Environmental testing, as a type of reliability testing, has evolved into a method of predicting how the product's use environment affects product performance and functionality. That is to say, before the product is put on the market, the environmental test is used to assess the degree of environmental impact on the product; when the function of the product is affected, the environmental test is used to identify the cause and take measures to protect the product from environmental impact to maintain Product reliability. These tests have far exceeded their original purpose, and are now widely used in the research and development of materials and products, various inspections in the production process, inspection before transportation and quality control after transportation, and are also used for analysis Defects in the actual use of the product and improvements in new products. Environmental testing is very effective for testing methods and maintaining product reliability.
The environmental tests we are talking about here are narrowly defined, and actually refer to artificially simulated environmental tests (hereinafter referred to as environmental tests). Broadly speaking, environmental tests are basically divided into three categories: "natural exposure test", "manual simulation test" and "field operation test". The natural exposure test is to test the sample for long-term exposure to natural environmental conditions. On-site operation test is to test the sample device at various typical use sites and make it in normal operation state. These two tests can directly reflect the performance and reliability of the product in actual use, and are also the basis for verifying the accuracy of manual simulation tests. However, the test period is long, it takes a lot of manpower and material resources, and the former test conditions can not be controlled, affecting the reproducibility of the test, and sometimes can not keep up with the development of the product; the latter data feedback is slow. Therefore, in order to identify the adaptability of the product to the environment in a relatively short period of time, artificial simulation environmental tests are mostly used in scientific research and production work, that is, one or more environmental factors are simulated in the laboratory test equipment (box or room) Role, and be appropriately strengthened. The determination of the test conditions of the manual simulation test requires that it not only can simulate the authenticity of the main factors in the environment, but also play a certain acceleration role in time, but the degree of acceleration should not change the law of the actual damage mechanism of the product. For this reason, the test conditions and methods of manual simulation tests must be organically linked to the level and value of product environmental conditions.
As shown in Figure 1, the simple division of environmental testing can be divided into "climate environmental testing", "mechanical environmental testing" and "comprehensive environmental testing". Climate-related environmental tests include environmental stress tests such as temperature, humidity and pressure, while mechanical environmental tests include environmental stress tests such as shock and vibration, and comprehensive environmental tests are stress tests that integrate environmental factors such as climate and machinery.
Figure 1 Types of environmental tests
Environmental test
Mechanical environmental test
Comprehensive environmental test
Climate Environment Test
Environmental stress: impact, vibration, collision, acceleration, high noise, high wind
Environmental stress: environmental factors combining mechanical environment and climatic environment
Environmental stress: temperature and humidity, gas, salt fog, wind and rain, pressure, solar radiation
Of course, tests conducted using environmental test equipment cannot completely reproduce the environment in which the product is used and simulate all environmental factors. Here, the limitations of environmental tests must be understood. An environmental test consisting of a single factor (temperature, humidity, pressure, vibration, shock, or a substance such as salt) is called a simple environmental test. In fact, it is very difficult to create a completely single environment, and most test environments are very complicated. Therefore, when designing the test conditions, the tester needs to select the most important environmental factor that has the greatest impact on the product, so the environmental test can only be an artificial environment that is still very different from the real environment.
In general, product defects are caused by the following aspects:
a. The concentration and diversity of raw materials, friction, wear, stress, heat, current and electric field strength, these factors will affect the performance of certain aspects of the product;
b. Factors caused by product characteristics (raw materials, manufacturing processes, structural components, and mass production) during product design and manufacturing;
c. Stress generated by the external environment.
Therefore, the test conditions must be based on specific product conditions, which are different for different products. If the product under test has changed, the corresponding environmental test will also change.
2. Temperature stress
Environmental stress conditions can cause product failure. Hughes Airlines (USA) technical documentation clearly shows the relationship between failure and environmental stress (see Figure 2). It can be seen that the failure caused by temperature and humidity environmental stress accounts for about 60% of all environmental stress-induced failures among various stress effects, and there is a close relationship between temperature stress and failure.
At present, in all parts of the world, no matter from land to sea or from high altitude to space, electronic and electrical products and other fields are widely used. As the temperature decreases with increasing altitude or in high latitudes in winter, or some products are located close to refrigeration components, equipment or systems, or some products themselves include refrigeration components, equipment or systems, resulting in a low-temperature environment. Low temperature will have different degrees of harmful effects on almost all materials. The physical properties and electrical properties of the various materials that make up the product will change, resulting in temporary or permanent performance degradation and even failure.
Similarly, the high temperature caused by the natural high temperature in low-latitude tropical regions, the increase in solar radiation, the temperature rise caused by poor ventilation, and the temperature rise caused by the self-heating of the heat dissipation samples during use, high temperatures will reduce the reliability of the electronic assembly, and the mechanical structure The seals, rubber parts and plastic parts will be rapidly aging and deteriorating under the effect of high temperature and sun exposure. The structure, physical properties and electrical properties of various other materials will also change greatly, resulting in temporary or permanent damage and Performance changes.
