Evaluation of single-fibre strength distribution from fibre bundle strength

A new modified Weibull distribution function has been suggested for analysing the strength of fibres used as reinforcements for advanced composites. The function provides an upper and a lower strength limit and is characterized by two shape and two location parameters. A method for determining the parameters of this distribution from the analysis of the tensile curves of fibre bundles has also been developed. Application of the method to the experimental results on Thornel-300 carbon fibres shows that the shape parameters become modified in the case of bundles.     

The conflicts between strength and toughness

The attainment of both strength and toughness is a vital requirement for most structural materials; unfortunately these properties are generally mutually exclusive. Although the quest continues for stronger and harder materials, these have little to no use as bulk structural materials without appropriate fracture resistance. It is the lower-strength, and hence higher-toughness, materials that find use for most safety-critical applications where premature or, worse still, catastrophic fracture is unacceptable. For these reasons, the development of strong and tough (damage-tolerant) materials has traditionally been an exercise in compromise between hardness versus ductility. Drawing examples from metallic glasses, natural and biological materials, and structural and biomimetic ceramics, we examine some of the newer strategies in dealing with this conflict. Specifically, we focus on the interplay between the mechanisms that individually contribute to strength and toughness, noting that these phenomena can originate from very different lengthscales in a material's structural architecture. We show how these new and natural materials can defeat the conflict of strength versus toughness and achieve unprecedented levels of damage tolerance within their respective material classes.     

Residual strength and toughness of damaged composites

A study has been made of the effect of prior damage on the tensile strength and toughness of a number of carbon and glass fibre reinforced plastics materials. The methods of introducing this prior damage were by tension fatigue cycling, by transverse compression, by repeated impact, and by stress corrosion. The effects of the damage were assessed by measuring the un-notched and notched tensile strengths and, in some instances, the work of fracture of notched samples. Acoustic emission monitoring and microstructural studies were carried out in support of the mechanical tests. It appears that under various circumstances, the effects of microstructural damage may result in independent changes in the notched and un-notched strengths of a composite, although the pattern of changes is not simple. The toughness to strength ratio,K _Q/_f, may rise or fall, depending on the material and the damaging conditions. However, the results for all of the tests presented here still fall within the 90% confidence limits for theK _Q/_f relationship previously identified.     

Flexural strength and fracture toughness of sea ice

A series of mid-winter experiments were carried out on the ice in the rubble field around Tarsiut Island in the Beaufort Sea. The tests included grain structure determinations, salinity and density of the ice, small beam flexural strength and fracture toughness. Typical values for flexural strength and fracture toughness were 0.6–1.0 MPa and 100–140 kPa m^{$\text{1}\text{2}$} respectively. Both properties were dependent on brine volume and depth in the ice sheet. In comparing these results with identical tests on finegrained freshwater ice it was found that for comparable loading conditions, the strength of the sea ice was significantly lower than the strength of the freshwater ice, whereas the fracture toughness of the sea ice was higher than the fracture toughness of the freshwater ice.     

Fracture strength and toughness of some phenolic concretes

The strength and toughness of a range of phenolic resin based concretes have been evaluated as a function of matrix resin type, content and catalyst ratios. Two resin systems were examined using resin: filler ratios of 4:1, 5: 1, 6:1, 7: 1 and 8: 1 by weight. Catalyst ratios were varied between 4 and 10% by weight of total resin. Through grading mixes were designed using four single sized silica sands of nominal particle sizes 2.4 and 1.2 mm and also 600 and 300 m in combination with a microfiller with a maximum particle size of 150 m. Gap gradings were also produced using two of the sand components (1.2 mm and 300 m) and the microfiller. Microfillers employed included silica flour, china clay and spheriglass 5000. Strength ranged from 4.7 to 7.9 MPa measured in axial tension and 21.3 to 31.4 MPa measured in flexure using four point bending. Fracture toughness, evaluated as the critical strain energy release rate, G,,, was determined using precracked double torsion specimens. Typical values ranged from 0.13 to 0.22 kJ m^–2.[/p]     

