Shock hardening

The shock-hardening response (total dislocation density) of Cu-8.7 Ge [43] is found to vary as a function of pulse duration as shown in Fig. 7.9. 

Another aspect of laser-driven vaporization and plasma formation that should be considered is the momentum transferred to the surface. In leaving the surface at high speed the ejected material carries away a substantial amount of momentum. This appears as a recoil momentum of the surface and, since it occurs during the short duration of the laser irradiation, it results in a substantial impulse to the surface. This process has been used to shock-harden certain materials. An ablative coating is placed on the area to be shock-hardened and it is shuck by a pulsed laser beam. The impulse due to the removal of the ablative coating produces the desired hardening. The hansfer of an impulse by laser irradiation is another subject of major research interest, in part due to its potential value as a laser weapons effect. 

As the concentration of defects increases under shock loading, we cannot exceed a certain critical value, after which a fragile destruction takes place. In metals, this usually occurs after the third shock treatment. It has been shown [59] that a thermal treatment at 350 °C leads to annealing of the defects and micro-cracks, which allows to repeat the shock hardening. Six cycles of such shock/temperature treatment (STT) of Steel 45 (containing C 0.45, Si 0.27, Cr 0.25, Mn 0.65, Ni 0.25 %, and the balance of Fe) increased its Vickers hardness from Hy = 156 for the initial state up to 7/v=418. It is very important technologically that low-temperature heating does increase not only the hardness but also the plasticity. 

Shock loading in most metals and alloys produces greater hardening than quasi-static deformation to the same total strain, particularly if the metal undergoes a polymorphic phase transition, such as is observed in pure iron [1]-[10]. Figure 6.1 compares the stress-strain response of an annealed... 

The initial strain hardening rate (df/dy)o is given as a function of plastic strain rate for strain rates up to 10 s (which includes shock compression to 5.4 GPa) as [38]... 

An important aspect of micromechanical evolution under conditions of shock-wave compression is the influence of shock-wave amplitude and pulse duration on residual strength. These effects are usually determined by shock-recovery experiments, a subject treated elsewhere in this book. Nevertheless, there are aspects of this subject that fit naturally into concepts associated with micromechanical constitutive behavior as discussed in this chapter. A brief discussion of shock-amplitude and pulse-duration hardening is presented here. 

Gray and Follansbee [44] quasi-statically tested OFE copper samples that had been shock loaded to 10 GPa and pulse durations of 0.1 fis, 1 /rs, and 2 fus. The quasi-static stress-strain curves are shown in Fig. 7.10 with the response of annealed starting copper included for comparison. The yield strength of shock-loaded copper is observed to increase with pulse duration, as the work-hardening rate is seen to systematically decrease. 

Dieter, G.E., Hardening Effect Produced with Shock Waves, in Strengthening Mechanisms in Solids, American Society of Metals, Metals Park, Ohio, 1962, pp. 279-340. 

Performance of Cured Resin Sockets. Poured resin sockets may be moved when the resin has hardened. After ambient or elevated temperature cure recommended by the manufacturer, resin sockets should develop the nominal strength of the rope and should also withstand, without cracking or breakage, shock loading sufficient to break the rope. Manufacturers of resin socketing material should be required to test to these criteria before resin materials are approved for this end use. 

Multiplexed diode laser sensors have also been applied for measurements of gas temperature, velocity, and H2O partial pressures in hypervelocity air flows at the Calspan University of Buffalo Research Center s (CUBRC) Large Energy National Shock Tunnel (LENS Tunnel) in Buffalo, New York [12]. The sensors were developed to provide quantitative characterization of the facility operation and, in particular, the freestream flow properties as a function of time. The measurements were recorded using a hardened probe, which contained critical optical components and photodetectors, that was installed directly into the hypersonic shock-tunnel near the nozzle exit to minimize complications due to boundary layers and facility vibration. 

This mixt is more powerful than TNT or Tetranitroaniline, but is insensitive to shock or friction and burns without detonation when dropped in a red hot crucible or ignited by flame. Its ignition temp is 194-208°. It cannot be stored for longer than 24-48 hours, as bubbles, resulting from decomposition of the H2 02, are evolved and the mass hardens. Moreover its explosive power decreased after 48 hours of standing (Ref 1)... 

Principal applications of Detasheet expls include field demolition, underwater explosions, seismic prospecting and metal cutting and hardening. For example, manganese (Hadfield) steel can be hardened by inducing a shock wave in the metal thru detonation of Detasheet placed direcdy on the metal s surface (Ref 5, p 5)... 

A coarse texture may also develop as a result of heat shock, which involves alternate thawing and freezing of the water in the ice cream owing to temperature fluctuations in the hardening and storage cabinet. This results in a reduction of the textural quality of the ice cream. 

Evans et al. [43] carried out 4 MeV electron irradiations of 14 different epoxy resins at 77 K which were selected from a large number of resin systems after screening tests on thermal shock at cryogenic temperatures [44]. The results of flexural tests show that most of these irradiated resins possess only moderate resistance to radiation. Takamura and Kato [45] tried to irradiate the bisphenol-A type epoxy resins with various hardeners at 5 K in a fission reactor and reported that the compressive strength of these epoxy resins decreased sharply after a combined neutron and y-ray irradiation equivalent to a dose of about 107 Gy. 

---