Low velocity impact

The behavior of materials under dynamic load is of considerable importance and interest in most mechanical analyses of design problems where these loads exist. The complex workings of the dynamic behavior problem can best be appreciated by summarizing the range of interactions of dynamic loads that exist for all the different types of materials. Dynamic loads involve the interactions of creep and relaxation loads, vibratory and transient fatigue loads, low-velocity impacts measurable sometimes in milliseconds, high-velocity impacts measurable in microseconds, and hypervelocity impacts as summarized in Fig. 2-4. 

Ulven, C. A. and Vaidya, U. K. Post-fire low velocity impact response of marine grade sandwich composites, Composites Part A 2006, 37, 997-1004. 

We saw in the previous section that Model I is insensitive to initiation. In particular, we reviewed results showing that, when the detonation is caused by flyer plate impact, the plate velocity required for initiation decreases to a limiting value of about 3.9 km/s as the mass of the flyer plate is increased. This behavior is consistent with the known insensitivity of crystalline explosives to high velocity impacts. In this section the initiation characteristics of Model I are examined more closely in the vicinity of the threshold for initiation. We find that the detonation begins behind a compaction front that is subsequently overrun by the detonation front. ° Hence the behavior of the model is qualitatively similar to that observed in experimental studies of homogeneous initiation and delayed detonation associated with low velocity impacts. 

We know that under certain conditions explosives will be initiated if they are subjected to impact. Low-velocity impact has caused many accidental explo-... 

Thus, the higher the local pressure, the higher the local melting point, and the lower the critical stress to produce ignition. They then go on (in Ref. 6) to analyze the mechanical conditions and pressure or stress distributions in both thick and thin explosive charges undergoing inelastic flow due to low-velocity impact. The comparative database they use in these analyses is from impact machine tests. They find that for most explosives the critical temperatures, T c, are between 400 and 600°C, and that the critical diameter of a hot spot is between 10 and 10 cm. 

We have seen in the previous sections how burning or deflagration can be initiated in an explosive. If the decomposition reaction is completed at shock velocities in the explosive, that is called a detonation. The initiation of chemical reaction in a detonation is similar to what we saw with low-velocity impact. The shock front compresses the unreacted explosive material, causing local shear failure and inelastic flow (Ref. 7). These processes create hot spots that grow into complete reaction. The difference in the case of detonation is that the ensuing reaction is completed at a much higher rate. 

Wasson J. T. and Rubin A. E. (1985) Formation of mesosiderites by low-velocity impacts as a natural consequence of planet formation. Nature 318, 168—170. 

A considerable amount of experimental work has been done on the initiation of solid explosives by low-velocity impact (a few m/sec). Although the data for various situations are valuable as regards the practical use of the explosives, understanding of impact initiation is still far from clear. Ideally, if the various physical and chemical parameters of a reactive material were known, it would be possible to predict the sensitivity of the material to any mechanical stimulus. However, this ideal is far from being achieved, and at present the only reliable approach is to conduct experiments. 

Rhodes. M. D.. Williams, J. G.. and Starnes, J. H, (1976). Effect of low velocity impact damage on the compressive strength of graphitc-epoxy hat-stiffened panels. Report N.AS.A TM X-73988 (TN D-8411). December 1976. 

Marshall. A. P.. and Bouadi, H, (1993). Low-velocity impact damage on thick-section graphite epoxy laminated plates. J. Reinforced PUtstics Composites. Dec. 1993, 12. 12, 1281 1294. 

Wey, A. C.. and Kessler. L. W. (1992), Quantitative measurement of delamination area in low-velocity impacted composites using acoustic microscopy. Review of Progress in QNDE. Plenum Pre.ss. Vol. 11. pp. 1413 1419. 

Adams, R. D., and Cawley. P. (1986), Low-velocity impact inspection of bonded structures. Proc. Int. Conf, Structural Adhesives in Engineering, pp. 139 142. 

Diamanti K, Hodgkinson JM, Soutis C. Detection of low-velocity impact damage in composite plates using Lamb waves. Struct Health Monit J 2004 3(1) 33—41. 

Figure 9.2 Individual delaminations caused by low-velocity impact.

Theoretical models for residual strength of low-velocity impacted panels have addressed the three failure mechanisms notch-type , sublaminate buckling and... 

Davies GAO, Hitchings D, Wang J. Prediction of threshold impact energy in quasi-isotropic carbon/epoxy composite laminates under low-velocity impact. Compos. Sci Technol 2000 60(l) l-7. 

Zhang X, Bianchi E, liu H. Predicting low-velocity impact damage in composites by a quasi-static load model with cohesive interface elements. Aeronaut J 2012 116(1186) 1350-67. 

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