Low High Test Research Articles

Cumulative Damage Rule under Two-Step Sliding Wear of White Metal

Sliding wear experiments were performed under two-step compressive loading in order to clarify the effect of load fluctuation on the wear life of the white metal bearing. Cumulative damag under two-step wear experiment cannot be predicted by a linear damage rule, since it is affected by the load sequence. The value of cumultive damage is less than unity in the high-low test, on the other hand in the low-high test the reverse is true. The constitutive equation for the wear test was proposed.The equation is capable to predict wear lives under constant loading as well as wear damages in the two-step sliding wear experiment.

高温強度 ひずみ範囲分割概念に基づくクリープ疲労損傷則の３１６ＬＣ鋼への適用

The creep-fatigue life prediction model previously proposed for evaluating Mod.9Cr-1Mo steel under variable strainings was applied to 316LC steel. In the proposed model the small crack growth curves in a smooth specimen are predicted in accordance with the applied straining conditions by using the material parameters and they are used as the damage accumulation curves. The feature of this model is its potential to evaluate the material damage and the residual life under arbitrary variable creep-fatigue strainings. For examining whether or not it is possible to determine the material parameters for 316LC steel based on the same creep-fatigue life prediction model as for Mod.9Cr-1Mo steel, creep-fatigue tests, crack growth tests, and high-low (HL) and low-high (LH) type two-step variable straining tests were conducted with PP (fast-fast) and CP (slow-fast) type strain waveforms at 700°C and 800°C. It was suggested that the crack initiation life cannot be negligible not only in PP type low strain ranges but also in CP type low strain ranges whereas it is negligible in CP type low strain ranges for Mod.9Cr-1Mo steel, and the model was partly modified to describe the initial crack growth behavior of 316LC steel. The remaining life in HL and LH tests was well predicted by the modified proposed model.

CREEP-FATIGUE DAMAGE RULE UNDER VARIABLE STRAINING BASED ON STRAIN RANGE PARTITIONING CONCEPT

The outline of the creep-fatigue life prediction model proposed by the authors and its applicability to the life prediction under variable strainings were summarized. In the model, crack growth curves in smooth specimens were predicted by the strain range partitioned crack growth rate equations, and these crack growth curves were used as damage accumulation curves. The sequential loading effect on creep-fatigue properties was evaluated considering the strain history dependency of cumulative damage curves. Results of two-step sequential straining tests such as High-Low and Low-High tests on Mod. 9Cr-1Mo, SUS304 and 316LC steel were presented and it was shown that the proposed model can be more successfully applied to the creep-fatigue life prediction under variable strainings than the conventional linear damage rule. Furthermore, in order to certify the validity of the model, the correspondence between predicted crack growth behaviors and experimentally obtained behaviors was examined. Surface crack growth behaviors in smooth bar specimens were observed for Mod. 9Cr-1Mo and 316LC steel and compared with those predicted. It was found that the predicted crack growth curves corresponded well with the relationships between the maximum crack length and the life ratio. Consequently, it was concluded that the proposed model can evaluate the material damage and the residual life under variable creep-fatigue strainings.

Life Prediction Methodology for Composite Structures. Part I—Constant Amplitude and Two-Stress Level Fatigue

A strength-based wearout model is presented for predicting the residual strength and life of composite structures subjected to constant amplitude or two-stress level fatigue loadings. It is assumed that the structure undergoes proportional loading, that its residual strength is a monotonically decreasing function of the number of loading cycles, and that both the life distribution due to continuous constant amplitude cycling and the residual strength distribution after an arbitrary load history may be represented by two parameter Weibull functions. The model also incorporates a "cycle mix factor" to account for the degradation of strength and life that may be associated with frequent changes in the stress amplitude of the loading. Experimental results from two-stress level, uniaxially loaded laminates are used to evaluate the model's predictive capability for sequencing effects, as in a low-high or high-low test, and for cycle mix effects, as in a low-high-low-high repeating test. Good correlation between theory and experiment is obtained for all loadings and laminates studied.

A new high-temperature creep-fatigue damage rule based on the strain range partitioning concept

A new life prediction model based on the strain range partitioned crack growth rate equations is presented which can evaluate the material damage and the residual life under variable creep-fatigue strainings. Two-step sequential creep-fatigue straining tests such as High-Low and Low-High tests were conducted on Mod. 9Cr-1Mo steel and type 316LC stainless steel in order to examine the validity of the proposed model. It was found that the proposed model can be more successfully applied to the creep-fatigue life prediction under variable strainings than the conventional life prediction methods.

