INCREASING THE LIMIT OF USABILITY OF CFRP TUBES BY BUILT-IN STRESSES

The limit of usability of a CFRP structure is often determined by first ply failure (FPF) indicated by interfibre/matrix cracking. It will be shown how for certain types of CFRP structures this crack initiation threshold can be delayed. The process used here is prestraining in analogy to that Of prestressed concrete. It is a so-called thermo-mechanical after treatment, inducing advantageous compression built-in stresses, i.e. negative strains transverse to the fibres. Cylindrical tube specimens were wound, prestrained and submitted to hydraulic pressurisation or spin load testing. Acoustic emission monitoring and crack counting following sectioning confirmed an increase in crack threshold level for this specimen type from epsilon(hoop) almost-equal-to 0.8% to 1.3%, which is higher than the operational strain, normally defined as tensile failure strain e(parallel-to) of the fibre divided by the usual design factor j(u) = 1.5. Theoretical investigations were done in support of the design of the prestraining device and of the tube specimen, as well as an evaluation of experimental data.