Investigation of Brittle Fracture in Steel by

Means of Ultra High Speed Photography

J. G. A. de Graaf

The propagation of brittle fracture in steel was studied with the aid of ultra high-speed photographyusing an AVCO streak camera and a Barr and Stroud rotating mirror-framing camera. Also, the dy-namic stress pattern caused by the running crack was investigated with the aid of a photostress sheet gluedonto the surface of the steel plate, using the framing camera. From the experiments it was found thatbrittle fracture in steel propagates in an intermittent way. The apparatus and the optical methods aredescribed and the striking results are given.

1. Introduction

Brittle fracture in metals has been studied extensivelyby many investigators, but although ample practicalknowledge has been obtained, the actual mechanism ofinitiation and propagation of a brittle fracture in steelis still not wholly understood.

In 1961 Sleeswijk and Verbraakl. 2 observed frommetallographic investigations that the distribution ofmechanical twins along a brittle fracture in steel wassimilar to the distribution of mechanical twins inducedby explosive deformation. It was therefore concludedthat a running brittle fracture created stress waves.A detailed study of the dynamical brittle fracture prob-lem in the Metal Research Institute T.N.O. wasthereupon initiated by Verbraak.

With electronic means, e.g., strain gauges glued onthe test plate which is fractured, it is only possible tofind average velocities of fracture propagation; thedetailed propagation, however, cannot be studied inthis way. Therefore, the experimental observationthere was done by the method of high-speed photo-graphy. As the relevant velocities are 1-2 mm/,usecfor the fracture and about 5 mm/,usec for the emittedstress waves the application of ultra high-speed cinema-tography was necessary.

The high-speed photography part of the investigationwas carried out by the author and his co-workers in thesection for high-speed photography of the CentralTechnical Institute T.N.O. in close cooperation with ateam from the Metal Research Institute T.N.O.

A preliminary account of the work was given byVerbraak and de Graaf3 at the 6th International Con-gress on High-Speed Photography. In the present

The author is with the Central Technical Institute T.N.O.,The Hague, The Netherlands.

Received 3 March 1964.

paper a complete survey will be given of the methodsof high-speed photography which were used for thisresearch, followed by some of the most striking results.For a more detailed discussion of the conclusions withrespect to the dynamical aspects of brittle fracture,reference is made to a recent paper of van Elst.4

The research was carried out under contract withEuratom and the U.S. Joint R and D Board.

II. Principle of MeasurementAll experiments were performed on brittle fracture in

steel plates 30 cm X 35 cm and thickness 22 mm undertensile stress of 12 kg/mm 2 transverse to the antici-pated fracture, in a Robertson apparatus (Fig. 1).

Test temperatures down to -40'C could be reachedby cooling the steel plates with solid carbon dioxide.The fracture was initiated by the impact of a steel pinfrom a bolt gun, the neighborhood of the initiationregion of the fracture always being cooled with liquidnitrogen.

The ultra high-speed photographic recording of theevent was done partly by means of a streak camera, andpartly by means of a rotating mirror-framing camera.

First a continuous recording of the fracture propaga-tion was made in order to obtain an insight into thedetailed velocity behavior as occurring at differenttemperatures of the test plate. This continuous re-cording of the running fracture was done by means ofthe streak camera.

These experiments showed that, if the test tempera-ture was not too far below the temperature of the brittle-ductilefracture transition, the propagation was stepwise.The length of the steps was "large" enough to be re-corded with the framing camera with sufficient detailat framing rates of 1-2 million frames per second.

When the fracture was very brittle, i.e., test tempera-ture far below the brittle-ductile transition temperature,

November 1964 / Vol. 3, No. 11 / APPLIED OPTICS 1223

During the experiments a number of single pictureswere also made by means of the open shutter methodusing a light flash of about 1-/isec duration.

The optical methods used for the investigations willnow be described in detail.

