COMPLEX STRESS DISTRIBUTIONS IN ENGINEERING MATERIALS. 175 
Bridgman™ * worked with extremely high-fluid pressures applied to 
the curved surface of a rod of circular section, on the outside of plugged 
hollow cylinders, and to the inside of heavy cylinders. All tests were to 
rupture. Brittle materials were also tested. Little numerical data is 
given. The original paper should be consulted since it does not lend itself 
to abstraction, and the results are very remarkable. It is doubtful 
whether the deductions, that all the theories of strength are not valid under 
certain conditions, are justified by these experiments. 
9. Brittle Materials under Combined Stresses. 
Carus Wilson found the tensile strength of cast iron to be 10-4, and 
the shearing stress at fracture to be 5-46 tons per square inch, ratio 1°9. 
Platt and Hayward found the ratio to be 2:2. The mean crushing strength 
was 41°5 tons per square inch. The rupture of cast iron in compression by 
shearing is well known, and he appeared to consider that it also held for 
tension. 
Izod*? gives the ultimate shear stress of cast iron from 1:1 to 15 times 
the ultimate tensile strength. 
Scoble *! © fractured round cast-iron bars by combined bending and 
torsion. The calculated stresses followed neither law, but the angles of 
fracture agreed well with the planes of maximum principal stress. On 
the assumption of a redistribution of stress by yield the maximum princi- 
pal stress varied 10 per cent. on either side of the mean value. Hardened 
cast-steel bars were elastic to fracture. At least two tests were made on 
each bar. ‘The maximum principal stress was nearly constant at fracture 
for each bar, and the bar broke along the plane of maximum principal 
stress with extreme accuracy. 
Williams attempted to determine the effect of fluid pressure on the 
strength of rock salt and hard aluminium. He claimed to disprove the 
Poncelet Theory, but the range of the experiments was too limited to draw 
further conclusions. 
Griibler®* used cement mortar formed round a central shaft and 
covered by a clamp which carried torsion levers. The shear stress was the 
same at all points at the same distance from the axis. The cement failed 
by tension, but never by shearing. 
Kédrmén™ compressed marble and sandstone and supplied latera 
pressure by means of glycerine under pressure. He quotes Mohr’s Law 
as the shear stress law. With no side pressure these stones behave as 
brittle materials, but with a pressure of 700 atmospheres the material 
becomes perfectly plastic, and the elastic limitis raised. Further deforma- 
tion is possible after the elastic limit if the lateral pressure is increased, 
but the effect is rapidly diminished at high pressures. The stones flowed 
on planes at 45 degrees to the axis. Permanent set may take place by 
relative shearing of the crystals for low values of the lateral pressure, or by 
internal changes in the crystals at high values. The first kind of failure 
occurs at a maximum value of the shear stress which depends on the 
normal stress, but the second form takes place at a limiting constant 
shear stress, and the material hardens. 
according tofTheory (a). The results for steel are ratios depending on the tensile 
strength, and would be high unless the first yield was detected. Their cylinders 
increased in diameter at yield, Turner’s diminished. : 
