Creep/fatigue crack growth testing standardisation assessment and validation for specimens containing weld mismatch

Abstract

Testing and modelling of creep and creep/fatigue in welded components are essential tools for improving safe lifing methodologies. Key factors which determine a successful methodology for modelling failure in engineering components are good testing data and the development of appropriate and accurate correlating parameters to treat the results in a the unified and verifiable manner. This work considers the first two aspects and identifies important issues and improvements in the development and standardisation in creep and creep/fatigue crack growth testing or welds. Under the auspices of the Versailles Agreement on Materials and Standards (VAMAS) committee a Code of Practice on creep crack growth of components has been developed. The procedure identifies methods of testing for non-standard, welded feature test components in VAMAS TWA31 Technical Working Area on Creep and Creep/Fatigue Crack Growth of weldments containing residual stress. The overview highlights the important elements in these pre-standardisation collaborative efforts by presenting the methods of analyses and example of their application to standard fracture mechanics weld and feature type plate and pipe specimens.     

Crack growth predictions in a complex helicopter component under spectrum loading

Fatigue crack growth predictions have been made on a helicopter round‐robin crack configuration. The crack configuration was a small corner defect at the edge of a large central hole in a flanged plate made of 7010 aluminium alloy and the component was subjected to a simulated helicopter spectrum loading. The crack growth rate data and the stress‐intensity factor (K) solution for the crack configuration were provided in the round‐robin. The FASTRAN life‐prediction code was used to predict fatigue crack growth under various load histories on the aluminium alloy, such as Rotorix and Asterix, on both compact tension C(T) specimens and the complex crack configuration. A BEASY three‐dimensional stress‐intensity factor solution for the round‐robin problem was also provided for this paper and is compared with the original K solution. Comparisons are made between measured and predicted fatigue crack growth lives for both crack configurations. The predicted lives for the C(T) specimens were 15–30% longer than the measured lives; and crack growth in the round‐robin configuration agreed very well in the early stages of crack growth, but the life was 30% short of the test results at the final crack length.     

Interpretation of creep crack initiation and growth data for weldments

Creep crack growth testing of macroscopically homogeneous materials is well established and standardised test procedures are detailed in ASTM E1457. In ASTM E1457 the use of the compact tension C(T) specimen is specified and crack growth data are interpreted using the fracture mechanics parameter C^∗. The resulting benchmark crack growth data are used in assessment procedures, together with estimates of the value of C^∗ in the component, to predict creep crack growth behaviour. In this work, the results of a series of creep crack growth tests performed on a Type 316 stainless steel weldment at a temperature of 550 °C are presented. The initial crack is located within the heat affected zone (HAZ) of the weldment. Since there are currently no agreed methods for determining C^∗ in inhomogeneous laboratory specimens, this paper examines the application of procedures in ASTM E1457 for the characterisation of crack growth in weldments. In addition, the creep toughness parameter is evaluated for the material. It is shown that the creep crack growth rates in the weldment may be described by the C^∗ values obtained following ASTM E1457 in conjunction with parent material properties. Comparison of the results with similar data for Type 316H stainless steel parent material shows that the crack growth rates for the crack located within the HAZ are higher and the initiation times lower than the parent values, for the range of test conditions examined. Simple analytical models based on ductility exhaustion arguments have been shown to bound the crack initiation and growth data for the weldment.     

Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

Abstract

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.     

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).     

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.     

Improved test method and analytical modelling for fatigue crack growth in coarse‐grain titanium alloy with rough fatigue surfaces

