The cohesive and adhesive strength of ice

The plane-strain fracture-energy test developed by Andrews and Stevenson has been applied to the study of ice adhering to substrates of stainless steel, titanium and anodised aluminium. In most cases the fracture is cohesive through the ice, and therefore yields a cohesive fracture energy (critical energy release rate). The value of this fracture energy, however, is dependent upon the nature of the substrate, stainless steel giving significantly lower values than titanium. The fracture energy is also affected by the rate of formation of the ice and by the rate of testing. Many of these effects can be traced to the influence of the substrate on the air-bubble content of the ice layer. At testing temperatures approaching the melting point of ice, a transition in fracture mode is observed from cohesive to adhesive, and the fracture energy diminishes. The addition of small amounts of sodium fluoride to the water from which the ice is formed, lowers the transition temperature to –5° C, and emphasizes the transition to the adhesive failure mode.     

Interfacial strength and fracture energy of individual carbon nanofibers in epoxy matrix as a function of surface conditions

The interfacial shear strength (IFSS) and fracture energy of individual carbon nanofibers embedded in epoxy were obtained for different surface conditions and treatments by novel, MEMS-based, nanoscale fiber pull-out experiments. As-grown vapor grown carbon nanofibers (VGCNFs) with turbostratic surface and 5 nm peak-to-valley surface roughness exhibited high IFSS and interfacial fracture energy, averaging 106 ± 29 MPa and 1.9 ± 0.9 J/m², respectively. Subsequent high temperature heat treatment and graphitization resulted in drastically reduced IFSS of 66 ± 10 MPa and interfacial fracture energy of 0.65 ± 0.14 J/m². The smaller IFSS values and the reduced standard deviation were due to significant reduction of the fiber surface roughness to 1–2 nm, as well as a decrease in surface defect density during conversion of turbostratic and amorphous carbon to highly ordered graphitic carbon. For both grades of VGCNFs failure was adhesive with clear nanofiber surfaces after debonding. Oxidative functionalization of high temperature heat-treated VGCNFs resulted in much higher IFSS of 189 ± 15 MPa and interfacial fracture energy of 3.3 ± 1.0 J/m². The debond surfaces of functionalized nanofibers had signs of matrix residue and/or shearing of the outer graphitic layer of the VGCNFs, namely the failure mode was a combination of cohesive matrix and/or cohesive fiber failure which contributed to the high IFSS. For all three grades of VGCNFs the IFSS was independent of fiber length and diameter. The findings of this experimental study emphasized the critical role of nanofiber surface morphology and chemistry in determining the shear strength and fracture energy of nanofiber interfaces, and shed light to prior composite-level strength and fracture toughness measurements.     

Ice nucleation in water droplets on glass surfaces: From micro- to macro-scale

Ice nucleation encountered in engineering systems is often induced by solid/water interfaces. When classical nucleation theory is used to analyze ice nucleation in such systems, the uniformity of interfaces that contribute to ice nucleation must be carefully considered, because classical nucleation theory cannot be directly applied to non-uniform interfaces. In this study, to discuss the uniformity of ice nucleating activity of solid/water interfaces, ice nucleation in water droplets prepared on glass surfaces was investigated for various droplet sizes from micrometer to sub-millimeter. When the interfacial area between water and the glass surface was smaller than 1 × 10⁻¹⁰ m², the ice nucleation temperature showed scatter of about 2 °C, suggesting uniformity of the interface. However, when the interfacial area was larger than 1 × 10⁻⁸ m², the ice nucleation temperature showed large scatter, suggesting the ice nucleating activity was no longer uniform.     

Fracture of a ridged multi-year Arctic sea ice floe

As part of the 1994 Sea Ice Mechanics Initiative experimental program, fracture experiments were carried out on an 80 m diameter ridged multi-year (MY) ice floe in the Beaufort Sea. An edge cracked, quasi-circular ridged floe was subjected to both cyclic and ramp loading sequences using a steel flat jack. Load, crack opening displacement, acoustical and seismic measurements were made during the experiments. The objective was to gain further insight into the fracture and constitutive properties of MY sea ice. Accurate predictions of the strength of MY sea ice and the forces developed during interactions between MY sea ice and floating or fixed structures are sought. Such interactions include MY ice floe collisions with offshore structures and ships. The fracture resistance of MY ice is determined to be within the range 23 < G_c < 47 J/m² for a 80 m diameter ridged MY floe. This fracture energy is similar to values obtained for the fracture of FY sea ice both in the Arctic and the Antarctic.     

