The delamination buckling and growth behaviors of a cross-ply composite laminate with damage induced by low velocity impact are investigated optically using three-dimensional digital image correlation (3D-DIC) method. For the 3D deformation measurement, the 3D-DIC setup comprised of two CCD cameras was adopted. The rectangle specimen was impacted under the impact energy of 7.0 J using a drop-weight testing machine, and the impact damage was detected by means of X-ray nondestructive evaluation (NDE) technique. The 3D deformation field measured with the optical system clearly reveals that the delamination buckling characteristic of the specimen mainly appears local deformation mode under compression after impact test. Moreover, the behavior of delamination growth evaluated by the 3D-DIC optical method reasonably agrees with the NDE observed damage result after compression.
Composite material is widely used in the structure components of aircraft and spacecraft because of its high specific strength and lightweight property. The increasing applications of composite material inevitably cause the risks from low velocity impact (LVI) damage by foreign object, such as a dropped tool or runway debris. The impact damage, mostly delamination, is especially dangerous for the composite structure because it is difficult to detect by visual inspection on the surface [
Numerous researchers focused their attentions on the CAI strength prediction and buckling analysis of composite laminates with embedded delaminations by the experimental method and numerical simulation [
The optical measurement techniques, which possess high precision, noncontact, and full-field advantages, have been widely used for measuring the displacement or strain fields of composite laminate. Shadow moiré was adopted to measure the out-of-plane displacements of composite laminates with artificially manufactured delaminations under in-plane compressive loads [
In this study, for evaluating the delamination buckling of damaged composite laminate, 3D measurements on the deformation process were experimentally investigated with the 3D-DIC method. The delamination buckling behavior of the impacted laminated specimen was analyzed by this optical method. For validating the performance of the 3D-DIC optical measurement method, the experimental results of 3D measurement were compared with the X-ray nondestructive evaluation (NDE) testing results after compression.
The specimen used in this study was made from HS160/REM graphite/epoxy prepregs. The dimensions of specimen with the stacking sequence
After the impact, impact damage (delaminations, matrix cracks, fibre fractures) was detected by means of NDE techniques. To obtain the highly detailed pictures of impact damage, penetrant-enhanced X-ray radiography was utilized to detect the impacted specimen after a radioopaque zinc iodide solution to infiltrate the damaged areas. This method can enhance the contrast of damage, but the internal damage not connected to the surface cannot be impregnated with the zinc solution and remains undetected. Therefore, the impacted specimen was ultrasonically inspected by C-scanning and analyzed with a specialized full-volume ultrasonic technique, which can reconstruct the internal damage of impacted sample and provide the information about the size and depth of selected delaminations. In order to eliminate the masking effect under scanning deeper damage, the specimen was examined from the two sides and the obtained information was recombined into a single image. The total delamination area measured by means of X-ray NDE technique was 432.1 mm2 [
In CAI test, an antibuckling device was designed according to the recommendations of SACMA SRM 2R-94 [
Compression testing device.
Geometric dimensions of the steel plate
Photograph of the fixture
The impacted laminate was mounted into the antibuckling device with 0° direction parallel to the loading direction. The pair of steel plates were clamped in the slot of the base plate and supported at both sides by slide blocks and side supports with the knife edges. The gap between the knife edges could be adjusted to different thickness specimens through sliding the slide blocks. A servohydraulic testing machine with a maximum load capacity of 100 kN was employed. The load was applied on the specimen through a loading assembly. Four back-to-back strain gauges were attached on a high strength steel bar connected with the compression header to monitor the magnitude of load during compression.
To measure the specimen deformation, the 3D-DIC experimental setup shown in Figure
Experimental setup of 3D-DIC.
To acquire the speckle pattern, the white paint was sprayed onto the specimen surface to generate random speckle-like field before testing. In addition, two white light sources were used to illuminate the composite specimen surface from two symmetrical directions to obtain high quality speckle images, as shown in Figure
Before capturing the specimen images, the camera parameters were calibrated using a predetermined chessboard calibration pattern, which has Print the calibration pattern on an A4 paper with a laser printer and attach it to a flat glass plate. Take a few pairs of images of the calibration pattern under different positions and orientations by moving or rotating the plane. In our experiment, twelve pairs of images were taken in total by both cameras during the calibration process. Detect the coordinates of angular points in the images by the edge detection algorithm. Estimate the intrinsic and extrinsic parameters using the flexible camera calibration technique, which is originally developed by Zhang [
After calibration, the pairs of images of specimen were simultaneously recorded under the states of different compressive loads. The out-of-plane displacement fields can be obtained with the 3D-DIC software developed in Visual C++ by ourselves.
Figure
A pair of images of specimen surface captured by the two cameras.
Left image
Right image
To show the buckling behavior of specimen, the out-of-plane displacement fields evaluated with the optical measuring system are shown in Figure
Measurement results of out-of-plane displacement under different load stages.
Until the compressive load level reaches about 8.2 kN, the height of the bulged surface in 3D plot increases steadily. At the load of 9.18 kN, there is an evident increase of deformation, that is, a local buckling at the damaged area. The local buckling is even larger at the load of 11.36 kN because the delamination already propagates at this area and causes a considerable reduction of the specimen stiffness. It can be seen from the contour maps in Figure
Figure
Locations of the considered lines.
Figure
Out-of-plane displacement distribution of line
To determine the boundary of local buckling and its extension, an enlarged view of the out-of-plane displacement distribution on line
Figure
Displacement distribution on different vertical lines: (a)
However, the growth of local deformation on both lines
Figure
Relationship between the maximum out-of-plane displacement and compressive loads.
To confirm the growth of delamination, the X-ray NDE technique was carried out to inspect the delamination damage. Figure
X-radiographs of specimen before (a) and after (b) compression.
The delamination grows along the positive
It can be seen in Figure
The 3D measurement during the deformation process of graphite fiber/epoxy composite laminate with low velocity impact damage under compression was conducted using the 3D-DIC optical full-field measurement technique. The deformation behavior including delamination buckling and growth was evaluated with the optical measuring system.
Quantitative deformation profiles under different load stages were experimentally obtained. The specimen mainly presents the local deformation mode under CAI test with the designed testing device. Moreover, the local deformation only grows towards the positive
The experimental results confirm the effectiveness of the optical measuring method in gaining a better understanding of the deformation characteristics of the specimen. Therefore, the 3D-DIC method is a powerful and useful tool for quantitatively evaluating the delamination buckling behavior of the composite laminate during compression after impact.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are grateful to the National Natural Science Foundation of China (Grant number 11102134), the Colleges and Universities Science and Technology Development Foundation of Tianjin (Grant number 20130903), and the European Union Foundation (Grant number FP7-PEOPLE-ITN 238325) for financial support.