Compact Type Specimens Research Articles

CREEP‐FATIGUE CRACK GROWTH BEHAVIOR IN 1Cr‐1Mo‐0.25V STEELS. PART I: ESTIMATION OF CRACK TIP PARAMETERS

Abstract— Non‐linear finite element (FEM) analyses, involving various creep deformation laws, as well as experiments with hold times of 100 s and 15 min were performed on compact type specimens with stationary cracks. The work was aimed at developing accurate expressions for estimating the small‐scale creep parameters, (Ct)avg, for 1Cr‐1Mo‐0.25V steel at 538°C (1000°F). Here (Ct)avg is a representative value for the small‐scale creep parameter Ct, averaged over a hold period under the conditions of creep‐fatigue loading. These expressions were then applied to crack growth data obtained from specimens tested under the various hold times. When an elastic‐plastic‐primary creep‐secondary creep constitutive model was used in the FEM analyses, the calculated values of Ct compared well with the measured values. The FEM results also showed that the accumulated creep deformation during the hold time was not significantly reversed during the unloading portion of the creep‐fatigue cycle for this material. Therefore, a new method of estimating (Ct)avg is proposed on the basis of these numerical and experimental results.

The Effect of Water Flow Rate Upon the Environmentally Assisted Cracking Response of a Low-Alloy Steel

Abstract The effect of water flow rate upon the environmentally assisted cracking (EAC) response of a high-sulfur ferritic steel was studied at a temperature of 243°C. In contrast to earlier studies which employed compact-type specimens, this study employed relatively large tight semi-elliptical surface cracks tested under generally linear-elastic conditions. Flow velocities parallel to the crack as low as 1.58–1.84 m/s were effective in mitigating EAC.

Characterization of creep-fatigue crack growth behavior in 2.25 Cr-1 Mo steel using (C t)avg

Time-dependent creep-fatigue crack growth (CFCG) is an important consideration in the design and remaining life estimation of high temperature components. CFCG tests were carried out on compact type (CT) specimens of 2.25 Cr-1.0 Mo steel and its behavior, for hold times ranging from 10 seconds to 50 seconds, at 594°C (1100°F) was characterized using the average value of the C t-parameter, (C t)avg. The trends in the creep-crack growth (CCG) data for this material are also compared with the CFCG data. The analytically estimated values of (C t)avg are compared with the experimental values of (C t)avg obtained from the measured values of load-line deflection rates. It is also shown that even in the absence of accurate creep deformation constants, accurate estimates of the measured values of (C t)avg can be obtained in CT specimens

Characterization of creep-fatigue crack growth behavior under trapezoidal waveshape using Ct-parameter

An experimental study of creep-fatigue crack growth behavior of 1.25Cr-0.5Mo steel at 538°C (1000°F) under trapezoidal loading waveshapes has been carried out on compact type specimens. In creep-fatigue crack growth experiments, hold times ranged from 0 seconds to 24 hours with intermediate conditions of 10 seconds, 98 seconds, 10 minutes and 15 minutes. Time-dependent crack growth rate during the hold period, (da/dt)avg, is correlated with (Ct)avg. The values of (Ct)avg are estimated using the equation recently proposed for elastic-cyclic plastic-secondary creeping (EL-CPL-SC) materials. The (da/dt)avg vs. (Ct)avg data fall on a single trend which matches with the trend of da/dt vs. Ct creep crack growth data for the same material. A model is proposed for predicting both the creep crack growth behavior and the creep-fatigue crack growth behavior. The model is suitable for assessing the residual life and/or the safe inspection intervals of high-temperature components such as steam headers. Transition of crack tip damage patterns from oxidation to creep cavitation across the range of hold times examined is also discussed.

Two-Dimensional Crack Growth Simulation across the Fusion Line.

Crack growth analyses of inhomogeneous CT (Compact Type) specimens are carried out by means of the two-dimensional finite-element method. The CT specimen is made of a welded plate of A533B-and HT80-type steel. The fusion line exists perpendicular to the crack direction. By changing the ratio of the widths of the two metals, the three specimens are analyzed. Based on the experimental results, generation phase analyses are conducted. The J-integral is evaluated using several conventional equations, and their availabilities for inhomogeneous material are discussed. The results show that the effect of the fusion line is strong when it is located in front of the crack tip. The crack tip fields are compared with HRR (Hutchinson-Rice-Rosengren) solutions and their deviations from the HRR solutions are measured by the Q-factor. The change of the stable crack growth condition is discussed using Q-factor.