Fracture characteristics of metal/intermetallic laminar composites produced by reaction sintering and hot pressing

Metal/intermetallic layered composites were formed by a process recently developed in which a self-propagating, high-temperature synthesis reaction was initiated at the interface between dissimilar metal foils. After the reaction, one of the metal foils was entirely consumed, resulting in a metal/intermetallic laminar composite. This study details the tensile fracture characteristics of these unique composites. Fracture mechanism and failure energy were controlled by varying the intermetallic-to-metal volume ratio. Failure initiated with the formation of cracks in the intermetallic layer. For high intermetallic-to-metal ratios, the intermetallic crack release energy was too great to prevent cracks from propagating through the metal layer and propagating the crack into the adjacent intermetallic layers, leading to a fast, low energy fracture. For lower intermetallic-to-metal ratios, the metal layers adsorbed the intermetallic crack release energy and blunted the propagating crack. Final failure resulted by ductile fracture of the metal layer after extensive intermetallic cracking.     

Joining of carbon fibre-reinforced silicon nitride composites with 72Ag-26Cu-2Ti filler metal

Unidirectionally reinforced carbon fibre/Si₃N₄ matrix composites were joined with 72Ag-26Cu-2Ti filler metal. Joining interfaces were observed by SEM and analysed by energy dispersive spectroscopy. The strength of the joints was evaluated by four-point bending tests. Most of the interfaces of Si₄N₄ matrix/filler metal were firm without cracking and separation. At the interfaces, reaction between the composites and filler metal was limited. Only a concentration of titanium at Si₃N₄ matrix/filler metal interface was confirmed. On the fracture surfaces, many holes left as traces of pull-out of carbon fibres and pulled out fibres could be observed. The maximum joining strength and the average strength, measured by the bending test, were 159 MPa and 107 MPa, respectively. The pull-out process of fibres from the matrix and the reasons for the large scatter in the strength of joints, were discussed. The fibre pull-out behaviour could be related to fibre distribution density at the joining interface.     

Effect of carbon nanotube reinforcement on fracture strength of composite adhesive joints

This study investigated the use of carbon nanotubes (CNTs) as an epoxy adhesive additive for adhesive joints between steel–composite interfaces and composite–composite interfaces. The study also examined the effect of CNT functionalization to improve CNT dispersion and thus improve joint strength. Specimens were constructed by adhesively bonding two parallel coupons, with a starting crack at one end. The specimens were loaded to final failure in three-point bending for Mode II fracture. Critical strain energy release rate was used to compare fracture properties of each set of specimens. It was shown that additions of multi-walled CNTs on the order of 1 wt% with diameters on the order of 30 nm and lengths 5–20 μm enhanced fracture toughness for both steel–composite and composite–composite adhesive joints tested. However, other combinations of CNTs could significantly decrease fracture properties, likely due to agglomeration issues. Functionalization of nanotubes showed some limited promise. Scanning electron microscopy validated the improved dispersion of CNTs using functionalization, but also highlighted the shortening effects due to the harsh chemical treatment. In summary, the study illustrates the importance of various CNT parameters on fracture properties, and encourages further investigation and optimization of these parameters for applications of interest.     

