PSI - Issue 28

Ezio Cadoni et al. / Procedia Structural Integrity 28 (2020) 964–970 Author name / Structural Integrity Procedia 00 (2020) 000–000

965

2

Nomenclature

density of the material

ρ

incident pulse reflected pulse

I

R T

transmitted pulse C 0 elastic wave speed in the bar A 0 cross section of the input and output bars A cross section of the specimen L specimen gauge length ˙ strain-rate

2. Material and specimens

The material used in the current study is a commercial tungsten alloy characterised by high-density ( ρ = 16 925 kg / m 3 ) with the following chemical composition: C (4.20%), O (2.59%), Co (3.79%), Ni (7.98%), W (81.43%). The high strain-rate tests were performed on a round specimen having a diameter of 2mm and gauge length of 5mm. For the quasi-static tests the specimen had same diameter but longer gauge length (15mm) as shown in Fig. 1. The specimen ends had a M5 fillet for the connection to the testing machines.

3. Quasi-static testing

The tests were carried out by means of a Zwick / Roell - Z50 universal machine to obtain the stress-strain law under quasi-static condition (0.001 s − 1 ) under strain control. High precision displacement transducer (with 0.3 µ m resolution) to measure the specimen strain was used.

3.1. Quasi-static results

In Table 1 are reported the quasi-static results in terms of elastic modulus ( E ), proof strength ( f p , 0 . 2 ), ultimate tensile stress ( f u ), uniform strain ( u ), fracture stress ( f f ), fracture strain ( f ), reduction of area ( Z ) and modulus of toughness ( W u ). The stress versus strain curves are depicted in Figure 1. It can be observed how the tests are extremely repeatable showing a quasi-perfect elasto-plastic behaviour but highly brittle, in fact the maximum elongation is restricted to few percentage points.

Table 1. Quasi-static results. Specimen E ( GPa )

3 )

f p , 0 . 2 ( MPa )

f u ( MPa )

u ( % )

f f ( MPa )

f ( % )

Z ( % )

W u ( MJ / m

A20 A21 A22

359.40 291.57 271.23

1513 1527 1660

1772 1753 1759

0.71 0.77 0.75

1610 1592 1618

5.44 5.69 4.13

14.44 11.69 12.90

87.75 90.86 64.99

Average (STD)

307.40 (46.2)

1567 (81)

1761 (10)

0.74 (0.03)

1607 (13)

5.09 (0.84)

13.01 (1.38)

81.20 (14.12)

4. Dynamic testing

The dynamic tests at high strain-rate were carried our by means of a Split Hopkinson Tensile Bar (SHTB) (Cadoni et al. (2011)). It consists of two circular straight high strength steel bars, having a diameter of 10 mm, with a length of 9