Low Interfacial Strength Research Articles

Mechanical properties of Nd–Ba–Cu–O bulk superconductors

We investigated the effects of Nd422 and Ag particles on the mechanical properties in Nd–Ba–Cu–O bulk superconductors. Both Nd422 and Ag particles were effective in decreasing the amount of microcracks running along the c direction. In the case of Nd422, however, excessive Nd422 addition enhanced the crack propagation, resulting in the degradation of mechanical strength. In the case of Ag addition, the beneficial effect of its ductile mechanical property was not observed. This was due to a relatively large size of Ag particles and low interfacial strength between Ag and Nd123 matrix. It was remarkable that the Weibull coefficient of the sample with Ag2O addition exceeded 13, which is reliable enough for practical engineering applications.

Interphase formation and structure in phenolic composites containing ultra-high modulus polyethylene fibre

The acid-catalysed cure reaction of a phenolic resin at 80°C results in the trapping of water as micron-sized spherical domains within the resin. When untreated commercial ultra-high modulus polyethylene (UHMPE) fibre is used as reinforcement with this resin, the water is trapped at the interface due to the poor wet-out of the fibre by the resin. This produces low interfacial shear strengths. A short radio frequency plasma treatment of the fibre in oxygen or water increases the shear strength by more than a factor of 2. The water liberated on resin cure is not trapped at the interface of the plasma-treated fibres, due to the enhanced wet-out of the fibre by the phenolic resin prior to cure. The plasma-treated UHMPE shows surface restructuring by XPS over a period of a week, but there is no loss of interfacial shear strength when the phenolic composite is fabricated, consistent with rapid reorientation of the carbonyl and carboxyl functionality in the presence of the resin.

Hydroxyapatite Composite Resin Cement Augmentation of Pedicle Screw Fixation

Pedicle screw stability is poor in osteopenic vertebrae attributable, in part, to low screw-bone interface strength. The current authors examined cement augmentation using a low curing temperature hydroxyapatite and bis-phenol-A glycidol methacrylate-based composite resin. This cement may stiffen the screw-bone interface and reduce the harmful effects associated with polymethylmethacrylate regarding temperature and toxic monomer. Thirty-five lumbar vertebrae from human cadavers were instrumented with pedicle screws, with one pedicle previously injected with cement and the other as the control. Caudocephalad toggling of ± 1 mm for 1600 cycles was applied to the pedicle screws, and the resulting forces supported by the implant-bone interface were captured by a load cell. A curve was constructed from the peak caudal load for each cycle and three mechanical measures parameterized this curve: (1) initial load; (2) rate of load decay during the first 400 cycles; and (3) final load. The initial load increased by 16% as a result of cement augmentation, the final load increased by 65%, and the rate of load decay decreased by 59%. Cement augmentation of pedicle screws increased the stiffness and stability of the screw-bone interface.

Optimization of an electron beam remelting of HVOF sprayed alloys and carbides

Due to the lamella structure of the coating and the lack of metallurgical bonding, thermal sprayed coatings may have disadvantages such as low coating cohesive strength and low interface strength between the substrate and coating. It was considered that the electron beam remelting process is one of the most convenient processes to reduce or remove these disadvantages. The optimization of electron beam remelting process parameters and the evaluation of this process were investigated. Not only electron beam conditions, but also the thickness of copper substrate and electroplated Ni affect the penetration depth and the remelted width at the interface. Also, in order to reduce the number of pores and the unevenness of the surface morphology, it was found that relatively low fusing speed and homogeneous heating are preferable. Moreover, because the metallurgical bonding could dominate the interfacial strength, the interfacial shear strength is independent of the coating hardness. Thus, it is successful to deposit coatings, Stellite 6, Ni based self-fused alloy 4 and WC-12Co with an adhesion strength value of approximately 350 MPa.

