PSI - Issue 3

Jilali Nattaj et al. / Procedia Structural Integrity 3 (2017) 579–587 Author name / Structural Integrity Procedia 00 (2017) 000–000

582

4

3. Experimental methodology 3.1. Fatigue tests To determine the lifetime at break, fatigue tests at constant amplitude up to failure for three load levels, 352, 282 and 248 MPa, in stress controlled mode were performed on A36 material. 3.2. Static tests For each loading level (352, 282 and 248 MPa) ten samples were exposed to fatigue loading of 1000 cycles. Then, static tensile tests were provided to determine the residual strength of the material. The experiments showed a crack with a length close to 0.1 mm. It propagates regularly across the section and have defect size comparable to the grain size of the steel. The noticeable crack is at the end of the initiation stage for steel A36. The grain diameter is about 0.032 mm. 4. Parameters evaluation 4.1. Determination of ά and k parameters The factor α is proportional to N f /N f0 . Therefore, it can be represented by the equation (6): with “C” constant of proportionality that will be determined by experimental results. They allowed us to calculate the term k  by taking into consideration at least the results of two or three points of the experimental results to determine the parameter α k . Then, the parameter “k” is obtained. Prior to this, we will determine, using the unified theory approach, the number of break cycles of notched specimens under different loading levels which leads to the Wohler curve (∆σ, Nf) plotting. 4.2. Determination of the endurance limit: The notch decreases considerably the endurance limit of a specimen. So, the reduction of it can be determined by a coefficient K f given by: 0 f f N C    N         (6)

 

0

(7)

K



f

*

e

If we consider Kt as the coefficient of concentration of static stresses at the notch, the sensitivity of the material, denoted '' q '', at the notch is defined by Peterson (1974) as below:

f t K 1 q K 1   

(8)

In this paper we are considering a radius at the bottom of the specimen’s notch to be equal to 0.05 mm. Furthermore, the variation curve of the notch sensitivity index for steels with a resistance between 400 and 700 MPa (Lieurade &Lu) allows us to estimate the sensitivity index of steel A36 (σ u = 558 MPa) at q = 0.55. Then, it is proven that the presence of notch leads to a local stress concentration factor K t between 4 and 5 leading to the factor K f = 3.2.