Damage Nucleation Sites Research Articles

Shock and spallation behavior of ultrahigh molecular weight polyethylene

We investigate dynamic compression and fracture of ultra-high molecular weight polyethylene (UHMWPE) with plate impact experiments, high-speed velocimetry, and postmortem X-ray computed tomography. The Hugoniot equation of state of UHMWPE up to peak shock stress of ∼2.0 GPa is obtained via reverse impact. Spall strength and corresponding tensile strain rates are determined via symmetric impact. With increasing impact velocity, spall strength is nearly a constant (∼70 MPa) up at first, and then decreases continuously as a result of competition between increasing tensile strain rate, shock heating and strain softening; the damage degree increases first and then decreases rapidly at the expense of more damage nucleation sites. The void size distributions follow a power law. The small voids are closer to spheres and voids/cracks becomes flatter as void volume increases, likely because of the rearrangement and breakage of linear polymer chains of UHMWPE.

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.

Shock compression and spall damage of dendritic high-entropy alloy CoCrFeNiCu

Plate impact experiments are conducted on a dendritic dual face-center cubic phase high-entropy alloy (HEA) CoCrFeNiCu, consisting of Cu-lean dendritic (DR) and Cu-rich interdendritic regions (ID). Free surface velocity histories are obtained along with microstructure characterizations. The Hugoniot equation of state and spall strength at different shock stresses are determined. Dislocation slip is the main deformation mode for CoCrFeNiCu HEA, and the dislocation density of the Cu-rich interdendritic regions increases more significantly during shock compression. Both ductile and brittle damage modes are observed. With increasing impact velocity, the Cu-rich interdendritic regions with severe strain localizations and more defects provide more damage nucleation sites. The spall strength increases firstly, reaches its maximum at a peak shock stress of 5.8 GPa, and then decreases. Molecular dynamics simulations reveal that the abnormal dependence of spall strength on peak shock stress is a result of strain localization and thermal softening in the Cu-rich regions.

The influence of pearlite fraction on the shock properties of ferrite–pearlite steel microstructures: Insight into the effect of second-phase particles

The goal of this work is to investigate the effect of varying phase fractions on the overall spall strength and damage behavior of a material. Specifically, two plain carbon, ferrite–pearlite steels (1045 and A283) were subjected to spall recovery experiments to investigate the effect of pearlite fraction on spall strength and total damage. The A283 (20% pearlite) alloy exhibited a higher Hugoniot elastic limit and spall strength compared with 1045 (60% pearlite). Discontinuous and continuous yielding behaviors were observed at quasi-static and dynamic rates for A283 and 1045, respectively. The yielding behavior was connected to pearlite fraction and the prevalence of dislocation-emitting, ferrite/cementite interfaces. Postmortem characterization revealed cementite lamellae cracking within pearlite of 1045, suggesting that pearlite reduces spall strength by providing low-energy damage nucleation sites. The rate of damage growth and coalescence was similar between the two alloys; however, 1045 exhibited more continuous cracks than A283, which exhibited a greater prevalence of discrete voids.

Unveiling damage sites and fracture path in laser powder bed fusion AlSi10Mg: Comparison between horizontal and vertical loading directions

The laser powder bed fusion (LPBF) additively manufactured AlSi10Mg alloy has been subjected to numerous investigations addressing its microstructure and mechanical properties. However, there is still a gap in the understanding of the correlation between the microstructure and fracture behavior, in particular when the melt pool border is oriented differently to tensile load. In this work, damage nucleation sites and fracture path of as built LPBF AlSi10Mg were analyzed and compared for two loading directions. For the first time, the orientation of the melt pool border is shown to present negligible effect on a ductility exceeding 10%, in contrast to previous works. Through observation and statistical analysis of damage, it is found for both loading directions that the melt pool border does not exhibit damage localization. When the melt pool border is perpendicular to tensile load, the crack propagates frequently along the coarse melt pool due to easier damage growth in this relatively softer region. Nevertheless, the ductility is barely compromised owing to delayed damage coalescence across larger α-Al cells.

