# EASI-STRESS standardization of industrial residual stress measurement: benchmark specimen design

Wen Cui, Manuel Sánchez Poncela, Romain Badyka, Philipp Mayr, Philip J Withers, Matthew J Roy

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## To cite this version:

Wen Cui, Manuel Sánchez Poncela, Romain Badyka, Philipp Mayr, Philip J Withers, et al., EASI-STRESS standardization of industrial residual stress measurement: benchmark specimen design. ICRS 11 - The 11th International Conference of Residual Stresses, SF2M; IJL, Mar 2022, Nancy, France. hal-03997979

HAL Id: hal-03997979

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Submitted on 20 Feb 2023

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# EASI-STRESS STANDARDIZATION OF INDUSTRIAL RESIDUAL STRESS MEASUREMENT: BENCHMARK SPECIMEN DESIGN

Wen Cui<sup>a,e*</sup>, Manuel Sánchez Poncela<sup>b</sup>, Romain Badyka<sup>c</sup>, Philipp Mayr<sup>d</sup>, Ricardo Fernández Gutiérrez<sup>d</sup>, Philip J. Withers<sup>a</sup> and Matthew J. Roy<sup>e</sup>

## ABSTRACT

This paper concerns the European Activity for Standardization of Industrial Residual Stress Characterization (EASI-STRESS) Round Robin activities, aimed at harmonizing measurement techniques over the widest sources of residual stress in modern metal manufacturing. Over decades, much effort on residual stress standardization has been devoted to the techniques themselves, whilst little has been reported in the open literature on the definition and compatibility of test samples across different techniques, and on the logic behind specimens used in prior round robin. This makes the design of benchmark specimens a real challenge for the harmonization process. This paper reviews the design of all benchmark specimens employed for EASI-STRESS and addresses how they embrace challenges posted by the requirements for harmonization. The specimens represent a wide cross-section of how residual stresses manifest in metal processing, and at the same time maximise the number of measurement techniques that can be employed. These benchmark specimens can be easily reproduced and hence be readily adopted for future activities on residual stress measurement.

**Keywords:** residual stress; standardization; benchmark; sample design; round robin stress measurement.

## 1. Introduction

Round Robin residual stress measurement for metallic components have been on-going over the last two decades. In particular there have been two prominent activities; the first investigated residual stress measurement by neutron diffraction under the Versailles Project on Advanced Materials and Standards Technical Working Area 20 (VAMAS TWA 20) [1], the second, the European Network on Neutron Techniques Standardization for Structural Integrity (NeT) [2], focused on stress measurement in structural welds. A recent study also examined the interlaboratory reproducibility of contour method, on a simulate surface [3]. The unique aim of the European Activity for Standardization of Industrial Residual Stress Characterization (EASI-STRESS) is to harmonise measurement techniques over the widest origins of residual stress in

<footnote><sup>a</sup>Henry Royce Institute, Department of Materials, University of Manchester, Oxford Road, M13 9PL Manchester, UK</footnote>

<footnote><sup>b</sup>ArcelorMittal Innovación Investigación e Inversión, S.L.U. Residencia la Granda, s/n - 44318, La Granda, Gozón, Asturias, Spain</footnote>

<footnote><sup>c</sup>EDF Lab Les Renardières, MMC Department, Av. des Renardières, F-77250 Moret sur Loing, France</footnote>

<footnote><sup>d</sup>Nemak Linz GmbH, Zeppelinstraße 24, 4030 Linz, Austria</footnote>

<footnote><sup>e</sup>Henry Royce Institute, Department of Mechanical, Civil and Aerospace Engineering, University of Manchester, Oxford Road, M13 9PL, Manchester, UK</footnote>

<footnote>* Corresponding author. wen.cui@manchester.ac.uk</footnote>
metallic components which represent current manufacturing trends. For the purpose of harmonisation, beyond the design of components that can be replicated with relative ease, the round robin samples need to be amenable to the host of commonly employed laboratory techniques and high energy diffraction methods employed by large-scale research infrastructures (LRIs). This means benchmark specimens should be applicable to as many measurement techniques as possible considered within EASI-STRESS, namely deep hole drilling, laboratory X-ray diffraction, contour method, synchrotron and neutron diffraction. Additionally, they should have configurations suitable for numerical modelling, and their inherent residual stress fields need to represent a variety of sources, be well understood and predictable.

