Creep-fatigue test of a SA 316SS structure and comparative damage evaluations based upon elastic and inelastic approaches

A creep-fatigue in the reactor structures of a liquid metal reactor needs to be dealt with from a structural integrity point of view. This paper deals with the creep-fatigue damage evaluation of a typical geometrical discontinuity structure made of 316SS based upon both an elastic approach and an inelastic approach. The creep-fatigue structural tests of a 7 mm thick cylindrical structure and a 5 mm thick cylindrical structure have been carried out with a 1MN actuator and a 50 kW induction heater with a 1 h hold time at 550 °C. The strain behaviors, by an analytical method, and the damage evaluations were compared with those by tests. For an inelastic analysis by using the developed code NONSTA-VP, by implementing Chaboche's unified viscoplasticity model into ABAQUS, the methods to determine the material parameters are presented. The current comparisons show satisfactory results, but more efforts are needed to verify them in full.     

长期运行后汽轮机转子裂纹扩展行为的研究

对运行16年的30Cr1Mo1V亚临界汽轮机高中压转子进行解剖试验,采用直流电位法对材料在538℃下的蠕变裂纹、蠕变-疲劳裂纹萌生与扩展行为进行了研究,分析了不同初始应力强度因子对蠕变裂纹扩展孕育时间和蠕变裂纹扩展速率的影响,并对高温段和低温段的相关性能进行了比较,研究了不同保持时间对蠕变-疲劳裂纹扩展行为的影响,同时还分析了不同条件下裂纹扩展行为的时间或循环相关性．结果表明：疲劳缩短了蠕变-疲劳裂纹的扩展孕育期,加速了裂纹的扩展;载荷保持时间较短情况下,蠕变-疲劳裂纹扩展行为与循环相关;栽荷保持时间较长情况下,裂纹扩展行为与时间相关．     

Strain rate and hydrogen effects on crack growth from a notch in a Fe-high-Mn steel containing 1.1 wt% solute carbon

Effects of strain rate and hydrogen on crack propagation from a notch were investigated using a Fe-33Mn-1.1C steel by tension tests conducted at a cross head displacement speeds of 10⁻² and 10⁻⁴ mm/s. Decreasing cross head displacement speed reduced the elongation by promoting intergranular crack initiation at the notch tip, whereas the crack propagation path was unaffected by the strain rate. Intergranular cracking in the studied steel was mainly caused by plasticity-driven mechanism of dynamic strain aging (DSA) and plasticity-driven damage along grain boundaries. With the introduction of hydrogen, decrease in yield strength due to cracking at the notch tip before yielding as well as reduction in elongation were observed. Coexistence of several hydrogen embrittlement mechanisms, such as hydrogen enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) were observed at and further away from the notch tip resulting in hydrogen assisted intergranular fracture and cracking which was the key reason behind the ductility reduction.     

Creep fatigue crack behavior of two power plant steels

Crack initiation and crack growth under creep fatigue conditions were experimentally determined on a bainitic turbine rotor steel (30CrMoNiV4-11) and a martensitic pipe steel (X10CrMoVNb9-1). Side grooved compact tension (CT) specimens with 25 and 50 mm thickness as well as double edge notch tensile (DENT) specimens with 15 and 60 mm thickness have been tested in order to observe possible influences of geometry and thus to check the transferability of the specimen test results to the behavior of components.The creep fatigue crack test results can be described with the usual fracture mechanics parameters. A modified two-criteria-method can be used to estimate the crack initiation under creep fatigue conditions. The creep fatigue crack growth can be calculated from the accumulation of fatigue crack growth which is described by the Forman-law and creep crack growth which is described by the C^*-parameter.     

