bands是什么意思

bands的意思乐队。

例句：

1. Bands played German marches.

管乐队演奏了德国的进行曲。

2. Featuring new bands gives the show an edge.

以新乐队为主打是这次演出的一个亮点。

3. Several bands have namechecked Lee Hazelwood in interviews.

好几个乐队在采访时都提到了李•海佐伍德的名字。

4. The military bands began to play stirringly.

军乐队开始了鼓舞人心的演奏。

5. Local bands provide music for dancing.

当地的流行乐队为跳舞伴奏。

短语：

1、shadow bands 影带

2、set of bands 谱带组

3、Geneva bands 法衣领前下垂的两条白

4、parallel bands 平行带

5、deformation bands 变形带

6、transmission bands [无] 通频带

7、amateur bands [电] 业余波段

8、vocal bands 声带

9、warp bands 分条痕

10、bands of 一帮，成群

这是一片写的不错的

Effect of fiber architecture on flexural characteristics and fracture of fiber-reinforc

Vistasp M. Karbharia, Corresponding Author Contact Information, E-mail The Corresponding Author and Howard Strasslerb

aMaterials Science &Engineering Program, and Department of Structural Engineering, MC-0085, University of California San Diego, Room 105, Building 409, University Center, La Jolla, CA 92093-0085, USA.

bDepartment of Restorative Dentistry, Dental School, University of Maryland, Baltimore, MD, USA

Received 10 December 2005 revised 25 June 2006 accepted 31 August 2006. Available online 7 November 2006.

Abstract

Objective

The aim of this study was to compare and elucidate the differences in damage mechanisms and response of fiber-reinforced dental resin composites based on three different brandsnext term under flexural loading. The types of reinforcement consisted of a unidirectional E-glass prepreg (Splint-It from Jeneric/Petron Inc.), an ultrahigh molecular weight polyethylene fiber based biaxial braid (Connect, Kerr) and an ultrahigh molecular weight polyethylene fiber based leno-weave (Ribbond).

Methods

Three different commercially available fiber reinforcing systems were used to fabricate rectangular bars, with the fiber reinforcement close to the tensile face, which were tested in flexure with an emphasis on studying damage mechanisms and response. Eight specimens (n = 8) of each type were tested. Overall energy capacity as well as flexural strength and modulus were determined and results compared in light of the different abilities of the architectures used.

Results

Under flexural loading unreinforced and unidirectional prepreg reinforced dental composites failed in a brittle previous termfashion,next term whereas the braid and leno-weave reinforced materials underwent significant deformation without rupture. The braid reinforced specimens showed the highest peak load. The addition of the unidirectional to the matrix resulted in an average strain of 0.06 mm/mm which is 50% greater than the capacity of the unreinforced matrix, whereas the addition of the braid and leno-weave resulted in increases of 119 and 126%, respectively, emphasizing the higher capacity of both the UHM polyethylene fibers and the architectures to hold together without rupture under flexural loading. The addition of the fiber reinforcement substantially increases the level of strain energy in the specimens with the maximum being attained in the braid reinforced specimens with a 433% increase in energy absorption capability above the unreinforced case. The minimum scatter and highest consistency in response is seen in the leno-weave reinforced specimens due to the details of the architecture which restrict fabric shearing and movement during placement.

Significance

It is crucial that the appropriate selection of fiber architectures be made not just from a perspective of highest strength, but overall damage tolerance and energy absorption. Differences in weaves and architectures can result in substantially different performance and appropriate selection can mitigate premature and catastrophic failure. The study provides details of materials level response characteristics which are useful in selection of the fiber reinforcement based on specifics of application.

