344 CHAPTER 10 Materials
Incompressiontestsonaluminumanditsductilealloys,similardifficultiesareencounteredtothose
experiencedwithmildsteel.Thestress–straincurveisverysimilarintheelasticrangetothatobtained
inatensiletest,buttheultimatestrengthincompressioncannotbedetermined;indesignitsvalueis
assumedtocoincidewiththatintension.
Aluminumanditsalloyscansufferaformofcorrosionparticularlyinthesalt-ladenatmosphereof
coastalregions.Thesurfacebecomespittedandcoveredbyawhite,furrydeposit.Thiscanbeprevented
byanelectrolyticprocesscalledanodizing,whichcoversthesurfacewithaninertcoating.Aluminum
alloyswillalsocorrodeiftheyareplacedindirectcontactwithothermetals,suchassteel.Toprevent
this,plasticisinsertedbetweenthepossibleareasofcontact.
Brittle Materials
These include cast iron, high-strength steel, concrete, timber, ceramics, glass, and so on. The plastic
rangeforbrittlematerialsextendstoonlysmallvaluesofstrain.Atypicalstress–straincurveforabrittle
materialundertensionisshowninFig.10.15.Littleornoyieldingoccurs,andfracturetakesplacevery
shortlyaftertheelasticlimitisreached.
Thefractureofacylindricaltestpiecetakestheformofasinglefailureplaneapproximatelyper-
pendiculartothedirectionofloadingwithnovisible“necking”andanelongationoftheorderof2to
3percent.
Incompression,thestress–straincurveforabrittlematerialisverysimilartothatintensionexcept
that failure occurs at a much higher value of stress; for concrete, the ratio is of the order of 10:1.
This is thought to be due to the presence of microscopic cracks in the material, giving rise to high
stress concentrations which are more likely to have a greater effect in reducing tensile strength than
compressivestrength.
Composites
Fiber composites have stress–strain characteristics which indicate that they are brittle materials
(Fig. 10.16). There is little or no plasticity, and the modulus of elasticity is less than that of steel
Fig.10.15
Stress–strain curve for a brittle material.