Available at: http://digitalcommons.calpoly.edu/theses/842
Date of Award
MS in Aerospace Engineering
Impact strength is one of the most important structural properties for a designer to consider, but is often the most difficult to quantify or measure. A constant concern in the field of composites is the effect of foreign object impact damage because it is often undetectable by visual inspection. An impact can create interlaminar damage that often results in severe reductions in strength and instability of the structure. The main objective of this study is to determine the effectiveness of a damage arrestment device (DAD) on the mechanical behavior of composite sandwiches, following a low-velocity impact. A 7.56-lbf crosshead dropped from a height of 37.5-inches was considered for the low-velocity impact testing. In this study, the experimental and numerical analysis of composite sandwiches were investigated, which included static 4-point bend and vibration testing. Composite sandwiches were constructed utilizing four-plies of Advanced Composites Group LTM45EL/CF1803 bi-directional woven carbon fiber face sheets with a General Plastics Last-A-Foam FR-6710 rigid polyurethane core. Specimens were cured in an autoclave, using the manufacturer’s specified curing cycle.
In addition to the experimental and numerical analysis of composite sandwiches, developing and building a data acquisition (DAQ) system for the Dynatup 8250 drop weight impact tester was accomplished. Utilizing National Instruments signal conditioning hardware, in conjunction with LabView and MATLAB, complete testing software was developed and built to provide full data acquisition for an impact test. The testing hardware and software provide complete force vs. time history and crosshead acceleration of the impact event, as well as provide instantaneous impact velocity of the projectile. The testing hardware, software, and procedures were developed and built in the Aerospace Structures/Composites laboratory at Cal Poly for approximately 15% of the cost from the manufacturer.
In the first study, static 4-point bend testing was investigated to determine the residual flexural strength of composite sandwich beams following a low-velocity impact. Four different specimen cases were investigated in the 4-point bend test, with and without being impacted: first a control beam with no delamination or DAD, second a control beam with a centrally located 1-inch long initial delamination, third a DAD key beam with two transverse DADs centrally located 1-inch apart, and finally a DAD key beam with a centrally located initial delamination between two transverse DADs. The specimens used followed the ASTM D6272 standard test method. The specimens were 1-inch wide by 11-inch long beams. The experimental results showed that the presence of DAD keys significantly improved both the residual stiffness and ultimate strength of a composite sandwich structure that had been damaged under low-velocity impact loading, even with the presence of an initial face-core delamination.
In the second study, vibration testing was investigated as a means to detect a delamination in the structure and the effect of impact damage on the vibrational characteristics, such as damping, on composite sandwich plates. Four different specimen cases were investigated in the vibration test, both with and without being impacted: first a control plate with no delamination or DAD, second three control plates with varying 1-inch initial delamination locations at the 1st, 2nd, and 3rd bending-mode nodes, third a DAD key plate with one DAD running the entire length longitudinally along the center of the plate, and finally three DAD key plates with one DAD running the entire length longitudinally along the center of the plate and varying 1-inch delamination locations at the 1st, 2nd, and 3rd bending mode-nodes. The response accelerometer location was varied at 1-inch increments along the length of the plate. From the experimental results, it was determined that varying the location of the accelerometer had a significant effect on the detection of face-core delamination in a composite sandwich structure. Additionally, it was shown that damping characteristics significantly degraded in control case plates after a low-velocity impact, but they were better retained when a DAD key was added to the structure.
Numerical analysis utilizing the finite element method (FEM) was employed to validate experimental testing, as well as provide a means to examine the stress distribution and impact absorption of the structure. The impact event was modeled utilizing the LS-Dyna explicit FE solver, which generated complete force vs. time history of the impact event. Static 4-point bending and vibration analysis were solved utilizing the LS-Dyna implicit solver. Finally a damaged mesh was obtained from the explicit impact solution and subjected to subsequent static 4-point bending and vibration analysis to numerically determine the residual mechanical behavior after impact. All cases showed good agreement between the numerical, analytical, and experimental results.