Nanostructured Hydroxyapatite Coatings for Improved Adhesion and Corrosion Resistance for Medical Implants

ABSTRACTHydroxyapetite (HA) coating on medical implant has been used in commercial application for several decades. The coating, commercially made by thermal spray method, functions as a intermediate layer between human tissues and the metal implant. The coating can speed up early stage healing after operation but the life span is much limited by low interfacial bond strength, which comes from the dissolution of amorphous HA in human body fluid during its service. This amorphous phase is formed in coating process under high temperature. To overcome these problems, we have developed a novel room temperature electrophoretic deposition process to fabricate nanostructured HA coating. This nanostructured HA coating significantly improved coating's bond strength up to 50-60 MPa, 2-3 times better than the thermal sprayed HA coating. The nanostructured HA coating also has corrosion resistance 50-100 times higher than the conventional HA coating. X-ray diffraction shows that all the HA coating is fully crystalline phase. It is expected that the implants with the nanostructured HA coating will have much longer service life. Other benefits derived from this process include room temperature deposition, the ability to control the coating microstructure and phases, and low cost for production.

Mechanistic Design Framework for Spalling Distress

Spalling is a distress form in concrete pavements that often manifests as the breakdown of the joint of a slab within 15 cm (6 in.) of the joint or crack and can occur at both longitudinal and transverse joints. Efforts have been under way at Texas A&M University to formulate mechanistic spalling models derived from data gathered in recent Texas Department of Transportation studies related to spall development. Extensive field studies have led to the establishment of a spalling mechanism consisting of a step-by-step process that can be characterized with engineering mechanics. These findings indicate that spalling is the result of damage initiated in the form of a shear delamination that is oriented parallel to and at a shallow depth below the surface of the pavement. Conditions necessary for formation of the delaminations include low interfacial strength between the aggregate and mortar and sufficient evaporation of pore water from the hydrating concrete, resulting in differential drying shrinkage near the pavement surface. Delaminations have been noted to initiate early in the life of the pavement and, once formed, extend later into spalls as a result of incompressibles, freeze-thaw cycles, traffic loading, and other such effects. A design framework for delamination formation and subsequent spalling development is presented in a practical format in which to mechanistically design concrete pavement systems relative to spalling distress.

Interfacial characterization of plasma-spray coated calcium phosphate on Ti-6Al-4V.

Microstructural characterization was performed on plasma-spray coated calcium phosphate and its interface with Ti-6Al-4V using scanning and transmission electron microscopy. The amorphous calcium phosphate phase was found near the coating/substrate interface, usually in between the crystalline hydroxyapatite grains and the Ti-6Al-4V substrate. The presence of the amorphous calcium phosphate and the channels extending from the interface to the coating surface was attributed to the low interfacial bond strength in a physiological environment. There was an indication of titanium diffusing into the amorphous regions. However, no reaction phases were found between the coating and the Ti-6Al-4V substrate. In the coating area remote from the interface, no intergranular amorphous calcium phosphate phase was observed. The microstructure of the Ti-6Al-4V substrate near the interface was observed to be affected by heat generated during plasma spraying.

ＧＦＲＴＰの微視的破壊プロセスに及ぼす繊維表面処理の影響 微小格子法の適用

In-situ observation of microscopic fracture process around an open hole in short glass fiber reinforced thermoplastic GFRTP composites is conducted. The specimens with four different fiber surface treatments are used. In all the material systems, the first damage observed is fiber/matrix interfacial debonding at the fiber ends. It is observed that there is a difference in fracture process between the materials with higher and lower interfacial strength. In a material system with lower interfacial strength, debonding propagates along the fiber to the fiber center and fiber break is not observed. In a material system with higher interfacial strength, extensive debonding propagation is not observed and fiber break is observed. The micro-grid method is applied to measure the fiber displacement distribution. The shear-lag prediction is compared with the experimental results. The fiber displacement distribution obtained by the micro-grid method is found to agree well with the shear-lag prediction until extensive interfacial debonding occurs.

