Two-step Stress Research Articles

Very High Cycle Fatigue Damage of TC21 Titanium Alloy under High/Low Two-Step Stress Loading

Very high cycle fatigue (VHCF) tests were carried out under variable amplitude loading for TC21 titanium alloy. The first level of high amplitude loading was set as 950 MPa close to yield strength, and the second level of low amplitude loading was determined between 435 MPa and 500 MPa where fatigue cracks initiated at the specimen subsurface under constant amplitude. The results indicate that the high/low stress block significantly reduced the cumulative fatigue life of low stress amplitude, and the fatigue crack initiation site changed from the specimen subsurface under constant loading to the specimen surface under stress block. Based on continuum damage mechanics, the fatigue damage model of two-step stress block was established to estimate the fatigue damage process. The prediction of cumulative fatigue life generally agreed with the experimental data. The cumulative fatigue damage of the stress block was related to the stress amplitude and the cycle ratio, which determined the stress fatigue damage and its interaction damage. The surface crack initiation in the stress block accelerated fatigue damage of low stress amplitude, reducing the cumulative life.

Tunable Energy Absorbing Property of Bilayer Amorphous Glass Foam via Dry Powder Printing.

The research in this paper entails the design of material systems with tunable energy-absorbing properties. Hollow glass microspheres of different densities are layered using dry powder printing and subsequently sintered to form a cellular structure. The tunability of the bilayer foams is investigated using various combinations of hollow microspheres with different densities and different thickness ratios of the layers. The mechanical responses to quasi-static uniaxial compression of the bilayer foams are also investigated. These bilayer samples show different mechanical responses from uniform samples with a distinctive two-step stress-strain profile that includes a first and second plateau stress. The strain where the second plateau starts can be tuned by adjusting the thickness ratio of the two layers. The resulting tunable stress-strain profile demonstrates tunable energy absorption. The tunability is found to be more significant if the density values of each layer differ largely. For comparison, bilayer samples are fabricated using epoxy at the interface instead of a sintering process and a different mechanical response is shown from a sintered sample with the different stress-strain profile. Designing the layered foams allows tuning of the stress-strain profile, enabling desired energy-absorbing properties which are critical in diverse impact conditions.

Fatigue life and cyclic deformation behaviour of quenched and tempered steel AISI 4140 at two-step stress- and total-strain-controlled push–pull loading

Abstract The behaviour of steels at multi-step cyclic loading was explored up to now almost exclusively in fatigue-life-oriented investigations. Thus, only few works exist dealing with the cyclic deformation behaviour at two- and multi-step loading. Therefore, the cyclic deformation behaviour at two-step experiments with a single amplitude change (2-block experiments) and with multiple changes between two blocks of certain length and different amplitudes (multi-block experiments) was investigated in this work at the technically important steel AISI 4140 (German grade 42CrMo4).

Stress-dependent opening- and shear-mode propagation behavior of fatigue cracks in ultrafine-grained Cu fabricated by equal channel angular pressing

Constant and two-step stress amplitude tests were conducted to study the surface crack growth mechanism of ultrafine-grained copper processed by equal channel angular pressing (ECAP) in high- and low-cycle fatigue regimes. The crack growth direction varied depending on the location on the circumference of the round bar specimen and applied stress amplitude. On the surface where an intersection between the shear plane of final pressing and the specimen’s surface made an angle of 45° with respect to the loading axis, the crack grew with a 45° inclination to the loading axis at stress amplitudes ≥ 180 MPa. At stress amplitudes < 160 MPa, the crack grew nearly perpendicular to the loading axis. The physical background of various crack growth directions at high and low stresses is discussed based on the microstructural evolution caused by cyclic stressing and mixed-mode deformation at the crack tip.

