Significant Degradation Of Mechanical Properties Research Articles

Flexible polytetrafluoroethylene (PTFE) composite film with enhanced thermal conductivity and low interface thermal resistance for high frequency electronics

The significant degradation in mechanical properties and filling-gap performance of thermal interface materials (TIMs) under high filling ratios remained a prevalent problem. Here, a polytetrafluoroethylene (PTFE) based composite film with excellent flexibility at ultra-high hybrid fillers content (up to 90 wt%) was developed through the mechanical interlocking strategy. Due to the sufficient fibrosis of PTFE under shear stress, boron nitride (BN) and aluminum nitride (AlN) were firmly constrained inside the fiber cage. The hybrid fillers arranged into a seesaw-like structure during the calendering process, which facilitated the formation of thermal conductive network in both horizontal and vertical directions. Importantly, this unique embedded structure allowed for direct contact between the fillers and the heat source, resulting in a 90.45% decrease in interfacial thermal resistance. The resultant film possessed a high thermal conductivity (4.3 W/m·K, in-plane, 0.75 W/m·K, out-plane), low dielectric constant (3.1, 10 GHz), and excellent flexibility, making it suitable for application of interface heat dissipation in high-frequency electronic devices.

Digital Image Correlation Technique to Aid Monotonic and Cyclic Testing in a Noisy Environment during In Situ Electrochemical Hydrogen Charging

Hydrogen is receiving growing interest as an energy carrier to facilitate the shift to a green economy. However, hydrogen may cause the significant degradation of mechanical properties of structural materials, premature strain localisation, crack nucleation, and catastrophic fracture. Therefore, mechanical testing in hydrogenating conditions plays a vital role in material integrity assessment. Digital image correlation (DIC) is a versatile optical technique that is ideally suited for studying local deformation distribution under external stimuli. However, during mechanical testing with in situ electrochemical hydrogen charging, gas bubbles inherent to hydrogen recombination are created at the sample surface, causing significant errors in the DIC measurements, and posing significant challenges to researchers and practitioners utilising this technique for testing in harsh environments. A postprocessing technique for the digital removal of gas bubbles is presented and validated for severe charging conditions (−1400 mV vs. Ag/AgCl) under monotonic and cyclic loading conditions. Displacement fields and strain measurements are produced from the filtered images. An example application for measuring the crack tip opening displacement during a slow strain rate tensile test is presented. The limitations of the technique and a comparison to other bubble mitigation techniques are briefly discussed. It was concluded that the proposed filtering technique is highly effective in the digital removal of gas bubbles during in situ electrochemical hydrogen charging, enabling the use of DIC when the sample surface is almost completely obscured by gas bubbles.

Effects of recycling and hygrothermal environment on mechanical properties of thermoplastic composites

Recyclability of nine types of composites including natural and mineral fillers in polypropylene and nylon matrix have been studied through injection molding and mechanical grinding. No significant degradation in mechanical properties of natural fiber composites occurred after recycling or water absorption tests. In contrast, glass fiber composites showed decrease in tensile strength and modulus by up to 45% and 40%, respectively after five rounds of recycling mainly due to severe fiber attrition. Glass bubbles breakage occurred after recycling, which diminished its ability to provide lightweighting. Only glass fiber reinforced nylon composites were affected by hygrothermal conditioning, which were characterized as a function of recycling. At saturation point, water absorption for nylon composites was 7.7% by wt. after 45 days of immersion, which significantly affected the mechanical properties. Tensile strength and modulus of glass fiber nylon composites reduced from 88.4 MPa and 5.6 GPa to 36.2 MPa and 1.8 GPa, respectively.

Sensitivity to hydrogen embrittlement of AISI 4140 steel: A numerical study on fracture toughness

Contamination of steel structures, working in environments where the presence of hydrogen cannot be neglected, can lead to significant degradation of mechanical properties, in particular, fracture toughness. In order to estimate the local hydrogen concentration at the crack tip and to understand the embrittlement mechanism, numerical models are important tools to support experimental tests that are quite complex to perform. This paper presents the application of a cohesive zone model, which couples diffusion and mechanical fields, to study the hydrogen embrittlement on AISI 4140 steel. The total hydrogen concentration, sum of the contents of hydrogen present in the lattice and in dislocation traps, is the quantity governing the embrittling effect. The input parameters of the model were calibrated using experimental tests performed on steel samples; then, initial lattice concentration was calibrated based on hydrogen pre-charged tests. A sensitivity analysis was proposed, discussing the effects of material, environmental and testing input parameters. The analysis confirms the capability of this numerical tool in predicting the mechanical response in presence of hydrogen, highlighting its potential to be used for practical design and assessment.

