EuroFasteners September 2022

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Mechanical testing of bolts, screws and studs The testing of bolts, screws and studs to DIN EN ISO 898-1, ASTM F606-1 and DIN EN ISO 3506-1 standards is used to determine mechanical properties such as tensile strength, offset yield, fracture elongation or hardness. Some of the most important fasteners, bolts, screws and studs are considered universally important not only in industrial settings, but also for everyday purposes. They are used in the construction of complex machinery and equipment, as well as in vehicles and buildings. Since mechanical stress is concentrated on these types of con nection points, the integrity of fasteners is important, so bolts, screws and nuts are subject to strict safety requirements. Property classes of bolts and screws are identified with two numbers separated by a dot. The number to the left of the dot is the tensile strength in MPa divided by 100. The number to the right of the dot indicates the tenfold of the yield point or offset yield ratio. For example, a screw of property class 9.8 would have the following nominal values:

Purpose-designed tooling for fastener tensile testing

All variations of the tensile test must be performed according to ISO 6892-1, where the test speed does not exceed 25mm/min. In addition, the test speed does not exceed 10mm/min in the determination of yield point or offset yield. For full-size fasteners, the tests defined for the determination of offset yield, tensile strength and elongation after fracture may be combined in one tensile test. The methods to Vickers (ISO 6507-1), Brinell (ISO 6506-1) or Rockwell (ISO 6508-1) are permissible for the determination of hardness. For hardness testing to Vickers, a minimum load force of HV 10 applies. Hardness testing to Brinell is mainly performed with HBW 1/30 or HBW 2.5/187.5. For hardness testing to Rockwell, scale B (HRB) is used for fasten ers of property class 4.6 to 6.8, and scale C (HRC) for property classes 8.8 to 12.9. In the event of arbitration, the Vickers hardness test is used as reference. Hardness is either measured on the cross section or on the surface. In the first case, a cross section specimen is taken from the threaded area. Indentations are made near the fastener’s axis and the resulting hardness is also referred to as core hardness. When hardness is measured on the surface, the surface must be flat (eg on the head, on the end of the fastener or on the unthreaded shank) after removal of any coating. In the retempering test, Vickers hardness is measured before and after heat treatment. The reduction of hardness after retempering, if any, must be less than 20 Vickers units. In the decarburisation and carburisation test, hardness is tested to Vickers with a test force of 2.942 N (HV 0.3) on a longitudinal section through the thread axis. The thread zone is measured on three defined points with different distances to the surface. Differences in measured values that are either too small or too large are an indication of decarburisation or carburisation. ZwickRoell Ltd www.zwickroell.com

Nominal tensile strength: Rm = 9 x 100 MPa = 900 MPa

Yield point ratio: Re/Rm = 8 ÷ 10 = 0.8

Yield point: Re = 0.8 x 900 MPa = 720 MPa

The property class of a nut is identified with a single number, which corresponds to the highest property class of a screw with which the nut can be paired. A nut of property class 9 can be paired, for example, with a screw up to property class 9.8. Threaded fasteners are selected very carefully regarding their industrial application and are integrated carefully into the design.

Accurate determination (via tensile tests) of the elastic tensile modulus and the

limit of elastic loading is essential, since these values determine the limit forces required for a secure, reliable threaded connection.

Vickers hardness testing machine

EF 7/57

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September 2022

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