基于Microindentation的PEM燃料电池垫片材料机械性能评估

质子交换膜(PEM)燃料电池需要弹性体垫片来密封阳极和阴极的反应气体(氧气和氢气),垫片材料机械性能的稳定性对燃料电池操作及其性能至关重要。采用Microindentation方法研究了PEM燃料电池弹性体垫片材料(硅橡胶弹性体材料)的机械性能。实验研究了试样的加载和卸载行为,基于Hertz接触理论研究了弹性体垫片材料的变形行为,获得了材料的弹性模量、硬度和迟滞损失能。同时,采用有限元方法分析了试样在加载情况下的应力分布。结果表明,本研究所用的Microindentation技术及其分析方法为PEM燃料电池垫片材料机械性能评估提供了一种简单有效的方法。

Characterization of meso-scale mechanical properties of Longmaxi shale using grid microindentation experiments

Mechanical properties,such as the hardness H,Young’s modulus E,creep modulus C,and fracture toughness Kc,are essential parameters in the design of hydraulic fracturing systems for prospective shale gas formations.In this study,a practical methodology is presented for obtaining these properties through microindentation experiments combined with quantitative observations of the mineralogical phases using X-ray diffraction(XRD),scanning electron microscopy(SEM)with backscattered electron(BSE)imaging,and energy-dispersive X-ray spectroscopy(EDS)analyses.We apply this method in the case of three types of Longmaxi shales with different mineralogies(i.e.carbonate-,clay-,and quartz-rich,respectively),which allows us to determine the characteristic indentation depth,hc?8e10 mm,beyond which the mechanical response of the carbonate-rich shale is homogeneous and independent of its complex heterogeneous microstructure.Moreover,exploiting the results of a large number of indentation tests,we demonstrate that the indentation modulus M of the shale increases as a power-law of hardness H,and its creep modulus C increases linearly with H.We also compute the fracture toughness Kc from the indentation data by assuming a perfectly plastic behavior of the sample.Our results are in good agreement with independent measurements of Kc determined by microscratch tests.Finally,further tests on quartz-and clay-rich samples of the Longmaxi shale suggest further variations in the samples’mechanical properties depending on their burial conditions and the mechanical properties of their dominant mineral phases.

Micro-plasticity Constitutive Model Taking Account of Size Effects for Pure Aluminum by Microindentation

The macro-plasticity power function constitutive model （MPFCM）, the modified macro- plasticity power function constitutive model （MMPFCM） and the micro-plasticity constitutive model （MCM） taking the material intrinsic length were established to characterize the microindentation size effects of pure aluminum, respectively. The experimental results indicated MPFCM only determined precisely in the great indentation load. While a modified one named MMPFCM was subsequently established taking account of the parameters variation with the increase of indentation depth. The conventional dimensional analysis method was employed to determine the strength coefficient K and the strain hardening exponent n of this modified model. And then MCM taking account of size effects was proposed based on the Taylor dislocation model. The first- order steepest gradient descent method was adopted to obtain the material intrinsic length for the geometrically necessary dislocations. The parameters of MCM were identified by using the UMAT subroutine of ABAQUS software. The average absolute relative error of MCM is relatively lower than that of the macro-one. Although the precision of the modified one is also high, the applied scope is limited, only for the microindentation material. In addition, the intrinsic length 5.09 bun of pure aluminum is also obtained based on the strain gradient theory.

Superficial tribological transformation of a ferritic stainless steel by dynamic microindentation

Currently, the stainless steel starts to widen its fields of application in an extraordinary way in medicine and surgery, in the domestic utensils (flatware) and in heavy industry (petrochemistry, nuclear industry and transport). This work consists of making an experimental study on a type of ferritic stainless steel having undergone the test of the microindentation at controlled load and knowing the Superficial Tribological Transformations (STT) caused by this test. It is supposed that it is a simulation with the damages caused on the stainless steels which are in the environment (the effect of hail) or in industry (shot-blasting of the turbines). The analysis of the repeated shocks is based on the mechanical characterization (microhardness, microstructures of the impacts) and geometrical to see the evolution of the diameter and depth of the impacts according to the number of shocks (cycles of impact), in order to know the plastic deformation.

Ceramic Nanocomposites Reinforced with a High Volume Fraction of Carbon Nanotubes

Multi-walled carbon nanotubes（MWNTs） were incorporated into precursor-derived ceramics made from a polysilazane.A ceramic nanocomposite reinforced with about 35 vol%of carbon nanotubes（CNTs）was fabricated by infiltrating CNT-preform with liquid-phased polymeric precursor followed by pyrolysis.The nanocomposite has a dense structure without micro-cracks.The results reveal that the nanocomposite has lower indentation hardness but higher fracture energy than non-reinforced ceramic from the microindentation tests results.The effect of the CNTs on the mechanical properties of the nanocomposite should be discussed accordingly.

