高分子材料在器械上的常见失效与检测

结论：高分子的失效多数是渐进的，且与时间和环境强相关

金属的失效多数可以用强度和疲劳来解释，高分子材料不同 ——它的性能随时间、温度、环境介质而变化，很多失效不是「一下子坏了」，而是性能逐渐劣化到不能用。

这带来两个检测上的差别：

不能只测初始性能。 新料的数据说明不了使用几年后的状态。

要考虑环境因素。 温度、湿度、光照、化学介质都影响材料，而且往往是叠加作用。

常见的失效模式

「环境应力开裂」是高分子特有且容易被忽略的模式，它在应力不大、介质看起来无害的条件下也可能发生，而且往往在使用一段时间后才出现。

环境应力开裂

机理。 材料在拉应力作用下接触某些化学介质时，即使应力远低于强度、介质也不溶解材料，仍会出现开裂。

常见的介质。 某些清洁剂、消毒剂、油脂、药液、甚至某些标签胶。

应力的来源。 装配应力、成型残余应力、使用中的载荷。

检测方法。 在施加应变的条件下接触介质，观察开裂。

设计上的应对。 降低残余应力（成型工艺）、避免应力集中、选择抗环境应力开裂的材料、避免接触敏感介质。

「成型残余应力」是容易被忽略的应力来源。 注塑件内部可能有相当的残余应力，即使不受外载，接触介质后也可能开裂。退火处理能降低残余应力。

老化的检测

热氧老化。 在升温条件下加速氧化。

光老化。 紫外线照射的影响。

湿热老化。 温度加湿度的作用。

综合老化。 多因素同时作用，更接近实际。

评价指标。 力学性能（特别是断裂伸长率）、外观（变色、龟裂）、以及化学变化。

「断裂伸长率」是老化最敏感的指标，它通常比拉伸强度更早、更明显地下降，只测强度可能漏掉材料已经脆化的事实。

蠕变与应力松弛

蠕变。 恒定载荷下变形随时间增加。影响尺寸稳定和配合。

应力松弛。 恒定变形下应力随时间下降。影响卡扣、密封件、螺纹连接的保持力。

温度的影响。 温度升高显著加速蠕变和松弛。

检测方法。 在恒定载荷或恒定变形下长期测量。

实际的影响。 卡扣的保持力下降、密封件的密封力下降、螺栓的预紧力下降。

「卡扣保持力的下降」在使用几年后可能导致部件松脱，而这在新品测试中完全看不出来。

疲劳与磨损

疲劳。 高分子的疲劳与金属不同——发热明显，内部生热可能导致热失效而非机械失效。

加载频率的影响。 频率高时发热多，试验频率要控制。

磨损。 摩擦面的材料损失，与对偶材料、载荷、速度、润滑相关。

磨损颗粒。 在某些应用中颗粒是关注点。

「疲劳试验的频率」在高分子材料上要注意，频率过高导致试样发热，测出的寿命偏低且失效机理与实际不同。

检测项目的组织

基础性能。 拉伸、弯曲、冲击、硬度。

热性能。 软化温度、玻璃化转变、熔点、热变形温度。

老化。 各类老化后的性能保持。

环境相容。 与接触介质的相容、环境应力开裂。

长期性能。 蠕变、应力松弛。

低温性能。 低温冲击。

化学。 成分、添加剂、释放物质。

建议按「初始性能 + 环境作用后的性能」组织，而不只测初始性能。

设计与选材上的应对

材料选择。 按使用环境选择，不只按初始性能。

降低应力集中。 圆角、避免尖锐缺口。

控制残余应力。 成型工艺、退火。

避免敏感介质。 明确不相容的清洁剂和化学品。

留出性能余量。 考虑老化后的性能下降。

明确使用限制。 温度、化学品、光照的限制写进说明书。

试样的制备与状态

高分子材料的测试对试样状态很敏感：

试样来源。 从实际制件上取样还是单独注塑的标准试样。两者的分子取向和内应力不同，数据可能有差别。

取样方向。 注塑件有流动方向，不同方向的性能不同。

状态调节。 按规定的温湿度平衡一定时间，含湿量对某些材料的性能影响明显。

表面状态。 机加工的试样表面与模塑表面不同。

批次。 材料批次与制件批次的对应。

「从实际制件取样」的数据更接近产品的真实性能，虽然试样制备麻烦，但对判断产品的实际表现更有价值。

数据的使用

材料数据的用途包括：选材的依据；设计计算的输入；变更时的对比基线；失效分析的参考；以及供应商来料的验收。

「设计计算的输入」要注意用哪组数据 ——短期强度用于校核瞬时载荷，而长期承载要用蠕变数据，用错会导致设计偏于危险。

常见的准备不足

信息不全。 材料、工艺、使用环境的信息缺失，导致方案设计缺乏依据。

样品不具代表性。 试制样品或挑选样品，与量产状态有差距。

条件未明确。 测试条件没说清楚，数据的适用范围就不明确。

只看初始值。 忽略了环境和时间作用后的性能变化。

项目不完整。 遗漏了必要的项目，提交时才发现。

这五项在实际委托中出现频率较高，建议在项目启动时逐项确认，因为它们造成的返工成本远高于事前确认所花的时间。

我们的做法

做高分子材料相关测试时，我们建议不只测初始性能，还要测环境作用后的性能 ——老化、介质浸泡、温度循环之后重新测量，得到衰减的幅度。

断裂伸长率是我们建议必测的指标，因为它对老化和辐照最敏感，能较早反映材料的脆化。

对于接触清洁剂和消毒剂的部件，环境应力开裂的评价值得做，因为这类失效在实际使用中并不少见，而常规的力学测试发现不了。

如果你有高分子件需要做材料相关的检测，可以把材料信息和使用环境发过来一起讨论，或者直接联系：132 4819 8029。能力范围见服务介绍，送检要求见送检要求，流程见检测流程。

