Tension Hold Research Articles

交流電位差法を用いたＳＵＳ３０４とＴｉ‐１７の荷重制御下およびき裂中央開口変位制御下のクリープ疲労き裂伝ぱ試験

The a.c. electric potential method was adopted for the crack length measurement in high temperature fatigue tests. The method was found easy to get the same relationship between the electric potential and the crack length as that in the d.c. potential method, when a constant a.c. of the relatively low frequency of 93Hz was supplied to the plate specimens of SUS304 stainless steel and titanium alloy, Ti-17, with a center crack or double edge cracks. Fatigue crack propagation tests were conducted on SUS304 at 600°C and on Ti-17 at 482°C under the trapezoidal waveforms with tension hold time in the crack center opening displacement (CCOD) control as well as the load control conditions. A creep-dominated, time-dependent relationship between the crack propagation rate, dl/dN, and the creep J-integral range, ΔJc, in the CCOD control condition was in good agreement with that in the load control condition, although the CCOD control test was a kind of ΔJc-decreasing tests. The titanium alloy, Ti-17, showed a lower bound of the available dl/dN-ΔJc data band of some heat resisting steels.

ＳＵＳ３０４鋼の高温低サイクル疲労き裂伝ぱとフラクトグラフィ

High temperature low-cycle fatigue crack propagation behaviour of SUS 304 stainless steel under typical four types of reversed loading patterns (P-P, P-C, C-P and C-C type) was investigated, and the results were discussed in the light of fracture mechanics and electron fractography. The stress intensity factor range ΔK failed to describe the crack propagation behaviour adequately. In the case of loading with tension hold (i.e. C-P and C-C type), the crack propagation rate was successfully correlated with the modified J-integral J and agreed with the static creep crack propagation rate. The crack propagation rate for all of the four types of loading was successfully correlated in terms of the cyclic J-integral range ΔJ, and it was unnecessary to separate ΔJ into ΔJf and ΔJc. The crack propagation rate for the types of loading without tension hold or with short tension hold time (i.e. P-P and P-C type, and C-P and C-C type with comparatively short tension hold time) was successfully correlated with ΔJf, and that long tension hold time (i.e. C-P and C-C type with comparatively long tension hold time) was successfully correlated with ΔJc. The fracture surfaces of the case where the crack propagation rate was correlated with ΔJf were predominantly transgranular fatigue type, that is, fracture morphology and correlating fracture mechanics parameter (ΔJf) were of the same type (fatigue type). On the other hand, those of the case where crack propagation rate was correlated with ΔJc were transgranular fatigue type or intergranular creep type or mixed type depending on the condition, that is, fracture morphology and correlating fracture mechanics parameter (ΔJc) were of different type in some cases.There was no significant effect of creep-fatigue interaction on the crack propagation under the combined creep and fatigue conditions, and crack propagation may be assessed by a simple summation of fatigue component and creep component.

ＳＵＳ３０４鋼の高温低サイクル疲労における微小き裂の発生と初期成長の観察

An accurate prediction of fatigue fracture at elevated temperatures depends on the elucidation of the small crack initiation and propagation behavior. Experimental studies on the small cracks, however, have been quite limited. In this study, two types of high-temperature low-cycle fatigue tests, C-P type of a trapezoidal stress waveform with tension hold and P-P type of a high-frequency triangular stress waveform, were conducted by using smooth specimens of 304 stainless steel, and the behavior of initiation and early growth of surface small cracks was observed continuously during the tests with a high-temperature microscope. Furthermore, a detailed study was made by the interrupted tests with the replicas or scanning electron microscopy. The results obtained are summerized as follows. (1) The crack of C-P type fatigue initiates at a grain boundary between two triple points as it opens uniformly and gradually. (2) The crack growth to the adjacent grain boundary in the early stage of C-P type fatigue is caused through the same process as the crack initiation, showing a stepwise and discontinuous manner. (3) The process mentioned at (1) is the process of growth and coalescence of voids on the grain boundary owing to the grain boundary sliding. (4) The crack initiation of P-P type fatigue takes place at the intersection of slip bands and a grain boundary, and most of the cracks propagate along the grain boundaries in the early stage of fatigue. (5) The crack growth rate of P-P type fatigue decreases suddenly when the crack tip comes near to the triple point. (6) The fracture mechanisms of C-P type fatigue and P-P type fatigue are quite alike to that of creep and that of low cycle fatigue at room temperature, respectively.

