- Email: [email protected]
Reply to comments on “A two stage mechanism of ductile erosion”
406
I I.1.1.1KS
3 G. P. Tilly,
1,!I/ Z.. 3 (8)
5 J. Priemer. f-o~r.\c.l1~irthr,~i~./lt~, and I. Klcis. Ucw.
8 J. G. A. Bilter. Y G.
1llF
t~I~ITol~
b~~tw, 23 ( lY73) X7 96.
6 G. Schlaug. S,wr/x~,~io~~%wX/riww. 7 H. Uuemiii\
IO
Pommel.
zertriimmerung
It%or. 6 ( 1963)
Verlag
31 (1075)
1965.
Chemie.
Weinheim
und VDI
Verlap,
DiiwAdorf.
lY07.
359~371.
5 2 I.
StossuntersLlchl~~~~~~~ Stahlkueel-Slahlplattc und Strahlverschleiss.
Dis.wrrtrtio,l. Technical
in
Lusammcnhang
University.
Stuttgart.
mit
Strahlmittel-
IY67.
IO I. Kleis. Fr~x. Tu/liw~ /‘o/~~rr~~/~r~. /J~.sI.. 16X ( IYSY).
II
K. Wellinger
I2 I. Kleis. !4iw.
Received
and I-l. BreAcl. 13 (lY6Y)
It&w.
I?
( 1969)757
2X I.
IYY 215.
May 14, 1974
Reply to comments on “A two stage mechanism of ductile erosion”
In their comments on the two stage theory oferosion Kleis et ~11.draw attention to the fact that their values for the coefficient of restitution differ from those measured and reported earlier by Bitte?. In fact Table I shows that they decrease with speed but the speeds are unfortunately too low to be wholly relevant. Whilst the differences are significant it is still true that a large proportion of the energy prior to impact is available for the erosion process e.y. using Kleis’ value of K =0.42 gives over 80”,,. Kleis has apparently measured his coefficients on unfractured particles, however. when fracture occurs on impact some of the energy is used in projecting the particles outwards as well as backwards so that a larger fraction of intial energy may bc available for erosion than he has calculated. The comparisons made in Table I do not appear to be valid because V, is dependent on particle size and varies for different target materials. The eqns. (l-3) in no way invalidate the suggestions made in our earlier paper”. The suggestion that work hardening of the surf&e during the initial stages of erosion increases the coefficient of restitution energy is inferred in Bitter’s earlier work’. However. it is also true that during the incubation period of erosion the loss of material actually increases rather than decreases: this is the opposite of Kleis’ predictions and it is suggested that this is due to the fact that the changing of the surface topography plays a larger role in the erosion process than work hardening of the material. It is incorrectly suggested that a value off- equal to 1 for the fragmentation process. involves a large number of very small secondary particles. As defined in the earlier work such a value denotes that all impacting particles break into at least two pieces although in practice they normally shatter into many more. The sizes into which they break are of less importance than the proportion of primary particles that break. Recent work by Winters and Hutchings I3 showed that particles that break into at least two pieces are more effective in removing material than unfractured particles. In practice the fragments found experimentally are not mostly smaller than cl, as claimed by Kleis. In Fig. 5 of ref. 3 it can be seen that over 80”;, of the fragments are greater than 20 /I after impact of 135 11quartz at 1.OOOft/s. This paper also shows
LETTERS TO THE EDITOR
407
that d, is approximately 5 p for a velocity of 800 ft/s so that it is clear that the fragments are in fact mostly above this threshold value. The secondary impacts are not necessarily at glancing impact angles because they involve the removal of extruded lips of material produced during the primary process, thus many of the secondary impacts may occur at effectively normal angles due to this changing surface topography. Arguments related to a threshold particle size in primary erosion are not necessarily relevant to the secondary process because; (a) secondary erosion occurs at significantly higher velocities and (b) there is the above difference in mechanism of material removal. The two stage theory of erosion was postulated for quartz particles against ductile materials and is irrelevant to brittle materials. However, results obtained for nylon eroded by quartz, and steel eroded by diamond, glass and silicon carbon particles’, indicates that the theory probably applies to these systems as well. The results quoted by Kleis using iron pellets may well not suit the erosion theory developed for purely brittle particles. We are aware that the influence of particle size does not necessarily involve a flat erosion plateau for very large particle sizes and it was noted that