Issue 7

C. Colombo et alii, Frattura ed Integrità Strutturale, 7 (2009) 65-72; DOI: 10.3221/IGF-ESIS.07.05 69 In Tab. 2 the results of the tests are shown. In Fig. 6 the typical aspect of a DGM failure is shown. Some interrupted tensile and compressive tests were also executed, being the aim to investigate the development of the damage by means of observations at the SEM. These analyses allowed to assess the progressive nature of damage in the different layers of material. In particular cracks between the fibers and the matrix were observed with increasing length with the load, while their number remained about the same till the final rupture. However some broken fibers were noted in the Roving layer for high values of the stresses. In Fig. 7 some SEM images showing the damage evolution are shown: Figs 7a and 7b show the damage evolution in the Mat at 200MPa and 340 MPa: the interface crack evolution between the matrix and the fibers can be easily observed. Figs 7c and 7d s how the Roving and also in this case the most evident damage type is the formation of interface cracks. Specimen E (MPa) UTS (MPa) Elongation % Failure type Failure zone Tensile tests 1 30605 367 1,20 DGM Middle 2 30794 377 1,23 DGM Middle 3 31005 377 1,21 DGM Middle 4 30003 361 1,20 DGM Middle 5 28791 381 1,32 XGM Middle Mean value 30240 373 1,23 Specimen E (MPa) UCS (MPa) Elongation % Failure type Failure zone Compres- sion tests 1 31311 454 1,45 DGM Middle 2 27296 383 1,40 DGM Middle 3 26607 382 1,43 DGM Middle 4 26973 375 1,39 DGM Middle 5 31313 402 1,28 DGM Middle Mean value 28700 399 1,39 Table 2 : Summary of the results of the static tests (DGM= edge Delamination Gage Middle, XGM=eXplosive Gage Middle) (a) (b) Figure 6 : Typical aspect of a DGM failure: (a) tensile failure, (b) compression failure. Similar observations performed on compression specimens evidenced the same damage evolution. F ATIGUE CHARACTERIZATION xial fatigue tests (R=-1) were executed, being the aim the determination of the S-N curve of the material and to verify the existence of a fatigue limit. The specimens have constant section and their geometry is the same as in Fig.5. The specimens were considered run-out if failure did not occur till 10.000.000 cycles. Tests were performed in loop load control and the imposed frequency was 5 Hz; no temperature increment was observed. Fifteen specimens were used to determine the fatigue limit according the stair-case procedure while other nine specimens were tested to determine the leaning part of the S-N curve according to the procedure included in the ASTM E 739-91. In Fig. 8 t he results are shown: it is possible to observe the limited dispersion of the data. If the applied stress decrease from 120 MPa A