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... In general, ductile damage can be divided into three stages: cavitation nucleation, growth and coalescence. It was found by Yang et al. [17] that cavitation nucleation was not random. The nucleation occurred easily at grain boundary triple points where had a large difference of Taylor Factor value within α-phase. ...
... The intensity of the (110) peak of compressed irons is significantly reduced compared to that of the starting iron, likely due to the introduction of many dislocations under high strain rates. 41 ...
Understanding of dynamical responses and mechanical characteristics of metals and alloys at high strain rates holds significant importance in fundamental physics and optimizing the performance capabilities of materials. During high-speed impact scenarios, materials may be subjected to high pressure and plastic deformation, which have the potential to modulate their mechanical attributes. In this study, high-speed planar impact experiments were conducted to investigate the progressive alterations in the microstructures and mechanical properties in coarse-grain body-centered cubic (bcc) iron subjected to high-strain-rate (approximately 2.60–3.89 × 10⁶ s⁻¹) impact reaching approximately 15 GPa in a one-stage light-gas gun. The nanoindentation tests show that the nano-hardness of the post-shock iron improves 1.5 times from approximately 1.75–2.70 GPa. Microscopic analyses of the post-shock bcc-iron show no significant grain refinement but a noticeable increase in the twin boundaries (TBs) and low angle grain boundaries (LAGBs) proportion with increasing shock pressure. Therefore, the interaction between TBs, LAGBs, and dislocations in post-shock iron grains plays an important role in mediating its mechanical properties. Our findings serve as possible guidance for exploring the mechanical properties of single-crystalline and poly-crystalline iron-based materials, such as steel, with optimized mechanical performance.
... Titanium alloys have the advantages of low density, high specific strength, strong corrosion resistance, etc. Presently, titanium materials have been widely used in aerospace, medical, and chemical industries [1][2][3]. They will undoubtedly play an important role in military fields such as warheads and lightweight armor, due to the many advantages mentioned above [4,5]. ...
The application of titanium alloys in weaponry is increasingly widespread, due to their high specific strength and excellent corrosion resistance. The weapons such as armors must be subjected to intense shock loads caused by explosion and hyper-velocity collision, etc., during service. Therefore, their service performance is closely related to the shock-induced response characteristics of materials, especially the microstructural evolution during the shock pulses and its effect on the mechanical properties. This chapter introduces the research progress on the shock response of some typical titanium alloys such as Ti-6Al-4V, Ti-10V-2Fe-3Al, and Ti-3.5Al-10Mo-8V-1Fe. The effects of alloying composition (alloy type) and stress amplitude on the shock-induced mechanical response and microstructural evolution of titanium alloys are explored through soft recovery shock experiments, quasi-static reloading tests, as well as careful multi-scale microscopic analyses.
... However, inconsistencies between the stress and strain easily occur at the interface during loading, resulting in stress concentration at the phase interface due to the differences in the physical, mechanical properties, and grain orientation between the α phase and β phase. Therefore, the phase interface is the preferred site for crack nucleation and propagation under tensile [10] or fatigue loading [12], which is detrimental to the mechanical properties of the alloy [13,14]. The stress concentration caused by dislocation pile-up can be reduced by refining the α phase in titanium alloy [15], but the stress concentration at the interface has not been effectively alleviated. ...
The tensile deformation and fracture behavior of a particular semi-equiaxed microstructure (S-EM) in a near alpha titanium alloy TA19 are investigated by an in situ method. In the S-EM, the thin β lamellae grow through the equiaxed αp phase (αp), and the original αp/βtrans interface in the bimodal microstructure largely disappears, forming a blurry interface between the semi-equiaxed αp phase (equiaxed αp phase that is grew through by the thin β lamellae) and the transformed β microstructure (βtrans). The formation of dense slip bands inside the semi-equiaxed αp phase in the S-EM is inhibited by the thin β lamellae during the tensile deformation. The special characteristics of the S-EM reduce the stress concentration at the interface, and the crack initiation probability in the blurry semi-αp/βtrans interface decreased compared to the distinct αp/βtrans interface in a conventional equiaxed microstructure (EM). Moreover, the ultimate tensile strength of the S-EM is higher than that of the EM with a slight loss of plasticity.
Based on the crystal plasticity theory, the changes in TC4 titanium alloy before and after cutting, especially the changes in cutting forces, chip temperature, morphology, and quality, were analyzed in this study. Initially, a polycrystalline plasticity model for the TC4 titanium alloy was established. Following that, a crystal plasticity polycrystal finite element cutting simulation model is developed to perform cutting simulations. This model represents the mechanical behavior of the micro grains of TC4 titanium alloy during the deformation and cutting processes. Further, it validates the internal grain plastic deformation and the effect on the metallography of TC4 titanium alloy before and after cutting. The results show a good consistency between experimental and simulation data. The grain size increases gradually from the center to the edge of the cutting area, with the phenomenon of equiaxed grains becoming more apparent, leading to the formation of elongated fine grains. During the cutting process, grains interlock and fracture, exhibiting strong anisotropy.
