distribution showed a knee, and a small number of main cracks grew predominantly. The knee occurred when the crack length was comparable to one grain size. It was also shown that the surface small crack growth rate showed the minimum value when the cracks arrived at the grain boudaries. The above behavior resulted from that the grain boundaries temporarily impeded the small crack growth. Furthermore, the small crack growth rate was compared with the macroscopic crack growth rate. The law for the macroscopic crack growth was applicable to the small crack growth in slow-fast cycling, up to the crack length being one grain size. On the other hand, the small crack growth rate in fast-fast cycling was about ten times as large as the macroscopic crack growth rate in the region where the crack length was smaller than or comparable to the grain size.

A Fatigue Fracture Surface Analysis Map of the 18 Ni Maraging Steel

The fatigue fracture surface analysis map of a 18 Ni maraging steel (see Fig. 1) is presented for the aging temperature of 482°C and the stress ratio of 0.10. The map consists of the quantitative information obtained from the analysis of fatigue fracture surface and the curve of macroscopic crack growth rate vs. stress intensity factor reported in Ref. 4). It is aimed to estimate the mode of fatigue fracture and the rate and the direction of crack growth from a fracture surface of which a cause is unknown. The effectivity of the map for the above aim is confirmed under other kinds of testing conditions, such as with the prior austenitic grain sizes of 11 μm and 25μm, the aging temperatures of 482°C and 432°C, and the stress ratios of 0.10 and 0.70.

Study on Crack Growth Behavior in Impact Fatigue : Part 1, Impact Fatigue Crack Growth Characteristics of 0.1%C Aluminum Killed Steel

A series of experiments are conducted to investigate the impact fatigue crack growth behavior of O.1%C aluminum killed carbon steel conforming to JIS. S10C. And results obtained are discussed from several viewpoints in comparison with those in non impact fatigue. It is clear that macroscopic crack growth rate, da/dN, in impact fatigue is higher than that in non impact fatigue especially in relatively low Kmax or low ΔK region. Through quantitative analysis of fracture surface morphology, it is revealed that the higher crack growth rate in impact fatigue is sustained by the larger amount of intergranular fracture mode which accelarates crack growth rate to some extent. Other characteristical aspects in impact fatigue crack growth behavior are also described in detail.

18Niマルエージ鋼の疲れ破面解析図

A fatigue fracture surface-analysis map of 18Ni maraging steel (see Fig. 1) is proposed using the condition of the aging temperature of 482°C and the stress ratio of 0.1, which consists of the quantitative information obtained from the analysis of fatigue fracture surface and the curve of macroscopic crack growth rate versus stress intensity factor. The map intends to estimate the mode of fatigue fracture, the rate and the direction of crack growth, etc. from a fracture surface of which the cause is unkown. The effectivity of the map for the above aim is confirmed using the other four kinds of testing conditions, namely the prior austenitic grain sizes are 11 μm and 25 μm, the aging temperatures are 482°C and 432°C, and the stress ratios are 0.1 and 0.7.

Influence of absorbed moisture on fatigue crack propagation behaviour in polyamides

Abstract : The fatigue fracture surface morphology of nylon 66, nylon 6 and nylon 612 was examined to ascertain mechanisms of fatigue crack propagation (FCP) in these polymers. Attention was also given to noting any correlation between fracture surface markings and macroscopic fatigue crack growth rate data. In general, observed changes in fracture surface appearance reflected an increasing level of plastic deformation with increasing water content, particularly in N66 and N6. Classical fatigue striations were identified in specimens of N66 and N6 containing 1.7-5.7 wt% water. Other types of fracture lineage of unknown origin were also seen which can confound the interpretation of fatigue fracture topography. Unlike the cases of N6 and N66, the fracture of N612 was dominated by a microvoid coalescence mechanism at all moisture levels and at all Delta K levels examined. (Author)

Some Mechanistic Observations on the Crack Growth Characteristics of Pressure Vessel and Piping Steels in PWR Environment

Abstract The fatigue crack growth behavior of A533B and A508 pressure vessel steel and Types 304 and 316 steels used in reactor coolant piping has been studied in a pressurized water reactor environment at 288 C (550 F). The influence of stress ratio (Pmin/Pmax), frequency, ramp times, specimen orientation, and material microstructures were included in the study. While none of the materials showed evidence of static crack growth in the environment, the ferritic steels did show an enhanced fatigue crack growth rate at test frequencies of 5 cpm and lower. Based on fractographic examinations, the enhanced growth rate is not the result of environmentally induced intergranular or cleavage modes of crack propagation, instead, striation spacing measurements were found to agree with the macroscopic crack growth rate. This indicates that a time dependent environmental interaction exists which introduces a frequency dependent enhancement of the mechanically developed striations. Crack growth experiments using hold ...

Nucleation and propagation of fatigue cracks in Al-304 stainless steel laminate composite

A study has been made of fatigue crack nucleation and propagation in Al-stainless steel (30 vol%) laminate composites. A Paris type power relationship between the crack growth rate, da/dN, and the alternating stress intensity, ΔK, was obtained over the crack growth rates ranging from 10−7 to 10−4 mm/cycle, with an exponentm of 2.7. The cracks nucleated first in Al strips and then in stainless steel strips accompanied by some interface decohesion. The fatigue crack propagated in two stages. In the first stage, where the Al-steel interface was largely intact, the crack propagated in a plane strain mode (flat fracture surface with striations, each striation consisting of a cluster of interstriations). In the second stage, where there occurred extensive Al-steel interface delamination and the concomitant loss of mutual constraint, the crack propagated in the plane stress mode (slant fracture with voids). The crack growth was faster in Al than that in steel since the apparent striation spacing was larger in the former than in the latter. No one to one correspondence existed between the apparent striation spacing and the macroscopic crack growth rate. Thus, although, microscopically, the crack front was not planar; macroscopically, it could be regarded as planar, and a Paris type power relationship did characterize the macroscopic fatigue crack growth in this laminate system over the applied stress amplitude studied. Comparing the fatigue crack growth rates among Al-steel laminate, commercial or pure aluminium and 304 stainless steel, the Al-steel laminate has the lowest crack growth rate. This plus the weight and cost saving benefits make Al-steel laminate quite attractive.