Fatigue crack growth rate properties are typically determined by experimental methods in accordance with ASTM Standard E647. These traditional methods use standard notched specimens that are precracked under cyclic tensile loads before the main test. The data that are produced using this approach have been demonstrated elsewhere to be potentially adversely affected by the test method, particularly in the threshold region where load reduction (LR) methods are also required. Coarse‐grained materials that exhibit rough and tortuous fatigue surfaces have been observed to be strongly affected by the tensile precracking and LR, in part because the anomalies caused by crack closure and roughness‐induced closure become more important. The focus of the work reported in this paper was to further develop methods to determine more accurate fatigue crack growth rate properties from threshold through to fracture for coarse‐grained, β‐annealed, titanium alloy Ti‐6Al‐4V extra low interstitial thick plate material. A particular emphasis was put upon the threshold and near threshold region, which is of strong importance in the overall fatigue life of components. New approaches that differ from the ASTM Standard included compression precracking, LR starting from a lower load level and continuing the test beyond rates where crack growth would otherwise be considered below threshold. For the threshold regime, two LR methods were also investigated: the ASTM method and a method where the load is reduced with crack growth such that the crack mouth opening displacement is held constant, in an attempt to avoid remote closure. Constant amplitude fatigue crack growth rate data were produced from threshold to fracture for the titanium alloy at a variety of stress ratios. Spike overload tests were also conducted These data were then used to develop an improved analytical model to predict crack growth under spectrum loading and the predictions were found to correlate well with test results.     

Study of polyethylene pipe resins by a fatigue test that simulates crack propagation in a real pipe

A fatigue test that simulates the step-wise crack propagation found in pipes in the field, and uses a standard compact-tension specimen, was employed to study and rank crack resistance of various pipe resins. The thermal history during compression moulding of the test specimens strongly affected fracture kinetics. It was found that crack-resistant properties of in-service pipe were best reproduced if compression-moulded plaques were fast cooled under load. This procedure was used to prepare specimens from candidate pipe resins for fatigue testing. The resins were compared in terms of discontinuous crack growth kinetics. The ranking based on resistance to fatigue crack propagation correlated with results of a standard PENT creep test. However, fatigue failure times were an order of magnitude less than the standard creep times. After comparing the initiation and failure times of the resins with detailed kinetics of step-wisse crack propagation, a simplified and rapid procedure is proposed which calls for evaluating only the first jump after initiation.     

Description of creep and creep fatigue crack growth in 1% and 9% Cr steels

Creep crack growth tests on a 1CrMoV steel are presented, covering the aspects of specimen size, geometry and service-like stresses. To consider nonstationary loading in modern plants creep fatigue crack growth tests have been started. As test materials a 1CrMoV steel and a modern 9%Cr-steel were used. By means of a comparison of creep crack and creep fatigue crack results the effectiveness of the fracture mechanics parameters K₁, ΔK₁, and C* could be evaluated.     

Delayed failure of a commercial vitreous bonded alumina

The static fatigue resistance of a commercial low-cost alumina is evaluated. Flexural stress rupture results are compared to fracture mechanics crack growth testing. The original goal of this work was to develop satisfactory experimental procedures for the two test methods prior to more extensive testing on high-performance ceramics such as silicon nitride. The static fatigue trends measured by the two methods are comparable at 1000° C. The results are consistent with a model of static fatigue involving microcrack growth, coalescence and fracture due to stress corrosion. Creep deformations are very small, suggesting that creep fracture is not the mechanism of failure. A refined method for double torsion testing is presented.     

Creep–fatigue crack growth of intermetallic compound Ni3 Al(B) at elevated temperatures

Abstract

The creep–fatigue crack growth of Ni₃ Al(B) alloy was investigated at elevated temperatures in air under four different loading waveforms. Two types of time dependent damage mechanisms have been identified: oxidation and creep effects. As compared with fatigue crack growth in air at room temperature, the effect of oxidation at the crack tip on the crack growth acceleration is significant. Creep effects, on the other hand, are dominant for tensile holding and slow–fast loading waveforms. The complicated interaction between creep–fatigue, oxidation induced embrittlement, and oxide induced crack closure determined the different fatigue crack growth behaviours for different loading waveforms at elevated temperature. The relationship between the constants C and m in the Paris formula and loading waveform were examined, and the influence of loading waveform on the crack propagation were also discussed.     

