Testing and quality control in filament-wound composites - A425

 

Testing and quality control in filament-wound composites
Document Type	Article
Document Identifier	425
Tags	Deposition
Prerequisites	The factory Factory cells (where and how) Material deposition Filament winding

Introduction

Because Filament Winding (FW) components are often pressure-bearing and safety-critical (e.g., pressure vessels, piping, drive shafts), robust testing and inspection are essential. This article summarizes recognized test standards and quality-control approaches for filament-wound parts, highlighting practical methods for verifying structural integrity and compliance with design specifications.

Significance

The article categorizes testing into destructive and non-destructive methods. Mechanical evaluations include hoop and axial tensile tests, burst and proof-pressure tests, interlaminar shear strength, and fatigue assessment. Non-destructive evaluation (NDE) methods, such as ultrasonic inspection, acoustic emission monitoring, X-ray computed tomography, and thermography, are discussed with emphasis on defect detection (voids, delamination, fiber misalignment). Dimensional checks such as roundness, thickness variation, and surface finish are addressed alongside statistical process control and documentation practices. The article references relevant ASTM and ISO standards, linking each to its typical application. An additional section introduces digital quality monitoring, including sensor integration, in-situ tension tracking, and AI-assisted data analysis for real-time feedback.

Scope

Readers learn how to establish quality protocols that ensure filament-wound components meet performance and safety targets. The article provides decision guidance on selecting test methods based on product geometry and criticality, reinforcing the link between testing, certification, and continuous improvement.

Classification of Testing Approaches for Filament-Wound Structures

Testing and inspection of filament-wound composite structures are consistently divided into destructive and non-destructive approaches, reflecting their distinct roles in qualification and quality assurance. Destructive testing establishes, e.g., ultimate strength, failure modes, and damage tolerance limits, which are critical for certification and design validation. Non-destructive evaluation (NDE), by contrast, is aimed at identifying manufacturing-induced defects, monitoring damage evolution, and verifying structural integrity without compromising the component's usability. For pressure vessels and piping, these two approaches are complementary and are typically applied sequentially or in parallel throughout the manufacturing and qualification lifecycle.

Destructive Mechanical Testing Methods

Pressure, Tensile, and Structural Performance Tests

Destructive mechanical testing of filament-wound composites includes hoop tensile tests, axial tensile tests, burst pressure tests, and proof-pressure tests, each targeting specific aspects of structural response. Hoop and axial tensile tests isolate directional stiffness and strength contributions associated with winding angle and fiber orientation. The typically cylindrical geometry of filament-wound components may pose challenges, as part curvature can limit the fabrication of common specimen types, such as dogbone tensile specimens. Consequently, burst pressure testing is a widely used approach to evaluate the combined response of the laminate under internal pressure, revealing catastrophic failure mechanisms such as fiber rupture, matrix cracking, and delamination propagation. These tests are also effective in assessing the influence of localized damage, such as low-velocity impact, on residual strength in composite overwrapped pressure vessels. Proof-pressure testing, conducted at pressures below burst levels, serves as a structural integrity verification step and is widely prescribed in ASTM standards for filament-wound pipes and vessels.

Interlaminar Shear and Fatigue Testing

Interlaminar shear strength (ILSS) testing using short beam specimens provides a direct measure of the quality of bonding between adjacent layers formed during FW. Because ILSS is sensitive to void content, resin-rich or resin-starved regions, and incomplete consolidation, it is frequently used to evaluate the effects of winding tension, resin impregnation, and cure quality. Notably, it is permissible to use curved laminates as short beam specimens. Fatigue testing, often performed under cyclic pressure rather than mechanical loading, assesses stiffness degradation and progressive damage accumulation, which are critical considerations for pressure vessels subjected to repeated service cycles.

Non-Destructive Evaluation (NDE) Techniques

Ultrasonic Inspection and Associated Defect Types

Ultrasonic inspection is among the most widely applied NDE methods for filament-wound composites, capable of detecting voids, delaminations, and fiber misalignment through pulse-echo or through-transmission configurations.

Acoustic Emission Monitoring and Damage Mechanisms

Acoustic emission (AE) monitoring is used to detect active damage mechanisms rather than pre-existing defects. AE signals are generated by events such as matrix microcracking, fiber-matrix debonding, delamination growth, and fiber fracture during mechanical or pressure loading. AE is valuable during proof-pressure and burst testing, as it enables correlation between load levels and the onset of specific damage mechanisms. In filament-wound structures, AE monitoring can therefore be used to identify critical damage thresholds and to distinguish between benign manufacturing defects and damage that actively propagates under load.