with quartz above 300 p there was a further increase in the level of erosion. However, this is irrelevant to the industrial processes for which the work was conducted. Target materials such as libre-glass and glass do not exhibit a plateau at all as shown in ref. 2. It appears that Kleis has used unrealistically high erosion concentrations in his experiments e.g. in ref. 7 concentrations of 3-250 g/cm2/s have been used whereas values of 0.001-10 g/cm2/s were used in ref. 2 because the industrial process is believed to involve about 0.02 g/cm’/s. Values as high as 250 g/cm’/s are unlikely to occur in practice and such testing could probably introduce effects unrelated to the true erosion process e.g. it is possible that unrealistically high heating effects may be generated and there could be interference between impacting particles which would not otherwise occur. It is interesting to note that in our tests on nylon the target material was blackened and displaced by the centrifugal action of the erosion rig indicating that it had softened and heated during the process. Turning to the recent paper by Uuemois and Kleis’, it is stated that erosion rates are high because the hardness of the abrasive is much greater than that of the target material. In practice’ the situation is more complex and particles can erode materials of similar hardness e.g. tests using soft glass spheres (Vickers hardness 450) and irregular quartz (hardness 1100) produce similar levels of erosion when impacted against stainless steel (hardness 355). It is suggested that this behaviour is due to the nature of the secondary process.’ It should be noted that Uuemdis and Kleis’ used a relatively small range of velocities in their experiments (up to 330 m/s) so that some of their observations about the precise behaviour of the exponent m are based on restrictive data. In the discussion to a paper by Sheldonr4 it was commented that for erosion of metals, silicon carbide and quartz both produce a value of m of about 2.3 but that invariably there is a trend to a lower value (about 2.0) at the highest velocities. However, use of a best fitting straight line on a logarithmic graph will not introduce any significant error. The illustration of the impact process given in Fig. 8 is interesting in that it confirms the suggestions made in refs. 2 and 3 but it would be more convincing if the actual photographs had been shown. In conclusion we should like to thank Kleis and his colleagues for their inter-
408
LETTERS
TO THE EDITOR
esting comments. In general they have introduced fresh evidence for the two stage erosion process and drawn our attention to work which we were not aware of at the time the two stage theory was postulated. In all aspects the additional data give added support to the hypothesis. It appears that we are respectively studying different ends of the erosion spectrum. In connection with gas turbine engines we are concerned with erosion caused by relatively small particles at high velocities and low concentrations whereas Kleis appears to be concerned with erosion caused by large particles at lower velocities and high concentrations. This reflects the different practical applications of the two Groups. Such factors could produce differing relationships between the primary and secondary components of erosion. However. secondary erosion undoubtedly plays a major role under the conditions defined in our experiments and no other theories have yet been postulated that adequately predict experimental findings. G. P. Tilly Transport and Road Research Laboratory, Wendy National
Crowthorne, Berks RGlI
Sage Gas Turbine Establishment,
Pyestock,
Farnborough
1 12 As preceding paper. 13 R. E. Winter and I. M. Hutchings, Wear, 29 (2) (1974) 181-194. 14 G. L. Sheldon, J. Basic Eng., 92 D (1970) 619-626.
Received
6AU (Gt. Britain)
September
30, 1974
(Gt. Britain)
I I.1.1.1KS
3 G. P. Tilly,
1,!I/ Z.. 3 (8)
5 J. Priemer. f-o~r.\c.l1~irthr,~i~./lt~, and I. Klcis. Ucw.
8 J. G. A. Bilter. Y G.
1llF
t~I~ITol~
b~~tw, 23 ( lY73) X7 96.
6 G. Schlaug. S,wr/x~,~io~~%wX/riww. 7 H. Uuemiii\
IO
Pommel.
zertriimmerung
It%or. 6 ( 1963)
Verlag
31 (1075)
1965.
Chemie.
Weinheim
und VDI
Verlap,
DiiwAdorf.
lY07.
359~371.
5 2 I.
StossuntersLlchl~~~~~~~ Stahlkueel-Slahlplattc und Strahlverschleiss.
Dis.wrrtrtio,l. Technical
in
Lusammcnhang
University.
Stuttgart.
mit
Strahlmittel-
IY67.
IO I. Kleis. Fr~x. Tu/liw~ /‘o/~~rr~~/~r~. /J~.sI.. 16X ( IYSY).
II
K. Wellinger
I2 I. Kleis. !4iw.
Received
and I-l. BreAcl. 13 (lY6Y)
It&w.
I?
( 1969)757
2X I.
IYY 215.
