Crack Surface Morphology Research Articles

Processing and Mechanics of Aromatic Vitrimeric Composites at Elevated Temperatures and Healing Performance

Carbon fiber reinforced polymer (CFRP) composites are renowned for their exceptional mechanical properties, with applications in industries such as automotive, aerospace, medical, civil, and beyond. Despite these merits, a significant challenge in CFRPs lies in their repairability and maintenance. This study, for the first time, delves into the processing and self-healing capability of aromatic thermosetting co-polyester vitrimer-based carbon fiber composites through mechanical testing. Vitrimers are an emerging class of thermosetting polymers, which, owing to their exchangeable covalent bonds, enable the re-formation of bonds across cracks. The specific vitrimer chosen for this study is an aromatic thermosetting co-polyester (ATSP). The mechanical properties of samples were analyzed initially through three-point bending (3PB) testing at room temperature before and after healing (by curing samples for 2 h at 280 °C). Samples were also 3PB tested at 100 °C to analyze their mechanical properties at an elevated temperature for comparison to the samples tested at room temperature. To investigate the fracture properties, optical microscopy images of samples were taken after 3PB tests, which were analyzed to observe crack initiation and crack growth behavior. Through load–displacement curves from double cantilever beam (DCB) mechanical testing, the Mode I crack initiation fracture toughness values of self-healed composites and control composites were calculated to evaluate healing efficiency in ATSP CFRP composites cured at 280 °C for 2 h. Scanning electron microscopy (SEM) showed a similar surface morphology of cracks before and after self-healing. Micro-computed tomography (CT) X-ray imaging confirmed that the healed samples closely resembled the as-fabricated ones, with the exception of some manufacturing voids, caused by outgassing in the initial healing cycle. This research demonstrated the ability for the in situ repair of ATSP CFRPs by restoring the fracture toughness to values comparable to the pristine composite (~289 J/m2).

Calculation model of depth under quantification of surface morphology of dry shrinkage cracks in red clay

Cracks commonly form in clay owing to water loss, negatively affecting strength and hydraulic properties of soil, leading to rock and soil damage including landslides and dam instability. Understanding the changing patterns of the shrinkage crack depth is crucial for exploring slope infiltration patterns and determining key factors for slope stability. This study suggests a relationship between the surface and depth variations during cracking, and investigates the influence of the surface roughness on shrinkage cracks in saturated red clay. Regression analysis and machine learning algorithms were used to establish a depth prediction model. (1) Based on the surface morphology of shrinkage cracks, they are categorized into three types: linear, curved, and inflectional (linear and curved are the main types). (2) The surface roughness and development rate of surface cracks were positively correlated. (3) Based on the geometric parameters of the crack surface, a prediction model (error within 15%) for crack depth was established using multivariate nonlinear regression, providing a reference for the initial assessment of crack depth. These results provide a better understanding of the development patterns of shrinkage cracks, whose influence on slope stability is critical for mitigating the risks associated with slope instability and other soil failures.

Damage Mechanics Challenge: Predictions based on the phase field fracture model

In this work, we describe our contribution to the Purdue-SANDIA-LLNL Damage Mechanics Challenge. The phase field fracture model is adopted to blindly estimate the failure characteristics of the challenge test, an unconventional three-point bending experiment on an additively manufactured rock resembling a type of gypsum. The model is formulated in a variationally consistent fashion, incorporating a volumetric–deviatoric strain energy decomposition, and the numerical implementation adopts a monolithic unconditionally stable solution scheme. Our focus is on providing an efficient and simple yet rigorous approach capable of delivering accurate predictions based solely on physical parameters. Model inputs are Young’s modulus E, Poisson’s ratio ν, toughness Gc and strength σc (as determined by the choice of phase field length scale ℓ). We show that a single mode I three-point bending test is sufficient to calibrate the model, and that the calibrated model can then reliably predict the force versus displacement responses, crack paths and surface crack morphologies of more intricate three-point bending experiments that are inherently mixed-mode. Importantly, our peak load, crack trajectory and crack surface morphology predictions for the challenge test, submitted before the experimental data was released, show a remarkable agreement with experiments. The characteristics of the challenge, and how changes in these can impact the predictive abilities of phase field fracture models, are also discussed.

