Out-of-Autoclave Qualification: Pathways to Certify OoA Prepregs for Primary Structures

Introduction

Autoclaves have shaped the aerospace composites industry for four decades, but they are also its most expensive bottleneck. A large autoclave costs millions of dollars, consumes enormous energy, and limits part dimensions to the envelope of the vessel. Widebody aircraft programs need wing covers and fuselage panels that stretch beyond the practical size of even the largest production autoclaves, and every production rate increase competes for oven and autoclave capacity. Out-of-autoclave prepregs — materials cured in a vacuum bag under atmospheric pressure — are the industry's answer: parts as large as a vacuum bag can be, at void levels that approach autoclave quality.

Adopting OoA materials for primary structures is a certification exercise as much as a manufacturing decision. The qualification process must prove to authorities and customers that the material system is consistent, that its mechanical properties support the design allowables, and that the cured quality — especially porosity — stays within limits across real production variation. This article walks through how OoA prepregs work, the technical hurdles in qualifying them, and the statistical pathway from screening tests to A-basis allowables for primary structure certification.

How Out-of-Autoclave Prepregs Work

Conventional prepreg systems are designed to cure under several bars of autoclave pressure, which squeezes air and volatiles out of the laminate. An OoA prepreg must achieve the same low void content with only the roughly one bar of pressure available from vacuum bagging. It does this through a partially advanced resin chemistry: the resin B-stages to a point where the preform contains an engineered network of microscopic air channels. During the vacuum hold, air and moisture are drawn out through these channels before the resin gels; once gelled, the channels close and the laminate cures to a dense, low-porosity part.

The practical result is that OoA laminates routinely achieve void content below one percent in flat and gently curved panels, approaching the quality of autoclave-cured parts. The trade-off is processing discipline. Vacuum integrity is everything: a leaking baglet, an excessive ramp rate that cures the surface before the interior evacuates, or an inadequate vacuum hold time all trap air and produce porosity. Process control systems that monitor vacuum level, temperature uniformity and ramp rates are therefore central to OoA production, just as pressure control is central to autoclave curing.

OoA materials entered airframes through secondary structures — fairings, control surfaces and interior panels — where the certification burden is lighter. That experience base enables the move to primary structures: the same material system, process controls and inspection rigor are already qualified, and the primary-structure campaign concentrates on the additional tests, environmental conditions and statistical basis values required for flight-critical load paths. Airframers describe this as a staged adoption whose pace varies with program risk tolerance and part size.

The Qualification Challenge for Primary Structures

Certifying a material for primary structures requires demonstrating that its properties are reliable across batch variation, manufacturing tolerances and environmental exposure. The established framework — documented in composite handbooks such as CMH-17 and reflected in airworthiness guidance — organizes the effort in two layers: material qualification, which establishes the performance of the material system as produced by the supplier, and allowables development, which converts test data into statistically defined design values used across the airframe.

Porosity deserves special attention in an OoA campaign because it is the failure mode the process must guard against. Porosity degrades compressive strength and interlaminar shear most severely, and inspection methods such as ultrasonic C-scan and micrographic cross-sections are used both for development programs and for production quality control. Qualification testing therefore pairs mechanical testing with a rigorous porosity control strategy, and production parts typically carry defined acceptance thresholds tied to design analysis.

Batch-to-batch consistency is the statistical foundation of any qualification, and it deserves extra attention for OoA materials because the process window is narrower than autoclave curing. A material lot that cures acceptably in development may behave differently when resin age, fiber batch or bagging practice drift, and the qualification matrix is designed to capture that variation rather than hide it. Auditors and airframers therefore look for documented process capability: vacuum integrity records, temperature uniformity surveys, moisture control at layup and a supplier quality system that ties every delivered panel to its material and process history.