BN 첨가 AISI P20 쾌삭강의 개발

As products life cycles are becoming shorter, the reduction of die and mold manufacturing cost and time is becoming more crucial in the machinery, automotive, and electronics industries. Over the past decades, many initiatives have been made to develop high performance free-machining steels without significant degradation of mechanical properties. To develop a modified AISI P20 free-machining steel, we studied the effects of B, N, and S additives on the variations of the cutting forces and metal structures such as grain size, density, and distribution of free-machining inclusions. From a set of experiments, it was observed that an appropriate addition of B and N additives reduces the resulting cutting force by approximately 6.3% and delays the tool wear progress. During the solidification B and N additives form hBN precipitates, with a layered and planar structure, within the steel matrix. The hBN precipitates’ weak shear strength results in lowering the required milling force. It is also confirmed that machinability is prominently improved when a large number of microsized hBN precipitates are distributed uniformly in the steel matrix. This study could contribute to the development of high performance BN-added free-machining steels for die and mold applications.

Bond behavior degradation between FRP and masonry under aggressive environmental conditions

Application of Fibre Reinforced Polymers (FRPs) as external reinforcement for masonry structures has been found to be a promising technique, but the long-term behavior of such systems still needs attention. This study presents an overview of the experimental activities followed by the authors on durability of externally bonded FRP-masonry components subjected to water immersion and accelerated hygrothermal exposure. Both moist and hygrothermal environments were found to be extremely deteriorating in the studied materials and led to significant degradation of mechanical properties and loss in bond strength. The effect of mechanical surface treatment on the bond durability performance was also investigated.

Effect of alternating current (AC) stressing on the microstructure and mechanical properties of low-silver content solder interconnect

The low-cost low-silver-content solder is receiving high attention while its reliability is not well studied. In this work, a copper/tin-silver-copper/nickel (Cu/Sn0.3Ag0.7Cu/Ni) line-type solder interconnect was designed for investigation of the effect of alternating current (AC) on the microstructure and mechanical properties. Three dimensional (3D) X-ray microcomputed tomography tests showed voids and protruding IMCs formed in the solder/Ni and solder/Cu interface regions of the interconnect after prolonged AC stressing, respectively. The thickness growth of interfacial IMC layers followed linear laws with faster rates than those in thermal aged sample. Atomic migration of Cu atoms in Sn0.3Ag0.7Cu solder was estimated using mathematical models with a thermal gradient of 206.77 °C cm−1. Significant degradation in mechanical properties happened. Nano-indentation results showed the hardness and elastic modulus in different solder areas were in the following order: central solder > solder near solder/Ni interface > solder near solder/Cu interface. Four mechanisms can be used to explain these results of AC damages: 1) elevated temperature due to Joule heating, which enhanced grain coarsening and dislocations redistribution; 2) thermal fatigue due to cyclic loading, which damaged the interconnect strength by accumulated stress induced by mismatch in the coefficients of thermal expansion; 3) thermo-migration due to thermal gradient from Ni side to Cu side, which released the thermal stress at the solder/Ni interface and prevented further deterioration, while increasing the thermal stress at the solder/Cu interface and deteriorated the mechanical strength; 4) atomic migration due to chemical gradient, which induced the mass redistribution and contribute to the growth of IMCs layers.