Ill. Equipment and Dark-Field Techniques

A. Streak CameraFor the continuous recording of the fracture propaga-

tion a continuous access streak camera was used, manu-factured by AVCO, Type MC-300-2. This camera hasa rotating mirror which has the form of a hexagonalprism, driven by an air turbine. The maximum writingspeed is 3.9 mm/,4sec at 3000 rps of the rotating mirror.The effective aperture is f/4.5 to f/7. The mirrorspeed, i.e., the writing speed, was read on a Beckman-Berkeley electronic timer, using the magnetic pickupof the camera.

The main difficulties in working with a streak camerafor recording the velocity of fracture propagation werehow to prevent rewriting of the streak camera on thesame photographic plate and how to illuminate thefracture, the background having to remain absolutelydark. These problems were solved by using a rectan-gular flash light source and dark-field illumination,respectively.

The first-mentioned light source was especiallydeveloped for the purpose by Frtingel.*

It consisted of an electronic xenon flashlamp of65-Wsec energy and flash times of 50, 100, 150, and200 gsec, which produced an adequate rectangularintensity vs time curve (Fig. 2).

Fig. 1. Scheme of the experimental arrangement in a Robertsonapparatus. (a) General layout. (b) Initiation region for ex-periments when the fracture was started by a bolt-gun. (c)Initiation region for experiments when the fracture was started

by a detonator.

the steps were so small that only the streak cameracould be used.

In other cases both the streak and the framing camerawere used. Valuable information was obtained bycomparing the streak pictures with the pictures of theframing camera, and in this way a number of charac-teristics of the brittle fracture could be explained.

In most of the later experiments, however, when itwas known from earlier experiments that the steplength which could be expected was large enough, onlythe framing camera was used, as it was far simpler toevaluate the pictures obtained with this camera thanthe streak pictures.

The stress pattern around the moving fracture wasalso recorded because, according to Verbraak's con-ception, the stress waves are also an important factor inthe mechanism of brittle fracture. This part of theinvestigation was performed by means of optical meth-ods using photostress sheet glued onto the steel plateand photographic recording with a rotating mirror-framing camera.

Fig. 2. Intensity vs time curve of the rectangular flash.

This way of illuminating the object made it possibleto work without a camera shutter by setting the flashduration to a little less than /6 of the time of one revolu-tion of the rotating mirror. The light of this elec-tronic flashlamp was focused on the test plate withthe aid of a cylindrical mirror in the region where frac-ture was expected. The optical axis of the camerawas normal to the test plate and the light source wasset at an angle of 300 to the plate. The test plate washighly polished, and because of the oblique incidentlight the plate remained a dark field for the camerauntil the fracture occurred.

* Impulsphysics F. Frcngel, Inc., Hamburg-Rissen, Germany.

1224 APPLIED OPTICS / Vol. 3, No. 11 / November 1964

35 cm

PLANE PARALLEL

90 TON

HIGHLY POLISHED SURFACE

Fig. 3. Optical arrangement for the streak camera experiments.

The running fracture then became luminous becausethe shear lips and cleavage facets of the fracture causea scattering of the incident light into the camera. Goodcontrast was obtained even when the fracture wasnearly completely brittle and practically no shear lipswere present. As this type of streak camera is con-tinuous writing, no synchronization is needed of therotating mirror with the event, only the flash has to betriggered in such a way that the test plate is illuminatedwhen the running fracture passes through the field ofview of the camera. This synchronization was achievedwith the aid of an electrical contact which was closedby the bolt gun used for the initiation of the fracture.Of course, this method of synchronization requiredsome experience about initiation time and fracturevelocity of the various steels at the test temperatureused, but this caused no serious difficulties.

In the experiments at low temperature trouble wasexperienced from the formation of frost on the plate.To overcome this difficulty a special box was con-structed through which dry nitrogen was passed, whichmade it possible to work with temperatures of the testplate down to -40C. A drawing of the setup forthe streak photographic method is given in Fig. 3.

The streak photographs were made on sheet filmGevachrome 32 (22'DIN) 7 cm X 20 cm and developedin Agfa Eikonal 1:5. In general, the pictures were ofreally good quality, and it was possible to deduce fromthem with reasonable accuracy the average velocity ofthe fracture as well as the length and duration of thesteps. Some of the results are given in Sec. IV.