X-ray Computed Tomography and Internal Architecture Defects

X-ray computed tomography (CT) provides three-dimensional visualization of internal laminate architecture and is capable of resolving porosity distribution, fiber waviness, fiber misalignment, and local resin-rich regions. CT is a validation tool for characterizing complex internal features that are difficult to detect using conventional ultrasonic methods. Although its application is generally limited to laboratory-scale components, CT is used to benchmark manufacturing quality and to validate damage observations from destructive testing.

Infrared Thermography and Near-Surface Defects

Infrared thermography is primarily sensitive to near-surface defects and subsurface delaminations, which alter local heat flow and thermal diffusivity. Thermographic methods are particularly effective for rapid inspection over large surface areas, making them suitable for cylindrical filament-wound shells. Active thermography, in which an external heat source is applied, enhances defect detectability and is commonly used to screen for delamination and debonding in composite structures.

Dimensional Inspection and Process Control

Dimensional inspection complements mechanical testing and NDE methods by verifying geometric conformity. Measurements of roundness, wall thickness variation, and surface finish are essential for ensuring that filament-wound components meet design tolerances and performance expectations. Variations in thickness or roundness can indicate inconsistencies in winding tension, fiber placement accuracy, or mandrel alignment, all of which can influence mechanical performance. Packaging and handling of filament-wound products also follow standardized requirements to preserve structural integrity during transport. ASTM standards emphasize the use of statistical process control (SPC) and detailed documentation to track manufacturing variability and ensure traceability. These practices support correlation between process parameters and observed defects or performance deviations, reinforcing quality assurance frameworks for filament-wound products.

Standards Governing Testing and Manufacturing Practices

ASTM and ISO standards provide structured guidance linking test methods to specific applications. ASTM D2996 and ASTM D3299 define requirements for filament-wound pipes and corrosion-resistant tanks, including pressure testing, dimensional inspection, and documentation. ASTM D4018 supports raw material qualification by standardizing testing of continuous filament carbon and graphite fiber tows. ISO 1268-5 specifies procedures for producing filament-wound test plates, enabling consistent mechanical characterization. Together, these standards ensure that destructive testing, NDE, and dimensional inspection are applied in a coherent and reproducible manner.

Digital and Intelligent Quality Monitoring in Filament Winding

Robotic and advanced winding machines incorporate in-situ tension sensors, process parameter logging, and real-time feedback control, enabling tighter control over fiber placement and resin distribution. These systems reduce variability associated with manual or semi-automated processes, particularly for complex geometries. The application of AI and machine learning to FW and composite manufacturing is discussed as a means of analyzing large datasets generated during production and inspection. Machine learning techniques are used to identify patterns associated with defects, predict quality outcomes, and support process optimization. These digital approaches are presented as enhancements to established testing and standards-based frameworks, strengthening real-time decision-making and quality assurance rather than replacing conventional qualification methods.

References

A single unit of fibre reinforcement that cannot be separated further without breaking it. It often has a circular cross section that is formed by the dies used to create it or its precursor material. Many filaments are manufactured in parallel to produce a strand or tow. The number of filaments in a strand or tow is often references by how many thousands of filaments comprise the tow. e.g. an 18k tow is made up of ~18000 filaments.

The failure mode characterized by a partial or full separation of adjacent laminae. Delamination is unique to laminated structures, and can be induced by impact load, fatigue load, and quasistatic load. (Also known as interlaminar separation)

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

With regards to modelling & simulation, validation demonstrates the accuracy of a computational model — in the context of its intended use — to check that it represents the physics of the problem and the ‘reality of interest’ (needs are satisfied).

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

With regards to modelling & simulation, verification confirms the implementation of a computational model to ensure that it represents the mathematical model and equations used to describe it (requirements are met).

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

A quantitative measure of how a material will respond to transient thermal conditions. It is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity).

Materials with large thermal diffusivity have a high thermal conductivity relative to their capacity to store energy (heat capacity) and will rapidly distribute the thermal energy throughout its volume and rapidly change temperature to reach the new equilibrium temperature.

Materials with small thermal diffusivity have a low thermal conductivity relative to their capacity to store energy (heat capacity) and will have a sluggish temperature response to a change in thermal conditions (large temperature lag). This is because it takes more energy per unit volume to change the temperature, and the low thermal conductivity means it takes longer to distribute the thermal energy throughout its volume.

An engineering design activity that relates to the determination of material level properties that will be used in the structural design and certification.