Coating/substrate Modulus Mismatches and Margin Geometry Effects
on Contact Damage in Curved Brittle Coating Systems碎花连衣裙
Tarek Qasim
School of Mechanical Engineering, The University of Western Australia
贴春联的顺序
35 Stirling Highway, Crawley, WA 6009, Australia
Tarek.Qasim@uwa.edu.au爱国宣传语
Keywords: Margin geometry; Radial cracks; Brittle coating; Dental crowns
Abstract . The effects of coating/substrate modulus mismatch and margin geometry on contact damage in bi-layer systems were investigated. Following an earlier study, convex specimens having curvature of 12 mm inner coating diameter and 1mm thick brittle layer on a polymeric and dental co
mposite support bas were produced. Sample coating geometry at the margins was varied by grinding the edge of the glass shells in various shapes. The specimens were tested by applying single cycle load at the specimen’s axis of symmetry using flat indenter of low elastic modulus. The effects of margin geometry and support layer modulus on radial crack initiation and damage evolution was examined, with particular attention paid to the relevance of such damage to lifetime-limiting failures of all- ceramic dental crowns. Finite element modeling was ud to evaluate stress distribution in the glass coating. Experimental trends interrupted with peak maximum principal stress at the margins. The results of this study illustrate that the fracture behaviour of brittle layered structures is not dominated by certain variables. It is demonstrated that critical loads for initiation of radial cracks are nsitive to support layer modulus as well as margin geometry. Support layer modulus plays an important role in crack propagation and subquent damage patterns, especially at specimen side walls.
Introduction
Layered structures consisting of brittle coating materials are finding common usage in a variety of engineering applications such as tribological and electronic packaging, coated cutting tools and thermal barrier coatings [1-3] as well as biomechanical implant structures such as dental crowns, hip
and knee prosthes, heart valves and bone implants [4-7].The combination of a hard, brittle outer layer and a compliant, tough inner layer offers high wear resistance and damage tolerance, factors that are crucial to the lifetime of such structures. Previous studies have focud on failures of such bi-layer systems under indentation with “hard” spherical indenters with a large elastic modulus. Following similar studies by the author [8-12] using “soft” indenters with a lower elastic modulus in which a new form of fracture revealed, “margin cracks” starting at the specimen edges and propagating upward at the specimen side walls to form "mi-lunar" fracture around one side of the dome [11,12].The soft indenters deformed under loading which resulted in a distributed load over the samples, simulating loading of a crown during mastication of soft food – for example. The distributed load creates a large compression zone directly beneath the indenter, eliminating the tensile stress in that region that would otherwi lead to radial cracking. Fractures of this type have been reported in the clinical dental literature [13-16]. The issue of when this type of fracture “margin damage” becomes dominant as a function of coating/substrate modulus ratio and margins geometry are the point in focus of this paper.
In this study, I propo to examine the effect of a variant formulation for specimen margins – chamfered, radiud and square as shown in Fig.1, on radial cracks evolution in convex layer speci
mens, having polymeric and dental composite support bas (substrates). Specimens fabricated
Fig. 1 Schematic drawing showing indentation with a cylindrical indenter at the
specimen axis of symmetry, load P on a dome structure of inner radius r s consisting of
a brittle shell of thickness d supported by compliant ba extending depth h below the
margin edges.
using finite element modeling (FEM) and compared with experimental results of the system under consideration here. Details of the FEA procedures, using Abaqus V6.5 software, have been documented and demonstrated in previous studies [9-12]. The results of this study confirmed the effect of margin geometry having on radial cracks critical loads especially in the ca of softer (polymeric) substrate and on the growth of radial cracks to failure where the substrate modulus becomes more dominant
Materials and testing
The glass shells were produced in the glassblowing workshop at the School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia. They were produced from 12mm inner diameter 1mm thick borosilicate glass tubes (Schott Duran, Germany). This glass has a
Young’s modulus of 72.9 GPa. Following this, the inner concave surfaces of the glass shells were given an abrasion treatment with 50µm sand particles using a dental sandblast machine (P-G4000 Harnish and Reith, Czech Republic). Sandblasting reduces the strength of the glass clo to that of dental porcelain. Also, providing a more uniform density of surface flaw distribution, the procedure also facilitates bonding of the epoxy resin to the glass shell [11,12,17].
The glass shells were placed into a mould, and each concave interior filled with epoxy resin having a Young’s modulus of 3.4 GPa (R2512, ATL Composites, Australia) in layers of 3-4mm with a drying time of 24 hours allowed between each layer. This was done to avoid the shrinkage rate during cutting of the epoxy, and to avoid residual stress in the glass [8,10,17]. A final layer of approximately 6mm above the rim of the specimen was added. Similar to this procedure, some specimens filled with dental composites (D.C.) and cured using a blue light according to manufacturer specifications (Z100 Restorative, 3M ESPE), this dental composite have a Young’s modulus of 13.3 GPa.
