Production of Stress Waves with Nanosecond Laser Pulses Jay A. Fox

U.S. Army Mobility Equipment Research & Develop-• merit Center, Fort Belvoir, Virginia 22060.

Received 13 March 1974. The production of stress waves generated by the irradia­

tion of solids with short laser pulses has been demon­strated and studied over the past few years.1-7 As has previously been discussed,8 laser generated shock waves may be used to study material properties without some of the intrinsic difficulties of conventional flyer plate tech­niques. As an example of this, the effective Grüneisen parameter of a two-dimensional quartz phenolic has been

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determined9 with the aid of laser generated stresses. Other investigators have suggested that metals may be shock processed10'11 with short laser pulses to harden these materials.

Whatever the motivation for producing these laser in­duced stresses, relatively high peak stresses must be pro­duced. It has been stated5 that Q-switched lasers pro­duce stress magnitudes of much less than 1 kbar when ir­radiating a bare target. For that reason various tech­niques of coating targets have been devised4,5 to increase the peak stress. Most, if not all, of these irradiations were conducted under vacuum conditions to prevent air breakdown from decoupling the target from the laser beam.

However, in our last communication7 we demonstrated that relatively high peak stresses (3-4 kbar) could be de­tected at the back face of 1-mm thick aluminum targets, even when the targets were uncoated and irradiated with 30-nsec pulses in the presence of atmospheric air. It was also shown that peak stress depended on laser pulse dura­tion in an inverse manner; thereby suggesting that even greater stresses were possible if sufficiently short laser pulses could be obtained. In this letter we shall show that the magnitude of the stress wave can indeed be in­creased by this means. In particular, we shall report on another set of experiments in which 1-mm thick samples of 6061-T6 aluminum were irradiated in air with 1.5-nsec, 1.06-μm laser pulses. It also will be shown that target in­duced air breakdown may not be as limiting as previously thought in inducing high peak stresses; and, further, that the magnitude of these stresses is virtually independent of the surface condition of the target.

The experimental setup was essentially the same as that described in Ref. 7. The CGE VD 640 Q-switched glass laser at Battelle Columbus Laboratories, Columbus, Ohio, was used to produce energies up to 140 J with a pulse width (FWHM—full width at half-maximum) of 1.5 nsec. (These parameters were recorded for each laser shot.) The beam was focused with a 1-m lens onto the surface of the aluminum target, which was affixed to the quartz stress gauge. In all cases the laser beam diameter on target was 0.76 cm. Since this dimension was equal to the diameter of the gauge (0.46 cm) plus three times the thickness of the 1-mm targets, the stress wave could be regarded as one-dimensional during the writing time of the gauge (~110 nsec). Fluences on target were varied by inserting calibrated neutral density filters in the path of the beam. The estimated accuracy of fluence measure­ments was ±20%. The temporal record of a typical stress

Fig. 1. A typical stress wave as recorded by a quartz gauge af­fixed to the rear surface of a 1-mm thick aluminum sample. The fluence on target was 22 J/cm2, and the laser pulse width was 1.5

nsec.

wave is shown in Fig. 1. The fluence on target was 22 J / cm2, and the maximum stress was 3.1 kbar. The actual rise time of the signal was less than the indicated 4 nsec, but this measurement was limited by the response of the recording oscilloscope. The width at half-maximum am­plitude was about 12 nsec and, as will be shown, depend­ed to some extent on the incident fluence. It is inter­esting to note that the surface condition of the target did not affect the measured stress wave. Neither the magni­tude nor the duration of the stress was changed for a given fluence whether the target surface was polished to a mir­ror finish, sandpapered, or previously impacted by many laser pulses. This behavior is consistent with the results reported by Basov et al.,12 which show that the reflectivi­ty of metals irradiated with these high intensities sharply decreases to values approximately 0.1 of the normal value. This suggests that the original reflectivity of a surface may be unimportant in couping in high intensity laser ra­diation.

The dependence of peak stress on fluence is illustrated in Fig. 2. The magnitude of these stresses has been cor­rected for the slight acoustic impedance mismatch be­tween the aluminum targets and the quartz gauges and represents the stress in the target at the rear face. Peak stresses as great as 7 kbar were observed. This magni­tude is the largest rear surface stress reported for an un-coated target in air and is comparable with that generated in vacuum with the aid of transparent surface overlays. For comparison, a curve representing previously re­ported7 data (using 30-nsec laser pulses) is also presented on the same graph. It should be noted that the use of the shorter (1.5-nsec) laser pulse caused an increase of more than a factor of two in the stress level, a result predicted by our previously reported data.7 For large values of fluence, the peak stress in the 1.5-nsec case reached a constant value but did not decrease as in the 30-nsec case. The decrease had been previously attributed to plasma shielding caused by target induced air breakdown. This hypothesis agreed with a report13 of air breakdown in the presence of aluminum targets irradiated with 30-nsec 1.06-μm pulses at fluences of more than 150 J /cm 2 .

