PSI - Issue 2_B

Giovanni Meneghetti et al. / Procedia Structural Integrity 2 (2016) 2076–2083 G. Meneghetti / Structural Integrity Procedia 00 (2016) 000–000

2078

3

specimens, characterized by notch radii equal to 0.5-, 1- and 3-mm, in terms of the Q parameter, which has evaluated by means of an infrared camera allowing for a higher spatial resolution. As a result, all new fatigue data were rationalized by the same energy-based fatigue scatter band previously proposed by Meneghetti et al. (2013).

800

10 20

Plain material k=17.2; T  =1.19; T N,  =20.0 Hole, R=8 mm: k=8.9; T  =1.18; T N,  =4.3 Data from Menghetti et al (2013) Stair case: broken, unbroken

Axial load: k=18.9, T  =1.13, T N,  =10.0 Torsional load: k=18.7, T  =1.13, T N,  =9.02 Data from Meneghetti et al (2014)

Scatter bands:10% - 90% survival probabilities, from Meneghetti et al (2013)

Load ratio: -1

Q k 2.11 A,50% 

500

3

cycle) 0.133MJ/(m 



120 fatigue data T 4.50 T 2.04 N,Q Q  

k=5.8 T  =1.30 T N,  =4.5

Strain controlled U-notch, R=5 mm V-notch, R=3 mm 120 fatigue data

1

 an ,  a [MPa]

0.1 Q [MJ/(m 3 ·cycle)]

300

AISI 304L

Data from Meneghetti et al. (2013)

200

R= -1 10% - 90% survival probabilities.

Plain material

Stair case: broken; unbroken

Strain controlled Hole, R=8 mm U-notch, R=5 mm V-notch, R=3 mm

Axial load Torsional load Data from Meneghetti et al (2014)

AISI 304L

100

0.01

10 2

10 3

10 4 10 7 N f , number of cycles to failure 10 5 10 6

10 2

10 3

10 4 10 7 N f , number of cycles to failure 10 5 10 6

Fig. 1. Completely reversed axial and torsional fatigue test results relevant to AISI 304L steel specimens analysed in terms of net section stress amplitude (from Meneghetti et al. (2014)).

Fig. 2. Fatigue data reported in Fig. 1 analysed in terms of specific heat loss per cycle. Scatter band is defined for 10% and 90% survival probabilities (from Meneghetti et al. (2014)).

2. Material, specimens’ geometry and test conditions Constant amplitude, push-pull, stress controlled fatigue tests were carried out on specimens prepared from 4-mm thick hot rolled AISI 304L stainless steel sheets, according to the geometry shown in Fig. 3. Some mechanical properties of tested material (engineering proof stress R p0.2 , engineering tensile strength R m , elongation at break A%) are listed in Table 1. 3D linear elastic finite element analyses were carried out to calculate the net-section stress concentration factor, K tn , of specimens shown in Fig.3c, which resulted K tn =4.26, 7.39 and 8.96 for R=3, 1 and 0.5 mm, respectively.

(b)

45°

(c)

(a)

45°

163.5

8

8

9

46

46

38

R

R

50

20

50

150

150

Fig. 3. Specimen’s geometry for a) plain, b) severely (R=0.1, 0.5 mm)- and c) blunt notched specimens (R= 1 and 3 mm).

All fatigue tests were carried out by using a servo-hydraulic Schenck Hydropuls PSA 100 machine equipped with a 100 kN load cell and a Trio Sistemi RT3 digital controller. Plain specimens were tested to evaluate the material stress-life curve as well as the fatigue limit,  A,-1 , by means of a short stair case procedure at 10 million cycles. Specimen’s temperature was measured by using copper-constantan thermocouple wires having diameter 0.127 mm, which were fixed at the specimen’s centre by means of a silver-loaded conductive epoxy glue. Temperature signals generated by the thermocouples were acquired by means of a data logger Agilent Technologies HP 34970A operating at a maximum sample frequency, f acq , of 22 Hz (accuracy equal to 0.02 °C). Load test frequencies between 3 and 25 Hz were adopted depending on the applied stress level. In order to limit the stabilized temperature observed during the fatigue tests below 70°C, a blower was used to cool the specimens. About ten minutes before each cooling, the blower was switched off. At the same time the test frequency was appropriately reduced in order to