Crack Path Deflection Research Articles

Extraction and characterization of nano-silica particles to enhance mechanical properties of general-purpose unsaturated polyester resin

Abstract Unsaturated polyester resin (UPR) is the most versatile liquid polymer with a wide range of applications in every aspect of the industry but it has low impact strength, low elongation at break, and low toughness. Its mechanical properties can be enhanced with the addition of an optimum percentage of nano-silica fillers by using ASTM polymer test standards, which have been followed by various research groups. For this research, enhanced mechanical properties of the resin have been tested for 0.5, 1, 2, 3, and 4% amount by weight fraction of the nano-silica as fiber nanomaterial. The sugar cane bagasse ash was collected from the Wenji Sugar Factory and extracted with the required size of the particle, which is 10 nm. The ability of extraction is used to manipulate the particle size as the researcher needs. The aim is to determine the enhanced mechanical properties of the UPR by the addition of optimum nano-silica particles. Nanoparticles have the effect of filling porous regions, crack path deflection, and crack bridging capability of the material, which provides good adhesion with the matrix to increase the mechanical properties of composite materials. Experimental result dictates that 0.5% nano-silica addition with 10 nm particle size performs best by enhancing the mechanical properties of composite material up to 30.45% for tensile, 33% for compression, 17.8% for flexural, a slightly 10% improvement for impact test and it shows an overall 27% better performance than the pure UPR. Thermal stability and glass transition temperature were not influenced by the addition of nano-silica.

Comparison of fatigue crack growth behaviour in electron-beam and laser powder-bed-fusion Inconel 718

A comparative study on fatigue crack growth behaviour was made between the electron-beam powder-bed-fusion (PBF-EB) and laser powder-bed-fusion (PBF-LB) Inconel 718. The crack followed a transgranular path with a faster propagation rate for the PBF-EB Inconel 718, whereas a combination of intergranular and transgranular path was observed for the PBF-LB, and its slower rate being comparable to the wrought counterpart. The main fatigue crack in the PBF-EB Inconel 718 exhibited a sawtooth shaped path at the micro-scale, with intensive slip traces close to the crack surfaces owing to the very low work hardening rate. Based on the digital image correlation (DIC) analysis of the crack tip field, fatigue-crack sawtooth path in the PBF-EB Inconel 718 can be successfully predicted by using a strain energy density criterion, which dictates the crack growth follows the direction of minimum distance from the crack tip to elasto-plastic boundary. For the PBF-LB Inconel 718, the predominant fatigue crack was straight at low ΔK, but severe deflections occurred at the medium and high ΔK regimes. A clear correlation exists between the incipient intergranular cracks and the main crack path deflection. This suggests that once the accumulated damage in the plastic zone around the crack tip reaches the critical value, intergranular cracks can form the new front of the fatigue crack, causing the main crack path deflection. Elasto-plastic fracture mechanics parameters of rp and ΔCTOD, derived from the DIC-based crack tip field analysis, can qualitatively predict the lower crack growth rate of the PBF-LB Inconel 718.

On the cyclic deformation behavior and correlated damage mechanism of selective laser melted AlSi10Mg alloy holding different initial microstructures

The present work deals with the cyclic deformation behavior of AlSi10Mg alloy fabricated through selective laser melting. Toward this end, the printed material was heat treated and was tested through strain-controlled push–pull fatigue under different strain amplitudes. The highest strengths were recorded for the direct aged, as-built and solutionized microstructures, respectively. The directly aged microstructure was characterized by the transformation of the columnar solidified structure into the equiaxed one and formation of the nano-Si precipitates. This was led to the best fatigue performance where an outstanding balance between the fatigue life and strength level was recorded. The low aspect ratio of the melt pools intensified the Marangoni effect, therefore, caused concentration of the material flow on the wetting side. This led to crack initiation and propagation at the end of the melt pools and formation of balling defects. Balling defect was characterized as the initiation site in the case of as-built and directly aged specimens, the cone-like geometry of which resulted in crack path deflection and a better fatigue performance. For solution treated microstructure, the melt pools were dissolved and the lack of fusion holding a critical size and located close to the surface, was characterized as the main crack initiation site.

