subfigure above caption

Question

I want to have sub-figure exactly above the caption, but now I have it on the left side of the caption.

\begin{figure}[H]
\centering
\begin{subfigure}{0.45\textwidth}
\includegraphics[scale=0.7]{astmdowel}
\caption{} \label{fig:}
\end{subfigure}
\hspace*{0cm} % separation between the subfigures
\hfill
\begin{subfigure}{0.45\textwidth}
\includegraphics[scale=0.7]{BSdowel}
\caption{} \label{fig:}
\end{subfigure}
\caption{Specimen according to: a)ASTM D 5764 -97a b)BS EN 383:2007.} \label{fig:40}
\end{figure}

Thank you for your comments, I checked using fbox and the pictures don't have any extra space:

Here is working example:

\documentclass[12pt, a4paper]{report}
% ctrl+T comment, ctrl+U uncomment
\usepackage{tikz}
\usepackage{slashbox}
\usepackage{caption}
\usepackage{float}
\usepackage{enumerate}
\usepackage{bm}

\usepackage[export]{adjustbox}

\usepackage{multirow}
\usepackage{array}

\usepackage[labelformat=simple]{subcaption}
\renewcommand\thesubfigure{\alph{subfigure})}

\usepackage{fixltx2e}
\captionsetup[subfigure]{skip=1pt} %%global setting for subfigure
\usepackage{amsmath, siunitx} 



\usepackage{xcolor}
\usepackage{subcaption}
\usepackage{graphicx}
\usepackage[labelfont=it]{caption}
\usepackage[font=it]{caption}

%\usepackage{showframe}% http://ctan.org/pkg/showframe
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\makeatletter
\patchcmd{\@makechapterhead}{\vspace*{50\p@}}{}{}{}% Removes space above \chapter head
\patchcmd{\@makeschapterhead}{\vspace*{50\p@}}{}{}{}% Removes space above \chapter* head
\makeatother


\pretolerance=10000 % stop latex from braking the words
\tolerance=2000 % stop latex from braking the words
\emergencystretch=10pt % stop latex from braking the words
\usepackage{titlesec}
\usepackage{amsmath}
\usepackage{booktabs}
\usepackage{geometry}
 \geometry{a4paper,
bottom=25mm,
right=31.7mm,
 left=31.7mm,
 top=25mm,
 }
\DeclareMathSizes{10}{10}{10}{10}


\titleformat{\chapter}[display]
{\normalfont\huge\bfseries}{}{18pt}{\Huge}



\begin{document}

\begin{figure}[H]
\centering
\begin{subfigure}{0.45\textwidth}
\fbox{\includegraphics[scale=0.7]{astmdowel}}

\caption{} \label{fig:}
\end{subfigure}
\hspace*{0cm} % separation between the subfigures
\hfill
\begin{subfigure}{0.45\textwidth}
\fbox{\includegraphics[scale=0.7]{BSdowel}}
\caption{} \label{fig:}
\end{subfigure}
\caption{Specimen according to: a)ASTM D 5764 -97a b)BS EN 383:2007.} \label{fig:40}
\end{figure}

\end{document}

Answer 1

I can not reproduce your problem even with your original preamble.

Since in it are many packages as caption, subcaption loaded many times with non-consistent options I first clean up it, than comment all packages and your definitions which aren't necessary to show your image and than make caption and subcaptionoptions consistent.

In second step I make small changes in include graphics. Here I suggest to use instead option scale option width, which better handle centering of images on page (with it there is small offset - image are slightly shift to left).

Complete code is:

\documentclass[12pt, a4paper]{report}
% ctrl+T comment, ctrl+U uncomment
%\usepackage{tikz}
%\usepackage{slashbox}
%\usepackage{enumerate}
%\usepackage{bm}
%\usepackage[export]{adjustbox}

%\usepackage{array}
%\usepackage{booktabs}
%\usepackage{multirow}

%\usepackage{amsmath}
%\DeclareMathSizes{10}{10}{10}{10}

\usepackage{graphicx}
%\usepackage{float}
\usepackage[font=it]{caption}
\usepackage[labelformat=brace]{subcaption}
%\captionsetup[subfigure]{skip=1pt} %%global setting for subfigure
%\usepackage{siunitx}

%\usepackage{xcolor}

%\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
%\makeatletter
%\patchcmd{\@makechapterhead}{\vspace*{50\p@}}{}{}{}% Removes space above \chapter head
%\patchcmd{\@makeschapterhead}{\vspace*{50\p@}}{}{}{}% Removes space above \chapter* head
%\makeatother

%\pretolerance=10000 % stop latex from braking the words
%\tolerance=2000 % stop latex from braking the words
%\emergencystretch=10pt % stop latex from braking the words
%\usepackage{titlesec}

\usepackage{geometry}
\geometry{  a4paper,
            bottom=25mm,
            right=31.7mm,
            left=31.7mm,
            top=25mm,
            showframe% <-- for show page layout
            }

%\titleformat{\chapter}[display]
%{\normalfont\huge\bfseries}{}{18pt}{\Huge}


\begin{document}
    \begin{figure}%[htb]% <-- changed
    \centering
\begin{subfigure}{0.45\textwidth}% <-- changed
\includegraphics[width=\linewidth]{example-image-a}% <-- changed to example image
\caption{} \label{fig:}
\end{subfigure}
\hfil% <-- changed 
\begin{subfigure}{0.45\textwidth}
\includegraphics[width=\linewidth]{example-image-b}
\caption{} \label{fig:}
\end{subfigure}
\caption{Specimen according to: a)ASTM D 5764 -97a b)BS EN 383:2007.} \label{fig:40}
    \end{figure}
\end{document}

Result:

I suggest that first you test above MWE with your original images. if you not get the same positioned images, than you have something very strange in your images.