PSI - Issue 68

Danilo D’Andrea et al. / Procedia Structural Integrity 68 (2025) 746–755 D’Andrea et al./ Structural Integrity Procedia 00 (2025) 000–000

751

6

Filling speed

45 mm/s 0.45 mm

Nozzle diameter

Nozzle temperature

250°C

Build Plate temperature 90°C Thermal fields were acquired by using IRtech XT model IR thermal camera and were used to apply Risitano’s Thermographic Method (RTM) and Static Thermographic Method (STM) as it was made in (G. Risitano 2022). This camera has a thermal sensitivity of 80 mK and a temperature measure field between -20°C and 100°C. It presents a resolution of 382x288 px and it can acquire 20 frames per second. Thermal raw data were extracted using Timage Connect software. Data were processed in Python environment. Stepwise fatigue tests were performed with 2 Hz frequency and load ratio of 0.1, applying six load level between 25 MPa and 39 MPa for 10000 cycles each. Constant amplitude tests were performed with 2 Hz frequency and load ratio of 0.1. 4. Results and Discussions Static tensile tests have been conducted to evaluate the stress at which the first damage of the material occurs, using Static Thermographic Method (STM). Static tests were performed at a speed rate of 0.04 mm/s, which represents the best speed rate for evaluating thermal release, as it ensures adiabatic test conditions, with no energy dissipation from the sample to the environment. During the tests, the IR camera has been used to monitor the surface temperature variation of the specimen as the difference between the initial and the instantaneous temperature value. In Figure 5 are shown the stress-temperature curves of Nylon PA12 reinforced with 15% of carbon fibres. The stress curve was plotted against temperature variation and time to correlate the stress level with the energy release of the material. As reported in chapter 2, in order to highlight the inflection point between the first and the second thermal phase, an iterative method developed in python language was used. The stress measured when the inflection point appears is the limit stress, a characteristic of the material related to micro defects that plasticizes when a tensile load is applied. This algorithm is based on the optimization of the determination coefficient of the bilinear model and on the comparison with that of the individual regression lines. To filter the thermal signal, a median filter with a window of 5 elements at a time was used. As shown in Figure 5, the limit stress lim is the point between the linear decrease in temperature (red zone) and the linear decrease that does not follow Lord Kelvin's law (blue zone). By considering four tests conducted under the same conditions, the value of the limit stress determined by the STM is equal to σ lim = 30 ± 1.3 MPa. Stress limits, yielding stress and ultimate stress calculated according to ASTM D638 standard resulting from tensile tests are reported in table 3. In Figure 5 reported typical stress and temperature’s trends over time. Table 3. Tensile tests results Specimen Yielding Stress [MPa] Ultimate Stress [MPa] Stress limit [MPa] 001 40.83 35.4 31.7 002 41.4 35.0 29.7 003 40.5 37.1 28.7 004 43.2 38.7 30.5 Mean value 41.5 36.5 30.1 Standard deviation 1.2 1.7 1.3