The dynamic failure behavior of titanium alloys has received increasing attention due to their promising application in high-strain-rate service environments. In this research, a series of plate impact experiments were carried out on a metastable β titanium alloy Ti–2Al-9.2Mo–2Fe (2A2F) to reveal the influences of multiple shock and β grain size on its spall fracture behavior. The results show that the spall behavior of 2A2F is significantly dependent on the number of impacts and β grain size. For the alloy with a small grain size, after first shocked at 6 and 12 GPa, the spall strength is increased by more than 60% during the second shock loading process. However, the pre-shock process has little effect on the spall strength of the alloy with the larger grain size. The microstructure analyses show that dislocation proliferation and β-to-α″/ω phase transformations occurred during the shock pulse. The α″ variants and fishbone-like α″ structures were also formed in 2A2F at the higher shock stress. These shock-induced substructures strengthen the β grains, leading to the shift in the nucleation site and propagation path of the spall micro-damages and the improvement of the spall strength.
Shock compression and spallation of a eutectic high-entropy alloy (HEA) AlCoCrFeNi2.1 with lamellar structure are investigated via plate impact loading with free-surface velocity measurements. The as-cast and postmortem samples are characterized with transmission electron microscopy, electron back-scatter diffraction and scanning electron microscopy. An accurate Hugoniot equation of state is determined. After shock compression to ∼12 GPa, both the L12 and B2 phases retain their ordered structures. Dense dislocations in the {111} slip planes, stacking faults and deformation twins are found in the L12 phase, along with fewer dislocations in the {110} slip bands in the B2 phase. Shock-induced deformation twinning within the L12 phase of this HEA is observed as a new deformation mechanism under various loading conditions. For spallation, both ductile and brittle damage modes are observed. The micro voids and cracks prefer to nucleate at the phase boundaries chiefly, then in the B2 phase. Under similar shock stress, the spall strength of AlCoCrFeNi2.1 HEA is about 40% higher than those of other reported dual-phase HEAs due to the high stability of its semi-coherent phase boundaries.
A series of sweeping detonation experiments were conducted to study the grain boundary effects during the primary spallation of high-purity copper cylinder. The free surface velocity profile of the shocked samples was measured by Doppler pins systems. The soft-recovered samples were characterized by optical and electron backscatter diffraction microscopy, and the effects of microstructures like grain boundaries, and crystal orientation on spall behavior were investigated. The results indicated that the critical stress of deformation twinning in cylindrical copper increased. The nucleation sites of spallation damage were determined by the joint influence of the grain orientation (Taylor factor) and the angle between grain boundaries and radial impact-stress direction. Voids were prone to nucleating at the grain boundaries perpendicular to the radial impact-stress direction. Nevertheless, the number of voids nucleated at boundaries was relatively different from the results obtained from the plate impact experiment and plate sweeping detonation experiment, which is a result of the curvature that existed in the cylindrical copper and the obliquity of the impact-stress direction during sweeping detonation loading.
The evolution behavior of α lamellae and evolution relationship between α lamellae and β phase in TC17 alloy during isothermal heating and subsequent compression were investigated. The results show that the α/β interface of TC17 alloy is made up of stable terrace plane and movable ledge plane. The ledge plane migrates along the length direction of α lamellae during isothermal heating, which results in rapid decrease of length and slow decrease of thickness of α lamellae. During compression, the continuous dynamic recrystallization (CDRX) are the main evolution mechanism of α and β phases. The slip systems of β phase are easily activated and the slip in β phase (i.e. (110)[1̅11]) will promote the slip in α phase (i.e. (0001)[112̅0]) at the initial stage of compression. Thus the CDRX of β phase is preferentially initiated by more activated slip and it will promotes the CDRX of α phase, then the HABs are formed in α lamellae. Meanwhile, the β stabilized elements Cr and Mo diffuse into the α/α boundaries in a short-time deformation. The α→β phase transformation is promoted by the enrichment of Cr and Mo elements and finally the α lamellae are separated with further compression.