Indentation tests were carried out in air at room temperature using Teflon indenter of modulus 0.48 GPa of cylindrical shape of 10.0 mm diameter and 10.0 mm height. Indenters were mounted in a screw-driven mechanical testing machine (Instron 4301, Instron Corp, Canton, MA). The tests
As - Polished Grinding trim杂技表演
Sand-blasted Square
Radiud
h
r
s
d
single cycle indentation was performed on each specimen. Some 4 to 8 parate tests were run for each margin geometry and substrate material. During loading, the specimens were viewed from the side using a video camera, such that the side walls of the specimen were within the field of view at all times. No delamination was obrved between the glass and the epoxy layers in any of the tests until ultimate failure, attesting to good bonding.
Results and Discussion
Prior to loading specimens were visually tested for pre existing cracks at the coating, no cracking was evident in the coatings including the margins for all tested specimens in this study. At higher loads due to the engulfment of the contact zone by the soft Teflon indenter, the crack evolution was difficult to follow in situ. The specimens were viewed from interrupted tests after unloading.
Figure 2 shows a quence of video clips during indentation in chamfered edge glass domes filled with dental composite, loaded parallel to the symmetry axis. During loading, a crack was obrved to pop-in between the margin of the specimen and the outer side of indentation area, as shown in Fig.
2(a). Multiple cracks occurred in the same fashion with increasing load, as in Fig. 3(b and c). The location of such cracks varied from specimen to specimen, but always pointed toward the contact centre. The initiation location of the cracks could not be determined, i.e. starting from the margins or from the indentation area, suggesting that crack initiation could occur anywhere in this region, depending on the availability of starting flows. Similar to this failure pattern was obrved for epoxy filled specimens with different failure patterns at higher loads as can be en in Fig.3.
Figure 3 compares cracks (failure) patterns in chamfered edge specimens after unloading at P = 1500 N (a) dental composite filled specimen and (b) epoxy filled specimen. In both cas, cracks have initiated at the edge of the specimen and propagated toward the contact centre. Note that the cracks tended to initiate more or less symmetrically, and multiple cracks occurred at higher loads as in Figs. 2 and 3. It is interesting, to indicate the initiation of condary cracks around the indentation area (e arrows in Fig. 3).
(a) (b) (c)
Fig. 2 Contact fractures in dental composite-filled glass domes of inner radius r s =
6 mm, chamfered edges, indented with Teflon indenter at load P : (a) 1000 N, (b) 1200 N, and (c) 15
00 N. Showing failure from radial cracks.
关联词练习Critical loads to initiate radial cracks in the glass sub-surface were measured. The initial cracks in the specimens “pop” in rapidly between the indentation area and the margins of the specimens, accompanied by an audible click (Fig. 2(a)). Data are plotted graphically in Fig. 4 for chamfered, radiud and square edge specimens, and for D.C. and epoxy substrates. As explained-previously, further increa in load does not extend the cracks onto the indentation area but instead more cracks initiated and linked with neighbouring radial arms and caud portion of the coating to parate from the substrate and, ultimately material dislodgment (Fig.3). This final stage in the failure process is indicated in Fig. 4 as “Failure loads”. Note that for epoxy filled specimens, margin geometry has no/less effect on the critical load for initiation of radial cracks. In both cas epoxy and dental composite filled specimens the chamfered specimens appear to be more susceptible to failure from radial cracking. This can be attributed to the loss coating material near the margins.
Figure 5 shows typical plots of maximum principal stress at an applied axial load of 1000N, for (a) Square margin, (b) radiud margin and (c) chamfered margin. Tensile stress are displayed. The plots clearly show the effect of different specimen margin geometry. Of primary interest is the broad t
ensile region outside the contact zone, confirming a strong transfer of load to the support edges. Also note the concentration of contours around the margin more in the ca of chamfered edges Fig. 5(a). The extensions of contours around the dome ba – a direct conquence of coating flexure, suggest that crack initiation could occur anywhere in this region, depending on the availability of starting flows. This explains the experimental obrvations discusd earlier.
Also of interest due to the large contact area under the soft indenter, the extended compression zone (white region) within the immediate contact engulfs the tensile area en previously using hard indenters, and conquently the area of peak tensile stress is shifted towards the margin.
Fig. 3 Contact fractures of chamfered edges glass domes of inner radius r s
= 6 mm, indented with Teflon indenter, at load P = 1500 N: (a) Filled with
Dental Composites - substrate, and (b) Filled with Epoxy – substrate, showing failure from radial cracks after unloading.
(a) (b)
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Fig. 5 Dental composite substrate, (a) Square margin, (b) radiud margin, (c) chamfered margin, all plotted on same scale. Little change in the stress from introducing the radius on the inner edge, however, chamfering of the margin increas the stress near the margin. Fig. 4 Histogram showing critical loads to initiate radial cracks and to
propagate the same cracks to failure at the edge of the glass dome, for same
(Teflon) indenter.
Initiation Failure
(a) (b) (c)
Acknowledgments
The author gratefully acknowledges Prof. Mark Bush, Prof. Zhao Hu and Dr Chris Ford (University of Western Australia) and Dr. Brian Lawn (NIST, Maryland) for many uful discussions. This work is supported by grant from the Australian Rearch Council (DP0878113).
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