VAMAS employed four specimens, consisting of a shrink-fit aluminium alloy ring and plug, a SiC-Al₂O₃ composite disc, a nickel alloy shot-peened plate and a ferritic steel weldment. These samples were selected to represent diverse stress states, including one from a non-metallic component, and were solely tested by the non-destructive neutron diffraction method [1, 4]. Details of these specimens and their suitability for techniques other than neutron diffraction were not fully reported. Rather than being focussed on any one technique, NeT predominantly examined residual stress in weldments. Nevertheless, it showed that a weldment could be a potential benchmark specimen applicable to most of the common techniques for residual stress measurement. There is currently an outstanding need for designing benchmark specimens for harmonising measurement techniques. This paper reviews the design and manufacture of four categories of benchmark specimens employed in EASI-STRESS, and addresses important criteria being considered behind each design.

## 2. Benchmark specimen designs

Macroscopic residual stresses in monolithic metallic engineering components arise due to chemical, thermal and plastically induced misfits between different regions within the component, or the interaction between misfitting parts within an assembly. The most accessible type of specimen is one that has a simple stress state and a uniform. A complex residual stress measurement might be a sample containing a triaxial internal stress field with both sharp and large gradients, and an inhomogeneous microstructure. A residual stress field in a manufactured metallic part can be complicated by factors including stress triaxiality driven by geometric features, stress gradients caused by inhomogeneous microstructure, along with chemical segregation of alloying ingredients and/or flaws such as porosity or coarse precipitates and inclusions. Fig. 1 shows an approach to separate out some of these aspects by selecting samples of increasing complexity in stress states and microstructures, extending from plane stress with a homogeneous microstructure to multiaxial and gradient stress states along with inhomogeneous microstructure, in an attempt to understand how different techniques tackle these distinct measurement challenges. To present recent trends in metal processing, specimens relevant to the following industrial processes are considered: 1) mechanical assembling or deformation; 2) metallic powder additive manufacturing; 3) gas tungsten arc welding; and 4) permanent mould casting. Consequently, four types of specimen are devised based on these key criteria:

* Have different origins of residual stress – to consider stresses imposed by external source, plastic deformation, incremental solidification and bulk solidification processes.

* Encompass different materials – to consider a variety of industrial relevant alloy systems, incorporating various strengthening mechanisms.

* Amenable to different techniques – suitable for laboratory diffraction and material removal techniques, as well as large facility diffraction methods.

* Reproducible – to be easily manufactured and reproduced.
<figure><img src="image_3.png" /><figcaption>Fig. 1 Schematic showing gradients in residual stress and microstructural complexity.</figcaption></figure>

## 2.1. Ferritic steel U shape and U bend samples

Residual stresses are those which are typified as those being internally balanced and self-equilibrating in a single engineering component. However, depending on the frame of reference, assemblies at scale can also be considered to fall within this description [5]. For example, a fastener holding together two parts applies external stresses to each part but the overall assembly provides a medium which contains an internal stress. With this concept, U-shaped samples have been designed such that external loading can create a 'tuneable' stress state that can be captured by diffraction-based measurement techniques. This internal stress state can be considered 'residual', as it is internally balanced and elastic in nature. The U shape specimens have been complemented by an additional sample (termed U bend) having similar geometries but which contain a fixed residual stress imparted by plastic deformation. While the tuneable flexure-type specimens are not suitable for destructive techniques, these bent samples are. This combined approach is one that accounts for open, accessible manufacturing and metrology best practice of the specimens themselves, and simple predictive computational models describing their stress state.

The 'U shape' specimens are redesigns of a prior LINX project (Innovation Fund Denmark) specimen intended to demonstrate externally imposed elastic stresses. They are made from S355J0Z35+N steel and cut into a U profile directly from a plate using electro-discharge machining to minimise stress changes at the cut surfaces and are thus nominally stress free. As shown in Fig. 2, the two 'legs' of the 'U' can be brought together or pushed further apart via a locking turnbuckle, imposing controllable elastic stresses either in compression or in tension at the extrados of the U shape. Included datum features facilitate accurate positioning of the component, which is crucial for synchrotron and neutron diffraction measurements, and they could also help to establish boundary conditions in numerical models. These datums (indicated in red, Fig. 2) are spherical synthetic ruby points at fixed positions about the samples; this makes them suitable for both tactile and optical metrology systems. A novelty of this configuration is that each flexure serves as its own 'stress-free' reference sample once the turnbuckle is removed, providing that applied external loads do not exceed the material's yield point. Owing to the simple symmetric geometry of the U shape, the stress response to the displacement of the U legs and the maximum allowed displacement for elastic deformation are easily calculated by finite element models.
<figure><img src="image_4.png" /><figcaption>Fig. 1 Design of U flexure benchmark specimens and configurations for compression and tension loading.</figcaption></figure>