B. Rotating Mirror-Framing Camera

For the cinematographic recording of the runningbrittle fracture a rotating mirror-framing camera wasused, manufactured by Barr and Stroud, Type CP5903, and described by Skinner.' This camera gives asequence of 30 pictures of 19-mm diam and a maximum

framing frequency of about 2 X 106 frames/sec at themaximum rotating speed of 5500 rps of the plane two-sided mirror.

In contrast to the streak camera this type of framingcamera is not continuous writing; the event has there-fore to be synchronized with the position of the mirror.The best way to achieve this synchronization is to startthe event from a signal of the camera when the mirror isin a well-defined position. A synchronization unitwhich makes this possible is delivered as a standard itemfor the camera by the manufacturer.

The illumination of the test plate was performed bymeans of an electronic flash light source, developed forthis purpose by John Hadland Ltd. * This light sourceprovided for light flashes of 150-Wsec energy and flashdurations of 20, 50, or 100 usec. The light from theMAZDA-FA5 xenon flash light source was reflected bya special mirror of 30-cm diam, and concentrated on theanticipated fracture path via an auxiliary cylindricallens.

BARR & STROUDFRAMING CAMERA

Fig. 4. Optical arrangement for the framing camera experi-ments.

The fracture was initiated by a Star 8 ICI detonatorwhich was exploded at the desired moment by means ofan electric pulse of 4 kV, using the triggering signal fromthe camera via an electronic retarder and high voltagegenerator. The flash also was triggered from the cam-era with a preset delay and flash duration such thatthe fracture path was illuminated when the fracture ranthrough the field of view of the camera. The bestcontrast was obtained when the optical arrangementwas analogous to that described for the streak camerarecordings, i.e., with polished surface of the steel plates(Fig. 4). (In contrast to the streak arrangement,however, the surface of the steel plates need not be

* John Hadland Ltd., Kings Langley, Herts., England.

November 1964 / Vol. 3, No. 11 / APPLIED OPTICS 1225

polished to optical quality.) However, for the ob-servation of plastic deformation preceding the runningfracture the steel plates were not polished, thoughthe same optical system was used.

The film material used for the experiments wasIlford HP 3 (27 DIN) 35 mm, which was developed inEikonal 1:20.

Good results were obtained with an aperture of theobjective lens (Ennalyt tele 125 mm) set at f/2.5,the effective aperture of the whole optical system of theframing camera then being about f/18.

C. Framing Camera. Recording of theDynamic Stress Pattern

For the recording of the dynamic stress pattern theBarr and Stroud framing camera was used in experi-ments in which photostress sheet was glued onto thesteel plates. The principle of this method is describedin detail by Cole et al.6

The sheet of photostress material had a thicknessof 0.305 cm and an optical strain coefficient of 0.083.For measuring the isoclinics Polaroid HN 32 polarizingfilters were used; and for recording the isochromaticsthe Polaroid polarizing filters combined with 0.25wavelength plate HNCP 37. It proved impossible touse the synchronization as described in Sec. III forthe reason that the detonation used for the initiationof the fracture caused false stress waves in the photo-stress material. The event therefore could not bestarted from a signal of the framing camera. Thismeans that the initiation of the fracture had to beeffected with the bolt gun already mentioned in Sec.IIIA for the streak photographic recordings.

The experiments were now carried out as follows.The flash duration was set equal to the time of half arevolution of the rotating mirror of the camera, whichwas focused on a relatively large part of the expectedfracture path. As the actual recording of the framingcamera takes place during one-sixth of half the revolu-tion time of the mirror, the fracture was recorded duringonly one-sixth of its passage across the total field ofview of the camera. The exact position of the fractureduring the recording was not a priori known as it wasdependent on the position of the mirror when the frac-ture entered the field of view.

