PSI - Issue 28

Giovanni Pio Pucillo et al. / Procedia Structural Integrity 28 (2020) 1998–2012 GP Pucillo et al. – Part I / Structural Integrity Procedia 00 (2019) 000 – 000

2007

10

the head and the foot were partially removed for this experiment, see Section 2), different residual strain values were measured by strain gauges glued at symmetric positions with respect to the rail longitudinal axis. In particular, it was found that the hoop residual strain at 90° is higher than that recorded at -90°: ε hoop,res (π/2) > ε hoop,res (- π/2), and this result agrees with the literature (Lowry 1991; Ball and Lowry 1998), where it is reported that lower magnitude of residual stresses are expected for short edge distance (see also Section 3 of (Pucillo et al. 2020) for more details). For completeness, Fig. 9-d shows the radial strain measured by the strain gauge installed along the longitudinal axis of the rail ( θ = 180°) at 3.5 mm from the hole edge; in this case, the signal has gone out of the reading range of the amplifier during the maximum hole expansion, therefore it was not possible to acquire the maximum intensity, whereas at the mandrel exit the signal has returned into the reading range of the amplifier, so the acquired residual strain of approximatively - 15200 μm/m is correct. 3.1.3. Cold expansion of hole #6 Strain gauges data acquired during the cold expansions of hole #6 are shown in Fig. 10. Fig. 10-a shows the hoop strain measured by strain gauges installed at 0° (the strain trend is the same of those of Fig. 8-c (hole #5) and Fig. 9 (hole #2)), while Fig. 10-b refers to hoop strain measurement at ±45° and +135°; even in this case, the signals of the strain gauges installed at 3.5 mm from hole edge (#6_T0_3.5_CE_6 and #6_T45_3.5_CE_6) were interrupted during the first step of the cold expansion process.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

-8000 -6000 -4000 -2000 0

#6-T45_3.5_CE-6 #6-T45_9_CE-6 #6-T135_5_CE-6 #6-T135_11_CE-6 #6-T-45_9_CE-6

#6-T0_3.5_CE-6 #6-T0_9_CE-6 #6-T0_16_CE-6

a

b

c

Strain [ μ m/m]

Strain [ μ m/m]

Strain [ μ m/m]

-20000 -18000 -16000 -14000 -12000 -10000

#6-R90_7_CE-6 #6-R180_3.5_CE-6

470 480 490 500 510 520 530 540 550

470 480 490 500 510 520 530 540 550

470 480 490 500 510 520 530 540 550

Time [s]

Time [s]

Time [s]

Fig. 10. Hoop (a, b) and radial (c) strain measured by strain gauges applied near the hole #6.

The comparison between the signals of strain gauges applied at the same radial distance (9 mm) from hole edge but installed at different angular locations (#6_T0_9_CE_6, #6_T45_9_CE_6, and #6_T-45_9_CE_6) reveals that the hoop residual strain at 0° is higher than the one at 45°, that is higher than the one at -45°: ε hoop,res (0) > ε hoop,res (π/4) > ε hoop,res (- π/4). Signals shown in Fig. 10-c refer to the radial strains measured by strain gauges installed at +90° and 180°. The strain trend is the same as in Fig. 9-d (hole #2), therefore the same considerations are worth.