Redesign essential to make best use of increased strengths

Tom Shelley reports on a just held design seminar on how to take best advantage from using high strength steels



Weights of fabrications can in some cases be halved by using high strength steels with yield stresses up to four or more times those of mild steels, but in many cases, it is necessary to adopt new design and manufacturing methods in order to achieve best results.

Substituting high strength steel for mild steel and just making it thinner is very unlikely to lead to good results. Possible buckling behaviour is likely to be different, requiring modifications to the design. In fabrication, it may well be desirable to consider roll forming, where high strength steels tend to behave better than mild steels, instead of simple bending and stamping, where high strength steels tend to exhibit greater spring back.

All of these issues and more were covered in seminars and a series of design manuals produced by Swedish Steel, which while tailored to the company's products, contain so much general and useful information that we expect them to be found on every leading mechanical engineer's bookshelf alongside the bearing catalogue from another major company of Swedish origin.

Hot rolled steels with a minimum guaranteed yield point below 310 MPa are usually designated mild steels, whole cold rolled and metal coated steels with a guaranteed yield point between 200 and 450 MPa are designated high strength steels. Swedish Steel make standard products with tensile strengths up to 1400 MPa and yield strengths up to 1200 MPa, and at the seminar, Joachim Larsson from SSAB Tunnplat mentioned 'nanosteels' with yield strengths of 2000 MPa, but then added the comment that such materials were, "Quite hard to use". On being questioned, he explained that if energy absorption is required, as in automotive crash protection, "High strength does not necessarily give benefits because of local buckling. There are instances where a side impact beam made with 1200 MPa yield stress steel absorbs less energy than one made with 1000 MPa yield stress. However, for some applications, such as crane booms, greater yield strength is of immediate benefit", provided, we gathered, the steel exhibits sufficient ductility to be able to absorb impact.

Proper understanding, based on computer modelling, we learned is essential in good design. For example, a piece of steel strip with a notch on each side actually has a slightly higher yield stress than one with a single notch, which is higher than a strip with no notch, because the notch introduces a 3D stress state. This did not mean notches are good, of course, because they lead to early failure and lower energy absorption under the stress strain curve. However, but it does mean they need to be factored into design calculations, as does speed of impact, which also has a marked effect on increasing effective yield stress.

Stiffeners often need to be formed into structural panels made of high strength steels, but good forming properties depend on the material being able to work harden and stretch uniformly, and not on low yield strength. Roll forming in stages is often preferable to stamping, and all materials can be welded using conventional methods and equipment, but optimum setup conditions are usually different, and it may be wise to turn to a different joining technique, such as using adhesives, in order to produce an optimum design.

The three design guides are: "Sheet Steel Handbook: Design and fabrication in high strength sheet steel" (214 pages), "Sheet Steel Forming Handbook: Size shearing and plastic forming" (108 pages) and "Sheet steel joining handbook: Joining of high strength steels", (165 pages).

SSAB Swedish Steel

Pointers

* Use of high strength steels can enable weight savings of as much as 50 per cent in some fabrications

* To ensure best results, it is usually necessary to redesign parts to make best use of material properties and it is likely to be desirable to change fabrication methods also.

Author
Tom Shelley

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