Fracture Toughness Of Carbon Nanotubes/Aluminum Matrix Composites

Carbon nanotube/aluminum matrix composites?CNT/Al?exhibited high strength,modulus and plasticity,which showed a great potential in lightweight structural applications.However,as an important mechanical property for materials applications,fracture toughness was still lack of relevant research.Besides,there were many methods to test the fracture toughness,and the applicability for CNT/Al composites wasn't clarified.In this paper,2024 aluminium alloy was used to study the applicability of different fracture toughness testing methods.Then CNT/Al-Cu-Mg composite was prepared by flake powder metallurgy route via element alloying and tested the fracture toughness through suitable testing methods,to study the influence of CNT on fracture toughness of composite.The fracture surface and crack morphology were observed to explore the toughening mechanism of CNT for CNT/Al-Cu-Mg composites.The main results are as follows:1)The comparison results of various fracture toughness testing methods showed that compared with the standard fracture toughness K_ICC test value of 43.37 MPa·m^1/2,the value of circumferential notched specimens was too low as 33.42 MPa·m^1/2,and the boundary effect model method had obtained very high value reaching 154.94MPa·m^1/2.It can be seen that the two methods were not suitable for the fracture toughness testing of 2024 aluminum alloy.Besides,both the fatigue crack and the size of the sample had effects on the test results of fracture toughness.Only when fatigue cracks existed and the size was large enough,can the reasonable fracture toughness be obtained.Therefore,further studies were conducted in accordance with the standard K_ICC testing methods.2)The fracture toughness test results showed that the value of 1.5wt.%CNT/Al-Cu-Mg composites was 24.90 MPa·m^1/2,which was higher than most traditional particle reinforced aluminum matrix composites,but lower than that of 2024 aluminum alloy.While the results showed that the value decreased with the increase of CNT content.3)The observation of fracture surface and crack morphology showed that CNT does not reduce the fracture toughness by forming microcracks,nor does it improve the fracture toughness via extrinsic toughening mechanisms such as bridging and pulling-out.Instead,CNT combined with matrix tightly during crack propagation and deformed synergistically,leads to the increase of stress concentration in micro regions that facilitate microcracks formation,results in the decrease of the fracture toughness.In summary,the applicability of different fracture toughness testing methods was compared and analyzed.The influence of nanoscale CNT on fracture behavior of Al matrix composites was studied,which provided experimental reference for fracture toughness testing and fracture behavior research of new nanoscale reinforced metal matrix composites.