Stress generation and evolution in oxide heteroepitaxy

Many physical properties of oxides can be changed by inducing lattice distortions in the crystal through heteroepitaxial growth of thin films. The average lattice strain can often be tuned by changing the film thickness or using suitable buffer layers between film and substrate. The exploitation of the full potential of strain engineering for sample or device fabrication rests on the understanding of the fundamental mechanisms of stress generation and evolution. For this study an optical measurement of the substrate curvature is used to monitor in situ how the stress builds up and relaxes during the growth of oxide thin films by pulsed laser deposition. The relaxation behavior is correlated with the growth mode, which is monitored simultaneously with reflection high-energy electron diffraction. The stress relaxation data is fitted and compared with theoretical models for stress evolution which were established for semiconductor epitaxy. The initial stage of the growth appears to be governed by surface stress and surface energy effects, while the subsequent stress relaxation is found to be fundamentally different between films grown on single-crystal substrates and on buffer layers. The first case can be rationalized with established theoretical models, but these models fail in the attempt to describe the growth on buffer layers. This is most probably due to the larger average density of crystalline defects in the buffer layers, which leads to a two-step stress relaxation mechanism, driven first by the nucleation and later by the migration of dislocation lines.

Experimental investigation of multi-step stress-relaxation-ageing of 7050 aluminium alloy for different pre-strained conditions

A novel insight into the whole two-step stress relaxation ageing process during T74 multi–step ageing treatment (120°C for 6h and subsequently 177°C for 7h), which is typically experienced by extra-large aircraft components that contain high residual stresses, has been established. Stress relaxation ageing (SRA) tests, tensile tests and transmission electron microscopy (TEM) were performed on AA7050 samples to determine the relationship between internal microstructure and macroscopic behaviour during the stress relaxation and precipitate evolution process. Samples were subjected to SRA at different initial stresses (220–360MPa) after being pre-strained to different extents (i.e. 0%, 1%, 3%). Room temperature tensile tests were then performed on interrupted SRA test specimens to examine the corresponding strengthening phenomenon. TEM was performed on a selection of peak–aged and T74 over–aged samples to study the precipitate distribution. At 120°C typical stress relaxation behaviour was observed and the data followed a logarithmic curve. Subsequently at 177°C, dislocation–creep dominated stress relaxation behaviour, with no apparent threshold stress, was observed. The absence of a threshold stress at 177°C may be attributed to the continuous over-ageing phenomenon. The effect of pre-deformation levels and initial stresses on SRA has also been investigated. Pre-stretching, which creates uniformly distributed dislocations, promotes stress relaxation and ageing. No significant influence of initial stress level on SRA was observed at 120°C, but noticeable effects were seen at 177°C. The calculated stress exponent n at 177°C is found independent of the initial stresses. These findings provide clear scientific guidance for residual stress reduction during the multi-step ageing process of AA7050 and provide the basis for residual stress prediction models.

Programming of One- and Two-Step Stress Recovery in a Poly(ester urethane).

This work demonstrates that phase-segregated poly(ester urethane) (PEU) with switching segments of crystallizable poly(1,4-butylene adipate) (PBA) can be programmed to generate two separate stress recovery events upon heating under constant strain conditions. For programming, two elongations are applied at different temperatures, followed by unloading and cooling. During the adjacent heating, two-step stress recovery is triggered. The results indicate that the magnitude of the stress recovery signals corresponds to the recovery of the two deformation stresses in reverse order. As demonstrated by further experiments, twofold stress recovery can be detected as long as the elongation at higher temperature exceeds the strain level of the deformation at lower temperature. Another finding includes that varying the lower deformation temperature enables a control over the stress recovery temperature and thus the implementation of so-called “temperature-memory effects”. Moreover, exerting only one elongation during programming enables a heating-initiated one-step stress recovery close to the deformation temperature. Based on these findings, such polymers may offer new technological opportunities in the fields of active assembly when used as fastening elements and in functional clothing when utilized for compression stockings.

