Low frequency fatigue crack growth behavior of a470 class 8 rotor steel at 538°C (1000°F)

Fatigue crack growth rate data were developed at various frequencies and hold times at maximum load for A470 Class 8 steel at 538°C (1000°F) by using an accelerated test method which involves alternating test frequency and temperature. These data were consistent with fatigue crack growth rate data obtained from the same material and developed according to the ASTM specification E-647-T78. This result suggests that there is no transient effect associated with the alternating test frequency and temperature and that the accelerated testing procedure can be used to expedite the development of elevated temperature fatigue crack growth rate data at very low frequencies and long hold times. At 538°C (1000°F) fatigue crack growth properties with hold time developed from both 1T-CT and multiple-edge-craek tension specimens fall in the same scatter band on the da/dN vs ΔK plot. This result indicates the applicability of ΔK to characterize the fatigue crack growth behavior with hold time at elevated temperature. Also, the model proposed by Saxena et al. was found to successfully predict the fatigue crack growth rate properties with 28 min hold time of the A470 Class 8 rotor steel at 538°C (1000°F).     

Life extension technology for steam pipe systems—I. Development of material properties

Material properties of A106B low-carbon steels were developed for life prediction analyses of steam pipes operated at elevated temperatures but in the sub-creep temperature range. Tensile, fracture toughness, fatigue crack growth rate and low-cycle fatigue properties were obtained on the piping steel at 24°C (75°F) and 288°C (550°F). The latter temperature corresponded to the highest operating temperature of nuclear plant steam piping. Increasing the test temperature from 24°C (75°F) to 288°C (550°F) decreased the yield strength and fracture toughness of the steel. Fatigue crack propagation rate properties at 24°C (75°F) and 288°C (550°F) were found to be comparable.

In the low-cycle fatigue tests, below a strain amplitude level of approximately 0.5%, cyclic softening was observed, while at higher strain levels, cyclic hardening was present. Based on the results of tensile and incremental-step fatigue testing, the strain-life curve was predicted. The predicted strain-life curve was found to be in agreement with the experimental result.

The fracture surfaces of fracture toughness specimens showed ductile fracture, while striations were observed on those of fatigue crack growth specimens. Fatigue striations were also observed on the fracture surfaces of low-cycle fatigue specimens. Fatigue initiation was associated with inclusions. It was shown that plastic straining in A106B steel could be detected by acoustic emission and by monitoring the eddy current response. These nondestructive evaluation techniques exhibit possibilities for in-situ monitoring of fatigue deformation.

While the development of material properties for the life prediction assessment of steam pipes is included in Part I of this paper, the establishment of a quantitative life prediction methodology and inspection criteria is contained in Part II. The developed life prediction methodology quantifies the effects of operating parameters on the remaining life of steam pipes using the material properties obtained in Part I.     

On the electrical potential analysis of a cracked fracture mechanics test specimen using the finite element method

The use of the finite element method to calculate electrical potential drop vs crack extension calibration curves for fracture mechanics test specimens is investigated. The influence of finite element mesh size and the use of singular elements at the crack tip are considered. Also, some of the details of the compact specimen geometry and electrical potential set-up are examined. Electrical potential solutions for very deep cracks in the compact specimen were obtained and extrapolated to a limiting case solution. A comparison of some of the solutions generated in this investigation with other available numerical solutions and experimentally measured data was very favorable.     

Effect of pre-charged hydrogen on fatigue crack growth of low alloy steel at 288 °C

Hydrogen effects on mechanical strength and crack growth were studied at high temperatures. The study was motivated by the fact that the environmentally assisted cracking (EAC) of pressure vessel steel SA508 Cl.3 in 288 °C water was suspected to be related to hydrogen embrittlement. Fatigue crack growth rate and tensile tests were performed with hydrogen pre-charged specimens at high temperatures. At 288 °C the fatigue crack growth rate of the hydrogen pre-charged specimen was faster than that of as-received; the fatigue fracture surface of hydrogen pre-charged specimen correspondingly showed EAC like feature. Meanwhile, ductile striation was evident for the case of as-received in both air and argon gas environments. In the dynamic strain aging (DSA) loading condition at 288 °C during tensile tests, the pre-charged hydrogen induced a marked softening (decrease in ultimate tensile strength; UTS) as well as a little ductility loss; this was accompanied by the macrocracks grown from microvoids/microcracks promoted by DSA and hydrogen. These experiments showed that hydrogen embrittlement is an effective mechanism of EAC not only at low temperature but also at the high temperature.     

