Crack growth resistance methodologies for austenitic stainless steels

The paper reviews the methodologies that are being used to quantify the effect of cracks in austenitic stainless steel structures. As a framework for discussion, these methodologies, i.e. net-section stress and tearing modulus, used to describe respectively the onset of crack extension and crack stability, are considered with regard to the behaviour of a circumferential through-wall crack in a piping system.     

Crack growth monitoring by strain sensing

A simple method, based on the changes in strain pattern which are induced in a body by crack extension, is proposed for the measurement and sensing of crack growth. Experimental evidence is presented to demonstrate its ability to monitor the growth of cracks emanating at the bore surface of thick-walled cylinders, and detect crack instability during fracture toughness tests.     

Study of creep cavitation in stainless steel weldment

Abstract

A study of creep cavities near reheat cracking in AISI Type 316H austenitic stainless steel headers, removed from prolonged high temperature operation in nuclear power plants, is reported. It is shown how application of scanning electron microscopy (SEM), cryogenic fractography and small angle neutron scattering (SANS) can be applied, in a complementary way, to observe and quantify creep cavitation. Creep cavities in the vicinity of the crack are found to be mainly surrounding inter-granular carbides. Trends in the size and area fraction of creep cavities relative to the crack path are quantified using optimised metallography. The SANS technique is found to be a very suitable method of quantifying creep cavitation within the size range up to 600 nm averaged over a larger gauge volume. It is shown that the cavity size distribution peaks in the region 100–300 nm, and this correlates closely with the quantitative SEM observations.     

Crack resistance of austenitic pipes with circumferential through-wall cracks

For a monotonously increasing load the correct evaluation of the crack resistance properties of a structure is essential for safety analyses. Considerable attention has been given to the through-wall case, this is generally believed to be the controlling case with regard to complete pipe failure. The maximum load conditions for circumferential crack growth in pipes under displacement-controlled loading has been determined. The need for crack resistance curves, measured on circumferentially through-wall cracked straight pipes of austenitic stainless steel 316 L under bending, is emphasized by the limitation in the data range from small specimens and by the differences in the fracture mechanics procedures. To address these issues and to improve calculational methods a joint fracture mechanics programme is being performed by Electricité de France, Novatome and Siemens-Interatom. The working programme contains experimental and theoretical investigations on the applicability of small-specimen data to real structures.     

Co-planar bi-metallic interdigitated electrode substrate for spin-coated organic solar cells

A bulk heterojunction organic solar cell with co-planar interdigitated electrodes was fabricated and tested. The co-planar electrodes had a separation distance of 1-3 μm and were fabricated from aluminum and nickel on a heavily oxidized silicon wafer using photolithography. The device was prepared by spin-coating MEH-PPV:PCBB in a 1:3 wt ratio with a total donor:acceptor solution concentration of 2.44%. The device demonstrated a strong photovoltaic response under AM1.5 illumination of 80 mW/cm² with an open circuit voltage of 0.704 V. The co-planar electrode design offers advantages in terms of electrode material selection and reliability as well as simplified device fabrication.     

Continuous monitoring of back wall stress corrosion cracking growth in sensitized type 304 stainless steel weldment by means of potential drop techniques

Stress corrosion cracking (SCC) tests on welded specimens of sensitized type 304SS with a thickness of 20 mm were performed in sodium thiosulphate solution at room temperature, with continuous monitoring of the SCC growth, using the techniques of modified induced current potential drop (MICPD), alternating current potential drop (ACPD) and direct current potential drop (DCPD). The MICPD and DCPD techniques permit continuous monitoring of the back wall SCC, which initiates from a fatigue pre-crack at a depth of about 4 mm, from which it propagates through more than 80% of the specimen thickness. The MICPD technique can decrease the effect of the current flowing in the direction of the crack length by focusing the induced current into the local area of measurement using induction coils, so that the sensitivity of the continuous monitoring of the back wall SCC is higher than that of the ACPD and DCPD techniques.     

Theoretical and experimental studies of a bi-metallic heat switch for space applications

This work presents theoretical and experimental studies on a passively actuated bi-metallic heat switch for space applications. The working principle of the heat switch is based on the differential thermal expansion of distinct metals. Analytical one-dimensional and two-dimensional heat conduction models are developed to predict the thermal resistance of the heat switch, which is a function of temperature. A non-dimensional parametric analysis is performed in order to study how the design parameters affect the total thermal resistance of the heat switch. The theoretical models are also compared with experimental data obtained from a prototype of the heat switch. The agreement between theory and experiments is good.     

