Available at: http://digitalcommons.calpoly.edu/theses/1327
Date of Award
MS in Aerospace Engineering
Dr. Faysal Kolkailah
The use of composite sandwich structures is rapidly increasing in the aerospace industry because of their increased strength-to-weight and stiffness-to-weight characteristics. The effects of low velocity impacts on these structures, however, are the main weakness that hinders further use of them in the industry because the damages from these loadings can often be catastrophic. Impact behavior of composite materials in general is a crucial consideration for a designer but can be difficult to describe theoretically. Because of this, experimental analysis is typically used to attempt to describe the behavior of composite sandwiches under impact loads. Experimental testing can still be unpredictable, however, because low velocity impacts can cause undetectable damage within the composites that weaken their structural integrity. This is an important issue with composite sandwich structures because interlaminar damage within the composite facesheets is typical with composites but the addition of a core material results in added failure modes. Because the core is typically a weaker material than the surrounding facesheet material, the core is easily damaged by the impact loads. The adhesion between the composite facesheets and the core material can also be a major region of concern for sandwich structures. Delamination of the facesheet from the core is a major issue when these structures are subjected to impact loads.
This study investigated, through experimental and numerical analysis, how varying the core and facesheet material combination affected the flexural strength of a composite sandwich subjected to low velocity impact. Carbon, hemp, aramid, and glass fiber materials as facesheets combined with honeycomb and foam as core materials were considered. Three layers of the same composite material were laid on the top and bottom of the core material to form each sandwich structure. This resulted in eight different sandwich designs. The carbon fiber/honeycomb sandwiches were then combined with the aramid fiber facesheets, keeping the same three layer facesheet design, to form two hybrid sandwich designs. This was done to attempt to improve the impact resistance and post-impact strength characteristics of the carbon fiber sandwiches. The two and one layer aramid fiber laminates on these hybrid sandwiches were always laid up on the outside of the structure. The sandwiches were cured using a composite press set to the recommended curing cycle for the composite facesheet material. The hybrid sandwiches were cured twice for the two different facesheet materials. The cured specimens were then cut into 3 inch by 10 inch sandwiches and 2/3 of them were subjected to an impact from a 7.56 lbf crosshead which was dropped from a height of 38.15 inches above the bottom of the specimen using a Dynatup 8250 drop weight machine.
The impacted specimen and the control specimen (1/3 of the specimens not subjected to an impact) were loaded in a four-point bend test per ASTM D7250 to determine the non-impacted and post-impact flexural strengths of these structures. Each sandwich was tested under two four-point bend loading conditions which resulted in two different extension values at the same 100 lbf loading value. The span between the two supports on the bottom of the sandwich was always 8 inches but the span between the two loading pins on the top of the sandwich changed between the two loading conditions. The 2/3 of the sandwiches that were tested after being impacted were subjected to bending loads in two different ways. Half of the specimens were subjected to four-point bending loads with the impact damage on the top facesheet (compressive surface) in between the loading pins; the other half were subjected to bending loads with the damage on the bottom facesheet (tensile surface).
Theoretical failure mode analysis was done for each sandwich to understand the comparisons between predicted and experimental failures. A numerical investigation was, also, completed using Abaqus to verify the results of the experimental tests. Non-impacted and impacted four-point bending models were constructed and mid-span deflection values were collected for comparison with the experimental testing results. Experimental and numerical results showed that carbon fiber sandwiches were the best sandwich design for overall composite sandwich bending strength; however, post-impact strengths could greatly improve. The hybrid sandwich designs improved post-impact behavior but more than three facesheet layers are necessary for significant improvement. Hemp facesheet sandwiches showed the best post-impact bending characteristics of any sandwich despite having the largest impact damage sizes. Glass and aramid fiber facesheet sandwiches resisted impact the best but this resulted in premature delamination failures that limited the potential of these structures. Honeycomb core materials outperformed foam in terms of ultimate bending loads but post-impact strengths were better for foam cores. Decent agreement between numerical and experimental results was found but poor material quality and high error in material properties testing results brought about larger disagreements for some sandwich designs.