Fall 2013
Material Engineering Lab ME251-0
Lab#8
Metal Tensile Testing
Instructor Name: Ali Ghazinezami
Student Name: Hassan AL-Tulaihi
WSU ID:V222W652
Due date: 11/06/2013
Class time: 1:30 to 4:30
INTRODUCTION:
Corrosion fatigue of structural metals and alloys considers as process that is influenced by the number of factors, including stress level and strain-stress state, material microstructure and environment Developments in fracture mechanics approaches have provided the better understanding of fatigue failure as phenomenon of crack-like defect nucleation and growth to some critical size. Now it is clearly that under certain circumstances corrosion fatigue lifetime may be controlled by the development and propagation of very small defects the knowledge of initial stages of given processes become significantly important for prediction of total durability of structure components under operating conditions. Recently in framework of this problem the relationship between anodic dissolution process on a cyclically deformed smooth surface and short corrosion fatigue crack growth behavior has been established. As further developments of this direction the present work is focused on the initial stage of corrosion fatigue with take into account of detailed consideration of electrochemical state variables of metal surface under cyclic deforming
Theory:
Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.(2)
The three components required for fatigue to occur are, applied stress of sufficient magnitude, large variation or fluctuation in applied stress, and sufficiently large number of cycles of the applied stress. (1)
A very useful way to visualize time to failure for a specific material is with the S-N curve. The "S-N" means stress verse cycles to failure, which when plotted use the stress amplitude, SA plotted on the vertical axis and the logarithm of the number of cycles to failure.(3)
Factors that affect fatigue-life (2)(3)
* Cyclic stress state: Depending on the complexity of the geometry and the loading, one or more properties of the stress state need to be considered, such as stress amplitude, mean stress, biaxiality, in-phase or out-of-phase shear stress, and load sequence,
* Geometry: Notches and variation in cross section throughout a part lead to stress concentrations where fatigue cracks initiate.
* Surface quality. Surface roughness cause microscopic stress concentrations that lower the fatigue strength. Compressive residual stresses can be introduced in the surface by e.g. shot peening to increase fatigue life. Such techniques for producing surface stress are often referred to as peening, whatever the mechanism used to produce the stress. Low plasticity burnishing, laser peening, and ultrasonic can also produce this surface compressive stress and can increase the fatigue life of the component. This improvement is normally observed only for high-cycle fatigue.
* Material Type: Fatigue life, as well as the behavior during cyclic loading, varies widely for different materials, e.g. composites and polymers differ markedly from metals.
* Residual stresses: Welding, cutting, casting, and other manufacturing processes involving heat or deformation can produce high levels of tensile residual stress, which decreases the fatigue strength. * Size and distribution of internal defects: Casting defects such as gas porosity, non-metallic inclusions and shrinkage voids can significantly reduce fatigue strength.
* Direction of loading: For non-isotropic materials, fatigue strength depends on the direction of the principal stress.
* Grain size: For most metals, smaller grains yield longer fatigue lives, however, the presence of surface defects or scratches will have a greater influence
