Spall Properties Research Articles

Effect of grain size on spall fracture of CrCoNi medium-entropy alloy under Taylor-wave loading

The effect of grain size on spall properties of medium-entropy alloy CrCoNi under Taylor-wave loading is investigated. Three different grain sizes, 300 μm, 20 μm, and 3 μm, are explored. The free surface velocity histories are measured to obtain such quantities as dynamic yield strength and spall strength. With decreasing grain size, there is a slight decrease in spall strength, but a significant increase in damage rate. The shock-recovered samples are characterized with scanning electron microscopy and electron backscatter diffraction. In coarse-grained samples, voids tend to nucleate within grain interiors, while in fine-grained samples, voids nucleate preferentially at grain boundaries (GBs). Molecular dynamics simulations show that the nucleation of intergranular or intragranular voids is dominated by GB–slip and slip–slip intersections, respectively. The higher GB density results in more extensive GB–slip intersections, consistent with the experimental results.

Dynamic spall properties of an additively manufactured, high-entropy alloy (CoCrFeMnNi)

The dynamic spall properties of an additively manufactured (AM), CoCrFeMnNi high-entropy alloy (HEA) were investigated as a function of processing defects. Laser Powder Bed Fusion (LPBF) was used to additively manufacture HEA samples that were subsequently subjected to shock loading through plate impact experiments. Seven different combinations of laser power and scan speeds were explored, ranging from 180 to 280 W and 925–1350 mm/s, respectively. All samples were considered within the bounds of lack-of-fusion and keyholing defects based on initial experiments, but exhibited varying degrees of solidification cracking and microstructural changes. Grain size and grain aspect ratio were found to modestly decrease with faster scan speeds and lower laser powers, while cracking associated with the AM process increased with faster scan speeds and higher laser powers. Spall strength and spall damage did not systematically trend with microstructural characteristics such as grain size, but did exhibit a relationship with pre-existing crack density. Spall properties were found to generally degrade with increasing crack density. Evidence of defect compaction was not observed in soft recovered spall samples, thus pre-existing cracks were proposed to degrade spall properties by acting as stress concentrators and preferred damage nucleation sites during tensile impact loading. Here, the presence of manufacturing-related defects, such as cracks, dominated the dynamic response; however, in the absence of these defects, microstructural changes like grain size and texture would be expected to control dynamic behavior.

Spall properties and damage mechanisms of a low-alloy steel fabricated via laser powder bed fusion

Plate impact experiments are conducted on a representative, low-alloy, 24CrNiMo steel (containing lath martensite substructures) fabricated via laser powder bed fusion and tempering, to investigate its spall strength and damage mechanism within a peak stress range of 5–11 GPa. The Hugoniot elastic limit (2.4 GPa) and spall strengths (3.9–4.2 GPa) are obtained from free surface velocities, and show negligible anisotropy. The microstructure before and after impact loading are characterized by electron backscatter diffraction and scanning electron microscopy. The length, width and orientation angle of cracks are quantified. Spall cracks are large in quantity, but small in mean propagation distance and opening displacement, especially at high peak stress. Cracks develop mainly along the {100} cleavage planes of laths of the same variant rather than along melt pool boundaries. The mismatch between the {100} cleavage planes across the ∼60°〈111〉 lath boundaries leads to high resistance of fine laths against crack propagation. The fine lath substructure is beneficial to the dynamic mechanical properties of low-alloy steels.

The Role of Second Phase Intermetallic Particles on the Spall Failure of 5083 Aluminum

5083 aluminum alloy is a light-weight and strain-hardened material used in high strain-rate applications such as those experienced under shock loading. Symmetric real-time (in situ) and end-state (ex situ recovery) plate impact shock experiments were conducted to study the spall response and the effects of microstructure on the spall properties of both 5083-H321 and 5083-ECAE + 30 % cold-rolled (CR) aluminum alloys shock loaded to approximately 1.46 GPa (~0.2 km/s) and 2.96 GPa (~0.4 km/s). The results show that mechanically processing the 5083-H321 aluminum by Equal Channel Angular Extrusion (ECAE), followed by subsequent CR significantly increases the Hugoniot Elastic Limit (HEL) by 78 %. However, this significant increase in HEL was at the expense of spall strength. The spall strength of the 5083-ECAE + 30 % CR aluminum dropped by 37 and 23 % when compared to their 5083-H321 aluminum counterpart at shock stresses of approximately 1.46 and 2.96 GPa respectively. This reduction in spall strength is attributed to the cracking and re-alignment of the manganese (Mn)–iron (Fe) rich second phase intermetallic particles during mechanical processing (i.e., ECAE and subsequent CR), which are consequently favorable to spallation.