Fatigue crack initiation at notches in SA 333 piping steel

Abstract

In this paper are reported two recent investigations into crack initiation at notch roots using techniques developed for remote monitoring of crack growth in high–temperature water environments. In the first, a notched compact–type specimen of a carbon steel, SA 333 Gr. 6, was monitored for crack initiation (defined as a 0·076 mm crack at the notch root) in a variety of water chemistries and testing conditions. The mechanical conditions at the notch root have been analysed using the Neuber–notch method, enabling a direct comparison to be made with the strain–controlled fatigue data curves used for a smooth specimen in the ASME Boiler and Pressure Vessel Code, Section III. In the second investigation a different method was considered for monitoring crack initiation. A modification of the electrical–potential technique, called the reversing dc electrical potential method, was used to obtain quantitative information on the initiation and early growth of small surface cracks in notched bars of a high–strength alloy at elevated temperature. Results obtained by the method are presented and discussed.

MST/72     

Results of a Japanese round robin program for creep crack growth using Gr.92 steel welds

High-Cr ferritic heat-resistant steels are commonly used for boiler components in ultra-super critical thermal power plants operated at about 600 °C. In the welded joints of these steels, Type-IV cracks initiate in the fine-grained HAZ during long-term use at high temperatures, causing their creep strength to decrease. To assist the standardization of the testing and evaluation method for creep crack growth (CCG) in the welded components, we conducted round robin tests (RRT) using 9Cr-0.5Mo-1.8 W-V-Nb steel (ASME Grade 92 steel) welded joint as part of the VAMAS TWA31 collaboration. The CCG tests were carried out using the CT specimen and the circumferentially-notched round bar specimen for both the base metal and welded joint of Gr.92 steel. Testing was performed at four different laboratories. The effects of specimen configuration, temperature, load, and stress triaxiality conditions on the crack growth rate and fracture life were investigated.     

Analysis of interfacial cracks in a TBC/superalloy system under thermal loading

Thermal barrier coatings (TBCs) provide thermal insulation to high temperature superalloys. Residual stresses develop in TBCs during cool down from processing temperatures and subsequent thermal cyclic loading due to the thermal expansion mismatch between the different layers (substrate, bond coat, and TBC). These residual stresses can initiate microcracks at the bond coat/TBC interface and can lead to debonding at the bond coat/TBC interface. The highest residual stresses occur at the interfaces. The effect of voids or crack like flaws at the interface can be responsible for initiating debonding and accelerate the oxidation process. The effect of interfacial microcracks has been investigated using the fracture mechanics approach. In particular, J-integral and the energy release rate G, for both mode I and mode II using the virtual crack extension method were evaluated. Two types of specimens were studied. The specimens were cooled down from processing temperature of 1000°C to 0°C. The variation of the properties as a function of temperature were used for the analysis. It was found that the use of temperature dependent properties in contrast to constant properties provide significantly different values of J-integral and G. For the stepped-disc specimen with an edge crack, crack growth is only due to mode II, while for the cylinder specimen with an internal crack, crack growth is due to mixed-mode loading. An important implication of this result is that edge delaminations in a disk specimen may only grow due to mode II conditions under pure thermal loading. Shear fracture characteristics of interfacial crack thus become important in the failure of the TBC.     

Calibrations for the electrical potential method of crack growth measurement by a direct electrical analogy

A direct analogue method is introduced for obtaining calibration curves relating increase in voltage and crack length for the electrical potential method of measuring crack growth. The numerical results obtained from the analogue are justified by comparison with theoretical predictions. The method is compared with other techniques for obtaining calibrations. Finally, results are presented for plate specimens with various central notches, the S.E.N. bend specimen and the C.K.S. fracture toughness specimen.     

