## 1 Introduction  
Currently, one of the main requirements for characterizing the response of materials to service loadings is the understanding of how and where damage occurs, while predicting the mechanical response of the material in different configurations. In real applications, materials are subjected to multiaxial loads. The multiaxiality of more conventional isotropic materials has been extensively studied and many analytical models have been proposed [1; 2; 3]. However, more advanced materials, such as polymer composite materials, have been gaining more and more interest over the last number of years, but their mechanical behavior has been mainly investigated under uniaxial stresses, mostly due to the challenges occurring when multiaxiality is taken into consideration [4; 5; 6]. Nevertheless, the anisotropy that characterizes these materials induces a significant complexity, and investigations regarding only uniaxial loading should not be considered as adequate. It is generally accepted that different multiaxial stress states and combinations of normal and shear stresses result in complex mechanical behaviors and, more significantly, in dissimilar damage sequences, which affect both the short-term and long-term response of the material. Damage in composites occurs through a gradual accumulation of matrix cracking, fiber-matrix interfacial debonding, delamination, and fiber failure [4]. However, the influence of multiaxiality on the damage development in composites is still not clear, and quantitative damage observations especially under multiaxial stresses are missing in the literature. Nevertheless, in order to be able to develop suitable damage criteria for composite materials, it is of paramount importance to study how the different multiaxiality affects their progressive damage both in a qualitative and quantitative way. There have been different approaches reported in literature for the investigation of multiaxial stresses in composite materials [7]. As [PERSON] et al. have already proposed [8], there are two different ways to introduce multiaxiality in composites--internally and externally. In flat laminates with different stacking sequences, internal multiaxiality can be developed even under simple external loading, taking advantage of the intrinsic anisotropy of the composite materials. External multiaxiality can be applied, for example, in tubular specimens under combined tension-torsion or in cruciform geometries loaded in two directions [9; 10; 11; 12; 13]. However, as far as the local developed stresses on the lamina level are the same, a similar mechanical behavior should be expected in all geometries [8; 14]. The potential for using simpler and more easily manufactured flat laminates, together with well-established uniaxial testing devices, to assess multiaxiality is explored in this work, with a few studies being already reported in literature, both for static [15; 16] and fatigue [17; 18] loading conditions. Two different carbon fiber reinforced epoxy (CFRE) angle-ply laminates were, therefore, selected to be tested under tensile quasi-static conditions. The selection of the lay-up was based on the developed multiaxial stresses in the off-axis layers. Special attention was given to the combination of normal and shear stresses in the off-axis layers, defined by bi-axiality ratios, \(\lambda\)[8]. Moreover, the laminates were selected in such a manner that \"clean\" stress fields can be developed, so that a direct link of the progressive damage with the \(\lambda\) ratios can be made, which is more difficult in multidirectional laminates, like in the case of quasi-isotropic laminates. Apart from the choice of the appropriate laminate configurations for the multiaxiality study, special attention is given to the selection of the experimental techniques for damage monitoring.