As designed, the U shapes are not amenable to destructive testing (e.g., the contour method). Plastically deformed ‘U bends’ employing the identical configuration and material are therefore produced as supplementary specimens, will be subject to all techniques considered within EASI-STRESS. These will help to characterise the effects of plasticity on the diffraction patterns. As shown schematically in Fig. 3, the U bends are produced by plastically bending steel blanks using a custom bending fixture replicating the Guided Bend Test fixture prescribed by ISO 5173 standard [6]. The blanks are electro-discharge machined from the parent material of the U shapes (flexures) so that residual stresses are solely developed by plastic deformation during bending. After bending, datum features similar to those for the U shapes have been installed to ease positioning for diffraction measurements. FEA has been employed to provide an estimate of the residual stress after bending. The ‘U shape’ and the ‘U bend’ together, are referred to as the ‘U’ samples in our study.

<figure><img src="image_5.png" /><figcaption>Fig. 2 Schematic of the custom bending fixture and a ‘U bend’ specimen plastically formed from the fixture.</figcaption></figure>

## 2.2. Additive manufactured steel arches

An arch containing a void or negative space was chosen to represent a common feature, which is also one of the design advantages, in additively manufacturing parts (Fig. 4). The component leads itself to establishing a triaxial stress state, which has been largely absent from additive manufactured benchmarks for residual stress evaluations to date. The configuration reduces the number of processing steps (e.g., tooling or machining) with relative bulky regions which eases residual stress measurement, particularly for laboratory-based techniques which require reliable material removal and favour simple geometries and uniform dimensions. For example, the
accuracy of the contour method relies heavily on the quality of the cutting planes which is easiest to achieve if the cutting path has a consistent material thickness.

<figure><img src="image_6.png" /><figcaption>Fig. 4. A photo showing printed benchmark arch specimens on a build plate, and images showing the dimensions and printing directions of layers.</figcaption></figure>

Two different laser-based, powder-bed fusion (L-PBF) machines are employed to examine, not only the final stress state of printed part, but also the reproducibility of the technology. Key parameters of the process are shown in Table 1. Stainless steel 316L powder was selected as the printing material for its well characterized response to L-PBF processes, as well as for its stable austenitic microstructure throughout the continuous thermal cycling in L-PBF. Samples are printed onto two build platforms made of 304 stainless steel and C45U (EN 1.1730) ferritic-pearlitic steel respectively, with the intent to leave the specimens attached after building, such that the round-robin exercise captures real stresses in components which are generated during the process. Stress-relieved samples held at 700 °C for 2 hours and passively cooled all in an inert atmosphere are also available. The specific treatment was selected to maximize stress relief whilst preserving the underlying microstructure. Previous observations [7] of L-PBF 316L suggests a reasonably stable microstructure with minor precipitates forming for long holding times (> 20 hours) at 700 °C.

<table><caption>Table 1. Key processing conditions for manufacturing the arch samples.</caption><thead><tr><th>Supplier</th><th>ArcelorMittal</th><th>Volume-E</th></tr></thead><tbody><tr><td>Laser</td><td>Spot size: 100 µm<br>VED: 50 J/mm³</td><td>Spot size: 160 µm<br>VED: 36 J/mm³</td></tr><tr><td>Parameters</td><td colspan="2">Scanning strategy: 67° rotation banding<br>Contouring, no additional surface reprocessing</td></tr><tr><td>Machine</td><td>MetalFAB1, 4 lasers</td><td>EOS M290, single laser</td></tr><tr><td>Layer thickness</td><td>50 µm</td><td>60 µm</td></tr><tr><td>Environment</td><td>Argon, 0.05% O₂ max.</td><td>Argon, 0.10% O₂ max.</td></tr><tr><td>Build plate</td><td>10 mm thick, 304 steel</td><td>36 mm thick, C45U steel</td></tr><tr><td>Recoating</td><td>Soft blade<br>12 s per layer</td><td>High speed steel blade<br>10 s per layer</td></tr><tr><td>316L Powder</td><td>Size: 15-45 µm</td><td>Size: 15-80 µm</td></tr></tbody></table>
## 2.3. Three-pass tungsten inert gas welded Inconel plates

