Defect, stress concentration around

Stress optical analysis for stress concentrations around moulding defects Stress/creep relaxation Adhesion studies... 

Various examples of using these materials for surface strain mapping have been presented in a recent publication [69]. The determination of stress concentrations around defects such as holes or notches in a deformed plate of the copolyurethane is shown in Figure 8.11. A circular hole and a notch of predetermined dimensions were accurately machined into a 3 mm thick specimen of the copolymer. The specimen was deformed in tension in the Raman spec-... 

A technique generally applied to characterize and prevent the capping and lamination of a material intended to be compacted is using the brittle fracture index (BFI). The BFI was designed by Hiestand et al. [31] and measures the ability of a material to relieve stress by plastic deformation around a defect. It is obtained by applying Equation (8) and compares the tensile strength of a tablet with a hole in its center (To), which acts as a built-in stress concentrator defect, with the tensile strength of a similar tablet without a hole (T), both at the same relative density ... 

If you ve ever taken any mechanics classes you probably recall that a crack acts as a stress concentrator. In a hypothetical flawless material the lines of stress are uniformly spaced out and a load is evenly borne by all the atoms or molecules in the object But the presence of a hole or a crack requires the stress to go around the opening (Figure 13-31). The stress concentration depends upon the size and shape of the defect. Ing-lis calculated the stress concentration factor for an elliptical hole to be given by Equation 13-23 ... 

The report of the inquiry [111] criticised the design and fabrication of the alterations made to the original pontoon. The actual cause of the accident was the failure of some tie bars in the detail around the jacking points. The failure was due to brittle fracture which initiated from severe notches such as a small radius curve at the fillet between the spade end and the shank of the tie bar. Weld defects and fatigue cracks were also present in tie bars subsequently recovered from the sea bed. The tie bars had been flame cut to shape and had weld repairs visible to the eye. There had been no post welding heat treatment of the steel. The steel complied with the original specification but tests showed low Charpy V notch impact values. Photo elastic tests indicated a stress concentration factor of 7 at the fillet between the spade end and the shank. The fracture was initiated in the opinion of the inquiry tribunal by the low ambient temperature of around 3°C. 

Structures constructed from alloys that exhibit this ductile-to-brittle behavior should be used only at temperatures above the transition temperature to avoid brittle and catastrophic failure. Classic examples of this type of failure were discussed in the case study found in Chapter 1. During World War II, a number of welded transport ships away from combat suddenly split in half. The vessels were constructed of a steel alloy that possessed adequate toughness according to room-temperature tensile tests. The brittle fractures occurred at relatively low ambient temperatures, at about 4°C (40°F), in the vicinity of the transition temperature of the alloy. Each fracture crack originated at some point of stress concentration, probably a sharp corner or fabrication defect, and then propagated around the entire girth of the ship. 

Nakagawa et al. (1999) studied theeffect ofvoids in butt joints subjected to thermal stresses and found that stresses around defects in the center of the joint are more significant than those near the free surface of the adhesive. Lang and Mallick (1999) and de Moura et al. (2006) studied the influence of gaps in the adhesive and found that a gap in the middle of the overlap has little effect on joint strength. That is because the stress is more concentrated at the ends of the overlap. However, if a ductile adhesive that makes use of the whole overlap is used, then a joint strength is expected. 

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