Elastic properties of Mn-rich α intermetallic phase in engineering aluminum alloys: An ab initio study

The α intermetallic phase can be found in almost all aluminum alloys used in engineering, primarily because of the presence of unwanted but unavoidable impurities from primary production or recycling. There are ample examples showing that α intermetallic particles act as damage nucleation sites during forming operations. To achieve an in-depth understanding of these particles as damage nucleation sites, it is important to know their thermomechanical behavior, as well as their interactions with the matrix during production and service. Despite their importance, however, the mechanical properties of the α intermetallic phase, such as the elastic modulus and the thermal expansion, have not been very well studied. Here, we apply ab initio methods to study the mechanical properties of the α intermetallic phase, with the focus on two polymorphs of the Mn-rich α phase: Al114Mn24 and Al108Mn24Si6. Besides the ground-state elastic properties, the temperature-dependent thermal expansion coefficient and bulk modulus are also calculated. As a case study, these calculated properties are used as input to an Eshelby-type eigenstrain model to evaluate the thermal residual stress of a spherical α-phase particle in an aluminum matrix during a cooling process.

Residual ferrite in martensitic stainless steels: The effect of mechanical strength contrast on ductility

Ductile damage process of a martensitic stainless steel with 15 vol% of residual ferrite and two populations of carbide particles is investigated using a combined multiscale experimental and modelling approach. Whereas Nb-rich carbides contribute to grain refinement, coarser Cr-rich carbides are preferential damage nucleation sites. Three different heat treatments are applied to partially dissolve Cr-carbide particles while keeping the same ratio of ferrite versus martensite volume fraction. Surprisingly, ductility decreases with decreasing volume fraction of Cr-carbides. Nanoindentation mapping indicates that the strength contrast between ferrite and martensite increases with carbide dissolution. According to finite element simulations of strain partitioning inside the two phase microstructure, the stress triaxiality in ferrite increases with the mechanical strength contrast. This promotes void nucleation and growth, reducing the fracture strain.

Study of Evolution of Asphalt Binder Microstructure Resulting from Aging and Tensile Loading

This paper presents findings from a study conducted to evaluate changes in the microstructure of asphalt binder resulting from aging as well as the effect of these changes on the evolution of damage resulting from tensile deformations. Two types of asphalt binders were aged with the rolling thin film oven and pressure aging vessel aging techniques. The microstructure and the microrheology of the six binders were obtained with atomic force microscopy (AFM) imaging and creep indentation experiments. This information was then used to perform numerical simulations to examine the effect of tensile strains on the internal stress distribution in the binder. Experimentally, a microloading apparatus was used to induce tensile strains in the samples of the asphalt binder while the binder was observed for damage with the use of AFM. The damage and changes in the binder microstructure resulting from the tensile load observed experimentally were compared with the results from the numerical simulations. Results suggest that localized regions of high stress intensity between different domains act as damage nucleation sites. Results also suggest that the differences in the rheological properties of the microdomains reduce with aging. As a result, the internal distribution of stresses becomes less heterogeneous, the magnitude of stress localization decreases, and there are fewer sites where damage nucleates.

Simulations of Damage, Crack Initiation, and Propagation in Interlayer Dielectric Structures: Understanding Assembly-Induced Fracture in Dies

Performance enhancement by lowering the dielectric constant of interlayer dielectric (ILD) materials often compromises the mechanical integrity of the dielectric stack. At the present time, fracture in the ILD stacks induced by assembly to either an organic substrate or a die stack (3-D) is an important reliability consideration. These interactions include what is popularly referred to as the chip-package interactions. In this paper, we develop insights on the potential crack initiation site within the ILD, die-substrate geometrical parameters that cause most damage, as well as insights on the manufacturing process that is critical to failure. Towards this end, we utilize analytical models based on classical elasticity theory as well as sophisticated numerical techniques that are capable of nucleating and propagating cracks at arbitrary locations within the structure without remeshing. Specifically, we analytically estimate the strength of singularities at all the possible multimaterial corners in the ILD stack to provide insight on the likely damage nucleation sites for various material configurations in the ILD stack. Two novel numerical approaches are used for fracture simulation. In the first, cracks are modeled as discontinuous enrichments over an underlying continuous behavioral approximation. In the second approach, the underlying material description is enriched with a cohesive damage description whose stiffness is evolved according to a prescribed damage law. Multilevel finite-element models are used to determine the load imposed on the ILD structure by the substrate. Maximum damage induced in the ILD stack by the above load is used as an indicator of the reliability risk. Parametric simulations are conducted by varying ILD material, die size, die thickness, as well as the solder material. Through analytical models of bonded assemblies, we identify groups of relevant dimensionless parameters to relate the numerically estimated damage in ILD stacks to the die/substrate material and geometrical parameters. We demonstrate that the damage in the ILD stack is least when the flexural rigidity of the die is matched to that of the assembled substrate. We also demonstrate that ILD damage is only weakly correlated to shear deformation on the die surface due to assembly. We generalize the above observations into mathematical fits (for use as design rules) relating damage in ILD stacks to ILD material choice, relative substrate flexural rigidity, and die size.

