Superior Fracture Research Articles

Microstructure, tensile properties and fracture toughness of friction stir welded AA7075-T651 aluminium alloy joints

Abstract The main objective of this investigation is to study the microstructure, tensile properties and fracture toughness of friction stir welded (FSW) butt joints of 10 mm thick AA7075-T651 plates. The microstructural features of stir zone (SZ), thermos-mechanically affected zone (TMAZ), heat affected zone (HAZ) were analyzed using optical microscopy technique. The tensile properties were evaluated using smooth and notch tensile specimens and compared to base metal properties. The microhardness survey was done across the weld cross section and correlated to the failure of tensile specimens. Compact tension (CT) specimens were used to evaluate the fracture toughness of welded joints. The fractured tensile and CT specimens were analyzed using scanning electron microscopy (SEM). Results showed that the FSW AA7075-T651 specimens welded using axial load of 12 kN, tool rotation speed of 750 rpm and welding speed of 30 mm/min exhibited 412 MPa tensile strength and 9% elongation. It showed 88 and 89% of base metal strength elongation. The joints showed fracture toughness of 23 MPa m1/2 which is 80% of base metal fracture toughness. The superior tensile and fracture toughness properties of joints are mainly attributed to the evolution of finer grains in SZ due to the stirring action of FSW tool.

Peptide-enhanced tough, resilient and adhesive eutectogels for highly reliable strain/pressure sensing under extreme conditions

Natural gels and biomimetic hydrogel materials have been able to achieve outstanding integrated mechanical properties due to the gain of natural biological structures. However, nearly every natural biological structure relies on water as solvents or carriers, which limits the possibility in extreme conditions, such as sub-zero temperatures and long-term application. Here, peptide-enhanced eutectic gels were synthesized by introducing α-helical “molecular spring” structure into deep eutectic solvent. The gel takes full advantage of the α-helical structure, achieving high tensile/compression, good resilience, superior fracture toughness, excellent fatigue resistance and strong adhesion, while it also inherits the benefits of the deep eutectic solvent and solves the problems of solvent volatilization and freezing. This enables unprecedentedly long and stable sensing of human motion or mechanical movement. The electrical signal shows almost no drift even after 10,000 deformations for 29 hours or in the −20 °C to 80 °C temperature range.

Facile method for preparing lamellar structured alumina-zirconia ceramics with high toughness

This study describes a facile and effective method for fabricating alumina-zirconia composites with innovative lamellar structure as well as outstanding mechanical strength/toughness. To this end, the particle-stabilized foam method has been utilized to synthesize alumina ceramic foams built by single-layer pore wall firstly, providing the basic lamellar structure for the subsequently mixing with zirconia particles and hot-press sintering to achieve robust alumina-zirconia ceramics. The unique lamellar microstructure is composed of alumina sheets and zirconia grain layers alternated consistently, resulting in superior fracture toughness of 9.38 MPa m1/2, high flexural strength of 1.01 GPa and Vickers hardness of 17.04 GPa at the relative density of 99.63%. This approach opens up new possibilities for creating alumina-zirconia ceramics with exceptional mechanical performance.

Enhanced Fracture Toughness and High-Temperature Strength of Directionally Solidified Mo-XC Alloys

The present article focuses on refractory metals strengthened by ceramic phases to improve the mechanical properties at ambient temperatures as well as at high temperatures. In this work, near-eutectic, directionally solidified (DS) Mo-34TiC and Mo-27ZrC alloys are investigated with regard to their microstructures, fracture toughness at room temperature and compressive strength at high temperatures. Superior fracture toughness as well as competitive strength compared with other Mo-XC composites are achieved. Compressive tests up to 1000 °C on longitudinal and transversal specimens show the significant impact of the anisotropy on the strength.

