Application of Digital Image Correlation Method to Study Dental Composite Shrinkage

Abstract:  Although resin-based composites are widely used in dental restoration, these materials shrink during polymerisation. Polymerisation shrinkage results in distortion of the restoration and bonded tooth and also generates internal stress at the resin–tooth interface. Digital image correlation (DIC) is used to determine the in-plane displacement field by matching different zones of two characterised pictures. The objective of this study was to examine the applicability of DIC in measuring the deformation of the composite restoration and the surrounding tooth. A preliminary experiment examined the shrinkage of composites in a simulated cavity using the DIC method. The measured shrinkage pattern was consistent with a corresponding finite element model. Subsequently the deformation of composite restorations on human molars was examined using this validated DIC method. The greatest deformation was found on the free occlusal surfaces, and the least on the gingival wall. The increased deformation on the post-cured images indicated that the shrinkage continued even after termination of light activation. DIC method facilitates a full-field measurement of shrinkage profile. These experimental results did not only demonstrate the spatial and temporal relationship of displacement in a dental restoration, but also provide validation of computational models to examine the polymerisation consequence.     

Analysis of the mechanical behaviour of adhesively bonded assemblies of composites under tensile–shear out-of-plane loads

The use of composite materials is a key element in energy reduction strategies, particularly in all areas of transportation. The study of the behaviour of assemblies of composites, including bonded assemblies, is of great importance. Complex 3D loadings are needed to analyse the response of both the assemblies and the composites. However, few experimental devices allow out-of-plane loading to be studied, except by using thick composite specimens which are not always representative of industrial applications. This paper presents some possibilities of an experimental device, using a modified Arcan test, and optimized hybrid bonded assemblies, which limit the influence of edge effects. The influence of some design parameters are presented in the case of naval composites, they underline the possibilities of the proposed strategy. The failure envelope curves in the normal stress–tangential stress plane for proportional monotonic out-of-plane loadings are presented in the case of a bonded assembly for different composites.     

Development and measurement of stress in polymer coatings

This paper reviews stress development mechanisms and stress measurement techniques for polymer coatings. Most polymeric coatings shrink during and after solidification due to chemical reaction, solvent evaporation, phase separation, or some combination thereof. Coating adhesion, however, prevents shrinkage from occurring freely; this frustration of in-plane shrinkage leads to a tensile stress in the plane of the coating. At the same time, stress accumulates, it may be relaxed by processes such as molecular motion. The measured stress at any time is the result of the competition between stress buildup from frustrated shrinkage and stress relief from relaxation. Accumulation of stress is a problem because it can lead to defects such as cracks. An understanding of stress development in various types of polymeric coating systems will lead to strategies for material selection, process optimization, and defect elimination. In this paper, background on stress development is provided, followed by an overview of stress measurement methods for polymer coatings. The remainder of the paper focuses on stress development during the drying and curing of polymer coatings, drawing many examples from previous stress measurement studies.     

Interface Stress-Induced Degradation of the Performance of Electrically Conductive Adhesives

Electrically conductive adhesives are of increasing importance as a powerful alternative to eutectic bonding. In the use of electrically conductive adhesives in soldering applications, a lack of conductivity of the adhesive bond between surface mounted devices (SMD) and the printed circuit board was frequently observed. This lack of conductivity caused failure of the unit because of an unacceptable series resistivity in the circuit. A failure analysis proved that the resistance of the contact between the adhesive and device depends on the stress situation in the bonded interface induced by the shrinkage of the adhesive during curing. Under stress, the formation of a resin segregation layer of low conductivity results. The outer shape of the electrical component as well as the SMD assembly process has strong influence on the electrical performance.     

粘结SmFe2合金的制备工艺与磁致伸缩性能研究

采用树脂粘结法制备了不同尺寸的SmFe2合金样品，研究了粘结剂的含量，模压压力，样品密度及样品尺寸对SmFe2合金的磁致伸缩性能的影响规律。结果表明：粘结剂含量和模压压力对粘结SmFe2合金的磁致伸缩性能具有较大影响，发棒材样品长度≥20mm时，样品尺寸对合金的磁致伸缩性能亦有较大影响。     

New urethane oligodimethacrylates with quaternary alkylammonium for formulating dental composites

