Fatigue crack propagation testing using subsized rotating bending specimens

In many industrial applications, mechanical properties characterization is needed yet sufficient amount of material for standardsized specimens is not available. Miniaturized specimen testing technique has to be employed. Currently there are a lot of effort in developing subsized specimen technique for impact, fracture toughness and tensile properties. Work on fatigue properties testing is limited and largely confined to stress/strain life tests. In the current work, evaluation of fatigue crack propagation behaviour using surface crack growth in a rotating bending rod has been attempted. Nine different rod geometries have been tested. The resulting fatigue crack propagation data is more sensitive to rod diameter than to rod length. Difference in crack growth behaviour can largely be understood when crack closure is taken into account. All in all, the crack propagation data obtained from these small sized rods agree well with those obtained from standard testing employing compact tension specimens. A more precise picture about the crack growth behaviour can be obtained if crack closure is considered. If crack closure is not monitored, rods with longer length and smaller diameter are more likely to give the conservative upper bound fatigue crack propagation behaviour.     

Fatigue studies on carbon steel piping materials and components: Indian PHWRs

This paper describes the results of fatigue studies on carbon steel piping materials and components of Indian Pressurized Heavy Water Reactors (PHWRs). The piping components include pipes and elbows, of outer diameter 219 mm, 324 mm and 406 mm, made of carbon steel (SA333 Gr.6 grade) material. Tests on actual pipes and elbows with part through notch were carried out to study the behaviour of crack growth under cyclic loading for different pipe sizes, notch aspect ratios, stress ratios, etc. During the tests, numbers of cycles for crack initiation from blunt notch were recorded with an accuracy of 0.1 mm. In conjunction with component tests, the experimental studies were also conducted on standard specimens to understand the effect of different variables such as size (thickness), type of specimen and components (elbow and pipe), welding, stress ratio, notch orientation on fatigue crack growth rate. The fatigue crack growth curve (da/dN versus ΔK) obtained from three-point bend specimen and pipe was compared with that given in ASME Section XI. The comparison shows that da/dN versus ΔK curves obtained from the specimen and pipe tests are nearly same. The analytical predictions for crack initiation and crack growth for the tested components were compared with experimental results. Such comparisons validate the modeling procedure for crack initiation and growth.     

Estimation of crack growth in the creep range during plastic cyclic loading

Crack growth investigations were performed on the creep-resistant steel 13 CrMo 4 4 in the fatigue and the creep fatigue regime, especially regarding the influence of creep damage on crack growth. To this effect, 2% creep strain was applied to the material at a temperature of 560°C. The crack propagation rate was determined as a function of the specimen shape, temperature, test frequency and hold times. In the case of compact tension (CT-)specimens, creep pretreatment does not affect crack growth. For center-cracked tension (CCT-)specimens, however, the creep pretreatment results in a considerable increase in the crack propagation rate. Hold times of 90 minutes at maximum loading cause an increase in da/dN due to further cavity nucleation. The hold time at which cavity nucleation might occur is evaluated. The dependency on frequency of crack growth may be evaluated by means of a linear superposition of creep and fatigue crack growth. The transition frequency above which pure fatigue crack growth occurs is calculated and the regimes of fatigue, creep and creep—fatigue interaction with environmental influences are characterized.     

Fatigue behavior of pipes containing multiple flaws in inner surface

In this work was studied the growth behavior of multiple cracks in the inner surface of pipes. The fatigue tests were performed using two kinds of test pipes, i.e., the straight pipes and bend pipes of AISI Type 304L stainless steel, having 320 mm in outer diameter and 35 mm in thickness approximately.The crack growth curves obtained by the fatigue tests were compared with the analytical curves of two kinds of crack growth prediction methods. One method is based on the ASME Boiler and Pressure Vessel Code, Sec. XI. Another method is based on the procedure in which the crack growth formula is applied to both the surface and thickness directions. The analytical crack growth curves predicted by the ASME Code are conservative for the test results of the straight and bend pipes. However the results of bend pipe test suggest that the procedure of the ASME Code may give an unconservative fatigue life under the certain condition.On the other hand, the test results of the straight pipes can be evaluated reasonably and those of the bend pipes can be evaluated conservatively by the latter method.     

