Thomas H. Hyde

Finite element-based analysis of experimentally identified parametric envelopes for stable keyhole plasma arc welding of a titanium alloy:

Finite element modelling of the experimentally obtained parametric envelopes for stable keyhole plasma arc welding was performed using a three-dimensional conical Gaussian heat source. Uncoupled steady-state and transient heat transfer analyses were performed to predict fusion zones and thermal histories. The heat source definition was validated against the experimentally obtained macrograph and thermal history for a representative stable keyhole condition. The paper investigates the relationships between the primary welding parameters, i.e. current, traverse speed, plasma gas flow rate and the weld efficiency, using inverse finite element modelling. The effect of the change of plasma gas flow rate on weld efficiency was estimated by iteratively changing the efficiency to match the experimental results. The numerical–experimental approach is proposed to establish relationships between welding parameters and weld efficiency which can be utilised to understand the underlying physics of keyhole plasma arc welding. The inversely identified relationships between key welding parameters can be useful in selecting the appropriate combination of weld parameters for stable keyhole welding.

Determination of creep and damage properties for P92 at 675 °C:

In order to predict the service life of components that operate at high temperatures, such as steam carrying pipes in conventional power plants, the material creep behaviour needs to be determined. There are little creep data available on grade P92 (9Cr2W) steel (a potential successor to P91) as it is a relatively new material; therefore a testing programme has been undertaken. This paper presents the results of uniaxial and notched bar creep tests on P92 parent material (PM) and P92 weld metal (WM) at 675u2009°C. The PM had higher failure times and lower minimum creep strain rates for tests in the same stress range (80–100u2009MPa) as the WM, but the PM and WM values tend to converge at high stress, with a significant difference between the failure times as applied stress decreases. The notch strengthening effect was found to decrease as the applied stress decreased. Processing of the test data including the calculation of the minimum creep strain rates has been performed to determine the material constants required for Norton’s steady state creep and both the Kachanov and the Liu and Murakami creep damage models. Material constant sets for creep of P92 PM and WM at 675u2009°C, including a parameter to describe the effect of a multiaxial stress state, have been obtained that give a good fit to the test data. Validation was achieved using finite element analysis.

Some considerations on specimen types for small sample creep tests

Abstract Commonly used small creep specimen types (i.e. conventional sub-size uniaxial specimens, impression creep specimens and small punch specimens) and test methods, are briefly described, including their capabilities and the methods used to process the data from these tests. The performances of these test types are compared and their relative advantages and disadvantages, for specific practical applications, are assessed. An alternative, novel, “more flexible”, small ring-type specimen test method is described. A significant advantage of the ring-type specimen is that it has a much greater equivalent gauge length (EGL), compared with those of other small specimen types. This specimen type allows small strains to be related to relatively large deformations; it is particularly suitable for creep tests at relatively low equivalent uniaxial stresses. Possible future exploitation of the small specimen test techniques is briefly addressed.

Theoretical basis and practical aspects of small specimen creep testing

Interest in and the application of small specimen creep test techniques are increasing. This is because it is only possible to obtain small samples of material in some situations, for example, the scoop samples that are removed from in-service components, the heat-affected zones that are created when welds are used to join components and the desire to produce only small amounts of material in alloy development programmes. It is therefore important to review and compare the theoretical basis and practical aspects of each of the small specimen creep testing methods, in order to clearly understand which of the methods is the best for any specific application. This article provides the theoretical basis for each commonly used test method.

Comparison of several optimisation strategies for the determination of material constants in the Chaboche visco-plasticity model

Determining representative material constant sets for models that can accurately predict the complex plasticity and creep behaviour of components undergoing cyclic loading is of great interest to many industries. The Chaboche unified visco-plasticity model is an example of a model that, with the correct modifications, shows much promise for this particular application. Methods to approximate material constant values in the Chaboche model have been well established; however, the need for optimisation of these parameters is vital due to assumptions made in the initial estimation process. Optimisation of a material constant set is conducted by fitting the predicted response to the experimental results of cyclic tests. It is expected that any experimental data set (found using the same values of test parameters such as temperature; the dependency of which is not accounted for in the original Chaboche model) should yield a single set of optimised material parameters for a given material. In practice, this may not be the case. Experimental test programs usually include multiple loading waveforms; therefore, it is often possible to optimise for separate, different sets of material constants for the same material operating under comparable conditions. Several optimisation strategies that utilise multiple sets of experimental data to form the objective functions in optimisation programs have been applied and critiqued. A procedure that evaluates objective functions derived from the multiple experimental data types simultaneously (i.e. in the same optimisation iteration) was found to give the most consistently high-quality fitting. In the present work, this is demonstrated using cyclic experimental data for a P91 steel at 600u2009°C. Similar strategies may be applied to many constitutive laws that require some form of optimisation to determine material constant values.