Ténacité et résistance à la propagation des fissures de fatigue de l'alliage de titane TA6V

This paper presents test results concerning plane strain toughness (critical stress intensity factor K IC) and the kinetics of crack propagation in Ti-6Al-4V alloy for various morphologies, acicular or equiaxed, resulting from different heat treatments carried out in β or α + β phase fields. It has been shown that acicular-type structures present higher toughnesses ( K IC may reach 110 MN m −3 2 ) than equiaxed grains structures ( K IC approximately 60 MN m −3 2 ). In the validity range of the Paris equation, da/ dJV = CΔK n , macroscopic crack growth rates, for identical states of stress intensity at the crack tip, are lower for acicular structures. Also, the threshold of no crack propagation is higher ( ΔK s of the order of 10 MN m −3 2 ) for such structures, in comparison with the structure of equiaxed grains ( ΔK s of the order of 8 MN m −3 2 ).

The relationship between striation spacing, macroscopic crack growth rate, and the low-cycle fatigue life of a Type 316 stainless steel at 625°C

The fracture surfaces of smooth and notched Type 316 stainless steel specimens tested under essentially low-cycle fatigue conditions in air at 625°C and a cyclic frequency of one per minute have been examined under a scanning electron microscope. Observations from notched specimens indicate that for plastic strain ranges above the smooth specimen fatigue limit, macroscopic crack growth per cycle and striation spacing are in good agreement. Below this strain level, striation spacing exceeds macroscopic growth rate. Smooth specimen failure occurs by the initiation and propagation of one or more surface cracks. Striation spacing data from these specimens imply that low-cycle fatigue is dominated by the crack propagation process.

Microstructural Effects in the Low-Cycle Fatigue of Fe-Ni-Cr Austenite

A series of low-cycle fatigue tests was performed at 593 deg C on Fe-20 Cr-10 Ni and Fe-20 Cr-20 Ni (in weight percent) austenitic alloys in both the cast and wrought conditions. The as-cast alloys exhibited substantially longer cyclic lives than the wrought alloys or wrought Type 304 stainless steel. An effect of Ni content on fatigue life was noted for the cast alloys, but not for the wrought. Striation, measurements indicated that the majority of the cyclic life was spent in crack initiation and early growth in all cases, and the superiority of the cast alloys was almost entirely due to a greater resistance to crack initiation. Macroscopic crack-growth rates were found to be essentially independent of composition and microstructure. A 1-min tension hold time per cycle produced a significant reduction in cyclic life in all cases except for the as-cast high Ni alloy. The decrease in life appeared to be associated with the initiation of cracks from localized deformation at grain boundaries.

Fractographic Study of Fatigue Crack Propagation

Fractographic studies were carried out on fatigue fracture of aluminum alloys and carbon steels. Special attention was given to the correlation between striation spacing and macroscopic crack growth rate and to the effect of stress change on the crack growth rate as determined by striation spacing measurements. Also the mechanism of formation of striations and tire tracks was examined by means of precisely matched electron fractographs of matching fracture surfaces. Some remarks are also made on the mechanisms of fatigue crack propagation.

Effect of thermomechanical processing on fatigue crack propagation

The effect of thermomechanical processing on fatigue crack propagation (FCP) is examined for 70/30 brass and 305 stainless steel. It is found that grain size and cold work induced changes in yield strength, ductility, and preferred orientation have a minor effect on FCP. Rather, cyclically stabilized properties of material in the crack tip plastic zone are believed to control the FCP process. Although mechanical processing fails to significantly alter the rate of FCP, it is apparently responsible for the unique fracture path observed in specimens oriented at an angle(A) to the rolling direction. Deviation of the crack path out of the plane of maximum net section stress is believed to be associated with mechanical fibering andJor crystallographic texturing effects. The complex fracture mode transition observed in cold worked 70/30 brass also is associated with the deformation texture of the starting material. For the cold-worked 305 stainless steel, striation spacings are correlated with the stress intensity range for specimens tested in the longitudinal, transverse, and “angle” orientations. Comparison of these data with corresponding macroscopic data indicate that an approximately one-to-one correspondence exists between macroscopic and microscopic fatigue crack growth rates over the range investigated.

Fractographic Studies of Fatigue Fracture of Metals

1. Correlation between crack growth rate and fracture surface topography.This section describes a part of the result of a co-operative investigation of Subcommittee on Fractography of Japan Society of Mechanical Engineers which is now under way with the aim of correlating microscopic fracture surface topography of different materials to macroscopic parameters. Materials studied are carbon steels, low alloy steels, stainless steels, cast steels, aluminum alloys, titanium alloys, copper alloys, bearing steels, case hardening steels and sintered alloys. Roughly speaking, fairly good correlation was obtained between spacing of fatigue striations and macroscopic crack growth rate with ductile materials in the range of growth rate around 0.03--3 microns/cycle (Fig.1). There was a tendency of striation spacing being larger than macroscopic rate in the range of lower growth rate and smaller in the range of higher growth rate. Above this range, dimples or stretched surfaces were predominant and, below this range, irregular striation-like markings(Fig.2) were main feature of the fracture surface, cleavage-like markings or grain boundary fracture being found occasionally.