Composite materials in tensile testing machine testingcommonquestionOverview

1. Preparation stage: it is easy to overlook problems andCorrespondStandard

(1) Frequently Asked Questions

1.Improper sample preparation: The sample is not placed according to the direction of the lamination of the composite material, resulting in disordered arrangement of the sample fibers; there are burrs and delamination on the edges of the sample, and it is easy to break from the defects during testing, affecting the accuracy of the data.

2.Improper specimen clamping: The clamping force is too small, and the sample slips during testing; the clamping force is too large, and the surface fibers of the sample are directly crushed, destroying the original structure of the material, resulting in distortion of the test results.

3.Environmental conditions are not controlled: The sample is not placed under standard temperature and humidity (usually 23°C ± 2°C, relative humidity 50% ± 5%). The performance of the composite material may fluctuate due to moisture absorption or temperature changes. For example, the tensile strength of glass fiber composite materials decreases after absorbing moisture.

(2) Need to follow standards

1.Sample preparation standards: Different composite materials have different reference standards. For example, the fiber material adopts GB/T 3354-2014 "Test Method for Tensile Properties of Oriented Fiber Reinforced Plastics", which specifies the sample size (such as long fiber composite material sample length ≥ 200mm, width 10-25mm) and cutting tools (a diamond saw blade is required to avoid fiber fluffing).

2.Clamping standards: Select an appropriate clamp according to the type of composite material. For example, brittle composite materials (carbon fiber reinforced plastics) need to use clamps with elastic pads to avoid clamping damage; the clamping position needs to be aligned with the specimen gauge line, and the deviation does not exceed 1mm.

3.environmental standards: Testing samples must be carried out in accordance with standards. For example, plastic samples must comply with GB/T 2918-2018 "Standard Environment for Condition Adjustment and Testing of Plastic Samples", and the samples must be placed in a standard environment before testing.

2. Operation stage: key issues and implementation standards

(1) Frequently Asked Questions

1.Loading rate is unreasonable: If the speed is too fast, the internal stress transfer of the composite material will be uneven, and brittle fracture will easily occur, and the measured tensile strength will be high; if the speed is too slow, the material will creep, resulting in low tensile strength.

2.Data collection is not timely: The sample did not record data simultaneously when it reached the yield point and fracture point, missing key performance parameters (such as yield strength, elongation at break). Especially composite materials are mostly brittle materials, and data at the moment of fracture are easily lost.

3.Specimen offset not corrected: During the test, the sample is laterally shifted due to a slight loosening of the clamp, causing the actual force direction to be inconsistent with the tensile axis, resulting in additional bending moments, causing deviations in the test results.

(2) Need to follow standards

1.Loading rate criteria: According to GB/T 3354-2014, the loading rate of different composite materials is different. For example, the tensile test rate of carbon fiber reinforced plastic is 2-5mm/min, and that of glass fiber reinforced plastic is 5-10mm/min. It must be set strictly according to the material type.

2.Data collection standards: In accordance with ISO 527-4:2004 "Determination of Tensile Properties of Plastics Part 4: Test Conditions for Isotropic and Orthotropic Fiber Reinforced Plastics", the data collection frequency is required to be no less than 10Hz to ensure the capture of key node data such as yielding and fracture.

3.Alignment standards: GB/T 1446-2005 General Principles states that before testing, the centering of the fixture needs to be adjusted to ensure that the coaxiality deviation between the specimen axis and the tensile axis of the testing machine is ≤0.5%. Whether the specimen deviates can be observed through preloading.

3. Data and Results Stage: Error-prone Points and Judgment Criteria

(1) Frequently Asked Questions

1.Improper data processing: Abnormal data (such as abnormal fracture strength values due to sample defects) are not eliminated, and the single test result is directly taken as the final value; when calculating the elastic modulus, the linear segment data is not selected, resulting in a deviation in the modulus value.

2.The basis for determining the result is wrong: Confusing the determination indicators of different standards, such as applying the tensile strength determination standard for metal materials to composite materials, ignoring the unique indicators such as interlaminar shear strength and fiber direction strength of composite materials.

3.Report incomplete information: The fiber type (such as carbon fiber, glass fiber), test parameters, matrix material, etc. of the composite material are not indicated in the report, resulting in a lack of traceability of the test results and the customer being unable to reproduce the test.

(2) Need to follow standards

1.Data processing standards: According to GB/T 3354-2014, multiple samples must be tested in each group, and the average value shall be obtained after eliminating outliers of dispersion; the elastic modulus calculation needs to select the linear segment in the stress-strain curve (usually the stress is 0.1%-0.5% of the rated strength interval).

2.Result criteria: Regarding the characteristics of composite materials, refer to GB/T 1040.4-2006 "Determination of tensile properties of plastics - Part 4: Test conditions for isotropic and orthotropic fiber-reinforced plastics". In addition to determining the tensile strength and elongation at break, it is also necessary to determine the interlayer shear strength (if the sample is a laminated structure) and the strength difference in different fiber directions.

3.reporting standards: Referring to the requirements of GB/T 1446-2005, the report must include sample information, testing environment, loading rate, original data curve, judgment standard number, etc., to ensure that the information is complete and traceable.

From the three core stages of preparation before testing, operation during testing, and data processing after testing, the above has sorted out the high-frequency issues and key points of standards that need to be followed in tensile testing machine testing of composite materials, providing basic guidance for standardizing the testing process and avoiding data deviations.However, there are many types of composite materials (such as carbon fiber composite materials, glass fiber composite materials, etc.), and the differences in characteristics of different materials will lead to different details and difficulties in detection - such as brittle fracture control of carbon fiber composite materials, response to the effects of moisture absorption of glass fiber composite materials, and interlayer stress distribution detection of laminated composite materials, etc., all of which require more targeted analysis.

In the future, Shandong Huayan testing machines will be launched for testing scenarios of different types of composite materials.Separate in-depth analysis of content,Help everyone respond to various testing needs more accurately.If you have specific concerns about composite material types or testing pain points, you can always provide feedback. We will optimize subsequent content based on actual needs and work with you to overcome technical problems in composite material testing.