ALTERNATING CURRENT STRESS MEASUREMENT ACSM A new product from TSC for measuring bolt tightness The ACSM technique has been developed for measurements of stress in threads, stress in tubular joints, residual stress in welded components and residual stress in forgings. In particular special purpose probes have been designed for the threads used in typical underwater clamp repairs. Two approaches have been developed so far for threads which either measure stress directly in a loaded threaded or alternatively determine the stress by comparison between the loaded section and the free end. Both approaches have been successfully tried in laboratory tests on 34mm and 40mm high strength bolts and the ACSM system has now been used for subsea. TSC are now in a position to offer the ACSM equipment, or conduct service work, on subsea clamps using ACSM where bolt loads can be measured and reported. The cost and time of these measurements are similar to ACFM U21 work but will need the ACSM system and a TSC operator. Stress probes are available for diver use as well as ROV deployment. Stress measurement of simple bodies such as tubular weld joints is also possible. The ACSM system received TYPE APPROVAL from BUREAU VERITAS in July 1998.     Stress / Load Sensor in air or subsea Stress sensor for tubular joints Load sensor for clamp bolts Surface stress for pressure vessels  The measurement of mechanical stress in a body is important in general terms as it is an indicator of residual strength or possible fatigue damage. In many cases the original stress in a component may change during service. It is important to be able to measure the stress periodically during the service life. In some cases, as in repairs, it is also an indicator of whether the repair is effective. This is particularly true of bolted/clamped repairs where bolt tightness is required at all times. In other situations, where clamps are used for restraint or support, bolt tightness is also important. A non-destructive measure of the load in a component, structure or bolt would be very advantageous. Mechanical stress can influence the magnetic domain distribution in ferrous metals. This feature, known as piezo-magnetism can be utilised to determine changes in the stress state on the surface of metal. Magnetic permeability changes of this sort can be measured using a.c. field measurement devices. These devices have improved recently to the point where very small changes in permeability and hence stresses are detectable. Thus changes in stress of the order of a few percent of the zero to yield stress range have been measured in the laboratory for structural steels. The very latest a.c. field measurement devices can now be used for non-contacting induced field measurements and this has provided the opportunity of developing an Alternating Current Stress Measurement (ACSM) sensor for use in the service. In order to exploit this stress measurement capability a new non-contacting probe system has been designed and tested. This system uses multiple induced fields and sensor coils together with materials data and software interpretations based on the theoretical analysis of anisotropic permeability. It also has modified ACFM electronics to give a completely new purpose made stress measurement device. The work was partly conducted using a DTI SMART award and was completed in August 1996.   Applications Products