Structural Integrity Evaluation of Polymer Composites using Optical Strain Measurement (ARAMIS)

Transcription

1 Structural Integrity Evaluation of Polymer Composites using Optical Strain Measurement (ARAMIS) Z Y Zhang M O W Richardson Centre of Excellence - Manufacturing (RCMI) Department of Mechanical and Manufacturing Engineering Portsmouth University, Anglesea Building, Portsmouth, Hampshire PO1 3DJ, UK ABSTRACT This study deals with damage evaluation using an optical deformation and strain measurement system (ARAMIS). Damage was introduced using an Instrumented Falling Weight Impact Test (IFWIT) machine. The incident impact energy was controlled to low magnitude so that the specimen was not penetrated and only internal damage was introduced. This internal damage can have an adverse effect on structural integrity, and potentially lead to catastrophic failure. This paper presents the theoretical aspects involved with ARAMIS and its capability of evaluating the internal damage. Surface strain concentrations at different loading levels are used to indicate the location and geometrical characteristics of both external and internal damage. Key words: NDT, damage, composites, optical strain measurement. Introduction Fibre reinforced polymer (FRP) composite materials exhibit distinct properties and have found ever-increasing applications as engineering components and structures in the fields of aircraft, rockets, military weapon systems and nuclear energy industries (1-2). In these fields it is critically important to evaluate the integrity and reliability of the components and structures in a non-destructive way because routinely employed mechanical property testing is destructive in nature and not practically and economically viable in most circumstances. Although conventional NDT techniques have been widely practised and show varying degrees of success, they invariably suffer from inherent limitations and shortcomings (3-5). Implementation of reliable and effective methods to establish the integrity of FRP composite components and structures has been regarded as an important challenge to the NDT community. This requirement has led to the necessity for research and development of more reliable and effective non-destructive testing and evaluation techniques. ARAMIS is an optical technique to measure the deformation and strain of the surface of an object before and after loading. It has advantages of simple preparation of the specimen, large measuring area, non-contact and full field measurement, material independent determination, full field and graphical results, three-dimensional results and good mobility. Its capabilities include materials testing, stability estimating, component dimensioning, non-linear behaviour examination, creep and ageing process characterisation and structural integrity evaluation. This paper describes the use of ARAMIS for evaluating impact induced damage in glass fibre reinforced unsaturated polyester composites. Theory ARAMIS is the trade name of an optical deformation and strain measurement system. The fundamental principle is based upon the fact that the distribution of grey scale values of a 1

2 rectangular area (facet) in the un-deformed state corresponds to the distribution of grey scale values of the same area in the deformed state as illustrated in Figure 1. Figure 1: Intensity distribution in the non-deformed (1) and the deformed (2) state ARAMIS combines the advantages of photogrammetry and the object grating method. Photogrammetry is one of the optical methods that can produce 3D-coordinates of surface points. The displacement vectors, local strain values and contour difference can be computed from the data when the object is deformed. Arranging the object points on the surface of a specimen like a grating, is a technique well known in experimental mechanics. Instead of an expendable line mesh, a stochastic pattern is applied to the surface using a graphite spray that allows high local resolution. The technique possesses many unique features including simple preparation of the specimen, large measuring area, non-contact and full field measurement, material independent determination, full field and graphical results and three-dimensional results. Figure 2: Optical arrangement of ARAMIS The optical arrangement for photogrammetry consists of two CCD cameras and a loading device where the specimen is located as illustrated in Figure 2.If the position of the two cameras and two homologous image points P 1 (x 1, y 1) and P 2 (x 2, y 2) are known, the corresponding object point P (X, Y, Z) can be calculated. This procedure is known as space intersection. A geometric model is then established which implements the transformation 2

3 from image points to object points. The parameters of the camera are necessary to establish the model that is calculated through a calibration procedure. As far as this is concerned, a special calibration object is needed that has small circular targets with known diameters attached on the surface. During the calibration process, it is required that the calibration object is recognized by the two cameras from several views. Displacement determination The displacement determination involves calculation of the object coordinates for each deformation state for point P. It is important that the point P with the coordinates (X, Y, Z) in the non-deformed state can be found in each deformed state with its new coordinates (Xt, Yt, Zt). Only in this case canthe displacement and strain can be calculated for this object point. The calculation of homologous image points of two deformation states recognized from two cameras can be achieved by a combination of three 2D displacement calculation processes. Here it is important that one image is defined as the reference state. After determining all of the 2D displacement fields, the homologous image points can be easily calculated. With this information, the object coordinates for each state can be produced by space intersection. Strain determination After the determination of the displacement field, the surface strain distribution can be numerically calculated. This is done by transformation of the 3D displacement distribution into a 2D displacement distribution. The strain can then be calculated in 2D space. The strain calculation can be split into two steps. The first requires that the tangential plane be calculated for each of the object points Pu and Pv. For this purpose, the object points in the neighborhood are used, as illustrated in Figure 3. This rectangular area is referred to as the object facet. The second requires the object points in these object facets be projected onto the tangential plane. This must be done in the direction of the normal vector of the tangential plane. Thus the problem can be defined as 2-dimensional, as shown in Figure 4. The calculation of the deformation gradient can be implemented using algorithms. Figure 3: Determination of the tangential plane 3

