STRENGTH OF MATERIALS 



245 



I 



piece of metal is strained to a point beyond its elastic limit 



it is permanently stretched and weakened. For this reason, it 



is never considered advisable to strain 



a chain to more than one-half the 



amount shown by the method given 



for computing the tensile strength. In 



other words, the proof test of a chain 



should be about 50% of the ultimate 



resistance of the weakest link. 



FIG. 103. 



If, for example, the tensile strength of a chain made from ^ in. 

 wrought iron is 40,000 Ibs. per square inch, the safe working strength 

 may be calculated as follows : 



Area = Diameter squared X .7854 = .5 X .5 X .7854 = .19635 



.19635 X 40,000 = 7854 

 7854 X 1.63 = 12,802 Ibs. = ultimate breaking strength 



12,802 X .50 = 6401 Ibs. = proof test, or safe working 

 . strength. 



FIG. 104. 



Questions 



STRESSES 



1. What name should be applied 

 to the stress produced at point A in 

 Fig. 103? 



2. To what stress are the legs of the table subjected in Fig. 104? 



3. To what stress is a boiler seam rivet subjected? (See Fig. 105.) 



4. What stresses are produced in the 

 main rod of a locomotive when the engine 

 is working? 



5. What stresses are produced in the 

 piston rod of a locomotive when working? 



6. To what stress are the stay bolts of a boiler subjected? 



7. To what stress are the sheets of a boiler subjected? 



8. To what stress does the blacksmith subject a piece of iron 

 when he strikes it a blow with his hammer? 



FIG. 105. 



