Decrease In Crack Growth Rate Research Articles

Increasing the Reliability of Aluminum Wirebonds by Thermal Treatments: Analysis and Implementation

In this work we studied the effect of a post-bonding thermal treatment on thick aluminum wirebonds lifetime in IGBT power modules during power cycling at different temperature swings. A reliability improvement of up to 44% is demonstrated for the bonds located at the center of the chip and seeing the highest thermal treatment temperature. The physical phenomena occurring during the thermal treatments and during power cycling are studied using microstructure analysis (Scanning Electron Microscopy techniques) and nanoindentation. Softening of the material results from recrystallization and/or recovery during thermal treatment, whereas during power cycling the thermal treated material hardens due to strain accumulation and defect formation. Despite the hardening our data suggests a decrease in crack growth rate at the second half of the testing time. We implemented an approach for applying thermal treatments during typical operating conditions in a converter by controlling the off-state losses of a device in a multichip configuration. The stability and the limits of this approach are studied experimentally and described by an electrothermal model. This study allows us to conclude on a realistic process window for online thermal treatments, by considering both material constrains and practical temperature control limits.

Crack Growth and Tolerance of Stainless Steel Canisters in the Marine Environment

Most of the dry storage cask systems that are used to contain nuclear fuel are austenitic stainless steel canisters. Past experience suggests that stainless steel is susceptible to stress corrosion cracking (SCC) in the presence of chloride salts. A crack growth rate (CGR) model has been developed and applied to evaluate the crack depth in stainless steel canisters over the timeframe of the storage at independent spent fuel storage installations. This study focuses on stainless steel canisters for dry storage systems in the two nuclear power plants in Taiwan. The crack depth was first evaluated using the CGR model, site climate data, and canister surface temperature. The critical crack size and depth were then determined from the structural tolerance assessment of the canister shells. It was found that the variations in the thermal and hydraulic properties of dry storage canisters produce large variations in the SCC initiation time but do not affect the surface temperature in the range of 55–60 °C. The CGR at the SCC initiation is high and the growth of flaws is significant. The surface temperature and CGR decrease with time. The total crack depth therefore may not vary greatly as a function of SCC initiation time. Overall, dry storage canisters show high structural tolerance to crack size and depth.

An experimental methodology to quantify the resistance of grain boundaries to fatigue crack growth in an AA2024 T351 Al-Cu Alloy

The growth behaviors of the short fatigue cracks initiated from the micro-notches, 30×15µm2, fabricated using FIB in some coarse grains, were investigated across grain boundaries in an AA2024-T351 Al-Cu-Mg alloy in four-point bend fatigue at 20Hz, R=0.1, and room temperature in air. The resistance to short crack growth at the grain boundary that the crack crossed was quantified from the measured decrease in growth rate at the boundary. It was found that the decrease in crack growth rate followed a linear relationship with the twist component (α) of crack deflection at the boundary, as revealed in EBSD and FIB experiments. The resistance of grain boundaries to short crack growth was revealed to a Weilbul-type function of α, which quantitatively verified Zhai's minimum α criteria for short fatigue crack growth across grain boundaries in planar slip alloys.

The influence of aqueous environment of coolant of power plants on cyclical crack resistance of steels

The results of investigations of cyclic corrosion crack resistance of carbon steel of grade 20 and low-alloyed pipe steels of grade 15GS and 12Kh1MF are presented. The variants of water standard conditions of power plants are used as test environments. A significant activating impact of the aquatic environment on the kinetics of growth of fatigue cracks of the steels under study is shown. The most significant corrosive effect is observed when tested in an aqueous environment with addition of organic acid. In the low-frequency range of cyclical loading (0.04–0.0008 Hz), the frequency does not influence the characteristics of the cyclic corrosion crack resistance of steels. To eliminate the influence of stress cycle asymmetry on the diagram of corrosion-fatigue crack resistance of steels, it was proposed to use as a parameter of crack growth rate (CGR) the effective range of stress intensity factor (SIF), functionally associated with the factor of stress cycle asymmetry. When the temperature of the medium is increased from 80 to 150°C and to 280°C, the form of kinetic diagram of cyclic corrosion crack resistance is changed, resulting in a decrease in crack growth rate at the middle and upper sections of the diagram.