Unraveling the Cracking Mechanisms of Air Plasma-Sprayed Thermal Barrier Coatings: An In-Situ SEM Investigation

In this research work, real-time three-point bending coupled with the scanning electron microscopy (SEM) technique were used to study the crack formation and growth of air plasma spraying (APS) thermal barrier coatings (TBCs). The acquired micrographs were then used to study the strain fields in the vicinity of the cracking region using digital image correlation (DIC) analysis. Fractography analysis for the fractured regions of the APS coatings was also discussed. Based on real-time observation, it was found that roughness of the coatings’ free surface (e.g., valleys) can promote the initiation of cracks since it acts as stress concentration points. Pores and splats features of the coating microstructure contribute to crack branching and crack path deflection, respectively. The former phenomenon (i.e., crack branching) negatively affects the lifetime of the TBC system as it results in an increased fracture area, while the latter can improve the fracture toughness of the coatings and its durability through improving the coating’s ability to dissipate the energy required for crack propagation.

Enhanced interlaminar fracture toughness of CF/PEEK laminates by interleaving CNT-decorated PEEK films

The weak interlaminar properties severely hinder the high-property applications of carbon fiber (CF) reinforced thermoplastic composites. Here, interlaminar properties were significantly enhanced by introducing carbon nanotube (CNT) decorated polyether ether ketone (PEEK) films into the interlayer. PEEK films decorated with different CNT contents were fabricated by the spraying and thermoforming processes. Tensile test results show that the films with a CNT surface density of 45 mg/m2 exhibit superior tensile strength and Young's modulus due to the higher crystallinity and larger crystalline size. The introduction of this modified film into unidirectional CF reinforced PEEK laminates leads to an increase by 101% to 1.67 kJ/m2 in interlaminar fracture toughness, while the interlaminar shear strength is retained. Fractographic analysis reveals that the increase in interlayer resin content and crack path deflection are primary factors contributing to interlaminar toughening. Molecular dynamics simulation demonstrates that incorporating CNT in the interlayer resin significantly improved the interfacial bonding strength between fiber and resin, thus resulting in crack path deflection.

Fracture toughness of wood and transparent wood biocomposites in the toughest LT-direction

Fracture toughness and mechanisms of crack growth are characterized for transparent wood polymer biocomposites and compared to native wood, with the crack normal to the fiber direction (LT fracture plane). Side-grooved specimen geometries generated pure mode I crack growth, whereas previous investigations commonly report 90° crack path deflection. Crack growth micromechanisms were analyzed by experimental fracture tests and in-situ microscopy observations. Large damage zones around the crack tip with fiber bundle bridging and pull-out were observed in the crack wake, justifying more advanced cohesive zone modeling suitable for composite materials design. The polymer matrix resulted in much higher fracture energy of the biocomposites compared to native wood due to increased local cohesive strength. This strength increased from the polymer contribution and more homogeneous stress distribution in the wood fibers.

3D fatigue crack path deflection and residual stresses in 17-4PH stainless steel rod

Damage tolerant structures require accurate fatigue crack growth rate models for life prediction. Central to these models are fracture mechanics similitude and empirically gathered growth rates. In this work, standard test methods failed to achieve similitude for 17-4PH stainless steel round-rod but succeeded for plate. Material product form differences are interrogated through constant ΔK tests, crack path analysis, residual stress (RS) characterization, and growth rate simulation. Analyses revealed that quench-induced RS in the round-rod promoted a non-planar, 3D crack path with closure effects. Strategies to mitigate the RS/path effects are evaluated by controlling constraint, closure, environment, and heat treatment.

Self-healing of matrix cracking and delamination damage assessment in microcapsules reinforced carbon fibre epoxy composite under flexural loading

Structural integrity of micro cracked CFRP composite structures can be re-established by incorporating the microcapsules based self-healing system into the composite. In the current study, in order to fabricate capsules reinforced CFRP composite, 0.54: 0.39: 0.07 vol fractions of matrix, carbon fibre and separately encapsulated epoxy, hardener microcapsules were employed. Effect of microcapsules addition and induced damage on the flexural strength of the composite were investigated. It was noticed that flexural strength decreases with the increase in capsules concentration and for 5 wt% capsules reinforced composite flexural strength reduced by 21.73%, 29.32% due to induce of matrix cracking and delamination damage respectively. In order to assess self-healing performance of the composite, different damage events were induced in the composite and healing efficiency was evaluated based on the recovery in flexural strength. Effect of capsules wt%, healing temperature and pressure on the healing efficiency were assessed and noticed higher healing efficiencies at elevated temperature and pressure healing conditions. Healing efficiencies of 71.30%, 54.21% were obtained at optimal healing conditions for matrix cracking, delamination damage recovery respectively. Crack path deflection, crack pinning and capsules rupture mechanisms were observed in the microstructural investigation of fracture surface of capsule reinforced composite.