Fatigue of cemented hip replacements under torsional loads

Fatigue resistance of hip replacement prostheses is becoming ever more important as the operation is carried out on younger and more active patients. Torsional loading of the implant, which occurs especially during activities, such as rising from a chair or climbing stairs, is implicated in the failure process. To examine fatigue failure of the implant–bone fixation, which is made using polymethylmethacrylate cement, an experimental model was designed and 16 specimens tested at torsional moments of 50 Nm and 80 Nm to both 1 and 2 million cycles. The numbers and lengths of cracks initiated under cyclic loading were quantified using dye penetrant to highlight the cracks and a profile projector to magnify them. The majority of cracks initiated from the PMMA/metal and PMMA/bone interfaces, more often than from pores in the PMMA. A bimaterial fracture mechanics analysis confirmed that the interfaces are too weak to sustain in vivo levels of cyclic loading. It is proposed that, under torsional loading, fatigue failure of PMMA fixated implants originates from pores located on the interfaces.     

Mechanical behavior and failure process during compressive and shear deformation of honeycomb composite at elevated temperatures

The mechanical behavior and failure mechanism of honeycomb composite consisting of Nomex honeycomb core and 2024Al alloy facesheets were investigated. The compressive and shear deformation behaviors of honeycomb composite were analyzed at temperatures ranged 25–300°C. The compressive and shear strengths of honeycomb composite decreased continuously with increasing temperature up to 300°C. The stress-strain curves obtained from the compressive and shear tests showed that the stress increased to a peak value and then decreased rapidly to a steady state value, which is nearly constant up to failure with increasing strain. The compressive deformation behavior of honeycomb composite was progressed by an elastic and plastic buckling of cell walls, debonding fracture at the interfaces of cell walls, and followed by a fracture of resin layer on cell walls. The shear deformation of honeycomb composite was progressed by an elastic shear deformation, plastic shear deformation, fracture of resin layer on cell walls, and followed by debonding fracture at core/facesheet interfaces. The shear strength of honeycomb composite showed strong anisotropy dependent on the loading direction. The shear strength in longitudinal direction was about 1.4 times higher compared to that in transversal direction due to the different thickness of cell walls mainly loaded during the shear deformation.     

Adherence-fracture energy of a glass-bonded thick-film conductor: effect of firing conditions

The effect of firing conditions on the adherence of a glass-bonded Pt-Au printed thick film conductor to a 96 wt % Al₂O₃ substrate was determined by a fracture mechanics measurement of the critical fracture energy for catastrophic thick film-substrate separation. The technique also demonstrated that separation by slow crack growth (delayed failure) occurred in this system. Analysis of the thick film microstructure and fracture surfaces showed that optimum adherence was primarily a result of a mechanically interlocked interface formed between the conductor metal and the glass bonding layer which, in turn, was strongly bonded to the alumina substrate. The two step decrease observed in _IC (from 3.7 to 0.2 J m^–2) with firing temperatures over 860° C resulted from the removal of this interlocking metal-glass interface brought on by metal sintering and glass migration to the substrate. Thus, at 860° C firing temperatures, adherence is controlled by cohesive fracture in the glass bonding phase while above 1000° C it is controlled by adhesive failure of the weak chemical-physical bond at the metal-glass interface.     

A fracture approach to thick film adhesion measurements

The constant-compliance double cantilever beam test for adhesive fracture energy has been adapted to the measurement of the adhesion of thick film metallizations on alumina. The test involves a single beam soldered to metallization strips and measures the opening-mode fracture energy, G _Ic. Solder-wire peel tests were also made on the same metallizations. Both the fracture and peel tests indicated failure in the thick films but near the film/alumina boundary and both sets of data exhibited a bimodal distribution. The magnitude of the fracture energies indicated failure usually occurred in the glass phase of the film but that film adherence was greatly enhanced by interlocking between the metal and glass phases. This interlocking, and thus the film adhesion, was strongly dependent on the firing temperature.     

AA5052薄板磁脉冲胶焊复合接头动态力学性能研究

目的 研究AA5052铝合金薄板在高速冲击载荷下的磁脉冲胶焊复合接头的动态力学性能,探究不同载荷速率对该胶焊复合接头力学和失效行为的影响规律.方法 利用磁脉冲焊接系统成功制备了胶焊复合连接试件.采用万能拉伸试验机、高速拉伸试验系统,结合全场应变测量系统,获得胶焊复合接头的力学性能规律,以及渐进失效过程和搭接区应变变化....     