Although evidence of breakdown was again observed with the 1.5-nsec pulses at fluences as low as 40 J/cm2 , similar decreases in peak stresses were not measured. Apparently the shorter extent of the 1.5-nsec pulse serves to at least partially negate the decoupling effect of the breakdown; i.e., even if breakdowns occur, by the time the plasmas can expand to cut off the rest of the pulse most of the laser energy has already coupled into the tar­get.

If it is true that the amount of laser energy reaching a target surface is not limited by target induced breakdown plasmas, then more impulse should be transferred to the target even at fluences above those necessary to initiate the plasma. Such a phenomenon has at least been indi­rectly observed. First of all, the peak stress continues to rise even for fluences much greater than the threshold for target induced air breakdown (40 J /cm 2 ) . Second, as seen from Fig. 3, the impulse imparted to the target also increases. (Here the impulse has been estimated by mea­suring the area under stress-time curves,14 and therefore does not include late-time effects such as blast waves and /or tranverse stresses.) Another feature of this graph is that more impulse is being transferred to the target even at fluence levels at which the peak stress has reached a nearly constant value. For example, although the peak stress only increases by 5% when the fluence changes from 111 J /cm 2 to 160 J /cm 2 , the impulse increases by 25%. This behavior is consistent with the broadening of plastic

Fig. 2. The dependence of stress on fluence. Both the results of this experiment performed with a 1.5-nsec laser pulse and a pre­viously reported7 set using a 30-nsec laser pulse are shown. The

30-nsec data points have been omitted for clarity.

Fig. 3. The variation of impulse transmitted to the target as a function of incident fluence. The impulse is expressed in arbi­trary units and has been estimated using the integral of the

stress waves as they traverse a target. Consider two such waves formed by the action of different fluences acting on the target. The wave created by the greater fluence can decay to nominally the same value of peak stress as the other wave (i.e., the elastic limit) but still impart more impulse because it has broadened to a greater extent and thus increased the area under the stress-time curve. In fact, it is experimentally observed that such a broadening does take place, and that the half-width of these laser in­duced stress waves does increase from 12 nsec at 20 J /cm 2

to 24 nsec at 168 J /cm 2 . This would seem to provide a reasonable explanation for the apparently conflicting high fluence behaviors of the (1.5-nsec) stress curve of Fig. 2 and the impulse curve of Fig. 3.

The details of the preceding qualitative discussion have been verified by calculations performed with the aid of the Pisces15 1 DL finite-difference Lagrangian code. A 1-mm thick sample of 6061-T6 aluminum target was spec­ified and a polynomial equation of state with a von Mises yield model was used. It was found that a steep-fronted triangular-shaped shock of 12-nsec width (FWHM) would decay into a 7-kbar elastic stress of varying temporal ex­tent, depending on the magnitude of the incident shock.

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stress-time curve.

To double the width of the rear face elastic stress to 24 nsec, an input of 21-kbar stress must be assumed.

It should be mentioned that, while this argument is consistent with the data, it is by no means the only possi­ble explanation of the apparent increase in impulse at high fluences. For example, it has been pointed out by the reviewer that breakdown in the air at the surface, ablation by the fireball, and shock loading by the blast wave can also provide a mechanism for impulse augmen­tation.

It is also not certain that shielding effects will not pre­dominate at higher fluences. What has been shown, how­ever, is that peak stresses of comparable magnitudes to those previously attained with the aid of surface coatings and vacuum conditions can be produced with the aid of 1.5-nsec laser pulses in an atmospheric air environment. It is suggested that shorter laser pulses may be even more successful in producing higher peak stresses, al­though the utility of such short duration stresses is open to questions.

The author wishes to express his appreciation to J. W. Beal, B. E. Campbell, and B. D. Trott of Battelle Colum­bus Laboratories for their aid during this project.

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(1968). 3. P. S. Peercy, E. D. Jones, J. C. Bushnell, and G. W. Gobeli,

Appl. Phys. Lett. 16, 120 (1970). 4. N. C. Anderholm, Appl. Phys. Lett. 16, 113 (1970). 5. J. D. O'Keefe and C. H. Skeen, Appl. Phys. Lett. 21, 464

(1972). 6. J. D. O'Keefe, C. H. Skeen, and C. M. York, J. Appl. Phys.

44,4622(1973). 7. J. A. Fox and D. N. Barr, Appl. Opt. 12, 2547 (1973). 8. A. J. Palmer and J. F. Asmus, Appl. Opt. 9, 227 (1969). 9. N. C. Anderhold and R. R. Boade, Appl. Phys. 43, 434 (1972).

10. B. P. Fairand, B. A. Wilcox, W. J. Gallagher, and D. N. Wil­liams, J. Appl. Phys. 43, 3893 (1972).

11. M. Siegrist, B. Adam, and F. Kneubühl, Phys. Lett. 42A, 352 (1973).

12. N. G. Basov, V. A. Boiko, O. N. Krokhin, O. G. Semenov, and G. V. Sklizkov, Sov. Phys. Tech. Phys. 13, 1581 (1969).

13. J. A. Fox and D. N. Barr, Appl. Phys. Lett. 22, 94 (1973). 14. Only areas under curves corresponding to clear, unambiguous

oscilloscope traces were measured. 15. The Pisces code is a product of Physics International Co.,

San Leandro, California.

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