English version

Conclusion. Metal failures can mostly be explained by strength and fatigue. Polymers differ: their properties change with time, temperature and chemical environment, and many failures are not a sudden break but a gradual decline until the material is no longer serviceable. Two testing consequences follow. Initial properties alone are insufficient, since data on new material say nothing about its state after years of service. And environmental factors must be considered, since temperature, humidity, light and chemical media all act on the material, frequently in combination.

Common failure modes. Environmental stress cracking arises from stress combined with a specific chemical medium. Ageing embrittlement arises from light, heat and oxidation. Creep arises from sustained constant load. Stress relaxation is the decay of stress under constant strain. Fatigue arises from repeated loading. Wear arises from friction. Hydrolysis degrades certain materials under heat and humidity. Plasticiser migration hardens and embrittles the material. And low-temperature embrittlement occurs below the transition range. Environmental stress cracking is specific to polymers and easily overlooked: it can occur at modest stress in media that appear harmless, and it typically appears only after a period of service.

Environmental stress cracking. The mechanism is that a material under tensile stress in contact with certain chemical media cracks even where the stress is well below its strength and the medium does not dissolve it. Common media include certain cleaning agents, disinfectants, oils, medicinal solutions and even some label adhesives. Stress sources include assembly stress, moulding residual stress and service loads. Testing applies strain while exposing the specimen to the medium and observes cracking. Design responses include reducing residual stress through moulding process, avoiding stress concentration, selecting materials resistant to environmental stress cracking, and avoiding contact with sensitive media. Moulding residual stress is an easily overlooked source: an injection moulded part can carry considerable internal stress and may crack on contact with a medium even without external load. Annealing reduces residual stress.

Testing ageing. Thermal oxidative ageing accelerates oxidation at elevated temperature. Light ageing covers ultraviolet exposure. Damp heat ageing combines temperature and humidity. Combined ageing applies several factors together and is closer to reality. Evaluation covers mechanical properties, elongation at break especially, appearance including discolouration and crazing, and chemical change. Elongation at break is the most sensitive ageing indicator, generally declining earlier and more markedly than tensile strength, so measuring strength alone can miss the fact that the material has already embrittled.

Creep and stress relaxation. Creep is increasing deformation under constant load, affecting dimensional stability and fit. Stress relaxation is decaying stress under constant strain, affecting the retention of clips, seals and threaded joints. Temperature accelerates both markedly. Testing measures over long periods under constant load or constant strain. The practical effects are loss of clip retention, loss of sealing force and loss of bolt preload. Loss of clip retention after some years can allow components to detach, and new-product testing reveals nothing of it.

Fatigue and wear. Polymer fatigue differs from metal fatigue in that heating is pronounced, and internal heat generation may cause thermal rather than mechanical failure. Loading frequency matters, since high frequency generates more heat and test frequency must be controlled. Wear covers material loss at friction surfaces and depends on the counterface, load, speed and lubrication. And wear particles are a concern in some applications. Test frequency deserves attention in polymer fatigue: excessive frequency heats the specimen, producing a shortened life through a mechanism unlike the real one.

Organising the test items. Basic properties cover tension, flexure, impact and hardness. Thermal properties cover softening temperature, glass transition, melting point and heat deflection temperature. Ageing covers property retention after each ageing regime. Environmental compatibility covers media compatibility and environmental stress cracking. Long-term properties cover creep and stress relaxation. Low-temperature properties cover cold impact. And chemistry covers composition, additives and released substances. Organise around initial properties plus properties after environmental exposure rather than initial properties alone.

Design and material responses. Select materials for the service environment rather than initial properties alone. Reduce stress concentration through radii and by avoiding sharp notches. Control residual stress through moulding process and annealing. Avoid sensitive media by identifying incompatible cleaning agents and chemicals. Leave performance margin allowing for decline through ageing. And state limitations of use, writing temperature, chemical and light restrictions into the instructions.

How we handle it. For polymer testing we suggest measuring properties after environmental exposure as well as initially, re-measuring after ageing, immersion and temperature cycling to quantify the decline. Elongation at break is an item we suggest always measuring, since it is the most sensitive to ageing and irradiation and reveals embrittlement earliest. For components contacting cleaning agents and disinfectants, environmental stress cracking assessment is worth performing, since such failures are not rare in service while routine mechanical testing cannot detect them.

Send us the material information and service environment and we will work through the testing. Phone or WeChat: +86 132 4819 8029.