Appendix chapter naming - titlesec/appendix clash

Question

I am using titlesec to apply formatting to my chapter and section headings. When I add an appendix to the document, the chapter name reads 'Chapter A', where as it should read 'Appendix A'

Running a \renewcommand after \begin{appendices} seems to have no effect. I believe there is a clash between titlesec and appendix, but I am not sure how to resolve it.

How can I redefine the appendix chapter name?

\documentclass[11pt,a4paper]{report}
\setlength{\headheight}{13.6pt}
\usepackage{lipsum}
\usepackage{fancyhdr}
\usepackage{titlesec}
\usepackage{appendix}

%Pagestyle setup
\pagestyle{fancyplain}
\fancyhf{}
\fancyfoot[R]{\thepage}
\fancyhead[R]{\fancyplain{}{\nouppercase\leftmark}}
\renewcommand{\headrulewidth}{0pt}

\usepackage{xpatch}
\xpatchcmd\chaptermark{. }{\ --}{}{\PatchFailed}

%Title formatting
\titleformat
{\chapter}[display]
    {\bfseries\LARGE}{Chapter \thechapter}{2ex}
    {
        \vspace{-0.5ex}
        \rule{\textwidth}{0.4pt}
        \centering
    }
    [
        \vspace{-1.5ex}
        \rule{\textwidth}{0.4pt}
        \vspace{-4ex} 
    ]

\begin{document}
\cleardoublepage
\pagenumbering{roman}
\chapter*{Abstract}
\addcontentsline{toc}{chapter}{Abstract}
\pagestyle{plain}
\lipsum

\appendixtitleon
\appendixtitletocon
\tableofcontents

\cleardoublepage
\pagenumbering{arabic}
\pagestyle{fancy}
\chapter{Introduction}
\lipsum

\clearpage
\pagestyle{plain}
\begin{appendices}
\renewcommand\chaptername{Appendix} %Has no effect
\renewcommand\appendixname{Appendix} %Also has no effect
\renewcommand\chaptertitlename{Appendix} %From titlesec documentation. Again, no effect
\chapter{The first appendix} 
\lipsum{}
\end{appendices}
\end{document}

Answer 1

If you want \renewcommand\chaptertitlename{...} to have an effect, you may want to build it in the definition of your custom chapter format.

\documentclass[11pt,a4paper]{report}
\setlength{\headheight}{13.6pt}
\usepackage{lipsum}
\usepackage{fancyhdr}
\usepackage{titlesec}
\usepackage{appendix}

%Pagestyle setup
\pagestyle{fancyplain}
\fancyhf{}
\fancyfoot[R]{\thepage}
\fancyhead[R]{\fancyplain{}{\nouppercase\leftmark}}
\renewcommand{\headrulewidth}{0pt}

\usepackage{xpatch}
\xpatchcmd\chaptermark{. }{\ --}{}{\PatchFailed}
\renewcommand\chaptertitlename{Chapter}
%Title formatting
\titleformat
{\chapter}[display]
    {\bfseries\LARGE}{\chaptertitlename\ \thechapter}{2ex}
    {
        \vspace{-0.5ex}
        \rule{\textwidth}{0.4pt}
        \centering
    }
    [
        \vspace{-1.5ex}
        \rule{\textwidth}{0.4pt}
        \vspace{-4ex} 
    ]

\begin{document}
\cleardoublepage
\pagenumbering{roman}
\chapter*{Abstract}
\addcontentsline{toc}{chapter}{Abstract}
\pagestyle{plain}
\lipsum

\appendixtitleon
\appendixtitletocon
\tableofcontents

\cleardoublepage
\pagenumbering{arabic}
\pagestyle{fancy}
\chapter{Introduction}
\lipsum

\clearpage
\pagestyle{plain}
\begin{appendices}
\renewcommand\chaptertitlename{Appendix} %From titlesec documentation. Again, no effect
\chapter{The first appendix} 
\lipsum{}
\end{appendices}
\end{document}