Amit Puri

Flexural Testing of Composite Sandwich Structures with Digital Speckle Photogrammetry

Composite sandwich structures are finding increasingly widespread use in fields ranging from aerospace and wind turbines to sports applications such as skis and surfboards. The high specific stiffness that composite sandwich structures can provide lends them well to these applications. However, the operational environment of these structures is frequently aggressive and often results in damage during service. The extent and effect of damage incurred is an important factor in the design and maintenance of composite sandwich structures. Failure of an individual component can be catastrophic for the rest of the structure. The purpose of this investigation was, firstly, to ascertain whether DSP was a viable technique for determining strain fields within composite sandwich structures. Secondly, to determine whether four point flexure would give rise to pure flexure between the central rollers, and if not, to understand what load conditions were present. This investigation was also carried out with a view to extend the investigation into the effect of defects on composite sandwich structures manufactured by RIFT. The grounds for selection of composite sandwich structures normally lie in their flexural performance. Reliable and accurate quantitative testing methods for evaluating the flexural performance of sandwich panels are needed if composite sandwich structures are to be used safely and effectively. In addition, methods to determine the effect of damage and defects on flexural behaviour of sandwich structures is particularly important for designing the repair and maintenance regimes of composite sandwich components.

Optical Strain Measurement Techniques to Assist in Life Monitoring of Power Plant Components

Sensors for monitoring creep strain in high-pressure steam pipes and other power plant components are subjected to very demanding environmental and operational conditions. It is important that the sensors are of a rugged design and that measurement can be made that only relates to creep movements in power plant components. The E.ON UK auto-reference creep management and control (ARCMAC) optical strain gauges have been designed to have this capability. These optical strain gauges are installed across sections of welded steam pipe and other plant components in locations that provide the best monitoring points to reveal the early onset of failure processes. Reported in this paper are recent developments to improve optical creep strain measurement to achieve a 65 microstrain accuracy level with an error of less than 10%. Also reported are trials of combining optical strain gauges with digital image correlation (DIC) to obtain detailed information of the creep strain distribution around the gauges. The DIC data for known defect geometries have been validated with finite element analysis.

Non-Destructive Analysis of Wind Turbine Blade Structural Integrity

The hollow profile of a wind turbine blade is subject to high levels of deformation when under the crushing pressure that is induced by flapwise loading. The load bearing box girder will ovalize under the pressure like forces and the shear sandwich webs and monolithic composite flanges are critical components whose failure may cause overall blade failure. This paper addresses the underlying causes of failure for these two parts by performing individual representative mechanical testing on them, and using digital image correlation (DIC) and acoustic emissions (AE) monitoring. Results show that the strength of the flange is primarily determined by the tensile strength of a relatively thin biaxial layer in this complex design. The buckling tests performed on sandwich material show that DIC is useful in determining the causes of failure, whilst AE monitoring has potential for in-situ monitoring of these structures, and is able to provide important data about the damage inside a blade.© 2009 ASME

ARCMAC Optical Strain Measurement for Creep Monitoring: Recent Improvements in Accuracy and Resolution

Remaining life of power station high pressure steam pipes is heavily dependant upon material creep rates. However, due to the difficulty in monitoring strain in these pipes as a result of the demanding operational conditions, a rugged optical strain gauge system has been developed. The current E.ON UK ARCMAC gauge system has been validated using the UK National Physical Laboratory standard grade extensometer and provides a strain measurement accuracy of 64 micro-strain with an error of <10%. This system uses precision optics, a CCD camera and a light source system to capture images of uniaxial and biaxial optical strain gauges on steam pipes during periodic maintenance. Further developments of the ARCMAC system have included the design, manufacture and validation of an advanced ARCMAC optical measurement system with improved sensor resolution and improved accuracy. Additionally, the methodology of image processing has been studied in order to reduce errors in both the existing and the new ARCMAC systems. Finally, Digital Image Correlation (DIC) has been used alongside ARCMAC gauges to monitor strain fields around welds and defects in steam pipes. Some of these techniques have also been used in a related study into strain monitoring in wind turbine blades.© 2009 ASME

Non-Destructive Evaluation of Damage Mechanisms in Composite Sandwich Structure

This paper presents the experimental results obtained of flexurally loaded wind turbine blade cross section material. All material was extracted from a wind turbine blade box girder and testing was conducted in four point configuration. The aim was to gain an understanding of the structural integrity of this lightweight material as it deforms in flexure. To allow for thorough analysis, digital image correlation (DIC) was used to produce full field strain maps of the deforming specimens. Results highlight the capability of the DIC technique to identify regions of failure, as well as the aspects responsible for them. Overall, the results present a foundation for tests on larger substructure, and eventually integration into manufacturing and maintenance aspects of the industry.

