PSI - Issue 7

Šulko Miroslav et al. / Procedia Structural Integrity 7 (2017) 262–267 Šulko Miroslav et Al./ Structural Integrity Procedia 00 (2017) 000–000

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On the surface of the cyclically loading specimens, from the end of the first stage of the fatigue process is possible metallographically to observe the slipping bands lying along the intersecting surface with the slip plane. The dislocation structure lying beneath these slipping bands, i.e. under the surface of intrusions and extrusions, substantially differs from the structure in the surrounding matrix which is identical to the internal structure of material. McGrath and Waldron (1974) have demonstrated significant differences between the surface and inner structures of the dislocation layers in the case of cyclically deformed samples. It is clear from the above analysis that the cyclic loading and the associated fatigue process of the material leave a trace in the surface layer of the material, which must be reflected by changing its mechanical properties. For steel (and other metallic materials), the surface layer does not have a homogeneous structure. The surface layer (as well as the whole material) is a mixture of structural phases - ferrite, perlite, ... The mechanical properties of the individual structural components are mutually different and it can be assumed that they will also react differently to the cyclic loading. In the case of surface layers very important role also play the processes of cyclic hardening respectively softening of material. Given the proven fact that their behavior during cyclic loading is not identical with the behavior of the entire material volume residue [1,2], it can be expected that changes in the mechanical properties of the surface layers may have a different character than the residual volume of the material. 3. Plan of the experiment and its results In order to find a diagnostic method sensitive to the changes in mechanical properties of the surface layers caused by cyclic deformation is important to satisfy the following requirements: • relative simplicity allowing its use in operational conditions • application way of the method to target of the surface layers of metallic materials • minimal influence of diagnostic material Based on the above analysis and formulated requirements was developed a method of measuring of the surface hardness of the material. In order to meet the hardness requirements of the surface material only, the micro hardness measurement and the small indentation hardness measurement are taken into account. Measurement of micro hardness by the Vickers method leaves the trace of indentation with a depth of 10 -3 mm at the material surface. The use of the Brinell method with 1 mm diameter of the bullet indentation that leaves impression of the 10 -2 mm depth and the trace of indentation is more favorable geometry and it does not require special surface preparation. The measurement of micro hardness by the Vickers method makes it possible to monitor the hardness of the micro structured phases but requires it adequate preparation of the surface of the material to be measured. The objective of the measurements is to diagnose the development of hardness of the surface layer of material during cyclic loading. The individual measurements should be carried out regularly after a number of load cycles of material have been completed. The shape of the test samples of the material is also very important. Because of the homogeneous state of stress on the surface, cylindrical cross-sectional test specimens were chosen as shown in (Fig. 2), although this shape is not ideal for forming a correct indentation. For chosen material of steel S355J0 (C – 0,2%, Mn - 1,6%, Si – 0,55%) and 24CrMoV55 (C – 0,25%, Mn – 0,6%, Si – 0,3%, Mo – 0,5%V – 0,7%, P – 0,04%, Cr – 1,6%) were first created fatigue lifetime curves for alternating cycle of uniaxial tensile stress (Basquin curve). Based on these curve was chosen the amplitudes of alternating stress loading during which the hardness of structural phases using Vickers method as well as hardness of surface layer by Brinell method was measured at regular intervals. Cyclical loading of material specimens was applied up to the fatigue crack. The hardness measurement interval was determined based on the known Basquin curve ( ) f a N f 2 = σ (1)

so that the hardness development at about 10%-15% intervals from the total fatigue lifetime to the fatigue crack could be measured. At each measurement, 8 indentations were evenly distributed around the circumference of the circular cross-section of the specimen to capture the scatter of the hardness in the structure of the surface of the