SIST EN ISO 18166:2026

Numerical welding simulation - Execution and documentation (ISO 18166:2026)

This document specifies the execution, validation, verification and documentation of a numerical welding simulation within the field of computational welding mechanics (CWM) and performed with a scientific computational tool (SCT).
This document is applicable to the thermal and mechanical finite element analysis (FEA) of arc, laser and electron beam welding processes for the purpose of calculating the effects of welding processes, and in particular, residual stresses and distortion, in support of structural integrity assessment.

Numerische Schweißsimulation - Ausführung und Dokumentation (ISO 18166:2026)

Dieses Dokument legt die Ausführung, Validierung, Verifizierung und Dokumentation einer numerischen Schweißsimulation zur rechnerischen Abbildung des Schweißprozesses (CWM, en: computational welding mechanics) fest, die mit einem wissenschaftlichen Rechentool (SCT, en: scientific computational tool) durchgeführt wird.
Dieses Dokument ist anwendbar für die thermische und mechanische Finite-Elemente-Methode (FEA, en: finite element analysis) von Lichtbogen-, Laser- und Elektronenstrahlschweißverfahren zum Zweck der Berechnung der Einflüsse von Schweißprozessen, insbesondere von Schweißeigenspannungen und Schweißverzügen, zur Unterstützung der Beurteilung der strukturellen Unversehrtheit.

Simulation numérique de soudage - Exécution et documentation (ISO 18166:2026)

Le présent document spécifie l'exécution, la validation, la vérification et la documentation d'une simulation numérique de soudage dans le domaine de la mécanique du solide et réalisée à l'aide d'un outil de calcul scientifique.
Le présent document est applicable à l'analyse thermique et mécanique par la méthode des éléments finis des procédés de soudage à l'arc, au laser et par faisceau d'électrons dans le but de calculer les conséquences des procédés de soudage, et notamment les contraintes et déformations résiduelles, en appui à l'évaluation de l'intégrité des structures.

Numerična simulacija varjenja - Izvedba in dokumentacija (ISO 18166:2026)

Ta dokument določa izvedbo, validacijo, verifikacijo in dokumentacijo numerične simulacije varjenja na področju računalniške mehanike varjenja (CWM) in izvedeno z znanstvenim računalniškim orodjem (SCT).
Ta dokument je uporaben za termično in mehansko analizo z metodo končnih elementov (FEA) procesov obločnega, laserskega in elektronskega žarkovnega varjenja z namenom izračuna učinkov varilnih procesov, zlasti preostalih napetosti in deformacij, kot podporo oceni strukturne celovitosti.

General Information

• Status: Published
• Public Enquiry End Date: 19-Mar-2025
• Publication Date: 06-Apr-2026
• ICS: 25.160.01 - Welding, brazing and soldering in general
• ICS: 35.240.50 - IT applications in industry
• Technical Committee: VAR - Welding
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 20-Feb-2026
• Due Date: 27-Apr-2026
• Completion Date: 07-Apr-2026
• Ref Project: EN ISO 18166:2026 - Numerical welding simulation - Execution and documentation (ISO 18166:2026)

Relations

• Replaces: SIST-TS CEN ISO/TS 18166:2016 - Numerical welding simulation - Execution and documentation (ISO/TS 18166:2016)
• Effective Date: 01-May-2026

Overview

The kSIST FprEN ISO 18166:2025:2025 standard, titled Numerical Welding Simulation - Execution and Documentation, provides comprehensive guidance for conducting and documenting numerical welding simulations within the field of Computational Welding Mechanics (CWM). Developed under the auspices of the European Committee for Standardization (CEN) and ISO Technical Committee 44 (Welding and allied processes), this standard updates and replaces the prior Technical Specification ISO/TS 18166:2016.

The standard primarily focuses on the thermal and mechanical finite element analysis (FEA) of fusion welding processes, such as arc, laser, and electron beam welding of metallic components. It establishes a consistent framework for execution, verification, validation, and reporting of numerical welding simulations to support the prediction of critical outcomes like residual stresses and welding distortion.

Key Topics

• Computational Welding Mechanics (CWM) Workflow
  Guidance on simulation execution including problem formulation, defining input parameters, geometry meshing, running simulations, monitoring computation, and post-processing results.

