title: Fracture toughness and microhardness of a composite: do they correlate?
abstract: OBJECTIVE
  Chipping and bulk fracture are major contributors in clinical failures of composite restorations. Fracture toughness (K(Ic)) quantifies susceptibility for fracture, but experimental determination is complicated. It would be beneficial for the dental community if a relatively simple experiment, such as microhardness (HK), could be used to screen composites for fracture resistance. This study explores a possible correlation between K(Ic) and HK.
  
  
  METHODS
  Composite cylinders (4mm diameter and approximately 7 mm long) were cured for five combinations of light intensity (I, microm W/cm(2)) and curing time (T, s) to achieve a range of different total light energy densities (I x T=100 x 10, 100 x 20, 300 x 20, 300 x 40, and 700 x 60 microm W s/cm(2)). A chevron-notch was cut in the median plane of the cylinders for the fracture toughness test, which was executed in a displacement control mode at 6 micro m/s cross-head speed (sample size 4). Knoop hardness was determined at the median plane of the cylinders (sample size 6). The tests were performed 15 min and 24h after curing.
  
  
  RESULTS
  Both the K(Ic) and HK increased with increased light energy density and storage time. Linear regression analysis indicated a strong correlation between HK and K(Ic) tested at the same time period (R(2)=0.97 and 0.90 for 15 min and 24h, respectively). The correlation became weaker between the different storage times (R(2)=0.71), indicating a change in fracture toughness and/or microhardness mechanisms.
  
  
  CONCLUSIONS
  Fracture toughness of a composite cannot be simply extrapolated from microhardness.
authors.affiliation: Department of Operative Dentistry, Chulalongkorn University, Bangkok, Thailand
authors.forename: D
authors.initials: D
authors.lastname: Tantbirojn
authors.forename: A
authors.initials: A
authors.lastname: Versluis
authors.forename: Y-S
authors.initials: YS
authors.lastname: Cheng
authors.forename: W H
authors.initials: WH
authors.lastname: Douglas
mesh_terms: D000465:Algorithms; D003188:Composite Resins / Q000737:chemistry* / Q000528:radiation effects; D055117:Curing Lights, Dental; D003764:Dental Materials / Q000737:chemistry* / Q000528:radiation effects; D003799:Dental Stress Analysis / Q000295:instrumentation; D006244:Hardness; D006801:Humans; D008027:Light; D008422:Materials Testing; D012822:Silicon Dioxide / Q000737:chemistry / Q000528:radiation effects; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D015040:Zirconium / Q000737:chemistry / Q000528:radiation effects
chemical_list: D003188:Composite Resins; D003764:Dental Materials; C083568:Z100 composite resin; D012822:Silicon Dioxide; D015040:Zirconium