Scaling of brittle failure: strength versus toughness

We study the scaling of strength and toughness in function of temperature, loading rate and system size, to investigate the difference between tensile failure and fracture failure. Molecular simulation is used to estimate the failure of intact and cracked bodies while varying temperature, strain rate and system size over many orders of magnitude, making it possible to identify scaling laws. Two materials are considered: an idealized toy model, for which a scaling law can be derived analytically, and a realistic molecular model of graphene. The results show that strength and toughness follow very similar scalings with temperature and loading rate, but differ markedly regarding the scaling with system size. Strength scales with the number of atoms whereas toughness scales with the number of cracks. It means that intermediate situations of moderate stress concentrations (e.g., notch) can exhibit not obvious size scaling, in-between those of strength and toughness. Following a theoretical analysis of failure as a thermally activated process, we could rationalize the observed scaling and formulate a general rate–temperature–size equivalence. The scaling law of the toy model can be derived rigorously but is not representative of real materials because of a force discontinuity in the potential. A more representative scaling law, valid for graphene, is proposed with a different exponent.     

Microstructural features affecting fracture toughness of high strength steels

The fracture toughness of quenched and tempered steels, such as AISI 4340, AISI 4130 and 300M, can be increased by 50–100% by minor changes in heat treating procedures. Certain microstructural features, particularly blocky ferrite, upper bahnte and twinned martensite plates, are deleterious to fracture toughness. Similarly, the presence of undissolved carbides and sulfide inclusions, which act as crack nuclei, can lower fracture toughness by 25–50%. Other microstructural constituents, such as lower bainte, autotempered martensite, and retained austenite can enhance fracture toughness. By controlling the amounts and distributions of the microstructural constituents, the fracture toughness values of AISI 4340, AISI 4130 and 300M can be raised to the fracture toughness level of 18Ni maraging steel at equivalent values of yield strength.     

Dynamic fracture toughness of a high strength armor steel

This paper summarizes the results of a research being carried out to determine fracture behavior both in static and dynamic conditions of high strength armor steel Armox500T. In this research, notched specimens were cut to be tested in three-point bending test. Specimens were pre-cracked by flexural fatigue. Thereafter, some specimens were tested in bending up to rupture to determine the static fracture toughness K_IC. To obtain fracture toughness in dynamic conditions, a split Hopkinson bar modified to perform three-point bending tests was used. In this device, displacements and velocities of the specimen were measured, as well as the rupture time by means of fracture detection sensors, glued to the specimens. After that, a numerical simulation of the test was performed by using LS DYNA hydrocode, obtaining stresses and strain histories around the crack tip. From these results, the stress intensity factor history was derived. By using the rupture time, measured by the sensors, the value of the fracture toughness computed was unrealistic. Therefore, the use of a numerical procedure to obtain the rupture time was decided, by comparing experimental results of velocities at the transmission bar with numerical results obtained with several rupture times. With this procedure, the computation of dynamic fracture toughness was possible. The method shows that the measurement of the dynamic fracture toughness is possible without the needs of using crack sensors or strain gauges. It can be observed that fracture toughness of this steel under static and dynamic conditions is quite similar.     

Probabilistic description of experimental statistical strength characteristics

Theoretical strength probabilistic characteristics are compared for uniaxial and biaxial stretching of a stochastically defective material with certain experimental data for some metals. The two can be matched if one selects the structural characteristics of the material, but reliable determination should be based on more representative experiments.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 28, No. 1, pp. 95–98, January–February, 1992.     