Specific Features of the Growth of Fatigue Cracks in 36G2S Steel of Drill Pipes After the Recovery Heat Treatment

We study the influence of repeated normalization of operating 36G2S steel of drill pipes on their structure, fatigue-cracks growth, and fractographic features of fracture. It is shown that the crack-growth rate decreases and brittle elements in the fracture surfaces disappear after the removal of the texture of ferritic precipitates and carbides from grain boundaries as a result of the repeated heat treatment of steel. At the same time, 5% of all investigated specimens do not exhibit any positive influence of the recovery heat treatment on the fatigue crack growth rate in the operating metal, which means that it is reasonable to perform the preliminary (prior heat treatment) control of the engineering state of the metal according to the parameters sensitive to scattered damage.

Thermomechanical fatigue crack growth in a cast polycrystalline superalloy

Thermomechanical fatigue (TMF) crack growth testing has been performed on the polycrystalline superalloy IN792. All tests were conducted in mechanical strain control in the temperature range between 100 and 750 °C. The influence of in-phase (IP) and out-of-phase (OP) TMF cycles was investigated as well as the influence of applying extended dwell times (up to 6 hours) at the maximum temperature. The crack growth rates were also evaluated based on linear elastic fracture mechanics and described as a function of the stress intensity factor KI. Without dwell time at the maximum temperature, the crack growth rates are generally higher for the OP-TMF cycle compared to the IP-TMF cycle, when equivalent nominal strain ranges are compared. However, due to the fact that the tests were conducted in mechanical strain control, the stress response is very different for the IP and OP cycles. Also the crack closure level differs significantly between the cycle types. By taking the stress response into account and comparing the crack growth rates for equivalent effective stress intensity factor rages ΔKeff defined as Kmax − Kclosure, very similar crack growth rates were actually noticed independent of whether an IP or OP cycle were used. While the introduction of a 6 hour dwell time significantly increased the crack growth rates for the IP-TMF cycle, a decrease in crack growth rates versus ΔKeff were actually seen for the OP-TMF cycle. The fracture behaviour during the different test conditions has been investigated using scanning electron microscopy.

Influence of continuum precipitates on intergranular fatigue crack growth of a P/M nickel-based superalloy

This paper examines the role of microstructure, in particular, the size and volume fraction of secondary (γ′s) and tertiary (γ′t) γ′ precipitates, on intergranular crack growth in P/M IN100. A series of different heat treatments were carried out on the as-received material to vary the γ′statistics through control of the different features of the solutioning, stabilization, and aging parts of the heat treatment cycles. Dwell fatigue crack growth experiments have been performed on specimens having as received as well as heat treated microstructures at 650°C and 700°C in air with a 0.5Hz loading frequency with and without a dwell time. Results of the as received material show that the dwell crack growth rate, while dependent on the temperature level, is independent of the hold time period. For the modified microstructures tested at 650°C, the dwell crack growth rate is shown to be sensitive to γ′ variations in the surrounding continuum. This sensitivity was illustrated by correlating the crack growth rate and the corresponding continuum yield which shows the dwell crack growth rate decreases with the increase in the yield strength. The influence of the continuum yield on dwell crack growth was first examined numerically by modelling an intergranular crack path surrounded by a continuum region represented by a coarse crystal plasticity model while the far field continuum is represented by an internal state variable model. The grain boundary path considers grain boundary sliding and a critical sliding length which is implemented as a fracture criterion. Results of this model show that the grain boundary sliding is influenced by the viscous strain developing in the surrounding continuum. An increase in the continuum yield strength accompanied with a lower viscous strain results in higher constraint on the grain boundary sliding and thus a decrease in the crack growth rate. This observation was further examined by calculating the individual contributions of the solid solution, γ′s, and γ′t to the total yield strength of the continuum.