Mixed-mode fracture behaviour of refractories with asymmetric wedge splitting test. Part II: Experimental case study

The asymmetric wedge splitting test for performing mixed-mode loading and its numerical evaluation has been presented in a companion paper (Part I). In this work (Part II), the influences of various levels of mode II loading on damage behaviour of refractories with different brittleness were experimentally investigated by comparing mode I and mixed-mode fractures under symmetric and asymmetric wedge splitting loading with seven different wedge angles. The digital image correlation technique was also used for strain maps visualization as well as the deformation parameters acquisition.With the increase of asymmetric wedge angle, the fracture behaviour becomes unstable what is associated with steeper load-displacement curves, more instantaneous energy release and restrained fracture process zone development. The in-plane shear loading contributes to the accelerated extension of the crack tip and its deviation from central plane. Meanwhile, the co-existing local shear stresses caused by the refractory's heterogeneity lead to crack path deflection as well.

Impact of test environment on the fracture resistance of cortical bone

Water is a crucial component of bone, affecting the interplay of collagen and minerals and contributing to bone's high strength and ductility. Dehydration has been shown to significantly effect osseous mechanical properties; however, studies comparing the effects of various dehydrating environments on fracture toughness of bone are scarce. Accordingly, the crack resistance curve (R-curve) behavior of human and sheep cortical bone was characterized in a bio-bath, in ambient pressure air, and in scanning electron microscopes (SEMs) under three different environmental conditions (water vapor pressure, air pressure, and high-vacuum). The aim of this work was to better understand the impact of test environment on both intrinsic and extrinsic toughening and hence crack initiation toughness, K0 and crack growth resistance, dK/dΔa. Results show significantly lower K0 values for samples that were tested inside SEMs combined with pronounced extrinsic toughening through microcracking and crack path deflections out of the mode I plane. Importantly, all three SEM test environments gave similar results, and thus it does not matter which type of SEM is used. Ex situ testing of hydrated samples revealed similar K0 for both environments but elevated crack growth resistance for testing in ambient air relative to the bio-bath. Our data reveals the experimental difficulties to directly observe microscale crack propagation in cortical bone that resembles the in vivo situation. Ex situ testing immersed in Hanks' Balanced Salt Solution (HBSS) with subsequent crack path analysis, while tedious, is thought to presents the most realistic picture of the in vivo structure-fracture property relations in biological tissue.

Analysis of near-tip fatigue crack path deflection in metallic materials

This article studies the plasticity-induced fatigue crack growth retardation in a steel caused by crack path deflection in the near-tip region. To this end, the finite element method was used to model the crack with its deflected kink. The results allow a calculation of the fatigue crack propagation rate for the two analyzed crack configurations, i.e.: (i) locally-multiaxial fatigue (main crack plus deflected kink) and (ii) uniaxial fatigue (only main crack). It is seen that the retardation effect rises with the increase of the initial deflected crack tip angle.

On the cleavage stress promoting crack path deflection and anisotropic fracture in cold drawn pearlitic steel

Crack path deflection is a phenomenon appearing in cold-drawn pearlitic steel wires as a consequence of the pearlite microstructure orientation induced by the manufacturing procedure, the deflection angle being approximately 90º, thereby representing a crack propagation step (fracture wall), a signal of splitting, delamination or debonding in the form of axial cracking (or micro-cracking) by means of a microscopic fracture mode in the form of enlarged and oriented cleavage in the direction of the wire axis (cold drawing direction). The described phenomenon is a signal of anisotropic fracture behaviour (strength anisotropy) associated with mixed-mode propagation. This paper demonstrates that, in addition to the oriented microstructure of the cold drawn pearlitic steels (at the two microstructural levels of pearlite colonies and lamellae), a sufficient level of stress triaxiality (constraint) is required, as in the case of cracked specimens in which a cleavage stress appears as a consequence of the singular stress distribution in the close vicinity of the crack tip (crack tip stress-strain field).