Crack growth rate in hydrogen pre-charged martensitic steels during slow strain rate tests

Crack growth rate in two high strength martensitic steels with the Mo contents of 0.43 wt.% and 1.06 wt.% was investigated by means of slow strain rate tests (SSRT) on compact tensile specimens after hydrogen pre-charging. It was found that the crack growth rate increased and the values of stress intensity factors K_IH and K_Imax decreased with the increase of pre-charged hydrogen concentration. The steel with higher Mo content showed much lower crack growth rate than the steel with lower Mo content. It could be attributed to more nano-sized precipitates that can act as the hydrogen trapping sites and mitigate hydrogen deleterious effects on crack growth rate and the K_IH and K_Imax values.     

Coverage-dependent electro-catalytic activity of Pt sub-monolayer/Au bi-metallic catalyst toward methanol oxidation

The coverage-dependent catalytic properties of a Pt nanofilm formed on a Au substrate were investigated for the electro-oxidation of methanol. The coverage of Pt nanofilm was precisely fabricated by the formation of coverage-controlled under potential deposition of Cu adlayer and followed by the surface limited redox replacement reaction with different Pt complex ions. The STM images exhibit the formation of Pt nano-film on Au substrate with different coverage. It was clearly shown that the activity of Pt nanofilm deposited on Au substrate toward the methanol electro-oxidation was highly sensitive to its surface coverage. Pt–Au bimetallic catalyst was found to become active at the Pt surface coverage near 0.2 and reached its maximum around 0.6. The electro-catalytic activity as well as CO tolerance on Pt–Au bimetallic system was found higher than those on a Pt electrode.     

Crack initiation and crack growth assessment of a high pressure steam chest

Extensive cracking had occurred in a number of high pressure steam chests. An assessment was undertaken based on the R5 British Energy methodology to assess the components for both creep–fatigue damage initiation and crack growth analysis to determine fitness for purpose. The analysis determined that the remaining base rupture endurance life of the component was greater then 1 million hours, however, due to the start-up and shutdown ramp rates, creep–fatigue damage greater then unity has occurred leading to crack initiation in a number of locations. These cracks were confirmed during internal inspection of the steam chest. A subsequent crack growth analysis determined that the component could safely be returned to service for the expected future life of the station.     

Comparative studies of stress-relief cracking in relaxation test specimens and in a full-scale weldment

An attempt is made to assess the susceptibility of stress-relief cracking (SRC) of simulated HAZ microstructures in heated specimens of SA508 Class 2 steel, using a notched bend relaxation test. The ability of this test to reproduce service conditions is explored by comparing cracking behaviour and the fractographic details of the cracks produced with observations made on stress-relief cracks in actual weldments of SA508 Class 2 obtained from the same manufacturer.It is shown that SRC in the relaxation test and in the weldments exhibited similar fractographic features and micromechanisms of cracking. From the test specimens, it was found that a critical value of stress-intensity factor was necessary to produce ‘severe’ cracking, and application of this value to the cracking observed in the weldments appears to be fully consistent with the details of weldment manufacture. It is proposed that a stress-intensity approach of this sort can provide a general method for the assessment of possible SRC in welded components.     

Residual stress relief and local embrittlement in a A533B reactor pressure vessel weldment

A suitable testing technique is developed for evaluating the effect of weld residual stress on fracture toughness in the HAZ region of a reactor pressure vessel steel (SA533 Grade B, Class 1) and for investigating its susceptibility to reheat and underclad cracking or local embrittlement in the coarse-grained HAZs during post weld heat treatment. Use of a fracture mechanics approach leads to an interpretation of the effect of weld residual stress, tempering due to PWHT and notch acuity on the HAZ fracture toughness. This is applied to the estimation of COD transition temperature in the actual component of the reactor pressure vessel in terms of a summation of the various changes in the COD transition temperature associated with several factors such as weld residual stress, tempering, notch acuity and environment. Detailed comments are given on the determination of an allowable crack size in an actual component.     

The study on ductile fracture of the over-matched weldment with mechanical heterogeneity

The ductile fracture of structural steel including weldment can be described as a progressive process with void nucleation, growth and coalescence. The effects of mechanical heterogeneity of the weldment were investigated experimentally on the ductile fracture behaviors of the base metal and the weld metal using round-bar tensile notched bars. The results show that the mechanical heterogeneity of weldment has certain effects on fracture strain, stress triaxiality at fracture, critical void growth and the material constant C. The smaller the distance between the notch root and the fusion line, the larger the fracture stain, the smaller the stress triaxiality and the larger the critical void growth rate.     

The evaluation of weldment creep strength reduction factors by experimental and numerical simulations

The current design rules for welds are usually based upon the uniaxial creep rupture strength data. The effects of the stress multiaxility and the corresponding stress redistribution process of welded components are relatively ignored. As the present high-temperature testing techniques require large resources when testing welded components in full scale, the simulation of the effects will rely more on the numerical modelling. To evaluate the weldment joint efficiency this paper has proposed a general procedure in which the spatial distribution of constitutive parameters is determined by uniaxial testing while the creep process of components is simulated by numerical methods. Finite element methods are employed in the creep analysis of an AISI 316 butt-welded joint in pressurised tubes with a creep soft weld. To interpret the rupture behaviour of the tubes, different criteria are used to predict the rupture life. On the basis of the predicted structural rupture performance equations, the weldment creep reduction factors are evaluated for different design lifetimes. The reduction factors defined by the ASME code principle are found to be non-conservative in this case.     

