Editorial

Welcome to the second issue of Strain under the aegis of Blackwell Publishing. Although not a ‘special’ issue as such, this edition of Strain is marked by the presence of a number of interesting papers linked together by their coverage of issues in load/health monitoring and failure analysis. All are distinguished by their emphasis on practical matters; the appropriate technology is discussed in each case and where theory is applied, the emphasis is on fitness for purpose rather than generality at the cost of simplicity. As a result, anyone interested in Structural Health Monitoring (SHM) or damage detection and prognosis, particularly for civil structures, will find much to interest them. The first paper by Scott, Gilbert and Moss is concerned with the load testing of floor slabs in a full scale building. This type of study is vital to the development of many Civil Engineering technologies, not least SHM. The study concerns the sixth floor of a full scale (22.5 metre square) concrete building housed at the Large Building Test Facility of the Building Research Establishment at Cardington, Beds. Measurements from within the floor slabs were obtained using instrumented bars – with internal strain gauges – as developed over the last 20 years at Durham University. A number of interesting results are obtained. The gauges were able to place limits on the extent of creep and tension stiffening which occurred over the short term in the structure. They were able to show that load transmission between slabs was limited. Perhaps most interestingly, from the SHM viewpoint, the gauges were able to detect/highlight the presence of new cracks and to recognize propagation of existing cracks. The second paper is by Sykes and Allison and details a case study relating to failure of a Gas Turbine Inlet Weatherhood which was in service on a North Sea Production Platform. The paper shows how essentially elementary theory combined with basic testing is sufficient to elucidate the cause of failure and to suggest a solution. A natural frequency calculation based on Rayleigh's method gives an estimate suggesting that aerodynamic excitation of the structure will be likely. The subsequent wind-tunnel testing of the structure – with a little extrapolation – shows that the wind velocity observed before failure was likely to excite stresses in the structure in excess of the yield stress. In order to reach the latter conclusion, the authors abandoned Rayleigh's method in favour of a wave equation analysis and one of the conclusions of the paper is that care must be taken in applying the more elementary method to the calculation of bending stresses and moments. The analysis also highlights the sensitivity of the structure to certain geometrical dimensions and suggests a means of stiffening the structure to overcome the aeroelastic excitation problem. The final contribution to the full papers is by Maguire and Philips. The study provides a cost–benefit analysis for the monitoring of large scale structures. The main drivers for structural integrity assessment: safety, economy, environmental and legal issues, are briefly discussed before the paper moves on to a number of case study illustrations. The main thrust is to show that the cost of monitoring is essentially an increasing function of the length and complexity (number of channels) of the measurement process and this is summarized nicely in the form of a graph. The three case studies range broadly over scale and cost and a number of measurement technologies are discussed with reference to the scale and to the required reliability of the sensors. In all cases, the benefit of the monitoring appears to outweigh the cost. Fernie and Warrior provide a technical note on high strain rate testing of composite materials. The paper is concerned with improving signal to noise ratio in drop weight tests for composite samples. The improvements are achieved by a number of modifications to the usual test strategy; for example, the load cell in their apparatus is mounted near the specimen rather than on the drop weight; this eliminates the fall in load during rebound of the impact head. One benefit of the improvements is that results can be displayed without filtering, always an advantage. The results from the specimens tested – both in tension and compression – are interesting. Marked increases in moduli and ultimate strength are observed, with the ultimate tensile strength over twice the value from a quasi-static test. This type of analysis provides valuable guidance for anyone concerned with damage prognosis in an impact situation – the authors observe a 190% increase in matrix cracks over the quasi-static test. Other highlights of this issue include the technical note by Laura, Gutierrez and Rossit, which provides some insight into the problem of stress analysis for spherically anisotropic media. It is shown that under some fairly generic conditions, the expression for the radial stress becomes a standard Euler type differential equation which yields an exact solution. Also of interest, particularly in the light of the recent articles on digital photoelasticity, is a review of Professor Ramesh's book on that subject. As usual, the issue is completed by an array of regular features including News, Developments and Innovations and the Diary of Events.