结论：冲击的判定是「有没有失效」，不是「有没有受伤」
冲击试验的判定通常看的是结构有没有发生影响使用的破坏。样品在冲击后外观完好、功能正常，这一项就算通过。
但通过不等于没有受损。冲击载荷可能在材料内部、焊缝根部或者连接界面留下肉眼看不到的损伤，这些损伤在冲击这一项里不构成失效，却会在随后的疲劳试验中成为裂纹的起始点。
这就是为什么按标准顺序执行时，疲劳试验的结果会比用全新样品做的结果差——差的那部分，正是冲击留下的隐性损伤。
隐性损伤的常见形态
| 形态 | 发生位置 | 外观是否可见 |
|---|---|---|
| 微裂纹 | 焊缝根部、截面突变处 | 通常不可见 |
| 局部塑性变形 | 受冲击点附近、管件壁面 | 轻微时不易察觉 |
| 连接松动 | 螺栓连接、铆接部位 | 需要检查扭矩才能发现 |
| 涂层开裂 | 弯曲部位、焊缝附近 | 可见，但常被当成外观问题 |
| 界面脱粘 | 复合材料、粘接结构 | 不可见 |
第四行值得单独说。涂层开裂经常被当成外观瑕疵处理，但它往往是基材已经发生塑性变形的外在表现——涂层比基材脆，基材变形到一定程度涂层才会裂。 所以冲击后发现涂层开裂，应当检查下方的基材状态，而不是简单归为表面问题。
第五行在碳纤维车架上尤其需要关注。复合材料的层间脱粘从外面完全看不出来，但承载能力已经下降。金属件挨一下可能只是凹一块，复合材料挨一下可能内部已经分层。
怎么在试验中尽量发现
实验室能做的有限，但有几件事是可行的：
冲击前后的对比测量。 记录关键尺寸和几何关系，冲击后重新测量。如果出现超出预期的变化，说明发生了塑性变形。
紧固件扭矩复查。 冲击后逐个检查关键螺栓的扭矩，松动往往是连接界面受损的信号。
功能性复查。 折叠、调节、制动这些机构在冲击后逐一操作一遍，感受有没有异常的卡顿或旷量。
涂层与表面状态记录。 拍照记录冲击点附近的表面状态，为后续判断留证据。
这些做法都不需要专用设备，但需要在试验方案里事先写明，否则试验人员不会额外做。
对后续试验安排的影响
标准规定的试验顺序是有依据的，不应当随意调整。但有两种情形值得注意：
一是摸底阶段可以拆开做。 摸底的目的是定位问题，用全新样品单独做疲劳，能更清楚地看出结构本身的疲劳能力，排除冲击的干扰。拿到这个基准之后，再按标准顺序做一遍，两者的差值就反映了结构对冲击损伤的敏感程度。
二是整改验证时要按完整顺序。 局部补强之后只做疲劳、不做前面的冲击，得到的数据是偏乐观的，不能用来证明整改有效。
设计上能做什么
对冲击损伤敏感的结构，通常有几个共同特征：截面变化突然、焊缝位于受力集中区、连接方式依赖单一紧固点、材料韧性不足。
改善方向相应地也清楚：增加过渡圆角让应力分布更平缓；把焊缝布置在应力较低的位置；关键连接采用冗余设计；在韧性和强度之间取一个平衡，而不是一味追求强度指标。
最后一点常被忽略。 强度高的材料未必韧性好，而冲击工况恰恰考验韧性。选材时只看抗拉强度，冲击表现可能反而变差。
隐性损伤与售后失效的关系
实际售后中出现的结构失效，相当一部分不是一次性破坏，而是累积损伤的结果：产品在使用中经历过一次较大的磕碰，当时看起来没事，若干个月后在正常使用中断裂。
这个过程和试验里冲击加疲劳的顺序是同一个机制。理解了这一点，就能理解为什么标准要把冲击放在疲劳之前——它模拟的正是真实使用中损伤累积的过程。
对企业的实际意义是：售后收到结构失效的反馈时，不要只看断裂位置，还要了解产品有没有经历过异常冲击。如果有，问题可能出在结构对冲击损伤的容忍度上，而不是常规强度不足。
说明书中的使用提示
对冲击损伤敏感的产品，说明书里应当有相应提示：遇到明显磕碰后建议检查哪些部位、发现什么现象应当停止使用并联系售后。
这一条对碳纤维等复合材料产品尤其重要，因为使用者无法从外观判断内部是否分层。金属件变形了能看见，复合材料损伤了看不见——使用者需要被告知这个区别，否则会按金属件的经验去判断，继续使用已经受损的产品。
复合材料产品的额外考虑
碳纤维等复合材料在轮椅上用得越来越多，它的损伤特性与金属有本质差别，检测安排上要相应调整。
金属件的损伤通常表现为可见的变形或裂纹，复合材料的主要损伤形式是层间分层，从外观完全看不出来。这意味着冲击之后的常规目视检查对复合材料几乎无效。
可行的补充手段包括：敲击法做初步筛查、对关键部位做超声或其他无损检测、或者在冲击后直接进行承载能力的对比测试。具体用哪种，取决于结构形式和成本承受度。但无论哪种，都不应当只靠目视就判定复合材料件冲击后完好。
我们的做法
我们执行冲击项目时，会在冲击前后做一次对比记录，包括关键尺寸、紧固件状态和表面情况。这些记录本身不改变冲击项的判定结论，但当后续疲劳出现异常时，它们是判断原因的依据。
如果委托方有摸底需求，我们会建议把疲劳分两种条件各做一次：一次用全新样品，一次按标准顺序在冲击之后做。两组数据的差异能直接说明结构对冲击损伤有多敏感，这个信息对设计迭代很有用，而单做一种条件是得不到的。
有需要可以把结构方案发过来先判断敏感位置，或者直接联系：132 4819 8029。检测能力见服务介绍与电动轮椅检测，标准信息见标准查询，案例见案例。
English version
Conclusion: impact testing judges failure, not injury
An impact test is normally judged on whether damage occurred that affects use. If the sample looks intact and functions normally afterwards, the item passes.