Modeling of mode-I fatigue crack growth in quasibrittle structures under cyclic compression

A cyclic cohesive zone model is proposed to simulate the mode-I crack growth in quasibrittle structures under compressive fatigue. The constitutive behavior of the cohesive elements is formulated for both tension and compression regimes. A strain-softening cohesive law is adopted for the tension regime whereas a plastic-type cohesive behavior is considered for the compression regime. It is shown that the proposed model is able to capture some essential fracture behaviors of quasibrittle structures under compressive fatigue, which include the onset of fatigue crack growth, the gradual decrease in crack growth rate, and the exhaustion of residual tensile stress over the cycles. Based on a fracture process zone (FPZ)-equivalence principle, it is further shown that the existing kinetics equation for tensile fatigue crack can be extended to the crack growth under cyclic compression.

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.

疲労き裂進展抑制鋼の溶接継手疲労特性

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels due to the high stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (F/M steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-harf to one-tenth in the thickness direction, compared to conventional steel. As a result, the fatigue life of an out-of-surface gusset welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low stress ratio, the region near the weld toe is under a high stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.

Theoretical-experimental model for predicting crack growth rate in structural alloys under combined action of fatigue and creep

Holding periods of 300 and 3600 s in a trapezoidal load cycle are shown to increase the crack growth rate dozens of times for alloy ÉP962 and several-fold for alloy ÉP742 at a temperature of 973 K. It is demonstrated that in the presence of the first portion on the creep crack growth diagram, whereon the crack growth rate decreases, the crack growth kinetics for a trapezoidal load cycle can be predicted using the hypothesis of the linear summation of fatigue and creep crack growth rates provided that the peculiarities of the first portion of the creep crack growth diagram are taken into account. Empirical approaches are proposed for determining the mean crack velocity in the first portion of the creep crack growth diagram.

An Experimental Study of the Crack Growth Behavior of 16MnR Pressure Vessel Steel

An experimental investigation was conducted on the crack growth behavior of a pressure vessel steel, 16MnR, in ambient air. Standard compact tension specimens were subjected to Mode I loading with several R-ratios and loading amplitudes. Three circular notch sizes ranging from very sharp notch to blunt notch were used. In addition to constant amplitude loading, experiments were conducted to study the influences of overload and loading sequence on crack growth. The results show that the R-ratio has an insignificant influence on the crack growth of the material. The size of the notch together with the R-ratio and loading amplitude has a great influence on the early crack growth from the notch. A single tensile overload during a constant amplitude loading experiment retards the crack growth significantly. Right after the application of an overload, the crack growth rate is higher than that of the stable crack growth observed in the constant amplitude loading. The crack growth rate decreases and reaches a minimum value before it gradually increases and reaches the stable crack growth curve. In high-low sequence loading with the maximum load in the second step lower than that of the first loading step, the preceding higher constant amplitude loading results in a significant crack growth retardation in the second loading step. This phenomenon is similar to the effect of a single tensile overload on the constant amplitude loading. An existing model making use of the stress intensity factor is discussed with respect to its capability to describe the observed crack growth behavior with the influence of overload and sequence loading.

The Role of Intermetallic Phases in Fatigue Crack Propagation Behavior of Al-Zn-Mg-Cu Alloys

The aim of the present work is to determine the role of intermetallic (IM) phases in the fatigue crack propagation behavior of hot-forged Al-Zn-Mg-Cu alloys in T73 condition. To generate differences in the volume fraction and coarseness of various IM particles, the (Fe+Si) impurity level is varied from 0.23 to 0.37 mass%. The fatigue crack propagation tests are conducted in air at ambient temperature and a stress ratio R of 0.1. Characterization of the fatigue fracture surfaces is performed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Quantified IM particles data for each alloy are then related to the fatigue properties and fractographic analysis results. It was found that almost all particles of the Fecontaining phases (primarily (Cu,Fe,Mn)Al3 and Al7Cu2Fe) are broken and not effective in hindering fatigue crack propagation. On the other hand, the Mg2Si and soluble phase particles smaller than those of the Fe-containing phases contribute beneficially to fatigue life. These particles increase the tortuosity of the crack path and retard the crack growth rate. The crack growth rate decreases as the volume fraction of coarse Fe-containing particles increases, because more secondary cracks are produced decreasing the effective stress intensity at the main crack tip.

