§11.1 Fatigue,Creep and Fracture 449 Fig.10.5(b).Non-ferrous metal specimens show this type of curve and hence components made from aluminium,copper and nickel,etc.,must always be designed for a finite life. Another important fact to note is that the results of laboratory experiments utilising plain, polished,test pieces cannot be applied directly to structures and components without modifi- cation of the intrinsic values obtained.Allowance will have to be made for many differences between the component in its working environment and in the laboratory test such as the surface finish,size,type of loading and effect of stress concentrations.These factors will reduce the intrinsic (i.e.plain specimen)fatigue strength value thus, 听=cccl (11.6) where ow is the "modified fatigue strength"or"modified fatigue limit",oN is the intrinsic value,Kf is the fatigue strength reduction factor (see 11.1.4)and Ca,C and Ce are factors allowing for size,surface finish,type of loading,etc. The types of fatigue loading in common usage include direct stress,where the material is repeatedly loaded in its axial direction;plane bending,where the material is bent about its neutral plane;rotating bending,where the specimen is being rotated and at the same time subjected to a bending moment;torsion,where the specimen is subjected to conditions which produce reversed or fluctuating torsional stresses and,finally,combined stress conditions, where two or more of the previous types of loading are operating simultaneously.It is therefore important that the method of stressing and type of machine used to carry out the fatigue test should always be quoted. Within a fairly wide range of approximately 100 cycles/min to 6000 cycles/min,the effect of speed of testing (i.e.frequency of load cycling)on the fatigue strength of metals is small but,nevertheless,frequency may be important,particularly in polymers and other materials which show a large hysteresis loss.Test details should,therefore,always include the frequency of the stress cycle,this being chosen so as not to affect the result obtained (depending upon the material under test)the form of test piece and the type of machine used.Further details regarding fatigue testing procedure are given in BS3518:Parts 1 to 5. Most fatigue tests are carried out at room temperature but often tests are also carried out at elevated or sub-zero temperatures depending upon the expected environmental operating conditions.At low temperatures the fatigue strength of metals show no deterioration and may even show a slight improvement,however,with increase in temperature,the fatigue strength decreases as creep effects are added to those of fatigue and this is revealed by a more pronounced effect of frequency of cycling and of mean stress since creep is both stress- and time-dependent. When carrying out elevated temperature tests in air,oxidation of the sample may take place producing a condition similar to corrosion fatigue.Under the action of the cyclic stress, protective oxide films are cracked allowing further and more severe attack by the corrosive media.Thus fatigue and corrosion together ensure continuous propagation of cracks,and materials which show a definite fatigue limit at room temperature will not do so at elevated temperatures or at ambient temperatures under corrosive conditions-see Fig.11.6. 11.1.2.PiSiN curves The fatigue life of a component as determined at a particular stress level is a very variable quantity so that seemingly identical specimens may give widely differing results.This scatter$11.1 Fatigue, Creep and Fracture 449 Fig. 10.5(b). Non-ferrous metal specimens show this type of curve and hence components made from aluminium, copper and nickel, etc., must always be designed for a finite life. Another important fact to note is that the results of laboratory experiments utilising plain, polished, test pieces cannot be applied directly to structures and components without modifi￾cation of the intrinsic values obtained. Allowance will have to be made for many differences between the component in its working environment and in the laboratory test such as the surface finish, size, type of loading and effect of stress concentrations. These factors will reduce the intrinsic (i.e. plain specimen) fatigue strength value thus, (11.6) where oh is the “modified fatigue strength” or “modified fatigue limit”, ON is the intrinsic value, Kf is the fatigue strength reduction factor (see $ 11.1.4) and C, Cb and C, are factors allowing for size, surface finish, type of loading, etc. The types of fatigue loading in common usage include direct stress, where the material is repeatedly loaded in its axial direction; plane bending, where the material is bent about its neutral plane; rotating bending, where the specimen is being rotated and at the same time subjected to a bending moment; torsion, where the specimen is subjected to conditions which produce reversed or fluctuating torsional stresses and, finally, combined stress conditions, where two or more of the previous types of loading are operating simultaneously. It is therefore important that the method of stressing and type of machine used to carry out the fatigue test should always be quoted. Within a fairly wide range of approximately 100 cycles/min to 6000 cycledmin, the effect of speed of testing (i.e. frequency of load cycling) on the fatigue strength of metals is small but, nevertheless, frequency may be important, particularly in polymers and other materials which show a large hysteresis loss. Test details should, therefore, always include the frequency of the stress cycle, this being chosen so as not to affect the result obtained (depending upon the material under test) the form of test piece and the type of machine used. Further details regarding fatigue testing procedure are given in BS3518: Parts 1 to 5. Most fatigue tests are carried out at room temperature but often tests are also carried out at elevated or sub-zero temperatures depending upon the expected environmental operating conditions. At low temperatures the fatigue strength of metals show no deterioration and may even show a slight improvement, however, with increase in temperature, the fatigue strength decreases as creep effects are added to those of fatigue and this is revealed by a more pronounced effect of frequency of cycling and of mean stress since creep is both stress￾and time-dependent . When carrying out elevated temperature tests in air, oxidation of the sample may take place producing a condition similar to corrosion fatigue. Under the action of the cyclic stress, protective oxide films are cracked allowing further and more severe attack by the corrosive media. Thus fatigue and corrosion together ensure continuous propagation of cracks, and materials which show a definite fatigue limit at room temperature will not do so at elevated temperatures or at ambient temperatures under corrosive conditions - see Fig. 11.6. I I .I .2. PISIN curves The fatigue life of a component as determined at a particular stress level is a very variable quantity so that seemingly identical specimens may give widely differing results. This scatter