There has been a challenge for many decades to understand how heterogeneities influence the behavior of materials under shock loading, eventually leading to spall formation and failure. Experimental, analytical, and computational techniques have matured to the point where systematic studies of materials with complex microstructures under shock loading and the associated failure mechanisms are feasible. This is enabled by more accurate diagnostics as well as characterization methods. As interest in complex materials grows, understanding and predicting the role of heterogeneities in determining the dynamic behavior becomes crucial. Early computational studies, hydrocodes, in particular, historically preclude any irregularities in the form of defects and impurities in the material microstructure for the sake of simplification and to retain the hydrodynamic conservation equations. Contemporary computational methods, notably molecular dynamics simulations, can overcome this limitation by incorporating inhomogeneities albeit at a much lower length and time scale. This review discusses literature that has focused on investigating the role of various imperfections in the shock and spall behavior, emphasizing mainly heterogeneities such as second-phase particles, inclusions, and voids under both shock compression and release. Pre-existing defects are found in most engineering materials, ranging from thermodynamically necessary vacancies, to interstitial and dislocation, to microstructural features such as inclusions, second phase particles, voids, grain boundaries, and triple junctions. This literature review explores the interaction of these heterogeneities under shock loading during compression and release. Systematic characterization of material heterogeneities before and after shock loading, along with direct measurements of Hugoniot elastic limit and spall strength, allows for more generalized theories to be formulated. Continuous improvement toward time-resolved, in situ experimental data strengthens the ability to elucidate upon results gathered from simulations and analytical models, thus improving the overall ability to understand and predict how materials behave under dynamic loading.
Laser shock loading is a more promising technology for investigating spallation damage in materials under shock-wave loading. In this paper, shock-induced spallation in a 90W–Ni–Fe alloy at an ultrahigh tensile strain rate of 10⁶ s⁻¹ is investigated using a superintense ultrafast laser facility. The spallation of the 90W–Ni–Fe alloy was dominated by a transgranular fracture of tungsten(W) particles with a high spall strength of 6.46 GPa. Here, we found an interesting phenomenon that the formation of nanograins inside W particles leads to a new mode of transcrystalline fracture of W particles during the laser shock loading. Futhermore, most voids were nucleated inside the W particles rather than at the W/γ-(Ni, Fe) matrix-phase interface. This result contradicts the fracture theory under quas i-static loading, which posits that the W/γ-(Ni, Fe) matrix-phase interface is not the preferred site for the initial failure under shock loading.
The dual-phase steel (martensite (M)/ferrite (F)) samples with different phase content were dynamic loaded simultaneously by one-stage light gas gun. The velocity of free surface particles was measured by Doppler pin system (DPS) during the loading experiment. The soft recovered samples were investigated with optical microscopy, nanoindentation, and EBSD techniques to study the effect of the phase content on dynamic damage evolution in dual-phase steel. The results show that the sample two (1000°C/60 min + 780°C/30 min+quenching) has a higher M area percentage (77.2%), larger M size, and smaller number of M and less M/F interfaces compared with the sample one (1000°C/60 min + 740°C/30 min+quenching, with M area percentage of 45.51%). Due to the reflection and transmission of shock wave at M/F interface, tensile stress will be generated inside M with higher shock impedance. Under the same dynamic loading conditions, the more M/F interface means the greater the probability of void nucleation inside M. Thus the sample two with less M/F interfaces has lower nucleation density and lower spallation strength. The microcrack propagation resistance increases with the increase in the area percentage of M and the size of M, which results in the lower damage evolution rate of the sample two. Meanwhile, the size of M will affect the direction of microcracks propagation. Each M with larger size in the sample two is composed of several prior austenite (A) grains, and the orientation of the martensite packets in prior A grains is very different. Therefore, the microcrack propagation in the sample two is limited to different regions, the direction of microcracks propagation is easy to be deflected.
In this paper, a unified viscoplastic constitutive model for Ti–6Al–4V alloy coupling with damage and softening mechanisms was established to predict the flow behaviors and damage evolution in hot tensile process. To obtain the flow behaviors of Ti–6Al–4V alloy, the hot tensile tests were performed at temperatures between 750 °C and 850 °C and strain rates of 0.01–1 s−1. Then the evolution of microstructure was investigated by scanning electron microscope (SEM) under hot tensile conditions. Otherwise, the macro-fracture morphology of the tensile specimen was observed by SEM. The flow stress and microstructure evolution were predicted based on the set of constitutive model. The constitutive model was embed in ABAQUS by coding a user-defined material subroutine. The results show that the flow stress increases with the temperature decreasing and the strain rate increasing. By comparing the experimental and calculated results, the flow stress and damage evolution can be accurately predicted by the constitutive model. The fracture due to damage can be well predicted by the simulation model, indicating the good predictability of the constitutive model.
The Fe50Mn30Co10Cr10 high-entropy alloy (HEA) was subjected to plate impact loading with a single-stage light gas gun and the Doppler pin system (DPS) was used to measure the velocity of free surface particles of the sample during loading. The effects of microstructure on the evolution of spallation of the Fe50Mn30Co10Cr10 HEA were investigated by means of electron back scattered diffraction (EBSD), optical microscope (OM) and X-ray diffraction (XRD) techniques. The results showed that the voids are neither nucleated at the martensite (M)/matrix interface nor nucleated in the M aggregated area (M area) with high impact resistance as predicted by impact dynamics, instead, voids nucleated inside the matrix phase with low impact resistance. The reason is that the lattice expansion during M transformation leads to the formation of compressive residual stress at the M/matrix interface as well as M area and tensile residual stress in the matrix, which promoted the preferential nucleation of voids in the matrix. Void nucleation in the matrix was not random: the tri-junction of general high angle grain boundaries (HAGBs) and the grain boundaries of adjacent grains with a large difference in Taylor Factor (TF) value were prone to stress concentration under tensile loading, which were the priority position for void nucleation. The void nucleation was inhibited by twin boundary (TB) due to its stable structure and low energy. However, the intersections of HAGBs and termination TB are the possible position for void nucleation due to its high interface energy. The compressive residual stress at the M/matrix interface and inside of M area has a closure effect on the microcrack, which caused the microcrack avoided the M area and propagated in the matrix and finally stopped at the M/matrix interface.