B-Spline Surface-Based Reduced-Order Modeling of Nonplanar Crack Growth in Structural Digital Twins

Nonplanar crack growth holds a critical role in aeronautical structures, necessitating effective analysis under mixed fatigue loading to assess structural integrity. This study introduces a reduced-order modeling (ROM) method for predicting nonplanar crack growth in structural digital twins. The method’s advantage lies in its representation of the entire crack surface morphology using a B-spline surface, which better captures its impact on crack growth. The symmetric Galerkin boundary element method–finite element method coupling method is adopted as a full-order method to generate the crack database. Isoparametric coordinates of the crack surface and stress intensity factor serve as input and output, respectively, for training the ROM, which integrates -means clustering, principal component analysis, and Gaussian process regression. The proposed approach is demonstrated using a rotorcraft-mast-like component. Results reveal superior fracture mechanics parameter prediction compared to the crack-front-based ROM. Furthermore, the method boasts three orders of magnitude greater efficiency than full-order simulation, enabling its coupling with approaches like Monte Carlo for probabilistic crack growth analysis. Future work entails integrating our method into the probabilistic framework of digital twins.

Experimental Study on Repairing Corroded Cracks by Electrophoretic Deposition

The corrosion of reinforced concrete could reduce its mechanical properties and cause concrete cracking. Electrophoretic deposition is a new method for repairing rust-cracked reinforced concrete. In this study, we observed the crack surface morphology, mass growth, epoxy resin film thickness, ultrasonic rate, and epoxy resin filling depth of rust-cracked reinforced mortar specimens during the repair process and investigated internal microstructure and phase composition of the repaired mortar, and clarified the principle behind the electrophoretic deposition of cationic epoxy resin and curing agent molecules in the repair solution. The result of research shows that longer repair times decreased the carbonization depth and water absorption of the specimens. After repair, the porosity of the surface mortar decreased, owing to the filling of epoxy resin, and the newly generated epoxy resin film had a dense structure. Thus, the carbonization resistance and waterproof performance of the repaired specimens improved. The repair process of electrophoretic deposition can be divided into the immersion, repair, and curing stages, and the physical and chemical principles of each stage were explained.

On the dependence of crack surface morphology and energy dissipation on microstructure in ductile plate tearing

The two distinct tearing mechanisms observed in ductile metal plates are the void-by-void advance of the crack tip, and the simultaneous interaction of multiple voids on the plane ahead of the crack tip. Void-by-void crack advance, which leads to a cup-cup crack surface morphology, is the dominant mechanism if the plate contains a low number of small void nucleation sites (i.e., second phase particles). Conversely, a large number and/or size of nucleation sites trigger the simultaneous interaction of multiple voids resulting in a slanted crack. The present work aims to provide further insight into the parameters controlling the mechanisms and energy dissipation of plate tearing by focusing on the shape and, thereby, the orientation of the nucleation sites. The study uses a two-dimensional plane strain finite element domain to model the cross section of a plate, subject to mode I tearing, with discretely modeled, randomly distributed, finite-sized elliptic void nucleation sites. The developed finite element setup can capture the dependence of the crack surface morphology on the microstructure of the plate. The simulation results confirm that cup-cup crack propagation develops by intense plastic straining throughout the thinning region of the plate. Conversely, slanted and cup-cone cracks propagate in thin localized shear deformation bands. The energy dissipation is, therefore, greater for cup-cup cracks. The study demonstrates that the damage-related microstructure has a significant role in determining the overall hardening capacity of a plate, which in turn dictates the tearing mode and energy.

The crack surface morphology investigation of S355J2 steel after bending-torsion fatigue

The paper describes the analysis of crack surface morphology of S355J2 steel specimens after bending-torsion fatigue. These experimental investigations of the surface topography were carried out using the focus variation microscope, an optical 3D measurement device. Selected results of measured fracture surfaces for S355J2 steel were analysed according to the surface texture ISO 25178 standard. Differences in roughness values for different loadings were demonstrated. For profile Rx and areal Sx parameters, characteristic relationships of fracture zones have been demonstrated. It has been shown that roughness profile Ra for the rupture area is higher than for the propagation area, as well as increases after both the LCF and HCF tests and increases with the torsional loading level. However, Sa in the propagation area increases and the rupture area decreases.