Effects of air-entraining agent and polypropylene fiber on the mechanical properties, autogenous shrinkage, and fracture properties of fully recycled aggregate concrete

The effects of air-entraining agent (AEA) and polypropylene fiber (PPF) on the autogenous shrinkage and fracture properties of fully recycled aggregate concrete (FRAC) are studied to obtain FRAC with low density, low autogenous shrinkage, and superior fracture properties. Six groups of FRAC with different AEA (0, 5%, 10%) and PPF (0, 1.2 kg/m3) contents were prepared for test. The results illustrate that AEA can slightly reduce density but that it has an adverse effect on the strength and fracture properties of FRAC. The incorporation of PPFs can reduce the adverse effect of AEA on compressive strength and splitting tensile strength, but it will increase the adverse effect on fracture properties. Furthermore, the addition of PPFs alone will significantly reduce compressive strength and splitting tensile strength, whereas flexural strength and fracture properties are increased. The addition of AEA can improve the adverse effect of PPFs on compressive strength and splitting tensile strength, but flexural strength and fracture properties will decrease rather than increase. The addition of 10% AEA can greatly reduce autogenous shrinkage, and 5% AEA can reduce autogenous shrinkage at an early age. Incorporating PPFs can further reduce autogenous shrinkage. The study mainly examines the effects of AEA and PPF on the autogenous shrinkage of FRAC, and provides some new ideas for producing high-performance FRAC. At the same time, the cooperative effect of AEA and PPF on the properties of FRAC is also studied.

Effect of RAP and glass fiber on mode I fracture behaviors of ultra-thin friction course

Ultra-thin friction course (UTFC) presents excellent performance in preventative maintenance of asphalt pavement. UTFC could be also used as the wearing course of newly constructed pavements. This study investigated the effects of reclaimed asphalt pavement (RAP) and glass fiber on the fracture behaviors of UTFC at the low and intermediate temperatures. At the low temperature (-10℃), fracture energy, stress intensity factor and cracking resistance index (CRI) were obtained to present the fracture behaviors of three types of UTFC mixtures. At the intermediate temperature (25℃), the size-independent fracture energy considering the tail part of the load–displacement curves were obtained. Besides, J-integral, cracking resistance index (CRI) and balanced cracking index (BCI) were obtained to characterize the intermediate fracture performance. Results indicated that at −10 ℃, the incorporation of RAP weakened the fracture resistance of UTFC; while the addition of glass fiber could counteract the adverse effect of RAP. Besides, CRI value of UTFC with glass fiber was the largest, indicating the superior fracture resistance at the low temperature. At 25 ℃, UTFC presented the best cracking resistance in terms of fracture energy, J-integral, CRI values and BCI values, indicating the incorporation of RAP could deteriorate the fracture resistance at the intermediate temperature. When both RAP and glass fiber were added in UTFC, the fracture performance was deteriorated furtherly, which may be possibly caused by the non-uniform distribution of fiber. On the other hand, the bonding strength between fiber and asphalt could be mitigated when temperature increased from −10 ℃ to 25 ℃. The size-independent fracture energy of UTFC was the largest, indicating the superior fracture resistance, followed by UTFC-50% RAP-F and UTFC. It was also reported that the incorporation of fiber improved the ability to tolerate deformation.

Shape-morphing into 3D curved surfaces with nacre-like composite architectures.

Inhomogeneous in-plane deformation of soft materials or cutting and folding of inextensible flat sheets enables shape-morphing from two dimensional (2D) to three-dimensional (3D), while the resulting structures often have weakened mechanical strength. Shells like nacre are known for the superior fracture toughness due to the “brick and mortar” composite layers, enabling stress redistribution and crack stopping. Here, we report an optimal and universal cutting and stacking strategy that transforms composite plies into 3D doubly curved shapes with nacre-like architectures. The multilayered laminate exhibits staggered cut distributions, while the interlaminar shear mitigates the cut-induced mechanical weakness. The experimentally consolidated hemispherical shells exhibit, on average, 37 and 69% increases of compression peak forces, versus those with random cut distributions, when compressed in different directions. Our approach opens a previously unidentified paradigm for shape-conforming arbitrarily curved surfaces while achieving high mechanical performance.