The Fe50Mn30Co10Cr10 high entropy alloy with different martensite phase content were dynamic loaded at 190 m/s and 420 m/s impact velocity respectively by one-stage light gas gun. The velocity of free surface particles was measured by Doppler pin system (DPS) experiment during loading. The effect of phase content on dynamic damage evolution in Fe50Mn30Co10Cr10 high entropy this alloy was firstly studied. Results showed that after hot-rolling and quenching (HRQ), the mass percentage of martensite in the HRQ sample was 28.3 wt%, and the size of martensite aggregated area (M area) was large (∼49 μm). The sample cold-rolled and quenched (CRQ) after HRQ treatment had more martensite (46 wt%), and M areas in the CRQ sample were uniformly distributed and relatively smaller (∼10 μm) than that of the HRQ sample. Voids nucleated in the matrix where the impact impedance was lower than the martensite. The compressive residual stress in the M area and tensile residual stress in the matrix induced by the martensitic transformation inhibited and promoted nucleation of voids in the M area and matrix. The uniform distributed tensile residual stress areas with smaller size in the CRQ sample was more, which led to the more nucleation sites, higher nucleation density, and higher initial damage rate. in the CRQ samples, Meanwhile, voids were difficult to coalesce due to larger space between voids and limit of growth by the small-sized M areas, thus the damage evolution rate of the CRQ sample was relatively lower. Therefore, the spall strength of the CRQ sample (1.89 GPa) was relatively smaller than that of the HRQ sample (2.21 GPa) under the impact velocity of 190 m/s. Microcrack propagation “avoided” the small-sized and uniformly distributed M area, so the turning path and direction deviation of microcrack were shorter and smaller. Then the microcracks in the CRQ sample tended to coalesced into a large-sized crack, which lead to a higher crack propagation rate and lower spall strength of the CRQ sample (1.98 GPa) under higher impact velocity of 420 m/s. Furthermore, the microcrack in the CRQ sample could propagate along a “coalescence path” that formed at the interface of the adjacent M areas, which may have further increased the rate of crack propagation, and resulted in the lower spall strength of the CRQ samples.
The dual‐phase steel sample obtained by step quenching (SQ) of AISI 1020 steel is shock loaded using one‐stage light‐gas gun experiments. The free surface particle velocity is measured by Doppler pin system (DPS) during the loading experiment. The effect of phase interface on spallation damage nucleation and evolution in dual‐phase steel is investigated with optical microscopy (OM), nanoindentation, and electron backscatter diffraction (EBSD) techniques. The evolution mechanism of spallation damage in dual‐phase steel is systematically analyzed. The results show voids nucleated within martensite rather than at the martensite/ferrite interface under dynamic loading. The nucleation of voids in martensite was not random, the martensite block boundaries with a great Taylor Factor (TF) mismatch and a large angle to the impact direction were the preferred nucleation positions of the voids. Voids nucleate, grow and further coalesce into microcracks. Most of the microcracks propagate across the phase interface and the propagation direction hardly deflects. Due to the existence of many martensite blocks with high‐angle grain interface boundaries inside the martensite, the resistance of microcracks propagation in martensite is greater than that in ferrite. Furthermore, the size of microcracks is related to the angle between the direction of martensite blocks and shock direction.
For ductile metals, dynamic fracture occurs principally through void nucleation, growth, and coalescence at heterogeneities in the microstructure. Previous experimental research on high purity metals has shown that microstructural features, such as grain boundaries, inclusions, vacancies, and heterogeneities, can act as initial void nucleation sites. In addition, other research on two-phase materials has also highlighted the importance of the properties of a second phase itself in determining the dynamic response of the overall material. However, previous research has not investigated the effects of the distribution of a second phase on damage nucleation and evolution. To approach this problem in a systematic manner, two copper alloys with 1% lead materials, with the same Pb concentration but different Pb distributions, have been investigated. A new CuPb alloy was cast with a more homogeneous distribution of Pb as compared to a CuPb where the Pb congregated in large “stringer” type configurations. These materials were shock loaded at ∼1.2 GPa and soft recovered. In-situ free surface velocity information, and post mortem metallography, reveals that even though the spall strength of both the materials were similar, the total extent and details of damage in the materials varied by 15%. This suggests that altering the distribution of Pb in the Cu matrix leads to the creation of more void nucleation sites and also changed the rate of void growth.
