Composite shrinkage stress as a function of specimen dimensions and compliance of the testing system

Witzel, Marcelo F.; Ballester, Rafael Y.; Meira, Josete B.C.; Lima, Raul G.; Braga, Roberto R.

doi:10.1016/j.dental.2006.01.016

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Volume 23, Issue 2 , Pages 204-210, February 2007

Composite shrinkage stress as a function of specimen dimensions and compliance of the testing system

Marcelo F. Witzel
- Department of Dental Materials, School of Dentistry, University of São Paulo, São Paulo, Brazil
Rafael Y. Ballester
- Department of Dental Materials, School of Dentistry, University of São Paulo, São Paulo, Brazil
Josete B.C. Meira
- Department of Dental Materials, School of Dentistry, University of São Paulo, São Paulo, Brazil
Raul G. Lima
- Department of Mechanical Engineering, University of São Paulo, São Paulo, Brazil
Roberto R. Braga
- Department of Dental Materials, School of Dentistry, University of São Paulo, São Paulo, Brazil
- Corresponding author at: Departamento de Materiais Dentários da FOUSP, Av. Prof. Lineu Prestes, 2227, São Paulo, SP 05508-900, Brazil. Tel.: +55 11 30917840x224; fax: +55 11 30917840x201.

Received 6 September 2005; accepted 11 January 2006.

Abstract

Objectives

Verify the influence of specimen dimensions on composite shrinkage stress in testing systems of known compliance and in situations where axial strain of bonding substrates was suppressed. Stress distribution was evaluated using finite element analysis (FEA).

Methods

A chemically activated composite (Bisfill 2B, Bisco) was inserted between the flat surfaces of two glass rods (2, 4 or 6mm diameter, D) attached to a universal testing machine. Specimen height (H) was defined by adjusting the distance between the rods (0.5, 1, 2 or 4mm). An extensometer was used to monitor the distance between them. Maximum force after 30min of polymerization was divided by the cross-sectional area of the rod to obtain nominal stress (σ_n). Mathematical equations were employed in order to estimate the ‘corrected nominal stress’ (σ_cor) that would be obtained in ideally rigid systems. Data were analyzed by two-way ANOVA/Tukey test (α=0.05) and regression analysis (stress versus ‘C factor’ and stress versus specimen volume). Axysimetrical 2D models were used to evaluate X-component stress distribution (σ_x) in the zero compliance condition.

Results

The interactions between D and H were significant for σ_n and σ_cor (p<0.001). For D=2mm, height influenced only σ_cor values. A direct correlation was found between stress and C factor (σ_n:R²=0.959; σ_cor:R²=0.923), but not between stress and volume. FEA evidenced the effect of boundary restraints on σ_x.

Significance

Specimen dimensions influenced test results, mostly regarding σ_cor. Stress values strongly increased with the confinement of the specimen probably due to stress concentration adjacent to the bonded interface.

Keywords: Shrinkage stress, Resin composite, Finite element analysis