۱٫ Introduction

۲٫ Materials and equipment

۳٫ Preliminary numerical analysis

۴٫ Preliminary quasi-static tests

۵٫ Improved quasi-static tests

۶٫ Fatigue tests

۷٫ Conclusions and Perspectives

Acknowledgements

References

Mechanical response of CFRP unidirectional prepreg was investigated under tensile quasi-static and fatigue loading. Firstly, the findings of an initial investigation on the effects of the end tabs and gripping condition are presented. A dialogue between numerical simulations and experimental tests led to avoid premature splitting failures in the clamps. Proper tensile failure is assessed in the gage length, at enhanced strength levels for the material considered. Consequently, the unidirectional material was successfully tested under tension-tension fatigue, at high load levels. Fatigue damage mechanisms arise at high stress amplitude, revealing that also a UD laminate can suffer from fatigue degradation along the fibres direction.

Introduction

In a multidirectional carbon fibres reinforced plastic (CFRP) composite laminate, the longitudinal plies govern the fatigue life performance of the laminate in most cases. These plies have long continuous fibres aligned with the load direction, thus the fibres give almost the entire contribution of the stiffness and strength of the ply. In order to predict the fatigue life of a multidirectional composite laminate it is therefore necessary to have a good estimation of the tensile strength and the fatigue behavior of the unidirectional (UD) plies, under uni-axial tension load along the fibres direction (0°). The identification of these basic mechanical properties requires extremely difficult tests: because of the high anisotropy of the material, splitting (shear failure of the matrix) parallel to the fibres takes place prior to tensile rupture. This damage mode initiates at the end of the clamped region of the specimen, leading to an abrupt brittle explosive failure outside the gage length, as showed in [1,2]. Hence, the validity of the test is severely affected and the effective tensile strength could be underestimated. The ASTM D3039/D3039M and ISO 527-1 standards test method for quasi-static tensile properties of CFRP composites suggest the use of tabs to provide a soft interface. The material can be aluminium, or a E-glass fibre reinforced plastic composite, in a [0/90]ns laminate configuration. The composite tab is commonly applied at 45° to the loading direction. Both standards recommend a tab length of at least 50 mm. The adoption of a tapered geometry for the tabs, with low bevel angle (7° to 10°) is proposed but not compulsory. The grips should overhang the end of the tabs by about 10 mm. No quantitative indications are given concerning the clamping pressure. Finally, the specimen must be rectangular shaped, because the dog-bone geometry induces spurious premature failure characterized by splitting along the fibres direction. These standards give only recommendations, and no definitive solution is stated to avoid the premature failure in the clamping region.