Fatigue cracks in Eurofer 97 steel: Part II. Comparison of small and long fatigue crack growth

The fatigue crack growth rate in the Eurofer 97 steel at room temperature was measured by two different methodologies. Small crack growth data were obtained using cylindrical specimens with a shallow notch and no artificial crack starters. The growth of semicircular cracks of length between 10-2000 μm was followed in symmetrical cycling with constant strain amplitude (R_ε = −1). Long crack data were measured using standard CT specimen and ASTM methodology, i.e. R = 0.1. The growth of cracks having the length in the range of 10-30 mm was measured. It is shown that the crack growth rates of both types of cracks are in a very good agreement if J-integral representation is used and usual assumptions of the crack closure effects are taken into account.     

The feasibility of small size specimens for testing of environmentally assisted cracking of irradiated materials and of materials under irradiation in reactor core

Environmentally assisted cracking (EAC) or, in other words, stress corrosion cracking (SCC) of in-core materials has become an increasingly important reason for the downtime and maintenance costs of nuclear power plants (NPPs). Use of small size specimens for stress corrosion testing of irradiated materials is necessary because handling of high dose rate materials is difficult and the availability of these materials is limited. A drawback of using small size specimens is that they do not in some cases fulfil the requirements of the relevant testing standards and sometimes their limited load-bearing capacity prevents corrosion fatigue tests and tests with static loading at reasonable K_I values. The test results show that the ductile fracture resistance curves of a Cu–Zr–Cr alloy are, to some extent, independent of the specimen geometry and size. However, the curves of small specimens deviate from the curves of larger specimens at high J values (large plastic zone relative to the remaining ligament) or when the crack growth exceeds about 30% of the remaining ligament. The size dependency of the tested Cu–Zr–Cr alloy seems to be a consequence of decreasing stress triaxiality as the size of the specimen is decreased. The results of the SCC tests of sensitized SIS 2333 stainless steel (equal to AISI 304) specimens in simulated boiling water reactor (BWR) water show that the plastic deformation of the remaining ligament of the specimen has no significant effect on the environmentally assisted crack growth rate. This indicates that stress corrosion testing is not limited by the specimen size. The size dependency in SCC tests should be further studied by conducting tests using various specimen sizes.     

Fracture toughness of narrow-gap welded joints in the nuclear pressure vessel steel 22 NiMoCr 37

The mechanical testing of narrow-gap welded joints in 100 and 200 mm thick sections of the steel 22 NiMoCr 37 has revealed that the weld metal, and not the heat affected zone (HAZ) or the weld metal-parent metal boundary. is the critical region. This modified gas-shielded welding process operates with a very low heat input of the order of 6.500 J cm⁻¹ pass⁻¹ and the combination of small diameter welding wires and high welding speeds contributes to the excellent joint properties in the as-welded condition.To investigate the effect of preheating and post-welding heat treatment on the mechanical properties of narrow-gap welds, tensile, notch impact, flat bend and fracture toughness test specimens were extracted from joints welded with the following conditions: (1) no preheating: no post-weld heat treatment; (2) no preheating: soaking at 300°C: (3) no preheating: stress-relief heat treatment at 600°C; (4) preheating 200–250°C; no post-weld heat treatment; (5) preheating 200–250°C; soaking at 300°C; (6) preheating 200–250°C; stress relief heat treatment at 600°C. Tensile testing at room temperature and at 250°C of round specimens oriented across the seam revealed the ultimate fracture to be always located in the base material remote from the welded zone. Although pores or slag inclusions had an influence on bend-test results of specimens in the as-welded condition, the results generally show failure free bends to 180°C with no evidence of cracking in the HAZ or at the fusion boundary.Using sharp-notched impact bend specimens with the notch located in the centre of the seam as well as in and across the HAZ, absorbed energy-test temperature curves have been determined for each welding condition. In comparison with the base material impact toughness, the weld exhibits superior toughness in the temperature range − 60 – 0°C, but yielded lower values at room temperature. After stress relieving at 600°C, the impact toughness of the weld reduced significantly, apparently due to precipitations occurring in the weld-metal microstructure. Test results from welded specimens with the no notch in the HAZ show this region to have superior notch impact toughness to the base material.Crack opening displacement (COD) specimens 45 × 90 × 380 mm with the fatigue crack located in the weld metal and in the HAZ were tested at 0 and 20°C using both the recommendation in BS DD 19: 1972 as well as acoustic emission measurements for the determination of COD values. For this method of fracture toughness testing it has been shown that the occurrence of a critical event must be clearly defined as corresponding to stable crack growth or alternatively to unstable crack propagation.     