Improved test method for very low fatigue‐crack‐growth‐rate data

Currently, in North America, the threshold crack‐growth regime is experimentally defined by using ASTM Standard E647, which has been shown in many cases to exhibit anomalies due to the load‐reduction (LR) test method. The test method has been shown to induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold. In this paper, the fatigue‐crack growth rate properties in the threshold and near‐threshold regimes for a titanium alloy, Ti‐6Al‐4V (STOA), are determined by using the LR test method and an improved test method. The improved method uses ‘compression–compression’ precracking, as developed by Pippan, Topper and others, to provide fatigue‐crack‐growth rate data under constant‐amplitude loading in the near‐threshold regime, without load‐history effects. Tests were conducted over a wide range in stress ratios (R = 0.1–0.7) on compact C(T) specimens for three different widths (25, 51 and 76 mm). The slitting method was used on 51 mm C(T) specimens to confirm that the material did not contain significant levels of residual stresses from forming and/or machining. A crack‐mouth‐opening‐displacement gage was used to monitor crack growth. Data from the ASTM LR method gave near‐threshold values that were found to be dependent upon the specimen width. However, data from the compression precracking constant amplitude (CPCA) loading method gave near‐threshold data independent of specimen width. A crack‐closure analysis was performed for both the LR and CPCA data, to correlate data at the various stress ratios. The CPCA data correlated well with the effective stress‐intensity factor range against rate relation, whereas the LR data exhibited significant threshold fanning with both stress ratio and specimen width.     

Numerical assessment of thick walled power plant components under creep fatigue load with advanced models for deformation and lifetime

Abstract

Renewable energy sources are changing the operation mode of conventional power plants significantly. Load changes cause fatigue deformation and damage, where traditionally creep effects dominated. In thick walled components the fatigue is concentrated on the inside (where temperature changes are quickest), but through plastification and stress rearrangement during operation the deformation behaviour of the entire component is affected. In this work, the time and temperature dependent deformation of components is described by finite element analysis based on a viscoplastic deformation Chaboche type model. Creep fatigue damage is evaluated by a mechanism based creep fatigue crack growth model and applied by a post-processor program with automatic load cycle detection. An introduction to verification efforts within the HWT-II test rig (at GKM Mannheim) and by full scale component test (MACPLUS project) are given.     

Evaluating creep cracking in welded fracture mechanics specimens

At present there lacks a unified approach in understanding the mechanistic role of weldments cracking in fracture mechanics specimens at elevated temperatures. The effects of residual stress, the development and crack tip damage due to it and the subsequent creep relaxation at high temperatures can be evaluated using relevant test data, numerical modelling and residual stress measurements. These problem areas are considered in this paper and discussed in context under the newly formed collaborative international effort in the Versailles Agreement of Materials and Standards committee, VAMAS TWA31. The plans for this collaborative effort are to evaluate tests at elevated temperatures in a number of high strength steels and also model and measure weldments containing residual stresses. The aim of the four year programme will be to make recommendations and establish pre-standardisation methods for testing, measuring and analysing creep crack initiation (CCI), creep crack growth (CCG), and low frequency creep fatigue crack growth (CFCG) (where creep dominates) characteristics in weldments containing residual stress. The fracture mechanics geometries that will be considered have already been validated in the previous TWA25 collaboration [VAMAS TWA25, Draft Code of Practice, ‘Creep/fatigue Crack Growth in Components’, VAMAS document. Nikbin K, editor. May 2005.] for testing of parent material. Examples of testing and analysis techniques are presented in this paper to highlight the future objectives for this work.     

Effect of primary α content on creep and creep crack growth behaviour of near α-Ti alloy

Abstract

The effect of primary α content on creep and creep crack growth behaviour of a near α-Ti alloy has been investigated at 600°C. The alloy was heat treated at different temperatures so as to obtain different volume fractions of equiaxed primary α in the range from 5 to 40%. Constant load creep tests were carried out at 600°C in the stress range 250–400 MPa until rupture of the specimens. Creep crack growth tests were carried out at 600°C and at an initial stress intensity level of 25 MPa m^1/2. Creep data reveal that minimum creep rate increases and time to rupture decreases with increase in primary α content indicating that higher primary α leads to creep weakening. On similar lines, maximum creep crack growth resistance is associated with the alloy with lowest primary α content (i.e. 5%). Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces in the interior of the specimen ahead of the crack tip.     

