Relationship of fragment size to normalized spall strength for materials

A systematic study is described which addresses the technical issues associated with launching plates to hypervelocities using compressed helium gas as the driving medium. Similar acceleration rates are used to drive constant-mass flier plates, while the material properties such as the shock impedance, yield strength, and spall strength are varied. Results of these experiments indicate that, due to the severity of loading, the flier plates have a tendency to fragment following peak acceleration. Each material has its own unique fragment distribution. More significantly, these experiments suggest a direct dependence of the mean fragment size to its normalized spall strength, i.e., the spall strength divided by its density. A method is also proposed for successfully launching intact plates to hypervelocities.     

The computer simulation of an explosive test rig to determine the spall strength of metals

Spall fracture is produced by a tensile wave after reflection of a compressive shock wave at a free surface. The original shock wave may be initiated by the detonation of explosive in contact with the material or by the impact of a high velocity projectile. An explosive test rig has been devised where a shock wave produced by explosive detonation is attenuated by propagation along a bar. The degree of spall fracture in a sample on the other end of the bar depends on the bar length. A computer simulation, using the EPIC-2 code, has been made and compared to published values of the free surface velocities for various bar lengths. Good agreement was obtained by careful choice of input parameters used in the γ-law burn of the explosive, and when correction of the experimental values was made for the change in free surface velocity that occurs when the material has a finite spall strenght. The computer simulation enabled extrapolation of the pressure values to be made to longer bar lengths, to obtain the limit of incipient spall fracture. Also, information was obtained on the shock pulse shape and the pressure distribution across the bar. The spall strength of a 6061-T6 aluminium alloy was found to be 1400 MPa, in good agreement with published values for a similar pulse duration.     

Models of spall fracture

Strength of Materials -     

The spall strength of silicon carbide and boron carbide ceramics processed by spark plasma sintering

The spall strength of silicon carbide (SiC) and boron carbide (B₄C) ceramics processed by Spark Plasma Sintering (SPS) has been studied as a function of the loading stress. In the course of the planar impact experiments, the velocity of either the sample free surface or of the sample–window interface was continuously monitored by a Velocity Interferometer System for Any Reflector (VISAR). With the increase of impact stress the spall strength of both ceramics, increases initially and then declines monotonously until it vanishes almost completely, as the impact stress approaches the respective Hugoniot Elasic Limit (HEL). The mechanisms that may account for that behavior and, in particular, the role of the compressive wing cracks in the onset of the spall strength decline are discussed.     

Improving the validity of test data obtained with data measurement systems

Translated from Izmeritel'naya Tekhnika, No. 8, pp. 10–12, August, 1988.     

Comparison of hypervelocity fragmentation and spall experiments with Tuler–Butcher spall and fragment size criteria

The present investigation compares predictive theories of dynamic spall and fragmentation with previously reported experimental data. In the experimental tests, aluminum spheres normally impacted thin aluminum plates at over approximately 4.5–7.5 km/s. Scaling features of the impact breakup phenomenon were explored through selected variation in sphere size and plate thickness. The principal diagnostic was high-resolution flash radiography. Fragment-size features of resulting fragment clouds were determined through detailed analysis of the recorded radiographs. Other investigators have measured the spall strengths for aluminum at comparable ultra-high strain rates. Spall strength amplitude and the corresponding strain rate dependence are principal results of the study. Existing dynamic fracture criteria are specialized here to the sphere impact spall and fragmentation event, and compared with empirical data. Velocity and strain rate scaling relations are developed for fragmentation size in the sphere impact event.     

Direct numerical simulation of ductile spall failure

Large-scale direct numerical simulations of void growth and coalescence from 3-dimensional distributions of void nucleating particles are used to investigate the effect of material strain hardening and strain rate sensitivity on spall response. The computational model spans multiple particle spacings in the in-plane directions, and several finite elements span the initial particle diameters in the mixed-zone Arbitrary Lagrange–Eulerian (ALE) simulations. The matrix material is represented by traditional plasticity models in which material failure is not permitted. The 1000\(+\) particles are represented by the same material model as the surrounding matrix except the particles have low tensile strength to permit fracture, which is used to simulate particle cracking or decohesion. Voids grow and coalesce naturally in the ALE framework, and the simulations produce dimpled failure surfaces similar to those observed experimentally in spalled samples. The strain hardening and strain rate sensitivity of the matrix material are altered to explore their influence on the void growth and coalescence processes and on the simulated free surface velocity. The details available from the computational model permit association of the longitudinal stress evolution with features on the free surface velocity profile.     

A grain-scale study of spall in brittle materials

The evolution of spall for a brittle material is investigated under variance of anisotropy, grain boundary fracture energy, and loading. Because spall occurs in the interior of the specimen, fundamental studies of crack nucleation and growth are needed to better understand surface velocity measurements. Within a cohesive approach to fracture, we illustrate that for anisotropic materials, increases in the fracture energy cause a transition in crack nucleation from triple-points to entire grain boundary facets. Analysis of idealized flaws reveals that while crack initiation and acceleration are strong functions of the fracture energy, flaws soon reach speeds on the order of the Rayleigh wave speed. Finally, simulated surface velocities of spalled configurations are correlated with microstructural evolution. These fundamental studies of nucleation, growth, and spall attempt to link atomic separation to the macroscopic spall strength and provide a computational framework to examine the evolution of spall and the impact on the simulated surface velocity field.     

Features of the measurement of current strength by a thermoelectric transducer

The problem of measuring a pulsed current using a thermoelectric transducer is considered. It is not possible to measure all currents by direct reading from a thermoelectric transducer. The measurement of some currents requires the use of appropriate coefficients. Translated from Izmeritel'naya Tekhnika, No. 2, pp. 38–39, February, 1999.     

The validity of ultrasonic pulse velocity testing of in-place concrete for strength

It is generally accepted that the use of ultrasonic methods for assessment of concrete suffers many disadvantages, particularly where a strength estimation is required. The absence of alternative non-destructive methods capable of giving significantly better results, however, means that despite these difficulties ultrasonics have a valuable role to play, especially in the field of in-place evaluation of structural concrete. It is thus worthwhile to examine in some detail those factors having a major influence on ‘field’ results, with the aim of assessing the confidence with which they may be viewed.     

Evaluation of concrete spall repairs by pullout test

Spalling concrete was simulated in the laboratory by utilizing pullout test methods generally used for the determination ofin situ concrete strength. The cracking patterns displayed by pullout test specimens typify the damage of concrete by spalling. All specimens were damaged by pullout testing, repaired with epoxy mortar and subjected to a second pullout test at a later time. The test programme showed that the overriding factor which governs successful repairs to concrete is the soundness of the repair plane.