HYGROTHERMAL DURABILITY OF ADHESIVELY BONDED FRP/STEEL JOINTS

The use of fiber reinforce polymer, FRP, bonding to strengthen and repair deteriorated steel structures is increasing owing to its unique advantages over traditional strengthening and repair techniques. However, the lack of knowledge regarding environmental durability of adhesively bonded FRP/steel joints still hinders the widespread application of this method in steel structures. A number of studies have reported significant degradation of mechanical properties of these joints in hot and wet environments. In addition to that, the mechanisms of failure have been observed empirically to change from cohesive failure in the adhesive to apparent interfacial failure with increasing amount of moisture. This study presents the results of an experimental and numerical investigation to predict the mechanical behavior of FRP/steel joints after hygrothermal aging. First, moisture diffusion kinetics and mechanical degradation of a two-part commercially available epoxy adhesive and Carbon FRP material were experimentally characterized over a wide range of temperature and humidity conditions. These parameters were then incorporated in a coupled 3D diffusion-mechanical finite element, FE, model. In addition, bonded double-lap shear joints of CFRP/steel were aged for up to a year and tested to failure. It is found that the presence of moisture for less than a critical period can increase the joint strength. However, prolonged exposure to the same moisture content degrades the load-carrying capacity of the joint.

A study of the behavior of bi-oriented PVC exposed to ionizing radiation and its possible use in nuclear applications

The paper discusses whether bi-oriented PVC, obtained by modifying the structures of polymers chains to enhance the mechanical properties of unplasticized PVC, could successfully replace metallic materials in industrial applications where radioactive fluids are processed and an intense field of ionizing radiation is present. Tests have been carried out in order to study the behavior of a commercial bi-oriented PVC when exposed to ionizing radiations. A numerical simulation allows comparing the effects of radiation expected on the pipe in nuclear industry applications with those resulting from the irradiation tests. Contamination and decontamination tests of bi-oriented PVC in contact with a radioactive solution have been performed too. Results show that the bi-oriented PVC can withstand high β and γ radiation doses (up to 100kGy) without showing significant degradation in mechanical properties; bi-orientation of polymers chains in the bulk of material is not affected even to much higher doses (250kGy); the decontamination of the material is satisfactory. The results suggest that tested commercial bi-oriented PVC could be considered in nuclear industry applications.

Crosslink density, oxidation and chain scission in retrieved, highly cross-linked UHMWPE tibial bearings

Irradiated, thermally stabilized, highly cross-linked UHMWPE bearings have demonstrated superior wear performance and improved in vitro oxidation resistance compared with terminally gamma-sterilized bearings, yet retrieval analysis reveals unanticipated in vivo oxidation in these materials despite fewer or no measurable free radicals. There has been little evidence to date that the oxidation mechanism in thermally stabilized materials is the same as that in conventional materials, and so it is unknown whether oxidation in these materials is leading to chain scission and a degradation of mechanical properties, molecular weight, and crosslink density. The aim of this study was to determine whether measured in vivo oxidation in retrieved, highly cross-linked tibial bearings corresponds with a decreasing crosslink density. Analysis of three tibial bearing materials revealed that crosslink density decreased following in vivo duration, and that the change in crosslink density was strongly correlated with oxidation. The results suggest that oxidation in highly cross-linked materials is causing chain scissions that may, in time, impact the material properties. If in vivo oxidation continues over longer durations, there is potential for a clinically significant degradation of mechanical properties.

Multi-factorial models of a carbon fibre/epoxy composite subjected to accelerated environmental ageing

Among materials being introduced in the aerospace industry, the carbon fibre reinforced plastics (CFRP) have a place of privilege because of their exceptional stiffness-to-mass ratio. However, the polymer-based matrix is vulnerable to damages by environmental conditions. This work exposes the experimental results of several accelerated environmental ageing protocols on CFRP panels. The main concern is to justify or reject by statistical means that a significant degradation of mechanical properties does occur over the time, and to establish a basic model to quantify the effects of different environmental factors of the composite ageing. The results considered here are the elastic properties evaluated over several weeks of accelerated artificial ageing. The stiffness degradation of the samples subjected to the aforementioned ageing protocols is statistically described by a non-linear multi-factorial model inspired by the Design of Experiments (DoE) theory. The evolution of constitutive properties (namely mass and elastic properties) over the time exhibits an asymptotic exponential increasing (or decreasing) pattern over the time. The usefulness of these mathematical models is their predictability, based only on theoretical considerations on moisture absorption. This path is further investigated in this paper, clearing up the way to a methodical prediction of ageing models.