• Simulation Focus Areas
  Covers thermal phenomena such as heat flow, thermal expansion, and thermal stresses; mechanical outcomes including inelastic strains, residual stress distributions, and welding-induced distortions.

• Exclusions
  Does not deeply address process-specific physics like heat source modeling (e.g., laser, welding arc), melt pool dynamics, or microstructural phase transformations, though references for these exist in related standards.

• Validation and Verification
  Emphasizes establishing robust model credibility through calibration, uncertainty quantification, validation experiments, and comparison with experimental or real-world data.

• Documentation
  Provides a structured documentation template (Annex B) to ensure consistent and comprehensive reporting of simulation results, facilitating transparency and reproducibility.

• Scientific Computation Tools (SCTs)
  Defines criteria for computational tools used in CWM to assure appropriate numerical accuracy, model fidelity, and reliability.

Applications

• Design Optimization
  Engineers can utilize numerical welding simulations to optimize weld procedures and joint designs, minimizing residual stresses and distortions that adversely affect structural integrity.

• Structural Integrity Assessment
  Numerical predictions help assess welded structures under operational loads, supporting safety evaluations and regulatory compliance.

• Material and Welding Process Development
  Simulations facilitate investigations into material behavior during welding, assisting in developing new alloys and advanced welding techniques.

• Cross-Industry Usage
  The standard’s software-agnostic and industry-independent approach makes it applicable across sectors such as automotive, aerospace, shipbuilding, construction, and energy.

Related Standards

• ISO/TR 25901:2007 - Provides general terminology and definitions related to welding and weldability.

• ISO/TR 25901-1:2016 - Contains terms and definitions specifically related to welding simulations and computational welding mechanics.

• Additional ISO and IEC standards on welding processes and materials provide complementary guidance on specific welding operations and metallurgical considerations.

Conclusion

kSIST FprEN ISO 18166:2025:2025 is a key international standard that standardizes the methodology and documentation for numerical welding simulations. By adopting this standard, organizations enhance the reliability, repeatability, and credibility of simulation results in welding engineering. This supports better decision-making in product design, quality assurance, and process development while fostering global best practices in computational welding mechanics.

Keywords: numerical welding simulation, computational welding mechanics, CWM, finite element analysis, FEA, welding distortion, residual stress, heat flow, weld process simulation, welding documentation, welding validation, welding verification, scientific computation tools.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2026
Nadomešča:
SIST-TS CEN ISO/TS 18166:2016
Numerična simulacija varjenja - Izvedba in dokumentacija (ISO 18166:2026)
Numerical welding simulation - Execution and documentation (ISO 18166:2026)
Numerische Schweißsimulation - Ausführung und Dokumentation (ISO 18166:2026)
Simulation numérique de soudage - Exécution et documentation (ISO 18166:2026)
Ta slovenski standard je istoveten z: EN ISO 18166:2026
ICS:
25.160.01 Varjenje, trdo in mehko Welding, brazing and
spajkanje na splošno soldering in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 18166
EUROPEAN STANDARD
NORME EUROPÉENNE
February 2026
EUROPÄISCHE NORM
ICS 35.240.50 Supersedes CEN ISO/TS 18166:2016
English Version
Numerical welding simulation - Execution and
documentation (ISO 18166:2026)
Simulation numérique de soudage - Exécution et Numerische Schweißsimulation - Ausführung und
documentation (ISO 18166:2026) Dokumentation (ISO 18166:2026)
This European Standard was approved by CEN on 10 January 2026.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18166:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 18166:2026) has been prepared by Technical Committee ISO/TC 44 "Welding
and allied processes" in collaboration with Technical Committee CEN/TC 121 “Welding and allied
processes” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by August 2026, and conflicting national standards shall
be withdrawn at the latest by August 2026.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes CEN ISO/TS 18166:2016.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 18166:2026 has been approved by CEN as EN ISO 18166:2026 without any modification.

