Compression, the stress that resists a crushing force, tends to  shorten or squeeze aircraft parts. The compressive strength of  a material is also measured in psi. Under a compressive load,  an undrilled member is stronger than an identical member  with holes drilled through it. However, if a plug of equivalent  or stronger material is fitted tightly in a drilled member, it  transfers compressive loads across the hole, and the member  carries approximately as large a load as if the hole were not  there. Thus, for compressive loads, the gross or total area may  be used in determining the stress in a member if all holes are  tightly plugged with equivalent or stronger material.  Shear is the stress that resists the force tending to cause one  layer of a material to slide over an adjacent layer. Two riveted plates in tension subject the rivets to a shearing force. Usually,  the shear strength of a material is either equal to or less than  its tensile or compressive strength. Shear stress concerns the  aviation technician chiefly from the standpoint of the rivet  and bolt applications, particularly when attaching sheet metal,  because if a rivet used in a shear application gives way, the  riveted or bolted parts are pushed sideways.  Bearing stress resists the force that the rivet or bolt places  on the hole. As a rule, the strength of the fastener should be  such that its total shear strength is approximately equal to  the total bearing strength of the sheet material.  [Figure 4-2] Torsion is the stress that produces twisting. While moving  the aircraft forward, the engine also tends to twist it to one  side, but other aircraft components hold it on course. Thus,  torsion is created. The torsional strength of a material is its  resistance to twisting or torque (twisting stress). The stresses  arising from this action are shear stresses caused by the  rotation of adjacent planes past each other around a common  involving tension must take into consideration the net area of  the member. Net area is defined as the gross area minus that  removed by drilling holes or by making other changes in the  section. Placing rivets or bolts in holes makes no appreciable  difference in added strength, as the rivets or bolts will not  transfer tensional loads across holes in which they are inserted.  Figure 4-1.  Stresses in aircraft structures.  Figure 4-2.  Bearing stress. 4-3 Compression Tension E. Bending A. Tension B. Compression C. Torsion D. Shear Rivets Top sheet is bearing against  the bottom sheet. Fasteners  are pressing top sheet against  bottom sheet. The force that  tries to pull the  two sheets apart Bearing stress Torsion  Shear Compression Bearing