Analyses of crystal growth, optical, electrical, thermal and mechanical properties of an excellent detector-grade Cd0.9Mn0.1Te: V crystal

A high-quality cadmium manganese tellurium (Cd0.9Mn0.1Te: V or VCMT) crystal was successfully grown via modified Te solution vertical Bridgman method with vanadium doping. The crystal structure and quality were evaluated by powder X-ray diffraction analysis. An infrared transmission spectroscope measured the transmittance of the crystal at 64%, which would suggest that the grown crystal possessed high purity and crystallinity. Ultraviolet-visible-near-infrared spectroscopy analysis obtained the forbidden band width of approximately 1.577 eV. The current-voltage test indicated that the VCMT crystal had a high resistivity of 2.07 × 1010 Ω·cm. Mechanical properties were measured by a Vickers microhardness tester. Crack surface morphology around the indentation was recorded. Furthermore, mechanical properties, such as microhardness, fracture toughness, brittleness index and yield strength were investigated and discussed. The thermal stability of the VCMT single crystal was determined by thermogravimetric analysis. A VCMT detector was fabricated with planar configuration structure, which showed a resolution of 11.62% of the 241Am at 59.5 keV peak.

The effect of grain size on the fatigue overload behaviour of nickel

The fatigue overload behaviours of coarse grain (~30 μm), ultrafine grain (360 nm) and nanocrystalline (~30 nm) Ni are compared under constant amplitude loading at R = 0.1 after a 100% overload. Synchrotron X-ray diffraction is applied to map the elastic crack-tip strain fields at the mid-thickness of a compact tension specimen of nanocrystalline Ni at various stages through the loading cycles, from which the variation in stress before, during and after overload is estimated. Digital image correlation is used to measure the crack length and the displacement fields at the specimen surfaces for both grain sizes, from which the fatigue crack growth (FCG) rate, crack opening displacement and stress intensity factor range are determined. The FCG for coarse grain Ni is most significantly retarded whereas the nanocrystalline Ni is least affected by the overload due to an increased yield stress and a more planar crack surface morphology. As a result, FCG retardation by plasticity, surface roughness and residual stress-induced crack closure are reduced.

Experimental Investigation of Crack Propagation Mechanisms in Commercially Pure Aluminium Plates

The crack surface morphology in tearing of ductile metal plates depends on the mechanical properties, chemical composition and the microstructure of the plate material as well as on the loading conditions and the specimen geometry. This study assesses the crack surface morphologies observed in commercially pure aluminium plates (Al 1050 H14). Mode I tearing was performed in both single and double edge notched tensile test setups with specimens cut from five different plates with different thickness t, viz. t ϵ {0.5, 1,3,4,5} mm. In each combination, the long axis of the notch was either parallel (0°) or perpendicular (90°) to the rolling direction. Fractographs showed that, for specimen thicknesses smaller than 5 mm, the crack had a cup-cup morphology almost throughout its entire length for 0° specimens, while slanted crack propagation was visible in some regions of 90° specimens. For t = 5 mm, on the other hand, the dominant crack morphology was slanted for both directions, the extent of the cup-cup regions being larger for 0° specimens. The crack morphologies were repeatable for each direction-thickness combination, while also being consistent for both test setups having different loading conditions.

Microstructure and mechanical properties of Al-12Si produced by selective laser melting

Al–12Si specimens are produced by selective laser melting (SLM) from gas atomized powders. Installation for the production of powder is original. All specimens were prepared using the EOSINT M 280 device. A fine cellular structure is observed with residual free Si along the cellular boundaries. Room temperature tensile tests reveal remarkable mechanical behavior: the samples show yield and tensile strengths of about 102 MPa and 425 MPa, respectively, along with fracture strain of 12%. The study of crack surface morphology was shown by the example of a sample. Except the spherical pores, the interface of the molten pool also appears on the fracture surface, which indicates a mixture of fragile and ductile fracture. Additionally, the agglomerated silicon group appears also on the fracture surface.

