In order to understand the mechanisms that cause the initiation and propagation of fatigue cracks, and besides, to characterize fatigue damage, other techniques apart from Optical Method are needed. High sensitivity is reached when detecting cracks, but are also useful to follow the propagation of cracks. The resolution of the Scanning Electron Microscopy (SEM) is better than 1nm, while that of the Transmission Electron Microscopy (TEM) is better than 0.1nm. Although SEM offers lower sensitivity than TEM, it is still better than optical methods, which have the same purpose.
Scanning Electron Microscope (SEM)
Electronic microscopes are often used to study the initiation of fatigue cracks as well as their growth behaviour, in order to determine the involved mechanisms. Fine and Chung divided the fatigue process into four stages, taking into account the structural changes caused by cyclic stress in metals:
1. Crack initiation: Initial evolution caused by fatigue damage.
2. Slip-band crack growth: Initial cracks deepen on planes that are subjected to high shear stress. This stage is also known as STAGE I CRACK GROWTH.
3. Crack growth on planes of high tensile stress: In this stage, known as STAGE II CRACK GROWTH, cracks grow perpendicular to the maximum tensile stress.
4. Final ductile fracture: If a crack continues growing, at some point it will reach a length where the resulting cross section will not be able to support the applied load.
In the case of steel alloys subjected to fatigue testing in non-corrosive environments, crack initiation can occur due to slip bands, extrusions/intrusions, grain boundaries, inclusions or porosity.
In Figure 1, 2 and 3, some micrographs obtained by SEM show the nucleation, initiation and propagation of a crack.
X-RAR DIFFRACTION METHOD
During the fatigue process, changes in composition and strain, as well as residual stresses can be evaluated by using X-Ray diffraction (XRD), which may lead to a better understanding of the process. The XRD method offers both, qualitative and quantitative analysis of samples: Lattice constants, size of crystallites and lattice strains can be determined, among other properties. Besides, stress distributions, preferred orientation, texture and other characteristics can be investigated.
To create the XRD phenomenon, x-rays must be scattered by some electrons of atoms with no wavelength change. Copper, cobalt, iron, chromium or similar metallic elements are often used to obtain monochromatic x-rays. Certain geometrical conditions, which are determined by Bragg’s law or Laue’s equations, must be fulfilled to produce a diffracted beam. As a result, a diffraction pattern will be created, where the position and the intensity of the present peaks give information about physical properties of the material.
Tool steels used nowadays in industrial applications must be hard and sharp, and have a good chipping resistance[12-15]. Besides, they need to be easy to sharpen, corrosion resistant, and have good finish. Therefore, this means that the raw material needs to be hard and have good hardenability.
Good toughness, corrosion resistance, grindability, mirror polishability and absence of large carbide inclusions are other important properties of tool steels, but it is complicated to produce materials that fulfil all of these requirements.
Besides the before mentioned spalling, chipping and catastrophic breakage are often the principal modes of failure, and hence, the factor that limits the service life of the tool: Examples of the three failure modes mentioned above are shown in Figure 20.
In the same way that different experimental tests exist to analyze the contact fatigue resistance of the materials, there is a testing method intended to evaluate the chipping resistance of the materials, which is called « punch-test ».
Due to the unclear initiation susceptibility of the CBB and U-bend tests (the most popular tests for initiation) on cold-work effects, it has been necessary to evaluate the data using new techniques. During the last two decades, the small-punch test has been used to evaluate the mechanical properties of materials[17, 18].
Some of the applications of this type of test are the evaluation of the ductile-to-brittle transition temperature (DBTT), yield strength, fracture toughness and creep properties, among others[17, 18]. In brief, the small-punch test uses a ball in order to deform a sample placed on a supporting hole and clamped between two dies, an upper and a lower (Figure 21) [17,18]. The dimensions of the sample, hole of the holder and ball, as well as the environmental and experimental conditions can vary in order to evaluate the influence of each of these parameters.
Moreover, Figure 22 shows an example of the results that can be obtained with this kind of test. On the one hand, the general view with the areas with different levels of deformation, and on the other hand, several points of microcrack initiation are shown.
The machine used to measure the microhardness of the different materials is a MATSUSAWA MXT-α microhardness tester, as it can be seen in Figure 32: According to the standard, the indentation in the material must be at least 20-25μm long to get a reliable microhardness measurement. In the case of the materials used in this thesis, a load of 300g has been enough to get an appropriate indentation.
STANDING CONTACT FATIGUE
The SCF tests were performed using an INSTROM 1272 hydraulic press, as it can be seen in Figure 35: The capacity of this press varies between 0 and 21kN. A cyclic load has been applied to the samples to perform the SCF tests, varying the maximum load but keeping 0,5kN as minimum load always, to ensure contact between the spherical indenter and the sample. The frequency used in all the tests has been 45Hz.
Table of contents :
1.1.1. DAMAGE MECHANISMS
1.1.2. DETECTION METHODS
1.2. CONTACT FATIGUE
1.2.1. STANDING CONTACT FATIGUE
1.3. CHIPPING RESISTANCE
3. EXPERIMENTAL APPROACHES
3.1. EXPERIMENTAL MATERIAL
3.2. EXPERIMENTAL EQUIPMENT
3.3. SAMPLE PREPARATION
3.4. TEST PROCEDURE
4. RESULTS AND DISCUSSION
6. FUTURE ASPECTS
STANDING CONTACT FATIGUE TESTS
AISI D2 – SVERKER
06CV – 220
06CV – 250
VANADIS 4 EXTRA