Abstract
Aim
To analyze the distribution of stresses at the bond interface of Molloplast-B soft-liner attached to PMMA acrylic surface of different geometries (smooth and rough) and at different load-application distances by 3D-FEA modeling of typical shear-bond test.
Methodology Three-dimensional finite element analysis (3D-FE) was performed utilizing Patran and Marc softwares (MSC.Software, USA). Models of Molloplast-B disk liner (diameter: 8 mm, thickness: 3 mm) bonded to smooth and rough acrylic geometries were designed. A total of 8 models (4 models for each surface geometry) were used to analyze Von Mises, maximum principal and shear stresses for the nodes corresponding to the vertical diameter of the acrylic–liner interface when applying a uniform shear-loading at different distances from the interface; 0.25, 0.5, 1, and 2 mm. Materials properties were assumed to be isotropic, homogeneous, linear and elastic.
Results
Shear loadings, at various distances from the bond-interfaces, produced different magnitudes of shear and tensile stresses. They were more uniform for both smooth and rough geometries when loading was applied at smaller distances from the interface (0.25 and 0.50). The maximum shear stress did not surpass maximum tensile stress in the areas subjected to the highest stress. These values were higher for the models with increased distances and bending moment associated with rough surfaces.
Conclusions
It can be concluded that in conducting shear bond tests, shear and tensile stresses are present regardless of the distance from the bond interface at which the shearing stress is applied. However, at increased distances from the bond interface, tensile stresses are greater than shear stresses. When shear stress is applied at shorter distances from the bond interface (0.25 or 0.50 mm), and regardless of bond-interface geometry; there is more uniform stress distribution and lower effect of bending moment. However, in vitro studies of bond strengths between soft liners and acrylic substrates obtained by shear test should be approached with caution, considering the distance of load application and the surface treatment of the substrate as the most important factors that could interfere with the shear test values.
1
Introduction
Resilient denture liners such as silicones are used as lining material for dental prostheses. They act as a cushion over the soft tissues, distributing the occlusal forces, retaining the prosthesis, enhancing the fit, and preventing trauma of sensitive mucosa .
For a long serviceable prosthesis, the liner should remain resilient, and well adhered to the acrylic base . Several factors can affect the bond strength between lining material and acrylic resin, like geometry of bond surface whether it is roughened or chemically treated (etched) , the use of bonding agents , and thickness of the lining material .
Many in vitro tests have been conducted to test the bond strength between PMMA denture base and resilient materials, such as shear , tensile , tensile and shear , and peel tests . There is still uncertainty in selecting the best test that clinically relates to the factors that affect the bond integrity of different material combination during use. Furthermore, it is not possible to conduct direct comparisons between these studies due to variations in their experimental protocols.
For shear bond test, slight variations can make critical differences in the bonding strength values . Complexity of the shear test in dentistry is based on the difficulty to interpret the interfacial bonding phenomenon . Conventional shear bond tests record an underestimated value of the average nominal stress . It indicates the critical stress at the sensitive region, and it is likely recorded at the edge of the bonded area, near the loading site, when a point-load is applied . Such nominal stress does not express peaks of stress just before failure. In addition to that, in vitro tests can only evaluate the effect of one variable alone and other factors need to be considered fixed in running the test.
Finite element method (FEM) has been suggested to evaluate the bond strength of dental materials by simulating mechanical tests (i.e. tensile or shear tests). It has the advantages of investigating the stress distribution at the bond interfaces . And it enables setting several testing conditions at one time, like the loading testing device, materials properties, interfaces geometries, interface loading distance, and possible sites of interfacial fracture.
The aim of the study was to analyze the distribution of debonding stresses at the bond interface of Molloplast-B soft-liner attached to PMMA acrylic surface of different geometries (smooth and rough) and at different load-application distances by 3D-FEA modeling of typical shear-bond test. Our null hypotheses stated that stress distribution is not affected by the bond interface geometry (hypothesis 1) or distance of load application (hypothesis 2).
2
Methodology
Eight 3D-models of a typical liner-acrylic configuration were produced using a finite element analysis software package ( Patran and Marc , MSC.Software, USA; version r2, 2005). Each model consisted of desk-shaped soft liner (diameter: 8 mm, thickness: 3 mm) bonded to either smooth (four models) or rough (four models) acrylic interfaces (diameter: 14.4 mm). The refined solid mesh of the models had 73568 elements and 76068 nodes, performed by eight-node hexahedral elements, decreased toward the leading edge (from 0.5 mm at the base to 0.18 mm edge at the interface) and six-node wedge elements which were obtained from four-node quadrilateral elements rotated at 360° angle around Z axis ( Fig. 1 ). The interface nodes were shared between the acrylic and the liner following the assumption of perfect bonding. For the rough models, the roughness was created by removing alternate elements from the faces at the bond interface of both acrylic and liner, resembling perfect mechanical interlocking between liner and acrylic elements.