In addition, in the process of product storage, transportation, use and installation, in addition to changes in natural climate, environmental temperature changes due to human social practices will also be encountered. For example, the equipment moves from a room with a relatively high temperature to a room with a relatively low temperature; or a room with a relatively low temperature to a room with a relatively high temperature; or a device used outdoors suddenly rains or is immersed after strong solar radiation Cold water; or extreme temperature rise causes solder reflow, or the temperature of surrounding devices rises rapidly when the motor is turned on, and the temperature of the surrounding devices suddenly drops when the motor is turned off; or the device may be connected to the power supply in a cold environment On the ground, it causes a steep temperature gradient inside the equipment. Cutting off the power in a low-temperature environment may cause a steep temperature gradient in the opposite direction inside the equipment; or when the aircraft takes off or lands, the temperature may appear on the external equipment of the aircraft. The ground changes drastically and so on. Due to the rapid temperature change, the product will be subjected to a certain thermal shock force. Under this thermal shock force, it will cause the coating layer of the electronic and electrical components to fall off, the sealing material cracks or even breaks, the filling material leaks, etc., causing electronic The electrical performance of components is reduced; for products composed of different materials, due to uneven heating of the product when the temperature changes, the product is deformed, cracked, broken, etc. Due to the large temperature difference caused by temperature changes, condensation or frost will occur on the surface of the product at low temperatures, and it will evaporate or melt at high temperatures. The result of repeated high and low temperature effects will lead to and accelerate product corrosion.
The following table gives the main types of failure induced by temperature stress.
Table 2 Main types of failure caused by temperature stress
Failure
Environmental stress conditions
Sensitive components and materials
main category
Classification (cause)
Failure mode
temperature
High temperature aging
Ageing
Tensile strength aging
Insulation aging
Temperature + time
Plastic, resin
Chemical changes
Thermal decomposition
temperature
Plastic, resin
Soften, melt, vaporize, sublimate
distortion
temperature
Metal, plastic, thermal fuse
High temperature oxidation
Structure of the oxide layer
Temperature + time
Connection point material
Thermal diffusion (metal compound structure)
Wire break
Temperature + time
Dissimilar metal connection
Intermediate damage
semiconductor
hot spot
Temperature, voltage, electronic energy
Heterogeneous material
Heat accumulation burning
(The remaining heat burns)
combustion
Heating + drying + time
Plastic (eg wood chips with vinylon and polyurethane paint)
puncture
inner
Short circuit
Poor insulation
High temperature (200 ~ 400 ℃)
Silver, gold, steel, magnesium, nickel, lead, palladium, platinum, tantalum, titanium, tungsten, aluminum
Non-intrinsic
Short circuit
Poor insulation
High temperature (400 ~ 1000 ℃)
Halides of copper, silver, iron, nickel, cobalt, manganese, gold, platinum and palladium
migrate
Electromigration
Disconnect, broken lead
Temperature (0.5Tm) + current (density is 106A / cm2)
For example tungsten, copper, aluminum (especially aluminum leads in integrated circuits)
spread
metal
Fatigue, damage
Temperature + Stress + Time
Springs, structural elements
plastic
Fatigue, damage
Temperature + Stress + Time
Springs, structural elements
Low temperature and brittle
metal
damage
Low temperature
Body-centered cubic crystals (eg copper, molybdenum, tungsten) and close-packed cubic crystals (eg zinc, titanium, magnesium) and their alloy
plastic
damage
Low temperature + low humidity
High glass transition temperature (eg cellulose ethylene ammonia), low elasticity amorphous (eg styrene, methyl acrylate)
Flux flow
Flux flow sticks to cold metal surface
Noise, poor connection
Low temperature
Especially components connected to printed circuit boards (eg switches, connecting devices)
Therefore, when discussing the product life related to temperature, the expression of “θ ℃ rule†is generally adopted. The specific application can be expressed as "10 ℃ rule", etc. For example, when the ambient temperature rises by 10 ℃, the product life will be reduced by half; when the ambient temperature rises by 20 ℃, the product life will be reduced to a quarter. This rule can explain how temperature affects product life (failure).
Conversely, one can use a higher ambient temperature or lower ambient temperature or the alternation of ultra-high temperature and ultra-low temperature to accelerate the occurrence of product failure within a short period of time, to determine the change in product characteristics and thermal expansion due to the dissimilar materials that make up the components Failure caused by different coefficients. It is mostly used in the development of screening tests, material property tests, limit tests, evaluation tests, quality confirmation tests, accelerated life aging tests, etc.
3. Temperature test equipment
The temperature test equipment is generally composed of working room, regulating equipment, auxiliary equipment and control system. Among them, the regulation system includes heaters, evaporators, air supply devices, etc., the auxiliary equipment is the main room refrigeration unit, and the control system includes temperature controllers, program setters, and safety alarm devices.
The heater is mostly realized by the electric heating method of the resistance wire or the electric heating tube; in addition to the natural heating of the drying oven for the high temperature test of the non-radiating sample, it is generally equipped with an air supply device for forced air circulation, and the air supply method is often used. The method of side-to-side and bottom-to-side or full-hole orifice top-to-side is important to the uniform performance of the entire test equipment.
