ISO 898-1: Tensile testing of bolts – Strength classes and testing requirements
The standard DIN EN ISO 898-1 is an important basis for the safe design of bolted connections in Mechanical engineering, automotive engineering, and industrial assemblies.
Key facts at a glance
DIN EN ISO 898-1 defines the Minimum mechanical requirements for screws made of unalloyed and alloyed steel with metric ISO threadIt specifies how strength classes 4.6 – 12.9 are to be determined and verified. The basis for this is the standard-compliant tensile test, in which characteristic values such as tensile strength, yield strength and uniform elongation are determined under defined test conditions.
Key aspects of the standard:
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Scope of application: Screws M3 to M39
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Classification via two-digit strength classes (e.g. 8.8)
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Verification of mechanical properties in tensile testing
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Calibration requirements for the testing machine according to ISO 7500-1
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Differentiation from ISO 898-2 (nuts) and ISO 3506 (stainless steels)
- Differentiation from ISO 6892-1 (metal tensile test) – instead of the actual cross-section, the nominal stress cross-section A is used.s,nom used
- ISO 898-1 covers tests on turned screws (Rp0,2) and whole screws (Rpf)
What does DIN EN ISO 898-1 regulate?
DIN EN ISO 898-1 defines the mechanical and physical properties of screws made of unalloyed or alloyed steel with metric ISO thread according to ISO 261.
The aim of the standard is to clearly assign screws to a strength class. This classification enables the safe selection of fasteners for defined mechanical loads.
The focus is on:
- Minimum tensile strength
- Minimum yield strength or 0,2% proof stress
- uniform stretching
- Elongation at break or cross-sectional reduction
The following are not included in the scope:
- Screws made of stainless steel (→ ISO 3506)
- Copper alloy fasteners
- certain coated screws, if the coating system affects the mechanical behavior
Classification within the ISO 898 series of standards
The ISO 898 series of standards consists of several parts:
- ISO 898-1: Mechanical properties of screws
- ISO 898-2: Mechanical properties of nuts
While ISO 898-1 defines the properties of the screw itself, ISO 898-2 specifies the requirements for nuts that are combined with these screws. Only when both components are designed according to the standard is the mechanical performance of the connection ensured.
In the international context, ISO 898-1 is often compared with ASTM F606, which also describes test procedures for mechanical fasteners, but with different test conditions and unit system.
Strength classes according to ISO 898-1 explained
The strength classes consist of two numbers separated by a dot, for example:
- 8.8
- 10.9
- 12.9
Meaning of the marking:
- First number × 100 = nominal minimum tensile strength in MPa, e.g., 8 x 100 = >800 MPa
- Second number = ratio of yield strength to tensile strength, e.g., 800 x 0,8 = >640
8.8 example:
- Tensile strength = 800 MPa (Rm)
- Yield strength = 0,8 × 800 MPa = >640 MPa (Rp0,2)
This system allows for a quick technical classification of the mechanical performance of a screw.
Higher classes such as 10.9 or 12.9 are used particularly in high-stress or safety-relevant applications.
Testing requirements according to DIN EN ISO 898-1
The mechanical properties are determined in a tensile test on the entire screw or on defined test pieces.
Key test parameters of ISO 898-1
- RmMaximum tensile stress until breakage
- Re or Rp0,2: Yield strength or 0,2% proof stress
- Agt: Uniform elongation up to maximum force
- Z: Fracture constriction of a sheared screw
The test is carried out under controlled conditions, usually at ambient temperature (approx. 23 °C).
Sample preparation and clamping
For screws with partial threads, it is essential to ensure that the thread does not introduce any local stress peaks into the evaluation. Therefore, in many cases, special test pieces with a defined shank area are used.
The clamping must:
- fleeing
- centric
- without additional bending moments
This is done to ensure results that comply with standards.
Requirements for testing machines according to ISO 898-1
Universal testing machines with suitable force and strain measurement capabilities are required for testing in accordance with standards.
Calibration
- Force measurement according to ISO 7500-1
- Strain measurement (if used) according to ISO 9513
At least accuracy class 1 is required, in particular to determine the yield strength precisely.
Testing staff area
Depending on the screw diameter and strength class, test loads in the range of 100 kN to 300 kN or above may be required.
Clamping devices for screws
Special screw clamping devices are used for tensile tests on screws. The following are important:
- centric force transmission
- high frame rigidity
- wear-resistant clamping systems
- Reliable absorption even in the event of spontaneous sample fractures of high-strength classes
Electromechanical or hydraulic universal testing machines with suitable screw clamping devices enable the standard-compliant execution of tests according to ISO 898-1.
Video: Tensile test on bolts according to ISO 898-1 using a universal testing machine
Testing software and evaluation
For reproducible testing procedures, standards-compliant testing software is required. This software should:
- Store test parameters according to ISO 898-1
- Automatically calculate key figures
- Document test reports in accordance with standards.
- Consider the machine's calibration status
Predefined testing methods facilitate the standardized execution of serial tests, particularly in the quality assurance environment.
FAQ on ISO 898-1
What does the ".9" mean in 10.9 or 12.9?
The second number describes the ratio of the yield strength to the tensile strength. At 10.9, the yield strength is 90% of the tensile strength.
Do screws need to be checked in the threaded area?
No. The thread creates local stress concentrations. For evaluating the mechanical properties, the load-bearing shank area is used, or a defined test piece is employed.
How often do screws need to be checked?
The standard itself does not define a testing frequency. This is determined by internal quality assurance guidelines, batch sizes, or customer-specific requirements.
Does ISO 898-1 also apply to stainless steel screws?
No. ISO 3506 applies to stainless steels. The strength class systems are not directly transferable.
What role does the calibration of the testing machine play?
A valid calibration according to ISO 7500-1 is a prerequisite for standard-compliant test results. Without traceable calibration, the determined characteristic values cannot be reliably documented. Screws are inherently safety-critical components; therefore, calibration must be carried out by an accredited company (Thank you) take place.