S042022 変動負荷速度下におけるセラミックスの動的疲労強度分布特性と破壊メカニズム([S04202]セラミックスおよびセラミックス系複合材料(2))

This paper presents probabilistic approach for estimating the dynamic fatigue strength inceramics under variable stress rate. First, a probabilistic model was proposed on the basis of the slow crack growth concept in conjunction with two-parameter Weibull distribution. Second, the four-point bending test was carried out for alumina ceramics under two-step stress rate at room temperature. The experiment result was compared with the predictions to verify the validity of the model. The fracture surface of the specimen after the bending test was also observed. It is seen that mirror, mist and hackle in the vicinity of the surface acting maximum stress. Therefore, the dynamic fatigue fractures under variable stress rate caused by slow crack growth from an initial crack. We constructed fracture probability - bending strength diagrams (F-S diagrams) with the experimental results using the model. The F-S diagrams were in good agreement with Weibull plots of the bending strength.

Applicability of Applied Stress Estimation Method Based on the Micro Hardness of Fatigue Fracture Surface of Stainless Steel without Striations

In the failure analysis of fatigue fracture, the estimation of applied stress by examining the fatigue fracture surface is important in order to investigate the cause of failure. A fractographic method using striations has been established. Striations, however, can be observed only when a crack propagated at relatively high rate. In the case of failure after a long period of operation under low stress condition, striations are rarely observed. The estimation of applied stress is almost impossible in such a case. Therefore, an estimation method which can be applied in the low stress intensity region where striations are not observed is desirable. In this study, the applicability of micro hardness measured directly on the fatigue fracture surface as a parameter for the stress estimation was examined. The hardness of fatigue fracture surface was correlated with the stress intensity factor range. In addition, as an approach to service load, the relationship between the hardness of fatigue fracture surface and K value was investigated using repeated two-step stress.

Effect of Notch Shape and Absorbed Hydrogen on the Fatigue Fracture in Two-step Stress Test below Fatigue Limit

Effect of Notch Shape and Absorbed Hydrogen on the Fatigue Fracture in Two-step Stress Test below Fatigue Limit

Fatigue strength of small-notched specimens under variable amplitude loading within the fatigue limit diagram

Although the fatigue limit diagram is defined in principle for constant stress amplitude, it is often considered that fatigue failure would not occur, even in varying loading, if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small-notched specimens that fatigue failure occurred in some special cases of variable amplitude loading, even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two-step stress and repeated two-step stress patterns in which the first step stress was chosen to be equal to the fatigue limit with zero mean stress and a mean stress was superposed on the second step stress. A non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of a non-propagating crack and a mean stress change.

Delayed failure in a shock-loaded silicon carbide

The shock response of a silicon carbide has been investigated using the methods of plate impact, and monitored using manganin stress gauges mounted so as to be responsive to lateral stress. Close to the impact face, a two-step stress response is observed, indicating the presence of delayed failure at the impact face. As the shock front moves through the target, the failure wave appears to slow, before arresting between 4 and 6mm from the impact face. Measured shear stresses (ahead of the failure front) are in good agreement with the calculated elastic response, and with similar measurements made by both ourselves in other grades of silicon carbide and other authors. In gauge traces where the failure wave was not observed, a slight decrease in lateral stress (and thus a corresponding increase in shear strength) has been noticed. A similar response in some metals (in combination with recovery work in other ceramics) has led us to suggest that a degree of plastic deformation, in combination with a more brittle response, has occurred.

Fatigue life and cyclic deformation behaviour of quenched and tempered steel AISI 4140 at two-step stress- and total-strain-controlled push–pull loading

Abstract The behaviour of steels at multi-step cyclic loading was explored up to now almost exclusively in fatigue-life-oriented investigations. Thus, only few works exist dealing with the cyclic deformation behaviour at two- and multi-step loading. Therefore, the cyclic deformation behaviour at two-step experiments with a single amplitude change (2-block experiments) and with multiple changes between two blocks of certain length and different amplitudes (multi-block experiments) was investigated in this work at the technically important steel AISI 4140 (German grade 42CrMo4).

