Issue 38

S. Hörrmann et alii, Frattura ed Integrità Strutturale, 38 (2016) 76-81; DOI: 10.3221/IGF-ESIS.38.10 79 Fiber Direction Various damage mechanisms could be observed and monitored through the use of a camera (resolution = 5 MPx, frame rate = 0.2 fps) during the fatigue loading, and through fractography analysis (optical microscope with magnification factor of max. 63x) after testing: breakage of the longitudinal tows inside of the folds, longitudinal splitting along the folds turning lines (corresponding to the A1 and A2 turning points in Fig. 2b), and delamination of the folds along the fold lamination longitudinal plane (corresponding to the fold junction line A1A2 in Fig. 2b). All these separate damage mechanisms are initiated at the location of the fold; they are concurrent and coupled (influence each other); they are initiated at approximately the same moment (number of cycles) during the fatigue loading. Each of them is a progressive damage mechanism, i.e. longitudinal splitting propagates along longitudinal and through-thickness direction, delamination propagates along longitudinal direction, and more individual tows will break during cyclic loading. To be noted that the same progressive damage mechanisms appear in the material without the fold defect; however, in this case, the damage initiation location can be anywhere along the width of the specimen, while in the case of defect material the onset takes place always at the fold location. By finite element analysis it was found that the increased stiffness within the fold does not lead to a stress concentration by itself. A multiaxial stress state is introduced within the fold by clamping effects and this leads to damage initiation of the folded region nearby the tabs. The longitudinal tows breakage will bring a corresponding stiffness drop, which can be recorded by the strain measurements. What is reported in the present manuscript is the initial stiffness drop, corresponding to the damage onset of the progressive damage events. There will of cause be load carrying capacity after damage onset (which is defined here as the initial longitudinal stiffness drop, found to be around 2 – 8 % of the initial stiffness of the material featuring the fold manufacturing defect before damage initiation. The recorded data for the final failure of the material (defined as the total loss of the load carrying capacity) needs further analysis and understanding, and it will be presented in further reports. In Fig. 3, damage onset points of the material with defect are compared to the corresponding onset SN curves of the material without defect. The tension-tension (T-T) results are normalized on the ultimate tensile strength; the tension- compression (T-C) and compression-compression (C-C) results are normalized on the ultimate compressive strength. The static ultimate strengths for specimens with folds are also included. For the C-C load case no data without defect is available at this moment, further testing is needed for this case. Yet, the results of the material without defect under T-C loading (the gray lines in Fig. 3c) can be used for preliminary comparison. (a) (b) (c) Figure 3 : Fiber direction loading SN curves, normalized on the static strength. Regarding the static results, it can be inferred from Fig. 3a that the tensile strength is slightly increased by the presence of the fold, compared to the strength without defect; this is because the additional fibers act as reinforcement for this loading case. The compressive strength with defect is reduced to 80 % of the strength without defect, see Fig. 3b,c; the cause of this has to be further investigated and understood. In fiber direction the fold is a local reinforcement, since locally more fibers are present; the reinforcing effect can be noted under tension loading, but not under compression. The distance c did not have a measurable influence on the static results; the same fracture load was measured for different c values.