canti <- function(xx, w=4, t=2)
{
  ##########################################################################
  #
  # CANTILEVER BEAM FUNCTION
  #
  # Authors: Sonja Surjanovic, Simon Fraser University
  #          Derek Bingham, Simon Fraser University
  # Questions/Comments: Please email Derek Bingham at dbingham@stat.sfu.ca.
  #
  # Copyright 2013. Derek Bingham, Simon Fraser University.
  #
  # THERE IS NO WARRANTY, EXPRESS OR IMPLIED. WE DO NOT ASSUME ANY LIABILITY
  # FOR THE USE OF THIS SOFTWARE.  If software is modified to produce
  # derivative works, such modified software should be clearly marked.
  # Additionally, this program is free software; you can redistribute it 
  # and/or modify it under the terms of the GNU General Public License as 
  # published by the Free Software Foundation; version 2.0 of the License. 
  # Accordingly, this program is distributed in the hope that it will be 
  # useful, but WITHOUT ANY WARRANTY; without even the implied warranty 
  # of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 
  # General Public License for more details.
  #
  # For function details and reference information, see:
  # http://www.sfu.ca/~ssurjano/
  #
  ##########################################################################
  #
  # OUTPUTS AND INPUTS:
  #
  # y  = c(D, S), where D = displacement and S = stress
  # xx = c(R, E, X, Y)
  # w  = width (optional)
  # t  = thickness (optional)
  #
  #########################################################################
  
  R <- xx[1]
  E <- xx[2]
  X <- xx[3]
  Y <- xx[4]
  
  L   <- 100
  D_0 <- 2.2535
  
  Sterm1 <- 600*Y / (w*(t^2))
  Sterm2 <- 600*X / ((w^2)*t)
  
  S <- Sterm1 + Sterm2 
  
  Dfact1 <- 4*(L^3) / (E*w*t)
  Dfact2 <- sqrt((Y/(t^2))^2 + (X/(w^2))^2)
  
  D <- Dfact1 * Dfact2
  
  y <- c(D, S)
  return(y)
}