This manner of recording, synchronized only withthe initiation of the fracture but not with the mirrorposition, proved to be no serious disadvantage as al-ways a fracture path of about 3 cm could be studied,which was sufficient for this purpose. The stresspattern during at least one step of the propagatingfracture could bestudied in detail and, furthermore, thestress waves caused by the stepwise fracture propaga-tion could be recorded when traveling through thefield of view of the camera.

As a rule the dynamic stress pattern was recordedon color film. It was impossible to illuminate theobject adequately for recording the stress pattern withthe earlier described optical arrangements, as thepolarizers caused a considerable loss of light. After

some experimenting a method was devised which gavevery good results. The principle of this method isshown in Fig. 5.

A reflective surface was applied between photostressand test plate by coating the side of the photostressto be glued to the test plate with an aluminium layer.The light incident from the 150-Wsec flash lightsource was reflected from this reflecting layer directlyinto the camera. The crossed polarizers caused an ex-tinction of the direct light, only the interference fringestherefore were recorded on the film.

With this method of dark-field illumination, excellentrecordings could be made on high-speed Ektachrome(23 DIN) or Gevacolor Mask N5 (17 DIN) color film.For a framing frequency of 1 million frames/sec thefront lens of the framing camera (Dallmeyer, focaldistance 51 mm, maximum aperture f/i.9) could be setat f/8 or f/4, respectively; so with this optical systemthe available amount of light was ample for the purpose.

D. Still Pictures

In the course of the research also a number of singlepictures were taken to study some details of the fracturepropagation. This was done by means of the openshutter method using a Linhof Technica camera.The light flashes were of Wsec and I-ysec durationfrom a Frilngel Strobokin synchronized with the frac-ture.

Both black-and-white and color pictures could betaken without difficulty using the optical arrangementof Fig. 4. The black-and-white pictures were made onGevachrome 32 (23 DIN) developed as usual, and theflashlamp filled with argon gas. The color pictures

BARR & STROUDFRAMING CAMERA

ANALYZER _-

Fig. 5. Optical arrangement for the recording of the stresspattern.

1226 APPLIED OPTICS / Vol. 3, No. 11 / November 1964

Table I. Temperature and Fracture Propagation Parameters

Maximum AverageTest Maximum halting fracture

temperature step length time velocity(IC) (mm) (jusec) (mm/,usec)

21 30 20 1.4

10 9 7 1.8

0 4.5 3 1.5

-10 3.3 1.8 1.6-20 2 1 1.4

-35 2 1 1.6

(b)

(c)

Fig. 6. Some examples of streak photographs of the propagatingfracture. (a) Fracture nearly 100% brittle. (b) Fracture at

temperature appreciably below transition temperature. (c)

Fracture at temperature slightly below transition temperature.

were made on Gevacolor-N5 (17 DIN) with forceddevelopment, and the flashlamp filled with krypton gas,which gives twice as much light as argon, the spectrumof the light being moreover better adapted to photo-graphy in color.

IV. Observations

Some streak recordings of the running fracture in alow carbon steel are reproduced in Fig. 6. The striatedappearance of the picture is caused by the fact that thecleavage facets and shear lips along the fracture pathshow a different light scattering behavior.

From the slope of the contour and the known writingspeed of the streak camera the average fracture velocitycan easily be deduced.

From measurements at different temperatures itcould be concluded that the average velocity of thebrittle fracture does not depend on the temperature,being always of the order of 1-2 mm/gsec. Thelength of the steps and the halting time between suc-cessive steps are temperature-dependent, however.

To illustrate these effects some of the results aresummarized in Table I.

The streak photographs, however, although givingample information about step length, halting time, and

As can be seen, both the step length and the halting time de-crease with decreasing temperature. If the fracture is very brittlethe steps are so small that they cannot be observed anymore.

fracture velocity, give no information about the actualmechanism of fracture propagation, e.g., plastic flowphenomena or microcracks. The framing camerarecordings gave more information about the pictures.

