PSI - Issue 7

Marton Groza et al. / Procedia Structural Integrity 7 (2017) 438–445 M. Groza et al. / Structural Integrity Procedia 00 (2017) 000–000

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• and the Defect Stress Gradient approach from Vincent et al. (2014), which applies a correction proportional to the local stress gradient on the local stresses computed by the chosen multiaxial fatigue criterion. The objective of the present study is to apply the DSG approach at the scale of a Nodular Cast Iron component, compare different methods for the computation of the local stresses and propose a modelling and post-processing methodology for the fatigue assessment of components with surface defects.

2. Nodular cast iron material and experimental details

Nodular cast iron (NCI) also known as ductile (DCI) or spheroidal graphite iron (SGI) has mechanical properties comparable to steel with a great number of advantages related to the manufacturing. For all these beneficial characteristics, the price has to be paid with the frequent occurrence of various internal and surface inhomogeneities, which dominate the fatigue failure mechanisms. All experimental results derive from the work of Nadot et al. (2004) and concern ferritic nodular cast iron. Similar materials are commonly used for structural components in the vehicle industry. The given material is ferrite dominated, with less than 5% perlite. Table 1. summarizes the most important material properties and the defect free fatigue limits for different loading scenarios. The defect area has been measured on the fracture surfaces of each test specimen, and the defect size has been characterized with the area parameter from Murakami (2002). These results are plotted in Fig 3.

Table 1. Material parameters for NCI, experimental data from Nadot et al. (2004).

2

0.2% ( ) p R MPa

0.2% ( ) p cy R MPa

( ) E GPa

( ) HV kgf mm

( m R MPa

)

% (%) A

ten D R

ten D R

tor D R

σ

σ

σ

, 1 ( −

)

, 0.1 (

)

. 1 ( −

)

MPa

MPa

MPa

14

170

300

162

250

180

380

385

510

3. Description of surface defects

Based on an initial statistical analysis of surface defects observed on nodular cast iron components in the vehicle industry, it has been found, that their size is generally in the 0.5-2.5 mm range. Even some of the large defects can be assessed as passable, if they are located in a position with low mechanical utilization. It has also been concluded that the different surface pores, shrinkages and sand drops can be modelled as an ellipsoid shaped notch from a mechanical standpoint. With surface measurements (calliper gauges, optical 3D scanning) their size and shape can be estimated in an effective manner with admissible precision during quality inspections on the production line. In the current study, the max area parameter is used to quantify the size of the defect, which is a modification of the well-known parameter of Murakami. It is defined as the maximum cross-sectional area of the surface defect. The utilization of this parameter is supported by the demands of the industry, it leads to conservative results in the calculations, and can be easily calculated on site, since it is independent from the direction of the Maximum Principal Stress at the given location. With the measurement the major and minor axis and the defect depth, the max area parameter can be estimated.

Fig. 1. Estimation of

max area based on visual inspections and surface measurements, and three different surface defects with their estimated size expressed in max area