OS11W0286 Fatigue properties of high strength steel under two-step stress amplitude

In order to clarify the influence factor on high-cycle fatigue of high strength steel SUJ2, the rotating bending fatigue tests have been carried out up to 10^8 cycles at room temperature. According to the experimental results, in the constant stress condition, the fatigue crack is initiated from a specimen's surface. On the other hand, in the case of two-step stress amplitude condition the fatigue crack is initiated in the specimen's surface parts. Furthermore, when the fatigue damage under the two-step stress condition is evaluated by a modified Miner's low, it will be better way to evaluate the fatigue life with S-N curve by surface failure and that by internal one respectively.

435 高強度材の超長寿命疲労特性に及ぼす 2 段変動荷重の影響

435 高強度材の超長寿命疲労特性に及ぼす 2 段変動荷重の影響

739 高強度鋼の二段応力下における疲労特性

In order to clarify the influence factor on high-cycle fatigue of High strength steel SUJ2,the fatigue test have been carried out up to (10)^8 cycles under rotating bending at room temperature. According to the experimental results, in the case of failure under the conventional stress condition, there is only one crack initiation point. On the other hand, in the case of two-step stress condition there are plural crack initiation points. Furthermore, when the fatigue damage under the two-step stress condition is evaluated by Modified Miner's low, it should be added with S-N curve by surface failure and that by internal one respectively.

Two-step stress methodology for monitoring the gate oxide degradation in MOS devices

A recently presented two-step stress method for the analysis of the degradation of SiO2 films is reviewed. By provoking the oxide breakdown in two stages, the test methodology evaluates the degradation of the oxide from statistical breakdown data (without any assumption on the microscopic nature of the oxide degradation) and allows a direct comparison of different tests and oxides of different thickness. The degradation of 8 and 4.3 nm thick oxides is analysed and compared when they are subjected to constant-current and constant-voltage stresses. The results are compared with those obtained using conventional electrical tests. Special attention is given to the evaluation of the degradation using stress induced leakage current data.

Influence of a low field with opposite polarity to the stress on the degradation of 4.5 nm thick SiO 2 films

The effects on the oxide degradation of a low field of opposite polarity to that used during the stress have been analysed. Two different methods have been used: the analysis of stress-induced leakage current data and a recently proposed two-step stress method. The last procedure suggests a relaxation of the degradation process.

Monitoring the degradation that causes the breakdown of ultrathin (&lt;5 nm) SiO&lt;sub&gt;2&lt;/sub&gt; gate oxides

The degradation of ultrathin oxides subjected to constant-current stresses is analyzed using two independent procedures. First, the injected charge to breakdown is estimated from the stress-induced leakage current (SILC) evolution during the stress. Second, the degradation that leads to the breakdown is directly measured using a two-step stress test. The evolution of the SILC during constant-current stresses proceeds at a rate that decreases with time. Moreover, under low current density stress conditions the normalized SILC at breakdown is no longer constant. However, our two-step test methodology shows that the degradation of the oxide evolves roughly linearly until the breakdown. These apparently contradictory results can be reconciled assuming that the degradation at breakdown is independent of the stress conditions and using the initial SILC generation rate to calculate the charge-to-breakdown by linear extrapolation. The implications for the use of SILC data as a degradation monitor are discussed.

Relation between defect generation, stress induced leakage current and soft breakdown in thin (

The normalized stress induced leakage current (SILC) measured when the oxide is subjected to low level constant-current stresses shows a tendency towards saturation at large charge fluences. To investigate the origin of this saturation, the degradation of the oxide has been analyzed using two independent methods: SILC data analysis and a two-step stress test. The results show that, although under low stress currents the SILC saturation is observed, the degradation (i.e., the generation of defects) proceeds until the soft breakdown (SBD) event takes place. The implications for the use of SILC data as degradation monitor are analyzed.