288 



STKKN<;TH OF AIATKKIALS 



Knots of any kind near the central portion <>t' the beam may determine 

 the fracture and cause the beam to break off almost squarely. N> 

 law has yet been determined which will give the effect of knots >i 



various sizes on the strength 

 of timber. 



Some characteristic failures 

 in flexure are shown in i 

 165. The lower beam si i >\v - ; i 

 normal failure on the tension 

 side. The two upper beams 

 show the fracture of a some- 

 what more brittle material, the 

 fracture bring intlueneeil by 

 the prrsrmv of knots. The 

 upper beam also shows a eom- 

 -im failure. 



|., ( , ),;:, Problem 188. A reetan.i:ular 



l>ine l>eam. width l.-IH in., hei-ht 



I.'.W in., ;in<l span :><> in., was tested in nYxure l>v In in_- .supported at l>oth en>i 

 loaded in the iniddlc, and tin- following data obtaim-d. Draw the .strain diagram, 

 plotting leads in pounds as ordinates ami d-tl-rti..n in inches as abscissa-. I 

 the elastic limit and ccinpute the liber stn-ss <.n the ..utrr lil.n at the elastic limit. 

 Also compute the modulus of rupture, the modulus of i-Ia>tiriy. and the modulus 

 of elastic ivsilit II.T. 



240. Shearing tests. The shearing strength of timber parallel to 

 the grain is usually measured by finding the force necessary to cause 

 a small projecting block of the material to shear off along the gram. 