炭素繊維／アルミニウム合金複合材料の引張強さおよび圧縮強さに及ぼす界面強さの影響

The effect of the interfacial shear strength between the carbon fibers and the matrix of carbon fiber reinforced aluminium alloys (CFRM:two PAN based high modulus and a PAN based high strength CFs, Vf=70%) on the bending strength and the compressive strength has been studied. As the interfacial shear strength increases from a lower level of 5 MPa to a higher level of 25 MPa, the tensile strength of the CFRMs contains high modulus CFs decreases rapidly from a high level of 1, 300 MPa to a low level of 500 MPa. However, as the interfacial shear strengths is varied in this region, the compressive strength of the same CFRM increases sharply from a level of 400 MPa to a level of 1, 600 MPa. The compressive strengths of the CFRMs show a high, constant level of 1, 600 MPa in the range of interfacial shear strengths over 40 MPa. CFRMs with low interfacial shear strengths show fracture at the interface and compressive collapse in compression tests. As the interfacial shear strength of the CFRMs increases, fracture at the interface is reduced, and the probability of shear fracture increases. These changes increase the compressive strengths of the CFRMs. When the level of interfacial shear strength is over 40 MPa and the compressive strengths of the CFRMs exhibit an almost constant level of 1, 600 MPa, in this case the compressive specimens show complete shear fracture mode. The interfacial shear strength is considered to depend on the content of Al4C3 accumulated at the interface in the CFRMs.

Interfaces in continuous filament-reinforced Al2O3/NiAl composites

Interfacial structures in a continuous Al2O3 filament-reinforced NiAl composite were investigated by transmission electron microscopy (TEM). A graphite phase, which is an artifact of the composite fabrication procedure, decorates the interfacial region of the composite. The presence of the graphite is believed to play a role in both the low interfacial bond strength in the as-fabricated composite and the further reduction in bond strength after 10 thermal cycles in the temperature range of 373 to 1373 K. In regions where the graphite phase was not present, there appeared to be an intimate bond between the NiAl matrix and the A12O3 filaments. Simulation of TEM diffraction contrast images based upon a three-dimensional (3D) finite element analysis was employed to investigate the nature of the residual strains in regions along the interface. The simulations suggested that radial residual strains within the Al2O3 filaments were randomly distributed along the interface. These strains are believed to be related to dislocation nucleation in the NiAl, which results from the relaxation of the thermally generated residual stresses.

Improvement of fibre/matrix bonding in carbon-fibre/acrylic composites by electron-irradiation: Concept of difunctional chemical coupling agent

In view of its many advantages over thermal processing, a new polymerization process by electron beam irradiation has been developed in the last 10 years by Aerospatiale. However, the ILSS values for carbon/acrylic composite materials are relatively low, indicating low interfacial strength. By using the concept of a difunctional chemical coupling agent, isocyanate urethane methacrylate and isocyanate urethane styrenic compounds have been synthesized and used as sizing agents on carbon fibres. ILSS and pull-out tests have been used to evaluate the fibre/matrix adhesion. These sizing agents substantially improve carbon-fibre/ acrylic-matrix bonding and the mechanical results are quite competitive with those of thermal processes. This leads us to suppose that polymerization processing by electron beam irradiation will be substantially developed in the future.

Functional interfaces for oxide/oxide composites

Potentially useful interfacial coatings for composites of sapphire fibers in polycrystalline aluminum oxide matrices are discussed, subject to the constraints of phase compatibility in high temperature oxidizing environments and low mechanical interface strength ( i.e. to allow pull-out). Results of preliminary tests to investigate feasibility of some candidate coatings ( β-aluminas, LaPO 4 and CaF 2) are presented. These systems exhibited interfacial debonding under certain conditions, as required for high toughness composites, although several issues remain to be resolved before they can be used successfully.

液晶ポリマー／ポリフェニレンサルファイド系ポリマーブレンド射出成形品の力学的特性とモルフォロジー

Mechanical properties of injection moldings of incompatible binary polymer blends are strongly affected by morphology in the moldings. In this study, structure and mechanical properties of injection moldings of liquid crystalline polymer (LCP)/polyphenylene sulfide (PPS) binary polymer blends were investigated. The two polymers are incompatible and form separate phases in the molding. The structure of separate phases is affected very much by the blending ratio. At the low LCP concentration, the LCP forms spherical domains in the core and fibrillar structures in the skin layer. At the high concentration, the layer-structure like LCP is formed. The modulus of moldings increases with increasing LCP concentration whereas the strength decreases because of low interface strength of moldings.