Mechanism of hydrogen-restrained crack propagation and practical application research of thermohydrogen treatment in a TiAl-based alloy

The mechanism of hydrogen-restrained crack propagation and practical application of thermohydrogen treatment in a TiAl-based alloy was investigated in this study. Hydrogenated and unhydrogenated alloys were subjected to high-temperature compression test, with a temperature range 1050–1200 °C and strain rate range 0.001–1 s⁻¹. The results showed that crack propagation was restrained due to hydrogen addition. The main mechanism of hydrogen-restrained crack propagation of such alloy was revealed that hydrogen-promoted lamella bending and hydrogen-decreased Young's modulus induced inter-lamellar cracks transforming into trans-lamellar cracks, decreasing cracks in the hydrogenated alloy. Additionally, hydrogen-induced mechanical twinning in γ-phase lamellae partly restrained inter-lamellar crack propagation. In the two-step forging process, the optimum forging parameters were determined. It was found that hydrogen could effectively restrain crack propagation during the two-step forging process. Hydrogen refined grains of the forged billets, which improved toughness of such billets. The hydrogen content of the forged hydrogenated billets could be decreased to a desired value, and the phase composition and content were basically identical to those of the initial unhydrogenated alloy.     

Influence of reference stress formulae on creep and creep-fatigue crack initiation and growth prediction in plate components

Creep and creep-fatigue crack growth in pre-cracked plates of 316L(N) austenitic stainless steel, containing a semi-elliptical surface defect and tested at 650 °C under combined axial and bending loading, are investigated. The results have been interpreted in terms of the creep fracture mechanics parameter C^∗ and compared with data obtained on standard compact tension (CT) specimens of the same material and batch. In making the assessments, the reference stress method has been used to determine C^∗. Several formulae exist for calculating the reference stress depending on whether it is based on a ‘global’ or a ‘local’ collapse mechanism and the assessment procedure adopted. When using this approach, it has been found that the most satisfactory comparison of crack growth rates with standard CT specimen data is obtained when the ‘global’ reference stress solution is used in conjunction with mean uniaxial creep properties. It has been found that the main effect of changing the fatigue cycle range from 0.1 to −1.0 is to cause an acceleration in the early stage of cracking.     

Effect of residual hydrogen content on the tensile properties and crack propagation behavior of a type 316 stainless steel

The tensile properties and crack propagation rate in a type 316 austenitic stainless steel prepared by vacuum induction melting method with different residual hydrogen contents (1.1–11.5 × 10⁻⁶) were systematically investigated in this research work. The room temperature tensile properties were measured under both regular tensile (12 mm/min) and slow tensile (0.01 mm/min) conditions, and the fracture properties of the tensile fractures with both rates were analyzed. It shows that the hydrogen induced plasticity loss of stainless steel strongly depends on the tensile rate. Under regular tensile condition, there is no plastic loss even when the hydrogen content is up to 11.5 × 10⁻⁶ while in the slow tensile condition, the plastic loss can be clearly identified rising with the increasing H contents. The fatigue crack propagation rate was tested at room temperature, and the crack growth rate formula (Paris) of the 316 stainless steels with varied H contents were obtained. The fatigue crack propagation rate test shows that the crack growth rate of the 316 stainless steel with 8.0–11.5 × 10⁻⁶ hydrogen is significantly higher than that of benchmark steel.     

Creep crack growth testing of P91 and P22 pipe bends

Laboratory component tests play an important role in the development of life assessment procedures for high temperature crack initiation and growth. Thus, the working programme of the project BE 1702 HIDA, which addressed the validation, expansion and harmonisation of existing procedures for high temperature defect assessment, included a comprehensive experimental programme with feature tests of components as its core. Because of their relevance for the high temperature industry, P91 and P22 were included in HIDA among five materials. This paper presents laboratory creep crack growth tests of P91 and P22 pipe bends, discusses the test experience and draws some conclusions for laboratory component tests in general. The components were prepared with spark-eroded notches at the outer surface. The test temperature was 625°C for P91 and 565°C for P22.     

Fatigue crack propagation under gaseous hydrogen in a precipitation-hardened martensitic stainless steel

A test device has been developed at P′ Institute in order to investigate the mechanical behaviour of structural materials under high pressure of gaseous hydrogen. In this paper, preliminary results on fatigue crack propagation in a martensitic stainless steel are presented. A tremendous fatigue crack growth enhancement is observed at high pressure (9 MPa). This enhancement is dependent of pressure. It is noticed that the maximum enhancement is associated with a brittle fracture mode. However no intergranular decohesion is noticed in this regime.     

