What are some of the most important things to consider when impact testing?

Let's discuss this.

锤击试验时，要考虑的最重要的事情是什么？

我们来讨论一下这个问题。

This is a very good question. The most important considerations can be broken down into those that are impact related and those that are response related. The excitation concerns are numerous. Only issues pertaining to hammer tip, trigger delay and double impacts are discussed here. However, other issues related to overload/underload of the analog to digital converter, poor utilization of the digitizer, and difficulties with testing nonlinear structures, are some additional concerns (but are not addressed in this article). The response concerns lie with the signal decay and the need for windows to minimize leakage. Let's first discuss the excitation issues and then the response issues.

这个问题问得很好。最重要的考虑因素可以划分为与锤击相关的以及与响应相关的。与激励相关的事项非常多。此处只讨论与锤头、触发延迟以及双击相关的事项。但是，关于模数转换器过载/欠载、数字化仪的不当使用、以及测试非线性结构的难点等其它事项是另外一些需要考虑的事项（但本文不做讲解）。关于响应方面的注意事项为信号延迟、加窗的必要性以减少泄漏。我们先讨论激励方面的问题，接下来再讨论响应方面的问题。

 

First of all, the hammer tip is largely responsible for the frequency spectrum that is excited. In general, the harder the tip, the wider the frequency range that is excited. The hammer tips typically used range from rubber to metal on the extremes with various intermediate tips such as the soft plastic, hard plastic, eraser, etc. Each of these tips is designed to have a certain amount of elastic deformation during impact. The total time duration of the tip impact is directly related to the corresponding frequency range that is excited. Generally, the shorter the length of the time pulse, the wider the frequency range that is excited (Figure 1 shows some typical tips).

首先在很大程度上力锤锤头决定了所激起的频谱。总的来讲，锤头越硬，激起的频率范围越宽。通常所用锤头的两个极端是橡胶锤头和金属锤头，还有各式各样的介于二者之间的锤头，如软塑料、硬塑料、橡皮锤头，等等。在锤击过程中所设计的每种锤头具有一定程度的弹性变形。锤头冲击的总时间段与激起的相应频率范围直接相关。一般来讲，时间脉冲长度越短，激起的频率范围越宽（图1展示了几种典型的锤头）。

 

While this is generally the case, often the local flexibility of the structure can play an important part of the total time of the impact and therefore can have an effect on the force spectrum imparted to the structure. You may have noticed this when testing structures that have dramatically varying stiffnesses throughout the structure. When impacting a stiff region, one input frequency spectrum is observed and a much different, narrower frequency range is excited when impacting more flexible regions. (The published impact tip frequency spectrum provided by the hammer manufacturer does not include any of the local structure flexibility effects.) Be careful !!!

尽管情况通常如此，但结构上的局部柔性常常会对冲击总时间产生重要的作用，于是会对施加于结构上的力谱产生影响。当对具有显著刚度变化的结构进行测试时，你或许已经注意到了这个问题。当敲击一个硬区域时，可以观察到一个输入谱，而当敲击一个更软区域时，可以激起一个非常不同的更窄的频率范围。（由力锤制造商提供的锤头频谱没有包括任何局部结构柔性影响。）务必注意！！！

Another somewhat important test consideration for impact testing relates to the need for pretrigger delay. Since the measurement initiation is controlled by the leading slope of the pulse, part of the pulse is lost and the resulting spectrum is distorted unless some pretrigger delay is specified. The effect of spectrum distortion is shown in Figure 2. The red pulse and resulting frequency spectrum are clearly different than the correct blue pulse and resulting frequency spectrum with pretrigger delay. This can cause some input frequency spectrum distortion which will have an effect on the computed FRF.

锤击试验另一个比较重要的测试注意事项是关于预触发延迟必要性的。因为测量的引发是由脉冲的前导沿控制的，除非设置一定的预触发延迟，否则会丢失部分脉冲、造成频谱失真。频谱失真的影响如图2所示。红色脉冲及所得频谱与具有预触发延迟的正确的蓝色脉冲及所得频谱显然是不同的。这会引起一定的输入频谱失真，对计算得到的频响产生影响。

Typically, a pretrigger delay of 1% to 5% of the time window is sufficient to eliminate this effect. Care must be exercised when specifying this delay since some FFT analyzers use a plus (+) delay for pretrigger delay while others use a minus (-) delay for pretrigger delay. This causes a totally incorrect frequency spectrum if not applied correctly. Check your time pulse to assure that the entire pulse is captured in the time signal. In addition, some analyzers use a percentage of the block whereas others use an absolute time value in seconds. If absolute time is used then this can cause problems especially when changing bandwidths during test setup. Be careful !!!