Manipulating mechanical properties of graphene/Al composites by an in-situ synthesized hybrid reinforcement strategy

The structural deterioration caused by the relatively weak out-of-plane bending stiffness and the chemically-active edge area of graphene limits its outperformance in strengthening for Al matrix composites (AMCs). Introducing one-dimensional (1D) carbon nanotubes (CNTs) to graphene/metal system is one of the promised strategies to complement the weakness of 2D graphene and make full use of the outstanding intrinsic properties of the both reinforcements. To date, such synergistic strengthening and toughening mechanisms are largely unknown. In this study, AMCs reinforced by a novel hybrid reinforcement, i.e., graphene nanosheets decorated with Cu nanoparticles and CNTs (Cu@GNS-CNTs), are fabricated by an in-situ synthesis method. The combined contrast experiments validated that the organically integrated reinforcing structure promotes the intrinsic load bearing capacity of GNS and the strain hardening capability of the Al matrix simultaneously. As a result, the composites achieved excellent tensile strength and uniform elongation with almost no loss. The strengthening mechanism originated primarily from the hybrid reinforcement exhibits superior load-transfer, fracture inhibition and dislocation storage capability by controlling the interface reaction to construct an effective interface structure without damaging the reinforcement. Our work identifies a promising structural modification strategy for 2D materials and provides mechanistic insights into the synergistic strengthening effect of graphene/CNTs hybrid reinforcement.

Mesh/membrane composite with superior mechanical performance: A deep learning-based design

The mesh/membrane composite structure in aerospace and engineering applications is often susceptible to degradation or failure due to lack of stiffness and strength. Here, we propose a method to design the mesh/membrane composite based on deep learning technique, trained with a database of thousands of structures from finite element simulations. Our method provides a programmable design solution to screen out desired candidates of mesh with superior fracture strength in specified relative density and porosity. Results show that the method can offer designs with different failure modes and crack regions for various target functionalities, while maintaining light weight and superior fracture strength. Building on the paradigm of smart design, we suggest potential strategies for designing the mesh/membrane composite with desired mechanical performance.

Microstructural Effects on J‐Integral Fracture Toughness of Welded High‐Mn Steels at 298 and 77 K

J‐integral fracture toughness tests are conducted on welded high‐Mn steels at room and cryogenic temperatures. The specimens are categorized into weld metal (WM) and heat‐affected zone (HAZ) to consider the different microstructural effects. The microstructure of WM is composed of dendrites formed during solidification, while the microstructure of HAZ consists of coarsened grains from the welding process. Both specimens show twinning behavior after J‐integral fracture toughness tests at 298 K and the superior K JIc fracture toughness of WM (389 MPa√m) due to the formation of plentiful twins at the crack tip. At 77 K, K JIc fracture toughness of HAZ (221 MPa√m) is lower than that of WM (264 MPa√m) due to different stacking fault energies (SFEs). Twinning still prevails in WM at 77 K, while ε‐martensite is formed in HAZ by relatively low SFE. The interface between γ‐matrix and ε‐martensite can act as a crack initiation site during the fracture toughness test, leading to the deterioration of J‐integral fracture toughness in HAZ.

Trifunctional Laves precipitates enabling dual-hierarchical FeCrAl alloys ultra-strong and ductile