When the observations of the stepwise propagation ofthe brittle fracture could be carried out with sufficient ac-curacy with the framing camera, i.e., when the haltingtimes were longer than about 2 Musec, the results obtainedwith the streak camera were completely confirmed by theframing camera recordings. A typical result, obtainedwith the dark-field illumination according to Fig. 4 isshown in Fig. 7. From this sequence of 30 framing cam-era pictures, taken with a frequency of about 1 millionframes/sec, it can be observed that, in a time intervalshorter than the time between two successive frames,a region of previously poor contrast suddenly becomesnoticeable ahead of the main fracture, e.g., in picture 18.In the pictures 19-23 neither the main fracture nor thenew steps proceed; the contrast of the new step, how-ever, gradually becomes more marked, until in picture22 it has become part of the main fracture. In thefollowing pictures the same development can be ob-served.

This mode of step realization is due to plastic flowin front of the main fracture, which can also clearly beseen in Fig. 8, probably in combination with the crea-tion of subsurface cracks which gradually break throughto the surface.

As was remarked earlier the propagation of the stresswaves is also important for elucidating the mechanismof the stepwise propagation.

An example of a sequence of pictures of the stresspattern recorded with the aid of the method describedin Sec. IIIC, is given in Fig. 9.

From this and similar recordings it should be noticedthat the emitted stress waves separate from the tip ofthe fracture and then proceed in the steel plate with avelocity of about 5 mm/ssec, i.e., the sound velocity,as could be expected. The distance between the suc-cessive emitted stress waves in all cases correspondedto a time which was in good agreement with the haltingtime between the steps, measured with the methodsmentioned earlier.

November 1964 / Vol. 3, No. 11 / APPLIED OPTICS 1227

(a)

30

- - 25

~ i 20

A l B 15

Fig. 7. Sequence of 30 pictures from a framing camera recording.

For further theoretical considerations about thisbehavior of the stresses, reference is again made tovan Elst.4

V. Conclusion

The results of this work on brittle fracture clearlyshow that the application of the method of high-speedphotography gives us a powerful tool to attack problemswhich otherwise cannot well be solved.

It is demonstrated that it is very well possible touse this method not only for the observation of fracturepropagation in transparent materials but also for thestudy of fracture in nontransparent materials if dark-field illumination is used.

It is also possible to use ultra high-speed cinema-tography to record the stress pattern on color film

Fig. 8. Single picture of the running fracture, time of exposure1 //Asec.

without the need of very high-intensity illumination ifthe dark-field technique is used as described in thispaper.

This investigation is part of work performed under acontract for Euratom/U.S. Agreement for Cooperation.The sponsor's kind permission to publish these results isgratefully acknowledged.

Our thanks are further due to C. A. Verbraak.Director of the M/letal Research Institute T.N.Owho originated this project; to H. C. van Elst and hisco-workers at the letal Research Institute T.N.O. andthe staff of the high-speed photography section of theCentral Technical Institute T.N.O., who actuallycarried out the theoretical and experimental researchon brittle fracture, of which the high-speed photo-graphic method described in this paper formed anessential part.

1228 APPLIED OPTICS / Vol. 3, No. 11 / November 1964

(a)

(b)

Fig. 9. Framing camera recording of (a) the stress pattern at the tip of a running fracture; (b) stress waves propagating ahead ofthe running fracture.

References1. A. W. Sleeswijk and C. A. Verbraak, Acta Met. 9,917 (1961).

2. C. A. Verbraak, Materialpruefung 3, 383 (1961).

3. C. A. Verbraak and J. G. A. de Graaf, Proc. 6th Internatl.

Congr. on High Speed Photography (Tjeenk Willink,Haarlam, Netherlands, 1963), p. 529.

4. H. C. van Elst, Trans. Metal. Soc. AIME 230, 460 (1964).

5. A. Skinner, J. Sci. Instr. 39, 336 (1962).

6. C. A. Cole Jr., J. F. Quinlan, and F. Zandman, Proc. 5thInternatl. Congr. on High Speed Photography (Soc.Motion Picture Television Engrs., New York, N.Y., 1962), p.250.

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