Response to Comment on “Adiabatic shear instability is not necessary for adhesion in cold spray”

Dynamics and kinetics of dislocations in Al and Al–Cu alloy under dynamic loading

We investigate spall damage of a mild carbon steel under high strain-rate loading, regarding the effects of peak stress, strain rate, and pulse duration on spall strength and damage, as well as related microstructure features, using gas gun plate impact, laser velocimetry, and electron backscatter diffraction analysis. Our experiments demonstrate strong dependences of spall strength on peak stress and strain rate, and its weak dependence on pulse duration. We establish numerical relations between damage and peak stress or pulse duration. Brittle and ductile spall fracture modes are observed at different loading conditions. Damage nucleates at grain boundaries and triple junctions, either as transgranular cleavage cracks or voids.

Spall damage of a mild carbon steel: Effects of peak stress, strain rate and pulse duration

Molecular dynamics studies of the roles of microstructure and thermal effects in spallation of aluminum

The effects of loading on spall strength of a mild steel are investigated systematically under the flat-top impact loading, including the effect of tensile stress history and the effect of shock-induced microstructure. Peak stress, strain rate, pulse duration, spall strength and tensile stress history are obtained directly or indirectly from free-surface velocity measurements, and the recovered samples are characterized with electron backscatter diffraction analysis. For different tensile stress histories, spall strength depends on peak stress or strain rate, depending whether spall occurs on the plateau or rising edge of a tensile pulse. Pulse duration has an effect on whether spall occurs (incipient spallation) but not spall strength. Low shock stresses induce small microstructure change (dislocations) which has weak effects on nucleation and propagation of brittle cleavage cracks, while high shock stresses lead to deformation twins (in addition to dislocations) which act as the source of crack nucleation. A model is proposed to predict the spall strength of the mild steel under arbitrary loading conditions (peak stress below the phase transition).

Spall strength of a mild carbon steel: Effects of tensile stress history and shock-induced microstructure

Spall failure of aluminum materials with different microstructures

Critical Damage Evolution model for spall failure of ductile metals

The effects of grain size on the spall response were investigated for high purity copper materials by plate-impact experiments including real-time measurements of the free surface velocity profiles as well as post-impact fractography studies on the soft-recovered samples. High purity copper plates were cold rolled and heat treated to produce recrystallized samples with average grain sizes of 78, 273 and 400 μm, respectively. The spall strength estimated from the free surface velocity profile is nearly constant with no significant effect on the grain size. However, differences are observed in the acceleration rate of velocity rebound beyond the minima. This may be attributed to the effect of grain size on the growth rate of damage. Metallographic analyses of the fracture surface show that the characteristic feature of the fracture surface clearly depends on the grain size. In the 78- and 273-μm samples, the fracture surfaces are decorated with large, high-density ductile dimples suggesting that the preferential failure mode is ductile intergranular fracture. In the 400-μm samples, the fracture surfaces have a rock candy appearance with small, high density brittle dimples as well as large ductile dimples suggesting that the fracture mode is a mix of both brittle intergranular fracture and ductile transgranular fracture.

Effect of Grain Size on the Spall Fracture Behaviour of Pure Copper under Plate‐Impact Loading

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024-T4 and 7075-T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075-T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024-T4 alloy. But the initiation fracture toughness and spall strength of 2024-T4 alloy are higher than those of 7075-T6 alloy in three-point bending and plate impact experiments, which indicates that 2024-T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024-T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl2 precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075-T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024-T4 and 7075-T6 alloys under plate impact are also discussed.

Yonggang Wang

Papers

Spall failure of aluminum materials with different microstructures

Critical Damage Evolution model for spall failure of ductile metals

Effect of Grain Size on the Spall Fracture Behaviour of Pure Copper under Plate‐Impact Loading

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Influence of porosity on nonlinear mechanical properties of unpoled porous Pb(Zr0.95Ti0.05)O3 ceramics under uniaxial compression