documentclass[preprint,12pt]{elsarticle}
%usepackage{hyperref}

journal{Test Journal}
begin{document}
begin{frontmatter}
title{The Title}
author{Prof. Big Ol' Wanker}
address{123 Happy Lane}
begin{abstract}
TEXT
end{abstract}
begin{keyword}
keyword 1 sep keyword 2 sep keyword 3
end{keyword}
end{frontmatter}

section{SECTION1TEST}
noindent cite: cite{Richards1931} \
citep: citep{Richards1931} \
citet: citet{Richards1931} \

bibliography{library}
bibliographystyle{elsarticle-num-names}

end{document}

@article{Richards1931,
author = {Richards, LA},
doi = {10.1063/1.1745010},
file = {:D$backslash$:/Papers/Richards - 1931 - Capillary Conduction of Liquids Through Porous Mediums - Journal of Applied Physics.pdf:pdf},
issn = {01486349},
journal = {Journal of Applied Physics},
pages = {318},
title = {{Capillary Conduction of Liquids Through Porous Mediums}},
url = {http://scitation.aip.org/content/aip/journal/jap/1/5/10.1063/1.1745010},
volume = {1},
year = {1931}
}

documentclass{elsarticle}

usepackage{hyperref}

makeatletter
providecommand{doi}[1]{%
  begingroup
    letbibinfo@secondoftwo
    urlstyle{rm}%
    href{http://dx.doi.org/#1}{%
      doi:discretionary{}{}{}%
      nolinkurl{#1}%
    }%
  endgroup
}
makeatother