24 Erik Moncrieff which seek to model the micro structure of the textile within the coating matrix. Crimp models,when properly calibrated,can achieve very high accuracy.As such these models are the most suitable for applications where design duration and cost is secondary to the performance of the structure.Satellite and other space structures are examples.In the architectural textile structure industry,complex crimp models suffer from slow performance and the infeasibility of adequate calibration. As stated earlier,two element types are mostly used in industrial lightweight structure design systems.These are the cable net (link)and continuum (usually triangulated)models.There is a controversy concerning the relative accuracy and appropriateness of these models.Both elements are used in our systems,as well as several others.We advocate the cable net model as being the most appropriate for current industrial practice on the following basis. The shear resistance of textile is extremely low and is customarily ignored by everyone.Poisson's ratio effects caused by the textile structure are usually small, but can be noticeable in some parts of some structures.Contrary to extensive belief the cable net model can,like the continuum model,be used to model this effect. It must be stressed,however,that for both methods the sophistication is primitive and limited by the difficulty in choosing Poisson's ratio values.Looking back to the biaxial test data in Fig.7 above it should be clear that the extraction of a single representative Poisson's ratio parameter is pure fantasy.Another element type which we have developed for textile structure modelling is cruciform based with link forces a function of the two cruciform link strains.We believe that in the medium future the use of automatically calibrated cruciform elements will become feasible.This will require the development of an integrated system linking the producers,test laboratories,software developers and design engineers.The fundamental test will be whether it is possible to use an automatically calibrated model to confidently predict any real biaxial test. (a) (b) (c) (d) Fig.8.Element types:(a)Composite crimp,(b)Cable net link,(c)Continuum triangle and (d)Cruciform It is important to stress again that extra complexity does not necessarily imply extra accuracy.In particular,complex models require many difficult to measure parameters.Which are usually unavailable.When parameters are unavailable,less complex models are no less accurate and maintain contact between the designer and the physical behaviour.24 Erik Moncrieff which seek to model the micro structure of the textile within the coating matrix. Crimp models, when properly calibrated, can achieve very high accuracy. As such these models are the most suitable for applications where design duration and cost is secondary to the performance of the structure. Satellite and other space structures are examples. In the architectural textile structure industry, complex crimp models suffer from slow performance and the infeasibility of adequate calibration. As stated earlier, two element types are mostly used in industrial lightweight structure design systems. These are the cable net (link) and continuum (usually triangulated) models. There is a controversy concerning the relative accuracy and appropriateness of these models. Both elements are used in our systems, as well as several others. We advocate the cable net model as being the most appropriate for current industrial practice on the following basis. The shear resistance of textile is extremely low and is customarily ignored by everyone. Poisson’s ratio effects caused by the textile structure are usually small, but can be noticeable in some parts of some structures. Contrary to extensive belief the cable net model can, like the continuum model, be used to model this effect. It must be stressed, however, that for both methods the sophistication is primitive and limited by the difficulty in choosing Poisson’s ratio values. Looking back to the biaxial test data in Fig. 7 above it should be clear that the extraction of a single representative Poisson’s ratio parameter is pure fantasy. Another element type which we have developed for textile structure modelling is cruciform based with link forces a function of the two cruciform link strains. We believe that in the medium future the use of automatically calibrated cruciform elements will become feasible. This will require the development of an integrated system linking the producers, test laboratories, software developers and design engineers. The fundamental test will be whether it is possible to use an automatically calibrated model to confidently predict any real biaxial test. Fig. 8. Element types: (a) Composite crimp, (b) Cable net link, (c) Continuum triangle and (d) Cruciform It is important to stress again that extra complexity does not necessarily imply extra accuracy. In particular, complex models require many difficult to measure parameters. Which are usually unavailable. When parameters are unavailable, less complex models are no less accurate and maintain contact between the designer and the physical behaviour. (a) (b) (c) (d)