An Analysis of the Fatigue/Creep Behavior of 304 Stainless Steel Using a Continuous Damage Approach

Abstract A recently suggested procedure for estimating the life of a material subjected to fatigue-creep loading is applied to analyze the behavior of Type 304 stainless steel on the basis of the experimental data already available in the literature. The predictive technique is based on a combination of the two separate damage functions for fatigue and for creep. The effect of the tension hold time in interspersed creep-fatigue loading on the material life is taken into account by considering the relaxation stress characteristics during the hold period. The technique also permits the evaluation of damage (or healing) due to a compression hold time. The correlation between predictions and available experimental results obtained on Type 304 SS from 538 to 649° C under various loading conditions is discussed.

Creep-fatigue evaluation of austenitic stainless steel for SNR 300 - present status and future efforts

For the investigation of the creep-fatigue behaviour of X6 CrNi 1811 (similar to AISI Type 304 SS) sequential creep-fatigue tests, fatigue hold time experiments as well as fatigue relaxation tests have been performed and evaluated by different life-time prediction methods. The linear damage accumulation rule gave a satisfactory description only for a limited range of experimental parameters. First results for an extrapolation to 240h tension hold time indicate that, compared with the results of other life-time prediction methods, the linear damage accumulation rule appears to be a suitable tool for design purposes.

Microstructural Development and Cracking Behavior of AISI 304 Stainless Steel Tested in Time Dependent Fatigue Modes

Transmission and scanning electron microscopy were used to study the substructural development, crack initiation and crack propagation of AISI 304 stainless steel tested in low-cycle fatigue with various hold times. Tests at 593°C, a strain rate of 4 E-03 s−1 and strain ranges of 0.5, 1.0 and 2.0 percent were interrupted at various fractions of the fatigue life, both during rapid hardening and during saturation. Cells developed during the first few percent of the fatigue life, depending on a threshold stress value, below which cells were not observed. The cell size had a different dependence on the relaxed tensile stress during rapid hardening than during saturation. A work-hardening model relating the peak saturation stress to strain and cell size also applied during the late stages of rapid hardening. The number of cycles to initiate a crack of one grain diameter and to propagate the crack to failure were proportionally reduced for one minute tension holds. For a 60 minute tension hold, crack propagation occupied a much smaller fraction of the fatigue life.

Effect of strain wave shape on high temperature fatigue life of a type 316 steel and application of the strain range partitioning method

Effects of strain rate and strain wave shape on low-cycle fatigue life of a Type 316 stainless steel were investigated at 600 and 700 °C. A great reduction in the fatigue life corresponded with a variation in the fracture mode. Especially extensive grain boundary microcracks such as wedge- and cavity-type cracks were observed in slow-fast sawtooth wave shape tests and in tension hold time tests, respectively. The test results were analyzed by the strain range partitioning method proposed by Manson, Halford and Hirschberg. In applying the method, a new technique of partitioning the inelastic strain range was proposed and used. Four component strain range vs life relationships were dependent on the testing temperature between 600 and 700 °C. However, the difference between 600 and 700 °C was eliminated by using normalized component strain range by dividing by ductility of creep rupture test and tensile test.

Accelerated creep-fatigue crack propagation in thermally aged type 316 stainless steel

Crack propagation experiments performed on thermally aged 316 stainless steel at 593°C have shown that an acceleration in the rate of crack propagation occurred when extended tension hold periods were included in the creep-fatigue cycle. The results demonstrate that the onset of accelerated crack propagation was coincident with the linkage of intergranular cavities associated with grain boundary precipitates and was accompanied by a change in failure mode. Analyses of the quantitative values obtained for average cavity spacing, \\ ̄ gl and size, ḡr, show that the critical conditions necessary for cavity linkage were satisfied when the crack tip opening displacement equaled or exceeded a value given by a( \\ ̄ gl − \\ ̄ gr). The crack propagation rate was found to revert to the continuous cycling value when the duration of the hold period was reduced. The results suggest that both intergranular precipitates and creep-fatigue cycling must be present to produce the accelerated crack propagation.

Hold-time effects in high temperature fatigue

A mechanism for creep-fatigue interaction is proposed. It relates to the nucleation of the cavities and the growth and coalescence of cavities in the grain boundaries under conditions of cyclic loading. It is shown that when certain conditions are met one hold-time cycle is enough to nucleate cavities in the grain boundary. The specifics of this cycle are identified (for example it should be unbalanced in favor of the tension hold). The cavitation damage can be removed by imposing a long compression hold on the specimen. A second condition for successful interaction between creep and fatigue is that the cavities should grow and link with their neighbors. This condition is satisfied when the crack-tip-opening displacement exceeds a critical value. The grain size, and the average size and spacing of the carbides in the boundary are the important microstructural parameters in the proposed model.

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.