From Screening to A-Basis Allowables

A typical qualification pathway proceeds through screened stages. Initial screening evaluates candidate resin systems with small coupons — lap shear, short-beam strength and micrographic void checks — to select materials that cure acceptably at vacuum pressure. The selected system then enters a full qualification matrix that spans multiple material batches, environmental conditions and test types, generating the statistically based values used in design.

Statistical basis values follow well-defined rules. An A-basis value is the strength level exceeded by 99 percent of the population with 95 percent confidence, while a B-basis value is exceeded by 90 percent of the population with 95 percent confidence. Primary structural design commonly uses B-basis values for typical airframe sizing, with A-basis retained for critical load conditions. Achieving these values requires testing across the environmental envelope — room temperature dry, elevated temperature wet, and cold conditions — because matrix-dominated properties such as compression and shear change substantially with temperature and moisture content.

Tests That Define an OoA Material System

The table below lists the mechanical tests that anchor a typical OoA qualification campaign, alongside the properties they establish.

Test	Property Established	Standard Reference
Uniaxial tension	Fiber-dominated tensile strength and modulus	ASTM D3039
Compression after impact	Damage tolerance with impact damage	ASTM D7136/D7137
Open-hole compression	Strength with a stress-raising hole	ASTM D6484
In-plane shear	Matrix-dominated shear response	ASTM D3518
Interlaminar shear	Layer-to-layer shear strength	ASTM D2344
Density and fiber content	Consolidation and fiber volume fraction	ASTM D792/D3171

Key Steps in an OoA Certification Pathway

• Material screening: Small-coupon trials confirm that a candidate resin system cures to low void content under vacuum bag pressure alone.
• Process definition: Vacuum hold time, ramp rates and temperature uniformity are documented and held as production controls.
• Multi-batch testing: The qualification matrix spans at least several material batches to capture real supplier variation.
• Environmental envelope: Tests run at room temperature dry, elevated temperature wet and cold conditions to bound service behavior.
• Statistical analysis: A-basis and B-basis values are computed with defined reliability and confidence levels.
• Acceptance criteria: Production parts carry porosity and quality limits tied to the design analysis.

Frequently Asked Questions

How close is OoA part quality to autoclave quality?

OoA laminates typically achieve void content below one percent, approaching autoclave quality in flat and gently curved panels. The qualification program verifies this through ultrasonic inspection and micrographic sectioning across multiple batches. The main caveat is process discipline: vacuum integrity, temperature uniformity and ramp control determine whether a given part actually reaches the low porosity the material system is capable of, which is why automated process monitoring is central to OoA production.

What is the difference between A-basis and B-basis allowables?

An A-basis value is the strength exceeded by 99 percent of the material population with 95 percent confidence; a B-basis value is exceeded by 90 percent with 95 percent confidence. B-basis is commonly used for typical airframe sizing, while A-basis is retained for critical load conditions. The statistical rules ensure the design allowables are conservative relative to real batch-to-batch variation, which is why multi-batch qualification testing matters.

Why is porosity a special concern for OoA materials?

Porosity is the failure mode that OoA processing must defeat, because only vacuum pressure is available to consolidate the laminate. Porosity degrades compressive strength and interlaminar shear most severely. Qualification campaigns combat it with a defined vacuum hold and ramp policy, ultrasonic and micrographic verification, and production acceptance thresholds tied to design analysis — so parts prove low porosity before they are released for assembly.

Conclusion

Out-of-autoclave prepregs have matured from a research curiosity into a qualification-ready production material. Their process economics — no autoclave capital, lower energy, part size limited only by vacuum bagging equipment — matter most for the large structures at the edge of autoclave capacity, and their certification pathway is now a well-understood exercise combining material screening, multi-batch mechanical testing, environmental coverage and statistical analysis. Companies that build the process discipline and test campaign into their capability early will be the suppliers qualified to serve the next generation of widebody and large-part programs.

YongXian supplies carbon fiber fabrics, prepregs and process materials for aerospace and industrial composite manufacturing. Explore our carbon fiber product range or contact our engineering team to discuss materials and process support for your qualification program.