Keywords: Fiber reinforcementDental compositeFlexureDamage toleranceArchitectureUnidirectionalBraidLeno-weave

Article Outline

1. Introduction

2. Materials and methods

3. Results

4. Discussion

5. Summary

References

1. Introduction

A range of fillers in particulate form have conventionally been used to improve performance characteristics, such as strength, toughness and wear resistance, Although the addition of fillers and recent changes in composition of resin composites have been noted to provide enhanced wear resistance [1] and [2], conventional filler based systems are still brittle as compared to metals. Sakaguchi et al. [3] reported that these were prone to early fracture with crack propagation rates in excess of those seen in porcelain. This is of concern since clinical observations have demonstrated that under forces generated during mastication the inner faces of restorations can be subject to high tensile stresses which cause premature fracture initiation and failure [4]. In recent years, fiber reinforcements in the form of ribbons have been introduced to address these deficiencies [5]. By etching and bonding to tooth structure with composite resins embedded with woven fibers adapted to the contours of teeth periodontal splints, endodontic posts, anterior and posterior fixed partial dentures, orthodontic retainers and reinforcement of single tooth restorations can be accomplished. While the science of fiber-reinforced polymer composites is well established, the application of these materials in dental applications is still new and aspects related to material characterization, cure kinetics and even placement of reinforcement are still not widely understood.

Due to the nature of filled polymer and ceramic systems that have been used conventionally, most material level tests designed and used extensively, for the characterization of dental materials, emphasize the brittle nature of materials response. In many cases the tests and the interpretation of results, are not suited to the class of fiber-reinforced polymeric composites, wherein aspects, such as fiber orientation, placement of fabric and even scale effects are extremely important. The difference in characteristics and the need to develop a fundamental understanding of response of continuous fiber and fabric, reinforced dental composites has recently been emphasized both through laboratory and clinical studies. Recent studies have addressed critical aspects, such as effects of fabric layer thickness ratios and configurations [6], fiber position and orientation [7] and even test specimen size [8]. However, the selection and use of continuous reinforcement is largely on an ad hoc basis, with diverse claims being made by manufacturers, without a thorough understanding of the materials based performance demands for the material by the specifics of an application (for example, the fabric architecture required for optimized performance of a post are very different from those for a bridge) or details of response characteristics at levels beyond those of mere “strength” and “modulus”. Further, each fabric is known to respond in different manner to manipulation and drape (i.e. conformance) to changes in substrate configuration [9]. The architecture of the fabrics permits movement of fibers or constraint thereof and even shearing of the structure, to different extents. Weave patterns have also been noted to be important in the selection of composite materials for dental applications based on the specifics of application [10]. Thus, clinically, when each of the different fabric configurations is used to reinforce dental composites, there are manipulation changes that occur to some of the fabric materials. For the biaxially braided material, the fiber orientation can change after cutting and embedment in the composite when adapting to tooth contours. The fibers in the ribbon spread out and separate from each other and become more oriented in a direction transverse to the longitudinal axis of the ribbon. When the leno-weave is cut and embedded in dental composites, the fiber yarns maintain their orientation and do not separate from each other when closely adapted to the contours of teeth. However, due to the orthogonal structure gaps can appear within the architecture providing local areas unreinforced with fiber reinforcement. The unidirectional glass fiber material does not closely adapt to the contours of teeth due to the rigidity of the fibers. It is difficult to manipulate the fibrous material which leaves the final composite material thickerfurther manipulation causes glass fiber separation with some visible fractures of the fibers themselves.

The aim of this study is to experimentally assess the flexural response of three commercial fiber/fabric reinforcement systems available for dental use and to compare performance based on different characteristics and to elucidate differences based on details of fabric architecture and fiber type.

2. Materials and methods

Three different fabric-reinforcing products, all in ribbon form, were used in this investigation. The first is a 3 mm wide unidirectional E-glass prepreg structure with no transverse reinforcement (Splint-It, Jeneric/Petron Inc.1) designated as set A, whereas the other two are formed of ultra-high molecular weight polyethylene fibers in the form of a 4 mm wide biaxial braid (Connect, Kerr), designated as set B and a 3 mm wide Leno-weave (Ribbond, WA), designated as set C. The first is a pure unidirectional which intrinsically gives the highest efficiency of reinforcement in the longitudinal direction with resin dominated response in the transverse direction. The second is a biaxial braid without axial fibers, which provides very good conformability and structure through the two sets of yarns forming a symmetrical array with the yarns oriented at a fixed angle from the braid axis. The third architecture has warp yarns crossed pair wise in a figure of eight pattern as filling yarns providing an open weave effect for controlled yarn slippage and good stability.