4 Figure 4: Projection onto the tangential plane Experimentation The pultruded GRP panel was provided by Euro-Projects (LTTC) Ltd. Its lay-up was found to be symmetrical by deplying the specimen. It consisted of roving in the centre, sandwiched with random glass fibre mat, woven fibre fabric and surface veil symmetrically arranged on both sides. Its fibre volume fraction was approximately The panel was cut into rectangular specimens with overall dimensions of 150 x 40 x 4 mm. An instrumented low velocity falling weight impact machine was employed to introduce reproducible damage using a chisel shaped striker with impact energy levels predetermined and controlled by varying the striker heights in order to introduce nonpenetration damage in specimens (6-7). ARAMIS HR from Gesellschaft fur Opticsche Mestechnik (GOM) was used for the measurement and is shown schematically in Figure 5. Figure 5: Schematic of the ARAMIS optical strain imaging technique and hardware Results and Discussion Visual Inspection Visual inspection is one of the oldest evaluation techniques and it continues to be a useful NDT tool due to the fact that it is simple, speedy and cost effective compared to other more sophisticated approaches. The representative surface features of impacted coupons on both front and backside are shown in Figure 6. Visually there appears to be no indication of damage present on the front surface, implying no defects. By contrast, cracks are visible on back surfaces indicating the presence of damage. Thus the technique qualitatively implies 4

5 that some damage has occurred but without any confirmation or indication of degradation below the surface. Front surface Back surface Figure 6: Pictures of an impacted coupon showing both front and back surfaces Damage evaluation using ARAMIS ARAMIS is an optical technique for measuring the deformation and strain of the surface of an object before and after loading. Random surface speckles (produced by spraying black paint on a white background) function as optical tracers. Left and right cameras capture the speckles images prior to loading and after each loading step. The representative speckle images are shown in Figure 7. Grabbed image (left) Grabbed image (right) Figure 7: Speckle images from left and right cameras Displacement determination involves calculating the object coordinates for each deformation state. The global deformation is then visualised in X, Y, and Z directions. It can be seen that three-dimensional deformations have been introduced due to the loading. The magnitude of deformation is illustrated in Figure 8. However there is no direct indication of the presence of damage and geometrical features from these profiles. Irregularities in the deformation profile may show some signs, but generally it is impossible to visualise the damage. 5

6 X displacement Y displacement Z displacement Figure 8: Deformation profiles in the X, Y, and Z directions After determination of the displacement field, the strain distribution is then numerically calculated. The strain profiles at three loading levels are shown in Figure 9. It can be seen that there are pronounced strain concentrations. The magnitude of revealed damage increases with increasing loading levels complete with consistent locations and geometrical features. When the coupon is subjected to mechanical loading, a strain concentration is introduced because the damaged area is mechanically weaker and more vulnerable compared to surrounding areas. Also the surface above a flaw or crack is less constrained leaving it freer to move or strain. These mechanical differences are picked up by ARAMIS allowing visualization and quantification of otherwise barely visible damage. The findings from ARAMIS show good agreement and correlation with those from Electronic Speckle Pattern Interferometry (ESPI) and Electric Shearography (ES) and other conventional nondestructive evaluation techniques (8-9). Y strain (low loading) Y strain (medium loading) Y strain (high loading) Figure 9: Damage visualisation by strain profiles Another capability of ARAMIS is in sectional analysis. It can virtually implement singular and multiple sectioning at any location vertically, horizontally and diagonally. As can be seen in Figure 10, the coupon under study can be virtually and vertically sectioned through the middle. The strain values and profile in the X-direction and incremental changes can then be visualized. The abrupt changes in strain indicated the presence of damage. 6

7 Figure 10: Sectional analysis of strain distributions Conclusions Low velocity impact loading can potentially lead initially to barely visible damage in polymer composites and it is important to be able to visualise such damage. Visual inspection can only provide superficial information on internal damage. In many circumstances global deformation and strain can be more readily visualised by ARAMIS, which is able to flag up, not only surface damage location and geometrical features, but also provide a strain "signature" to locate of damage internally. With advantages of simple specimen preparation, non-contact and full field measurement, material independent determination, three-dimensional results and good mobility, such a technique is clearly a viable NDT tool able to evaluate the structural integrity of polymer composites in combination with other NDT techniques. REFERENCES 1. R E Shalin, Polymer Matrix Composites, Chapman & Hall, London (1995). 2. J.Murphy, The reinforced plastics handbook, Elsevier Science Publishers Ltd, UK (1995). 3. R D Adams, P Cawley, NDT International, 21 (4), (1988). 4. W J Cantwell, J Morton, Journal of Strain Analysis, 27 (1), (1992). 5. Z Y Zhang, PhD Thesis, Loughborough University, (1999). 6. M O W Richardson, M J Wisheart, Composites Part A 27A (1996). 7. M O W Richardson, M J Wisheart, Journal of Materials Science 35, (1999). 8. M O W Richardson, Z Y Zhang, Composites Part A: Applied Science and Manufacturing, 29, (1998). 9. M O W Richardson, Z Y Zhang, Insight, 40, (1998). 7