International
Standard
ISO 18166
First edition
Numerical welding simulation —
2026-01
Execution and documentation
Simulation numérique de soudage — Exécution et documentation
Reference number
ISO 18166:2026(en) © ISO 2026
ISO 18166:2026(en)
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
ISO 18166:2026(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 3
5 Principle . 4
6 Scientific Computation Tools (SCTs) . 4
7 Required data for simulation . 5
8 Formulation of the problem and establishment of the simulation strategy . 5
9 Establishment of the input parameters . 8
9.1 Input data .8
9.2 Simulation template .8
10 Geometry and mesh . 8
10.1 Geometry and mesh of welded joint .8
10.2 Mesh size .8
10.3 Type of elements .8
10.4 Modelling of the filler material .9
11 Performing the simulation . 9
11.1 Code verification .9
11.2 Thermal and metallurgical computations .10
11.2.1 General .10
11.2.2 Focus on metallurgical transformations .10
11.2.3 Modelling of heat source.10
11.2.4 Boundary and initial thermal conditions .11
11.3 Thermomechanical computation for residual stresses prediction .11
11.3.1 General .11
11.3.2 Model parameters adjustments .11
11.3.3 Materials with phase transformations .11
11.3.4 Boundary and initial thermomechanical conditions .11
11.4 Monitoring the solution during computation. 12
12 Simulation post-processing .12
12.1 General . 12
12.2 Cross-section of fusion zone . 12
12.3 Transient evolution of temperatures . 12
12.4 Phases and residual stresses and strains distributions . 12
13 Comparing/challenging the results.13
13.1 General . 13
13.2 Calculation verification . 13
13.3 Validation .14
13.3.1 General .14
13.3.2 Validation process .14
13.3.3 Lack of knowledge . .14
13.3.4 Validation experiment guidelines . 15
13.3.5 Additional validation and verification activities . 15
14 Uncertainty quantification .15
15 Reporting/display of results .16
15.1 General .16

iii
ISO 18166:2026(en)
15.2 Objective of welding simulation .16
15.3 Material properties and input data .16
15.4 Geometry and mesh .16
15.5 Numerical model parameters .17
15.6 Analysis of results .17
Annex A (informative) Technical specification of scientific computation tools for numerical
welding simulation/computational weld mechanics .18
Annex B (informative) Documentation template.20
Annex C (informative) Heat source modelling and calibration .26
Annex D (informative) Guidelines for validation experiment .35
Annex E (informative) Characterizing, tracing, and managing uncertainty in computational
weld mechanics and real-world systems .37
Annex F (informative) Mechanical properties of materials .39
Bibliography .46

iv
ISO 18166:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document has been prepared by Technical Committee ISO/TC 44, Welding and allied processes, in
collaboration with the European Committee for Standardization (CEN) Technical Committee CEN/TC 121,
Welding and allied processes, in accordance with the Agreement on technical cooperation between ISO and
CEN (Vienna Agreement).
This first edition of ISO 18166 cancels and replaces ISO/TS 18166:2016, which has been technically revised.
Any feedback or questions on this document should be directed to the user’s
national standards body. A complete listing of these bodies can be found at
www.iso.org/members.html. Official interpretations of TC 44 documents, where they exist, are available
from this page: https://committee.iso.org/sites/tc44/home/interpretation.html.

v
ISO 18166:2026(en)
Introduction
This document is not intended for use in a specific industry or with a specific software. Commercial tools
are not excluded. This document is beneficial for the design, manufacturing and assessment of a wide range
of components if the physical phenomena, software and numerical methods meet the specifications of the
scientific computational tools (SCTs) defined in Annex A.
This document can be used by industrial entities to define their requirements for specific applications of
computational welding mechanics (CWM).

