Fracture Toughness Behavior Research Articles

Numerical study of mode I fracture toughness of carbon-fibre-reinforced plastic under an impact load

The main objective of this work is, by using cohesive zone modelling, to compute the fracture toughness behaviour in mode I of unidirectional carbon-fibre-reinforced plastic subjected to an impact load at 4.7 m/s. To perform this task, double-cantilever beam specimens were simulated, with its opening displacement and crack propagation being assessed, as well as the evolution of strain rate through the test. Therefore, by plotting the crack propagation, it was possible to calculate the fracture toughness in mode I ( GIC). A comparison of the numerical results with experimental tests previously performed by using a drop weight falling-wedge impact test equipment was made, allowing to infer that the numerical approach, based on a triangular cohesive zone modelling, is capable to predict the behaviour of such specimens under impact, accurately obtain GIC, and to determine the value of strain rate achieved through the test.

Mode I interlaminar fracture toughness behavior and mechanisms of bamboo

Bamboo, with its fast growth rate and outstanding mechanical properties, has received increasing attention as a green material for engineering applications. Compared to other mechanical properties, little is known about fracture toughness, especially the toughening mechanism of bamboo. In this study, the Mode I interlaminar fracture toughness of bamboo with different proportions of fiber cells (FCs) and parenchyma cells (PCs) was tested, using in situ SEM to investigate the intrinsic mechanisms and extrinsic mechanisms at the cellular level. The results showed that both initiation and crack growth energies of high fiber density region were the lowest, and the crack growth energy of middle fiber density region was the highest. The intrinsic toughening in bamboo is associated with plastic zone size and crack kinking which are governed by PCs. The extrinsic toughening is related to fiber bridging which is governed by FCs. Having the highest fracture toughness, the middle region of bamboo may be the best choice of natural fiber for manufacturing high performance green composite materials. This work fills the gaps in the knowledge of fracture toughness and mechanisms of bamboo, which is vital to the utilization of bamboo in the structural applications.

Microstructure and high temperature fracture toughness of NG-TIG welded Inconel 617B superalloy

In the present study, the microstructure, fracture toughness, and fracture behavior of Inconel 617B narrow gap tungsten inert gas (NG-TIG) welded joint were investigated systematically at the designed service temperature of 700 °C. Fracture toughness (J0.2) of base metal (BM) and heat affected zone (HAZ) was higher than that of weld metal (WM). In HAZ and BM, strain mainly localised at grain boundaries with large misorientation and there were lots of coincidence site lattice (CSL) ∑3 boundaries related to twins inside grains, which led to the much higher fracture toughness of BM and HAZ than WM. The high numbers of twins as well as the less serious strain localization at grain boundaries resulted in the most outstanding fracture toughness of BM.

Open porosity and grain size effect on the mechanical property of ZrB2-30SiC containing HfB2

In this research, SPS parameters effect on the densification, hardness, flexural strength and fracture toughness behavior of spark plasma sintered ZrB2-30 vol% SiC containing HfB2 composites were investigated. The ternary ZrB2-SiC-HfB2 composites were spark plasma sintered in different temperatures, times and pressures. Macro-hardness (Hv30.0) was measured by a Vickers indenter. Flexural strength was measured by the three-point bending test. It was found that by increasing temperature from 1650 to 1725 °C, fracture toughness improved nearly 1 MPa m0.5 and it reached to 7.5 MPa m0.5. More temperature rising up to 1800 °C resulted in 1.3 MPa m0.5 reduction in it. Sintering time from 4 to 9 min, firstly increases fracture toughness but at 14 min caused reduction in it. It was concluded that the optimum pressure to reach maximum fracture toughness was 30 MPa and more than it (40 MPa) has an inverse effect.