The aim of this study was to prepare urethane dimethacrylates containing quaternary alkyl (C16, C12) ammonium and polyethylene glycol short sequences (Mn, 400 g/mol) and to investigate their behaviour in some experimental formulations in order to evaluate their potential applicability in the dental composites field. The structure of urethane dimethacrylates has been confirmed by ¹H (¹³C) NMR and FTIR spectra, as well as by electrospray ionization tandem mass spectroscopy, and gel permeation chromatography measurements. The effects of the cationic macromers on the properties of the filled/non-filled composites were examined through FTIR, photoDSC, and specific measurements as volumetric polymerization shrinkage, water sorption/solubility, contact angle, mechanical parameters, and morphology. The monomer compositions based on cationic dimethacrylate (6.88–27.52 wt%), BisGMA-analogue (48.18–68.82 wt%) and TEGDMA (23.3 wt%) showed a good photoreactivity in terms of double bond conversion (DC, 50.07–68.81 %) and polymerization rate (R_p, 0.099–0.141 s^?1) measured by photoDSC compared to a control sample (BisGMA-1/TEGDMA: DC, 45.91 %; R_p, 0.162 s^?1), while the polymerization shrinkage increased in acceptable limits (5.37–7.74 vol%). The mechanical properties (compressive, flexural and diametral tensile strength) of the composite resin incorporating 70 wt% silanized zirconium silicate micro/nanopowder can be modulated by the initial co-monomer concentrations.     

Characterization of light-cured dimethacrylate resins modified with silsesquioxanes

Methacrylate-functionalized silsesquioxanes (SSQO) were synthesized by the hydrolytic condensation of 3-(methacryloxypropyl) trimethoxysilane (MPTMS) using dimethacrylate monomers as reactive solvent. Resins containing about 40 wt% SSQO displayed a modest increase in viscosity compared with the unmodified methacrylate monomer. This is attributed to a dilution of the reacting medium, which discourages bimolecular reactions that lead to the generation of high-molar mass oligomers and, consequently, prevents undesired increases of viscosity. Substitution of monomer with SSQO reduced the volumetric shrinkage during photopolymerization. The amount of leachable monomer detected in formulations containing SSQO was markedly reduced. This indicates that the unreacted monomer exists mainly as pendant chains instead of free leachable monomer. The presence of different proportions of SSQO in the methacrylate monomer resulted in no apparent change in the flexural modulus and in a modest decrease in the compressive yield stress of the polymerized materials. From results presented in this research, it emerges that methacrylate monomers modified with SSQO are attractive for applications that require reduced volumetric shrinkage and long storage periods, such as light-cured dental restorative composites.     

Polymer grafted hydroxyapatite whisker as a filler for dental composite resin with enhanced physical and mechanical properties

The objective of this study was to investigate the effect of surface graft polymerization of hydroxyapatite whisker (HW) on physical and mechanical properties of dental composite resin. Poly bisphenol A glycidyl methacrylate (Poly(Bis-GMA)) was grafted onto silanized hydroxyapatite whisker (SHW) via solution polymerization and the amount of the Poly(Bis-GMA) on the surface was effectively controlled by polymerization time. The obtained poly(Bis-GMA) grafted hydroxyapatite whisker (PGHW) with different polymer contents was filled into a resin matrix respectively, meanwhile the composites with HW and with SHW served as controls. Monomer conversion was characterized by Fourier transform infrared spectroscopy (FTIR) and volume shrinkage of the composite resin was measured with a density tester. Mechanical properties were tested with a universal testing machine. The results indicated that the composite filled with PGHW-1 h (graft ratio of poly(Bis-GMA): 8.5 wt.%) showed lower shrinkage and better mechanical properties, improving flexural strength by 6.5% and 11.9% compared with SHW filled composite and HW filled composite, respectively. However, PGHW with higher graft ratios aggregated seriously and formed defects in the composite, leading to deterioration of mechanical properties. It was revealed that the poly(Bis-GMA) on the surface of PGHW acted as a functional transition layer and enhanced interfacial compatibility and interaction between whisker and resin matrix, which facilitated the dispersion of PGHW in the composite and decreased the composite shrinkage. Thus, the graft polymerization of Bis-GMA on the surface of filler might be a promising modification method for the fabrication of dental materials.     

Flexible piezotronic strain sensor

Strain sensors based on individual ZnO piezoelectric fine-wires (PFWs; nanowires, microwires) have been fabricated by a simple, reliable, and cost-effective technique. The electromechanical sensor device consists of a single electrically connected PFW that is placed on the outer surface of a flexible polystyrene (PS) substrate and bonded at its two ends. The entire device is fully packaged by a polydimethylsiloxane (PDMS) thin layer. The PFW has Schottky contacts at its two ends but with distinctly different barrier heights. The I- V characteristic is highly sensitive to strain mainly due to the change in Schottky barrier height (SBH), which scales linear with strain. The change in SBH is suggested owing to the strain induced band structure change and piezoelectric effect. The experimental data can be well-described by the thermionic emission-diffusion model. A gauge factor of as high as 1250 has been demonstrated, which is 25% higher than the best gauge factor demonstrated for carbon nanotubes. The strain sensor developed here has applications in strain and stress measurements in cell biology, biomedical sciences, MEMS devices, structure monitoring, and more.     