Plate impact experiments were conducted to study the effects of peak shock stress and pulse duration on the spall response of fully annealed 1100 aluminum. The spall strength was observed to decrease as the pulse duration was increased from approximately 0.58 μs to 1.17 μs. Also, an increase in tensile unloading strain rate increases the spall strength. However, our results also show an increase in spall strength with increase in peak shock stress up to approximately 8.3 GPa, followed by a decrease in spall strength for higher shock stresses.
Photon Doppler Velocimetry was used to characterize the early time formation of the jet from a 65-mm diameter shaped charge with a copper liner. This tool provided a high fidelity, high temporal resolution technique to characterize the axial velocity evolution of the shaped charge jet's first motion to exit from the base of the conical liner. The measurements were then compared with continuum mechanics shock physics simulations, using both CTH and ALEGRA, to understand the jet formation process and evolution.
This book presents the papers given at a conference on the impact testing of metals. Topics considered at the conference included dynamic consolidation, the analysis of dislocation kinetics across shocks, high-strain-rate deformation, adiabatic shear band phenomena, dynamic fracture, explosive metal working, shock synthesis and the property modification of materials, and novel concepts and applications of high pressure.
Plate impact experiments have been carried out to examine the influence of grain boundary characteristics on the dynamic tensile response of Cu samples with grain sizes of 30, 60, 100, and 200 μm. The peak compressive stress is ∼1.50 GPa for all experiments, low enough to cause an early stage of incipient spall damage that is correlated to the surrounding microstructure in metallographic analysis. The experimental configuration used in this work permits real-time measurements of the sample free surface velocity histories, soft-recovery, and postimpact examination of the damaged microstructure. The resulting tensile damage in the recovered samples is examined using optical and electron microscopy along with micro x-ray tomography. The free surface velocity measurements are used to calculate spall strength values and show no significant effect of the grain size. However, differences are observed in the free surface velocity behavior after the pull-back minima, when reacceleration occurs. The magnitude of the spall peak and its acceleration rate are dependent upon the grain size. The quantitative, postimpact, metallographic analyses of recovered samples show that for the materials with grain sizes larger than 30 μm, the void volume fraction and the average void size increase with increasing grain size. In the 30 and 200 μm samples, void coalescence is observed to dominate the void growth behavior, whereas in 60 and 100 μm samples, void growth is dominated by the growth of isolated voids. Electron backscatter diffraction (EBSD) observations show that voids preferentially nucleate and grow at grain boundaries with high angle misorientation. However, special boundaries corresponding to Σl (low angle, < 5 °) and Σ3 (∼60 ° <111> misorientation) types are more resistant to void formation. Finally, micro x-ray tomography results show three dimensional (3D) views of the damage fields consistent with the two dimensional (2D) surface observations. Based on these findings, mechanisms for the void growth and coalescence are proposed.
The incipient spall behaviours of Cu-34%Zn-3%Pb leaded brass samples with annealed and cryogenic-treated conditions were loaded using one-stage light gas gun experiments. The effect of Pb-phase on dynamic damage nucleation in leaded brass specimens was investigated by means of optical microscopy, scanning electron microscopy and x-ray computer tomography. It was found that the voids of incipient spall were mainly nucleated in the interior of the lead (no tensile stress would be produced within lead according to the impact theory) instead of nucleated at the phase interface as expected by quasi-static damage fracture theory. A nucleation model is proposed in the present work that is the asymmetry high compression zones in the centre of the lead-phase were formed by the rarefaction wave convergence effects of matrix/quasi-spherical lead interface, which caused adiabatic temperature rise that exceeded melting point of lead due to severe plastic deformation, finally led to local melting and void nucleation. In addition, the spall strength and damage rate increased with the increase in the Pb-phase number.
The spall behaviors of Cu-34%Zn-3%Pb leaded brass samples with annealing and cryogenic treating conditions were dynamic loaded using one-stage light gas gun experiments under ∼1.515 GPa shock pressure. The effects of α/β phase interface and Pb/matrix phase interface on dynamic damage nucleation, growth, and coalescence in leaded brass specimens were investigated by multidimensional testing techniques. Experiment results showed that voids of incipient spall were mainly nucleated in the interior of the Pb-phases with signs of melting, and a small number of voids were nucleated in α-phase, and both are not nucleated at the interface which contradicts the damage fracture theory under quasi-static loading. Due to the effect of reflection and transmission of shock wave at the phase interface, when the shock wave propagates from α-phase with higher impedance to Pb-phase or β-phase with lower impedance, a tensile pulse will be formed within α-phase. If the tensile pulse has sufficient amplitude voids would be nucleated in α-phase. On the other hand, it is considered that the asymmetry high compression zones in the center of the lead-phase with low impedance were formed by the shock wave convergence effects of matrix/lead quasi-spherical interface, which caused adiabatic temperature rise exceeded melting point of lead due to severe plastic deformation, and finally led to local melting and void nucleation in the center of the lead-phase rather than the tensile stress.
