Home » What Is Property Class 8.8?

What Is Property Class 8.8?

Property class 8.8 is a high tensile steel fastener grade defined by ISO standards for metric fasteners.

The designation “8.8” defines two fundamental mechanical properties:

  • Minimum ultimate tensile strength

  • Minimum yield strength

This combination provides a reliable balance of strength, ductility, and predictable failure behaviour, which is why property class 8.8 is one of the most widely specified fastener grades in construction and engineering.

Understanding Strength Values in N/mm² (MPa)

Fastener strength is expressed in N/mm² (Newtons per square millimetre), which is numerically equivalent to MPa (Mega Pascals).

For metric fasteners, the property class marking defines both tensile and yield strength.

  • The first number, multiplied by 100, defines the minimum ultimate tensile strength

  • The second number defines the ratio of yield strength to tensile strength

Example: Property Class 8.8

  • Ultimate tensile strength:
    8 × 100 = 800 N/mm² (MPa)

  • Yield strength:
    0.8 × 800 = 640 N/mm² (MPa)

This means a property class 8.8 fastener will begin to plastically deform at approximately 640 MPa and will fail in tension at approximately 800 MPa, when tested under standard conditions.

Does Strength Change with Diameter or Length?

The material strength values (N/mm²) defined by property class do not change with:

  • Fastener diameter

  • Fastener length

  • Thread pitch (coarse or fine)

  • Fastener geometry

  • Surface finish

An M6 and an M24 property class 8.8 fastener are manufactured from material with the same minimum tensile and yield strength.

What does change

While strength (MPa) remains constant, the maximum load a fastener can carry increases with diameter.

This is because:

  • Larger fasteners have a greater tensile stress area

  • This allows higher absolute loads (kN) before failure

This distinction is critical:

  • Strength (MPa) = material property

  • Load capacity (kN) = size-dependent

Mechanical Behaviour of Property Class 8.8

Property class 8.8 fasteners are designed to operate primarily in the elastic region of the stress–strain curve when correctly tightened.

Key characteristics:

  • Good resistance to permanent deformation

  • Predictable yield point before failure

  • Tolerance to moderate shock and dynamic loading

  • Lower brittleness than higher tensile grades

This makes 8.8 well suited to joints where reliability and installation tolerance are important.

Typical Applications of Property Class 8.8

Property class 8.8 fasteners are commonly used in:

  • Structural steelwork

  • Construction assemblies

  • Mechanical engineering

  • Industrial machinery

  • OEM manufacturing

  • Maintenance and repair

They are particularly suitable where:

  • Loads are significant but not extreme

  • Joint movement or vibration may be present

  • Over-tightening risk must be minimised

Property Class 8.8 Finish Types and Corrosion Expectations

The mechanical strength of an 8.8 fastener is independent of its finish.
Finish selection affects corrosion behaviour, not tensile performance.

Zinc Plated Property Class 8.8

Zinc plating is the most common finish for property class 8.8 fasteners.

Characteristics:

  • Provides general corrosion protection

  • Suitable for indoor and dry environments

  • Improves shelf life and handling resistance

Zinc plated fasteners are often evaluated using salt spray testing (e.g. ISO 9227).

Important note on salt spray testing

Salt spray testing:

  • Measures coating durability, not service life

  • Does not replicate real-world corrosion exposure

  • Is best used for comparative testing only

A higher salt spray hour rating does not equate to a longer in-service lifespan.

Self Colour (Uncoated) Property Class 8.8

Self colour fasteners have no protective coating.

Characteristics:

  • No inherent corrosion protection

  • Suitable for controlled or protected environments

  • Often used where:

    • Dimensional accuracy is critical

    • Coatings may interfere with tolerances

    • Secondary treatments are applied

What Property Class 8.8 Is Not Suitable For

Property class 8.8 is not recommended where:

  • Maximum tensile strength is required

  • Aggressive corrosion environments exist

  • Marine or chloride-rich exposure is present

  • Extremely high preload is required

Selecting a higher strength grade should always be based on joint design, not assumption.

Related Technical Standards & Comparisons

  • Property Class 10.9 explained

  • Property Class 12.9 explained

  • Zinc plated vs self colour finishes

  • Metric coarse vs metric fine threads