Impact-Specific Essential Fracture Work of Banana Fiber Reinforced Low-Density Polyethylene Composites

The aim of this study is to investigate the behavior of banana fiber (BF)-low-density polyethylene (LDPE) composite fracture toughness. The LDPE pellets are transformed into powder form which is then functioned as a matrix reinforced with banana fiber (BF). The composites were formed by injection molding techniques which are followed by atmospheric-pressure annealing at 90°C for 24 hours. The composite fracture toughness behavior was evaluated using the essential work of fracture (EWF) approach. The results show that fracture toughness which is characterized by essential fracture work (we) value increases by the presence of BF up to 5 wt.%. However, the we value starts to decrease in the composite with BF content of 6 wt.%. There is a mismatch about the phenomenon of non-essential fracture work. Stress-whitened zones can be seen and observed but non-essential fracture work based on curves is a negative value.

Influence of different curing modes on flexural properties, fracture toughness, and wear behavior of dual-cure provisional resin-based composites.

We investigated the influence of different curing modes on the mechanical properties and wear behavior of dual-cure provisional resin-based composites (DCPRs). Three DCPRs and a self-curing bis-acryl provisional resin-based composite were used. Flexural strength (σF), elastic modulus (E), resilience (R), and fracture toughness (KIC) were measured. The specimens were fabricated with and without light irradiation, stored in distilled water at 37°C for 24 h, and subjected to 5,000 or 10,000 thermal cycles. For sliding impact wear testing, 12 specimens were prepared with and without light irradiation. The maximum facet depth and volume loss were determined using a noncontact profilometer. Some of the mechanical properties and wear behavior of DCPRs are affected by light irradiation. This study indicated that proper light irradiation is important in polymerization process of the DCPRs to enhance the wear resistance and some mechanical properties.

Fracture toughness behavior of low-C medium-Mn high-strength steel with submicron-scale laminated microstructure of tempered martensite and reversed austenite

Fracture toughness was studied in terms of crack-tip opening displacement (CTOD) in low-C medium-Mn high-strength steel at both room temperature and − 40 °C, and excellent fracture toughness was obtained. The critical CTOD values (δ) and crack extension (∆a) followed the relationship: δ = 0.01343 + 0.62315∆a0.47531 and δ = 0.07391 + 0.48466∆a0.60103 at room temperature and − 40 °C, respectively. With the decrease in test temperature from room temperature to − 40 °C, the corresponding δ when ∆a = 0.2 mm (δ0.2) was reduced from 0.30341 to 0.25813 mm, and intersecting point in the crack extension resistance curve with a 0.2-mm passivation line (δQ0.2BL) was reduced from 0.42132 to 0.33941 mm. The large fraction of high misorientation boundaries between tempered martensite effectively hindered the crack propagation and increased the fracture toughness. Furthermore, the submicron-scale complex laminated microstructure of tempered martensite and reversed austenite refined the effective fracture grain size, which inhibited crack propagation and led to high fracture toughness. Also, the excellent fracture toughness is attributed to strain-induced martensite transformation of reversed austenite in the small plastic deformation zone ahead of the crack tip, which absorbed the strain energy, relaxed the local stress concentration, suppressed the crack propagation, and enhanced the fracture toughness.

BN-Based Fiber Coatings by Wet-Chemical Coating

Fiber coatings for BN/SiC-and BN/Si3N4-bilayer systems were developed for the use in SiC/SiC composites. All coatings were produced with high process velocities of 500 m/h by a continuous roll-to-roll dip-coating process. The fiber surface was fully covered with a homogeneous coating and without fiber bridging. Tensile tests of fiber bundles were used to examine potential degradation of the fiber properties due to the application of the coatings. The coated fiber bundles showed a reduction of the maximum tensile load to 90.0 % for the BN/Si3N4 and to 86.7 % for the BN/SiC coating in comparison to the fiber bundle in the as-received state. A thermal treatment of the coated fiber bundles up to 1650 °C led to no reduction of their maximum tensile load. SiC/SiC composites were fabricated by polymer infiltration and pyrolysis. The flexural strength and strain of composites with BN/SiC fiber coating were improved to 467 MPa and 0.42 % in comparison to the composites without fiber coating. The composites with BN/SiC coating showed toughened fracture behavior with fiber pull-out effects.

