Tube hydroforming

As evident in the ULSAB programme, hydroformed tube has significant potential in the consideration of parts consolidation especially for the more rugged applications such as the 4 × 4 sector which also allows a little more freedom of construction. The background and other weight-saving technologies demonstrated by the ULSAB initiative are gathered together in Chapter 4 but from a design viewpoint it is instructive to

consider the recent study which evaluated the possible advantages in incorporating hydroformed structural elements within the Land Rover Freelander.

Described in a recent IBEC conference,16 it is worth highlighting to show how potentially good ideas can be evaluated under realistic conditions, while defraying costs and resources of two major organizations. In this instance the design data for the recently developed Freelander was immediately to hand and could be relatively easily modified to allow an immediate comparison of new and conventional structures.

The opportunity was also presented to allow full vehicle testing of a new concept rather than the body-only exercise with the ULSAB sedan, relying on FE modelling to predict performance.

The Land Rover Freelander was chosen as the focus for this programme principally due to the maturity of the development programme for the vehicle and the design package which allowed application to either smaller or larger products. Although a Land Rover (hitherto body-on-chassis design), the body is of monocoque or unitary construction, and the incorporation of a rigid sectional product seemed a natural choice for a rugged off-road performer.

The final configuration of hydroformed components incorporated in the design is shown in Fig. 2.34 and followed an extremely detailed study. It is worth mentioning that the normal procedure is to work to a controlled pre-development plan whereby the features of a new design are compared with the original, a cost-effective manufacturing route defined and rigorous testing of new components undertaken. The whole process is regulated with frequent timing reviews and concurrence obtained before proceeding through successive ‘gateways’ or decision points. These pre-concept stages constituting the ‘creative’ phase, gateways and process steps are illustrated in Fig. 2.35(a):

During this evaluation the advantages of the hydroformed sections will have been assessed, first, to confirm weight and space saving potential allowed by shape characteristics, Fig. 2.35(b) and second, as shown in Fig. 2.36 in comparison with other possible methods that could produce similar savings.

It was essential for comparisons involving joints and flange replacement that the welds were accurately modelled. For normal press steel box sections the assumption is made that flangeless sections are used and no allowance is made for distinguishing between alternative joining methods. However, it was critical for this new type of hydroformed joint that the joining method was represented more accurately and solid elements were used to represent adhesives and rigid bars positioned at the centre of flanges to simulate spot welds, Fig. 2.37.10 The various design iterations can then proceed to determine sections and joints which would  probably benefit most from alternative hydroformed sections.

Comparison of hydroformed with conventional parts and the stages in the manufacture from tube are shown in Fig. 2.38. The ‘Application’ phase comprised manufacture of the prototype parts illustrated in Fig. 2.38, using representative methods by a number of key tube hydroform suppliers, and finally a build and test programme to validate the advantages of the modified structure. The findings are summarized in Fig. 2.39.

The results shown in Fig. 2.39, and from crash and durability testing, demonstrated that the revised structure was equivalent in performance to the Freelander while torsional stiffness was markedly improved. However, starting a completely new model programme without the constraints of an existing body it is highly likely that far more significant weight savings and parts consolidation could have been achieved. Manufacturing feasibility was also demonstrated so opportunities can now be determined in the forward model programme.

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Materials for Automobile Bodies

Geoff Davies F.I.M., M.Sc. (Oxon)

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD

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