Second Phase Particle Distribution And Its EffectOn The Formability Of Aluminum Alloys

Aluminum alloys have been increasingly used in the automotive industry for fuel economy. The existence of second phase particles in aluminum alloys provides damage nucleation sites during formation processes and limits the formability. Characterization of second phase particle distribution and its effect on the formability of aluminum alloys is of great importance. Distribution of Feand Mn-based second phase particles in Al-Mg alloys AA5182 and AA5754 is captured using microscopy and image analysis. Spatial tessellation of particle images is conducted to quantify particle distribution, such as particle size, shape (aspect ratio) and clustering. Large particles are found more often in AA5182 than AA5754. The obtained particle distribution is then applied to a so-called damage percolation model to predict formability of both alloys in stretch flanging.

Damage Evolution and Stress Analysis in Zirconia Thermal Barrier Coatings during Cyclic and Isothermal Oxidation

The failure mechanisms of air‐plasma‐sprayed ZrO2 thermal barrier coatings with various microstructures were studied by microscopic techniques after thermal cycling. The elastic modulus (E) and hardness (H) of the coatings were measured as functions of the number of thermal cycles. Initially, both E and H increased by ∼60% with thermal cycling because of sintering effects. However, after ∼80 cycles (0.5 h at 980°C), the accumulated damage in the coatings led to a significant decrease of ∼20% of the maximum value in both E and H. These results were correlated with stresses measured by a spectroscopic technique to understand specific damage mechanisms. Stress measurement and analysis revealed that the stress distribution in the scale was a complex function of local interface geometry and damage in the top coat. Localized variations in geometry could lead to variations in measured hydrostatic stresses from −0.25 to −2.0 GPa in the oxide scale. Protrusions of the top ZrO2 coat into the bond coat were localized areas of high stress concentration and acted as damage‐nucleation sites during thermal and mechanical cycling. The net compressive hydrostatic stress in the oxide scale increased significantly as the scale spalled during thermal cycling.

Creep damage nucleation sites in ferrous alloys

The effect of impurities on creep damage nucleation in, and the hot ductility of, ferrous alloys was investigated. It was found that cavities will preferentially nucleate on sulfide particles, may nucleate on oxide and nitride particles depending on the chemistry of the particle-matrix interface, and never nucleate on carbide particles. Careful control of composition can therefore be used to decrease an alloy's susceptibility to creep damage nucleation. Further work should concentrate on quantifying the distribution of grain boundary sulfides and correlating this distribution with accumulated damage in components. This information can then be used for life prediction.A microradiographic technique utilizing synchrotron radiation was used to image grain boundary damage in several ferrous alloys. The main advantage of this technique is its ability to examine the volume of a bulk sample (1–2 mm thick) with reasonable resolution. Statistically significant results on the size and spatial distribution of creep damage can therefore be obtained from one sample. Predictions of damage accumulation models can be compared with experimental results on the distribution of damage using a combination of microradiography and image analysis.

Total internal reflection microscopy: a surface inspection technique

Structure at and near the surface of a transparent sample or in a film on a transparent substrate can be observed by illuminating the sample from within using a well-collimated polarized laser beam incident at an angle equal to or greater than the critical angle of the sample material and examining the air side of the surface using an optical microscope. Although the technique is similar to dark-field microscopy, additional information can be obtained here concerning the size and depth of scattering sites on or near the surface. This technique, total internal reflection microscopy (TIRM), is complementary to phase contrast (Nomarski) microscopy. Two TIRM microscopes are shown, one of which is used as an attachment to a commercial Nomarski microscope and the second of which is used in laser damage measurements. This surface inspection technique had been used to study surface polishing and cleaning methods, laser damage nucleation sites, ion milling of optical surfaces, and thin film inclusions. A biological application for liquid medium studies is suggested. A description of the electric fields present at and near the air sample interface is given.