Passing is not the same as being undamaged. Impact loading can leave damage inside the material, at weld roots or at joint interfaces that is invisible to the eye. It does not constitute failure of the impact item, but it becomes the initiation site for cracks in the fatigue test that follows.
This is why, when the prescribed sequence is followed, fatigue results are worse than those obtained on fresh samples. The difference is exactly the hidden damage left by impact.
Common forms of hidden damage
Microcracks form at weld roots and abrupt section changes and are generally invisible. Local plastic deformation occurs near the impact point and on tube walls, and is hard to notice when slight. Joint loosening at bolted or riveted connections is only found by checking torque. Coating cracking near bends and welds is visible but is commonly dismissed as a cosmetic matter. Interface debonding in composite or adhesive-bonded structures is invisible.
Coating cracking deserves separate mention. It is frequently treated as a surface blemish, but it is often the outward sign that the substrate beneath has already deformed plastically: the coating is more brittle than the substrate and cracks only once the substrate has deformed appreciably. Coating cracks after impact should prompt examination of the substrate rather than being written off as surface damage.
Debonding matters particularly for carbon composite frames. Interlaminar separation cannot be seen from outside, yet load-carrying capacity is already reduced. A metal part struck once may simply be dented; a composite part struck once may already be delaminated internally.
What can be found during testing
What a laboratory can detect is limited, but several things are practical.
Measure key dimensions and geometric relationships before and after impact. Changes beyond expectation indicate plastic deformation. Recheck the torque of key fasteners after impact; loosening is often a sign of damage at the joint. Operate every folding, adjustment and braking mechanism after impact and feel for unusual stiffness or free play. Photograph the surface condition around the impact point to preserve evidence for later judgement.
None of these requires special equipment, but they must be written into the test plan in advance, otherwise they will not be done.
Effect on the sequence of later tests
The prescribed test sequence exists for a reason and should not be rearranged casually. Two situations are worth noting.
During preliminary testing the items can be separated. The purpose there is to locate problems, and running fatigue on fresh samples shows the structure's inherent fatigue capacity without the influence of impact. With that baseline established, running the prescribed sequence afterwards gives a difference that indicates how sensitive the structure is to impact damage.
When verifying corrective action, the full sequence must be followed. Running only fatigue after local reinforcement, skipping the preceding impact, produces optimistic data that cannot demonstrate the fix works.
What can be done in design
Structures sensitive to impact damage tend to share certain features: abrupt section changes, welds located in high-stress regions, connections that depend on a single fastening point, and insufficient material toughness.
The corresponding improvements are equally clear. Add transition radii so stress is distributed more gradually. Place welds where stress is lower. Use redundancy at critical connections. Balance toughness against strength rather than pursuing strength figures alone.
That last point is often overlooked. A stronger material is not necessarily a tougher one, and impact loading tests toughness specifically. Selecting material on tensile strength alone can make impact behaviour worse.
How we handle it
We record a before-and-after comparison around impact testing, covering key dimensions, fastener condition and surface state. These records do not change the verdict on the impact item, but when fatigue results later look anomalous they are the basis for working out why.
Where a client wants preliminary data, we suggest running fatigue under both conditions: once on fresh samples and once in the prescribed sequence after impact. The difference between the two directly indicates how sensitive the structure is to impact damage, which is useful for design iteration and cannot be obtained from a single condition.
Send us the structural concept and we can indicate sensitive locations. Phone or WeChat: +86 132 4819 8029.