Structurally stitched NCF preforms: Quasi-static response

Fatigue and post-fatigue tensile behaviour of non-crimp stitched and unstitched carbon/epoxy composites

“…In Table 3 the value of average threshold σ 1 and σ 2 are collected for the specimens fatigued after different number of cycles. The graphs in Figure 15 and Figure 16 and the values of first damage threshold stress σ 1 in Table 3 show a significant amount of events recorded close to the maximum stress formerly set during fatigue tests (240 MPa), below this threshold the amount of noise is barely significant.The damage detected during quasi-static tensile tests in [12], below the stress level of 240 MPa, was not recorded in the present work because completely imparted during cyclic tests. The large part of the damage (AE events) appears in the present quasi-static tensile loading for stress levels above the maximum stress applied during fatigue.…”

“…Typical appearance of the stitched preforms is shown in Figure 1. It is shown [12] that the knitting device for the stitching process produces openings in the composite and these openings are naturally oriented along the global fibres orientation in the ply. Characteristics of the reinforcement, stitching yarn and non-structural sewing thread are listed in Table 1.…”

“…The term 'structural' means that the stitching yarn does not only consolidate the plies (as the non-structural one does) but forms also a through the thickness reinforcement. The quasi-static mechanical response of the considered stitched carbon composite, produced in a different batch, were experimentally investigated in [12].…”

“…The term 'structural' means that the stitching yarn does not only consolidate the plies (as the non-structural one does) but forms also a through the thickness reinforcement. The quasi-static mechanical response of the considered stitched carbon composite, produced in a different batch, were experimentally investigated in [12].The first part of the paper presents the characteristics of the two composites and the description of the experimental techniques applied. In the second part, the tensile-tensile fatigue life is presented and compared by Wöhler-like diagrams.…”

“…The stress was evaluated by the specimen cross-section area averaged from three measurements. In the direction 0 o and 90 o of both 5 / 23 composites, four different maximum stress levels were considered (see Table 2) in order to have a complete characterization, from values close the static strength (see [12]), to the stress level which did not lead to a complete failure before 2 million cycles. Complete failure means separation in two parts of the specimen during cyclic loading.…”