Significance and Use
This guide references requirements that are intended to control the quality of NDT data. The purpose of this guide, therefore, is not to establish acceptance criteria and therefore approve composite materials or components for aerospace service.
Certain Practices referenced in the guide are written so they can be specified on the engineering drawing, specification, purchase order, or contract, for example, Practice E 1742 (Radiography).
Acceptance Criteria—Determination about whether a composite material or component meets acceptance criteria and is suitable for aerospace service must be made by the cognizant engineering organization. When examinations are performed in accordance with the referenced documents in this guide, the engineering drawing, specification, purchase order, or contract shall indicate the acceptance criteria.
Accept/reject criteria shall consist of a listing of the expected kinds of imperfections and the rejection level for each.
The classification of the articles under test into zones for various accept/reject criteria shall be determined from contractual documents.
Rejection of Composite Articles—If the type, size, or quantities of defects are found to be outside the allowable limits specified by the drawing, purchase order, or contract, the composite article shall be separated from acceptable articles, appropriately identified as discrepant, and submitted for material review by the cognizant engineering organization, and dispositioned as (1) acceptable as is, (2) subject to further rework or repair to make the materials or component acceptable, or (3) scrapped when required by contractual documents.
Acceptance criteria and interpretation of result shall be defined in requirements documents prior to performing the examination. Advance agreement should be reached between the purchaser and supplier regarding the interpretation of the results of the examinations. All discontinuities having signals that exceed the rejection level as defined by the process requirements documents shall be rejected unless it is determined from the part drawing that the rejectable discontinuities will not remain in the finished part.
Life Cycle Considerations—The referenced NDT practices and test methods have demonstrated utility in quality assurance of PMCs during the life cycle of the product. The modern NDT paradigm that has evolved and matured over the last twenty years has been fully demonstrated to provide benefits from the application of NDT during: (a) product and process design and optimization, (b) on-line process control, (c) after manufacture inspection, (d) in-service inspection, (e) and health monitoring.
In-process NDT can be used for feedback process control since all tests are based upon measurements which do not damage the article under test.
The applicability of NDT methods to evaluate PMC materials and components during their life cycle is summarized in Tables 3 and 4.
General Geometry and Size Considerations—Part contour, curvature, and surface condition may limit the ability of certain tests to detect imperfections with the desired accuracy.
Reporting—Reports and records shall be specified by agreement between purchaser and supplier. It is recommended that any NDT report or archival record contain information, when available, about the material type, method of fabrication, manufacturers name, part number, lot, date of lay-up and/or of cure, date and pressure load of previous tests (for pressure vessels), and previous service history (for in-service and failed composite articles). Forwards and backwards compatibility of data, data availability, criticality (length of data retention), specification change, specification revision and date, software and hardware considerations will also govern how reporting is performed.
TABLE 3 Application Examples of Established NDT Methods During Life Cycle
| NDT Method | Application |
|---|---|
| Acoustic Emission | May be used for quality control of production and fabrication processes (for example, to evaluate adhesive bonding after lay-up winding or curing), for proof-testing of pressure vessels after fabrication, and for periodic in-service and health monitoring inspections prior to failure. |
| Computed Tomography | May be used as a post-fabrication metrological method to verify engineering tolerances. |
| Leak Testing | May be used to validate leak tightness following fabrication, and in-service re-qualification of pressure vessels. For example, helium leak detection can be used during composite article fabrication to detect and seal leaks permanently (preferable) or temporarily in such a manner to allow repair at a later time. Similarly, halogen gas leak detection has been used in production examination. |
| Radiography and Radioscopy | May be used during fabrication inspection to evaluate honeycomb core imperfections or discontinuities such as node bonds, core-to-core splices, core-to-structure splices, porosity, included material as well as verification of structural placement. Water included material bonded structure not for laminates. |
