function modify_INP(infile, outfile, new_E)
    % Open input file
    fid_in = fopen(infile, 'r');
    if fid_in == -1
        error('Could not open input file.');
    end
    
    % Open output file
    fid_out = fopen(outfile, 'w');
    if fid_out == -1
        fclose(fid_in);
        error('Could not create output file.');
    end
    
    % Read input file line by line
    while ~feof(fid_in)
        line = fgetl(fid_in);
        
        % Check for material definition lines and modify E1 and E2 and E3
        if contains(char(line), '0.1, 0.49') && ~isnan(new_E)
            fprintf(fid_out, '%f, 0.49\n',new_E);
        else
            fprintf(fid_out, '%s\n', line);
        end
    end