PSI - Issue 28

S. Henschel et al. / Procedia Structural Integrity 28 (2020) 1369–1377 S. Henschel et al. / Procedia Structural Integrity 00 (2020) 000–000

1370

2

Nomenclature

a α B F

crack length

angle between loading axis and normal on ideal crack plane

thickness of specimen

force

l N

length of the notches at the back face

Poisson’s ratio

ν

r , ϕ

polar coordinates defining the point of interest with respect to the crack tip

K I , K II

mode I and II stress intensity factors

K IQ , K IIQ

provisional values of mode I and II fracture toughness

u 1 , u 2 ε 1 , ε 2

displacement in directions 1 and 2

strains in directions 1 and 2

S ZW , S ZH

stretch zone width and stretch zone height

W

width of specimen

With both setups, low and medium-strength materials like polycarbonate Banks-Sills et al. (1982), composites (Rikards et al. (1998)) or aluminum were tested (Aoki et al. (1990)). Furthermore, fatigue crack growth tests were performed, i.e. on a high-strength steel (Richard’s setup: Link (1993)). However, the suitability of the specimen and loading device for testing high-strength steel under monotonic loading was not yet shown. Furthermore, it was unclear how slight modifications of the specimen geometry will change the calculations of the stress intensity factor. The present authors measured the fracture toughness of the high-strength quenched and tempered steel 42CrMo4 at di ff erent loading rates and temperatures (e.g. Henschel et al. (2016); Henschel and Krüger (2016),). It was observed that the crack path can be deflected towards agglomerations of non-metallic inclusions (Henschel and Krüger (2015)). This process can lead to a mixed-mode loading at the crack tip. However, the fracture toughness for defined mixed mode loading was not measured. The first aim of this study is the experimental determination of mode I / II fracture toughness for a high-strength quenched and tempered steel (42CrMo4). To this end, strain gauges were applied. Since the specimen was a modified version of Richard’s original design, formulas for calculating the stress intensity factors were adapted. This was achieved by performing finite element analyses. The second aim is the investigation of the crack growth in the material. To this end, fracture surfaces were char acterized by means of scanning electron microscopy. The blunting of the crack tip was determined by measuring the stretch zone width ( S ZW ) and stretch zone height ( S ZH ).

2. Material and Methods

The chemical composition of the investigated 42CrMo4 steel is shown in Table 1. The processes of melting the steel, casting and solidification were performed in a steel casting simulator. The Ar atmosphere within this device was fully controlled. The steel was melted by an induction heating system which was also used to stir the melt within the crucible made of Al 2 O 3 / Al-Mg-spinel. Details of the steel casting simulator can be found in Aneziris et al. (2013). The evolutions of temperature and oxygen activity in the last 20 min before casting are given in Figure 1. The steel melt was cast through a spaghetti-type filter (C-bonded Al 2 O 3 ), which was produced by a alginate-based robo gel

Table 1. Chemical composition of investigated steel. C Cr Mo Mn

Si

Al

S

P

Fe

0.41

1.03

0.19

0.77

0.25

0.021

0.031

0.012

balance