The term dynamic failure is used to distinguish special characteristics of failure of materials subjected to dynamic loading. The major feature that differentiates dynamic failure from quasi-static behavior is the presence of stress waves. These waves arise due to the applied load or due to the stresses released from a crack tip at fracture. Several phenomena are particularly relevant to dynamic failure, including:

Microstructural Aspects of Dynamic Failure

Heusler Alloys: Past, Properties, New Alloys, And Prospects

The inelastic response and residual mechanical properties acquired from most shock compressed solids are quite different from those acquired from quasi-static or moderate strain rates. For...

Structure-Property Relationships under Extreme Dynamic Environments: Shock Recovery Experiments

It is a well-known fact that the mechanical response of metals depends upon the temperature, the velocity of deformation, the previous deformation undergone, and the stress state, among other parameters. As a result, various attempts have been made to incorporate these parameters into a single equation that would have the capability of predicting the response of a specific metal under a wide range of circumstances.

Metallurgical Effects of Shock and Pressure Waves in Metals

An investigation has been made into the effects of prestraining on the impact response and twinning structure of 304L stainless steel by means of a split Hopkinson bar and TEM metallography. Annealed 304L stainless steel was pre-deformed at prestrains of 0.15 and 0.3 using a Saginomiya 100 metal forming machine, providing specimens with different microstructural parameters and mechanical properties. Cylindrical specimens were machined from the pre-deformed material and tested at room temperature at strain rates of 10 -3 , 8 x 10 2 , 2.3 × 10 3 and 4.8 x 10 3 s -1 , with the true strains varying from 0.1 to 0.3. The results indicate that the overall stress-strain response was strongly dependent on applied strain rate and prestrain level. Flow stress increased with strain and strain rate for each prestrain condition. The higher prestrain specimen was generally stronger because of the different initial microstructure and degree of work hardening. Strain-rate sensitivity and activation volume varied with prestrain and work hardening stress (a - σ y ). Increased prestrain and work hardening stress resulted in increased strain-rate sensitivity, but the inverse was observed for activation volume. TEM thin foil observation revealed deformation twinning was operative in 304L stainless steel over a broad strain and strain rate range, and can combine with prestrain effects for different work hardening characteristics and impact strength enhancement. Twin density was observed to increase with prestrain and work hardening stress.

Effects of Prestrainning on the Impact Response and Twinning Structure of 304L Stainless Steel

A novel experimental set-up for obtaining shock-wave superposition is described. Detonation is simultaneously initiated at the two opposite sides of an explosive layer placed directly in contact with a steel block. It was possible to take advantage of thea (bcc) → e (hcp) →α (bcc) transformation occurring in iron at approximately 13 GPa in a unique fashion. The pressure of the two converging pulses is approximately 13 GPa, resulting in little or no transformation. However, in the region of superposition of the two waves the pressure is much higher.Post explosionem observation of the microstructure allowed an easy identification of the region of superposition, because of the profuse transformation “debris”. Events conducted for three thicknesses of the explosive layers allowed conclusions to be drawn about the changes of rarefaction and attenuation rates. The interactions occurring between the waves due to simultaneous superposition and transformation are thought to be responsible for internal spalling.

A technique for obtaining shock-wave parameters using wave superposition in low-carbon steel

The attenuation of planar shock waves generated by plate impact was monitored by their decay throughout massive nickel blocks. This was accomplished, during the passage of the wave, by manganin piezoresistive gages connected to oscilloscopes and, in the post-shocked condition, by hardness measurements and TEM observations at various distances from the impact surface in the nickel blocks. The nickel systems exhibited different metallurgical microstructures before shock loading: preshocked (grain size 150 μm), annealed (grain size 150 μm)and annealed (grain size 32 μm). For each system two different initial shock pressures were used: 10 and 25 GPa. The pulse duration was held constant at 2 μs. The experimental records of oscilloscopes showed that there are no significant effects of grain size and pre-deformation on the attenuation in nickel. The observed attenuation was compared with the calculated one according to hydrodynamic theory and poor agreement was found, An “accumulation” model based on the conservation of energy is presented herein to explain the dissipative processes of shock waves in metals.

Chen-Yih Hsu

Papers

A technique for obtaining shock-wave parameters using wave superposition in low-carbon steel

The Attenuation of Shock Waves in Nickel