PROPERTIES OF IRON AND STEEL CONSTRUCTIONS. 167 



quicker and more rapid the lower the temperature of the metal when under 

 mechanical treatment. Tensile resistance Fairbairn found to diminish 

 from 202°, where it was about the same as at ordinary temperature, a low 

 red heat, by about 17 per cent.; up to ordinary red heat, by about 34 per 

 cent. Experiments at the Franklin Institute found the ultimate tensilere- 

 sistance, at 575° lowered by 0.66, and at 700° by 0.33 from the ordinary 

 value. Bauschinger observed the strength of puddled plate, transverse to 

 the direction of rolling, to be at red heat 780 kil. (2,700 ordinary), and of 

 rolled iron along the fibres, 50 (4,430 ordinary). 



These results are of importance with respect to constructions exposed to 

 fire. Kirchweiger, of Hanover, regards the diminution of tensile strength 

 by heating as the cause of boiler explosions; attempting to prove at the 

 same time that a boiler filled with water may become red-hot. Bauschinger 

 thinks it possible that the continual variations and differences of tempera- 

 ture of the outer and inner surfaces may diminish the cohesion of the 

 laminae of the plate; tte inner laminae bearing a disproportionate share of 

 the strain, and the shearing resistance being lessened. 



A frequent theme of discussion is the influence of cold upon resistance 

 to sudden changes of stress — shocks in particular. It cannot be denied 

 that more axles and wheels break in winter than in summer. Styffe main- 

 tains that rupture is often due to the fact that the parts are held fast, and, 

 therefore, cannot yield to the contracting influence of the cold: again, for 

 tires, axles and rails, the effect of shocks is increased by the diminished 

 elasticity of the ground. 



Sandberg, in an appendix to the English translation of Styffe's work, 

 maintains that these are not the principal causes of breaking. He laid 

 iron rails upon granite supperts which lay upon granite rocks, so that the 

 elasticity of the foundations might be the same in any season. The two 

 halves of these rails were tested by blows with a 380 kil. ball at — 12° in 

 winter, and -f 29° in summer; and it was found that at — 12° the rail could 

 withstand only ^ of what it could at -f- 29°. This showed, at least, that 

 there are some kinds of iron that are weakened by frost. Styffe had tested 

 only under dead loads, and in this respect his results were trustworthy. 



Sandberg also found this peculiar result: that Aberdare rails, which 

 bore in summer 20 per cent, more strain than those from Creusot, in winter 

 had 30 per cent, less strength. This could be explained on the hypothesis 

 of a difference in constitution which affected the strength unequally. Fair- 

 bairn had already shown the unfavorable effect of phosphorus and sulphur 

 at low temperature; and Sandberg thought it possible that different results 

 would have been reached had the metal been free from phosphorus. 



Unfortunately the chemical constitution of the rails was not determined ; 

 but it seems likely, that phosphorus, which always diminishes resistance to 

 shock, may operate more actively at a low temperature. Its effect also in- 

 creases under high heat. Styffe found that the grain of a screw-bolt of 

 phosphor-iron was so affected, that a single blow of the hammer broke it. 