vi
International Standard ISO 18166:2026(en)
Numerical welding simulation — Execution and
documentation
1 Scope
This document specifies the execution, validation, verification and documentation of a numerical welding
simulation within the field of computational welding mechanics (CWM) and performed with a scientific
computational tool (SCT).
This document is applicable to the thermal and mechanical finite element analysis (FEA) of arc, laser and
electron beam welding processes for the purpose of calculating the effects of welding processes, and in
particular, residual stresses and distortion, in support of structural integrity assessment.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/TR 25901-1, Welding and allied processes — Vocabulary — Part 1: General terms
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/TR 25901-1 and the following
apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
accuracy
closeness of agreement between a measured quantity value and a true quantity value of a measurand
Note 1 to entry: The term measurand is defined by the VIM (ISO/IEC Guide 99:2007, 2.3) as a “quantity intended to be
measured”.
[SOURCE: ISO/IEC Guide 99 :2007, 2.13, modified — Note 1 to entry has been added.]
3.2
calculation strategy
set of modelling (and simulation) choices to perform a numerical simulation
Note 1 to entry: A calculation strategy defines the choice of physical models and of the coupling physics between
models, the correlations, the discretization both spatial (mesh) and temporal (time step), the calculation options.
3.3
calibration
process of adjusting modelling parameter values of the scientific computing tool
Note 1 to entry: Calibration improves agreement between the calculated values and the reference values.

ISO 18166:2026(en)
3.4
distortion
permanent deformation resulting in a change in shape of a solid body
3.5
evaluation criteria
essential metrics used to assess and compare the relative significance of each phenomenon
Note 1 to entry: The importance rank of a particular phenomenon is a measure of its relative influence on the selected
evaluation criteria.
Note 2 to entry: Adapted from Reference [17].
3.6
greedy algorithm
algorithm that follows the problem-solving heuristic of making the locally optimal choice at each stage
Note 1 to entry: In many cases, a greedy strategy does not produce an optimal solution, but a greedy heuristic can
yield locally optimal solutions that approximate a globally optimal solution in a reasonable amount of time.
3.7
heat flux
rate at which thermal energy is transferred through a unit area of surface
3.8
heat source
spatial and temporal numerical distribution of the thermal energy transferred to the weldment
by the welding process
3.9
numerical simulation
implementation of one or more scientific computing tools (3.11), with calculation strategies (3.2) and input
data, to produce numerical results describing the evolution of a physical situation
3.10
power density
amount of thermal power absorbed or generated per unit volume
3.11
scientific computing tool
SCT
software for numerical simulation (3.9) of physical phenomena
Note 1 to entry: An SCT may consist of one or more solvers and include pre- and post-processors.
Note 2 to entry: SCTs use computational methods to solve science and engineering problems.
Note 3 to entry: Refer to Annex A for technical specifications of SCTs.
3.12
reference scientific computing tool
scientific computing tool (3.11) for which the predictive performance is considered to be superior to that
expected of the scientific computing tool to be validated
3.13
scope of utilization
situations and scenarios studied using the scientific computing tool (3.11) for computational welding
mechanics
3.14
spatial discretization
distribution and type of the geometric units for subdividing the geometric model

ISO 18166:2026(en)
3.15
temporal discretization
step size and number of time units for subdividing the duration being modelled
3.16
validation case
data set considered to be pertinent and selected for carrying out separate effects or integral validation of a
scientific computing tool (3.11)
Note 1 to entry: Data set can be experimental test, operating experience feedback, simulation using a reference
scientific computing tool (3.12), analytical solution, etc.
3.17
validation experiment
experiment designed to validate the simulation results taking into account all relevant data and their
uncertainty
3.18
validation file
document in which all the results of the validation of a scientific computing tool (3.11) are inventoried
3.19
verification file
document in which all the results of the verification of a scientific computing tool (3.11) are inventoried
4 Abbreviated terms
For the purposes of this document, the abbreviated terms given in Table 1 apply.
Table 1 — Abbreviated terms
Abbreviated term Definition
2D two dimensional
3D three dimensional
CTE coefficient of thermal expansion
CWM computational weld mechanics
EBW electron beam welding
FEA finite element analysis
GMAW gas metal arc welding
GTAW gas tungsten arc welding
HAZ heat-affected zone
PIRT phenomena identification ranking table
PWHT post weld heat treatment
QI quantities of interest
SAW submerged arc welding
SCT scientific computation tool
SMAW shielded metal arc welding
WPS welding procedure specification
WPQR welding procedure qualification record

