High Temperature Miniature Specimen Test Methods

By Wei Sun, Zhufeng Yue, Guoyan Zhou and 2 more

High Temperature Miniature Specimen Test Methods, for the first time in book format, focuses on a comprehensive and thorough introduction to a range of high temperature, miniaturized test methods at elevated temperatures which are used to obtain “bulk creep or fatigue properties from a small volume of material. Complicated mathematics and modelling are not involved. It is intended to be of use to a wide range of audience of engineers (e.g. designers, manufacturers, metallurgists, stress analysts), researchers (e.g. materials scientists) and students (undergraduate and postgraduate) in the field of high-temperature material and structural integrity assessment. Specific novel features of the book include 1] theoretical basis of each test method; 2], data interpretation method of each test method; and 3] specific application of each test method.

• Provides the theoretical basis of each test method
• Includes the data interpretation method of each test method
• Presents specific applications and the limitations of each test method, along with opportunities for future developments

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 28, 2023
• ISBN: 9780443218989

Wei Sun

Wei Sun Ph.D DS.c was a Professor of Mechanical Engineering at the University of Nottingham, and has been working on creep, fatigue, cyclic plasticity, and the miniaturized specimen test methods at high temperatures for > 25 years. He has supervised 40 Ph.D projects (> 10 related to high temperature small specimen testing). He is an author of 260 international journal articles (63 related to high-temperature miniature specimen tests), 170 conference contributions (14 plenary/keynote lectures) and one textbook (Applied Creep Mechanics. McGraw-Hill 2013). He became Charted Engineer in 1998, a Fellow of The Institution of Mechanical Engineers in 2002, and a Fellow of The Institute of Materials in 2009. Prof. Sun has been an Emeritus Professor at the University of Nottingham since he retired in 2020, and currently is a member of EU CEN Impression Creep Standard Committe.

Book preview

1: Introduction

Abstract

This opening chapter, Chapter 1, briefly addresses requirements for miniaturized specimen test techniques and their use in determining high-temperature material properties from a small volume of material, the role of miniaturized testing in a high-temperature component life assessment, and the scope of the book, with a brief highlight of each chapter.

Keywords

High temperature; Life assessment; Material properties; Miniature specimen testing; Structural integrity

1.1. Conventional creep test specimen requirements

1.1.1. Full-size cylindrical uniaxial specimen test

There are many components in power plants, chemical plants, aero-engines, superplastic and forming dies, for example, which operate at temperatures high enough for creep to occur. To predict the creep behavior of these components, bulk material creep properties are required. A typical conventional creep test specimen that is often used to obtain bulk material creep properties is shown in Fig. 1.1. To produce data representative of bulk material properties, the rule of thumb is for the cross-sectional area of the gauge section to be large compared with the test material's metallurgic features, such as the grain size. Certainly, there is a need to obtain better quantification than this. However, in many cases the rule of thumb greatly overestimates the requirement for the gauge section size. Also, relatively large diameters (typically 8–10 mm) and gauge lengths (typically 40–80 mm) may be required to achieve the high sensitivity and accuracy of the strain measurement required. Having a relatively large gauge length allows high accuracy in mean strain measurements. Methods used for specimen manufacture and alignment, load control, temperature control (temporal and spatial), data recording, and so on are well-established and standardized [1–3].

1.1.2. Subsize cylindrical uniaxial specimen test

An example of the use of conventional subsize specimens is those employed in the electric power research institute (EPRI)/central electricity generating board (CEGB) remaining life project [4]. The geometry of the subsize specimen is similar to that of the full-size standard specimen (Fig. 1.1), but small diameters, typically 1–3 mm, and gauge lengths, typically 5–15 mm, were adopted that were sometimes electron beam–welded into conventional end pieces. Tests were carried out in inert atmospheres, and specialized loading frames were used to accommodate the low loads required. Data obtained (for 0.5Cr0.5Mo0.25V, 1Cr0.5Mo, 1.25Cr0.5Mo, and 2.25Cr1Mo steel) compared favorably with those of conventional creep tests [4].

Figure 1.1  Typical conventional, uniaxial creep test specimen. Full size : d GL ≈ 8–10 mm; GL ≈ 40–80 mm. Subsize:  d GL ≈ 1–3 mm; GL ≈ 5–15 mm. GL, gauge length.

Provided grain sizes are not too large, specimen diameters as small as 1 mm can be used to produce bulk material creep properties. Small gauge lengths (<10 mm) can reduce strain measurement sensitivities significantly compared with conventional creep test specimens and make strain measurements sensitive to relatively small variations in temperature. The effects of specimen misalignment are greater when specimen diameters are small. In addition, specimen manufacture is more complicated and expensive than that for conventional full-size specimens.

1.2. Need to extract material properties from small volume of material

In some situations, only small volumes of material are available, such as when new alloys are being developed or when small material samples (e.g., boat samples or scoop samples [5,6]) are extracted from service equipment to estimate the remaining life of the component. Also, in multipass welds, for example, small material subzones are created, such as the heat-affected zone (HAZ) generated in the parent material, or columnar and equiaxed zones generated in the weld metal (Fig. 1.2) [7]. Under these circumstances, the volume of material available will not be large enough to produce conventional specimens with the required (relatively large) dimensions.

