航天用刚挠印制板可靠性研究

通过分析影响航天用刚挠印制板可靠性的主要因素分层和金属化孔的质量,从刚挠印制板的设计、工艺控制等角度提出适合于航天用刚挠印制板的材料、关键工序工艺条件以及特殊检验要求。通过振动试验、加速度试验、加严热冲击试验、焊接后高低温循环试验等试验证明了高可靠性刚挠印制板可以应用于航天器产品,并给出航天用刚挠印制板具体的材料、生产工艺及关键工艺参数建议。     

Friction and wear behaviors of B4C/6061Al composite

The friction and wear behaviors of B₄C/6061Al composite were studied by considering the effect of sliding time, applied load, sliding velocity and heat treatment. The results show that, when the sliding time, applied load and sliding velocity reach critical values (namely 120 min, 30 N and 240 r min⁻¹, respectively), the mass loss and friction coefficient (COF) increase significantly. Severe delamination wear is the main wear mechanism after sliding for 120 min and under an applied load of 30 N. While fretting wear happens at a sliding velocity of 240 r min⁻¹. After solution-treated at 550 °C for 1 h and then aged at 180 °C for 15 h, the composite shows the highest wear resistance owing to the precipitation of β″ (Mg₂Si) phases in the matrix and the strong interface bonding between B₄C particles and the matrix alloy.     

On anisotropy of ferritic ODS alloys

Abstract

Ferritic oxide dispersion strengthened (ODS) alloys are candidate materials to be used as cladding for long term fast reactors, due to their high strength at high temperature and good swelling and irradiation resistances. The fabrication of cladding tubes is usually made by a succession of cold deformation steps where a deformation induced anisotropic microstructure could take place, which would affect the mechanical behaviour of the tube. The characterisation of this microstructural anisotropy is one of the key issues in the development of cladding ODS tubes. In this paper, the microstructural anisotropy of a Fe–14Cr–ODS extruded bar and a Fe–12Cr–ODS plate is characterised and its effect on the mechanical properties is analysed by tensile, impact and small punch testing. In both materials, a reduction of the ductility is observed in the transverse specimens. In addition, the fracture behaviour seems to be strongly dependent on the location of the crack plane regarding the elongated grained microstructure.     

Bridging effect on mode I fatigue delamination behavior in composite laminates

This paper discusses the bridging effect of fibres on mode I fatigue delamination growth in unidirectional and multidirectional polymer composite laminates based on a series of double cantilever beam (DCB) tests. From the results, there is sufficient evidence that fibre bridging can decrease the crack growth rate da/dN significantly, and using only one fatigue resistance curve to determine the delamination behavior in composite materials with large-scale fibre bridging may be inadequate. The bridging created in fatigue delamination is different from that of quasi-static delamination at the same crack length. So it is incorrect to use the resistance curve (R-curve) from quasi-static delamination tests to normalize fatigue delamination results.     

Embedded flaws for crack path control in composite laminates

An experimental investigation was conducted on using small flaws purposefully introduced into composite laminates to control growth of interlaminar cracks and through-thickness crack branching. Mode I crack growth specimens were used to study branching through 0°, 90° and 45° plies. The results showed that crack growth through 0° plies could be promoted by a ply gap, but this was not as controllable as combining a ply gap with a pre-crack to create a “crack branch flaw”. Crack branching through 45° plies could be controlled using crack branch flaws, and also promoted controllably using ply gaps. Crack branching through 90° plies was seen without any flaws, but was better controlled with embedded delaminations. Using these outcomes, crack branching through two quasi-isotropic laminates was demonstrated. The results have application to improved damage tolerance and fracture toughness, by taking advantage of high toughness crack growth mechanisms.     

Anchorage resistance of CFRP strips externally bonded to various cementitious substrates

This paper presents a study on the anchorage capacity of Carbon Fiber Reinforced Polymer (CFRP) strips bonded to a cementitious substrate used for concrete surface reprofiling. The structural strengthening of a large-scale prestressed concrete girder in the framework of a bridge retrofitting project by means of prestressed CFRP strips required the levelling of an initial negative camber of about 2–4 cm. Both midspan and girder end situations were investigated with lap-shear and prestress force-releasing tests. Four different solutions regarding the levelling material, i.e. three mortars applied by hand as well as dry shotcrete, were tested. The results in terms of strain, slip and total anchorage resistance are presented and compared. In the end, dry shotcrete is recommended for the girder application. In addition to a very convincing bond behavior, the application is, despite the necessity of involving a specialized company from the field, clearly less time-consuming and easier. The retained solution represents an interesting approach for future applications in bridge retrofitting when an even surface is necessary for bonding CFRP strips.     

On the crack and delamination risk optimization of a Si-interposer for LED packaging

3D-integration becomes more and more an important issue for advanced LED packaging solutions as it is a great challenge for the thermo-mechanical reliability to remove heat from LEDs to the environment by heat spreading or specialized cooling technologies. Thermal copper-TSVs provide an elegant solution to effectively transfer heat from LED to the heat spreading structures on the backside of a substrate. But, the use of copper-TSVs generates also novel challenges for reliability as well as also for reliability analysis and prediction, i.e. to manage multiple failure modes acting combined – interface delamination, cracking and fatigue, in particular. In this case, the thermal expansion mismatch between copper and silicon yields to risky stress situations.To overcome cracking and delamination risks in the vicinity of thermal copper-TSVs the authors performed extensive simulative work by means of fracture mechanics approaches – an interaction integral approach within a simulative DoE and the X-FEM methodology to help clarifying crack propagation paths in silicon. The results provided a good insight into the role of model parameters for further optimizations of the intended thermal TSV-approaches in LED packaging applications.     

Improving the interlaminar properties of polymer composites using a situ accumulation method to construct the multi-scale reinforcement of carbon nanofibers/carbon fibers

A novel method was developed to realize the situ accumulation of carbon nanofibers (CNFs) in the carbon fiber reinforced polymer composites (CFRPs) to construct the multi-scale reinforcement for improving the interlaminar properties. In this method, the prepreg was sealed by the nanomicroporous nylon membrane, and the excess resin was extracted from the prepreg by the vacuum-assisted method. It was found that the use of nylon membrane resulted in effective CNFs accumulation, especially in the interlayer by scanning electron microscopy. Short-beam strength tests and the end-notched flexure tests were conducted respectively to evaluate the interlaminar properties of CFRPs under shear loading. The results indicated that the interlaminar shear strength (ILSS) and the Mode II interlaminar fracture toughness (G_IIC) of CFRPs made by the filtering membrane-assisted method remarkably increased compared with those prepared without using filtering membrane.