Shrinkage vector determination of dental composite by μCT images

Hypotheses about contraction patterns of light-curing dental composites are controversial. The purpose of this study was to develop a method to determine the direction and extent of polymerization shrinkage. Cavities were restored with a modified composite that contained traceable particles, and digitized before and after light-curing with micro-computed tomography (μCT). The resulting displacement vector fields based on the elastic registration of μCT images (uncured vs. cured) were examined and used to calculate the changes. In unbonded restorations, the displacement vectors pointed inward, but not perfectly, to the center of mass. Bonded restorations exhibited two contraction patterns: either toward one side of the cavity or toward the top-surface of the restoration, and showed significant higher deformation than unbonded group (p < 0.01). The total vector length histogram also indicated the boundary condition is critical to the pattern of polymerization shrinkage. This method has great potential in validating current models of shrinkage.     

Luting Cement,the Stronghold or the Weak Link in Ceramic Restorations?

A luting agent for fixing indirect dental restorations has to be a sophisticated material as, apart from being biocompatible, it must allow complete seating, provide mechanical support, has to secure the retention and to seal the gap between restoration and tooth structure for many years of function in an aggressive bio‐chemical and mechanical environment. Because of their adhesive potential, the latest generations of dental cements, the glass‐ionomer and resin‐based composites offer improved results. However, a major disadvantage of both classes of materials is their setting, being accompanied by shrinkage, which eventually leads to fracturing of the brittle ceramic. Apparently conflicting interests like being at the same time flexible as well as rigid, hold for luting cements for full ceramic restorations. The interaction between layer thickness, curing shrinkage stress, visco‐elasticity, module and strength for a thin adhesive cement layer will be discussed in the perspective of durable functioning of ceramic restorations.     

Analytical analyses of stress transfer in fibre-reinforced composites with bonded and debonded fibre ends

The elastic stress transfer from the matrix to the fibre is analysed analytically for fibre-reinforced composites when the loading direction is parallel to the fibre axis. The fibres with bonded lateral interfaces and (1) debonded and (2) bonded ends are considered in the present study. For the case of debonded ends, the present solutions contain refinements of the previously derived analytical solutions. For the case of bonded ends, unlike the numerical solutions derived previously, the present analytical solutions are ready to be used for further analyses. The results show that the stress transfer is more effective when the fibre has higher Young's modulus or longer length. Also, compared to the debonded ends case, the stress transfer is more effective and the stress distribution is more uniform when the ends are bonded to the matrix.     

超声应力检测技术综述

应力检测是保证机械装置稳定可靠运行的必要手段,而考虑到机械装置结构的复杂性与工作环境的多样性,其对检测方法的要求较高。超声应力检测以其高精度、无害性、强适应性等优点在连续介质与非连续介质内外应力检测中得到了广泛的关注与应用。文章针对超声应力检测中最主要的三种方法:声弹性效应法、声反射系数法以及非线性超声检测法,结合国内外学者的研究成果,从其工作原理、发展过程与实际应用等方面分别进行综述,总结其发展现状,并对其未来发展提出展望。     

体积收缩率对光固化树脂基复合材料收缩应力的数值模拟

依据SiC晶须增强树脂基复合材料的光固化实验结果，运用材料的温度应力原理，利用降温等效体积应变与光固化体积收缩率关系的假设，建立了有限元模型。研究并分析了树脂基体中组分配比、光固化层厚和晶须增强体对成型件收缩应力的影响。结果表明：树脂基体中组分配比和晶须增强体对收缩应力的影响很大，而光固化层厚对收缩应力的影响较小。     

Water sorption and mechanical properties of acrylic based composites

Four commercial bisphenol-glycidilmethacrylate based composites used mainly for dental applications have been investigated. Differential thermal analysis performed on samples aged in water for different times indicated a small residual monomer reactivity which disappeared after ageing. A further crosslinking reaction facilitated by water plasticization and a monomer loss could be the main reasons for such 8 phenomenon. The embrittlement of these materials with ageing time has been detected from flexural mechanical properties. Water sorption/desorption experiments have been performed on G!! the materials studied at different temperatures. The decrease of diffusion coefficients with increasing water content together with the microscopic analysis of the fracture surfaces demonstrated good filler/matrix adhesion for all the four composites. The decrease of water diffusion coefficients with time for Miradapt, Silar and Adaptic has been explained on the base of the presence in the polymeric networks of different density regions due to inhomogeneous polymerization. This hypothesis is also in line with the Miradapt, Silar and Adaptic hysteresis phenomena observed in the sorption/desorption cycles.     

