PSI - Issue 28

NikolayA. Makhutov et al. / Procedia Structural Integrity 28 (2020) 1347–1359 N.Makhutov, D.Reznikov/ Structural Integrity Procedia 00 (2019) 000–000

1358 12

Table 1.Characteristics of strength and ductility under static and dynamic tension of the smooth specimen

t , о С

y , МPа

ψ f , %

Loading regime

σ

m

e f

f e

f 

y 

1 2 3 4

+ 20 - 80 + 20 - 80

252 317 533 665

0,225 0,195 0,127 0,105

0,78 0,77 0,65 0,32

54,3 53,1 47,8 27,7

619 487 244

1,00 1,26 2,11 2,64

629 526 436 140

96

It can be seen from Fig. 1 that with decreasing temperature and increasing strain rate е  , the yield strength σ y increases while the strain hardening exponent m and ductility e f , ψ f decrease. In this case the decrease in ductility characteristics e f , ψ f , f e is more intense than the increase in yield strength. Since the strain hardening m is relatively small and decreases, the fracture energy f  decreases significantly (by factor of 4.5) if the loading regimes 1 and 4 are compared. If we move from the tension (or bending) of a smooth specimen (Fig. 2a, 2b) to loading of a notched specimen (Fig. 2c) it is necessary to account for: - the influence of stresses concentration K t , K σ and strains K t , K е according to expressions (10) - (14), (19), (20) for various nominal stresses σ n and loads P ; - effect of stress triaxiality at the notch root (according to Fig. 5) which causes an increase in the resistance to plastic strains (by factor of I c ) according to expressions (24) and (32) and a decrease in plastic fracture strains (by factor of D e c ) according to expression (30).

Fig. 6. The dependencies of stress K σ and strain K e concentration factors for the V-notched Sharpy specimen for two design cases: static loading at the room temperature t=+20 0 C and dynamic loading at the low temperature t =-80 0 С The following parameter 2 1 c c    ; 3 0   ; K t =5; b =10mm; h =8mm; L 0 =70mm; k D =1; μ=0.5 were taken in calculations. The strain rate and local strain e max c at the notch root was estimated as:

D

c e е е K     ; max c e

.

(35)

e c

e K e 

 

n

e

c

I

c

The results of calculations (in analogy with Table. 1) are presented in Table. 2. Figure 6 shows the changes in the stress K σ and strain K e concentration factors for the V-notched Sharpy specimen subjected to two loading regimes according to Fig.1 and table 2: static loading at room temperature t =+20 0 C (curve 1) and dynamic loading at the lower temperature t =-80 0 С (curve 4). The fracture energy of the Sharpy specimen subjected to dynamic low temperature loading (Regime 4) reduces 5,4-fold in comparison with the fracture energy when the specimen is subjected to static loading under room temperature (Regime 1). This reduction is by 20 per cent higher than the similar reduction in the fracture energy for