Electrical crack length measurement and the temperature dependence of the Mode I fracture toughness of carbon fibre reinforced plastics

The conventional optical crack length measurement in fracture toughness testing is unsuitable for tests carried out in a chamber, like temperature tests. An electrical method has therefore been developed which determines the crack length on the basis of a change of electrical resistivity. Compared with optical measurements this method has proved to be very accurate. With this method the Mode I fracture toughness of a carbon fibre reinforced epoxy composite was determined over the temperature range from −55°C to 120°C. It was found that the fracture toughness rises with increasing temperature. At low temperatures the values were constant or increased slightly, with minimum at about 0°C.     

Elastic-plastic and failure properties of a unidirectional carbon/PMR-15 composite at room and elevated temperatures

A series of off-axis tensile tests at room and elevated temperatures have been conducted up to 316°C (600°F) to determine the elastic and plastic properties of a unidirectional carbon/PMR15 composite as a function of temperature. The transverse tensile and shear strengths of the composite as a function of temperature have also been determined. The effect of the specimen preparation process (type of machining) on the strength properties of the composite has also been evaluated. It has been shown that elastic (with the exception of Poisson ratios ν₁₂ and ν₂₁), plastic, and strength properties of the composite are significantly affected by elevated temperatures. It has also been demonstrated that the quality of machining can noticeably influence the normal and shear strength data at room and elevated temperatures. Even if the quality of machining is very high, failure of the specimens can occur either in the gage or grip sections. At room temperature, all specimens failed in the grip areas influencing the transverse tensile and shear strength measurements. However, the type of specimen failure does not noticeably affect the strength data at elevated temperatures. The transverse tensile and shear strength properties of the composite at room temperature could only be estimated by extrapolating the normal and shear strength vs temperature curves to room temperature.     

Toughness characterization of damageable ceramic matrix composites

As for concrete and many other heterogeneous materials, the damageable behavior of numerous ceramic matrix composites (CMCs) renders their toughness characterization particularly difficult. However, the need to compare CMCs' resistance to crack propagation has given rise to a new type of toughness test based on the use of a mixed CT–DCB specimen associated with steel frames and named Steel Framed Assisted Tension (SFAT). This type of specimen, whose shape and dimensions were adjusted by numerical simulation, allows the development and the steady state propagation of the process zone, while preventing the occurrence of damage outside the vicinity of notch or macrocrack tips. A study of the use of steel frames glued on each side of the specimens allowed the choice of a sufficiently rigid and resistant glue offering a good repeatability of the tests at a wide range of loading speeds. A compliance calibration procedure has been defined for SFAT specimens in view of the need to apply the method to anisotropic composites. Testing glass/epoxy composites with this procedure has shown its validity and pointed out the influence of the notch length on the R curves which can be derived in terms of crack growth release rate G from the related load–displacement curves. In addition, examination of the resulting G_R curves shows the possible use of various parameters to represent the tested material toughness. Finally, the whole testing procedure has been evaluated on the 2D–SiC/SiC and 2D–C/SiC composites.     

Characterization of the elevated temperature static load crack extension behavior of type 304 stainless steel

Creep crack extension rates in Type 304 stainless steel, obtained as a function of temperature over the range 650–800°C and as a function of specimen geometry at 750°C, are empirically correlated with both the net section stress and the apparent stress intensity factor. The results indicate that the stress intensity correlation is strongly dependent on specimen geometry, whereas the net section stress correlation appears to be generally valid. A direct correspondence between crack extension and local (crack tip) displacement is noted when creep crack extension rates at 750°C are compared with COD obtained from actual castings of the crack tip. By introducing the concept of a miniature creep specimen at the crack tip, a physical model for creep crack growth is developed, based on local stress relaxation and strain accumulation, that is consistent with both experimental observation and existing theories of steady state creep.     