Mechanical response of the TiAl/steel brazed joint under impact load

Mesnager impact tests of TiAl base metal and TiAl/steel brazed joints were conducted at room temperature (293 K) and elevated temperature (623 K). Impact strength, impact energy, fracture path, and the behavior of the reaction phases were studied. For the room temperature test, average impact energy and strength of the joint are 71.9% and 84.2% of the TiAl base metal, respectively; which are 62.5% and 75.3% of TiAl base metal, respectively, at 623 K. Fracture path and crack propagation process analysis show, when subjected to the impact load, cracks germinate at the interface of Ag-based solid solution/AlCu₂Ti particles, grow up and propagate into the Al–Cu–Ti brittle reaction layers, then propagate into the TiAl base metal, and result in failure.     

The influence of substrate chemistry on the adhesion of electrolessly deposited Ni(P) on metal oxide coated ceramics

The adhesion of electrolessly deposited Ni(P) on alumina ceramic substrates which were coated with thin SiO₂, SnO₂, TiO₂, Al₂O₃, Y₂O₃, ZrO₂ and (In,Sn)O_x (ITO) films was studied. The adhesion was measured with the aid of the 90° peel test. Strong adhesion of Ni(P) was found for the substrates with ZrO₂ and Al₂O₃ coatings and weak adhesion for the substrates with SiO₂, TiO₂, SnO₂, Y₂O₃ and ITO coatings. The fracture path and the type of interfacial bonding were analysed using scanning electron microscopy, energy-dispersive analysis of X-rays and X-ray photoelectron spectroscopy. In the case of the strongly adhering samples, fracture took place through the metal layer and along the interface. In the case of the weakly adhering samples only interfacial failure was observed between the Ni(P) layer and the metal oxide coating. Cross-section transmission electron microscopy studies of the interfaces suggested that the differences in peel energy values are caused by differences in micromechanical interlocking at the metal oxide-Ni(P) interface. In addition, a weak boundary layer which was found to be present at the Ni(P)-alumina interface was absent in the case of the strongly adhering samples with the ZrO₂ substrate coating.     

Interfacial fracture of dentin adhesively bonded to quartz-fiber reinforced composite

The paper presents the results of an experimental study of interfacial failure in a multilayered structure consisting of a dentin/resin cement/quartz-fiber reinforced composite (FRC). Slices of dentin close to the pulp chamber were sandwiched by two half-circle discs made of a quartz-fiber reinforced composite, bonded with bonding agent (All-bond 2, BISCO, Schaumburg) and resin cement (Duo-link, BISCO, Schaumburg) to make Brazil-nut sandwich specimens for interfacial toughness testing. Interfacial fracture toughness (strain energy release rate, G) was measured as a function of mode mixity by changing loading angles from 0° to 15°. The interfacial fracture surfaces were then examined using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) to determine the failure modes when loading angles changed. A computational model was also developed to calculate the driving forces, stress intensity factors and mode mixities. Interfacial toughness increased from ≈ 1.5 to 3.2 J/m² when the loading angle increases from ≈ 0 to 15°. The hybridized dentin/cement interface appeared to be tougher than the resin cement/quartz-fiber reinforced epoxy. The Brazil-nut sandwich specimen was a suitable method to investigate the mechanical integrity of dentin/cement/FRC interfaces.     

Surface energy controlled α–γ–α transformation texture and microstructure character study in ULC steels alloyed with Mn and Al

In this article, an ultralow-carbon steel grade alloyed with Mn and Al has been investigated during α–γ–α transformation annealing in vacuum. Typical texture and microstructure has evolved as a monolayer of grains on the outer surface of transformation-annealed sheets. This monolayer consists of <100>//ND and <110>//ND fibre, which is very different from the bulk texture components. The selective driving force is believed to reside in the anisotropy of surface energy at the metal–vapour interface. The grain morphology is very different from the bulk grains. Moreover, 30–40% of the grain boundary interfaces observed in the RD–TD surface sections are tilt incoherent <110> 70.5° boundaries, which are known to exhibit reduced interface energy. Hence, the conclusion can be drawn that the orientation selection of surface grains is strongly controlled by minimization of the interface energy; both metal/vapour and metal/metal interfaces play a roll in this.     

