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 reinforcement; Dental composite; Flexure; Damage tolerance; Architecture; Unidirectional; Braid; Leno-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 thicker; further 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.
Display Full Size version of this image (28K)
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.
Display Full Size version of this image (25K)
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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由于市场的开放，出口纺织品数量的增加，现行标准已不能满足产品质量和市场变化的要求。大多数的合资企业、独资企业以及有出口任务的企业，采用协议标准，按供需双方的协议合同考核和验收产品。而习惯于依赖国家标准或行业标准的企业，声称没有标准制约了企业的产品开发。
纵观国内纺织品市场尤其是制成品和服装市场上涌现出的很多被消费者认可的名牌产 品，其生产企业无一不是执行严格的技术标准和检验制度，无一不是采用优于国家标准和行业标准的企业内控标准。名牌产品是以优良的产品质量为基础，以高水准的标准为支撑，这些共识和实践对促进纺织工业的技术进步和产品质量的提高起到了积极的作用。
我国的纺织品标准现状
与纺织工业的发展相适应，我国纺织标准化工作不断地得到完善和提高，取得不小的成绩，纺织标准化工作为适应国家的经济建设和纺织工业的发展需要做出了应有的贡献。表现在：
1、从纺织材料到制成品和服装的标准已形成体系和规模。截止2002 年底，共有纺织品和服装标准885 个(不包括纤维原料标准)，其中国家标准383个，纺织行业标准502个，形成了以产品标准为主体，以基础标准相配套的纺织标准体系，包括术语符号标准、试验方法标准、物质标准和产品标准四类，涉及纤维、纱线、长丝、织物、纺织制品和服装等内容，从数量和覆盖面上基本满足了纺织品和服装的生产和贸易需要。
2、纺织品标准的采标率列为前位，基础标准与国际接轨。根据国家有关部门的统计，对国际标准的采标率，国内平均水平约为44％，而纺织品的采标率达80％。ISO中有关纺织品和服装的标准约有280多个。纺织行业对这些国际标准进行了研究，已经在不同程度上采用或已列入年度采用计划。除采用国际标准外，还不同程度的采用了国外先进国家的标准，如美国标准、英国标准、德国标准和日本标准等。特别是基础的、通用的术语标准和方法标准基本上采用了国际标准和国外先进标准，使制定的国家标准达到了国际标准或相当于国际标准的水平。
3、各类标准发挥了巨大作用。与国际接轨的基础标准，对统一纺织工业科技术语、统一纺织材料和产品的检测手段、统一规范产品的性能指标起到了重要的作用。特别是依据这些检测方法试验出具的数据不仅在全国范围内具有可比性，而且也得到了国外客户的认可，对纺织品贸易起到了不可低估的作用。制定的大量的纺织产品标准，适应了产品的发展和需要，解决了无标生产的问题，为企业的大量产品进入市场提供了技术依据。
4、企业的标准化理念对提高产品质量起到了关键作用。从1989年《标准化法》实施以来，企业的标准化工作逐步加强，参与标准化的热情越来越高涨。
但是，随着近年来市场经济的发展，现有的标准体制和标准内容逐渐显现出了其弊端，具体表现在：
1、在原有计划经济体制下，我国的纺织产品标准是生产型的，标准的制定以指导生产为主要出发点，技术要求与生产工艺紧密相联，指标定的过细过死，特别是标准的制修订速度滞后于产品的开发速度。有些企业认为标准水平太低，而有的企业却认为标准指标过高，形成了对标准的不同要求和评价。
2、随着纺织制品的成品化成为趋势，消费者对服饰和家庭装饰水平要求的提高，原料质量与制成品质量不配套的问题日益突出。例如，面料标准的色牢度差，水洗尺寸变化率大，缺乏实用性能考核指标等，由于标准不衔接引起的纠纷时有发生，消费者的投诉难以得到解决。