The effects of different peak compression stresses (2–5 GPa) on the spallation behaviour of high purity copper cylinder during sweeping detonation were examined by Electron Backscatter Diffraction Microscopy, Doppler Pins System and Optical Microscopy techniques. The velocity history of inner surface and the characteristics of void distributions in spalled copper cylinder were investigated. The results indicated that the spall strength of copper in these experiments was less than that revealed in previous reports concerning plate impact loading. The geometry of cylindrical copper and the obliquity of incident shock during sweeping detonation may be the main reasons. Different loading stresses seemed to be responsible for the characteristics of the resultant damage fields, and the maximum damage degree increased with increasing shock stress. Spall planes in different cross-sections of sample loaded with the same shock stress of 3.29 GPa were found, and the distance from the initiation end has little effect on the maximum damage degree (the maximum damage range from 12 to 14%), which means that the spallation behaviour was stable along the direction parallel to the detonation propagation direction under the same shock stress.
The spall behaviors of high-purity copper samples with different heat treatment histories were investigated using optical microscopy and X-ray computer tomography (XRCT). The spall samples were obtained by sliding detonation experiments at low pressures (2 to 4 GPa). It was found that the spall planes created by sliding detonation in this experiment are similar to the spall planes created by plate impact test, except for more secondary damage residual around the main spall plane. The results of damage degree, the shape, and the distributions of voids obtained by the means of metallography (2D) and XRCT (3D) statistics were consistent. For similar microstructure, the maximum damage degree and damage zone width increase with increasing shock stress. Whereas the ranges of voids distribution parallel to the shock stress direction decreases with the increasing of shock stress. For the shock stress is similar, the shape of voids in annealed samples are closed to spheres, their mean flatness is 0.51. The voids in samples with thermo-mechanical treatment histories are sheet like with mean flatness 0.16. The difference in grain size (40 and 9 μm) may be the main reason of such difference.
For ductile metals, dynamic fracture occurs through void nucleation, growth, and coalescence. Previous experimental works in high purity metals have shown that microstructural features such as grain boundaries, inclusions, vacancies, and heterogeneities can act as initial void nucleation sites. However, for materials of engineering significance, those with, second phase particles it is less clear what the role of a soft second phase will be on damage nucleation and evolution. To approach this problem in a systematic manner, two materials have been investigated: high purity copper and copper with 1% lead. These materials have been shock loaded at ∼1.5 GPa and soft recovered. In-situ free surface velocity information and post mortem metallography reveals the presence of a high number of small voids in CuPb in comparison to a lower number of large voids in Cu. This suggests that damage evolution is nucleation dominated in the CuPb and growth dominated in the pure Cu.
The effect of crystalline structure on intergranular failure during shock loading has been examined. A suite of dynamic tensile experiments, using plate-impact testing, were conducted on copper (face-centered cubic) and tantalum (body-centered cubic) specimens with different grain sizes (30–200 μm). These experiments were designed to probe void nucleation, growth, and coalescence processes that for these materials are known to lead to failure. For the grain sizes examined in the study, post-impact metallographic analyses show that in copper specimens, during the early stages of deformation, voids were present primarily at general or low-coincidence, high-angle grain boundaries (GBs), irrespective of grain size. In tantalum, while some voids developed along the GBs, an increasing amount of transgranular damage was observed as the grain size increased. A scenario based on the availability of potential nucleation sites and number of slip systems inherent to each crystalline structure is discussed. The role that this availability plays in either promoting or hindering plastic processes leading to damage nucleation and growth is then examined.
The deformation and failure of bulk Cu–Nb nanocomposites with a nominal layer thickness of 135 nm was investigated under planar shock loading. It was observed that little substructural evolution was evident after shock compression to a peak stress of 7 GPa, while specimens were fully spalled after loading to 7 GPa under free surface conditions. In these fully spalled specimens, the characteristics of ductile failure that formed on the fracture surface were dependent upon the processing route of the nanocomposite. Specifically, process-induced grain-shape differences due to dissimilar rolling passes are linked with differences in the failure response. In addition, incipient failure was also observed. Numerous nanovoids, 20 nm or less in size, nucleated and aligned in a row in the middle of Cu layers. Due to the reflection of the shock wave at the Cu–Nb interfaces, incipient voids tend to nucleate within the Cu phase, which has a higher impedance and lower spall strength than Nb. This occurs rather than nucleation along the Cu–Nb interfaces or in the Nb phase. This finding contradicts the general thinking of failure starting from interfaces, and indicates that the Cu–Nb interfaces are stable under dynamic loading. It is postulated that numerous voids nucleate in the Cu layers under shock loading, then lead to failure through their growth and coalescence.