Influence of temperature on tensile deformation and fracture of unfilled and short-glass-fiber filled poly(cyano aryl ether)

Tensile deformation and fracture of injection-molded poly(cyano aryl ether) thermoplastic (UR-PCAE) and its short-glass-fiber reinforced composite (SGF-PCAE) were studied with particular attention to the effect of testing temperature T. PCAE exhibited high performance in stiffness and thermal resistance in the dynamic mechanical analysis. The tensile strength for UR-PCAE specimens was decreased to about a half with an increase of T from - 40° C to 100° C. SGF-PCAE exhibited brittle-like fracture in the temperature range stated above, and a large decrease in failure strain and strain energy ductility index. Compact tension type specimens showed low fracture toughness Jt: UR-PCAE provided a minimum value at 60°C, but a large increase in Jt with a further increase in T. For SGF-PCAE, however, Jt showed a slight increase from 23°C to 100°C. Fracture surface observation revealed that the matrix ductility considerably affected the dependence of T on the toughness.

Fatigue Crack Growth in a High Tensile Strength Steel ;n Seawater and Several Other Environments.

A study was made to explain why the fatigue crack growth (FCG) in the intermediate ΔK regime is accelerated in seawater and other liquid environments, compared with in air. Fatigue crack growth rates (FCGRs) were measured under various environmental conditions using compact type specimens cut from a 667 MPa-tensil-strength steel plate. Anodic and cathodic polarization at -0.6 and -0.8VSCE, and removal of dissolved oxygen and salts exerted minimal influence on the FCGR in synthetic seawater (SSW). The FCG-acceleration was observed also in several nonaqueous liquids, though the FCGR in a liquid containing no hydrogen atom in its molucular structure was almost the same as the FCGR in air. The set of results showed that the FCGR was increased by hydrogen formed in a process other than a corrosion reaction. At an overprotecting potential of -1.2VSCE in SSW, the amount of hydrogen absorbed into the steel was measured as 0.06ppm or less, and hydrogen-embrittlement type stress-corrosion-cracking (HE-SCC) did not take place at a stress intensity factor or 117MPa √m in a sustained load test. In such an environment, a marked FCG-acceleration was observed, and the FCGR was not influenced by stress ratio, if the crack closure effect was compensated. The observation of fracture surfaces showed little effect of environments. These results indicated that the FCG-acceleration was not caused by the superimposition of HE-SCC. The hydrogen-assisted fatigue-crack-growth (HA-FCG) model was proposed: Atomic hydrogen is dissociated from water or other environmental substances adsorbed onto steel surface, and then is absorbed into the steel to activate the FCG process.

Characterization of creep-fatigue crack growth behavior under trapezoidal waveshape using C t -parameter

An experimental study of creep-fatigue crack growth behavior of 1.25Cr-0.5Mo steel at 538°C (1000°F) under trapezoidal loading waveshapes has been carried out on compact type specimens. In creep-fatigue crack growth experiments, hold times ranged from 0 seconds to 24 hours with intermediate conditions of 10 seconds, 98 seconds, 10 minutes and 15 minutes. Time-dependent crack growth rate during the hold period, (da/dt)avg, is correlated with (C t )avg. The values of (C t )avg are estimated using the equation recently proposed for elastic-cyclic plastic-secondary creeping (EL-CPL-SC) materials. The (da/dt)avg vs. (C t )avg data fall on a single trend which matches with the trend of da/dt vs. C t creep crack growth data for the same material. A model is proposed for predicting both the creep crack growth behavior and the creep-fatigue crack growth behavior. The model is suitable for assessing the residual life and/or the safe inspection intervals of high-temperature components such as steam headers. Transition of crack tip damage patterns from oxidation to creep cavitation across the range of hold times examined is also discussed.

鉄系焼結材料の疲労き裂進展挙動

The behavior of fatigue crack growth of sintered iron and steel compacts were investigated in comparing with that of the nodular graphite cast irons for various stress ratios and specimen thickness by using 1/2 compact type specimens, prepared by single pressing, sintering and machining.The results obtained were summarized as follows:1) Under the constant stress ratio, the relationship between fatigue crack growth rate(da/dN) and stress intensity factor range(ΔK) of sintered compacts was dependent on the density and the structure. However, the relationship between da/dN and ΔK/E normalized by Young's modulus(E) was linear independently of the density in the sintered iron compacts. Under the same ΔK, da/dN of the sintered compacts was slightly bigger than that of the nodular graphite cast irons.2) As the stress ratio is increased, the fatigue crack growth curves of sintered compacts were shifted to the lower stress intensity factor range. The threshold in stress intensity factor(ΔKth) was decreased with an increase in the stress ratio.3) Effects of specimen thickness on the fatigue crack growth in sintered compacts did not appear in the range of 10.0mm to 12.5mm.