Influence of Loading Conditions on Cyclic Compressive Fatigue Behavior of an Al<sub>2</sub>O<sub>3</sub> Matrix Composite

An Al2O3 matrix composites, i.e. partially stabilized zirconia toughening alumina (ZTA) reinforced by SiC particle (ZTA-SiCP), was prepared by hot pressing (HP). Fatigue behavior of ZTA-SiCP under cyclic compressive loads was investigated on different loading conditions. The application of cyclic compressive loads to a notched specimen led to a stable crack growth along the notch plane in a direction normal to the far-field compressive axis. Irreversible damages in the main form of microcrack were induced at the stress concentration zone during compression loading, and it led to high residual tensile stresses ahead of the notch root upon unloading. Nucleation and growth of a model I fatigue crack were caused by the residual tensile stresses at the notch root. Along with propagation of the fatigue crack, a gradual decrease in crack growth rate was shown due to the crack closure caused by accumulating of debris particles within the wake of growing crack, and thus led to the crack arrested at last. The fatigue crack length was investigated as a function of notch length, the maximum compressive stress, stress range and load frequency.

Changes in lattice-strain profiles around a fatigue crack through the retardation period after overloading

An overload was applied during a fatigue-crack-growth experiment, resulting in a temporary decrease in crack-growth rates, i.e., a retardation period. Neutron diffraction was used to investigate the retardation phenomenon by mapping the changes in the lattice-strain profiles around the fatigue-crack tip in a series of compact-tension (CT) specimens, which were fatigued to various stages through the retardation period after the overload. Following the overload, compressive-strain fields were observed along the loading direction close to the crack tip. As the crack grows out of the retardation period, the residual compressive strains decreased.

Theoretical-Experimental Model for Predicting Crack Growth Rate in Structural Alloys of Aircraft Gas Turbine Engine Disks with Regard to Fatigue and Creep

Hold times of 300 and 3600 s under trapezoidal cyclic loading are shown to result in an increase in crack growth rates by many factors of ten for EP962 alloy and several times for EP742 alloy at a temperature 973 K. It has been found that in case the creep crack growth diagram has a first segment where the crack growth rate decreases, the crack growth kinetics under trapezoidal cyclic loading can be predicted by means of the hypothesis of linear summation of fatigue and creep crack growth rates considering special features of the first segment of the creep crack growth diagram. The authors put forward empirical approaches to determining the mean rate on the first segment of the creep crack growth diagram.

Simulation Of Microcrack Growth Under Multiaxial Random Loading AndComparison With Experimental Results

In this paper a model designed to simulate the growth of microcracks under the influence of cyclic loading is presented. Considering fatigue crack growth microstructural barriers as well as the state of stress play an essential role. The crack growth is initially dominated by shear stresses leading to microstructurally short cracks (stage I). As the tip of the microcrack approaches a grain boundary the crack growth rate decreases. The transition from stage I to stage I1 crack growth is also considered in the model as the crack reaches a specific length and continues to grow under the influence of maximum normal stresses (physically short cracks). The comparison of the simulation results to experimental results of variable amplitude loading for bending, torsion, multiaxial proportional loading and multiaxial non-correlated loading applied to notched specimens of 42CrMo4V reveals a close match with the experimental results.