ISO 18166:2026(en)
5 Principle
The thermomechanical numerical simulation of welding is mainly based on the finite element method. It
consists of a WPS implemented in an SCT for CWM, pre- and post-processing tools, and verification and
validation methods (see References [18] and [19]).
The CWM problem is generally defined as a three-dimensional solid element model employing a moving
heat source with simultaneous calculation of temperature, microstructure, displacement and stresses,
utilizing elastoplastic constitutive law based on material properties ranging from room temperature up to
the melting temperature.
It requires the geometric modelling of the part to be joined in the form of a mesh, the modelling of the
initial and boundary conditions and the definition of materials behaviours. From the spatial and temporal
discretization, the SCT allows the resolution of a heat transfer problem with transient heat source, with
possibly the determination of metallurgical transformations and the thermomechanical calculation of
residual stresses, strains and distortions. The pre- and post-processing tools may be different from the SCT.
However, it is the set of tools used for the intended studies that is the subject of the recommendations of this
document.
6 Scientific Computation Tools (SCTs)
An SCT for numerical welding simulation has specific capabilities compared to conventional FEA software.
The SCT shall enable the calculation of the quantities of interest with an uncertainty level appropriate to
intended use of the studies. The SCT shall enable the implementation solution of a CWM problem following
all the recommendations of this document.
In order to follow all the recommendations of this document the SCT shall:
— create 2D and 3D meshes of single and multi-pass welded joints (see Clause 10);
— visualize and export the quantities of interest (see Clause 8);
— define and verify the heat source according to space and time coordinates (see 11.2.3);
— access to solver parameters of accuracy, convergence, numerical schemes in order to be able to carry out
a spatial and temporal convergence study (see 11.1);
— simulate the spatio-temporal evolution of the temperature (see 11.2.3);
— simulate the phase transformations if any (see 11.3.3);
— couple thermomechanics with metallurgical effects (see 11.2).
The SCT should:
— simulate the addition of filler metal;
— activate and deactivate elements during a simulation;
— consider viscous and creep effects especially when PWHT is of concern;
— simulate the temperature and the cyclic behaviour of the materials, recovery effects, and transformation
induced plasticity.
The SCT should use and have a library of:
— validated and verified material data;
— cases for verification and validation.
The user may also add their own data and their own verification and validation cases.

ISO 18166:2026(en)
7 Required data for simulation
In order to ensure a representative modelling, the user shall have enough information on the way the
welding was carried out that may be obtained from a welding book, from the description and qualification of
the welding procedures specifications (WPS) or from a production report.
The user shall have access to the following:
— fabrication procedure records, detailing how the structure was manufactured, such as intermediate
machining operations and shaving;
— design/construction drawings defining the nominal component geometry and dimensions;
— weld groove geometry (from drawings or WPS);
— weld procedure information, type of process, heat input per unit length of weld, welding voltage, current
and welding speed, deposit flow rate, type of cover gas, filler metal, welding position, number and
arrangement of passes, their trajectory, their sequence as well as the requirements for finishing and root
passes, and buttering;
— the characteristics of external clamping devices, interpass and pre and post temperatures requirements;
— applied mechanical restraint;
— appropriate thermomechanical properties from test certificates (or specifications);
— plant survey data characterising the constructed weldment geometry and dimensions (actual dimensions,
distortion effects, root penetration, presence of cap etc.);
— an etched macrograph showing a cross section of the weldment normal to the welding direction which
may be used to define the local geometry, number of weld passes, weld pass sequence and possibly the
weld heat input, hardness, metallography;
— construction records – photographs, weld dressing, inspection certificates (radiographs, etc.), repairs,
PWHT conditions, proof test conditions.
If any of this information is not available, the user may use data from a similar WPS or obtained from expert
advice, that shall be justified.
8 Formulation of the problem and establishment of the simulation strategy
This Clause gives the user a method regarding the quantities of interest, to select the predominant physical
phenomena, identifying the sources of uncertainty, and deducing the appropriate assumptions and modelling
strategies. Annex E offers supplementary guidance and informational material.
The selection of physical phenomena results from a whole process, which depends on the problem and the
objectives of calculation, and that shall be justified by the user.
By identifying the physical phenomena, prioritizing them, specifying the state of knowledge, the available
data, the uncertainties, and the maturity of the models and codes, the user should find the appropriate
assumptions and modelling strategies. To do this, the following questions should be answered:
— What is the objective of the welding simulation?
— How will the results be used and what level of conservatism and confidence is required?
— What are the quantities of interest?
— What are the physical phenomena involved? Are they separable?
— What are their effects on the quantities of interest?
— Are tests to observe them available? What kind of tests, with separable or integral effects?