鉄系焼結材料の疲労破面観察

The relationship between the behavior of fatigue crack growth and the pattern of fractured surface was investigated on the sintered irons and alloyed steels for various stress ratios by using 1/2 compact type specimens. The results obtained were summarized as follows:(1) The fracture pattern in the fatigue crack growth on the sintered ferrous materials consisted of shear and intergranular fractures, striation like pattern, and pore surface.(2) The fracture surface reflected the shear and local intergranular fractures in the region of low crack growth rate, while most of the fracture surface were occupied by the straiation like pattern in the region of high crack growth rate.(3) As the stress ratio increased, the fracture surface in the fatigue crack growth became coarser and many microcracks appeared within the facets. This behavior was independent on the density and the addtional alloying elements.

Creep crack growth verification testing in Alloy 800H tubular components

A method for determining the creep crack growth, CCG, and stress rupture behaviour of Alloy 800H tubular components containing longitudinal notches at 800°C is described. The presence of the notch is found to systematically weaken the tube, the degree of weakening dependent upon the notch length and depth. The creep crack growth rates, determined from a specially adapted potential drop technique are compared with those obtained from conventional compact tension type specimens. Using the stress intensity factor, K I, and the C * parameter as the basis of comparison it is found that the latter gives excellent correlation between the specimen and component behaviour. Finally attention is drawn to the potential dangers of predicting the component creep crack growth behaviour from the data obtained using conventional specimens for a structure sensitive material such as Alloy 800H and conversely to the advantages of the component type CCG tests developed in the present work.

Evaluation of crack-tip parameters for characterizing creep crack growth: Results of the ASTM round-robin programme

The results of a round-robin programme to evaluate crack-tip parameters such as K, C* and Ct for characterizing creep crack growth behaviour are presented. The programme was sponsored by the E24 Committee on Fracture Mechanics Test Methods of the American Society for Testing and Materials (ASTM). The tests were made on a 1Cr-1Mo-0.25V turbine rotor steel (ASTM grade A470 class 8) and included specimens of two thicknesses from the compact type (CT) and the centre crack tension (CCT) geometries. The test temperatures were 538°C and 593°C (1000°F and 1100°F). All tests were found to be in the extensive creep regime where the C*-integral and the Ct parameter are identical. It was shown that all creep crack growth data from CT specimens correlated uniquely with the C*-integral or the Ct parameter and no unique correlation was found between creep crack growth rate and K. The creep crack growth rates in 6.3 mm (0.25 in) plane-sided CT specimens were about two to three times slower than those in 25.4 mm (1 in) th...

Crack propagation behavior under creep conditions

The creep crack propagation behavior of a Cr−Mo−V rotor steel has been investigated at 538°C using time-dependent fracture mechanics concepts. The creep crack propagation lives and the creep deflection rates of double-edge-notched (DEN) specimens were estimated using previous data from compact-type specimens. The predicted crack growth lives and deflection rates compared favorably with the experimental data. When plotted as a function of the C t parameter, the experimentally determined creep crack propagation rates of DEN specimens were found to be in agreement with those of compact and center-crack-tension specimens. These results provide further experimental verification for the validity of the C t parameter for characterizing creep crack growth behavior. Some discrepancies between the predicted and the observed behavior are attributed to primary creep deformation behavior which was not considered in estimating the value of C t .

切欠き付シートモールディングコンパウンド複合材の破壊挙動

Effect of notch tip radius on fracture behavior of compact tension type specimens of sheet-molding-compound (SMC) composites was studied. Notch tip radii (ρ) were from 0.09mm to 2.1mm. Acoustic emission (AE) measurements and microscopic observations were carried out to examine damage formation procedure around the notch tip. Optical observations by a dye-penetration method exhibited that fracture patterns in the specimens of ρ≤0.56mm were quite different from those in case of ρ=2.1mm. For ρ≤0.56mm, a main-crack initiated at the notch tip and grew in parallel with generation of many sub-cracks surrounding the main-crack tip. For ρ=2.1mm, on the contrary, during main-crack growth, sub-cracks propagated in radial directions. With increasing notch tip radius, there were observed little increase in the maximum load, AE cumulative event counts and standard deviation of AE source locations. Strain energy at the maximum load slightly increased with increasing notch tip radius in a range 0.09mm≤ρ≤0.56mm. States of fiber strands at the notch tip affected parameters on strength more significantly than notch tip radii within ρ=2.1mm.

Fracture toughness evaluation for advanced materials using chevron-notched compact specimens

To utilize conventional testing methods and a variety of software appropriate to fracture mechanics tests, it is desired to develop a test method using compact-type specimens for ceramics. For ceramics, however, it is difficult practically to introduce a precrack into the specimen. In this study the utility and applicability of a chevron-notched specimen method and a compression damage method are discussed.