The fatigue behavior and delamination properties in fiber reinforced aramid laminates

The fuselage-wing intersection suffers from the cyclic bending moment of variable amplitude. Therefore, the influence of cyclic bending moment on the delamination and the fatigue crack propagation behavior in AFRP/ AI laminate of fuselage-wing was investigated in this study. The cyclic bending moment fatigue test in AFRP/ AI laminate was performed with five levels of bending moment. The shape and size of the delamination zone formed along the fatigue crack between aluminum sheet and aramid fiber-adhesive layer were measured by an ultrasonic C-scan. The relationships between da/dN and △K, between the cyclic bending moment and the delamination zone size, and between the fiber bridging behavior and the delamination zone were studied. As results, fiber failures were not observed in the delamination zone in this study; the fiber bridging modification factor increases and the fatigue crack growth rate decrease; and the shape of delamination zone is semi-elliptic with the contour decreasing non-linearly toward the crack tip.

Mikrorissentstehung und Mikrorisswachstum in Aluminiumlegierungen bei zyklischer Beanspruchung– Werkstoffliche Untersuchung und Simulation

Gezeigt wird die Modellierung und die Simulation des Mikrorisswachstums unter schwingender Beanspruchung. Mikrostrukturelle Barrieren sowie der Spannungszustand spielen dabei eine essentielle Rolle. Der metallische Werkstoff setzt sich in der Simulation aus hexagonalen Körnern zusammen, wobei die kristallographische Orientierung in jedem Korn anders ist. Das Risswachstum erfolgt zunächst bestimmt durch die Schubspannung in stage I (microstructurally short cracks). Mit Annäherung der Rissspitze an eine Korngrenze verlangsamt sich das Risswachstum. Nach Erreichen einer spezifischen Risslänge erfolgt das Risswachstum im stage II, d. h. unter Normalspannungseinfluss (physically small crack). Das Modell wird auf den Werkstoff AlMgSi1 angewendet. Hohlzylinder dieses Werkstoffs werden durch Zug-Druck, Torsion und mehrachsig beansprucht. Das simulierte Risswachstum zeigt im Hinblick auf die Rissorientierung und die Häufigkeitsverteilung eine gute Übereinstimmung mit den im Experiment erzielten Ergebnissen. Microcrack Initiation and Microcrack Growth in Aluminium Alloys Subjected to Cyclic Loading – Material Analysis and Simulation In this paper a model designed to simulate the growth of microcracks under the influence of cyclic loading is presented. Considering fatigue crack growth microstructural barriers as well as the state of stress play an essential role. The polycristalline metal was modelled as an aggregate of hexagonal grains with each of the grains showing a different crystallographic orientation. The crack growth is initially dominated by shear stresses leading to microstructurally short cracks (stage I). As the tip of the microcrack approaches a grain boundary the crack growth rate decreases. The transition from stage I to stage II crack growth is also considered in the model as the crack reaches a specific length and continues to grow under the influence of normal stresses (physically short cracks). The model is applied to tubular specimens of the aluminum alloy AlMgSi1 which are subjected to tension and torsion as well as to combined tension-torsion loading cycles. In terms of the microcrack distribution as a function of their orientation the simulated crack growth behaviour reveals a close match with the experimental results.

Delayed Fracture Properties of 1500 MPa Bainite/Martensite Dual-phase High Strength Steel and Its Hydrogen Traps.

It is very imperative to improve delayed fracture properties of high strength steel, which may enlarge its usage. The published literature shows that the susceptibility to hydrogen embrittlement of a novel 1 500 MPa bainite/martensite dual-phase high strength steel is inferior to that of conventional quench-tempered high strength steel. The stress corrosion cracking (SCC) in a 3.5% NaCl solution for novel 1 500 MPa bainite/ martensite dual-phase high strength steel was investigated in this paper by using modified wedge-opening-loading (WOL) specimens. The experimental results show that KISCC for novel 1 500 MPa bainite/martensite dual-phase high strength steel is larger than 50 MPa·m1/2, exceeding conventional high strength steel. Its crack growth rate (da/dt)II is about 1×10−5 mm/s, which is less than that of conventional high strength steel. Hydrogen trapping phenomena in the steel were investigated by electrochemical permeation technique. The lath boundaries and stable retained austenite are beneficial hydrogen trap, slowing down the segregation of hydrogen on the crack tip, hence KISCC increases and crack growth rate decreases.