The mechanisms of fatigue phenomenon, therefore, might be clarified by obtaining some correct information concerning the behaviors of the plastic region. The crack closure could also occur during growth. In order to better understand these thermal effects and to make a connection with the initiation and the propagation of the fatigue crack, a thermomechanical model is developed. If a plastic flaw exists at the tip, the structure is not endangered because the metal mass surrounding the crack will support the stress. The same has been found in AlCuMg alloys.Next
This paper studies the thermal effects associated with the propagation of a fatigue crack in a gigacycle fatigue regime. The temperature field was measured on the surface of the specimen, by means of an infrared camera. . Their results proved that the fractography-based evaluation method assisted by fracture mechanics is viable for exploring the fatigue failure process. Thus, the aim of this chapter is to offer the-state-of-the-art of the fatigue crack initiation mechanisms in metals. As the temperature increases, a higher stress is required for a crack to propagate.Next
So, the study of the thermal dissipation during the test appears a promising method to improve the understanding of the damage and failure mechanism in fatigue and to determine the number of cycles at initiation. Massimo Manfredini Bonfiglioli Industrial Gearmotors, Bologna, Italy The total life time during fatigue generally can be divided into two regimes: one for crack initiation and another one for crack propagation. The fatigue life of metals in which cracks initiate from an inclusion can then be predicted using the adjusted values of α and c, if the inclusion size is known. The relationship between the temperature and the stress required for a crack to propagate is called the crack arrest curve, which is shown on Figure 2 as Curve D. Crack initiation during fatigue is promoted, for example, by microstructures which tend to develop an inhomogeneous deformation which is usually more pronounced in coarse grained material. Different micromechanical explanations have been proposed in the literature: local grain refinement, carbide decohesion, matrix fragmentation, hydrogen embrittlement, numerous cyclic pressure and formation of persistent slip bands are the most famous proposals.Next
In microstructures with a more homogeneous slip distribution, cracks will mainly nucleate at large inclusions. There is a stress level below which a crack will not propagate at any temperature. All alloys mentioned here showed large inclusions of Mg 2Si particles 5 to 20 µm in dimension which had an oval shape and were elongated in the extrusion direction. It is generally recognized that the plastic deformation always precedes the fatigue failure and the plastic region in the matrix palys an important role in the nucleation and propagation of fatigue cracks. Results show that the crack was initiated at the phase interfaces and grain boundaries, and the crack propagated along the weakened channel formed by the deformed γ' phase and the oxide. At the crack initiation, the surface temperature suddenly increases whatever the localization of the initiation , which allows the determination of the number of cycles at the crack initiation and the number of cycles devoted to the fatigue crack propagation.Next
They contained Mn and differed in their dispersion. Integration of the Paris law allows one to predict the number of cycles to crack initiation. This is the reason for the high value of α. When testing aluminum alloys it has been found that flow stress varies only slightly with grain size. The fatigue crack initiation in high-strength steels at surface inclusions is linked to high stress amplitudes. The influence of surface roughness, internal inclusions, hardness, and residual stress on the fatigue strength are studied and discussed.Next
The aim of the paper is to gain insight regarding the factors that lead to the transition of the crack initiation site from the surface to the interior. Two kinds of initiation modes of microcracks could exist. This work may provide new insights into thermal fatigue mechanisms and advanced superalloy design. Total Materia remains the only tool which will be used for this purpose. In the gigacycle fatigue domain, more than 92% of the total life is devoted to the initiation of the crack.
The temperature fields measured by infrared thermography show a significant and very local increase in the temperature just before fracture. The commercial alloy contained also 0. Total Materia has allowed us to solve in a definite way all problems we had for the search of alternate materials in foreign countries. It should be noted that smaller grain size, higher temperature, and lower stress tend to mitigate crack initiation. The maximum thermal stress was evaluated by the finite element method, and the evolutions of the microstructures and the changes of mechanical properties during thermal fatigue process were characterized by scanning electron microscopy, transmission electron microscopy, and nanoindentation.
The resistance to fatigue crack initiation may be better than it is for in homogeneously deforming microstructures, but the crack propagation resistance can be lower in this case. Thenumber of cycles to failure, Nf, was 93 776. Tests were performed at ambient temperature using a piezoelectric fatigue system 20 kHz. According to our hypothesis, the main component isgrain renement, which leads to a local decrease in thethreshold value for crack initiation. The stress field at the crack tip and the degree of oxidation reaction together determine the rate of crack growth. In contrast,Figure 1c shows a fracture surface with a subsurfaceinclusion which has a calculated Kmax of 2. Low stress amplitudes are associated with a crack initiation at subsurface inclusions.