表面粗糙度对国产316LN钢低周疲劳性能的影响

表面粗糙度对材料服役过程中表面缺陷的形成有着重要影响,由于实验室一般采用光滑试样测试疲劳性能,而真实主管道经过多道工序,其表面粗糙度远大于实验室测试试样,这会给主管道的设计、使用带来风险。本文通过测试不同粗糙度316LN不锈钢的疲劳性能,分析粗糙度(分别为0.08、0.4、1.0μm)对疲劳寿命的影响。结果表明,随着粗糙度的增加,疲劳寿命会相应减少,两者在双对数坐标下呈线性关系。通过断口观察对比和裂纹萌生机理分析可知,粗糙度增加会引起应力集中,增加微裂纹的萌生速度,从而导致疲劳寿命下降。     

An investigation of the degradation characteristics for casting stainless steel, CF8M, under high temperatures

Stainless steel castings are used in pipes and valves subjected to high pressure and temperatures. The primary coolant system of a nuclear power plant is made of a stainless steel casting and the operating temperatures are in the range of 290–330°C. If the coolant system is exposed to these temperature ranges for a long period, it may be possible to experience degradation of the material. The present investigation is concerned with the degradation characteristics of CF8M (cast duplex stainless steel), exposed to the thermal and σ-phase degradation temperatures, 430 and 700°C, respectively. After the CF8M specimens are held 100–3600 h at 430°C for the thermally degraded specimens and maintained 20 min to 150 h at 700°C for the σ-phase degraded specimens, respectively, all specimens are water quenched. Each specimen of the thermally and σ-phase degraded materials is classified into five classes depending on the holding time at the given temperatures. In order to investigate the characteristics of degradation, microstructure, micro Vickers hardness, tensile, impact tests, and fatigue crack growth tests are performed for each class of the specimens. From the present investigation the following results were obtained: (1) the difference between the thermally and σ-phase degraded specimens can be distinguished through their microstructures, (2) hardness and tensile strength are increased with degradation, while elongation, reduction area, and impact energy are decreased by increasing the degradation, (3) the fatigue crack growth rate (FCG) of the σ-phase degradation at 700°C is larger than that of the thermally degraded specimens, and (4) the FCG for both thermally and σ-phase degraded specimens are larger than those of the virgin (nondegraded) specimens.     

Reactor vessel cladding separate effects studies

The existence of a layer of tough weld overlay cladding on the interior of a light-water reactor pressure vessel could mitigate damage caused during certain overcooling transients. The potential benefit of the cladding is that it could keep a short surface flaw, which would otherwise become long, from growing either by impeding crack initiation or by arresting a running crack. Two aspects critical to cladding behavior will be reported: irradiation effects on cladding toughness and the response of mechanically loaded, flawed structures in the presence of cladding.A two-phase irradiation experiment is being conducted. In the first phase, Charpy impact and tensile specimens from a single wire, submerged-arc stainless steel weld overlay were irradiated to 2 × 10²³ neutrons/m² (>1 MeV) at 288°C. Typical, good quality pressure vessel cladding exhibited very little irradiation-induced degradation. However, ductile-to-brittle transition behavior, caused by temperature-dependent failure of the residual δ-ferrite, was observed. In contrast, specimens from a highly diluted, poor quality weldment were markedly embrittled. In the second phase of irradiations, now in progress, a commercially produced three-wire series arc weldment will be evaluated under identical irradiation and testing conditions as the first series. In addition, 0.5T compact specimens of both weldments and higher fluences will be examined.A two-phase program is also being conducted utilizing relatively large bend specimens that have been clad and flawed on the tension surface. The testing rationale is that if a surface flaw is pinned by the cladding and cannot grow longer, it will also not grow beyond a certain depth, thereby arresting the entire flaw in a stress field in which it would otherwise propagate through the specimen. The results of phase one showed that single wire cladding with low-to-moderate toughness appeared to have a limited ability to mitigate crack propagation. For the second phase, three-wire cladding has been deposited on a base plate with a very high ductile-to-brittle transition temperature allowing testing to ascertain the crack inhibiting capability of tough upper shelf cladding.     

