Stress impact

The resulting mouldings and extrudates are consequently anisotropic and mouldings can be four to five times as strong in one direction as in another direction. This can lead to planes of weakness and easy fracture when subject to shock (impact) stresses. Generally such orientation is undesirable but there are at least two instances of its being of value ... 

Fig. 3.4. Representative stress-particle velocity relations such as those shown in the relatively low shock-pressure region are used to determine impact stresses with good precision.

The stress acting on particles is due to a relative velocity between the particles and the fluid. If their mean velocities also differ, contact between the particles or between a particle and the tank wall or the impeller elements leads to impact stress. However, this impact stress is negligible if the density differences and the particle concentrations are low. 

A method for the prediction of cushion curves for polymer packaging foams from a single impact stress-strain curve is proposed. The method is valid if there is a master curve for the increasing stress part of the stress-strain curve. For closed-cell polymer foams that deform by yielding... 

A study was made of the impact and recovery behaviour of three HDPE closed-cell foams with varying densities. Impact stress-strain curves were measured using a falling striker impact rig and the recovery monitored from 10s after the impact. Cell deformation was observed during compression and recovery using SEM. Recovery was found to occur by the viscoelastic straightening of the buckled faces and to be incomplete due to plastic deformation in the structure. 6 refs. 

These findings clearly show the significance of the material function, since the material reaction to similar stress conditions in the pipe bend can vary greatly. In contrast to PP, PMMA and PS apparently are attrited by the same mechanisms in effect under normal impact stress conditions. This also clarifies that it is only possible to deduce the stress mode from the process function but that without knowledge of the material function no conclusions concerning the governing attrition mechanisms can be drawn. 

Figure 9.18. Nonnalized largest fragment mass vs. nondimensional impact stress (P,) (After Arakawa [39].)...

K-quality WC-Co 88 to 96 cutting tools for short-chip materials, drilling plates for impact stress applications... 

A typical signal from the center gauge in the target array is shown in Figure 22. The trace rose first to the impact stress in the acrylic and then to the stress in the explosive sample. The spike results from the impedance mismatch of the adhesive bond with the Plexiglas and lead azide. 

As indicated, the experimental technique measures the impact stress directly and requires only the assumptions of continuity of mass and momentum at the impact interface to derive the Hugoniot. No steady-state assumptions were made, and the derivation of the Hugoniot was independent of the measured wave speed. Consequently, the measurements were considered to be an accurate representation of the response of the unreacted explosive. 

At low stresses the measured wave velocities were less than 1.23 km/sec. At higher impact stresses both the shock velocity and the amplitude of the propagated wave increased with distance. The change in wave velocity from a value less than that for unreacted material to the detonation velocity was abrupt. 

The threshold for the detection of reaction in the dextrinated lead azide (3.4 g/ml) subjected to 3,5-psec pulses was dependent upon sample thickness (Figure 25). No reaction was noted in 1-mm-thick samples even at the highest impact stresses (8.9 kbar) tested, whereas the threshold for 4-mm-thick samples was between 4 and 6 kbar. 

Comparison of initiation-threshold measurements suggests that there is a minimum thickness, or a run-up distance, before detonation occurs and that this is independent of pulse-width for stresses up to 10 kbars. For long pulses (3.5 psec) in the gas gun, no evidence of detonation was detected for 1-mm-fhick samples. Detonation occurred with run-up distances in the range of 1-2 mm for impact stresses of 8.9 kbar. For stresses greater than 6.0 kbar, evidence of detonation was noted after a 2-mm run. In the thin-flyer-plate experiments at stresses of 8 kbar, dextrinated lead azide displayed a 2-usec initiation delay. Voreck and coworkers [8] determined that a similar minimum thickness of lead azide is required for complete detonation in an explosive train consisting of NOL-130, lead azide, and RDX (Figure 15). 

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