The distribution and orientation of collagen fibres in unstretched and biaxially stretched pig aortic media have been determined by an X-ray diffraction technique in order to analyse the reorientation of collagen fibres in response to the presence of a notch in biaxially stretched samples. The results show that collagen fibres align themselves following the force trajectories. This results in preferential fibre orientation perpendicular to the advancing crack tip, in agreement with the theoretical predictions of stress concentration effect due to the presence of an elliptical notch in an elastic plate.