ISO 18166:2026(en)
— Is the method to model them known? What is the level of knowledge regarding the availability of data
and models of the computational codes?
— How does the model capabilities of modelling and lack of knowledge, impact the capacity to simulate the
dominant physical phenomena?
— Do the materials exhibit phase transformations? Have they been heat treated before welding? Are viscous
phenomena to be taken into account?
— How precisely should the process be modelled?
— What are the time and space scales, what physical phenomena are steady-state, transient, spatial 2D, 3D,
axisymmetric, and what are the appropriate modelling details?
To answer these questions, users should use Table 2 and Table 3 (see References [15], [16] and [17]) below
with the help of other experts in the same field. A three-level scale is used to rank:
— the effects of phenomena, model parameters on quantities of interest;
— the level of knowledge regarding the availability of data and models.
Table 2 — Importance level (I) of physical phenomena on quantities of interest
Rank Weight Definition – Effect Implication
The phenomenon has a significant or The phenomenon should be explicitly considered
High (H) 1,0 dominant impact on any of the evalua- in experimental programs and modelled with
tion criteria. high accuracy in computational tools.
The phenomenon has only a moderate Experimental studies and modelling are required,
Medium (M) 0,5
impact on the evaluation criteria. but the scope and accuracy may be compromised.
The phenomenon should be exhibited experi-
The phenomenon has small or no impact
Low (L) 0,0 mentally and considered in computational tools.
on the evaluation criteria.
However, almost any model is sufficient.
NOTE The overall importance level (I) for a phenomenon is calculated from the votes of each participating expert
through Formula (1):
10,,NN05 00, N
HM L
I (1)
NNN
HM L
where N , N and N refer to the numbers of “High”, “Medium” and “Low” importance votes, respectively and the
H M L
numerical values are the weights defined for these importance levels; see Table 2.
Table 3 — Level of knowledge (K) regarding availability of data and models
Rank Weight Availability of data Availability of models
The phenomenon is well understood.
Data obtained for the welding configu- Models that are validated for application to weld-
Adequate (A) 1,0
ration are available in sufficient range, ing configuration are available.
quantity and quality.
The phenomenon is understood. Data
obtained for the welding configuration The phenomenon may only be modelled with
are available, but not in sufficient range, moderate uncertainty or approximately modelled,
Some (S) 0,5 quantity or quality. Alternatively, data e.g. by lower order models or models for similar
pertinent to other conditions exist and phenomena that maybe extrapolated to the weld-
maybe extrapolated to the welding con- ing configuration.
figuration.
No validated models exist. Modelling the phenom-
The phenomenon is not well understood.
None (N) 0,0 enon is currently either not possible or is possible
No relevant data exist.
only with large uncertainty.
ISO 18166:2026(en)
NOTE Likewise, the overall knowledge levels (K) with regard to data and models were calculated separately
through Formula (2):
10,,NN05 00, N
AS N
K  (2)
NNN
AS N
where N , N and N refer to the numbers of Adequate, Some and None level of knowledge votes, respectively, and the
A S N
numerical values are the weights defined for the level of knowledge; see Table 3.
The user, with the help of other experts regarding the quantities of interest of the welding configuration,
identifies the physical phenomena, and ranks them in terms of impact on the evaluation criteria on quantities
of interest. The user identifies the model and data and ranks them in terms of availability and level of
knowledge. Based on the importance ranking and knowledge level, the user determines the phenomena that
need further consideration and the simulation strategy.
The evaluation criteria are essential metrics used to assess and compare the relative significance of each
phenomenon. A graphical representation [I (Importance Level) vs. K (Knowledge Level)] is shown in Figure 1
th
and the relative importance of the i physical phenomena is assessed by Formula (3):
D M
IK11K

ii i
R  (3)
i
D M
 
max IK11K

ii i
 
i
D M
where I is the importance level of the i th phenomenon, and K and K are the knowledge levels for
i i i
data and models, respectively.
Figure 1 — Graphical representation [I (Importance Level) vs K (Knowledge Level)]
For example, for phenomena with high impact on the quantities of interest but with no relevant data or
model, the user should then supplement the data to reduce uncertainties or modify the model by ensuring
a conservative approach. Another example, the heat source power per unit volume distribution modelling
has a high importance on the level of residual stresses. Thus, an adequate level of knowledge of mechanical
behaviour as a function of temperature is required to achieve best-estimate simulations (see Annex F).