Use of small specimen creep data in component life management: a review

Small specimen creep testing techniques are novel mechanical test techniques that have been developed over the past 25 years. They mainly include the sub-size uniaxial test, the small punch creep test, the impression creep test, the small ring creep test and the two-bar creep test. This paper outlines the current methods in practice for data interpretation as well as the state-of-the-art procedures for conducting the tests. Case studies for the use of impression creep testing and material strength ranking of creep resistant steels are reviewed along with the requirement for the standardisation of the impression creep test method. A database of small specimen creep testing is required to prove the validity of the tests.

A Simplified Method for Predicting the Creep Crack Growth in P91 Welds at 650 °C

This article uses a numerical method for predicting the creep crack growth in compact tension (CT) specimens, including those containing welds. A series of incremental steady-state finite-element (FE) analyses were performed, using Nortons law to represent the creep behaviour, in order to simulate the progressive creep crack growth process. Validation of the FE predictions was achieved by comparing the creep crack growth test results obtained for parent material (PM) and cross-weld CT specimens removed from a P91 weld; the tests were carried out at 650 °C. Experimental results show that the creep crack growth rates for cross-weld specimens are higher than those for PM specimens by a factor of ˜4, for the same value of C*. Good agreement was obtained between the FE predictions and the experimental results for both the PM and cross-weld CT specimens. The FE results obtained using this approach are also in good agreement with those obtained from continuum damage analyses. The advantage of the proposed approach is that it requires much fewer material constants for use in the numerical predictions, compared with the damage mechanics approach.

Residual stress distribution in a Ti-6Al-4V T-joint weld measured using synchrotron X-ray diffraction

To improve the manufacturing quality of welded structures, to prevent failures at weld joints and to predict their lifetime, measurements of the residual stresses generated by welding in the structures are extremely useful. The residual stresses may reduce the component life due to phenomena that occur at low applied stresses such as brittle fracture, fatigue and stress corrosion cracking. Welded thin Ti–6Al–4V panel components are commonly found in aero-engine assemblies and the weld integrity and reliability are critical. In this work, the residual stress distributions in a welded thin Ti–6Al–4V T-joint were measured by the newly developed SScanSS program with synchrotron X-ray diffraction technique. The measurement performed in this study, which included a large number of measurement points, has mapped a complete stress field in a thin sheet T-joint weld. It has not only provided improved understanding of residual stress in such a joint but also filled the missing link between the residual stress obtained by numerical modelling and their validation. The results have shown that the longitudinal stresses play the most important role in the residual stress distribution over the flange and high tensile stresses appear in the region near the weld zone. The measured results were compared with the numerically predicted results and these showed good agreement.

Process modelling and optimization of keyhole plasma arc welding of thin Ti-6Al-4V

This article presents a comprehensive piece of research work focused on the development, validation and application of finite element modelling capability for the prediction and optimization of robotic keyhole plasma arc welding of Ti-6Al-4V thin structures. Experimental and computational investigations were carried out to characterize, develop, optimize and validate various aspects of the finite element modelling. The experimental investigations cover the determination of welding parameter envelopes using a robotic welding cell and the measurements of thermal history, distortion, residual stress and weld pool profile. The computational investigations include the development and validation of finite element models as well as the development and validation of a fully automated welding sequence optimization tool using a genetic algorithm approach. The work provides useful guidance and generic methodologies for optimum design of thin and complex lightweight structures and has formed a basis for the development of a framework on structural integrity assessment and component lifing of thin structures fabricated by welding. The optimization tool has significant potential to be conveniently modified to suit other optimization objectives and/or welding processes.

The effects of geometrical inaccuracies of the experimental set-up on small punch creep test results

The small punch creep testing technique is able to provide creep properties from a very small amount of material. However, a universal and robust technique, to convert small punch creep testing results to corresponding uniaxial creep test data, has still not been established. In addition, the experimental output can be affected by several sources of uncertainty, such as friction between the components of the test rig and the specimen, and inaccuracies in the geometry of the experimental set-up and the testing procedures. This article reports the results of three-dimensional elastic/creep finite element analyses of small punch creep testing, taking into account geometrical inaccuracies in the initial punch position and the loading direction. The results of the calculations show that the initial position of the punch and the loading direction can considerably affect the variation in the specimen’s central deflection with time and the final time to failure. The minimum displacement rate was found to decrease when the punch moves away from the centre of the specimen and when the angle between the loading direction and the axis of the test rig increases. The time to failure increases when the punch deviates from the perfect axi-symmetric configuration. The effects of the direction of the load increase as the initial distance of the punch from the centre of the specimen increases. Analytical correlations, corresponding to the inaccuracies investigated, are also proposed.