Thermal stability of the engineered interfaces in Wf/W composites

Application of tungsten as a structural material is severely restricted due to its inherent brittleness. Recently, a novel toughening method for tungsten was proposed by the authors using tungsten wires as reinforcement. The idea is analogous to the fiber-reinforced ceramic–matrix composites theory which utilizes the internal energy dissipation caused by the debonding and frictional sliding at the fiber/matrix interfaces to absorb strain energy and to redistribute stress concentrations over an extended volume. To maximize the energy dissipation, the interfaces need to be engineered by coating which can withstand thermal exposure during service. In this work, we studied the thermal stability of various interfacial coatings after heat treatment. Microstructural change and the effect on mechanical properties were investigated by means of electron microscopy and fiber push-out tests. The results show that the microstructural phases of the analyzed interfaces remained relatively stable under thermal exposure of 800 °C for 10 h. Under such thermal exposure, the fracture energy of the Er/W multilayer and the ZrO _x /Zr multilayer were affected by less than 10%, while it was increased by 40% for the ZrO _x /W bilayer. The fracture energy of the C/W dual layer was decreased by a factor of 4, whereas for the Cu/W multilayer case it was increased by a factor of 2.     

Prediction of Delamination Onset and Critical Force in Carbon/Epoxy Panels Impacted by Ice Spheres

Polymer matrix composite structures are exposed to a variety of impact threats including hail ice. Internal delamination damage created by these impacts can exist in a form that is visually undetectable. This paper establishes an analysis methodology for predicting the onset of delamination damage in toughened carbon/epoxy composite laminates when impacted by high velocity ice spheres (hailstones). Experiments and analytical work focused on ice sphere impact onto composite panels have determined the failure threshold energy as a function of varying ice diameter and panel thickness, and have established the ability to predict the onset of delamination using cohesive elements in explicit dynamic finite element analysis. A critical force associated with damage onset was found to be independent of the ice diameter and thus can be expressed as a function of basic panel-describing parameters, namely bending rigidity and interlaminar fracture energy. Critical force can be used as a failure criterion in simpler models (e.g., shell elements) when predicting the onset of delamination by high speed spherical ice impact.     

Optimization of bond strength between gold alloy and porcelain through a composite interlayer obtained by powder metallurgy

The purpose of this study was to evaluate the influence of a composite interlayer (at the metal-ceramic interface) on the shear bond strength of a metal-ceramic composite when compared with a conventional porcelain fused to metal (PFM).Several metal-ceramic composites specimens were produced by hot pressing. To identify which was the best composition for the interlayer several composites, with different relations of metal/ceramic volume fraction, were bonded to metal and to ceramic substrates. The bond strength of the composites to substrates was assessed by the means of a shear test performed in a universal test machine (crosshead speed: 0.5 mm/min) until fracture. Some interfaces of fractured specimens as well as undestroyed interface specimens were examined with optical microscope and scanning electron microscope (SEM/EDS).The shear bond strength results for all composites bonded to metal and to ceramic substrates were significantly higher (>150 MPa) than those registered in the upper range of conventional porcelain fused to metal (PFM) techniques (∼80 MPa). The use of a composite interlayer proved to enhance metal/ceramic adhesion in 160%.     

Fractographic examinations of fracture in polycrystalline S2 ice

Fractographic examinations were carried out on the fracture surfaces of both single-edge notched bend (SENB) and wedge-loaded compact tension (WLCT) specimens of S2 freshwater ice. Formvar solutions provided an effective means of making replicas that revealed various patterns of the fracture surfaces. The fracture modes consisted of both cleavage and brittle intergranular fracture, with cleavage fracture dominating. The cleavage planes of the S2 ice were mainly the {0 0 0 1} and {10¯1 1} planes under the experimental conditions for this study. Kinks forming new grain boundaries were found on the fracture surfaces of polycrystalline S2 ice for the first time. Kinking is regarded as a possible mechanism of plastic deformation for polycrystalline ice and to partially account for the high fracture energy of S2 ice found in this study.