Multiple specimens of the fabrics were carefully measured and weighed and the average basis weight of the biaxial braid was determined to be 1.03 × 10−4 g/mm2 whereas that for the leno-weave was 1.42 × 10−4 g/mm2. It was noted that the unidirectional had an aerial weight of 2.2 times that of the other two. Rectangular test bars of size 2 mm × 2 mm × 48 mm were constructed from layered placement of a flowable composite resin (Virtuoso FloRestore, Demat) in polysiloxane molds, with glass slides held on top with rubber bands and light cured for 60 s using a Kulzer UniXS laboratory polymerization lamp. In the case of sets B and C the fabric was first wetted and then placed on the first layer of the flowable composite resin such that the fiber reinforcement was placed between 0.25 and 0.5 mm from the bottom surface (which would be used as the tensile surface in flexural testing). The addition of higher modulus material at or near the tensile surface is known from elementary mechanics of materials to increase flexural performance and has been verified for dental composite materials by Ellakwa et al. [11] and [12]. Care was taken to maintain alignment of the fibers and fabric structure and not cause wrinkling or lateral movement which would affect overall performance characteristics. The fabric reinforced specimens had only a single layer of reinforcement near the bottom surface with the rest of the specimen having no fiber reinforcement. This general configuration for flexural specimens has been used previously by Kanie et al. [13]. In the current investigation, fiber weight fraction in the single layer was between 37 and 42% but is significantly lower if determined on the basis of the full thickness of the overall specimen. Unreinforced bars of the resin were also fabricated the same way for comparison and were designated as set D.

Eight specimens (n = 8) from each set were tested in three-point flexure using a span of 16 mm which provides a span to depth (l/d) ratio of 16, which is recommended by ASTM D 790-03 [14]. It is noted that flexural characteristics can be substantially affected by choice of the l/d ratio which intrinsically sets the balance between shear and bending moment, with shear dominating on shorter spans. Load was introduced through a rounded crosshead indenter placed in two positions—parallel to the test specimen span (P1) and perpendicular to the test specimen span (P2). The load head indenter was of 4 mm total length. This was done to assess effects of load introduction since ribbon architecture had fibers at different orientations. Tests were conducted at a displacement rate of 1 mm/min and a minimum of eight tests were conducted for each set. Loading was continued till either the specimen showed catastrophic rupture or the specimen attained a negative slope of load versus displacement with the load drop continuing slowly past peak to below 85% of the peak load. This level was chosen to exceed the 0.05 mm/mm strain limitation of apparent failure recommended by ASTM D790-03 [14] so as to enable an assessment of ductility of the specimens. Specimens were carefully examined for cracking, crazing and other damage.

The flexure strength was determined as

Click to view the MathML source (1)

where P is the applied load (or peak load if rupture did not occur), L the span length between supports and b and d are the width and thickness of the specimens, respectively.

While the tangent modulus of elasticity is often used to determine the modulus of specimens, by drawing a tangent to the steepest initial straight-line portion of the load-deflection curve to measure the slope, m, which is then used as

Click to view the MathML source (2)

in the current case a majority of the specimens show significant changes in slopes very early in the response curve indicating microcracking and non-linearity. Since these occur fairly early the modulus determined from the initial tangent has significant statistical variation. In order to determine a more consistent measure of modulus the secant modulus of elasticity as defined in ASTM D790-03 [14] is used herein, with the secant being drawn between the origin and the point of maximum load to determine the slope m, which is then used in Eq. (2). This also has the advantage of providing a characteristic that incorporates the deformation capability, thereby differentiating between specimens that reach a maximum load at low deformation (such as, the unreinforced composite and the unidirectional reinforced composite) and those that show significant deformation prior to attainment of peak load (such as, the specimens reinforced with the braid and leno-weave).

The matrix material is generically more brittle than the fiber and usually has a lower ultimate strain. Thus, as the specimen bends the matrix is likely to develop a series of cracks with the initiation and propagation of cracks depending not just on the type and positioning of the reinforcement, but also on the strain capacity of the neat resin areas. It is thus of use to compute the strain in the composite under flexural load and this can be determined as

Click to view the MathML source (3)

where D is the midspan displacement.

