intersection with the curve of a line that is 

 parallel to the straight-line portion of the curve 

 and intersects the 0-stress axis at a strain equal 

 to a specified offset. Offset is usually specified 

 as 0.27o. Where stress-strain behavior of a materi- 

 al is knovm. Yield Strength may be given as the 

 stress corresponding to a specified strain, a 

 quantity that can be determined by direct measure- 

 ment and without a Stress-Strain Diagram. This 

 method is used primarily in determining Tensile 

 Yield Strength of copper and copper alloys. Speci- 

 fied deformation is 0.5% extension under load which 

 corresponds to an offset of about 0.35%. Without 

 a Stress-Strain Diagram, however. Yield Strength 

 at a specified offset cannot be compared with Yield 

 Strength at a specified strain under load with any 

 degree of certainty. Yield Strength may also be 

 determined as the stress required to produce a 

 specified Permanent Set, but this method involves 

 trial-and -error procedure and is seldom used. 

 Depending on type of loading involved. Yield Strength 

 may be known as Compressive Yield Strength, Flexural 



Yield Strength, Shear Yield Strength, Tensile Yield 

 Strength or Torsional Yield Strength. Yield 

 Strength, alone, is generally assumed to refer to 

 Tensile Yield Strength. 



YIELD STRENGTH ELONGATION . A measure of Ductility 

 of a material. The strain or Elongation correspond- 

 ing to the Yield Strength. 



YOUNG'S MODULUS . The modulus of elasticity of a 

 material or the quotient obtained by dividing the 

 stress per square inch by the elongation in one 

 inch caused by this stress. 



"Z" NUMBER. The atomic number of an element. 



ZOOSPORE . A motile, usually naked and flagellated 

 asexual spore (as of an alga). (20) 



ZYGOTE . A fertilized egg resulting from the union 

 of two gametes of opposite king, ovum and sperm. (19) 



131 