通常，1%～5%时间窗的预触发延迟足以消除这种影响。当设置这个延迟时，必须要小心仔细，因为有些FFT分析仪用正（+）延迟表示预触发，而另一些则用负（-）延迟表示预触发延迟。如果使用不当，会导致完全错误的频谱。检查时域脉冲来确信在时域信号中捕捉到了完整的脉冲。另外，有些分析仪利用块百分比，而另一些则是用以秒表示的绝对时间值。如果用了绝对时间，则可能会引起问题，特别是在试验设置过程中改变带宽时。务必要小心！！！

 

Another annoying impact testing problem is the double impact. The double impact generally causes a non-uniform, non-flat input force spectrum. Two typical double impacts are shown in Figure 3. The "ripple" in the spectrum is not desirable especially if the force spectrum dips substantially. A drop of 30 dB or more is cause for concern especially if it occurs at a resonant peak - and it often does.

双击是另一个令人厌烦的锤击测试问题。双击通常引起非一致、非平坦的输入力谱。两种典型的双击如图3所示。谱上的“纹波”是我们所不期望的，如果力谱显著降落尤甚。降落30dB或者更多要引起注意，特别是如果它发生在共振峰的位置 — 并且这经常会发生。

The reason for the double impact is generally from two possibilities. First, many double impacts occur due to new or inexperienced impact testers. It takes some time to get accustomed to swinging the hammer - it is a much different technique than driving nails! But even with experience, sometimes a double impact is unavoidable. Often, with lightly damped structures, the response of the structure is so fast that the hammer can not move away from the structure due to the response of the structure. In these cases, double impacts are unavoidable. The problem is that often the impact spectrum will have significant drop out at the major resonances of the structure. This can produce undesirable effects and must be avoided. One possible technique to overcome the double impact problem is to use the principle of reciprocity. The impact and response locations can be swapped thereby eliminating the double impact problem. This can often solve the problem but many times mass loading effects can become an separate issue.

造成双击的原因通常有两种可能。第一，很多双击是由锤击试验的新手或者不熟练人员造成的。需要花点时间来熟悉挥动力锤 — 它是一个不同于楔钉子的技术活！但是即使有经验，有时双击也不可避免。对于小阻尼结构，结构的响应是如此之快以至于力锤常常来不及从结构上移开。这种情况下，双击不可避免。问题在于，在结构的主要共振频率上锤击力谱常常具有显著的降落。这会带来我们所不期望的影响，务必要避免。克服双击问题的一种可能技术是利用互易性原理。锤击位置和响应位置互换，以避免双击问题。这通常能解决问题，但很多时候，附加质量影响会成为一个单独的问题。

 

The last major concern relates to the response and the need for the exponential window. The response of the system may not decay to zero within the sample interval of the FFT. When this is the case, then leakage can occur unless a window is used. The most appropriate window is the exponential window but should only be used when necessary. Many times the window is not necessary if the signal naturally decays within the sample interval.

最后一个主要的关心问题是关于响应、加指数窗必要性的。在FFT的采样时间段内系统的响应可能没有衰减到零。当出现这种情况时，除非使用窗函数，否则会发生泄漏。最合适的窗函数是指数窗，但只有在必要时才可以使用。很多时候，在采样时间段内，如果信号自然衰减到零，则没有必要加窗函数。

 

 

Often, the data acquisition system can be setup to allow this to happen. Two signal processing parameters should always be explored before using the window. The bandwidth selection can be changed which has a direct effect on the total time required to capture data. If you halve the bandwidth, you double the time sample. Another approach to increase the total time of the sample interval is to change the total number of samples for the acquisition. Both of these two signal processing parameters allow more time data to be collected and should always be explored prior to the use of the window. However, a window may still be required to minimize the effects of leakage if the signal does not die out by the end of the sample interval. Figure 4 shows one time signal in blue that requires an exponential window whereas the red time signal and sample interval does not require a window (or at least substantially reduces the need for the window).

数据采集系统常常可以设置为允许这种情况发生。在加窗之前，总应该探讨两个信号处理参数。可以改变带宽选择，它对采集数据所需的总时间有直接的影响。如果带宽减半，则时间加倍。另一种增加采样总时间的办法是改变采集的总样本点数。这两种信号处理参数允许采集更多的时域数据，并且在用窗函数之前，总应该先试一下。但是如果信号在采样时间段的尾部没有衰减完，则仍然需要一个窗函数来减少泄漏的影响。图4中所示，蓝色的时域信号需要指数窗，而红色的时域信号和采样时间段不需要窗函数（或者说，至少很大程度上减少了加窗的必要性）。

I hope this helps to explain some of the more important concerns when impact testing. If you have any other questions about modal analysis, just ask me.

我希望这有助于解释锤击试验时的某些重要的关心事项。如果你有关于模态分析的任何其他问题，尽管问我好了。