title: A critical view on biaxial and short-beam uniaxial flexural strength tests applied to resin composites using Weibull, fractographic and finite element analyses.
abstract: OBJECTIVE
  To evaluate the biaxial and short-beam uniaxial strength tests applied to resin composites based upon their Weibull parameters, fractographic features and stress distribution.
  
  
  METHODS
  Disk- (15 mm x 1 mm) and beam-shaped specimens (10 mm x 2 mm x 1 mm) of three commercial composites (Concept/Vigodent, CA; Heliomolar/Ivoclar-Vivadent, HE; Z250/3M ESPE, FZ) were prepared. After 48h dry storage at 37 degrees C, disks and beams were submitted to piston-on-three-balls (BI) and three-point bending (UNI) tests, respectively. Data were analyzed by Weibull statistics. Fractured surfaces were observed under stereomicroscope and scanning electron microscope. Maximum principal stress (sigma(1)) distribution was determined by finite element analysis (FEA). Maximum sigma(1-BI) and sigma(1-UNI) were compared to FZ strengths calculated by applying the average failure loads to the analytical equations (sigma(a-BI) and sigma(a-UNI)).
  
  
  RESULTS
  For BI, characteristic strengths were: 169.9a (FZ), 122.4b (CA) and 104.8c (HE), and for UNI were: 160.3a (FZ), 98.2b (CA) and 91.6b (HE). Weibull moduli (m) were similar within the same test. CA and HE presented statistically higher m for BI. Surface pores (BI) and edge flaws (UNI) were the most frequent fracture origins. sigma(1-BI) was 14% lower than sigma(a-BI). sigma(1-UNI) was 43% higher than sigma(a-UNI).
  
  
  CONCLUSIONS
  Compared to the short-beam uniaxial test, the biaxial test detected more differences among composites and displayed less data scattering for two of the tested materials. Also, biaxial strength was closer to the material's strength estimated by FEA.
authors.affiliation: Department of Dental Materials, University of São Paulo, Av. Prof. Lineu Prestes, 2227, Cidade Universitária, 05508-000 São Paulo, SP, Brazil.
authors.forename: Barbara
authors.initials: B
authors.lastname: Pick
authors.forename: Josete B C
authors.initials: JB
authors.lastname: Meira
authors.forename: Larissa
authors.initials: L
authors.lastname: Driemeier
authors.forename: Roberto R
authors.initials: RR
authors.lastname: Braga
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry; D003188:Composite Resins / Q000737:chemistry*; D003764:Dental Materials / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation; D020342:Finite Element Analysis; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D008956:Models, Chemical; D018583:Pliability; D011140:Polyurethanes / Q000737:chemistry; D016008:Statistical Distributions; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature; D013718:Tensile Strength; D013997:Time Factors
chemical_list: D000180:Acrylic Resins; D003188:Composite Resins; C069352:Concept resin; D003764:Dental Materials; C444701:Filtek Z250; C513708:Heliomolar; D011140:Polyurethanes