PSI - Issue 83

Davide D’Andrea et al. / Procedia Structural Integrity 83 (2026) 256–264

258

and a larger dispersion in results. Crisafulli et al., (2024) reported ultimate stress for both types of specimens and observed a decrease in it of 10.3%. These values are then used to determine plastic work ratio according to the approach of Risitano et al., (2015).

Table 1. Specimens’ mechanical properties.

Type

Young’s modulus, E [GPa] Ultimate stress, σ U [MPa]

Traditionally manufactured

190±4.4

714±11

Additively manufactured

169±7.5

640±8

Two stepwise fatigue tests were performed with the same load history on each specimens batch. These tests consisted in applying an increasing maximum stress level, ranging from 200 to 340 MPa with stress increases of 10 MPa and a stress ratio R= -1. Each cyclic stress block lasted 10000 cycles, with a testing frequency of 10 Hz. These information are reported in Table 2. Stepwise fatigue tests summaryTable 2. The differences in maximum stress levels are due to the different response of AM material to the applied load.

Table 2. Stepwise fatigue tests summary.

Stress increase [MPa]

Cycles per block [//]

Frequency, f [Hz]

Stress Ratio, R

Maximum stress levels [MPa]

Specimens’ batch

ID

Traditionally manufactured AISI 316L Additively manufactured AISI 316L

Step_AISI316L_Trad01 Step_AISI316L_Trad02 Step_AISI316L_AM01 Step_AISI316L_AM02

200÷330 200÷330 200÷340 200÷320

10

-1

10

10000

The proposed methodology differs from what is typically analyzed in stepwise fatigue tests. Usually, these analyses are focused on the first or second phases of temperature’s trend over cycles and take into account stabilization temperatures measured for each stress level (Curà et al., (2005); Fargione et al., (2002); Huang et al., (2017); Wei et al., (2024)). In this case, in addition to the stabilization temperature, which gives fundamental information about fatigue limit and fatigue life, the number of cycles leading to failure and characterizing the third phase of temperature’s trend was experimentally measured and adopted to get information about the failure behavior of the material. 3. Theoretical background 3.1. Risitano’s Thermographic Method and crack propagation modeling Risitano’s Thermographic Method (RTM) consists of monitoring the surface temperature of specimens subjected to fatigue loading. During cyclic loading, the energy dissipated in the material results in a temperature increase that is correlated with the magnitude of the applied load. Figure 2-a shows the temperature trend as a function of the number of cycles during a constant amplitude (CA) fatigue test. As illustrated in the figure, three distinct phases can be identified. The first phase is characterized by a temperature increase until a stabilization value is reached at N A . This is followed by a second phase, in which the temperature remains approximately constant. This phase ends at the number of cycles N B . After this point, a sudden temperature rise occurs, corresponding to the third phase, which precedes the final fracture of the specimen at N f . The area below temperature’s trend is a material property and can be pointed out as the Energy Parameter . The same behavior can be observed in stepwise fatigue tests (Figure 2-b), which consists in applying multiple stress levels on a single specimen in order to evaluate their associated stabilization temperatures. More information on RTM is available in Fargione et al., (2002).