Developments in Combined ARCMAC and Strain Mapping Systems for Creep Measurement

The E.ON UK ARCMAC gauge has been developed to provide for point-to-point biaxial creep strain measurements. Research is continuing to provide for strain field mapping about the ARCMAC point-to-point monitoring system. The aim is to have comprehensive monitoring of creep strain in high temperature steam pipes including bends, joints, welds and other pipe structural features. A requirement is that all creep strain measurements made satisfy the standards of the UK National Physical Laboratory that maintain liaison with the standards authorities in other countries. Creep strain and other monitoring equipment for power-station steam pipes need to be rugged and suitable for the many different locations in which they are to be installed. Appropriate equipment is also required for capturing data at different times from the installed gauges. The use of strain mapping is particularly required when monitoring non uniform sections of pipes. Results to be presented will show the current state-of-art measurement techniques now available.Copyright

Recent Developments in Methods to Study Creep Strain Variations in Power Station Steam Plant

In power station steam plant, experience has shown that portable monitoring equipment is ideal for reliable point-to point measurement of creep strain and also acquisition of strain mapping information. On going laboratory tests have shown positive initial results in that direction; the results being validated against metrology standards. In addition to the monitoring of in-service strain in steam pipes, the study of the effects of strain distribution across welded pipe sections and other known early failure locations on steam pipes and other plant components is growing in interest. An existing strain monitoring method employs the E.ON UK ARCMAC gauge system with a capability of both uniaxial and biaxial strain measurements. For strain mapping, digital image correlation (DIC) is also incorporated and integrated with the ARCMAC system. In an effort to further improve the ARMAC system portability, the use of a digital single-lens reflex (DSLR) camera system is investigated. The scope of a combined ARCMAC/DIC system to provide for both point-to-point and strain mapping of piping features is studied. Identification of a feature using a combination of ARCMAC and DIC techniques is presented here. Additionally, the investigation of an edge-finding technique which can complement the existing ARCMAC system is presented.Copyright

Predicting lifetimes of components in power station engineering plant

Abstract Today, there is an increasing demand for power station plant to be operated cost effectively and to reliably maintain required electrical supply. This requires minimising the risk of having to shut down the plant for emergency repairs. Of paramount importance is plant operating integrity and well prepared and executed maintenance programmes. The present paper reports on the recent developments to the auto-reference creep management and control system used by E.ON UK. This includes achieving biaxial strain measurement with increased resolution and the employment of digital image correlation.

Monitoring Creep Strain in Power Station Engineering Plant

Cost effective and reliable operation of a power station plant and achieving low carbon dioxide emissions can be very dependent on maintenance activities. Planning maintenance to minimise the down-time of the plant is a key cost factor. It is important to have the latest data as to the age and state of the components and parts at the time of plant shut down. Indeed, deciding on the best time to shut down the plant for maintenance can depend upon trends in these data. For steam pipes, the required ageing information is the creep strain rate. These creep strain data obtained at shut down provide key information as to the needed replacement of pipes to maintain reliable plant operation. This paper presents the E.ON UK ARCMAC creep strain measurement systems that are being further developed by Imperial College London with the use of DIC and other techniques.

Resolution of Creep Strain Measurements Using the ARCMAC Strain Monitoring System

Essential is that creep strain monitoring sensors need to be of a rugged design well able to endure the demanding conditions related to the environment of high-pressure steam pipes and other power plant components. A useful monitoring method employs the E.ON UK ARCMAC gauge system with a capability of both uniaxial and biaxial strain measurements. The choice of monitoring location, for example, can be to welded pipe sections and other known early failure locations on steam pipes and other plant components. The ARCMAC system has been refined over the years to improve the accuracy of measurement. In this paper, recent developments have allowed the strain measurement accuracy to be improved to 64 micro-strain with an error of <10%. Also, recently, high temperature trials of the combined ARCMAC/DIC have commenced and early findings are reported.Copyright