The synergy of ultra-high strength and large ductility is a vital requirement in particular for single-phase structural alloys; unfortunately these two properties are generally mutually exclusive. The hard-yet-deformable precipitates in alloys not only act as the strengthener that can block dislocation motion thus enhance the work hardening capability to strengthen materials, but also act as the stress buffer via self-deformation that releases local stress concentrations and as the cracking modulator that triggers multiple micromechanisms that resist crack propagation to ductilize materials. Here, taking the advanced FeCrAl-based alloy as an example, we confirmed a strategy that combines these triple functions of precipitates in a dual-hierarchical alloy by controllably embedding the multi-scaled hard-yet-deformable Laves precipitates in the lamellar coarse-grained matrix, where the lamellar boundary decorated by equiaxed ultra-fine grains serving as crack arresters. The proposed design concept of dual-hierarchically heterogeneous structure can achieve a substantial increase in yield strength up to the ultrahigh level ∼1.0 GPa by far with enhanced uniform elongation. These trifunctional Laves precipitates exhibit the interface-controlled mechanisms transited from dislocation cutting through coherent nanoscaled Laves precipitates to stacking fault-mediated plasticity of incoherent micro-scaled Laves precipitates. The present dual-hierarchical design strategy opens a new avenue to engineer materials with exceptional properties for superior fracture resistance.

Unbreakable Hydrogels with Self-Recoverable 10 200% Stretchability.

Design of tough hydrogels maintaining structural integrity under multivariable mechanical loads remains hugely challenging because the anticipated characteristics such as stretchability, strength, toughness, and fracture resistance can hardly be compatible. Herein, a simple but robust hydrogel network formed by copolymerization of divinyl benzene with acrylamide in micellar solutions for ultra-high fracture resistance and self-recoverable stretchability is proposed. The network provides dynamic association of hydrophobic domains and homogeneous crosslinking of hydrophilic chains, which shows step-by-step deformation process. The dynamic associations allow recoverable small deformations, then the homogeneous crosslinking ensures reversible unfolding and alignment of polymer chains to self-strengthen for ultra-large deformations without crack propagations. The resultant hydrogels exhibit comprehensive unbreakable feature with self-recoverable ultra-high stretchability (100% recovery from 10 200% strain), superior fracture resistance (toughness > 26kJ m-2 ), and anticrack propagation and fatigue (fatigue threshold: ≈2.5kJ m-2 ). Even the prenotched hydrogels can undergo tens cyclic loads at 10 200% strain and thousands cyclic loads at 200% strain without noticeable changes in mechanical performance. The robust network prepared from homogeneous hydrophobic crosslinking provides a facile approach and a new mechanism to explore tough hydrogels with superior antifracture and extreme self-recoverable deformability for diverse applications.

Significantly toughened PZT-based piezoelectric ceramics simultaneously keeping excellent electrical properties via incorporating trace amount of yttria-partially-stabilized zirconia

Piezoelectric ceramics are prone to cracking during functioning due to the repetitively applied high stress. Hence, toughened ceramics with robust microstructures are highly desired for resisting the potential cracks. Toughening of Lead Zirconium Titanate (PZT) based piezoelectric ceramics with high d33·g33 for ignition applications is investigated in this work. Yttria-partially-stabilized zirconia (YPSZ) particles were incorporated into the PZT-based piezoelectric ceramics via conventional solid state reaction. Measurements of mechanical properties of as-sintered ceramics indicate that incorporation of an appropriate amount of YPSZ significantly improves the fracture toughness of PZT-based ceramics by 38.7%, meanwhile the piezoelectric coefficient still remains at a high level with a decrease of less than 7%. When doping 0.5 wt% YPSZ, the optimal comprehensive properties can be obtained as follows: fracture toughness KIC = 1.41 MPa·m1/2, Vickers hardness HV = 4.60 GPa, piezoelectric coefficient d33 = 462 pC/N, planar electromechanical coupling factor kp = 60.1%, and dielectric constant ε33T/ε0 = 1750. This successful toughening mechanism paves a way in developing functional ceramics simultaneously possessing superior fracture toughness and electrical properties via doping an appropriate content of YPSZ submicron particles.

Clinical Experience with the New EASY (Endoscopic Approach to the Symphysis) Preparation Technique for Injuries of the Anterior Pelvic Ring.