金属材料裂纹冲击韧性评定方法研究

工程结构在复杂环境下长期服役后,裂纹的萌生和发展难以避免。对于需要承受爆炸与冲击的工程结构,需要对所用材料的裂纹冲击韧性进行评定,避免裂纹出现可能导致的冲击脆性断裂。基于Instron VHS高速材料试验机,开发了一套材料动态断裂试验装置,测量了4种具有高冲击功金属材料的裂纹冲击韧脆转变过程,并研究了影响金属材料裂纹冲击韧脆转变速率的因素。发现材料夏比冲击功并不能完全反应裂纹的冲击韧性,预制裂纹与否、试样约束方式和试样裂尖约束因子都会影响金属材料裂纹冲击试验中的韧脆转变速率。基于以上研究结果,提出了金属材料裂纹冲击韧性评定方法的基本思路。      

Short fatigue crack propagation and effect of notch plastic field

In engineering application, almost all structures and components contain notches or holes. They often experience severe fatigue loading, and have been recognized as a potential site for small fatigue crack initiation and propagation. In this paper, the effects of notch plastic field on small fatigue crack initiation and propagation from notch member, under cyclic tensile loading control, are investigated. Experiment shows that small crack initiates from notch specimen at far higher rate than that of smooth specimen; small crack propagation is still faster than that of smooth specimen within notch plastic field, though this difference is progressively lessening; beyond notch plastic zone, small crack growth rate is approaching long crack growth rate. Analysis via finite element and analytical method reveals that notch plasticity has key influence on small crack initiation, crack tip generated plasticity has critical impact on small crack propagation within notch plastic field, while plasticity induced crack closure has dominant effect on crack propagation out of notch plastic field. A comparison between experimental and analytical results is made to identify the mechanisms of small fatigue crack initiation and propagation within notch plastic field.     

Time dependent ductile crack extension in A533B class I steel

Four wide plate specimens manufactured in A533B Class I, 90 mm thick by 500 mm wide containing through-thickness or semi-elliptical surface fatigue cracks were tested at +70°C. These specimens were subjected to a series of increasing applied loads, each of 100 h duration, until failure. Testing was performed using a computer interactive 40 MN load controlled tensile testing rig. Values of the fracture toughness parameters J and crack tip opening displacement (CTOD) were derived from the recorded values of applied load, plate extension and crack mouth opening displacement.The influence of loading rate, degree of yield containment and crack orientation on the time dependent behaviour is assessed and compared with data obtained from wide plate and bend tests under monotonic loading and from bend tests conducted with a variable loading rate, with hold periods, under crack mouth opening control. Interpretation of the results provides a clearer understanding of low temperature time dependent ductile crack extension and enables the identification of the conditions under which this phenomenon is apparent, to allow the necessary adjustments to failure assessments.     

Experiments on the transferability of creep crack growth laws to tubes

The present study served to investigate creep crack growth experiments for a tube with a 180° surface crack in circumferential direction. The tube had an outer diameter of 197 mm and a wall thickness of 23.5 mm. It was loaded with four-point bending at 700°C. No solutions for calculation of the C *-integral were available in the literature for this testing geometry, so new approaches had to be found. An approximation method, the so-called limit analysis technique was used for this purpose. It requires the plastic limit load of a specimen and the displacement rate of the load application points as the experimental input parameters. Before the limit analysis technique was applied to the tube under bending, results from earlier creep crack growth experiments were used to verify this technique. The C * values determined from the limit analysis were compared with values based on the numerical calculations of the EPRI handbook. A good agreement was found for both calculation methods. It therefore seems justified to apply the limit load procedure also to the tube loaded in bending. The calculation on the basis of the limit analysis was performed in two ways, which differed by the type of crack front modelling. The simpler model yielded results that were too conservative in comparison with the experimental results of CT specimens. The application of the second model, which better covers the true crack front, provides values for da/dt versus C * which are actually within the scatter band of CT specimen results. This procedure therefore permits the transfer of standard specimen results to components.     

Monitoring of inside surface crack growth by strain measurement of the outside surface: A feasibility study

Cracks detected by in-service inspections are not always removed when they are judged to be not hazardous according to fitness-for-service evaluations. In order to secure the integrity of the cracked components, it is important to confirm that the cracks do not grow notably beyond the growth prediction conducted for the judgement. However, due to the limitation of accuracy of size determination by the current inspection techniques such as ultrasonic testing, it is difficult to know how much the cracks have grown since their previous measurement. In this study, feasibility of a crack growth monitoring method (outside strain monitoring method) was evaluated by finite element analyses and experiments. When a pipe deforms elastically due to internal pressure, the strain at its outside surface increases. The magnitude of strain near the crack differs from that at an uncracked portion, and the difference depends on the crack size. Elastic finite element analyses were performed for cracked pipes under internal pressure for various crack sizes. It was shown that, by measuring the change in strain at the outside surface of the cracked pipe, the crack size and how much the crack grew can be identified. In the experiment, cracked pipes were subjected to static internal pressure and strains for eight cracks of different sizes were measured. It was revealed that the maximum error was 0.44 mm for the estimation of crack depth of 4 mm and 0.28 mm for the estimation of 1 mm crack growth in the depth direction.     

