用提交按钮提交form问题验证没通过依然提交表单

<form id="sb" action="${pageContext.request.contextPath }/admin/loanContract/addOrUpdate.do" method="post" onsubmit="submitTest();">
<div class="j_dd">
            <div class="clearfix">
            <span class="fTit left">交易订单号：</span><input type="text" class="txt left ui-autocomplete-input" id="tradeCode" name="tradeCode" onkeyup="value=value.replace(/[W]/g,'') "onbeforepaste="clipboardData.setData('text',clipboardData.getData('text').replace(/[^d]/g,''))" style="300px;"/>*<span style="color:red; display:block; padding-left:85px; clear:both; padding-top:5px;" id="sp" name="sp"></span>
            </div>
            <div class="clearfix" style="margin-top:10px">
                <span class="fTit left">备注：</span>
                <textarea id="tradeIntro" name="tradeIntro" style="305px;"></textarea>
            </div>

            <div class="bigBox clearfix">
               <button class="tjbtn"  type="submit" id="sbm">提 交</button>
               <a href="${pageContext.request.contextPath }/admin/loanContract/loanItem.do?contents=platform" title="" class="tjbtn">取 消</a>
            </div>
</form>

<script type="text/javascript">
    // Ajax校验数据库该交易订单是否已存在
    var orderInfoConfirm = {
    modifyOrderInfo : function(){        
    $("#tradeCode").blur(function(){
        
        var tradeCode = $("#tradeCode").val();
        if(tradeCode ==""){
            $("#sp").html("请输入交易订单号!");
            return;
        }
        
        $.ajax({
            url:"checkTradeCode.do",
            data:{tradeCode:tradeCode},
            dataType:"json",
            success:function(data){
                var arr = eval(data);
                $("#sp").html(arr.code);
            }
        });
        
    });
    },
    submitTest : function(){
    $("#sbm").click(function(){    
        if(tradeCode ==""){
            $("#sp").html("请输入交易订单号!");
            return false;
        }
        var sp = $("#sp").html();     
        if(sp == "交易订单已存在！"||sp == "请输入交易订单号!"){    
            return false;
        }
        else return true;
    });
    }
    };



    $(document).ready(function() {
        orderInfoConfirm.modifyOrderInfo();
        orderInfoConfirm.submitTest();
    });
    
    
</script>