On the micromechanism of inclusion driven ductile fracture and its implications on fracture toughness

The objective is to identify the micromechanisms of ductile crack advance, and isolate the key microstructural and material parameters that affect these micromechanisms and fracture toughness of ductile structural materials. Three dimensional, finite element, finite deformation, small scale yielding calculations of mode I crack growth are carried out for ductile material matrix containing two populations of void nucleating particles using an elasto-viscoplastic constitutive framework for progressively cavitating solid. The larger particles or inclusions that result in void nucleation at an early stage are modeled discretely while smaller particles that require large strains to nucleate voids are homogeneously distributed. The size, spacing and volume fraction of inclusions introduce microstructure-based length-scales. In the calculations, ductile crack growth is computed and fracture toughness is characterized. Several features of crack growth behavior and dependence of fracture toughness on microstructural and material parameters observed in experiments, naturally emerge in our calculations. The extent to which the microstructural and material parameters affect the micromechanisms of ductile crack advance and, hence, the macroscopic fracture toughness of the material is discussed. The results presented provide guidelines for microstructural engineering to increase ductile fracture toughness, for example, the results show that for a material with small inclusions, increasing the mean inclusion spacing has a greater effect on fracture toughness than for a material with large inclusions.

Carbon fiber reinforced ultra-high temperature ceramic matrix composites: A review

Abstract Recent studies on carbon fiber-reinforced ultra-high temperature ceramic matrix (C/UHTC) composites fabricated by hot-pressing, chemical vapor infiltration, polymer impregnation and pyrolysis, and melt infiltration (MI) are reviewed. Various efforts have been made to improve these preparation processes and to combine two or more of these because they have both the advantages and disadvantages in terms of the processing time, operating temperature, and the porosity of the resulting C/UHTC composites. In addition, the parameters governing the fracture toughness, thermal conductivity, and recession behavior (in oxidizing environments) of these composites have been discussed. This review demonstrates that C/UHTC composites with Zr- or Hf-based UHTC matrices fabricated via MI are potential candidates for advanced heat-resistant structural materials.

Finite element modeling on fracture toughness of 3D angle-interlock woven carbon/epoxy composites at microstructure level

Fracture toughness behaviors of 3D angle-interlock woven composites were characterized with Mode I double cantilever beam test and finite element analysis (FEA). Load–displacement curves, strain energy release rates and fracture morphologies were obtained from tests and compared with FEA. We found that the 3D woven fabric structure increases strain energy release rates. The pullout and breakages of the yarns through thickness direction are the main mechanisms for the higher fracture toughness than laminates. The tradeoff between higher fracture toughness and lower in-plane stiffness should be considered for designing the 3D woven composites.

Translaminar fracture toughness and fatigue crack growth characterization of carbon‐epoxy plain weave laminates

An experimental investigation of mode‐I translaminar fracture toughness and fatigue crack growth behavior of a carbon fiber‐epoxy plain weave laminate manufactured by resin infusion under flexible tooling (RIFT) is presented in this article. Pre‐cracked compact tension (CT) specimens were used to perform both quasi‐static and fatigue tests. Different data reduction techniques for fracture toughness calculation were used and compared with each other. The ASTM E399 test method was modified to account for the material orthotropy and specimen geometry effects using a correction function based on a numerical evaluation of the strain energy release rate. The proposed modification shows good agreement against other experimental methods found in the literature and its application was validated for fatigue tests. Fatigue testing shows that failure in undesired modes is likely to occur prior to translaminar fracture, which was attributed to a higher tensile fatigue threshold than compression or shear fatigue threshold presented by the composite in analysis. Increasing the specimens’ initial notch length was a solution for avoiding these types of failure. The experimental results were compiled in the form of a Paris curve, and their particularities were discussed. A fractographic analysis was carried out to define damage patterns and its evolution process in both types of tests. POLYM. COMPOS., 40:3791–3804, 2019. © 2019 Society of Plastics Engineers