Many components in currently operating power plants, which were originally designed for a service life of about 25 years, were overdesigned [8] and the safety factors used were such that significantly more than 25 years of operation are possible. Hence, there is a large financial incentive to use nondestructive evaluation methods to assist in assessing the remaining life of various components and extend the plant operating life. Also, methods being developed, such as those that use scoop samples, will enable more reliable life assessments to be made for new designs of high-temperature components. Part of the assessment process consists of determining the current creep strength of the materials from which components are made. As part of the remaining life assessment process, it is possible for many components to extract the required small scoop or boat samples without significantly affecting the strength of the component. Typical dimensions of the small scoop samples are shown in Fig. 1.3, which also shows a scoop sampling machine. Also, HAZs in welds are narrow (typically 2–4 mm) and the columnar and equiaxed regions in multipass welds are even smaller. In some alloy development programs, it may be possible to produce only small quantities of materials from which to compare and quantify the required relative properties. Therefore, as small as they are, the HAZs, weld zones, alloy development, and scoop sample materials are all that may be available to perform the required material assessments. Thus, small-scale or miniaturized specimen test methods have been developed.

Figure 1.2  Schematic of cross-section of a multi-pass weld: an example of heterogeneous material structures: [7]. HAZ, heat-affected zone.

Figure 1.3  Scoop sample: (a) extraction, (b) close-up image, and (c) typical dimensions.

Literature reviews [9,10] have identified several small specimen creep test types in use, including the subsidized, conventional uniaxial specimen, small punch specimen, impression creep specimen, ring-type specimen, and two-bar specimen (Fig. 1.4a–e).

Figure 1.4  Schematics of miniaturized specimens and load modes: (a) Subsize uniaxial, (b) small punch, (c) impression, (d) ring, and (e) two-bar.

1.3. Requirements for material evaluation and structural integrity

1.3.1. General background

The ability to measure creep properties from a small volume of material has the potential to support the rapid and economic development of new high-temperature exotic alloys [11]. Similarly, data from small volumes of materials have direct input into remaining life studies [12], improving the accuracy of life prediction. Such data can also be used to generate creep constitutive laws for local structures such as irradiated materials and composites as well as those generated in weldments and substrate-coating systems. [13].

1.3.2. Fusion materials

Fission neutrons for materials testing have been available for decades in hundreds of experimental reactors worldwide [14]. An extensive database of irradiated materials is available. However, unfortunately, experimental fusion reactors for materials testing do not exist. Testing facilities with a 14-MeV neutron source for irradiating candidate materials under fusion reactor conditions with control of the temperature of the irradiated material have been under development for 4 decades and have become an urgent need and crucial feature in world fusion road maps.

Optimization of the limited testing space makes the use of small specimens indispensable. The limited testing volume with required neutron fluxes in accelerator-driven, fusion-relevant neutron sources drove the development of small specimens for fusion applications.

This need was already identified in 1983 when the first review of the state of the art of these techniques was done. The development has continued steadily in various laboratories worldwide, yielding similar results but without a standard procedure. Since then, more than 10 specific symposia have taken place, but no harmonization of the small specimen test technique (SSTT) has been accomplished. The nuclear industry, especially the fusion industry, found that the lack of common uses in SSTT is preventing a comparison and optimal exchange of data.

In 2017, the International Atomic Agency launched a coordinated research project (CRP) series entitled Toward the standardization of small specimen test techniques for fusion applications, focused on harmonizing the most relevant five SSTTs. Half of the work was already performed in the first phase of the CRP; the task was expected to be finalized in its second phase, starting in 2022 [15,16].

1.3.3. Condition monitoring and life management

To enhance the practice of assessing the condition of creep aging components, the requirement was proposed for the more proactive use of small specimen testing methods in conjunction with other assessment techniques [17] for an in-service condition assessment of power plant materials, notably earlier in the plant life cycle and within a holistic life assessment framework. This is intended to provide a means of calibrating the time-dependent response of the component or system being monitored, supplying a key reference in-service strain rate measurement, or material property evaluation, which can subsequently be used with other traditionally deployed assessment methods to define a more targeted and cost-effective forward inspection plan. According to this, methods for how small specimen creep testing methods and other complementary tools can be used in a new and structured approach to life management were also proposed [18,19]. However, to date, miniaturized specimen test data for onsite in-service materials are extremely limited.

1.3.4. Gas turbine blades

To date, few applications of miniaturized specimen testing for gas turbine blades have been available [20,21]. Fig. 1.5 shows a service-exposed industrial gas turbine blade made from conventionally cast nickel-base superalloy IN738, with positions for metallographic examinations and small-scale specimens (Fig. 1.6), removed from the blade root and airfoil sections, tested at 850°C [22]. The service-induced microstructural changes resulted in reduced tensile and low cycle fatigue properties in the airfoil section compared with the blade root section. Based on the fatigue data, the residual fatigue life of the gas turbine blade was