Here's a juxtaposition of three possible solutions: with tabularx, with tabular, and with tabular*. Since there's no need for automatic line breaks in any cells, using tabularx seems like overkill. You should choose between either the tabular and the tabular* solution.
What I would really recommend you do (and what's implemented in all three solutions below) is to use the line-drawing macros of the booktabs package instead of \hline and \cline. I would also provide a bit more structure to the header material, say by placing information about units of measurement on lines of their own. Finally, consider using the \SI and \si macros of the siunitx package to typeset scientific units (with and without numbers).

\documentclass{article}
\usepackage[letterpaper,margin=1in]{geometry} % choose page parameters suitably
\usepackage{tabularx,ragged2e,siunitx,booktabs}
\newcolumntype{C}{>{\Centering\arraybackslash}X}
\usepackage[skip=0.333\baselineskip]{caption} % optional
\begin{document}
\begin{table}
\caption{Table with \texttt{tabularx}} \label{tab:tabularx}
\begin{tabularx}{\linewidth}{@{} l *{6}{C}@{}}
\toprule
Material & \multicolumn{6}{c@{}}{Test} \\
\cmidrule(l){2-7}
& \multicolumn{2}{c}{Impact at \SI{10}{\joule}}
& \multicolumn{2}{c}{Impact at \SI{14}{\joule}}
& \multicolumn{2}{c}{QSI} \\
\cmidrule(lr){2-3} \cmidrule(lr){4-5} \cmidrule(l){6-7}
& $F_i$ & Load drop & $F_i$ & Load drop & $F_Q$ & Load drop \\
& (\si{\kilo\newton}) & (\%) & (\si{\kilo\newton}) & (\%) & (\si{\kilo\newton}) & (\%) \\
\midrule
LTHIN\textsubscript{LVI} & 3.24 $\pm$ 0.02 & 19.75 $\pm$ 0.55 & 3.41 $\pm$ 0.25 & 22.80 $\pm$ 1.40 & 3.32 & Multiple \\
LV1\textsubscript{LVI} & 3.19 $\pm$ 0.04 & 14.64 $\pm$ 2.55 & 3.24 $\pm$ 0.22 & 11.34 $\pm$ 4.46 & 3.10 & Multiple \\
LV2\textsubscript{LVI} & 3.51 $\pm$ 0.15 & 8.17 $\pm$ 2.36 & 3.95 $\pm$ 0.51 & 10.80 $\pm$ 1.28 & None & None \\
\bottomrule
\end{tabularx}
\bigskip\bigskip
\caption{Table with \texttt{tabular}} \label{tab:tabular}
\centering
\begin{tabular}{@{} l *{6}{c} @{}}
\toprule
Material & \multicolumn{6}{c@{}}{Test} \\
\cmidrule(l){2-7}
& \multicolumn{2}{c}{Impact at \SI{10}{\joule}}
& \multicolumn{2}{c}{Impact at \SI{14}{\joule}}
& \multicolumn{2}{c}{QSI} \\
\cmidrule(lr){2-3} \cmidrule(lr){4-5} \cmidrule(l){6-7}
& $F_i$ & Load drop & $F_i$ & Load drop & $F_Q$ & Load drop \\
& (\si{\kilo\newton}) & (\%) & (\si{\kilo\newton}) & (\%) & (\si{\kilo\newton}) & (\%) \\
\midrule
LTHIN\textsubscript{LVI} & 3.24 $\pm$ 0.02 & 19.75 $\pm$ 0.55 & 3.41 $\pm$ 0.25 & 22.80 $\pm$ 1.40 & 3.32 & Multiple \\
LV1\textsubscript{LVI} & 3.19 $\pm$ 0.04 & 14.64 $\pm$ 2.55 & 3.24 $\pm$ 0.22 & 11.34 $\pm$ 4.46 & 3.10 & Multiple \\
LV2\textsubscript{LVI} & 3.51 $\pm$ 0.15 & 8.17 $\pm$ 2.36 & 3.95 $\pm$ 0.51 & 10.80 $\pm$ 1.28 & None & None \\
\bottomrule
\end{tabular}
\bigskip\bigskip
\caption{Table with \texttt{tabular*}} \label{tab:tabular*}
\setlength\tabcolsep{0pt}
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}} l *{6}{c}}
\toprule
Material & \multicolumn{6}{c}{Test} \\
\cmidrule(l){2-7}
& \multicolumn{2}{c}{Impact at \SI{10}{\joule}}
& \multicolumn{2}{c}{Impact at \SI{14}{\joule}}
& \multicolumn{2}{c}{QSI} \\
\cmidrule{2-3} \cmidrule{4-5} \cmidrule{6-7}
& $F_i$ & Load drop & $F_i$ & Load drop & $F_Q$ & Load drop \\
& (\si{\kilo\newton}) & (\%) & (\si{\kilo\newton}) & (\%) & (\si{\kilo\newton}) & (\%) \\
\midrule
LTHIN\textsubscript{LVI} & 3.24 $\pm$ 0.02 & 19.75 $\pm$ 0.55 & 3.41 $\pm$ 0.25 & 22.80 $\pm$ 1.40 & 3.32 & Multiple \\
LV1\textsubscript{LVI} & 3.19 $\pm$ 0.04 & 14.64 $\pm$ 2.55 & 3.24 $\pm$ 0.22 & 11.34 $\pm$ 4.46 & 3.10 & Multiple \\
LV2\textsubscript{LVI} & 3.51 $\pm$ 0.15 & 8.17 $\pm$ 2.36 & 3.95 $\pm$ 0.51 & 10.80 $\pm$ 1.28 & None & None \\
\bottomrule
\end{tabular*}
\end{table}
\end{document}
tabularxif you have noXcolumns? This table looks like it would be fine without usingtabularxat all. Also you should be usingbooktabsfor its rules. (And it's helpful to post compilable documents on the site, instead of code fragments.) – Alan Munn Apr 21 '19 at 17:27