For composite design, it is desirable to have data covering the failure envelope from Mode I to Mode II. The existing standard procedures for quasi-static testing (ISO 15024 for Mode I and ISO 15114 for Mode II) have recently been shown to be adaptable for the respective fatigue tests under displacement control in round robin tests performed by Technical Committee 4 of the European Structural Integrity Society. The Calibrated End-Loaded Split (C-ELS) test set-up developed for Mode II as described in the ISO standard further allows to perform a Fixed-Ratio Mixed Mode I/II (FRMM) test by simply inverting the loading direction compared to mode II. This test is now explored with quasi-static and fatigue loading for selected carbon fibre reinforced polymer-matrix (CFRP) composite laminates. Recent fatigue delamination analysis showed that a modified Hartman-Schijve approach can be used to achieve power law representations of fatigue crack growth in composites with exponents of around 2. This is significantly lower than the exponent values seen for composites with the common Paris law data representation that can amount to values in the range between about 3 and 10. Thus, the modified Hartman-Schijve approach may yield data for a damage tolerant design philosophy requiring the assessment of delamination growth in composite structures. Based on these findings, the applicability of this data representation approach to various CFRP composite laminates is investigated. Cyclic FRMM I/II tests were carried out and analysed with both, the common Paris law and the modified Hartman-Schijve approach.