Axial micro-cracking paths and locally anisotropic hydrogen embrittlement behaviour in cold drawn pearlitic steel wires

This paper analyses the anisotropic fracture behaviour exhibited by initially-smooth samples of cold drawn pearlitic steel wires (commercial prestressing steel wires) subjected to tensile tests in aggressive environment promoted by a corrosion cell. Two testing rates were used: one moderately fast (0.1 mm/min) and another moderately slow (0.01 mm/min). The electro-chemical potential ranged between –1100 mV SCE and –1400 mV SCE (cathodic conditions) to promote hydrogen embrittlement (HE). Axial micro-cracking paths (local crack path deflection) and locally anisotropic HE behaviour was exhibited by the specimens but the phenomenon is strictly local, because the triaxiality level is not high enough to produce global crack path deflection in the presence of hydrogen. The anisotropic fracture behaviour shows marked deviations of the fracture path from the initial fracture zone till the fracture end zone, such deviations being higher than those observed in the specimens failing in inert environment.

Fabrication of (TixZr1−x)B2-(ZrxTi1−x)N composites by reactive spark plasma sintering of ZrB2-TiN

In this study, microstructural and mechanical properties of (ZrxTi1−x)N-(TixZr1−x)B2 composite fabricated by reactive spark plasma sintering (SPS) of ZrB2 and TiN was investigated. ZrB2 and TiN starting materials with equimolar ratio were sintered at 2000 °C for 8 min under a pressure of 30 MPa. An expansion (about 4%) was observed during the sintering process of the composite in the temperature range between 1290 °C and 1625 °C, which was attributed to the reaction of the starting materials. XRD pattern of the fabricated composite revealed the formation of TiB2 and ZrN phases after the sintering process. It was found that (Ti0.91Zr0.09)B2 and (Zr0.92Ti0.08)N phases are formed after sintering. High values of Vickers microhardness (28 ± 0.2 GPa), flexural strength (706 ± 51 MPa) and fracture toughness (8.49 ± 0.92 MPa m1/2) were achieved for this composite. Fracture of the (ZrxTi1−x)N and (TixZr1−x)B2 phases and crack path deflection mechanisms was founded as dominant toughening mechanisms in this composite.

Role of carbon nanotubes on mechanical properties of SiC-B4C-Ni hybrid composites fabricated by reactive spark plasma sintering

The present study was conducted to investigate the microstructural and mechanical properties of SiC-45 vol%B4C-10 vol%Ni and SiC-45 vol% B4C-10 vol%Ni-5vol% CNT in-situ composites fabricated through reactive spark plasma sintering (SPS) method at 1650 °C for 5 min. The phase and microstructural characterization of the composites were evaluated utilizing x-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Ultimately, microhardness, flexural strength, and fracture toughness of the composites were measured. Densification behavior of the samples revealed that the initial shrinkage of the composites took place at about 750 °C, which is related to the reaction between Ni and SiC phases. XRD results confirmed the formation of Ni2Si phase. The relative density of 98.3 ± 0.32% was achieved for the SiC-B4C-10Ni sample; this value was 97.8 ± 0.62% for the SiC-B4C-10Ni-5CNT sample. The flexural strength of 362 ± 23 MPa was achieved for the SiC-B4C-10Ni sample; this value was 369 ± 15 MPa for the SiC-B4C-10Ni-5CNT sample. The comparison between the fabricated composites concerning the fracture toughness indicated further fracture toughness of the SiC-B4C-10Ni-5CNT sample (5.82 ± 0.32 MPa m1/2) than that of the SiC-B4C-10Ni sample (3.35 ± 0.56 MPa m1/2). Formation of micro-cracks, crack path deflection, and CNT bridging was the main toughening mechanisms in the SiC-B4C-10Ni-5CNT sample.

Cleavage Stress Producing Notch-Induced Anisotropic Fracture and Crack Path Deflection in Cold Drawn Pearlitic Steel

The fracture performance of axisymmetric notched samples taken from pearlitic steels with different levels of cold-drawing is studied. To this end, a real manufacture chain was stopped in the course of the process (on-site in the factory), and samples of all intermediate stages were extracted from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (prestressing steel wire). Thus, the drawing intensity or straining level (represented by the yield strength) is treated as the key variable to elucidate the consequences of manufacturing on the posterior fracture issues. On the basis of a materials science approach, the clearly anisotropic fracture behavior of heavily drawn steels (exhibiting deflection in the fracture surface) is rationalized on the basis of the markedly oriented pearlitic microstructure of the cold-drawn steel that influences the operative micromechanism of fracture. In addition, a finite element analysis of the stress distribution at the fracture instant allows the computation of the cleavage annular stress required to produce anisotropic fracture behavior and thus crack path deflection associated with mixed-mode cracking. Results show that such a stress is the variable governing initiation and propagation of anisotropic fracture by cleavage (a specially oriented and enlarged cleavage fracture) appearing along the wire axis direction in the case of sharply-notched samples of heavily drawn pearlitic steels.