| Shearography | May be used in quality assurance, material optimization, and manufacturing process control. |
| Strain Measurement | May be used during proof testing before placement into service, or during periodic re-qualification. Can be destructive depending on the strain thresholds reached during test. |
| Thermography | May be used to follow imperfection or discontinuity growth during service. If video thermographic equipment is used, systems that are being dynamically tested or used can be examined in real-time. |
| Ultrasound | Automatic recording systems allow parts to be removed from a processing line when defect severity exceeds established limits. Measurement of the apparent attenuation in composite materials is useful in applications such as comparison of crystallinity and fiber loading in different lots, or the assessment of environmental degradation. Most common method applied for laminar oriented defect detection such as impact damage causing delamination fiber fracturing, included material, and porosity. |
| Visual NDT | Used primarily for quality inspections of composite materials and components upon receipt (after fabrication and before installation). |
TABLE 4 Application of Established NDT Methods During the Life Cycle of Polymeric Matrix Composites
| Defect | Product and Process Design and Optimization | On-Line Process Control | After Manufacture Inspection | In-Service Inspection | Health Monitoring |
|---|---|---|---|---|---|
| Acoustic Emission | X | X | X | X | X |
| Computed Tomography | X | ||||
| Leak Testing | X | X | XA | ||
| Radiography and Radioscopy | X | X | X | X | |
| Shearography | X | X | X | X | |
| Strain Measurement | X | X | |||
| Thermography | X | X | |||
| Ultrasound | X | X | X | X | X |
| Visual | X | X | X | X | X |
A Applicable to composites used in storage and distribution of fluids and gases, for example, filament-wound pressure vessels.
Scope
1.1 This guide provides information to help engineers select appropriate nondestructive testing (NDT) methods to characterize aerospace polymer matrix composites (PMCs). This guide does not intend to describe every inspection technology. Rather, emphasis is placed on established NDT methods that have been developed into consensus standards and that are currently used by industry. Specific practices and test methods are not described in detail, but are referenced. The referenced NDT practices and test methods have demonstrated utility in quality assurance of PMCs during process design and optimization, process control, after manufacture inspection, in-service inspection, and health monitoring.
1.2 This guide does not specify accept-reject criteria and is not intended to be used as a means for approving composite materials or components for service.
1.3 This guide covers the following established NDT methods as applied to polymeric matrix composites: Acoustic Emission, Computed Tomography, Leak Testing, Radiography, Radioscopy, Shearography, Strain Measurement (contact methods), Thermography, Ultrasound, and Visual NDT.
1.4 The value of this guide consists of the narrative descriptions of general procedures and significance and use sections for established NDT methods as applied to polymer matrix composites. Additional information is provided about the use of currently active standard documents (an emphasis is placed on applicable standard guides, practices, and test methods of ASTM Committee E07 on Nondestructive Testing), geometry and size considerations, safety and hazards considerations, and information about physical reference standards.
1.5 To ensure proper use of the referenced standard documents, there are recognized NDT specialists that are certified in accordance with industry and company NDT specifications. It is recommended that a NDT specialist be a part of any composite component design, quality assurance, in-service maintenance or damage examination.
1.6 This guide summarizes the application of NDT methods to fiber- and fabric-reinforced polymeric matrix composites. The composites of interest are primarily, but not exclusively limited to those containing high modulus (greater than 20 GPa (3×106 psi)) fibers. Furthermore, an emphasis is placed on composites with continuous (versus discontinuous) fiber reinforcement.
1.7 This guide is applicable to polymeric matrix composites containing but not limited to bismaleimide, epoxy, phenolic, poly(amide imide), polybenzimidazole, polyester (thermosetting and thermoplastic), poly(ether ether ketone), poly(ether imide), polyimide (thermosetting and thermoplastic), poly(phenylene sulfide), or polysulfone matrices; and alumina, aramid, boron, carbon, glass, quartz, or silicon carbide fibers.
1.8 The composite materials considered herein include uniaxial laminae, cross-ply laminates, angle-ply laminates, and structural sandwich constructions. The composite components made therefrom include filament-wound pressure vessels, flight control surfaces, and various structural composites.
1.9 The values stated in SI units are to be regarded as the standard. The values given in parentheses are provided for information only.
1.10 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
