StoryBoard.Stop方法为什么无反映

Question

<Window x:Class="wpfShopping.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="650" Width="398" Loaded="Window_Loaded" MouseLeftButtonDown="Window_MouseLeftButtonDown">
    <Canvas>

        <StackPanel Canvas.Top="0" x:Name="sp1" Orientation="Horizontal">
            <StackPanel.RenderTransform>
                <TransformGroup>
                    <TranslateTransform X="0" Y="0"/>
                </TransformGroup>
            </StackPanel.RenderTransform>
            <Image Source="icons/1.ico"></Image>
            <Image Source="icons/2.ico"></Image>
            <Image Source="icons/4.ico"></Image>
        </StackPanel>
        <StackPanel Canvas.Top="125" x:Name="sp2" Orientation="Horizontal">
            <StackPanel.RenderTransform>
                <TransformGroup>
                    <TranslateTransform X="0" Y="0"/>
                </TransformGroup>
            </StackPanel.RenderTransform>
            <Image Source="icons/52.ico"></Image>
            <Image Source="icons/53.ico"></Image>
            <Image Source="icons/54.ico"></Image>
        </StackPanel>
        <StackPanel Canvas.Top="250" x:Name="sp3" Orientation="Horizontal">
            <StackPanel.RenderTransform>
                <TransformGroup>
                    <TranslateTransform X="0" Y="0"/>
                </TransformGroup>
            </StackPanel.RenderTransform>
            <Image Source="icons/11.ico"></Image>
            <Image Source="icons/12.ico"></Image>
            <Image Source="icons/13.ico"></Image>
        </StackPanel>
        <StackPanel Canvas.Top="375" x:Name="sp4" Orientation="Horizontal">
            <StackPanel.RenderTransform>
                <TransformGroup>
                    <TranslateTransform X="0" Y="0"/>
                </TransformGroup>
            </StackPanel.RenderTransform>
            <Image Source="icons/s1.ico"></Image>
            <Image Source="icons/s2.ico"></Image>
            <Image Source="icons/s3.ico"></Image>
        </StackPanel>
        <Button x:Name="btntest" Content="stop" Click="btntest_Click"></Button>
    </Canvas>
</Window>
cs：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using System.Windows.Media.Animation;

namespace wpfShopping
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        Storyboard sb = new Storyboard();
        public MainWindow()
        {
            InitializeComponent();
            NameScope.SetNameScope(this, new NameScope());
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {

        }

        private void Window_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {

            StackPanel sp = (e.Source as Image).Parent as StackPanel;
            CreateAnimation(sp);

        }
        private void CreateAnimation(StackPanel sp)
        {

            DoubleAnimation da = new DoubleAnimation();
            da.From = 0; da.To = 200;
            da.Duration = TimeSpan.FromSeconds(5);

            Storyboard sb = new Storyboard();
            //sb.RepeatBehavior = RepeatBehavior.Forever;
            //sb.AutoReverse = true;
            sb.Children.Add(da);

            this.RegisterName(sp.Name, sp);
            Storyboard.SetTargetName(da, sp.Name);
            Storyboard.SetTargetProperty(da, new PropertyPath("(UIElement.RenderTransform).(TransformGroup.Children)[0].(TranslateTransform.X)"));

            try
            {

                sb.Begin(this, true);
                // sb.GetCurrentState(sp1);
                // sb.Stop(sp1);
            }
            catch (Exception ex)
            {
                //MessageBox.Show(ex.Message);
            }

        }

        void sb_CurrentTimeInvalidated(object sender, EventArgs e)
        {
            // string str = sender.ToString();

        }

        private void btntest_Click(object sender, RoutedEventArgs e)
        {
            sb.Stop(this);
        }
    }
}
点击Stop为什么无反应啊，请教哥们姐们！