Detection of embedded fatigue cracks in Inconel weld overlay and the evaluation of the minimum thickness of the weld overlay using eddy current testing

This study evaluates the applicability of eddy current testing to the detection and sizing of fatigue cracks embedded in Inconel weld overlays. Welded plate specimens, which model head penetration welds and their weld overlays, are fabricated, and fatigue cracks are artificially introduced into the specimens. Eddy current inspections are performed using a uniform eddy current probe driven with 10 kHz, and all of the fatigue cracks are detected with clear signals. Subsequent numerical inversions estimate that the minimum thicknesses of the weld overlays are 1.47, 2.17, and 2.23 mm, whereas true thicknesses revealed through destructive testing are 1.51, 3.25, and 2.10 mm, respectively. Thicknesses are also evaluated using potential drop and ultrasonic testing methods; the results demonstrate that eddy current testing is the most efficient of the three methods.     

Cyclic crack growth evaluation of a 20MnMoNi55 piping steel in high-oxygen reactor water

Samples of a low alloy steel piping material taken from the full scale corrosion fatigue test loop of the Heissdampfreaktor (HDR) plant have been tested at 240°C in high oxygen reactor water. The small-scale specimens (CT25) were exposed to a similar loading spectrum to that which has been used in the full-scale corrosion fatigue tests at the HDR-plant. During the autoclave tests cyclic crack growth rates were determined. Fracture surface investigations were performed not only for the laboratory test specimens but also for the fracture surface of a sample taken from the HDR test loop piping after the full scale test. In this paper the autoclave testing results and fracture surface observations are presented and compared to those obtained in the HDR piping tests.     

Fatigue cracks in Eurofer 97 steel: Part I. Nucleation and small crack growth kinetics

Fatigue crack nucleation and growth were studied in the Eurofer 97 ferritic-martensitic steel at room temperature. Cylindrical specimens with a shallow notch and no artificial crack starters were used. The constant strain amplitude cycling was adopted. First fatigue cracks nucleate at about 5% of the fatigue life along the surface slip bands. If a crack overcome the barrier of the first high angle boundary, its growth is regular and an exponential growth law is observed. This law may be used for the residual fatigue life prediction based on the small crack growth kinetics.     

Gaseous hydrogen embrittlement of PH 13-8 Mo steel

In this study, notched tensile and fatigue crack growth tests in gaseous hydrogen were performed on PH 13-8 Mo stainless steel specimens at room temperature. These specimens were susceptible to hydrogen embrittlement (HE), but at different degrees, depending on the aging conditions or the microstructures of the alloys. In hydrogen, the accelerated fatigue crack growth rate (FCGR) usually accompanied a reduced notched tensile strength (NTS) of the specimens, i.e., the faster the FCGR the lower the NTS. It was proposed that the same fracture mechanism could be applied to these two different types of specimens, regardless of the loading conditions. Rapid fatigue crack growth and high NTS loss were found in the H800 (426 °C under-aged) and H900 (482 °C peak-aged) specimens. The HE susceptibility of the steel was reduced by increasing the aging temperature above 593 °C, which was attributed to the increased amount of austenite in the structure. Extensive quasi-cleavage fracture was observed for the specimens that were deteriorated severely by HE.     

Surveillance of defects under cyclic loads and during hot and cold hydro-tests by acoustic emission, UT and potential drop techniques (German HDR-programme)

In the framework of the German HDR reactor safety programme a selected area of the reactor pressure vessel was loaded with 9500 thermal cycles. In 3 saw cut regions several crack fields were generated and monitored with on-line and off-line NDE-procedures. With potential drop and ultrasonic testing, crack front profiles could be described off-line with sufficient resolution. During the trials the crack growth was measured with stationary potential drop and ultrasound probes. With on-line acoustic emission crack growth of about 5 mm could be detected. On the basis of an interpretation model a classification of the acoustic emission signals in crack growth and crack surface friction phenomena was possible. The crack growth during the cooling phase is caused by fatigue; the reason for crack growth during the heating up phase is corrosion influence. All cracks are growing at different time periods and have different stress conditions.