High-energy injuries of the anterior pelvic ring and especially disruptions of the pubic symphysis usually require surgical stabilization. There is a conflict between biomechanical stability and size in the surgical approach. We have previously presented a minimally invasive approach for symphyseal plating. Our current endoscopic surgical technique is presented step-by-step and is illustrated in a video. The results and courses of the first seven patients treated with the EASY preparation are presented. In all seven patients, the EASY preparation was successful. In four of the seven patients with an anterior pelvic ring injury, we performed a complete endoscopic plate osteosynthesis of the pubic symphysis. One patient gave consent only for endoscopic preparation, in one patient, we converted to open plate due to ventilation problems, and in one patient, the superior pubic rami fracture was stable. There were no surgical complications. The EASY is a feasible minimally invasive preparation technique to stabilize anterior pelvic ring injuries in the most biomechanically stable manner by performing plate osteosynthesis. However, further scientific monitoring is necessary to improve the technique and validate our data.

Pediatric Cranial Dog Bite Injuries: More than Meets the Eye.

Case PresentationA six-month-old female presented to a community hospital with small lacerations to the scalp, face, and left eyelid from a dog bite injury. Computed tomography imaging revealed an impacted right frontal bone fracture and left superior orbital fracture, prompting transfer, neurosurgical repair, and infectious disease management of the injury.DiscussionThis report highlights the importance of having a high level of suspicion for deeper injury in pediatric and especially infant craniofacial dog bites, obtaining radiographic imaging, and initiating appropriate multidisciplinary triage to prevent life-threatening infection and complications.

Interaction of rod decussation and crack growth in enamel

Enamel possesses ingenious hierarchical structure that gives rise to superior fracture resistance. Despite considerable efforts devoted to characterization of fracture behavior of enamel, the role of rod decussation in fracture of enamel is largely unknown. In this study, the features of rod decussation in the inner enamel are experimentally identified, and analyses of crack growth in enamel are carried out using a micromechanical model of enamel, in which the structural features of the outer enamel and rod decussation of the inner enamel are incorporated. We carry out calculations within a framework based on the extended finite element method, and the crack growth and crack path selection are natural outcomes of imposed loading. We show that crack deflection in enamel is controlled by rod decussation. For crack growth in the parazone, the crack path is oriented along the axis of enamel rods, leading to gross crack deflection. The microstructure of inner enamel with intermediate inclination angle enables multiple crack deflections, giving rise to enhanced toughness. For crack growth in the diazone, the transition in orientation of crack deflection occurs as inclination angle increases. The relatively straight crack path emerges in the case of the microstructure of enamel with intermediate inclination angle, leading to weak fracture resistance. It is further found that compared with the diazone, the gross crack deflection in the parazone provides greater contribution to fracture resistance of enamel. The findings of this study provide a good mechanistic understanding of the role of rod decussation in enamel.

Nacre-liked material with tough and post-tunable mechanical properties

• A universal fabrication methodology for post-tunable mechanical properties materials. • Wide tunable range without the need for specific functional groups. • Stable and reversible ‘brick and mortar’ structure. • Superior fracture toughness. Biomaterials, often imparted time-dependent mechanical properties, which are promising in fields ranging from sensors to robotics. Here, a facile method was proposed to fabricate post-tunable mechanical properties composites based on hydrogels and ceramic nanofiller. The wide tunable range of Young's modulus (27.3 kPa to 3.5 GPa) and ultimate stress (173 kPa to 102 MPa) can be achieved by combining solvent absorption and evaporation process with platelets reinforcement effect. Additionally, a large fracture toughness (∼32,000 J m - 2 ) is obtained as a result of the nacre-liked “brick and mortar” structure introduced by shear force during fabrication. The superior flexibility and designability of this material were demonstrated via actuators, portable structure, and metamaterials. Above all, this study provides a new thought to fabricate tough materials with post-tunable mechanical properties.

