Stress concentration point

Under cyclic or repeated stress conditions, rupture of protective oxide films that prevent corrosion takes place at a greater rate than that at which new protec tive films can be formed. Such a situation frequently resiilts in formation of anodic areas at the points of rupture these produce pits that serve as stress-concentration points for the origin or cracks that cause ultimate failure. 

Figure 8.64. Concentration coefficient of the maximum principal stress vs. the highest stress concentration point. Particles types are labeled as in Figure 8.63. [Adapted, by permission, from Mitsui S, Kihara H, Yoshimi S, Okamoto Y, Polym. Engng. Sci., 36, No. 17, 1996, 2241-6.]...

An inside radius in a steel part is easy to produce using any machining method (milling, turning, or grinding). This radius is also important for the life expectancy of the steel part, as it avoids stress concentration points (stress risers) and consequent breakage. The radius on the core pin will then produce a radius hi the molded piece where, in normal design practice, a chamfer would be expected (Fig. 5.8). 

In this type of weld, inevitably some stress concentration points are caused due to unwanted but real notches, which are particularly prone to initiate and propagate fatigue cracks. The presence of pressure pulses due to pumps or due to cavitation phenomena with rapid evaporation ( flashing ) tends to enhance this tendency. Some real-life cases are ... 

All corners in reinforced PPS parts should have a radius of 60% of the wall thickness. This will eliminate stress concentration points and potential sites for crack initiation, and it will also help to create smoother flow patterns during filling to reduce the potential for molded-in stresses and void formation. Reinforced PPS is a particularly notch-sensitive semicrystalline thermoplastic, so sharp inside corners create crack initiation points and must he avoided in part designs. 

The use of mechanical fasteners generally means that holes have to be drilled in the materials to be joined. This may weaken the material, provide stress concentration points, and introduce sites for corrosion. These problems can be avoided by the use of an adhesive, which thus ensures the integrity of the components. 

Stresses imposed on pump by pipe loads. Examine pipe design to determine stress concentration point. Redesign piping to eliminate loads. 

Crazing usually occurs when a number of microvoids nucleate at high stress concentration points (i.e., small cracks, air bubbles, voids). The microvoids tend to develop along the plane perpendicular to the maximum principal stress direction, and instead of coalescing to form a crack, they remain separated by fibrils of plastically deformed material, forming a craze. 

For the cements, the modulus at 1% strain varied between 3.2 and 5.6 GPa (tensile) and between 1.6 and 2.3 GPa (compression). This parameter tends to increase as the HA content is incremented. The ultimate tensile and compressive strengths (UTS and UCS) tend to increase as the HA content becomes higher. This is true up to 20% of HA (up to 16-20 MPa for UTS and 70-98 MPa for UCS, depending on the solid/liquid ratio). Above that values, both UTS and UCS tend to decrease, as the ceramic particles start to act as stress concentration points, giving rise to more brittle materials with lower UTS and UCS. When compared with commercial acrylic bone cements, it can be concluded that the developed cements show similar (or slightly lower) UTS and similar or superior UCS and modulus. 

This transfer function has enough information between the input stress waves and the stress fluctuation at the joint end that is the stress concentration point in the static case. 

Applying fracture mechanics to impact problems has other difficulties. Local finer meshes, which are often used near stress concentration points in static finite element methods, are not suitable for dynamic calculation because the step of time integral depends on the smallest mesh size in the analytical model, and total calculation time increases drastically even if only one small mesh is used. Therefore, finite element analysis for fracture mechanics, which needs finer meshes near the crack tips for calculating the stress intensity factors, consumes time and computer resources. 

---