title: What do microtensile bond strength values of adhesives mean?
abstract: OBJECTIVE
  To test the hypothesis that stress distribution is more complex than generally assumed during microtensile testing by determining the stress level in the adhesive region of a virtual dentin-adhesive-composite stick using FEA (finite element analysis).
  
  
  METHODS
  A 3D FEA model simulating a dentin-adhesive-composite stick was analyzed. The length of the composite and the dentin was 5.0 mm each and the thickness of the adhesive layer was 0.02 mm. For the stress analysis, either only one lateral side of the stick or both end surfaces were attached. A 20-N load was then exerted on the stick with its 1.0 mm2 cross-sectional area, and von Mises stresses were calculated.
  
  
  RESULTS
  Large variations in stress levels existed. The highest stresses were located in the dentin and composite sections, near the adhesive interface. The stress level in these regions in the stick attached to one lateral side was more than 5 times higher than the 20 MPa stress calculated by dividing the force with the crosssectional area. For the specimen glued to the ends of the sticks, the stress level differences at the bonded interfaces were around 22 MPa, which decreased to 12 to 14 MPa in the center of the adhesive. Thus, this load condition yielded von Mises stress levels at the interface that were closer to the expected stress level than were the lateral-side attached specimens.
  
  
  CONCLUSIONS
  The calculated stress levels were higher and more complex than the strength values obtained by dividing the load at failure by the cross-sectional area. Reported strength values from microtensile tests therefore do not represent the true bond strength values at either the dentin/adhesive or adhesive/composite interface. However, the high stress levels in dentin and composite may explain the cohesive failures reported in the literature.
authors.affiliation: Department of Restorative Dental Sciences, College of Dentistry, University of Florida, Gainesville, FL 32610, USA. ksoderholm@dental.ufl.edu
authors.forename: Karl-Johan
authors.initials: KJ
authors.lastname: Söderholm
authors.forename: Saulo
authors.initials: S
authors.lastname: Geraldeli
authors.forename: Chiayi
authors.initials: C
authors.lastname: Shen
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D003198:Computer Simulation; D001840:Dental Bonding / Q000706:statistics & numerical data*; D003738:Dental Cements / Q000737:chemistry*; D003764:Dental Materials / Q000737:chemistry*; D003804:Dentin / Q000648:ultrastructure*; D055119:Elastic Modulus; D020342:Finite Element Analysis; D006801:Humans; D008422:Materials Testing; D018583:Pliability; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength; D014584:User-Computer Interface
chemical_list: D003188:Composite Resins; D003738:Dental Cements; D003764:Dental Materials