Synthesis of a novel MoSS+Mo5SiB2+Mo5Si3 based Mo–Si–B alloy and its enhanced fracture toughness

Mo–Si–B based alloys are promising ultrahigh-temperature structural materials for turbine blades. Different from traditional MoSS (Mo solid solution)+Mo5SiB2+Mo3Si based alloy, present paper focuses on a novel non-equilibrium MoSS + Mo5SiB2+Mo5Si3 based alloy, which shows more potential for excellent comprehensive performance. This alloy was synthesized by SPS (spark plasma sintering) technique using pure Mo and Mo5SiB2+Mo5Si3 powders as raw materials. SEM, EDS, XRD, EPMA, single-edge notched bending and nano-identation tests were used to reveal the formation mechanism of this alloy and evaluate its fracture toughness. The results showed that this MoSS + Mo5SiB2+Mo5Si3 based alloy was composed of MoSS, Mo5SiB2, Mo5Si3 and a little Mo3Si. Mo5SiB2+Mo3Si halos were found around the original Mo5SiB2+Mo5Si3 particles as a result of inter-diffusion between Mo and Mo5SiB2+Mo5Si3 during sintering processes. Compared with traditional cast or reaction sintered MoSS + Mo5SiB2+Mo3Si based alloy with the same composition, this MoSS + Mo5SiB2+Mo5Si3 based alloy showed superior fracture toughness benefiting from its higher MoSS content, larger coarseness of MoSS and lower Si concentration in MoSS.

Cu2+ Release from Polylactic Acid Coating on Titanium Reduces Bone Implant-Related Infection.

Implant-related infection (IRI) is a major problem in orthopedics. Copper (Cu) is an essential trace element with strong bactericidal activity and, thus, presents potential for reducing IRI. The present study explored a straightforward strategy for releasing Cu2+ from titanium (Ti) implants, and we conducted a preliminary study to assess the feasibility of this approach in clinical translation. Polylactic acid (PLA) coatings containing different concentrations of copper ions were prepared on Ti discs. The antibacterial activity and biocompatibility of the copper ion-incorporated Ti implants were evaluated using Staphylococcus aureus (S. aureus), bone marrow mesenchymal stem cells (BMSCs) and animal models. In vitro, the coatings produced burst release of Cu2+ in 12 h, and inhibited S. aureus growth in a dose-dependent manner. The coatings prepared from PLA solutions containing 0.5 or 1.0 mg/mL reduced the viability and osteogenic differentiation of BMSCs, but these effects were negated after the coatings were immersed in culture medium for 6 h. Four weeks after implantation, the Cu-free K-wires challenged with S. aureus had persistent infection and inferior fracture healing to the other three groups, while Cu-coated wires had no evidence of infection. Furthermore, the Cu-coated wires placed in rabbits without S. aureus challenge showed superior fracture healing to the other three groups. These results suggest that PLA coatings containing Cu2+ may be an effective design for reducing IRI without adversely affecting adjacent bone healing.

Effect of corrosion on cavitation erosion behavior of HVOF sprayed cobalt-based coatings

Cobalt-based coatings have been widely applied to provide guidance to cavitation erosion (CE) and corrosion resistance since the coatings possessing superior mechanical and anti-corrosion properties. In this study, we prepared cobalt-based alloy (Stellite 21) coating and WC-17Co coating on 1Cr18Ni9Ti by HVOF. The CE resistances were evaluated in deionized water and 3.5 wt% NaCl solution (NaCl solution), and the anti-corrosion properties were studied using polarization tests. Results show that the WC-17Co coating had superior CE resistance than cobalt-based alloy coating in deionized water because of superior microhardness and fracture toughness characteristics. The WC-17Co coating presented much loose corrosion products (W/Co-oxides) in NaCl solution, which prone to be removed by the mechanical effect of the CE and accelerated the coating damage. On the contrary, the compact Cr oxides formed on cobalt-based alloy coating surface in NaCl solution could seal the pores, preventing to formation of erosion pits, and mitigate the damage of CE. Therefore, the cobalt-based alloy coating exhibited the best CE resistance in NaCl solution and had the potential to prevent CE in seawater.