Interlaminar shear properties of graphite fiber, high‐performance resin composites

AbstractShear properties of laminates consisting of graphite fiber (Celion 6000, Celion 3000, and T‐300) and several resins (epoxy, hot melt‐bismaleimide, solvent bismaleimide, polyimide, and polystyrylpyridine (PSP)) were measured using the short‐beam test. The interlaminar shear strength of epoxy composites was hightest. All other resin composites showed considerably lower shear properties. The shear strengths deereased with temperature, with the loss up to 100°C being more pronounced. The PSP composites did not show a loss in strength in the temperature range investigated (room temperature to 250°C). Boiling the composite samples in water for 24 h resuled in negligible reduction in shear strength in all cases. The interlaminar shear properties depended on the fiberresin interfacial bond, the wetting characteristics of the resin, and the resin meechanical properties. Attempts were made to determine to what extent each of these factors controlled the shear properties of the composites studies. Thus, the interfacial bond between the resin and the single fiber was determined. Results showed that the lower shear strength of polyimide, bisimides and PSP composites, as compared to epoxy resin composites, seemed to be due to their corresponding low interfacial bond strengths. The subsequent decrease in shear strength with temperature appeared to be directly correlated to the decrease in bond strength. The wettability of carbon fiber tow with all these resins was also determined. The small variations in wetting characteristics of the resin did not seem to justify the rather large differences in shear‐strength properties observed.

Strength of the Cement-Bone Interface

The fixation of total joint components to bone using acrylic bone cement is by the penetration of the cement into the microstructure of cancellous bone to achieve a mechanical interlock. It has been shown that the method of cement application and the preparation of the cancellous surface significantly affects both the tensile and shear strengths of the cement-bone interface. Doughy cement finger-packed on an uncleaned surface resulted in a very low interface strength compared to a low-viscosity cement made to penetrate a cleaned bone surface. Maximum strengths were achieved for cancellous bone cleaned by using either a high-intensity water lavage or a polyethylene brush and by facilitating penetration of the cement for distances of 5 to 10 mm into the bone.

Graphite/Aluminum: An Evaluation of State-of-the-Art Material

The tensile and fracture toughness properties of a unidirectional, 28.3 volume percent graphite/aluminum composite have been measured and compared to boron/aluminum composites. The longitudinal and transverse tensile strengths in four-point bending were determined to be 105.6 ksi (728 MPa) and 7.19 ksi (49.5 MPa), respectively. The longitudinal strength is only slightly lower than that which would be expected from a boron/ aluminum composite of similar volume fraction. The low transverse tensile strength was shown to be the result of a very weak filament-matrix inter face. The apparent fracture toughness, Kc', of this graphite/aluminum com posite was found to be 23.48 ksi √in (25.83 MPa √ m) which is consider ably higher than the fracture toughness of a boron/aluminum composite with similar volume fraction. While the reason for this unexpectedly high fracture toughness is not definitively identified, the evidence suggests that the source may be the low filament-matrix interface strength.

Fracture mechanism transitions in laminate composites

Possible effects of using standard metallurgical fracture toughness specimens to measure the fracture toughness KIC of laminates and high-volume-fraction composites have been analysed. It has been shown that fracture can take place either through the laminates or by delamination. The method of load application causes tensile stresses perpendicular to the laminates and shear stresses parallel to the laminates, either of which can cause pre-existing flaws in the laminate interface to propagate or the bond strength to be exceeded. Expressions have been determined that permit prediction of the mechanism of crack extension, provided that certain geometric and materials parameters are known. They indicate that delamination is more likely for small specimens, short cracks, high transverse fracture toughness, and low interfacial toughnesses and strengths. Thus, geometric factors can play a more important role in fracture of composites than in isotropic materials, and care must be exercised when assessing the results of toughness tests.