Effect of hold-time on low cycle fatigue behaviour of nitrogen bearing 316L stainless steel

Axial strain controlled low cycle fatigue (LCF) behaviour and creep–fatigue interaction behaviour of 316L(N) stainless steel (SS) in solution annealed condition has been investigated at various strain amplitudes. Creep–fatigue interaction behaviour of the material has been evaluated by employing tension and compression holds at peak strain. Tension holds up to 90 min were studied. Under all the testing conditions, the material showed initial hardening followed by stress saturation. Hold-time conditions generally showed lower cyclic stress response compared to continuous cycling. The decrease in cyclic stress response with hold-time is attributed to enhanced recovery of the substructure and increase in the grain boundary damage accumulated during the stress relaxation period. The fatigue life is observed to be lower in tensile-hold conditions and the endurance decreased with increase in the duration of the hold. The factors that contribute to the decrease in fatigue life with hold-time have been identified from metallographic studies as the development of creep cracks and cavities and crack initiation and propagation assisted by oxidation. Creep–fatigue damage values based on the damage summation rule have been computed and compared with the RCC-MR bi-linear creep–fatigue interaction diagram suggested for nitrogen alloyed 316L SS at 600°C.     

Retardation of fatigue crack propagation by indentation technique

This paper used indentations to retard crack development and thus to prolong the crack growth life. The growth retardation resulting from the indentation-induced strain hardening and the crack closure due to indentation-induced residual stresses were explored. The retardation tests using 3–10 kN indentation loads were performed on different thickness specimens of AISI 4130 low alloy steel and AISI 304 stainless steel. These loads were applied using a hemispherical indenter to both sides of the expected crack path on the specimen surfaces. Loads of 4.5 kN and greater increased the microhardness at and around the indentation position, indicating that at those loads the hardness or the strain hardening contributed to retarding crack growth. In addition, all the loads caused different levels of crack closure. The greater the loads, the stronger were the crack closure effect and the accompanying growth retardation. In the 3.5 mm thick AISI 4130 specimens, a 10 kN load exerted the strongest growth retardation to arrest the post-indentation crack propagation.     

Dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation

With a validated fluid-structure-fracture coupling approach, this paper studied the dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation. Fracture of tube was judged by a bivariate failure criterion derived from the underlying failure mechanism at high strain rate conditions. A programed burn approach based on the CJ theory was applied to simulate gaseous detonation. The coupling between detonation wave and tube was realized by penalty contact algorithm with an improved contact stiffness calculation formula. It was demonstrated that the peak pressure at tube edge is 29% higher than that at the middle of tube face. The dominant crack driving force comes from the specific vibration and deformation modes of square tube, where the deformed round section of tube corresponds to the maximum stress wave that travels behind the flexural waves on the tube. Above mechanism makes the backward cracks branch or turn before the forward cracks and the speeds of front and back branch cracks comparable to each other, which is opposite or different from the cases of round tubes. The crack behaviors with different initial flaw locations and detonation pressures were summarized and identified in detail. The forward crack speed can be up to 900 m/s, while the backward crack speeds are generally 65%–85% of above and the branch cracks run at about 100 m/s. In addition, the crack speed has a certain increase immediately after crack branching or turning. Among the three initial flaw location cases, the tube with initial flaw at the middle of face is most resistant to crack propagation under internal detonations.     

