Issue 48

C. E. Cruz Gonzalez et alii, Frattura ed Integrità Strutturale, 48 (2019) 530-544; DOI: 10.3221/IGF-ESIS.48.51 535 Mechanical testing of Single lap shear (SLS) tests with 12.7 and 50.0 mm of overlap specimens Before fatigue testing under the standard ASTM D3166 ‒ 99 [19] it is necessary to know the maximum tensile load to set the parameters. The procedures used for testing the adhesive bonded joint were according the standard ASTM D1002 [18]. Rectangular tabs (25.4×6.4 or 2.9 mm) were bonded on the specimen extremes to assure an axial loading during testing. An INSTRON 4482 testing machine with flat knurled grips was used for the specimen testing. The rate of testing (2.4 kN min -1 or approximately 1.2 mm-min -1 ) was settled according to the standard ASTM D1002 [18]. It is known that polymeric materials may be rate sensitive, in this case, tests were carried out at temperatures below glass transition temperature (T g ) where rate sensitivity is negligible [20]. Additionally, foil strain gauges (Vishay EP-08-250BG-120) were bonded to the surface following the work of Karachalios et al [21]. The bond (adherend-strain gauges) remained for 24 h, and then the connecting cables were soldered to the extensometer terminals and connected to a P3 strain system. Then, SLS specimens, were tested in fatigue following ASTM D3166- 99(2012) [19] procedures. An MTS 810 with flat knurled grips machine was used. The parameters were 0.1 of reversibility load ratio (R) and 30 Hz of frequency at 30, 50 and 70 % of the maximum load obtained in the SLS test. Both, 12.7 and 50.0 overlap length specimens were tested to build the S-N graphs. In this case, load-cycle graphs were built according to Ashcroft [22] and ten specimens were tested until they fractured or reaches 10 6 cycles. Post Fracture Analysis After single lap shear and fatigue test, a visual inspection was performed on each fractured specimen to determine the fracture mode. An Optima V20 stereoscope was used to observe the fractured surfaces at magnification of 20x. Post fracture analysis was performed to complement the visual inspection, for the determination of the fracture mode upon the remaining adhesive on the surface. Post fracture analysis was performed with a JEOL LV600 scanning electron microscope operated at 15 kV with secondary electrons signal and working distance range was from 9 to 11 mm. The magnifications used were range from 100 x, 1000 x and 10000 x. Figure 3: Stress-strain graphs for the DC-80, Betamate 120 and MP5520 adhesives. R ESULTS Adhesive mechanical properties ig. 3 which is a stress-strain chart depicts the relationships between these for every adhesive. It means, that each adhesive could behave (deform) different when is subjected to an external stress. The DC-80 adhesive (blue dashed graph in Fig. 3) has a tensile strength of 43.44±0.88 MPa, 1.8±0.3% of elongation, a Poisson ratio 0.336 and a Young modulus of 2383±54MPa. On the other hand, the Betamate 120 adhesive (red dashed graph in Fig. 3), had a tensile strength of 23.24±0.50 MPa, 2.3±0.02% of elongation, a Poisson ratio 0.292 and a Young modulus of 1371.8±23.4 MPa. Finally, the MP55420 adhesive (black continuous lined graph in Fig. 3), had a tensile strength of 12.00±0.25 MPa, 20.3±0.45% of elongation, a Poisson ratio 0.305 and a Young modulus of 471.55±4.88 MPa. The results were similar to F