Microstructural effects on the spall properties of ECAE-processed AZ31B magnesium alloy

Time-resolved normal plate impact experiments and spall recovery experiments were conducted to study the spall behavior of AZ31B-4E magnesium alloy processed via Equal-Channel Angular Extrusion (ECAE). The spall strength and incipient spall damage in the specimens were measured at different shock stresses using 51 mm and 105 mm bore gas guns. The Hugoniot Elastic Limit (HEL) was measured to be approximately 181 ± 3 MPa. The spall strengths extracted from the free surface velocity profiles of the shocked specimens were found to decrease by 5% for shock stresses ranging from 1.7 GPa to 4.6 GPa. However, this reduction in spall strength may fall within the experimental error. Post-test fractographic examinations of recovered specimens revealed that spall failure originated at micrometer-size intermetallic inclusions and propagated through the material by cavitation events with a very limited growth of voids. It was concluded that the strengthening of AZ31B-4E magnesium alloy by the ECAE-process resulted in adverse effects on its microstructure and spall behavior because of the process-induced cracking of intermetallic inclusions and their weak interface strengths.

Spall behavior of rolled aluminum 5083-H116 plate

The spall properties of rolled Al 5083-H116 plate are investigated using symmetric plate impact experiments over the stress range 1.5–6.2 GPa. Rear free surface velocity measurements made employing Velocity Interferometer System for Any Reflector interferometry reveal velocity profiles with clear signals of the Hugoniot elastic limit (HEL) and velocity pullback, indicative of a transition from elastic to plastic behavior and spalling. Experiments were performed on samples obtained both through the thickness and along all of the three principal axes of the rolled plate. For impact through the thickness, the average values of the HEL and spall strength are 0.43 GPa and 0.81 GPa, respectively. Decreasing the flyer plate and sample thicknesses resulted in an increased spall strength value of 0.95 GPa, while the HEL remained the same. The spall strength along the longitudinal (rolling) direction was 1.06 GPa versus 0.95 GPa for impact along either transverse direction. Spall damage for this impact direction often propagated away from the spall plane in the direction of impact and along the grain boundaries. For impact through the thickness, the fracture surface revealed a mixed mode of ductile and intergranular fracture that was not present for the fracture surface in the other two directions. This mixed fracture mode seems to correspond to a shoulder observed in the free surface velocity traces after the pullback. In all cases, cracked brittle inclusions were observed near the spall damage regions, indicating their role in nucleating voids during spall failure.

The spall strength of condensed matter

Two conditions are proposed which place constraints on the processes of dynamic spall in condensed media, and determine inequalities which bound the spall strength, fragment size, and failure time. Spall is defined as rupture within a body due to stress states in excess of the tensile strength of the material. The first is a horizon condition which establishes a domain of communication, consistent with the time to failure, within which spall must be independent of the surrounding environment. The second is an energy condition which requires that the potential and kinetic energy associated with the tensile loading process exceed the fracture energy of the material. Equality in the relations established from these conditions corresponds to energy-limited spall and provides specific analytic expressions for the spall properties. Inequality implies flaw-limited spall and requires more detailed material property information before spall can be characterized. Energy-limited spall is determined by the material fracture toughness in brittle solids and the material flow stress in ductile solids. Calculated spall properties, assuming energy-limited spall, compare well with experimental spall data for various materials. Under certain conditions, a transition from brittle to ductile spall (definition in text) with increasing strain rate is predicted. Comparison is made with spall data on 6061-T6 aluminum for which a brittle-to-ductile transition is predicted to occur at a critical strain rate of approximately 4 × 10 5 s −1. Energy-limited spall in liquids within their range of Newtonian fluid behavior is governed by surface energy and viscosity. Spall is predicted to be dominated by surface energy at low strain rates and viscous dissipation at high rates. Examples of each appear to exist within the scant experimental spall data available for liquids.