Experimental study of effect of post processing on fracture toughness and fatigue crack growth performance of selective laser melting Ti-6Al-4V

For Ti-6Al-4V, a titanium alloy increasingly used in aerospace structure, selective laser melting (SLM) is an attractive additive manufacturing technology, which is attributed to its complex construction capability with high accuracy and good surface quality. In order to obtain qualified mechanical properties, SLM parameters and post processing should be tailored for diverse service conditions. Fracture toughness and fatigue crack growth (FCG) behavior are critical characteristics for damage tolerance evaluation of such metallic structures, and they are affected by post processing technologies significantly. The objective of this study is to obtain the fracture toughness and fatigue crack growth behavior of Ti-6Al-4V manufactured by SLM, and to evaluate the influence of post-SLM thermomechanical treatment and surface machining. Fracture toughness and FCG tests were performed for SLM Ti-6Al-4V in three types of post processing status: as-built, heat treated and hot isostatically pressed (HIPed), respectively. Specimens with as-built and machined surface were tested. The microstructure and fractography were analyzed as well in order to investigate the relevance among manufacture process, microstructure and mechanical properties. The results demonstrate that as-built SLM Ti-6Al-4V presents poor ductility and FCG behavior due to martensitic microstructure and residual stresses. Both heat treatment and hot isostatic pressing improve the plane-stress fracture toughness and FCG performance considerably, while surface machining shows slight effect.

Mode II fracture toughness of asymmetric metal-composite adhesive joints

The paper presents an experimental investigation of the mode II fracture toughness behavior of dissimilar metal-composite adhesive joints using the end-notched flexure (ENF) test. The adhesive joint under study consists of a thin titanium sheet joined with a thin CFRP laminate and is envisioned tobe applied in the hybrid laminar flow control system of future aircraft. Four different industrial technologies for the manufacturing of the joint areevaluated; co-bonding with and without adhesive and secondary bonding using either a thermoset or a thermoplastic composite. The vacuum-assisted resin transfer molding (VARTM) technique is employed for the manufacturing of the titanium-CFRP joint. After manufacturing, the joint is stiffened from its both sides with two aluminum backing beams to prevent large deformations during the subsequent ENF tests. Towards the fracture toughness determination from the experimental data, an analytical model recently reported by the authors is applied; that model considers the bending-extension coupling of each sub-laminate of the joint as well as the effect of the manufacturing-induced residual thermal stresses. The load-displacement behaviors, failure patterns, and fracture toughness performances for each of the four manufacturing options (MO) investigated are presented and compared.

Fracture toughness and R-curve behavior of laminated ZrB2–SiC/SiCw ceramic

Laminated ZrB2–SiC/SiCw (LZSw) was prepared via tap-casting and hot-pressing processes. The R-curve behavior of the ceramic materials was evaluated using indentation-strength method. The flexural strength and fracture toughness values of the materials were found to reach up to 607 MPa and 14.5 MPa·m1/2, respectively, when employing SiC whiskers as a strong interface. The indentation strength was observed to decrease upon an increase in the indentation load, and compared with the steep decreasing trend exhibited by the monolithic ZrB2–SiC (ZS20), LZSw showed a slow decreasing trend. LZSw showed a distinct upward trend in its R-curve, which was found to be dependent on the indentation collapse caused by its laminated structure and the increasing crack propagation path as a result of crack deflection and branching mechanisms.

An Innovative Industrial Process for Forging 7050 Al Alloy

The effect of an unconventional thermal treatment method aimed to improve toughness behavior in Al alloys is reported. The method involves solution heat treating and an intermediate warm working step, before final ageing thermal treatment on a AA7050 high resistance aluminum alloy. Results show the possibility to increase fracture toughness behavior without tensile and conductivity (IACS) properties loss by adopting a warm deformation process instead of the standard cold deformation. Moreover, the adoption of an intermediate warm deformation instead of standard cold deformation, allows to reduce material microstructural grain-size heterogeneity.