This paper introduces the concept of effective structural size in titanium alloys and its importance with respect to material production routes and component lifing/design. Traditionally, process route optimization has relied on optical microscopy, which may be misleading when predicting mechanical properties. Similarly, continuum mechanics and current lifing methods are based on empirical data analysis. The advent of advanced material characterization techniques, e.g. EBSD combined with crystal plasticity modelling, has the potential to provide the next generation of mechanistically sound methods that more accurately predict material behaviour in complex loading regimes. These benefits are reviewed in the context of industrial application. Crystal plasticity modelling techniques are presented and a particular structural unit - termed a rogue grain - in a model single-phase titanium alloy is considered. Cold dwell under both strain and stress control is then assessed in the structural unit.
A suite of impact experiments was conducted to assess spatial variability in the dynamic properties of tantalum, on length scales of tens of microns to a few millimeters. Two different sample types were used: tantalum processed to yield a uniform refined grain structure (grain size ∼20μm) with a strong axisymmetric {111} crystallographic texture, and tantalum processed to yield an equiaxial structure with grain size ∼42μm. Impact experiments were conducted loading the samples to stress levels from 6 to 12GPa, which are well above the Hugoniot Elastic Limit (HEL), then pulling the sample into sufficient tension to produce spall. These stress levels were specifically chosen to investigate the spall behavior of tantalum at levels ranging from the incipient spall stage to significantly above the spall strength, focusing on microstructural phenomena. A recently developed spatially resolved velocity interferometer known as the line-imaging VISAR allowed the point-to-point variability of the spall strength to be determined. Specifically, we have been able to determine in real time the nucleation and growth of void defect structures that lead to the eventual spallation or delaminating of the plate. Experiments indicate that the nucleation and growth process is time-dependent and heterogeneous since a time-dependent distribution of defects is measured. This strongly suggests that the spall strength of the material is not a single-valued function. When fitted to Weibull failure statistics, the results indicate a similar mean value and variability for the spall strength of both types of tantalum. The spatial dependence of the material distension of the spalled tantalum is also deduced, in the approximation of uniaxial strain.
This paper presents results on the dynamics of damage in copper under incipient spall conditions for multicrystalline specimens. Specimens were annealed from polycrystalline material to reduce the number of grain boundaries the shock wave traverses in passing through the specimen. The specimens incorporated unique fiducials that permit accurate correlation of pre‐shot characterization of the grain orientations, grain size etc. with the location of the dynamic diagnostics and with post‐shot metallography. The dynamic diagnostics—Transient Imaging Displacement Interferometry (TIDI), point VISAR, line VISAR—were all precisely aligned on the specimen surface to view regions of interest revealed in the pre‐shot characterization. Initial analyses demonstrate the wealth of information obtained from this experimental approach. Examples include observation that regions in the surface microstructure most likely to be damaged based on pre‐shot characterization show some of the largest displacements during the shock‐loading. Post‐shot microscopy shows damage in the same places. Another observation is there appears to be localized plastic deformation that occurs during the first compression wave which then remains and evolves through several cycles of compression and release from the spall plane.
The influence of increasing strain rate on the mechanical behavior and deformation substructures in metals and alloys that deform predominately by slip is very similar to that seen following quasi-static deformation at increasingly lower temperatures or due to a decrease in stacking-fault energy (γsf). Deformation at higher rates (a) produces more uniform dislocation distributions for the same amount of strain, (b) hinders the formation of discrete dislocation cells, (c) decreases cell size, and (d) increases misorientation, with more dislocations trapped within cell interiors. The suppression of thermally activated dislocation processes in this regime can lead to stresses high enough to activate and grow deformation twins even in high-stacking-fault-energy, face-centered-cubic metals. In this review, examples of the high-strain-rate mechanical behavior and the deformation substructure evolution observed in a range of materials following high and shock-loading strain rates are presented and compared with ...
The indentation load-displacement behavior of six materials tested with a Berkovich indenter has been carefully documented to establish an improved method for determining hardness and elastic modulus from indentation load-displacement data. The materials included fused silica, soda–lime glass, and single crystals of aluminum, tungsten, quartz, and sapphire. It is shown that the load–displacement curves during unloading in these materials are not linear, even in the initial stages, thereby suggesting that the flat punch approximation used so often in the analysis of unloading data is not entirely adequate. An analysis technique is presented that accounts for the curvature in the unloading data and provides a physically justifiable procedure for determining the depth which should be used in conjunction with the indenter shape function to establish the contact area at peak load. The hardnesses and elastic moduli of the six materials are computed using the analysis procedure and compared with values determined by independent means to assess the accuracy of the method. The results show that with good technique, moduli can be measured to within 5%.
ABSTRACTA crystal plasticity model for near-alpha hcp titanium alloys embodying a quasi-cleavage failure mechanism is presented and employed to investigate the conditions necessary in order for facet nucleation to occur in cold-dwell fatigue. A model polycrystal is used to investigate the effects of combinations of crystallographic orientations (and in particular, a rogue grain combination), the essential role of (cold) creep during hold periods in the loading cycle and the more damaging effect of a load hold rather than a strain hold in facet nucleation. Direct comparisons of model predictions are made with dwell fatigue test results. More generally, the crystal model for faceting is found to be consistent with a range of experimental observations.
