Andrew D. Nurse

Full-field automated photoelasticity by use of a three-wavelength approach to phase stepping

An overdeterministic least-squares phase-stepping method for automated photoelasticity is described. Problems associated with isochromatic-isoclinic interaction are solved by use of a three-wavelength method to calculate the value of the isochromatic parameter and the isoclinic angle. The ramped isoclinic phase map can now be unwrapped to give the orientation of the principal stresses with respect to a reference axis of the polariscope unambiguously. A three-wavelength approach to determination of the absolute value of the isochromatic parameter is shown to give reliable results also.

Completely automated determination of two-dimensional photoelastic parameters using load stepping

The new approach to phase-stepping photoelasticity known as ‘‘load stepping’’ is used to determine automatically the isochromatic parameter a and the isoclinic angle ?. There is no need for the user to calibrate the results other than to convert the isochromatic parameter into a principal stress difference using the material fringe constant. Four phase-stepped images are collected using a circular polariscope for each of three load steps, which differ by small equal increments. A ramped phase map for the isochromatic parameter is produced in the range -?<??? that can be unwrapped using conventional techniques. Calibrated values of a can then be calculated using the phase map of ??, which is the incremental change in the isochromatic value between the different steps. The value of the isoclinic angle is automatically determined in the correct range -?/2<?????/2 without unwrapping. The results obtained from experimental testing of a disk-in-compression specimen using transmission photoelasticity presented in comparison with theoretical solutions demonstrate the accuracy of the new approach. In another test performed on a bar in torsion using reflection photoelasticity we demonstrate how determination of the isoclinic angle in the range -?/2<???/2 gives results superior to those obtained if the isoclinic angle is determined in the wrapped range -?/4<???/4 .

Characterization of a single delamination using geometric moments and genetic algorithms

The application of genetic algorithms for quantitative characterization of a single delamination in composite laminated panels is discussed. The damage identification procedure is formulated as an input-output inverse problem through which system parameters are identified. The input of the inverse problem, the normalized central moments (NCM), is calculated from the surface displacements of a finite element (FE) representation of a delaminated panel in a vacuum chamber. The output parameters, the planar location, diameter, and depth of the flaw, are the solution to the inverse problem to characterize the idealized circular debonding crack. The inverse problem is solved as an optimization procedure. The objective function of the optimization algorithm is defined as the squared difference of the NCM obtained from a FE model with an actual delamination and from a FE model with a trial debonding. The optimum crack parameters are found by minimizing the objective function through the use of a novel implementation of real-coded adaptive range genetic algorithms. Numerical examples are presented to show the effectiveness of the proposed identification procedure.

Experimental stress intensity factors for cracks in a thin plate with stiffeners

Abstract The problem of cracked plates stiffened by three-dimensional stringers is investigated using transmission photoelasticity. Models were produced of the hole-in-the-plate geometry stiffened by a combination of stringers transverse and/or parallel to the applied tensile load. Cracks of different lengths emanating from one edge of the hole and approaching a stringer were examined. These cases represent geometry and loading conditions for which it would normally be very difficult to obtain results using analytical methods. The stringer-stiffened plates show a consistent reduction in the non-dimensional stress intensity factor of about 20 per cent irrespective of the arrangement of stringers.

Detection and sizing of delamination cracks in composite panels using speckle interferometry and genetic algorithms

A real-time system for analysing data from speckle interferometers, and speckle shearing interferometers, has been applied to the problem of detecting and sizing sub-surface delamination cracks in carbon fiber composite panels subjected to vacuum loading. Interferograms are continuously recorded by a CCD camera at a rate of 60 frames s-1 with temporal phase shifting carried out at the same rate. Wrapped phase maps are calculated and displayed at 15 frames s-1. These are unwrapped using a temporal phase unwrapping algorithm to provide a real-time display of the relevant displacement component. The damage identification procedure is formulated as an input output inverse problem through which system parameters are identified. The input of the inverse problem, the geometric moments (GM), is calculated from the surface displacement fields. The objective function of the optimization algorithm is defined as the squared difference between the GM obtained from the sample under test and that from a finite element (FE) model with a trial debonding. The optimum crack parameters are found by minimizing the objective function through the use of a novel implementation of real-coded genetic algorithms (ARGAs). The performance of the measurement procedure and analysis algorithms is evaluated using a set of carbon-fiber epoxy composite samples with systematically-varying and well-characterized defect geometries.

Photoelastic determination of fatigue crack stress intensity factors

A program of research was undertaken to develop a method to find stress intensity factors of fatigue cracks propagating in metal structures using the method of photoelastic coatings. The method is described in this paper along with test results to demonstrate how it can be used in the analysis of fatigue crack growth.