Influence of Porosity on Fracture Toughness of Hydroxyapatite/Multi-Walled Carbon Nanotubes Biocomposite Materials

The paper analyzes the influence of porosity of hydroxyapatite (HA)/multi-wall carbon nanotubes (MWCNTs) composites on their fracture toughness. It is found that two competing factors affect the resulting fracture toughness. On the one hand, MWCNTs, which are stronger than the surrounding HA matrix, promote the increase in the fracture toughness owing to the possible crack path deflection from the planar geometry, that leads to a decrease in the crack driving force. On the other hand, the higher MWCNT concentration leads to the reduction in the composite porosity. The computer simulation of the crack formation in HA composite with different porosity shows that the lower porosity provided by the sintering activation due to the MWCNT addition, leads to a reduction in the fracture toughness of ceramics. It is shown that the MWCNT addition in the amount of less than 0.5 wt.% does not lead to a significant growth in the fracture toughness due to these two competing factors.

On the cleavage hoop stress responsible for crack path deflection and notch-induced anisotropic fracture in cold drawn pearlitic steel: Revisiting John Ford’s Monument Valley

In this paper a study is presented of the fracture behaviour in the presence of notches of high-strength pearlitic steel with different cold drawing degree. To this end, axisymmetric notched samples with a circumferentially-shaped notch of very distinct geometries (sharp and blunt, deep and shallow) were taken from wires with different levels of drawing from the initial hot rolled bar (not cold drawn at all) to the final commercial product (prestressing steel wire: heavily cold drawn). Experimental results show that cold drawn steels exhibit a markedly anisotropic fracture behaviour in the form of fracture path deviation from the original transverse fracture path perpendicular to the wire axis or cold drawing direction. Present paper proposes a key variable governing the notch-induced anisotropic fracture process: the cleavage hoop stress as the relevant factor promoting this sort of fracture path deviation in the most heavily cold drawn pearlitic steel wires

Crack path-engineered 2D octet-truss lattice with bio-inspired crack deflection

Abstract Engineered lattice materials with pre-designed crack paths can provide critical component protection and tunable fracture energy. However, most existing crack path design methods require a change in the local topology of the material, which induces inconsistency in the material density and additional topological design complexity. In this study, the authors propose an optical programing method via digital light processing (DLP) to engineer the crack path in lattice materials without altering the material’s topological design. In this method, the effect of curing light intensity on the mechanical properties of the material is studied to develop an effective crack path engineering method, followed by the generation of a bio-inspired crack path deflection on an octet-truss lattice structure. Based on a derived theoretical model of material properties, a crack path engineering method is proposed by applying a grayscale on the desired crack path locations in DLP printing layer images. To mimic the weak crack deflection interfaces of natural materials, vertical crack deflection zones are designed with different relative positions on the octet-truss lattice structures. Consequently, a 15%–152% increase in quantitively controllable fracture energy is observed.

Microstructural characterization of Mg–Al–O rich nanophase strengthened Fe–Cr alloys

Fe–9Cr ODS alloys elaborated by mechanical milling (MM) and followed by hot isostatic pressing (HIP) and hot rolling (HR) have been studied using transmission electron microscopy (TEM) and atom probe tomography (APT) to understand their microstructural characterization. The as-HIP alloy exhibited equiaxed ferrite and contained nanoparticles and low density of dislocations. The as-HR alloy manifested elongated ferrite and contained nanoclusters and high density of dislocations. Characteristics of Mg–Al–O rich nanophase may give insight into competitive behavior between nanofeatures’ constituents which involves nucleation of Mg–Al–O cluster as core at first and then segregation of W and Y at peripheral location. Fracture morphology of tensile test specimens at room temperature is also revealed by scanning electron microscope (SEM). That for the as-HIP alloy is a single ductile fracture (dimples), while the as-HR alloy exhibits a mixed mode of cleavage and ductile fracture characterized by steps, facets and limited ductile ridges (dimples). The SEM observations indicate that Mg–Al–O rich nanophase can serve as a source of dimples and cracks formation (in both alloys), and especially that significant impediment of crack propagation (only in the as-HR alloy) which result in deflection or termination of crack path and growth of the steps. The HR process obviously enhances the mechanical properties of Fe–9Cr ODS alloy owing to the formation of high number density nanoclusters and dislocations, as well as refinement of grains. The contributions to yield strength arising from various strengthening mechanisms are addressed.