Fracture toughness enhancement of carbon fiber–reinforced polymer composites utilizing additive manufacturing fabrication

The present work reports a novel approach to enhance the fracture resistance and notch sensitivity of carbon fiber-reinforced polymer composites utilizing additive manufacturing (3-D printing) fabrication. The 3-D printed composites utilize carbon fiber bundles to reinforce nylon/chopped fiber resin in a multilayered structure configuration. Single-edge (60°) notched samples were printed using Mark Two printer. Three reinforcement schemes were designed and used to manufacture the specimens. The focus was placed on selective reinforcement at the crack tip to arrest crack initiation. The mechanical properties, fracture toughness, and fracture behavior of the printed composites were evaluated. It was found that wrapping fiber around the notch effectively blunted the notch and redirected crack propagation away from the notch tip, thereby lengthening the crack path and leading to improved fracture resistance. It was also found that such improvement reaches a saturation level. Excessive notch reinforcement beyond optimal limit can reverse the gains in fracture resistance due to notch-targeted reinforcement. Examination of the fracture surface morphology of the printed composites reveals lack of fusion of the sizing of the individual continuous carbon fiber bundles and the lack of adhesion between the matrix layers (nylon/chopped fiber resin) and the adjacent carbon fiber bundle reinforcement. Damage to the fibers within the carbon bundle was also observed. Thus, a synergetic effect of the carbon fiber bundles reinforcement and the matrix requires more optimization to manufacture carbon-reinforced polymer composites using 3-D printing.

Structural influences of two-dimensional and three-dimensional carbon/epoxy composites on mode I fracture toughness behaviors with rate effects on damage evolution

This paper reports the mode I interlaminar fracture toughness and fracture mechanisms of two-dimensional (2D) plain woven composite and three-dimensional (3D) angle-interlock woven composite. The fracture toughness behaviors were tested with double cantilever beam method at the different loading rates from 0.5 to 100 mm/min. Critical strain energy release rate was calculated to compare the difference between the 2D and the 3D woven composites. The fractographs were photographed with scanned electronic microscopy and optical microscopy to show the fracture morphologies. We found that the 3D angle-interlock woven composite has high fracture toughness than that of 2D woven composite. The binder yarns resist the crack initiation and propagation to increase the fracture toughness. While the lower in-plane stiffness of the 3D woven composites should be considered fully for designing the 3D woven composites.

Fracture Toughness and Tribological Wear Behaviour of Micro Alloyed Pearlitic- Ferritic Ductile Cast Iron

Fracture Toughness and Tribological Wear Behaviour of Micro Alloyed Pearlitic- Ferritic Ductile Cast Iron

Fracture behaviour of ceramic–metallic glass gradient transition coating

A new composite coating material of ceramic (Al2O3-40 wt% TiO2)–metallic glass (Fe56Cr23Mo13B8) gradient transition coating was successfully prepared by atmospheric plasma spraying technology, and a new theory of laminar–columnar structure gradient transition synergistic enhancement effect was proposed. The microstructure and element distribution of the composite coating were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA). The fracture toughness and fracture behaviour of the composite coating were analysed via micro-hardness and three-point bending (3PB) tests. The results showed that the stress release of the ceramic–metallic glass gradient transition coating was stable compared with that of the conventional gradient coating in the stage of acute deformation, and the coating exhibited better fracture toughness. Different areas of the ceramic–metallic glass gradient transition coating exhibited different fracture behaviours. Additionally, the ceramic layer was made of columnar crystals, and the metal–glass layer was lamellar. The laminar–columnar structure gradient transition synergistic enhancement effect improved the anti-crack growth and fracture toughness. This study provides a new and viable option for the improvement of thermal spraying ceramic composite coatings.