Creep-Fatigue Properties of Mod.9Cr-1Mo Steel Under Variable Straining

Two-step variable straining tests, which included CP- (slow-fast) type and PC- (fast-slow) type tests, were conducted at 600°C in air for Mod.9Cr-1Mo steel. In the tests, CP and PC-type strain range level varied from high to low for the high-low test and from low to high for the low-high test. The deviations of the experimental data from the linear damage rule prediction were observed. These experimental results and the effect of the strain waveform on the creep-fatigue properties under variable straining were well explained by the life prediction model previously proposed by the authors in which the strain range partitioned crack growth rate equations were applied.

変動ひずみ条件下におけるＭｏｄ．９Ｃｒ‐１Ｍｏ鋼のクリープ疲労特性に及ぼすひずみ波形の影響

For the purpose of clarifying the effect of strain waveform on creep-fatigue property under variable straining, two-step variable straining (high-low and low-high) tests were conducted on Mod. 9Cr-1Mo steel at 600°C in air under PC (fast-slow) and CC (slow-slow) type strain waveforms, and the experimental results were compared with the previously reported results obtained under PP (fast-fast) and CP (slow-fast) type strain waveforms. In both high-low and low-high tests, 1.0% was chosen as the higher strain range, and 0.8% and 0.4% as the lower strain ranges.The main results obtained in the present study are as follows.(1) Independent of strain waveform, the high-low and low-high test results could be predicted by the linear damage rule (LDR) when the lower strain range was 0.8%, whereas the deviation of the experimental data from LDR prediction was found when the lower strain range was 0.4%.(2) Such a deviation of the experimental data from LDR prediction was similar to that in CP type strain waveform, but was much smaller than that in PP type strain waveform.(3) The previously proposed creep-fatigue life prediction model under variable straining for the PP and CP data can be successfully applied to PC and CC type strain waveforms, suggesting that in Mod. 9Cr-1Mo steel 2(a0)pp=2.5×10-4mm and 2(a0)cp≈2(a0)pc≈2(a0)cc≈10-3mm.

How to increase the life of moulds that work at high temperature and delay the appearance of thermal fatigue: Cugat, M. Deform. Metal. 1990 16, (Special Edition 2), 19–24 (in Spanish)

A new life prediction model based on the strain range partitioned crack growth rate equations is presented which can evaluate the material damage and the residual life under variable creep-fatigue strainings. Two-step sequential creep-fatigue straining tests such as High-Low and Low-High tests were conducted on Mod. 9Cr-1Mo steel and type 316LC stainless steel in order to examine the validity of the proposed model. It was found that the proposed model can be more successfully applied to the creep-fatigue life prediction under variable strainings than the conventional life prediction methods.

Transient cyclic stress-strain response and cumulative damage in Cu-16at.% A1 single crystals fatigued under variable straining

In order to investigate the transient cyclic stress-strain response and cumulative damage behavior of Cu-16at.% A1 alloy, single crystals with a common single slip orientation were cyclically deformed under variable straining. Studies were made of hardening behavior, strain bursts, hysteresis loops, friction and back stresses, dislocation structures, damage accumulation and life behavior. During ascending step tests, hardening naturally occurred and the strain was found to be accommodated not by increasing the plastic strain of current active slip bands but by increasing the volume fraction of active slip bands. During descending step tests, however, the plastic strain per individual slip band decreases with decrease of strain amplitude. The transient response after a reduction of strain amplitude in Cu-16at.% Al single crystals is not softening as commonly occurs but hardening. Hardening in this case seems to be caused both by hardening of individual slip bands and by the decrease of the volume fraction of active slip bands, which is also an indication of hardening. With regard to damage behavior, in low-high step tests, the number of cracks was found to increase remarkably during high amplitude cycling whereas the number of cracks did not change much after the step to low amplitude cycling during a high-low test. Miner summations of fatigue life for H-L step tests were found to be smaller than unity because the stresses associated with the structure produced at high amplitude were high and the plastic strain localized in active slip bands increased with the decrease of the volume fraction of active slip bands, caused by continuous cycling at the reduced strain amplitude. In L-H step tests, the Miner summations for fatigue life were found to be greater than unity because the hardening and the damage accumulation associated with low amplitude cycling were relatively insignificant as compared to that at high amplitude and fresh damage developed during the high amplitude sequence. The Miner summation behavior in these alloy single crystals was found to be more typical of that in commercial metals, and different from behavior in copper.

Fatigue deformation behaviour and dislocation structures under two-step cyclic loading.

Fatigue tests with two-step cyclic loading (Low-High and High-Low stress change tests) were carried out on mild steel and stainless steel specimens with different slip characters. Plastic strain range Δep changed rapidly with the stress change, and it was investigated from a stand-point of rearrangement of dislocation structures, and compared with the analytical values by dislocation dynamics. The dislocation structures were rearranged easily in the Low-High stress change test. On the other hand, in the High-Low test, the specimen fractured before full rearrangement of dislocation because of surface micro-cracks growth resulting from initial high stress amplitude, but the rearrangement was performed sufficiently by the removal of cracks and by sufficient cycling of second low-stress amplitude. The rate of dislocation rearrangement was expressed clearly by the change of cell structure area and was higher in mild steel because of its ease of cross-slip.