Creep crack growth of high temperature weldments

The experimental programme of the EC supported project (SMT 2070) SOTA aimed at addressing a technical and industrial need to provide guidelines for creep crack growth (CCG) testing and data analysis of weldments. Mechanical and creep properties were determined on two pressure vessel steels of P22 (2.25Cr1Mo) and P91 (9Cr1MoVNb). The specimens were taken from pipe welds for weld metal (WM) tests, and simulated heat affected zone (HAZ) material for the HAZ tests.The CCG tests were carried out on cross-weld compact tension (CT) specimens machined out from weldment of pipes. The tests were done at 550 and 600°C on P22 and P91 materials, respectively. The CT specimens were notched using electrical discharge method, for a sharp starter crack. This method of initiating sharp starter crack was chosen to make sure that all partners will test specimens with starter crack location as specified in the work programme to study crack initiation and growth in WM and HAZ (both in the centre and type IV region). The CCG tests were carried out following the ASTM E1457-92 [ASTM E1457-92, Standard test method for measurement of creep crack growth rates in metals, ASTM, Philadelphia, PA 19103, USA]. The partners assessed their data and sent both assessed and their raw data to be further assessed centrally. All the data from partners were analysed and compared with those of partners' own assessed data.The present paper reports on the analyses of CCG data obtained in the programme including six laboratories from six European countries. The programme addresses the differences and difficulties in testing and the assessment of weldments, and provides guidelines for harmonisation of testing procedures for reliable data production for remanent life assessment of plants with welded components.     

Simulation of thermal fatigue damage in a 316L model pipe component

To contribute to the development of improved methods for assessing possible thermal fatigue damage in nuclear plant piping systems, a unique set of crack growth data has been generated for tubular test pieces in 316L(N) stainless steel subjected to cyclic thermal loads in a specially designed rig. By accurate modelling of the thermal loads and non-linear material behaviour using the finite element method, it was possible to reliably estimate the number of cycles to initiation, using standard isothermal fatigue life curves. To simulate crack growth, an engineering method was applied using published K solutions for semi-elliptical surface cracks and via 3-D elastic–plastic cracked-body analysis of selected scenarios. It was established that conservative estimates of the thermal fatigue crack growth can be obtained using the engineering model in conjunction with an upper bound fatigue crack growth law.     

Fracture investigations on piping system having large through-wall circumferential crack

In level-3 Leak-Before-Break (LBB) analysis, stability of a postulated through-wall circumferential crack is demonstrated by simplified fracture mechanics calculations. Detailed experimental studies, conducted by the authors, have revealed that the conventional assessment procedure used to demonstrate LBB is too conservative. There is a large factor of safety due to system indeterminacy. It was observed that the critical load of a cracked piping system (with even a large through-wall circumferential crack of about 120°) is of the order of 75–90% of the collapse load of the uncracked piping system. Reduction in load carrying capacity is even less for a piping system having an off-centre crack. This article discusses the above-mentioned aspects in detail. Detailed 3-D elastic-plastic finite element analyses of some of these tests were performed. The suitability of these numerical results to predict crack initiation load in light of the experimental data is discussed.     

Remaining life assessment and optimal design of statically indeterminate pipe system with circumferential crack

This paper examines the J–R curve theory in elastic-plastic fracture mechanics and crack extension resistance in order to analyze the large-scale yielding of statically indeterminate pipe structure with a circumferential crack. The dJ/da characteristic can be utilized to measure that the crack is stable ductile fracture or unstable ductile fracture and to analyze the crack growth rate. The total propagation time of the crack until collapse can be computationally estimated by numerical analysis that is presented in this paper. The further analysis of dJ/da and optimization is to maximize the crack propagation time during the crack growth before reaching plastic collapse. Both of the center-point crack and end-point crack are investigating cases for analyzing the mechanics and engineering design. These analysis and design strategies developed in this paper are useful for the safety performance of a structural pipe under crack deformation.     

Overview of the HIDA Project

HIDA is a recently completed four-year-duration large research, development and validation project aimed at harmonising and expanding current high temperature crack assessment procedures and practices with a view to developing a unified European/International procedure. The project was started in 1996 and completed in early 2000. The work involved analysis of currently available published and unpublished data and generation of data from various standard fracture mechanics and validation type feature specimen tests on a range of steels used in high temperature plants. The project covered creep, high temperature fatigue and creep-fatigue interaction.This paper outlines the background issues and methodology applied to achieve the project objectives. Details are covered in other HIDA papers presented at the first HIDA Conference (held at CEA Saclay, France, in April 1998),¹ and also in the HIDA papers being presented at this Conference.The project Consortium now intends to expand this work through a new European Commission funded project ‘HIDA Applicability’ to incorporate sensitivity and probabilistic assessment.     