A systematic study to quantify the effects of specific microstructural features on the spall behavior of 99.999 pct copper
has revealed a strong dependence of the failure processes on length scale. Shock loading experiments with Cu flyer plates
at velocities ranging from 300 to 2000 m/s (or impact pressures from 5 to 45 GPa) using a 35-mm single/two-stage light gas
gun revealed that single crystals exhibit a higher spallation resistance than fine-grained polycrystals and internally oxidized
single crystals. However, in contrast to previously reported results, the fine-grained (∼8-µm) polycrystalline samples exhibit
lower damage resistance than the coarse-grained (50- and 133-µm) samples. These observations have been analyzed in the context
of the length scale inherent in each of these microstructures, and modeled using an analytical model developed recently.
Fracturing, or scabbing, of a material near a free surface as the result of a transient compressional stress wave of high intensity impinging on that surface has been observed for many years; however, little quantitative data that relate the fracture to the nature of the stress wave and the physical properties of the material seem to exist. The phenomenon has been investigated for five metals, 1020 steel, 4130 steel, 24S-T4 aluminum alloy, brass, and copper, by using an explosive charge to induce a high intensity stress wave in the metal. The distribution of pressure within the wave was determined by a modified Hopkinson-bar type of experiment.
Scabbing has been found to be governed principally by the spatial distribution of pressure within the wave and a critical normal fracture stress σc that is characteristic of the material and perhaps the state of stress. Numerical values of σc were obtained for each of the five metals.
A crystal plasticity model for hcp materials is presented which is based on dislocation glide and pinning. Slip is assumed to occur on basal and prismatic systems, and dislocation pinning through the generation of geometrically necessary dislocations (GNDs). Elastic anisotropy and, through the coupling of GNDs with slip rate, physically-based lengthscale effects are included.A model polycrystal representative of the alloy Ti–6Al, which shows creep and strain rate effects at 20 °C, is developed and it is shown that the primary effect of elastic anisotropy during subsequent plastic flow is to increase local, grain-level, accumulated slip. Lengthscale effects, however, are shown to lead to quite considerable increases in grain-boundary stresses, and to the re-distribution of accumulated slip local to grain boundaries. In particular, an initially highly non-uniform slip distribution tends to be made more uniform through the hardening effect of sessile GNDs at grain boundaries.The concept of a rogue grain combination is presented; that is, a ‘primary’ grain with c-axis orientation at or near parallel to macro-level loading, together with adjacent grains with a prismatic plane orientated at about 70° to the normal to the load direction. This particular combination of orientations leads to the highest stresses normal to the primary basal, together with high levels of accumulated slip in adjacent grains.The presence of a rogue grain combination in cycles both with and without cold dwell is examined. The cycle with cold dwell is shown to be the more damaging, and a possible criterion for facet nucleation in Ti-alloys is introduced.
This paper reviews recent attempts to construct a microstatistical fracture mechanics; that is, a methodology that relates the kinetics of material failure on the microstructural level to continuum mechanics. The approach is to introduce microstructural descriptions of damage into the continuum constitutive relations as internal state variables. The microstructural damage descriptions are based on dynamic and quasi-static experiments with carefully controlled load amplitudes and durations. The resulting constitutive relations describe the nucleation, growth, and coalescence of the microscopic voids and cracks, and therefore in principle describe both quasi-static and dynamic fracture on the continuum scale. The paper describes several such kinetics models in detail, shows examples of several engineering applications, and discusses the link between microstatistical fracture mechanics and continuum fracture mechanics.
Dynamic failure of metals
- M A Meyers
- C T Aimone
M.A. Meyers, C.T. Aimone, Dynamic failure of metals, Progress. Mater. Sci. 28 (3)
(1983) 1-96.
- C Williams
- B Love
C. Williams, B. Love, Dynamic Failure of Materials, Dyn. Fail. Mater. A Rev. (2010).
Dynamic failure in two-phase materials
- S J Fensin
- E K Walker
- E K Cerreta
- C P Trujillo
- D T Martinez
- G T Gray
S.J. Fensin, E.K. Walker, E.K. Cerreta, C.P. Trujillo, D.T. Martinez, G.T. Gray,
Dynamic failure in two-phase materials, J. Appl. Phys. 118 (23) (2015)
033513-033555, http://dx.doi.org/10.1063/1.493810.
The role of interfaces on dynamic damage in two phase metals
- E Cerreta
- S Fensin
- G Gray Iii
E. Cerreta, S. Fensin, G. Gray Iii, et al., The role of interfaces on dynamic damage in
two phase metals, AIP Conf. Proc. 1426 (1) (2012) 5001, http://dx.doi.org/10.
1063/1.368652.