The toughness of a material can be related to both its ductility and its ultimate strength. This is an important performance characteristic and is often represented in terms of strain energy, U, which represents the work done to cause a deformation. This is essentially the area under the load-deformation curve and can be calculated as

Click to view the MathML source (4)

where P is the applied load and x is the deformation. In the case of the present investigation, two levels of strain energy are calculated to enable an assessment of the two response types. In the first, strain energy is computed to the deformation level corresponding to peak load (which is also the fracture load for sets A and D). In the case of specimens that show significant inelastic deformation (sets B and C) strain energy is also computed till a point corresponding to a deformation of 11.5 mm at which point the load shows a 15% drop from the peak. Post-peak response in flexural has earlier been reported by Alander et al. [8].

3. Results

The application of flexural loading was seen to result in two different macroscopic forms of response. In the case of specimens from sets A and D (reinforced with a unidirectional fabric and unreinforced) failure was catastrophic, in brittle fashion, at peak load, whereas in the case of specimens from sets B and C the attainment of peak load was followed by a very slow decrease in load with increasing displacement, representative of inelastic or plastic, deformation. Typical response curves are shown in Fig. 1 as an example.

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Fig. 1. Typical flexural response.

The variation in flexural strength (plotted here in terms of stress at peak load) with type of specimen and load introduction method is shown in Fig. 2. The highest strength was achieved by specimens with the braided fabric wherein on average a 125% increase over the unreinforced specimens was attained. Statistical analysis with ANOVA and Tukey's post hoc test revealed that method of load introduction did not affect the results and that further there were no significant differences in overall peak strength results between sets A and B (specimens containing the unidirectional and braided fabrics). Significant differences (p <0.003) were noted between sets B and C. It is, however, noted that in sets B and C, failure did not occur at the peak load, with load slowly decreasing with increase in midpoint deflection. A comparison of flexural stresses for these systems at peak load and load corresponding to a deflection of 11.5 mm is shown in Fig. 3. As can be seen the two systems show significant inelastic deformation with drops of only 12.8, 12.1, 11.7 and 9.5% from the peak, emphasizing the stable, ductile and non-catastrophic, post-peak response in these systems.

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Fig. 2. Flexural strength at peak load.

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Fig. 3. Comparison of flexural stresses in specimens having non-catastrophic failure modes.

A comparison of secant modulus (measured to the peak load) for the different sets is shown in Fig. 4. As can be seen, with the exception of the unidirectional system, the apparent moduli were lower than that of the unreinforced specimens. It is also noted that although the Tukey post hoc tests do not show a significant difference due to orientation of load indenter, the level for the unidirectionals is only 0.1022 compared to 1 for the others. Removal of a single outlier from P1 results in p <0.007 indicating a strong effect of orientation of the indenter with the secant modulus being 17.7% lower with the indenter placed parallel to the fibers, which results in splitting between fibers and uneven fracture with less pullout.

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Fig. 4. Comparison of secant moduli under flexural loading.

As was noted previously, both the unreinforced samples (set D) and the unidirectional prepreg reinforced specimens (set A) failed in catastrophic fashion at deformation levels significantly less than those at which the other two sets reached the inelastic peak. Since sets B and C did not fracture but showed large deformation with some partial depth cracking through the matrix it is important to be able to compare the levels of strain attained on the tension face using Eq. (3). This comparison is shown in Fig. 5 at the level of peak load (which is the fracture/failure load for sets A and D). While the addition of the unidirectional to the matrix resulted in an average strain of 0.06 mm/mm which is 50% greater than the capacity of the unreinforced matrix, the addition of the braid and leno-weave resulted in increases of 119 and 126%, respectively, emphasizing the higher capacity of both the UHMW polyethylene fibers and the architectures to hold together without rupture under flexural loading. It should be noted, as a reference, that the strain at the point at which the tests on sets B and C were stopped, at a midpoint deflection of 11.5 mm, was 0.135 mm/mm, which represents a 233% increase over the level attained by the unreinforced matrix. The us

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