A microdebonding study of the high-temperature oxidation embrittlement of a cross-ply glass-ceramic/SiC composite

A constant cross-section specimen with adhesively bonded tabs has been used for an investigation of the high-temperature tensile behavior of a cross-plied glass-ceramic-matrix composite consisting of CAS-II reinforced with Nicalon SiC fiber. Oxidation of the exposed interfaces along matrix cracks in 0 ° plies lowers the composite failure strain at 800 °C to the 0 ° ply matrix-cracking strain. Scanning electron microscopy and microdebonding analysis of the fracture surfaces indicate that the embrittlement process is the result of oxidation of the carbon-rich interphase as the matrix crack encounters 0 ° ply fibers, the interphase subsequently fuses with a higher bond strength and the crack grows through the fibers. Planar cracks grow inwards from the surface, covering the entire fracture surface given enough time (or sufficient strain). Degradation of the fibers does not appear to contribute to the embrittlement. Transverse plies crack at a lower strain than does the matrix in the 0 ° plies. However, it appears that oxygen does not enter 90 ° ply cracks in sufficient quantity to produce oxidation embrittlement, at least up to the 0 ° matrix-cracking strain. The strain to crack the 90 ° plies does not decrease significantly at high temperatures despite the fact that the cracks are primarily in the fiber/matrix interphase as they grow across the 90 ° plies.     

Physical models for cleavage fracture at various temperatures—Bases for local approach to fracture of HSLA steel

The author proposes the critical events controlling cleavage at various temperatures: at a very low temperature (−196 °C), critical event is the nucleation of a crack in ferrite at the precrack tip. At a moderate low temperature (around −100 °C), the critical event is the propagation of a carbide crack into the ferrite grain. With increasing temperature (around DBTT −80 °C), the carbide crack eligible to propagate into the ferrite grain should be the one initiated by a critical strain higher than that to initiate a carbide crack at low temperatures. The higher critical strain increases the flow stress by work hardening for making up the effect of lowering yield stress. At a higher temperature (−30 °C) after the crack tip is blunted to more than 60 μm and a fibrous crack extends, the critical event for cleavage fracture is the propagation of a grain-sized crack.     

AUSTEMPERING OF A SILICON MANGANESE CAST STEEL

The influence of austempering on the microstructure and mechanical properties of an alloyed cast steel containing high silicon (3.00%) and high manganese (2.00%) was studied. The influence of microstructure on the plain strain fracture toughness of this new steel was also examined. The test results show that by using a suitable austempering process, i.e., by austenitizing at 1010°C (1850°F) for 2 hr and then subsequently austempering at 316°C (600°F) for 6 hr, it is possible to produce more than 80% austenite in the matrix of the material. Such a large percentage of austenite in the matrix made the steel almost nonmagnetic. Austempering resulted in a significant improvement in mechanical properties as well as fracture toughness of the material. The potential applications of this steel are in naval structural components, aircraft, and automotive components.     

Experimentally determined key curves for fracture specimens

Crack extension during fracture toughness tests of ferritic structural steels cannot be determined from measurements of unloading compliance or electric potential change when the specimen is dynamically tested. Measurements of crack extension in fracture toughness tests are also very difficult when the test temperature is high or the test environment is aggressive. To circumvent this limitation, researchers for years have been developing key curve and normalization function methods to estimate crack extension in standard elastic-plastic fracture toughness test geometries. In the key curve method (Ernst et al., 1979; Joyce et al., 1980) a load-displacement curve is measured for a so-called `source' specimen that is sub size or has a blunt notch so that the crack will not initiate during elastic-plastic loading. The load and displacement are then converted to normalized stress-strain units to obtain a key curve that can be used to predict crack extension in geometrically similar `target' specimens of same material loaded at similar loading rates and tested under similar environmental conditions. More recently Landes and coworkers (Herrera and Landes, 1990; Landes et al., 1991) proposed the normalization data reduction technique – Annex A15 of ASTM 1820 specification – that presents an alternative to the standard E1820 unloading compliance procedure. Although the normalization method works well in many cases, it has serious drawbacks: the load, displacement and crack length at the end of the test must be measured; the prescribed functional form that is fitted to the initial and final data may not be accurate for all materials; and the iterative method of inferring crack length from the combination of the data and the normalization function is complex. The compliance ratio (CR) method developed in this paper determines key curves for predicting crack extension as follows. First, a statically loaded source specimen with the unloading compliance procedure specified in ASTM 1820. Second, the so-called CR load-displacement curve is calculated for the source specimen, which is the load-displacement record that would have been obtained if the crack had not extended. Third, non-dimensionalizing the CR load by the maximum load and the displacement by the elastic displacement at the maximum load, P ^* _i/P _max and v _i/v _{el max} from the source specimen yields the adjusted key curve. Analysis of extensive data shows that the key curve is independent of notch type, initial crack length and temperature. But it is dependent on specimen size and steel type. Assuming that the key curves of the source and target specimens are one and the same, the compliance of the target specimens are calculated with a reverse application of the compliance ratio method, and the crack length is obtained using the equations in ASTM E1820. The CR Method is found to be much simpler than the normalization method described in the Annex A15 of ASTM 1820. With the compliance ratio method, Joyce et al. (2001) successfully predicted crack extension in dynamically loaded specimens using a key curve of a statically loaded specimen.     

