Published on 21 January 2013
The fatigue strengths of joints in steel plate are well established (BS 5400:Part 10:1980 gives typical design data), but little information is available on joints in the rectangular hollow sections (RHS) used by a flour milling equipment manufacturer. To confirm that its designs were sound, the company, asked TWI to carry out a project to establish the fatigue strength of steel RHS T joints.
RHS joints are used extensively in the company's flour sifter frames. These machines comprise two rows of sieve boxes, with a central section in between, which houses an electric motor driving an out-of-balance mass. When the mass rotates, the entire sifter oscillates with a circular motion of typically 85mm diameter at a frequency of 5Hz.
The machine completes many millions of cycles during its life, so there was a need for accurate design data for the MIG welded frames fabricated from 80mm square tube.
TWI tests established the fatigue properties of the joints in the frame, allowing optimisation of the design.
Methods of increasing the fatigue strength were also considered. Weld toe grinding, the simplest and most widely adopted method, was not appropriate because of the limited wall thickness of the tubes (3.6mm).
TWI recommended needle peening as an alternative. This uses a conventional pneumatic needle gun, of the type widely used for descaling welds, holding a bunch of 2mm diameter wires with rounded tips. The peening treatment introduces a compressive residual stress in the crucial weld toe region. Previous studies within TWI's core Research Programme had shown the method to be effective in increasing the fatigue strengths of fillet welded joints in 12.5mm thickness steel.
Repeat tests with the RHS joints gave a tenfold increase in fatigue life - a very useful improvement which was adopted in production on selected sifter frames.
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