Multimodal probe for optical coherence tomography epidetection and micron-scale indentation

We present a multimodal ferrule-top sensor designed to perform the integrated epidetection of Optical Coberence Tomognphy(OCT)depth-profiles and micron-scale indentation by all-optical detection.By scarning a sample under the probe,we can obtain structural crosse soction images and identify a region of interest in a nonhomogencous sample.Then,with the same probe and setup,we can immediately target that area with a series of spherical indentation measurements,in which the applied load is known with aμN precision,the indentation depth with sub-/m precision and a maximum contact radius of 100 pm.Thanks to the visualization of the internal structure of the sample,we can gain a better insi ght into the observed mechanical behavior.The ability to impart a small,confined load,and perfomn OCT A scans at the same time,could lead to an altemative,high transverse resolution,Optical Coherence Elastography(OCE)sensor.

Effect of Precipitation on the Hardness of Ternary Metallic Glass

The present study reports effects of annealing treatment on the hardness of Ce75Al23Si2 rare earth-based metallic glasses (REMG). Then specimens were annealed at 100°C, 200°C, 250°C, 270°C, and 290°C five different temperatures for 30 minutes. After that, three different characteristic methods, including microindentation, X-ray Diffraction (XRD), and scanning electrical microscope (SEM) were conducted on the as received REMG sample and five annealed samples. XRD data demonstrate that the crystallization occurs in the sample at the annealing temperature as low as 200°C. The microindentation measurement shows that hardness of the REMG sample does not change at all before crystallization occurs in the sample and increases with the annealing temperature in the range of 200°C to 290°C. The average crystal size in the annealed samples was estimated using Debye-Scherrer equation to be 28-42 nm, in consistent with the SEM observation, indicating that nano-crystalline domains may give rise to the enhancement of hardness.

Defects Interaction on the Mechanical Properties during Transition Formation of (Mo, Cr)<SUB>3</SUB>Si Intermetallic Alloys

Molybdenum silicides alloys with different Mo and Cr additions were produced by the arc cast method. The microstructure revealed mostly single phase structure. Mechanical properties were evaluated in the alloys, showing a decreasing behavior on microhardness. Fracture toughness values were obtained from cracks produced by Vickers indentation technique, showing that ternary alloying did not have a significant effect. Vacancy studies demonstrated that thermal vacancies along the transition line slightly affected the mechanical behavior.

Biodegradable magnesium pins enhanced the healing of transverse patellar fracture in rabbits

Displaced fractures of patella often require open reduction surgery and internal fixation to restore the extensor continuity and articular congruity.Fracture fixation with biodegradable magnesium(Mg)pins enhanced fracture healing.We hypothesized that fixation with Mg pins and their degradation over time would enhance healing of patellar fracture radiologically,mechanically,and histologically.Transverse patellar fracture surgery was performed on thirty-two 18-weeks old female New Zealand White Rabbits.The fracture was fixed with a pin made of stainless steel or pure Mg,and a figure-of-eight stainless steel band wire.Samples were harvested at week 8 or 12,and assessed with microCT,tensile testing,microindentation,and histology.Microarchitectural analysis showed that Mg group showed 12%higher in the ratio of bone volume to tissue volume at week 8,and 38.4%higher of bone volume at week 12.Tensile testing showed that the failure load and stiffness of Mg group were 66.9%and 104%higher than the control group at week 8,respectively.At week 12,Mg group was 60.8%higher in ultimate strength than the control group.Microindentation showed that,compared to the Control group,Mg group showed 49.9%higher Vickers hardness and 31%higher elastic modulus at week 8 and 12,respectively.At week 12,the new bone of Mg group remodelled to laminar bone,but those of the control group remained woven bone-like.Fixation of transverse patellar fracture with Mg pins and its degradation enhanced new bone formation and mechanical properties of the repaired patella compared to the Control group.

Shear instability in heterogeneous nanolayered Cu/Zr composites

Ultrastrong nanolayered metallic composites are usually subjected to low ductility due to plastic instability during deformation.Here we investigated the shear instability of a newly designed heterogeneous nanolayered Cu/Zr composites by microindentation.The heterogeneity in size was generated by inserting a few thin Cu-Zr bilayers with an individual layer thickness of 2.5-10 nm into the interface region of the Cu/Zr layered composites with an individual layer thickness of 100 nm.The microindentation tests showed that multiple shear bands appeared in the heterogeneous composite with one bilayer,whereas only a single shear band was formed in that with two or three bilayers.Most importantly,the layer strain in the multi-shear band region is much smaller than that in the single-shear band area.For example,the strain of the 100 nm layers within the shear band in the composite with one 10 nm bilayer could reach as low as 2.8,which was less than half of that in the composite with three 10 nm bilayers,i.e.,6.1.These fndings demonstrated that strain delocalization can be achieved through shear band multiplication if an appropriate number of thin bilayers were used as interlayers in the 100 nm Cu/Zr composites.Besides,compared with the homogeneous composite with an individual layer thickness of 100 nm and the bimodal composite which is composed of alternating one 100 nm Cu-Zr bilayer and two 10 nm CuZr bilayers,the heterogeneous composite with one bilayer displayed a higher strength(2.15 GPa)and a favorable resistance to strain localization.