The effect of thermal exposure on the electrical conductivity and static mechanical behavior of several age hardenable aluminum alloys

Aluminum alloys 2014-T6, 2024-T3, 6061-T6, 7050-T7451, and 7075-T6 were thermally exposed at different times (1 min to 20 days) and temperatures 177–482 °C (350–900 F). This study was conducted to simulate the effects of heat damage on aluminum alloys and to determine the correlations existing between the static mechanical and electrical conductivity properties. Results indicate that at the temperatures below 260 °C (500 F) all five alloys showed clear correlations between the mechanical and physical properties.     

A Superplastic Al-Li-Cu-Mg-Zr Powder Alloy with High Hardness and Modulus

Structure/property studies were made on an experimental Al-3.18% Li-4.29% Cu-1.17% Mg-0.18% Zr powder alloy, which is of the low density/high modulus type. Alloy powder was made by the P&W/GPD rapid solidification rate (RSR) process, canned, and extruded to bar. The density was 2.458 × 10⁶ g/m³. The material was solution-treated, and aged at 149°C (300°F), 171°C (340°F), and 193°C (380°F), using hardness tests to determine the aging curves. Testpieces solution-treated at 516°C (961°F) showed an average yield strength (0.2% offset) of 43.3 ksi (299 MPa) and ultimate tensile strength of 50.0 ksi (345 MPa), with 1% elongation, which increased to 73.0 ksi (503 MPa) and 73.1 ksi (504 MPa), respectively, with only 0.2% elongation, on peak aging at 193°C (380°F), with a modulus of elasticity of 11.4 × 10⁶ psi (78.3 GPa). Hardness values reached 90–92 R_B on aging at 149–193°C (300–380°F). The as-extruded alloy showed superplastic behavior at 400–500°C (752–932°F) with elongations of 80–185% on 25.6 mm, peaking at 450°C (842°F). An RSR Al-2.53% Li-2.82% Mn-0.02% Zr extruded alloy showed only 18–23% elongation at 400–500°C (752–932°F).     

Fracture behavior of 9% nickel high-strength steel at various temperatures: Part I. Tensile tests

Fracture behavior of a 9% nickel 1000 MPa grade high-strength steel was investigated with tensile tests at various temperatures. Four critical stresses were found, which determined the fracture behaviors at various temperatures. Various fracture behaviors could be classified into three categories: (1) at −196 °C, a longitudinal crack initiated from the center of the necking region and propagated along the tensile direction to the regions close to both ends of the necking, it then changed the orientation and developed into two transverse cracks which propagated into opposite directions on two separated cross-sections. (2) In the range of −30 °C to 20 °C, the fracture surfaces were composed of typical center-fibrous-initiation region, middle shear-radical region and outer shear-lip region. (3) In the range of −150 °C to −60 °C, the middle shear-radiation region showed a very rough pattern with several convex ridges. Fracture mechanisms were analyzed by combining various fracture morphologies with FEM-calculated results of stress and strain.     

Strain induced fracture in low strength steels

This paper attempts to investigate the role of strain in the fracture of low strength steel in the temperature range of −196 to 28°C and at the crack tip strain rate range of 10⁻⁵ to 10°/sec. The tensile test data is suitably processed to estimate the fracture strain in the fracture toughness specimen. The micro-fractographic observations help to identify the microfracture mechanisms and relate them with the governing fracture criteria. Even for the non-microvoid coalescence mode of fracture in the alloys in the temperature region of −140 to −77°C, a significant role of plastic strain in fracture is identified. The significance of the critical strain zone for fracture is discussed and it has been compared with the process zone at fracture.