ISO 18166:2026(en)
9 Establishment of the input parameters
9.1 Input data
From the data of the physical problem to be simulated, in particular the configuration and the simulation
strategy, the user establishes the material data to be considered (thermo-physical, thermo-mechanical, and
thermo-metallurgical material properties across the temperatures of interest), the parameters of the model
and the solver, the applicable boundary conditions, the geometrical modelling and the simulation domain.
9.2 Simulation template
The user should link the input data in a preparation frame for the simulation data. The user should
summarise the objective of the simulation (by specifying the quantities of interest), the hypotheses and
choices of modelling and simulation, the verifications and validations carried out and the results obtained.
The user may use the form given as example in Annex B.
In addition, the user may make full or partial use of a subroutine automating preprocessing operations in the
SCT such as data setting from the template in Annex B, thus limiting user related copying and interpretation
errors.
10 Geometry and mesh
10.1 Geometry and mesh of welded joint
The user shall represent the geometry of the weld and the size of the passes.
The user should use a three-dimensional (3D) geometric model. However, depending on the configuration,
the user may assume a two-dimensional (2D) plane strain state perpendicular to the weld or use 2D
axisymmetric modelling. The user should then justify these assumptions and assess the related uncertainties
and the impact on the quantities of interest.
10.2 Mesh size
A sufficient degree of mesh refinement should be used, in the weld and adjacent HAZ regions, to model the
gradients of temperature that occur during welding and to resolve the evolution of residual stresses.
Outside of the weld and HAZ region, the user should prudently coarsen the mesh in order to contain the
model size, while ensuring that the mesh is sufficiently refined in the areas of structural concern, which may
not necessarily be local to the weld.
The user may perform preliminary mesh refinement studies to optimise the analysis requirements. Spatial
discretization shall be considered with the evolution of physical phenomena and quantities of interest over
time and space.
10.3 Type of elements
First order finite elements should be used for thermal simulation. In thermomechanical simulation, finite
elements suitable for materials with elastoplastic behaviour according to the Von Mises criterion shall be
used. For example, the user may use sub-integrated second order elements or selectively integrated first
order elements.
The mesh for thermomechanical simulation may be a different mesh than one used for thermal simulation.
The temperature fields of the thermal simulation are then transferred to the mesh for thermomechanical
simulation. The transfer method shall ensure the integrity of transferred field. For that purpose, the user
should perform sensitivity analyses to evaluate the impact on the quantities of interest. Similarly, the
user may choose a thermal simulation mesh different from the one used during heat source fitting such
as described in Annex C. The user should then ensure that changing size of the mesh does not significantly
affect the thermal predictions or the power transmitted to the weldment.

ISO 18166:2026(en)
10.4 Modelling of the filler material
A method of activating and deactivating the bead elements should be used to simulate the weld material
deposition for thermomechanical calculation. For mechanical calculation, among other methods,
deactivation may be realized by reducing the Young's modulus value by three orders of magnitude from its
ambient temperature value, or by choosing a Young's modulus value that is close to its melting temperature
value. Activation may be accomplished by assigning the elements their original properties and behaviour. As
passes are added, the beads of which the elements have been deactivated are sequentially added back to the
model. The user may use activation/deactivation with both imposed temperature or imposed heat flux.
In addition, the user should:
— ensure that the volume of the deposited material is consistent with a macrograph of the weldment cross-
section;
— activate elements when they’re below the solidus temperature or, as a minimum, activate each pass at
the beginning of the deposition;
— assign properties to deactivated elements that do not significantly impact the heat transfer and
mechanical rigidity of the structure.
11 Performing the simulation
11.1 Code verification
Code verification is a formalized process to determine if equations are resolved correctly. The user should
carry out verification test(s) and should not entirely rely on the verifications carried out by the SCT sup
...