May 14, 1974
Reply to comments on “A two stage mechanism of ductile erosion”
In their comments on the two stage theory oferosion Kleis et ~11.draw attention to the fact that their values for the coefficient of restitution differ from those measured and reported earlier by Bitte?. In fact Table I shows that they decrease with speed but the speeds are unfortunately too low to be wholly relevant. Whilst the differences are significant it is still true that a large proportion of the energy prior to impact is available for the erosion process e.y. using Kleis’ value of K =0.42 gives over 80”,,. Kleis has apparently measured his coefficients on unfractured particles, however. when fracture occurs on impact some of the energy is used in projecting the particles outwards as well as backwards so that a larger fraction of intial energy may bc available for erosion than he has calculated. The comparisons made in Table I do not appear to be valid because V, is dependent on particle size and varies for different target materials. The eqns. (l-3) in no way invalidate the suggestions made in our earlier paper”. The suggestion that work hardening of the surf&e during the initial stages of erosion increases the coefficient of restitution energy is inferred in Bitter’s earlier work’. However. it is also true that during the incubation period of erosion the loss of material actually increases rather than decreases: this is the opposite of Kleis’ predictions and it is suggested that this is due to the fact that the changing of the surface topography plays a larger role in the erosion process than work hardening of the material. It is incorrectly suggested that a value off- equal to 1 for the fragmentation process. involves a large number of very small secondary particles. As defined in the earlier work such a value denotes that all impacting particles break into at least two pieces although in practice they normally shatter into many more. The sizes into which they break are of less importance than the proportion of primary particles that break. Recent work by Winters and Hutchings I3 showed that particles that break into at least two pieces are more effective in removing material than unfractured particles. In practice the fragments found experimentally are not mostly smaller than cl, as claimed by Kleis. In Fig. 5 of ref. 3 it can be seen that over 80”;, of the fragments are greater than 20 /I after impact of 135 11quartz at 1.OOOft/s. This paper also shows
LETTERS TO THE EDITOR
407
that d, is approximately 5 p for a velocity of 800 ft/s so that it is clear that the fragments are in fact mostly above this threshold value. The secondary impacts are not necessarily at glancing impact angles because they involve the removal of extruded lips of material produced during the primary process, thus many of the secondary impacts may occur at effectively normal angles due to this changing surface topography. Arguments related to a threshold particle size in primary erosion are not necessarily relevant to the secondary process because; (a) secondary erosion occurs at significantly higher velocities and (b) there is the above difference in mechanism of material removal. The two stage theory of erosion was postulated for quartz particles against ductile materials and is irrelevant to brittle materials. However, results obtained for nylon eroded by quartz, and steel eroded by diamond, glass and silicon carbon particles’, indicates that the theory probably applies to these systems as well. The results quoted by Kleis using iron pellets may well not suit the erosion theory developed for purely brittle particles. We are aware that the influence of particle size does not necessarily involve a flat erosion plateau for very large particle sizes and it was noted that with quartz above 300 p there was a further increase in the level of erosion. However, this is irrelevant to the industrial processes for which the work was conducted. Target materials such as libre-glass and glass do not exhibit a plateau at all as shown in ref. 2. It appears that Kleis has used unrealistically high erosion concentrations in his experiments e.g. in ref. 7 concentrations of 3-250 g/cm2/s have been used whereas values of 0.001-10 g/cm2/s were used in ref. 2 because the industrial process is believed to involve about 0.02 g/cm’/s. Values as high as 250 g/cm’/s are unlikely to occur in practice and such testing could probably introduce effects unrelated to the true erosion process e.g. it is possible that unrealistically high heating effects may be generated and there could be interference between impacting particles which would not otherwise occur. It is interesting to note that in our tests on nylon the target material was blackened and displaced by the centrifugal action of the erosion rig indicating that it had softened and heated during the process. Turning to the recent paper by Uuemois and Kleis’, it is stated that erosion rates are high because the hardness of the abrasive is much greater than that of the target material. In practice’ the situation is more complex and particles can erode materials of similar hardness e.g. tests using soft glass spheres (Vickers hardness 450) and irregular quartz (hardness 1100) produce similar levels of erosion when impacted against stainless steel (hardness 355). It is suggested that this behaviour is due to the nature of the secondary process.’ It should be noted that Uuemdis and Kleis’ used a relatively small range of velocities in their experiments (up to 330 m/s) so that some of their observations about the precise behaviour of the exponent m are based on restrictive data. In the discussion to a paper by Sheldonr4 it was commented that for erosion of metals, silicon carbide and quartz both produce a value of m of about 2.3 but that invariably there is a trend to a lower value (about 2.0) at the highest velocities. However, use of a best fitting straight line on a logarithmic graph will not introduce any significant error. The illustration of the impact process given in Fig. 8 is interesting in that it confirms the suggestions made in refs. 2 and 3 but it would be more convincing if the actual photographs had been shown. In conclusion we should like to thank Kleis and his colleagues for their inter-
408
LETTERS
TO THE EDITOR
esting comments. In general they have introduced fresh evidence for the two stage erosion process and drawn our attention to work which we were not aware of at the time the two stage theory was postulated. In all aspects the additional data give added support to the hypothesis. It appears that we are respectively studying different ends of the erosion spectrum. In connection with gas turbine engines we are concerned with erosion caused by relatively small particles at high velocities and low concentrations whereas Kleis appears to be concerned with erosion caused by large particles at lower velocities and high concentrations. This reflects the different practical applications of the two Groups. Such factors could produce differing relationships between the primary and secondary components of erosion. However. secondary erosion undoubtedly plays a major role under the conditions defined in our experiments and no other theories have yet been postulated that adequately predict experimental findings. G. P. Tilly Transport and Road Research Laboratory, Wendy National
Crowthorne, Berks RGlI
Sage Gas Turbine Establishment,
Pyestock,
Farnborough
1 12 As preceding paper. 13 R. E. Winter and I. M. Hutchings, Wear, 29 (2) (1974) 181-194. 14 G. L. Sheldon, J. Basic Eng., 92 D (1970) 619-626.
Received
6AU (Gt. Britain)
September
30, 1974
(Gt. Britain)