Composites Today

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Quality issues related to usage of expired and off-spec carbon fiber prepreg

ALLRed & Associates Inc. the New York based fabricator of CFRP composites shapes and structural components having trade name DragonPlate® has shared some information regarding quality of carbon fibre prepreg and its implications in the final product.

It is well known in the composites industry that off-spec and expired surplus prepreg and other materials are sold at significantly discounted prices. However, the buyer should beware. These seemingly great deals come at a huge risk to the product quality. Using expired prepreg carbon fiber can lead to several problems and compromises in the quality and performance of the final product. Here are some of the critical issues associated with using expired prepreg carbon fiber:

1.  Resin degradation:  The resin in prepreg carbon fiber can degrade over time due to exposure to heat, light, and oxygen. This degradation can cause a loss of resin properties, including its ability to bond with carbon fibers. As a result, the mechanical properties of the composite may be compromised, leading to reduced strength, stiffness, and impact resistance.

2.  Cure cycle issues:  Prepreg carbon fiber requires a specific cure cycle involving controlled temperature and pressure to activate and set the resin. The curing process allows the resin to flow, bond with the carbon fibers, and solidify to form a strong composite. When using expired prepreg, the resin may not cure properly or require longer curing times and higher temperatures to achieve the desired properties. This can result in inadequate consolidation, improper resin flow, or incomplete bonding, leading to weaker joints and reduced performance.

3.  Reduced tackiness:  Tackiness refers to the adhesive quality of the resin in the prepreg carbon fiber. It is essential for proper placement and consolidation during the manufacturing process. Over time, the tackiness of the resin can decrease, making it challenging to handle and position the prepreg material accurately. This can lead to difficulties in achieving proper fiber alignment, causing inconsistencies in the laminate structure and potentially creating weak spots or voids within the composite.

4.  Moisture absorption:  Carbon fiber composites absorb moisture from the surrounding environment. This moisture absorption can lead to dimensional changes, reduced mechanical properties, and potential damage to the composite structure. Expired prepreg carbon fiber may have a higher moisture absorption rate due to resin degradation or changes in the carbon fiber surface. Moisture can weaken the composite and contribute to problems such as delamination, reduced strength, and decreased fatigue life.

5.  Structural integrity issues:  The combination of resin degradation, incomplete curing, and moisture absorption can compromise the structural integrity of the composite. The degraded resin may not provide the necessary strength and bonding between the carbon fibers, leading to reduced load-bearing capacity and increased susceptibility to failure. Moisture absorption can further exacerbate these issues by causing swelling, dimensional changes, and weakening the composite structure, making it less reliable and potentially unsafe for critical applications.

6.  Quality control challenges:  Using expired prepreg carbon fiber introduces uncertainties and challenges in quality control. Manufacturers establish expiration dates based on extensive testing and evaluation to ensure consistent and reliable performance. By using expired material, the manufacturer deviates from the recommended guidelines, making it difficult to assess the quality, predict the behavior, and guarantee the performance of the composite. This can increase variability, manufacturing rejects, and potential safety concerns.

In summary, expired prepreg carbon fiber can lead to resin degradation, cure cycle issues, reduced tackiness, moisture absorption, structural integrity problems, and quality control challenges. These issues can compromise the composite material’s mechanical properties, reliability, and safety. It is crucial to adhere to the manufacturer’s guidelines and use prepreg materials within their specified shelf life to ensure optimal performance and quality.

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