Environmental test chamber loading

Chamber measurement Supply air quality Background concentration Sink effects/recovery Air exchange ratio Air tightness of the environmental test chamber Internal air mixing Air velocity Accuracy of temperature, RH and air exehange ratio Product loading factor... 

Load Frame, Force and Strain Measurement, Data Acquisition The test method uses a standard load frame with a hydraulic or screw drive loading mechanism and standard force transducers. Force is applied transversely to produce a bending moment. Extension is measured by deflectometers in the gage section and strain is measured using bonded resistance strain gage rosettes to determine both longitudinal and transverse strains. If required, an environmental test chamber may be used to control humidity and ambient temperature. Data collection should be done with a minimum of 50-Hz response and an accuracy of 0.1 % for all data. 

In creep tests, a specimen is subjected to a constant load, and the strain is measured as a function of time. The test specimen in a laboratory setup can be a plastic film or bar clamped at one end to a rigid support while the load is applied suddenly at the other end (Figure 13.2). The elongation may be measured at time intervals using a cathetometer or a traveling microscope. Measurements may be conducted in an environmental chamber. 

A strain-controlled or fully confined recovery test was conducted. It started immediately after completion of the programming with the same loading device and environmental chamber. The chamber was heated at a ramp rate of 0.6°C/min. The heating was continued until the temperature starting the programming (71 °C or 79 °C) was reached. After that, the temperature was maintained constant for several hours and the stress was continuously recorded. This process was stopped when further stress recovery was negligible. 

Creep can be defined as deformation that occurs over time when the material is subjected to constant stress at constant temperature. In this investigation, the creep was measured in tension. The molded tensile bars with 0.5-in. taper were placed in an Instron 1331 servohydraulic testing machine, in load control, using a fixed mean level of 120 kg, and an amplitude of zero. The elevated test temperature of 80°C is achieved using a Thermotron environmental chamber. Testing is controlled by an IBM-compatible PC running Instron MAX software. Failure times (hours to creep rupture) were averaged for three specimens. 

Stress-strain behavior represents the response of a material to loading. Tests are performed on a universal testing machine (UTM), sometimes referred to as a tensile tester because of the primary mode of deformation used to characterize this form of behavior. Specimens are typically deformed at a constant speed, for reasons explained later. Since the properties vary significantly with temperature, tests may be conducted within an environmental chamber to obtain data at elevated and subambient temperatures. The most common information obtained from these tests are the modulus and tensile strength. 

Load frames for tensile testing are available in a range from less than 1 kN to over 300 kN. Environmental chambers that fit around the sample can be used to study low- or high-temperature behaviour, and are typically available to control temperatures in the range - 70-250°C. 

Cooling coils were installed on the loading and support push-rods to protect frame electronics and load cells from high-temperatures. An aluminum environmental chamber, connected to the push-rods by a bellows assembly, was used to maintain a controlled environment. Test frames were connected to a back-up-power supply system to provide uninterrupted power during the event of a ptower failure. 

To demonstrate the reliability of the testing system and the reproducibility of the data, stress-strain curves were measured in five series of tests with batch, loading method, environment, strain rate and adhesive type as variables. All tests were performed in environmental chambers in which RH and temperature were carefully controlled. Loading and data acquisition were computer controlled unless noted otherwise. In the following, the authors use the term initial strain rate as defined in the ASTM Test Method D 882. 

Variations on the basie testing configurations abound. Most instruments apply load vertically, but some are horizontal. Multiple sets of grips and load cells permit the simultaneous testing of duplicate specimens. Load cells come in various sizes, shapes, and sensitivities. The jaws that grip the sample may be mechanically, pneumatically, or hydraulically operated. Extensometers may be attached physically to the sample or track the movement of fiducial marks optically. Specimens may be enclosed in an environmental chamber, the interior of which may be heated or cooled, or filled with various media. 

Films 0.5 mm thick were compression molded from pellets into plaques as described previously. The plaques were aged for 7 to 12 days at ambient temperature before further processing. Microtensile specimens were cut from the plaques according to ASTM 1708, and the specimens were loaded in uniaxial tension to targeted strains with an MTS Alliance RT/30 electromechanical testing machine. Stretching temperature was controlled by using an Instron model 3119-009 environmental chamber. The crosshead speed for all specimens was 22.3 mm min (100% min for the as-cut microtensile specimen). 

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