Impact-fatigue behavior of composite tube/metal end fitting bonded joints

The relatively high specific strengths and moduli of advanced composite materials make them an attractive option for use in high speed industrial equipment. Hence, a potential market exists for integrating composites into existing machinery to replace critical metallic components. An application of such is the replacement of a high speed reciprocating steel bodymaker ram with one fabricated from a filament wound carbon fiber tube with bonded steel end fittings. Due to the dynamic forces which are inherent in the operation of high speed machinery, components are often subjected to impact, fatigue, and combined high cycle impact-fatigue. The behavior of composites as well as adhesives subjected to either impact or fatigue is well established. However, the combined impact-fatigue behavior of adhesive joints between filament wound carbon-fiber reinforced epoxy tubes and steel end fittings has not been investigated and is the focus of this study.

An impact-fatigue testing machine was designed and fabricated specifically for these tests. The impact pulses generated by the machine closely resemble those of a conventional drop weight impact test machine. In addition, the impact-fatigue machine is capable of completing high cycle impact-fatigue tests (10⁶ impacts) within a relatively short period of time by operating in excess of 10 impacts per second. Tests were performed at several impact load levels ranging from 15% to as much as 40% of the joint $\text{U}$ltimate $\text{S}$tatic $\text{C}$ompressive $\text{F}$ailure -$\text{L}$oad. These impact load levels were monitored throughout the specimen lifetime. Furthermore, three bondline thicknesses were investigated to attain an initial indication of the sensitivity to bondline thickness variation.

Results indicate that the specimens exhibit an initial plateau region for a number of cycles during which time no decrease in load carrying capacity is measured. After a critical number of impacts, damage becomes apparent as the sample is no longer capable of maintaining the initial load. At this point, these constant-displacement tests show a load drop and a corresponding compliance change is noted as the sample begins to show less resistance to impact. Post-impact compression test results also show this drop in strength and modulus. Further, the percentage of the $\text{U}$ltimate $\text{S}$tatic -$\text{C}$ompressive -$\text{F}$ailure Load being applied dynamically determines the length of the plateau as well as the rate of degradation. Thus, the determination of the performance of composite tube/metal end fitting bonded joints using this impact-fatigue test approach gives information critical for lifetime design of dynamically loaded composite components.