<section class="expandable-onClick">
    <div class="element">
        <h2>tytul</h2>
    </div>
    <p class="excerpt">excerpt</p>
</section>


<iframe class="thebox"></iframe>


$( ".expandable-onClick .element" ).click(function() {
    var urlTitle = $( this ).children('h2').text();
    var url ="partials/main/" + urlTitle + ".html";
    $( this ).parent().toggleClass("expanded");
    $( this ).parent().append($('<iframe class="thebox"></iframe>'));
    $('.thebox').attr( "src", encodeURIComponent( url ));

});