Cyclic strain-induced crack growth in graphite during electrochemical testing in propylene carbonate-based Li-ion battery electrolytes

Lithiation/de-lithiation cycles induced cracks in isostatically pressed graphite samples subjected to constant-load bending tests while simultaneously conducting cyclic voltammetry (CV) experiments in propylene carbonate-based Li-ion battery electrolyte solutions. A large cyclic strain of Δe = 0.95% was generated by lithiation/de-lithiation cycles as determined by micro-Raman measurements carried out concurrently with CV experiments. In-situ optical microscopy measurements of crack lengths, a, showed that the crack-growth rate, da/dt, depended on the stress intensity factor at the crack tip and could be expressed as da/dt = AΔK I . A two-stage crack-growth behaviour was determined with n = 51.3 in the first stage. However, lower crack propagation rates observed in the second stage (n = 9.9) were affected by crack closure due to (1) a rough fracture surface morphology of the graphite cracks, and (2) the deposition of solid electrolyte reduction products on facetted crack surfaces, where both the factors likely contributed to reducing ΔK to ΔK eff.

Hydrogen enhanced fatigue in full scale metallic vessel tests – Results from the MATHRYCE project

This study investigates the fatigue life of CrMo pressure vessels for hydrogen storage by hydraulic and hydrogen pressure cycle tests. Two different sized cylinders have been tested; 35 L inner volume and 28 MPa working pressure (WP) and 198 L volume and 41 MPa WP. On the inner surface of the cylinders U-shaped notches of different depths were machined by electro discharge machining technique. The initial notch sizes were designed based on a two stage fatigue predictive model based on fracture mechanics to develop through wall cracks in the deepest notches after about 50,000 hydraulic cycles together with crack propagation of the intermediate notches and crack initiation in the smallest. The cylinders were cycled between the nominal pressure of 2 MPa and the WP until leak before break (LBB). Strain gauges were placed at the external surface of the cylinders in correspondence of the internally machined notches. On the notches which developed through wall, the strain showed a progressive decrease followed by an increase of the hoop strain during the final stage of crack propagation until LBB failure. Hydrogen effect was clearly identified by the reduction in the number of cycles to failure comparing tests in hydrogen and in oil. Subsequent failure analysis at the end of each test revealed a typical trans-granular fatigue crack surface morphology (with fatigue striations) for tests in oil, while quasi cleavage and intergranular fracture appearance were found for hydrogen tests.

Crack surface morphology and grain misorientation in fatigued aluminium alloy AA7050 samples after interrupted ageing and retrogression-reageing treatments

We present the investigation of fatigue crack surface morphology and crystallographic grain orientation in samples of aluminium alloy AA7050 that had been subjected to interrupted ageing (T6I4-65) and retrogression-reageing (RRA) heat treatments. Both T6I4-65 and RRA ageing treatments generate bimodal (particle size) microstructural features. A single tensile overload was applied and its influence on the crystallographic grain orientation was evaluated using electron backscatter diffraction (EBSD). The results reveal that T6I4-65 and RRA heat treatments contribute to the increased occurrence of planar slip features in the form of small and large flat facets, respectively. T6I4-65 condition leads to a greater degree of lattice misorientation in the vicinity of fatigue crack tip than the RRA condition which is likely to be the evidence of increased plastic deformation due to the overload.

Three-dimensional characterization of microstructurally small fatigue-crack evolution using quantitative fractography combined with post-mortem X-ray tomography and high-energy X-ray diffraction microscopy

An experimental methodology based on post-mortem measurements is proposed to quantify rates of propagation and crack-surface crystallography of a 3-D, naturally nucleated, microstructurally small fatigue crack (MSFC) in a polycrystalline aluminum alloy (Al–Mg–Si). The post-mortem characterization involves: scanning electron microscopy-based fractography to measure crack-front projections (marker bands) at known cycle counts during the load history, X-ray computed tomography to provide high-resolution reconstructions of the 3-D crack-surface morphology, and near-field high-energy X-ray diffraction microscopy to provide 3-D grain geometries and orientations adjacent to fatigue-crack surfaces. Local MSFC-propagation rates are measured by accounting for the 3-D crack-surface morphology and varied by two orders of magnitude in the Al–Mg–Si specimen. Both intergranular and transgranular MSFC evolution were observed, with the latter occurring along a wide range of crystallographic planes. The findings demonstrate: (i) the complexity and variability of 3-D MSFC evolution in the Al–Mg–Si alloy; and (ii) the viability of the post-mortem characterization approach for quantifying 3-D MSFC evolution in polycrystalline alloys.

