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Understanding SAE J2649: The Standard for Strain-Life Fatigue Data Exchange
SAE J2649 is a recommended practice that defines a common file format for exchanging controlled periodic overload fatigue test data. These tests consist of a single large fatigue cycle (the overload) followed by a larger number of smaller cycles. The format provides a simple, text-based structure for capturing material properties, test conditions, and measured data. This article explains the key aspects of the SAE J2649 format, including mandatory and optional tags, data layout, and best practices for implementation.
Overview of SAE J2649
The SAE J2649 format is designed to facilitate consistent and unambiguous exchange of strain-life fatigue data between organizations, software tools, and databases. It uses a plain text file where each line is either a comment (beginning with #), a tag (special identifier lines starting with #SYMBOL=), or a data line.
Tests following this format involve two repeating block types:
- An overload cycle – fully reversed, high-amplitude cycle.
- A series of smaller cycles that share a common mean stress and amplitude.
The data file captures both the small-cycle block details and the overload cycle characteristics on a single data line per specimen.
🔍 Engineering Design Insight
The simple text-based format allows easy readability and manual editing while supporting automated parsing. The use of mandatory tags ensures baseline completeness, and optional tags enable detailed material characterization. Comments and blank lines enhance clarity without breaking format compatibility.
The simple text-based format allows easy readability and manual editing while supporting automated parsing. The use of mandatory tags ensures baseline completeness, and optional tags enable detailed material characterization. Comments and blank lines enhance clarity without breaking format compatibility.
File Format Structure and Usage
Mandatory Tags
The following tags are required for every valid SAE J2649 file:
| Tag | Description | Example |
|---|---|---|
#FileType= strain_life_overload |
Identifies the file as following this standard. | #FileType= strain_life_overload |
#DataType= raw or fitted |
Indicates whether data are experimental (raw) or derived from fitting (fitted). |
#DataType= raw |
#NAME= |
Unique material identifier. Multiple names can be used. | #NAME= SAE1045 |
#UNITS= ksi or MPa |
Stress units used throughout the file (case insensitive). | #UNITS= KSI |
#E= |
Average first loading elastic modulus from fatigue tests. | #E= 29500. (ksi) |
Optional Tags for Material Properties
Additional tags can be included to provide richer material characterization:
| Tag | Description |
|---|---|
#Sy= |
Monotonic 0.2% offset tensile yield stress |
#Su= |
Monotonic tensile ultimate stress |
#%RA= |
Percent reduction in area at failure |
#HB= |
Brinell hardness |
#monotonic_K= |
Strain hardening coefficient |
#monotonic_n= |
Strain hardening exponent |
#LowerYield= |
Average lower yield stress |
#FractureStrength= |
True fracture stress |
#FractureStrain= |
True fracture strain |
#MaterialForm= |
Raw material form (e.g., plate, rod) |
#SpecimenForm= |
Fatigue specimen geometry |
#PlasticStrain= measured or calculated |
Indicates origin of plastic strain values |
#FailureDef= |
Definition of failure (e.g., load drop, separation) |
Data Line Layout
Each data line corresponds to one test specimen. The columns are separated by spaces and contain parameters for both the small cycles (SC) and the overload cycles (OL). A sample line from Appendix A of SAE J2649 is shown below:
0.00075 4347.7 10000 20.0 0.00390 36.0 0.0000625 0.00500 10000 60.0 0.0 0.00270 28371.
The column definitions are (in order):
- SC e Amp – small cycle engineering strain amplitude
- Fail Blocks – total block repetitions to failure
- SC per block – number of small cycles per block
- SC S Amp – small cycle stress amplitude
- SC Mean e – small cycle mean strain
- SC Mean S – small cycle mean stress
- SC Plastic e Amp – small cycle plastic strain amplitude
- OL e Amp – overload cycle engineering strain amplitude
- CA-OL Life – constant amplitude life of the overload cycle alone
- OL S Amp – overload cycle stress amplitude
- OL Mean S – overload cycle mean stress
- OL Plastic e Amp – overload cycle plastic strain amplitude
- OL initial elastic modulus – initial modulus of the overload cycle
Comments and Special Markers
All lines beginning with # are treated as comments, unless they match the tag syntax. Inline comments can also be added after data lines. To denote a suspended test (runout), append #runout to the data line.
Best Practices and Frequently Asked Questions
⚠️ Common Mistakes
- Omitting mandatory tags (
#FileType,#DataType,#NAME,#UNITS,#E). - Writing strain values as percentages instead of absolute (e.g., 0.5% should be
0.005). - Incorrect spacing or missing delimiters between values.
- Forgetting to mark runout tests with
#runout. - Mixing stress units within the same file.
- Using unofficial tag names.
For reliable data exchange, always use the exact tag names, specify units clearly, include descriptive comments, and ensure parsers handle whitespace variability.
What are the mandatory tags in SAE J2649?
The mandatory tags are
#FileType= strain_life_overload, #DataType= (with value raw or fitted), #NAME=, #UNITS= (ksi or MPa), and #E=. These tags ensure that the file can be correctly interpreted.How do I indicate a runout (suspended test) in the data?
Append
#runout at the end of the corresponding data line. For example: 0.00070 4347.7 50000 17.0 0.00390 … #runout. Without this marker, the test is assumed to have failed.What is the difference between raw and fitted data?
Raw data (
#DataType= raw) are directly measured experimental values. Fitted data (#DataType= fitted) are derived from a mathematical model or interpolation and are not original measurements.How should strain values be reported – as percentages or absolute?
Strain values must be given as absolute dimensionless numbers (e.g., 0.005 for 0.5% strain). Using percentages directly (0.5) is a common error and will lead to misinterpretation.
For more details, refer to the full SAE J2649 standard and the sample file in Appendix A.