Creep crack initiation and propagation in type 304 stainless steel at 873 K

Creep fracture tests were performed on type 304 stainless steel using compact tension type specimens at 873 K in an air environment. Both the creep crack initiation time and the creep crack growth rate were measured as functions of various fracture parameters such as the stress intensity factor, the net section stress, the C ∗ integral and the crack-tip-opening displacement. It is shown that it is possible to correlate the creep crack initiation time with the C ∗ integral; however, the results obtained from experiments are not in agreement with those predicted from the creep crack initiation model based on the critical strain failure criterion. The reasons for this discrepancy are discussed. It is also shown that the creep crack growth rate can be correlated with the C ∗ integral.

Mechanisms of creep crack growth in 1 wt% antimony-copper: Implications for fracture parameters

A copper alloy with 1% (by weight) antimony was used as a model material and tested at 400°C to study the mechanisms of creep crack growth. At this temperature, the creep deformation in this material was dominated by secondary and tertiary creep. The expression for estimating C t (a crack tip parameter for creep conditions) in a compact type specimen used for testing was modified to include tertiary creep deformation. Extensive damage characterization was conducted on tested creep crack growth specimens using optical metallography and scanning electron microscopy. The following observations were derived from the test results. The creep crack growth rates correlated with C t only when intense cavitation damage was restricted to a region approximately 1.3 mm in size near the crack tip and no crack branching occurred. It was observed that the average diameter, areal density, and percent of grain boundary area cavitated decreased as function of distance from the crack tip. From these results it is argued that simultaneous nucleation and growth of cavities occur on grain boundary facets during the creep crack growth process. Percent grain boundary area cavitated is proposed as the most reasonable measure of creep damage. The critical amount of damage for crack extension appears to depend on the magnitude of the C t parameter.

Estimation of theCtParameter: Experimental Implications

AbstractThis paper examines the evaluation and interpretation of the Ct parameter that has been proposed for correlating crack growth under non-steady-state creep conditions. The study has been carried out by finite element analysis of the commonly used compact-type fracture specimen with stationary cracks.It is shown that the loss of constraint of the creep zone growing at the crack tip region due to intense creep at the loading pins can have a significant effect on the results of the load line displacement rate which are used to calculate Ct. Since Ct is identical to C* under extensive creep conditions, these effects also seriously impact determination of other commonly used creep crack growth parameters. This study has given some insight into the proper care that should be taken in obtaining experimentally measured and numerically calculated load line displacement rates in fracture specimens so as to eliminate the undesired effects of intense creep at the loading pins and/or between the crack tip and loading pins. It is shown that compact-type specimens with a crack size to width ratio greater than 0.5 and loaded with a dead weight are best suited for obtaining creep crack growth rate data. A relationship between Ct and the crack tip opening displacement rate has also been derived that provides a more complete framework for the analytical and experimental evaluation of the Ct parameter.

Mode I stress-intensity factor measurements by the electrical strain gage.

This paper describes the method of determining the Mode I stress-intensity factor (KI) by using the electrical strain gage. When fracture mechanics is applied to engineering, The stress-intensity factor near the crack tip must be accurately determined by way of a convenient measuring method of the strain. Usually, electrical strain gages are used for the strain measurement of the structure material. Thus the strain gage is used in this study. The type of strain gage used in this study is the chain strain gage consisting of 5 measuring grids. It is used in the experiments which examine the center-cracked test specimens (CCT) and single-edge-cracked plate tension specimens (SECT) as well as the compact-type specimens (CT). The accuracy of the stress-intensity factor (KI) determined by the experiments is within ±10 percent, compared to the analytical value.

Applicability of ModifiedJas a Fracture Parameter for Polycarbonate

AbstractThe applicability of modified J-integral, JM, as a fracture parameter for plain and impact-modified polycarbonate was examined. Specific issues were:1. Influence of the non-unique tensile stress-strain behavior, which is characteristic of these materials, on the applicability of JM.2. Methods of estimating JM in fracture specimens given the load—load-point deflection diagram.3. Fracture mechanisms that cause ductile and brittle crack growth in this class of materials.Crack growth resistance (JM-R) curves were obtained from compact-type (CT) specimens of multiple sizes and specimens of another geometry (single edge notch [SEN]). The JM-R curves were consistent for crack extensions up to 2.5 mm for CT specimens of various sizes and were independent of geometry for crack initiation and a limited amount of crack growth (1.5 mm). The JM-R curve for toughened polycarbonate showed a considerably higher resistance to ductile tearing than that of the plain polycarbonate. The fracture mechanisms in both types of materials were characterized using a scanning electron microscope.