Microstructure and Mechanical Properties of Electron Beam Welded Alloy J75

Microstructure and mechanical properties of electron beam welded alloy J75 were studied under as-welded and post-weld aging treatment (PWAT) conditions. The results showed that high-quality welds were produced by electron beam welding. Under as-welded condition, a fine dendritic structure consisting of gamma dendrite matrix and Laves phase was observed in the welds. Better mechanical properties were obtained in the weld zone than that of base metal because of the fine size of the dendritic structure. After PWAT, a discontinuous distribution of γ′ particles existed in the dendritic structure. The presence of a γ′ depletion zone in the dendrite core resulted in a significant degradation of mechanical properties of the weld.

Fabrication and characterization of micro-dimple array on Al2O3 surfaces by using a micro-tooling

Abstract Al2O3 is a material with high potential for use in biomedical implants because of its low wear rate and excellent biocompatibility. The micro-dimpling surface technique is one of the most advanced surface modification techniques available for the optimisation of tribological performance. A well-defined dimple surface fabricated on an Al2O3 substrate could further improve its tribological properties, thus extending the durability of an implant. In this study, we fabricated a well-defined micro-dimple pattern on an Al2O3 surface by using a CNC micro machine, and characterised the fabricated dimple parameters. Mechanical analyses were conducted to investigate whether there are any changes in mechanical properties, and XRD testing was performed to detect the presence of foreign wear particles due to the fabrication procedures. Finally a friction testing was conducted replicating ceramic-on-ceramic hip joints. The outcomes of the study indicate the successful fabrication of three micro-dimple patterns averaging in diameter of 403.59 µm (SD 6.6325 µm) and averaging in depth of 20.97 µm (SD 2.1368 µm). There was a change in hardness, toughness and residual strength found in subsequent mechanical testing; nevertheless, no foreign wear debris was detected. Therefore, we conclude that the precision fabrication of a micro-dimple array of Al2O3 surfaces by using micro-tooling can be successfully executed without any significant degradation of mechanical properties. The tribology test showed a significant reduction in friction coefficient with these micro-dimple arrays.

Experimental Study on Fire Resistance of Building Seismic Rubber Bearings

Eight building seismic rubber bearings (four uninsulated lead rubber bearings, one insulated lead rubber bearing, two uninsulated natural rubber bearings, and one insulated natural rubber bearing) were experimentally investigated in fire tests following the ISO834 standard heating process. The effects of vertical load and the duration of fire on the failure mode and residual mechanical properties of the rubber bearings were analyzed. The experimental results show that (1) the fire resistance of rubber bearings without insulation fails to exceed 1.5 h; (2) the duration of fire exposure has significant influence on the mechanical-properties degradation of rubber bearings; (3) the effect of vertical load, to which rubber bearings were subjected during the fire, on the residual mechanical properties and fire resistance is inevident; and (4) the proposed insulation is successful in preventing rubber bearings from significant degradation in mechanical properties in fires.

Evaluation of neutron irradiated silicon carbide and silicon carbide composites

The effects of fast neutron irradiation on SiC and SiC composites have been studied. The materials used were chemical vapor deposition (CVD) SiC and SiC/SiC composites reinforced with either Hi-Nicalon™ Type-S, Hi-Nicalon™ or Sylramic™ fibers fabricated by chemical vapor infiltration. A statistically significant population of flexural samples were irradiated up to 4.6 × 10 25 n/m 2 ( E > 0.1 MeV) at 300, 500, and 800 °C in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Dimensions and weights of the flexural bars were measured before and after the neutron irradiation. Mechanical properties were evaluated by four point flexural testing. Volume increase was seen for all bend bars following neutron irradiation. The magnitude of swelling depended on irradiation temperature and material, while it was nearly independent of irradiation fluence over the fluence range studied. Flexural strength of CVD SiC increased following irradiation depending on irradiation temperature. Over the temperature range studied, no significant degradation in mechanical properties was seen for composites fabricated with Hi-Nicalon™ Type-S, while composites reinforced with Hi-Nicalon™ or Sylramic fibers showed significant degradation. The effects of irradiation on the Weibull failure statistics are also presented suggesting a reduction in the Weibull modulus upon irradiation. The cause of this potential reduction is not known.