The bond interface areas of both the smooth and rough models were 50.26 and 148.70 mm 2 , respectively. For the smooth interface, it was calculated using the area enclosed by a circle. For the rough area, it was conducted by the same procedure, taking into account the element edges which promote the interlocking ( Fig. 2 ).
The way the experimental specimens were fixed into the shear jig was simulated in the FEA method. All models were constrained in three directions ( X , Y and Z ) at nodes on the side-and-back-surface of the acrylic, simulating actual experimental setup. And a uniform distributed load was applied on the liner elements corresponding to the area in contact with the sliding part of the experimental shear jig at four different distances from the bond interface. The 4 models of each acrylic interface (smooth and rough) were investigated at these distances (one model at each distance). These distances were (in mm) 0.25, 0.50, 1.0, and 2.0 ( Table 1 ). They were selected to investigate and minimize the bending stresses and stress concentrations. A 37.6 N was used as nominal force to control the loading. The force was selected as the maximum shear force recorded by Hatamleh and Watts to debond Molloplast-B liner from an acrylic substrate. The boundary conditions applied to the models are presented in Fig. 3 . The acrylic and liner were considered isotropic, homogeneous, linear and elastic. Their properties are presented in Table 2 as reported by two previous studies .
| Bond surface (code) | Loading distance (mm) | Interface bond area (mm 2 ) |
|---|---|---|
| Smooth (S 1 ) | 0.25 | Each is 50.26 |
| Smooth (S 2 ) | 0.50 | |
| Smooth (S 3 ) | 1.0 | |
| Smooth (S 4 ) | 2.0 | |
| Rough (R 1 ) | 0.25 | Each is 148.70 |
| Rough (R 2 ) | 0.50 | |
| Rough (R 3 ) | 1.0 | |
| Rough (R 4 ) | 2.0 | |
Von Mises stresses at the nodes corresponding to the acrylic–liner interface were analyzed by color-coded fringe plots, where each color represents a particular range of values. The color range and values were fixed for all models. This study did not focus on the real values but on predicting the mode of failure based on stress distribution of the models. Maximum principal (tensile) and maximum shear stresses affecting the bond integrity between the liner and the acrylic interface were investigated in vertical direction through the line represented as A–B in Fig. 4 , which was reported in previous study as the region where the stresses are likely to cause failure .
2
Methodology
Eight 3D-models of a typical liner-acrylic configuration were produced using a finite element analysis software package ( Patran and Marc , MSC.Software, USA; version r2, 2005). Each model consisted of desk-shaped soft liner (diameter: 8 mm, thickness: 3 mm) bonded to either smooth (four models) or rough (four models) acrylic interfaces (diameter: 14.4 mm). The refined solid mesh of the models had 73568 elements and 76068 nodes, performed by eight-node hexahedral elements, decreased toward the leading edge (from 0.5 mm at the base to 0.18 mm edge at the interface) and six-node wedge elements which were obtained from four-node quadrilateral elements rotated at 360° angle around Z axis ( Fig. 1 ). The interface nodes were shared between the acrylic and the liner following the assumption of perfect bonding. For the rough models, the roughness was created by removing alternate elements from the faces at the bond interface of both acrylic and liner, resembling perfect mechanical interlocking between liner and acrylic elements.
The bond interface areas of both the smooth and rough models were 50.26 and 148.70 mm 2 , respectively. For the smooth interface, it was calculated using the area enclosed by a circle. For the rough area, it was conducted by the same procedure, taking into account the element edges which promote the interlocking ( Fig. 2 ).
The way the experimental specimens were fixed into the shear jig was simulated in the FEA method. All models were constrained in three directions ( X , Y and Z ) at nodes on the side-and-back-surface of the acrylic, simulating actual experimental setup. And a uniform distributed load was applied on the liner elements corresponding to the area in contact with the sliding part of the experimental shear jig at four different distances from the bond interface. The 4 models of each acrylic interface (smooth and rough) were investigated at these distances (one model at each distance). These distances were (in mm) 0.25, 0.50, 1.0, and 2.0 ( Table 1 ). They were selected to investigate and minimize the bending stresses and stress concentrations. A 37.6 N was used as nominal force to control the loading. The force was selected as the maximum shear force recorded by Hatamleh and Watts to debond Molloplast-B liner from an acrylic substrate. The boundary conditions applied to the models are presented in Fig. 3 . The acrylic and liner were considered isotropic, homogeneous, linear and elastic. Their properties are presented in Table 2 as reported by two previous studies .