A review of mechanical behaviour and stress effects in hard superconductors

The mechanical properties of type II superconducting materials are reviewed as well as the effect of stress on the superconducting properties of these materials. The bcc alloys Nb-Ti and Nb-Zr exhibit good strength and extensive ductility at room temperature. Mechanical tests on these alloys at 4.2 K revealed serrated stress-strain curves, non-linear elastic effects, and reduced ductility. The non-linear behaviour is probably due to twinning and de-twinning or a reversible stress-induced martensitic transformation. The brittle A-15 compound superconductors, such as Nb₃Sn and V₃ Ga, exhibit unusual elastic properties and structural instabilities at cryogenic temperatures. p]Multifilamentary composites consisting of superconducting filaments in a normal metal matrix are normally used for superconducting devices. The mechanical properties of alloy and compound composites, tapes, as well as composites of niobium carbonitride chemically vapour deposited on high strength carbon fibres are presented. Hysteretic stress-strain behaviour in the metal matrix composites produces significant heat generation, an effect which may lead to degradation in performance of high field magnets. Measurements of the critical current density, J_c, under stress in a magnetic field are reported. Modest stress-reversible degradation in J_c is observed in Nb-Ti composites while more serious degradation is found in Nb₃Sn sample.The importance of mechanical behaviour on device performance is discussed.     

Effect of film thickness on the performance of photopolymers as holographic recording materials

An important issue in developing applications for photopolymers in holography is the effect of film thickness on recording properties. Now it is possible to create these samples with a much wider range of thickness (d = 20-1400 mum) than was previously available. We exploit these recent advances in photopolymer processing to systematically evaluate how the dynamic range of a photopolymer depends on its thickness. The results illustrate that sample performance increases linearly with thickness as predicted by standard models of volume holography. However, above a critical thickness sample performance degrades, and the angular response of recorded plane-wave holograms shows evidence of grating curvature. These distortions are likely the result of photopolymer shrinkage, which in thicker samples occurs in a nonuniform fashion. This problem limits the performance of these photopolymers and is likely to be an issue for any photopolymer that undergoes comparable polymerization shrinkage.     

A modified analysis for stress transfer in fibre-reinforced composites with bonded fibre ends

The elastic stress transfer from the matrix to the embedded fibre in fibre-reinforced composites has been analysed previously when the loading direction is parallel to the fibre axis and the fibre is bonded to the matrix. Stress transfer occurs both at the interface along the fibre length and at the ends of the fibre. However, the boundary condition at the bonded ends is ambiguous, and various assumptions have been made to obtain solutions for this stress transfer problem. To satisfy more rigorously the boundary condition for the bonded ends, a new technique of assuming imaginary fibres in the composite is proposed in the present study. Compared to the previous analytical solution, the present analytical solution bears more physical meaning and is in better agreement with numerical and experimental results     

Characterization of interlaminar shear failures of graphite/epoxy composite materials

Research undertaken to develop a more fundamental understanding of interlaminar shear failure in laminated graphite/epoxy composites is described. A test method capable of producing a state of pure interlaminar shear stress within a specified region of a composite test specimen was devised. The test method consisted of the four-point flexural testing of a unique test sample constructed of a coupon of Hercules AS4/3501-6, unidirectional, graphite/epoxy material bonded between two strips of steel using a room-temperature-curing epoxy adhesive. The major advantage of the test method is that the interlaminar shear failure of the composite coupon results from an induced state of pure shear stress, rather than from damage resulting from a complex stress state affecting the region of loading as typically occurs in the case of conventional flexural-type shear tests. The resulting shear failures were characterized with respect to fracture surface appearance, mode of failure, and stress state on the failure plane. A specially designed crack detection device was used to determine the site of fracture initiation and the direction of crack propagation.     

Comparison of shear strength tests on AV119 epoxy-joined carbon/carbon composites

The results of an experimental investigation on carbon/carbon composites (C/C) bonded joints tested in shear at room temperature, under seven different configurations, are presented. The samples have been joined by an epoxy adhesive (AV119): this adhesive is not to be considered as a suitable joining material for C/C for high temperature applications, but just as a model joining material chosen to obtain several bonded samples in a short time. Advantages and disadvantages of each configuration are discussed from an experimental point of view. A finite element analysis is also performed to compare the stress distribution obtained within the joint for the different testing geometries. It is shown that the measured values of the apparent shear strength decrease with the maximum opening stress estimated within the middle of the joint.