Crack initiation mechanisms in copper polycrystals cycled under constant strain amplitudes and in step tests

Crack nucleation mechanisms at plastic strain amplitudes of 10 −3 and 10 −4 are reported for polycrystalline pure copper. In specimens cycled at 10 −3, cell structures are formed and, as well known, cracks nucleate at the grain boundaries. At the lower amplitude the dislocation structures consist of persistent slip bands (PSBs) and loop patches. Nevertheless, while some cracks do nucleate in the PSBs, the grain boundaries are the preferred site, rather surprisingly, for both crack nucleation and crack propagation. Crack nucleation mechanisms for low-high and high-low step tests using these two amplitudes were also explored. In low-high tests, for a rather wide range of fatigue life fractions the subsequent cycles at the high amplitude tend to ignore the damage produced first at the low amplitude. The potency of the damage interaction is much greater in high-low tests for the conditions studied here.

ひずみ振幅変動に伴って生ずる繰返し応力‐ひずみ挙動と疲労寿命

The effects of slip character in metal polycrystals on the cyclic hardening and softening behaviors were investigated under the multiple repeated tests at two strain levels, and the fatigue lives of copper and α-brass in the double repeated fatigue tests were considered.The main results obtained are as follows:(1) The stress range after changing the strain amplitude approaches gradually to the inherent value of the virgin material for S10C and copper which show a wavy slip mode. On the other hand, for α-brass having planar slip character, the stress range approaches to the same value as that of the virgin material as the strain level is increased. However, as the strain level is decreased, the stress range reaches the value larger than the inherent value of the virgin material after the transient increase in the early stage. These results can be explained by considering the slip character of materials and the aging effect in α-brass.(2) For S10C and copper, the stress range for a given plastic strain range just after the change in the strain amplitude corresponds to the state of hardening just before the change. On the other hand, for α-brass the former does not always correspond to the latter. This characteristic property for the cyclically deformed α-brass can be explained by considering both the slip character and the aging effect of this material.(3) The cyclic stress-strain relation for α-brass of planar slipping mode is sensitively affected by its strain history, while for copper of wavy slip character this relation is scarcely dependent on it.(4) The double repeated fatigue tests show that the cumulative cycle ratio does not depend on the stressing sequence for copper, while for α-brass it is larger than one in the High-Low test, but less than one in the Low-High test. These test results correspond well with the cyclic strain hardening and softening behaviors shown in (1) and (2).

Fatigue Damage of Aluminum Alloy at High Temperature

Temperature is one of the most important factors which affect the fatigue strength, and many researches have hitherto been conducted on the fatigue strength at various temperatures, but little is known about the fatigue strength under fluctuating stress amplitude at high temperature. In this research, notched and unnotched specimens made of aluminum alloy 7075-T6 were tested at room temperature, 200°C and 300°C. Constant stress fatigue test and Low-High and High-Low type two-stress fluctuating fatigue tests were conducted for each test condition. Discussion was made on the cumulative cycle ratio at high temperature. It has been claryfied that the cumulative cycle ratio is less than 1.0 in the High-Low test and it is greater than 1.0 in the Low-High test at room temperature and 200°C. But, at 300°C, the cumulative cycle ratio is greater than 1.0 in the High-Low test and it is less than 1.0 in the Low-High test. The test result can be explained by the variation of the metallurgical structure of the used material caused during the experiment. An additional experiment was conducted to show the appropriateness of the explanation.

Fatigue Crack Propagation Behaviors of 18-8 Austenitic Steel under Double Repeated Stresses

Rotating bending fatigue tests under constant stress and two stress levels were carried out on notched 18-8 austenitic steel at room and elevated temperatures, and fatigue crack propagation behaviors during the tests were investigated. The cumulative cycle ratios were discussed from the experimental results.The temperature dependence of fatigue fracture mechanism was studied by means of electron fractographic observations.The main results obtained are as follows:(1) On High-Low tests at room temperature the delay time in continued cracking was observed after stress change, while at 600°C this phenomenon was not observed.(2) Cumulative cycle ratios were almost unaffected by stress sequence. The fatigue life under double repeated stresses should be discussed not only from the delay time, but also from the differences in crack growth behaviors at low and high stress levels.(3) The regular striations peculiar to fatigue fracture were detected on the fracture surface tested at elevated temperature, while at room temperature these were not observed. This fact might be explained by the presence of the strain induced martensite detected at room temperature test.