PSI - Issue 84

D. Rossi et al. / Procedia Structural Integrity 84 (2026) 337–343

341

a)

b) Fig. 2. Calibration curves of (a) 25 cm flat-jack; (b) 33 cm flat-jack; for gap δ = 4 mm and δ = 5 mm (adapted from Rossi et al., 2025) Table 1 summarizes the results obtained from the FJTs performed using both the 25 cm (M25) and 33 cm (M33) devices. For each test, the externally applied compressive stress P ext was progressively increased from 2 MPa to 4 MPa, and the corresponding flat-jack pressure p , calibration factor K m , correction factor K a , and estimated compressive stress f m were determined. For both flat-jack sizes, an increase in P ext resulted in higher internal flat-jack pressures and, consequently, higher estimated stresses. The calibration factor K m generally increased with pressure, reflecting the nonlinear mechanical response of the flat-jack, while the correction factor K a remained constant for each test configuration. Overall, the estimated stress values followed the trend of the applied external stress, with differences observed between the two flat-jack sizes in terms of consistency and magnitude of the results.

Table 1. Summary of FJT results for 25 cm (M25) and 33 cm (M33) devices. P ext p K m

K a

f m

MPa

Bar 23.9 34.7 41.3 33.0 35.8 46.2 37.6 48.5 62.3 19.9 29.1 41.7 23.4 35.1 49.0 30.5 44.5 59.0 30.2 39.8 53.9 27.1 38.0 50.6

-

-

MPa 1.33 1.98 2.41 1.74 1.91 2.55 2.23 2.98 3.93 0.97 1.45 2.16 1.36 2.10 3.06 1.81 2.93 4.11 2.09 2.81 3.98 1.72 2.50 3.47

2 3 4 2 3 4 2 3 4 2 3 4 2 3 4 2 3 4 2 3 4 2 3 4

0.65 0.66 0.68 0.66 0.66 0.69 0.67 0.69 0.71 0.64 0.65 0.68 0.64 0.66 0.69 0.62 0.69 0.73 0.73 0.74 0.78 0.71 0.74 0.77

M25-1

0.86

M25-2

0.80

M25-3

0.89

M25-4

0.77

M25-5

0.90

M33-1

0.95

M33-2

0.95

M33-3

0.96

Figure 3 shows the mean relative error ( E m ) and the coefficient of variation (CoV) obtained using the 25 cm (M25) and 33 cm (M33) flat-jacks at different applied stress levels. For all loading conditions, the M25 flat-jack exhibits significantly higher mean errors, with E m values ranging between approximately 28% and 31%, accompanied by a relatively large dispersion, as indicated by CoV values between 22% and 28%. In contrast, the results obtained with