Issue 78

S. Vantadori et alii, Fracture and Structural Integrity, 78 (2026) 432-453; DOI: 10.3221/IGF-ESIS.78.25

For each test, the specimen heads are secured on the testing machine through special jaws that prevent slippage. A cyclic axial tensile load is applied, being characterised by a sinusoidal shape, with a frequency of 80 Hz, and a fatigue ratio  0.10 R . Ten stress levels are selected, defined in terms of maximum stress as percentages of the ultimate tensile strength (from 55% to 6%). Three fatigue tests are conducted at each stress level. Each test is terminated either upon specimen failure or when the number of loading cycles reaches 2(10) 6 . However, if a run-out is observed, no further tests are performed at that stress level, and testing proceeds to the next lower stress level. Fatigue crack propagation tests The fatigue crack propagation behaviour of the AMC is evaluated in accordance with ASTM E647-23a [32], allowing the fatigue crack propagation rate curve to be determined, as well as the threshold stress intensity factor range, th K  , using the load-shedding technique. The samples used are standard C(T) specimens, whose sizes are reported in Fig. 4. The specimens are machined from the same metal plate used for the tensile samples, with a thickness of 10 mm. For these tests, the same testing machine used for the cyclic tensile tests is employed, but in this case, it is equipped with pins specifically designed for the aforementioned specimens. For each test, the specimen is secured on the testing machine through pins inserted in the specimen holes, and a clip gauge is mounted on the notch mouth, in order to measure the crack length throughout the experiment using the compliance method. Constant amplitude sinusoidal loading is considered, with a frequency of 30Hz and three different values of the fatigue ratio, R , that is: 0.10, 0.50 and 0.75. Each specimen is precracked before testing to remove the effect of the machined notch and obtain a sharpened crack. The fatigue crack propagation rate curves are obtained using three specimens for each R value. For the determination of th K  , the load-decreasing parameter, used in the load-shedding procedure, is set to -0.15 mm -1 , with two specimens tested under the same conditions.

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Figure 4: Fatigue crack propagation tests: geometry of AMC specimens (sizes in mm).

R ESULTS AND DISCUSSION

Tensile behaviour he results of the tensile tests are shown in Fig. 5, where stress is calculated based on the actual cross-sectional area of the specimen, and apparent strain is derived from the crosshead displacement. The toe region, located at the beginning of the curve and caused by specimen seating, is adjusted by extending the linear region to the zero-stress axis. The intersection point is considered the correct zero-strain point, and all the apparent strains are measured from that point. The obtained stress-strain curves exhibit the typical behaviour of a ductile material, with an initial linear elastic region followed by yielding, strain hardening, necking, and, finally, fracture. The yield strength is determined by using the offset method by considering an offset of 0.2%. Therefore, the yielding stress, 0.2% y  , is equal to 521 ± 1 MPa, corresponding to an elastic apparent strain of 3.87 ± 0.03%. The ultimate tensile strength, u  , is equal to 584 ± 1MPa, while the plastic apparent strain is equal to 11.97 ± 0.37%. The fracture strength, f  , is equal to 557 ± 4 MPa. T

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