Welding is a technique widely used in engineered metallic structures in industries. Welding inevitably causes local variations in temperature and microstructure which result in the generation of residual stress. Understanding the stress state at and around the weld bead is important for controlling and predicting the lifetime of welded components. However, the coarse and textured microstructure of heat affected zone in welded components challenges the measurement process, particularly for diffraction techniques, which often exam small volume of materials containing limit numbers of grains. The complex stress state imposed can also challenge destructive lab-based techniques. It is in this context that a NeT TG6 sample, which has been well studied by neutron diffraction [8], has been proposed for inclusion in this work.

The sample is a nickel-based alloy plate (Alloy 600) with a 3-pass slot weld made using an automated gas tungsten inert gas weld (GTAW/TIG) with a compatible, nickel-based Alloy 82 filler material. The dimension of the plate is 150×200×12 mm with a weld bead dimension equal to 76×5×5 mm, as per Fig. 5. An automated TIG welding machine was employed for welding the plates at the Électricité de France (EDF) laboratory in Chatou, France. Full processing conditions are defined in [9]. Previous studies on TG6 samples have shown variations in texture, and their grains distributed over a wide size range, increasing from 20 µm in heat unaffected zone in the parent material to 200 µm in the weld bead. These factors, however, are not necessarily of concern for diffraction-based techniques as good agreement in residual stress distribution has been achieved in previous inter-laboratory comparisons under NeT [8].

<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) A photo showing the benchmark NeT weldment specimen, and (b) a SEM micrograph showing the microstructure of the cross-section cut along the blue line highlighted in (a).</figcaption></figure>

## 2.4. Cast and quenched aluminium wedges

Casting is a well-established manufacturing process for aluminium components in the automotive sector. Inherent thermal gradients during solidification and subsequent heat treatments induce residual strains, and thus stresses which can lead to premature component failures. A cast aluminium specimen is chosen to be one benchmark specimen, aiming to gain better understanding of stresses generated by casting processes and the development of potential mitigation strategies.

Specimens are wedge-shaped, AlSi7Cu0.5Mg (EN AC-45500) alloy manufactured by Nemak by permanent mould casting and have the dimensions shown in Fig. 6. One specimen is provided in the as-cast condition; one after solution heat treatment and water quenching (referred to as T4); and a third following an additional age-hardening treatment (referred to as T6). The material was chosen to conform with the companion industrial component investigated under a different work package in EASI-STRESS. The proposed wedge design has characteristic casting features including different wall thicknesses and an associated grain size
distribution and therefore represents an ideal reference sample for cast aluminium components commonly employed in the automotive sector. In addition, since they originate from a casting trial geometry for permanent mould casting, these benchmark specimens have known, industrially relevant, thermal histories. Depending on the region of interest, casting wall thickness, and solidification time, the average grain size of a wedge varies, varying from 435 to 573 µm for specimens produced for EASI-STRESS. Preliminary destructive sectioning has been performed to ensure the process parameters are sufficient to generate detectable strains for a wide range of stress measurement techniques.

<figure><img src="image_8.png" /><figcaption>Fig. 6. (a) Dimensions of the benchmark wedge specimen, and (b) a cross-section showing wall thicknesses.</figcaption></figure>

**3. Summary**

Four categories of benchmark specimens suitable for quantification of residual stress by experimental and computational means have been developed. These specimen designs represent a wide cross-section of how residual stresses manifest in modern manufacturing for metallic components. They include different sources of residual stress across a wide range of metallic materials, and are therefore pertinent to a wide array of industries. They include plasticity-induced misfit strains that can occur during forging operations (U bend), through to the thermally-induced misfit strains and microstructure evolution associated with solidification events (wedge samples). These benchmark samples have been devised on the basis such that as many measurement techniques as possible can be employed, be they non-destructive such as those provided by large-scale synchrotron and neutron facilities, as well as semi destructive laboratory techniques, both within EASI-STRESS and by others. A number of duplicate samples are being made to enable wider engagement in round robin studies to evaluate, or develop, new residual stress measurement techniques. In addition, their simple design and associated manufacturing details mean that additional samples can be made at a later date as required.

**Acknowledgements**

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 953219.
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