Most of the refrigeration methods used are mechanical refrigeration. The refrigerator has a semi-closed or fully enclosed form. The cooling circuit also has a water-cooled or air-cooled form. The refrigerant is now mostly environmentally friendly. The throttling method uses a capillary or electronic expansion valve, which can be improved. Accuracy and longevity of the circuit, and can work continuously for a long time.
The control system now generally adopts PID mode, so that the heating power is automatically adjusted and controlled according to the actual temperature of the device and the set temperature deviation, to achieve stepless adjustment, and the temperature fluctuation is small; the display part is also upgraded from the previous digital display mode to the LCD touch screen mode. For example, the P gauge of the ESPEC Group uses a 6.54-inch TFT color LCD display screen, which is clear and beautiful, and is more convenient to operate.
In addition, the insulation layer of the equipment is made of glass wool or polyurethane foam layer, which has small thermal conductivity and good temperature resistance. Temperature detection adopts thermocouple, platinum resistance and thermistor methods, etc. with high accuracy.
The temperature test needs to be considered thoroughly, so it is also important to consider the selection of appropriate test equipment.
1. The test range must be required to meet the product failure possibility test range, that is, whether it is a high temperature box, a low temperature box, or a thermal shock box, it should be able to meet the extreme temperature conditions specified in the test requirements.
2. The test equipment must be selected based on the principle that the volume of the test sample should not exceed 1/5 of the working volume of the test equipment.
3. To ensure the uniformity of the temperature in the test area, choose whether to use forced air circulation or unforced air circulation according to the heat dissipation of the sample. For example, the LG-type drying oven produced by Shanghai ESPEC adopts hot air natural convection method, which is particularly suitable for powder drying, while the LC-type drying oven adopts hot air forced circulation method. The difference in the temperature distribution of the equipment will have a greater impact on the test results. When a larger sample is used, or the number of samples tested at the same time is larger, the test results will vary greatly depending on the location, so it should be Choose equipment with the best temperature uniformity possible. For example, the Z series thermostats produced by Shanghai ESPEC have a uniformity of 2.0 ° C.
4. The sample temperature can be measured by upwind or downwind sensor. The position and control method of the temperature control sensor of the equipment can be selected, and the appropriate equipment should be selected according to the test requirements.
5. To prevent the sample from absorbing or releasing heat caused by heat radiation or heat conduction in the test area, it is required that the heating or cooling system of the equipment has no effect on the sample.
6. It is easy to record and display, and it is required to install automatic counters, indicator lights, recording equipment and automatic shut-off instruments during the cycle test.
7. Facilitate sample placement. A sample holder for placing or hanging can be required, and the sample holder will not change its mechanical properties due to high and low temperature changes.
8. There should be lead holes for introducing the test power into the equipment working room.
9. Regardless of the damage to the samples and the safety of the test personnel, there must be protective measures. For example, there are observation windows and lighting, and alarm devices such as power failure, water shortage, over-temperature protection, and operator protection.
10. Do you need remote monitoring function?
Matters needing attention when using temperature test equipment:
1. When conducting a large sample test, there will be temperature and humidity deviations in the ascending and descending airflows, so you should carefully consider where to place the samples. Place it in the center of the working space of the test equipment as much as possible. The samples must not contact and overlap each other, and a certain interval should be left to allow air circulation; and the samples should be easy to move during the test and replace the samples during the test.
2. Check for volatile substances such as oil and gas in the test area, and check for odorless substances. The test results of such substances must be confirmed in advance.
3. Pay attention to confirm the temperature condition of the test sample in the test area.
4. In order to keep the temperature in the temperature zone consistent, it is necessary to ensure the test environment temperature and equipment power supply fluctuation as little as possible, to ensure that the test sample does not generate heat radiation and absorb heat, so as to ensure the temperature of the test zone is stable.
5. After the temperature test equipment finishes the test, quickly removing the test sample will cause unnecessary stress to the sample and may obtain unexpected results. Therefore, the sample must be removed after the test sample has cooled to ambient temperature.
6. The thermal conductivity of the installation of the test sample and the support frame should be low to ensure that the test sample and the installation and support frame are in an insulated state.
The test content of different products is determined jointly by the test requester, designer and tester. From the perspective of providing test equipment that meets the test, establish a technical specification framework for the test equipment. The test equipment supplier can provide specific specifications of the equipment based on this framework. Every test equipment that can simulate the test environment generally has a measurement system, so it is very easy to use off-the-shelf equipment and purchase new equipment. Simple test equipment can also be manufactured by yourself, but the cost, reliability of the test results, the degree of precise control and the safety factors of continuous operation should be considered.
Conclusion
Environmental testing is a process that can add value to a product and cannot be ignored. As the most basic type of environmental testing, temperature-based environmental testing has a wide range of applications and greatly improves the environmental adaptability and reliability of military and civilian products in China, and directly leads to the improvement of product quality. This article gives a simple and basic introduction to temperature stress and related temperature-type environmental testing equipment. Accurate selection and use according to usage requirements can achieve the true purpose of environmental testing.
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Naval Granville Test Equipment Co., Ltd. http: // telephone 87081827
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