Crystallographic orientation and temperature effects on the fatigue crack growth rate and resulting fracture surface morphology in PWA1484 single crystal superalloy

ABSTRACTThis research looked at the effect of crystallographic orientation and temperature on the fatigue crack growth rate and the resulting fracture surface morphology in PWA1484 single crystal superalloy. Two groups of single edge notch tension specimens, one group with controlled secondary orientations and one group with uncontrolled secondary orientation, were tested at temperatures from 649°C to 982°C at R‐ratios of 0.1 and 0.7. It was found that the effect of temperature on the crack growth rate becomes more pronounced as the crack driving force increases while the secondary orientation and R‐ratio effects on the crack growth rate increase with increasing temperature. Two types of crack surface morphology were seen during fractography. The first was a precipitate avoidance (γ′ avoidance) morphology that was rolling but still predominantly flat when observed on a larger scale. In <001> primary oriented specimens, this fracture mode tended to follow the precipitate/matrix faces (microscopically cubic) while macroscopically staying essentially normal to the applied loads. The second mode was a form of cleavage (γ′ shearing) and occurred predominantly on octahedral crystallographic planes.

Influences of ellipse cooling holes on creep crack initiation and surface morphology of nickel based superalloys

A creep damage model is developed with the consideration of cavitation and material microstructure degradation as damage factors. Five models are studied with different ratios between the length of ellipse hole axis parallel (denoted by y) and perpendicular to the loading direction (denoted by x), where y maintains constant. Two loading conditions are taken into account: constant external load and constant net section stress. The creep crack initiation time increases with increasing y/x ratio at both loading conditions. The stress distribution morphologies are quite similar for all the models at the start of creep where there are high stress strips between holes in the adjacent rows and low stress strips between holes in the same rows. During the creep, stress relaxation and redistribution induce some differences in the morphologies for different models at the crack initiation time. The damage distributions provide important information on crack initiation and propagation path.

Automated visualization of surface morphology of cracks by means of induced current potential drop technique

This paper presents the applicability of induced current potential drop technique to visualize surface morphology of multiple and/or branched cracks. Measurements of potential drop (PD) were performed around intergranular stress corrosion cracks. Data analysis software to extract crack locations from the PD data was developed. The usefulness of the software was investigated by comparing the analyzed results with a photo of the measured crack. The analyzed crack morphology showed good agreement with the photo of the crack though small crack located near greater cracks was not extracted. Moreover, a small crack, which was difficult to be identified from the photo of the crack, was identified with this software.

Atomistic study of anisotropic effect on two-dimensional dynamic crack

We adopt molecular dynamics (MD) method to extensively study the dynamical process during the crack propagation along two crystallographic directions in the two-dimensional close-packed system. The dependence of crack initiation time on the loading rates is investigated in comparison with continuum analysis. By calculating the displacement and stress field, the results are in excellent agreement with the asymptotic continuum solution of low-speed propagating crack. Moreover, the crack-tip velocity is numerically attained and associated with the instability of crack surface morphology, which results from the strongly anisotropic behavior. Further analysis remarkably observes the crack-branching healing process in that the dislocation emission absorbs the concentrated strain energy of crack tip.

Tribological characteristics and self-repairing effect of hydroxy-magnesium silicate on various surface roughness friction pairs

Tribological characteristics and self-repairing effect of hydroxy-magnesium silicate (HMS) dispersed in lubricant oil on steel-to-steel friction pairs with various surface roughness were analyzed. The friction-reduction, anti-wear and self-repairing performance of various surface roughness friction pairs were examined by friction testing machine. An operation comparison was made between SJ10W-40 lubricant with and without HMS. The surface morphology and elementary composition of the grinding cracks were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results show that the lubrication state changes from boundary lubrication into mixed lubrication after operation in lubricant with HMS. The friction-reduction, anti-wear and self-repairing performance of the friction pairs with various surface roughness are distinctly different. There is a repairing film whose material is different from substrate material on the grinding cracks. In addition, Si, Mg, O, Al and other elements are deposited on the repairing film which contains nanocrystals of these elements. And HMS self-repairing material possesses superior performance of friction-reduction, anti-wear and self-repairing effects.