谷歌领航的设计风潮

作为TMT公司总部设计风潮的领航者自1998年谷歌在美国加州门罗帕克一个车库内创建,后来搬入硅谷山景圆形剧场（AmPhitheatre）大街,及至今日，谷歌已在40个国家和地区拥有70个办事处,谷歌一直力求每个办事处的空间设计各不相同．其根据所在地风俗定制的主题显示了谷歌在办公空间设计上花费的心思。     

Evaluation of adhesion strength and toughness of fluoridated hydroxyapatite coatings

Adhesion strength and fracture toughness are two crucial mechanical properties for bioceramic coatings on metal implants directly affecting successful implantation and long-term stability. In this study, the adhesion strength of sol-gel derived FHA coatings on Ti6Al4V substrates was measured by pull-out tensile test, and the toughness was assessed by energy release method. With increase of the degree of fluoridation, the adhesion strength increases up to about 40% and the fracture toughness increases about 200 to 300%. Contrary to the wide-spread belief, it is interesting to note that after soaking in the Tris-buffered physiological saline solution (for 21 days), the adhesion strength increases about 60% as compared with the as-deposited coating, instead of decreasing. The mechanism of the increase is discussed.     

Intermittent bonding for high toughness/ high strength composites

High strength and high toughness are usually mutually exclusive in brittle filament/brittle matrix composites. The high tensile strength characteristic of strong interfacial filament/matrix bonding can, however, be combined with the high fracture toughness of weak interfacial bonding, when the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibres. The strong regions ensure that the filament strength is picked up; weak areas randomly in the path of running cracks serve to blunt them by the Cook/Gordon mechanism which, in turn, produces long pull-out lengths with an associated large contribution to toughness. Boron-epoxy composites of volume fraction 0.20 to 0.25 have been made in this way which have fracture toughnesses of over 200 kJ m^?2, whilst retaining rule of mixtures tensile strengths (～ 650 MN m^?2). At the volume fractions used, this apparently representsK _IC values greater than 100 MN m^?3/2. An analysis is presented for toughness and strength which demonstrates, in broad terms, the effects of varying the coating parameters of concern. Results show that the “toughness” of interfaces is an important parameter, differences in which may not be shown up in terms of interfacial “strength”. The choice of coating material is crucial in getting the desired effect. Some observations are made upon methods of measuring the components of toughness in composites.     

Compliant threads maximize spider silk connection strength and toughness

Millions of years of evolution have adapted spider webs to achieve a range of functionalities, including the well-known capture of prey, with efficient use of material. One feature that has escaped extensive investigation is the silk-on-silk connection joints within spider webs, particularly from a structural mechanics perspective. We report a joint theoretical and computational analysis of an idealized silk-on-silk fibre junction. By modifying the theory of multiple peeling, we quantitatively compare the performance of the system while systematically increasing the rigidity of the anchor thread, by both scaling the stress–strain response and the introduction of an applied pre-strain. The results of our study indicate that compliance is a virtue—the more extensible the anchorage, the tougher and stronger the connection becomes. In consideration of the theoretical model, in comparison with rigid substrates, a compliant anchorage enormously increases the effective adhesion strength (work required to detach), independent of the adhered thread itself, attributed to a nonlinear alignment between thread and anchor (contact peeling angle). The results can direct novel engineering design principles to achieve possible load transfer from compliant fibre-to-fibre anchorages, be they silk-on-silk or another, as-yet undeveloped, system.     

基于断裂韧性预报复合材料层合板的开孔拉伸强度

开孔层合板的强度预报往往取决于孔边的临界长度,它不仅与材料性能,而且与铺层、孔径都有关。本文基于线弹性断裂力学,提出了一种预报对称铺层层合板开孔拉伸强度的新方法,只需提供正交层合板的断裂韧性和无缺口层合板的拉伸强度,显著降低对实验数据的依赖性。首先,将临界长度表作为层合板断裂韧性和无缺口拉伸强度的函数,再通过正交层合板[90/0]_8s的紧凑拉伸试验和虚拟裂纹闭合技术,确定出0°层断裂韧性,进而计算得到任意对称铺层层合板的断裂韧性。本文测试了T300/7901层合板[0/±45/90]_2s和[0/±30/±60/90]_s的开孔拉伸强度,孔径分别为3 mm、6 mm和9 mm。理论预报结果与试验值吻合较好,最大误差为15.2%,满足工程应用需求。      

Hybrid fiber influence on strength and toughness of RC beams

In this paper, a newly developed method of evaluating the shear toughness of fiber reinforced RC beams in terms of its post-peak nominal average equivalent shear strength was investigated. This method is simple and the advantages are significant.