3、以前采标的指导思想是结合中国国情，考虑到国内现有设备和工艺条件，因此使我国采标的多数标准为“ 非等效”或“参照”。除基础标准接轨程度较高外，尽管有不少的产品标准前言中写明是采用国际或国外先进标准，但仅有少数指标甚至个别指标与国外标准一致，或采用的试验方法是采用国际标准的，因此，大多数产品标准的指标和水平没有真正与国外接轨。
4、由于市场的开放，出口纺织品数量的增加，现行标准已不能满足产品质量和市场变化的要求。大多数的合资企业、独资企业以及有出口任务的企业，采用协议标准，按供需双方的协议合同考核和验收产品。而习惯于依赖国家标准或行业标准的企业，声称没有标准制约了企业的产品开发。
与国外先进标准的差距
首先，形成的标准体系不同。ISO或国外的国家层面上的纺织标准，主要内容是基础类标准，重在统一术语，统一试验方法，统一评定手段，使各方提供的数据具有可比性和通用性。形成的是以基础标准为主体，再加上以最终用途产品配套的相关产品标准的标准体系。在产品标准中仅规定产品的性能指标和引用的试验方法标准。对大量的产品而言，国外是没有国家标准的，主要由企业根据产品的用途或购货方给予的价格，与购货方在合同或协议中规定产品的规格、性能指标、检验规则、包装等内容。
我国现行的纺织产品标准有不少是计划经济体制时的产物，形成的标准体系以原料或工艺划分的产品标准为主，目前主要分为棉纺织印染、毛纺织品、麻纺织品、丝产品、针织品、线带、化纤、色织布。近年来也以用途制定标准，但所占比例极小。标准中除性能指标外，还包括出厂检验、型式检验、复验等检验规则的内容，形成了各类原料产品“纱 线―――本色布―――印染布”的标准链。
其次，标准发挥的职能不同。国外将国家层面上的公开标准作为交货、验收的技术依据，从指导用户购买产品的角度和需要来制定，人们称之为贸易型标准。企业标准才是作为组织生产的技术依据。这种贸易型标准的技术内容规定的比较简明，比较笼统，比较灵活。
与之相反，我国大多数的产品标准的职能是用以组织生产的依据，从指导企业生产的角度的需要来确定，人们称之为生产型标准。为了便于企业生产，标准在技术内容方面，一般都规定的比较具体，比较详细，比较死。
随着市场经济的发展，纺织产品的新品种不断涌现，决定了简明灵活的贸易型标准更能符合市场的需要。我国的生产型标准范围较窄，覆盖的产品种类较少，造成标准的数量不少，但仍跟不上产品的发展速度。
第三，标准水平有差距。由于标准的职能不同，标准技术内容，如在考核项目的设置上，在性能指标的水平上等均有一定的差距。
国外根据最终用途制定的面料标准，考核项目更接近于服用实际，如耐磨、纱线滑移阻力、起毛起球、耐光色牢度等。我国的面料标准还缺少诸如接缝滑移、起毛起球、干洗尺寸变化、耐光色牢度等考核指标，不能适应人们对服用产品舒适美观性的要求。对服装的考核主要侧重服装的规格偏差、色差、缝制、疵点等外观质量，判定产品等级时忽略了 构成服装的主要元素―――面料和里料。
我国按生产型标准理念制定的标准，不能适用贸易关系超出生产方和购货方这种情况，例如，按染料类别和工艺制定不同的色牢度等级，在贸易交货验收中确定考核依据较为困难。而国外标准的质量指标控制严格，色牢度普遍高于国内指标1～1.5级，尤其是摩擦色牢度相差更多。
翻开产品标准，为数不少的标准文本中写有“优等品相当于国际先进水平，一等品相当于国际一般水平”等，实际上仅是个别单项指标水平达到国际水平，但综合性能达不到；还有个别标为采标的标准，其内容与国外标准相差甚远。
第四，国外标准形成了技术壁垒。随着贸易壁垒逐渐减小，各国都在借助于TBT有关条款规定，制造技术壁垒。而制造技术壁垒的有效途径就是法规和标准。欧洲议会和欧盟委员会2002年7月19日共同颁布的指令2002 ／61／EC―《对欧盟委员会关于限制某些危险物质和制剂(偶氮染料)的销售和使用的指令76／769／EEC的第 19次修改令》，连同欧盟委员会2002年5月15日颁布的关于修改并发布授权纺织产品使用欧共体生态标签(Ec o-label)的决定(2002／371／EC)，欧盟在为纺织品和日用消费品的市场准入构筑完整的“绿色屏障” 方面迈出了两个重大的步伐。中国作为全球最大的纺织品生产和出口国，可能受到的影响显然是不可低估的。
由于诸多原因，在进口纺织品中不乏有劣质产品和不合格产品。但我国技术法规和强制性标准欠缺，不能有效监督进口产品的质量。2000 年就着手制定的《纺织品基本安全技术要求》至今还未批准，对国外的不良产品起不到抵挡作用。
楼上的不错,就是看不懂
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