A fatigue-to-creep correlation in air for application to environmental stress cracking of polyethylene

The present study was undertaken to determine whether the correlation between fatigue and creep established for polyethylene in air could be extended to environmental liquids. Fatigue and creep tests under various conditions of stress, R-ratio (defined as the ratio of minimum to maximum load in the fatigue loading cycle), and frequency were performed in air and in Igepal solutions. The load–displacement curves indicated that stepwise fatigue crack growth in air was preserved in Igepal solutions at 50 °C, the temperature specified for the ASTM standard. In air, systematically decreasing the dynamic component of fatigue loading by increasing the R-ratio to R = 1 (creep) steadily increased the lifetime. In contrast, the lifetime in Igepal was affected to a much smaller extent. The fatigue to creep correlation in air was previously established primarily for tests at 21 °C. Before testing the correlation in Igepal, it was necessary to establish the correlation in air at 50 °C. Microscopic methods were used to verify stepwise crack growth by the sequential formation and breakdown of a craze zone, and to confirm the fatigue to creep correlation. The crack growth rate under various loading conditions was related to the maximum stress and R-ratio by a power law relationship. Alternatively, a strain rate approach, which considered a creep contribution and a fatigue acceleration factor that depended only on strain rate, reliably correlated fatigue and creep in air at 50 °C under most loading conditions of stress, R-ratio and frequency. The exceptions were fatigue loading under conditions of R = 0.1 and frequency less than 1 Hz. It was speculated that compression and bending of highly extended craze fibrils were responsible for unexpectedly high crack speeds.     

Verfahren zur vollständigen Ermittlung der R‐Abhängigkeit des Rissausbreitungsverhaltens mit nur einer Probe

Procedure for the determination of the complete R‐dependency of the crack growth behaviour with only one specimen A new concept for fatigue crack propagation tests has been developed. Using a single specimen, it is possible to determine fatigue crack growth curves (da/dN ‐ ΔK) for every stress ratio between R = 0.9 and R = ‐1. Additionally, the new concept also provides threshold values for fatigue crack growth for different values of R and K_max. In combination with a continuous crack length measurement tool (such as the DC potential drop method) this testing procedure can be performed with minimal effort of personnel and time. The test procedure consists of a sequence of K_max‐constant tests with decreasing crack growth rates. As the applied K_max is increasing stepwise there should be no load history effects. According to the procedures described in the ASTM Standard E 647, the results using this new testing procedure fit very well to the da/dN ‐ ΔK curves generated with different specimens. The tests also fulfil all the requirements of ASTM Standard E 647.     

HIDA activity on 2¼ Cr1Mo steel

Historically, the 2¼CrlMo steel is one of the first type of low alloy steels used in high temperature plant. The objective of the present study is to test uniaxial, standard fracture mechanics and feature specimens from the same batch of material as well as test welded and ex-service materials and samples with mechanically induced residual stress. The creep and creep/fatigue crack initiation and growth results using fracture mechanics modelling techniques will then be used to validate the ‘HIDA’ procedure for high-temperature defect assessment. This paper presents an outline of the testing programme. The initial results for this steel from both static and cyclic loading conditions are presented for the test temperature of 565°C. The feature tests, simulating actual components, consist of three industrially relevant pipe types which are pre-notched and internally pressurised. In addition some of these pipes are being tested under four point bending. Early results of the X-ray and magnetic measurements to characterise creep damage are also presented. Initial results of short term laboratory data derived from uniaxial tests as well as compact tension specimens are presented.     

Fatigue crack propagation behaviour of Al–Zn–Ce alloys

This paper presents the investigation on fatigue crack growth behaviour of Al–Zn and Al–Zn–Ce alloys. Fatigue tests were carried out on as‐cast and heat‐treated CT specimens according to ASTM E647 testing standard. The test results showed that the addition of rare earth element (cerium) and heat treatments (T6 and T5) had very strong influence on fatigue strength. This enhancement was due to metallurgical changes in the alloy system. Cerium eliminates the porosities and refines microstructures of the alloy, showing the improved fatigue crack growth behaviour. In addition, the fatigue fractured specimens were examined using a scanning electron microscope to clarify the fracture initiation points.