Hydrogen embrittlement and associated surface crack growth in fine-grained equiatomic CoCrFeMnNi high-entropy alloys with different annealing temperatures evaluated by tensile testing under in situ hydrogen charging

The effect of the annealing temperature after cold rolling on hydrogen embrittlement resistance was investigated with a face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy using tensile testing under electrochemical hydrogen charging. Decreasing annealing temperature from 800 °C to 750 °C decreased grain sizes from 3.2 to 2.1 μm, and resulted in the σ phase formation. Interestingly, the specimen annealed at 800 °C, which had coarser grains, showed a lower hydrogen embrittlement susceptibility than the specimen annealed at 750 °C, although hydrogen-assisted intergranular fracture was observed in both annealing conditions. Because the interface between the FCC matrix and σ was more susceptible to hydrogen than the grain boundary, the presence of the matrix/σ interface significantly assisted hydrogen-induced mechanical degradation. In terms of intergranular cracking, crack growth occurred via small crack initiation near a larger crack tip and subsequent crack coalescence, which has been observed in various steels and FCC alloys that contained hydrogen.     

Creep crack growth of seam-welded P22 and P91 pipes with artificial defects. Part I. Experimental study and post-test metallography

This paper describes the experimental work conducted on creep and fatigue crack growth behaviour in axially-notched, seam-welded pipes. The key objective of this work is to examine how the results from feature tests compare with standard laboratory specimen data using different European high temperature assessment methods, the final aim being to contribute towards the development and validation of a unified European procedure for estimating crack growth behaviour and remaining life of high temperature components. Within the context of this work, two steels P22 (2 1/4Cr1Mo) and P91 (9Cr1MoVNb) were studied at 565 and 625 °C, respectively.Two pipes were tested for each material; one pipe was subjected to a constant gas pressure (SP) and the other was tested under low cyclic pressure (CP) (10⁻⁴ Hz). Each pipe contained three axially machined notches, one in the heat affected zone (HAZ) and two in the base metal. The direct current (DC) potential drop (PD) technique was successfully applied to monitor crack growth during the tests. In both steels, the defect situated in the HAZ exhibited a higher crack growth rate, confirming that the HAZ is more vulnerable to crack initiation and propagation than the parent metal. Post-test metallography analysis showed that creep cavitation damage is the main mechanism governing the crack propagation. In the case of the cyclic tests for which the selected frequency was close to, or slightly higher than, the frequencies encountered in high temperature plants, metallographic observations showed no noticeable effect of cyclic loading in terms of transgranular crack growth.The data analysis of the experimental data obtained in this work are presented in Part II of this paper [Creep Crack Growth of Seam-welded P22 and P91 Pipes with Artificial defects—Part II. Data analysis. Second International HIDA Conference, Advances in Defects Assessment in High Temperature Plant, MPA, Stuttgart, Germany, 4–6 October, 2000].     

Ex situ characterization and modelling of fatigue crack propagation in catalyst coated membrane composites for fuel cell applications

Interactions between catalyst layers and membrane are known to influence the mechanical properties of catalyst coated membrane (CCM) composites used in fuel cells, and can further affect their fatigue-driven mechanical fracture — an important lifetime-limiting failure mode in automotive applications. Here, the fracture propagation phenomenon in CCMs is characterized through a series of ex situ experiments and microstructural investigations conducted across a range of stress, temperature (23-70 °C), and relative humidity (50–90%) conditions relevant to low-temperature polymer electrolyte fuel cells. In comparison to pure membranes, the crack propagation rates are slightly arrested in CCMs through mechanical reinforcement offered by the catalyst layers; however, the membrane layer still controls the overall crack growth trends through its temperature and humidity dependent ductile fracture characterized by confined yielding around the fracture surface. Local interfacial delamination and severe electrode cracking are found to accompany the CCM crack propagation, which aids membrane fracture by loss of local reinforcement. A Paris law based fracture modelling framework, incorporating the elastic-viscoplastic mechanical response of CCMs, is developed to semi-analytically evaluate one-dimensional crack growth rate during cyclic loading, and provides reasonably accurate predictions for the present ex situ problem.