Hot tensile properties of simulated heat-affected zone microstructures of 9Cr---1Mo weldment

The heat-affected zones (HAZ) of 9Cr---1Mo steel weldments consist of coarse-grain martensite with δ-ferrite, coarse-grain martensite, fine-grain martensite and intercritical structure. These HAZ microstructures have been simulated with respect to microstructure, hardness and grain size by isothermal heat treatment cycles in the temperature range 973–1573 K. Tensile tests at a strain rate of 3 × 10⁻⁴s⁻¹ over the range 298–873 K showed that 0·2% YS and UTS values for the various microstructural conditions were in the descending order of coarse-grain martensite, coarse-grain martensite with δ-ferrite, fine-grain martensite, base metal and intercritical structure. The presence of δ-ferrite in the coarse-grain region of HAZ restricted the grain growth, the increase in strength and the decrease in ductility. This minimises the problem of cracking in the coarse-grain region of HAZ in this steel which is often encountered in low-alloy Cr---Mo ferritic steels. The regions with intercritical structure showed the lowest strength and fracture energy. Fine-grain martensite exhibited a good combination of strength and ductility in the HAZ.     

Geometry and orientation effects on ductile crack growth resistance

Elastic-plastic resistance curves have been measured on 20 mm thick plate to API 5LX65 specifications for the three-point bend and centre-cracked tension geometries in the transverse orientation, and a comparison made with their longitudinal counterparts. An additional series of tests employing side grooving, in order to eliminate shear lips for these geometries, was also conducted. The results from these tests indicate that a dramatic decrease in the slope of the resistance curve results from the removal of the shear lips. However, toughness values relating to the initiation of tearing appear little affected by the side grooving.A distinct geometry effect between the tension and bend configuration is apparent even with the absence of shear lips. The tension geometry would appear to display a consistently higher resistance to crack growth than the equivalent bend configuration. However, the difference between the crack growth curves for the side grooved bend and tension geometries is small when compared to the effect that the shear lips have on the propagation resistance.Ductile instabilities were induced on side grooved centre-cracked tension plates. A ductile instability analysis for this geometry was deduced and a good correlation between theory and practice achieved. The importance of the compliance of the test machine is emphasised in this analysis.Calculations have also been made concerning the tearing instability parameter T and estimates made of the crack opening angles for the various configurations, based on the initial slope of the resistance curves.     

基于塑料光纤裂缝传感器的裂缝开度预测

针对混凝土工程裂缝难以准确预测的问题,结合利用塑料光纤裂缝监测技术所得到的试验数据,运用多项式回归和权值分配等数学方法,建立了一种考虑到裂缝与塑料光纤呈不同夹角时的影响的裂缝开度预测模型,并将模型预测的裂缝开度值与实测值进行比较分析。结果表明,预测值与实测值具有较好的拟合精度,可见将该模型与塑料光纤裂缝监测技术结合用于预测裂缝扩展宽度具有可行性,且预测精度较高。     

基于裂缝口开度的水工混凝土结构裂缝性态监控方法

针对复杂工况下带缝水工混凝土结构的裂缝扩展判据难以用于工程实际,且常规有限元法扩展法需不断进行网格重建的问题,推导了包含裂尖效应的渗流—应力耦合的扩展有限元模型,以此分析并建立了裂缝口开度与宏观裂尖开度在裂缝扩展全过程的关系,提出了基于粘着力的宏观裂尖判别法,并从数值分析角度验证了宏观裂尖开度临界值的无尺寸效应,利用其与裂缝口开度的关系建立了水工混凝土结构裂缝起裂、稳定扩展、失稳扩展全过程的监控方法,最终通过算例验证了该方法的合理性。     

Fatigue crack propagation characteristics of submerged arc weldment material in a simulated PWR environment at the outlet coolant temperature

The present work compares the fatigue crack growth data for A508 Cl.III material in a high and low bulk sulphate PWR water environment at 325°C, the PWR coolant outlet temperature. Through careful fractography and a knowledge of the non-metallic sulphide distribution, it emerges that environmentally assisted crack (EAC) growth exhibiting plateau growth behaviour mainly occurs, irrespective of sulphate level over the range 70 ppb (10⁻⁹) to 1000 ppb and chloride level over the range 40 to 300 ppb, when a growing fatigue crack encounters large non-metallic sulphide inclusion clusters.