Processing and Properties of ZrO2‐Containing Reaction‐Bonded Aluminum Oxide with High Initial Aluminum Contents

The reaction‐bonded aluminum oxide (RBAO) process relies upon the oxidation of Al/Al2O3 powder compacts, and many of its associated advantages stem from the presence of the aluminum in the green powder. Higher aluminum contents in the starting powders allow for higher green strengths, densities, and lower overall shrinkage, all while producing a fine‐grained, high‐strength sintered material. However, it is evident that the reaction and sintering of ZrO2‐containing RBAO with higher aluminum contents are more challenging. Therefore, in this study, the effects of aluminum content on the processing, structure, and properties of RBAO ceramics were comprehensively characterized. It was found that RBAO samples with high aluminum contents were more prone to cracking during reaction and even when successfully fired were not able to be sintered to full density. Despite these characteristics, RBAO samples with increasing aluminum contents showed no significant degradation in mechanical properties.

Cracks and precipitate phases in 321 stainless steel weld of flue gas pipe

The present work studies the AISI304 weld cracking mechanism of AISI321 austenite stainless steel of flue gas pipe, which served up to 60,000 h at 650–700 °C. The performance and microstructure of failed material were characterized by impact test, optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD). The results show that cracks ware caused by embrittlement of weld metal and thermal stress. Needle like σ-phase precipitation during service is primarily responsible for significant degradation in mechanical properties and microstructures of weld metal. Needle type σ-phase is a severe stress-raiser and plays a role as crack initiator at the interface with matrix. In addition, maximum equivalent stress is about 175 MPa in the elbow which is near to the yield stress of the steel. It is highly possible that a crack will be initiated under above stress condition and will propagate faster along embrittled σ-phase. Several approaches should be employed to control σ-phase precipitation in weld. These approaches include: decreasing content of ferrite and M 23C 6 carbide in weld and selecting Nb added weld wire during welding.

Application of the boundary shape perturbation method to stress analysis of laminated composites with ply waviness

Ply waviness is a manufacturing defect that occurs in thick fibre-reinforced composite laminates during the curing process. It is known to cause significant degradation of mechanical properties and fatigue life, particularly under compressive loading. To predict failure in the vicinity of the wavy layer, accurate information about stresses at the undulating interfaces is needed. In this paper, an analytical approach to determination of stresses in laminated composites with ply waviness subjected to compressive or flexural loading is presented. Using the boundary shape perturbation method, 3D elasticity problem for a laminate with undulating interfaces is transformed into a sequence of 3D elasticity problems for a reference laminate with perfectly flat layers. The application of the presented analytical approach to a particular problem is illustrated with the example of a transversally loaded laminated plate with a wavy layer.

Application of the boundary shape perturbation method to stress analysis of laminated composites with ply waviness

Ply waviness is a manufacturing defect that occurs in thick fibre-reinforced composite laminates during the curing process. It is known to cause significant degradation of mechanical properties and fatigue life, particularly under compressive loading. To predict failure in the vicinity of the wavy layer, accurate information about stresses at the undulating interfaces is needed. In this paper, an analytical approach to determination of stresses in laminated composites with ply waviness subjected to compressive or flexural loading is presented. Using the boundary shape perturbation method, 3D elasticity problem for a laminate with undulating interfaces is transformed into a sequence of 3D elasticity problems for a reference laminate with perfectly flat layers. The application of the presented analytical approach to a particular problem is illustrated with the example of a transversally loaded laminated plate with a wavy layer.

Analysis of failed reformer tubes

Reformer tubes from a fertilizer plant made of modified HK 40 steel which failed after 4 years service were investigated for failure mechanism and life evaluation. The investigation included hot tensile tests, hardness measurement, dimensional measurement, microscopy and a few accelerated creep tests. Analysis revealed that longitudinal cracks found in the tubes were caused by overheating because of inadequate feed flow caused by the choking of damaged catalyst. To avoid such choking, precautions should be taken while charging the catalyst that no broken piece of catalyst or any external material goes along. Nitrogen which is used for cooling down the tubes and which was retained inside during idle periods should be dry enough. Overheating during service is primarily responsible for significant degradation in mechanical properties and microstructures in the bottom portion of the reformer tubes.