Composite Materials Strength Determination Within the Current Certification Methodology for Aircraft Structures

Abstract

Although several composite failure criteria have been proposed over the years to predict the static strength of polymer composites, including micromechanical and first-ply failure theories, none of them has demonstrated an ability to predict the onset of damage across the wide range of load conditions and structural configurations. For that reason, current aerospace practice uses empirically derived laminate-based allowables to demonstrate compliance with regulatory requirements for static strength. Some examples of certification by analysis supported by test evidence as well as certification by test are given, and the fundamental differences are highlighted in the context of the building block philosophy. The lack of accepted failure criteria and general material-degradation models for the prediction of damage initiation and propagation provides the background for the current methodology, which relies either on testing alone or on analysis supported by extensive test evidence.

Lamination Parameters for Sandwich and Hybrid Material Composites

“…Facesheet failure will be determined in this paper using a common technique employed in industry known as the angle minus longitudinal (AML) method [26]. AML refers to the percentage of 45°p lies (angle) minus the percentage of plies at 0°(longitudinal), a metric for which laminates of similar values tend to possess the same failure modes in notched or damaged compression tests.…”

Design allowables of composite laminates: A review

“…An overview in abbreviated form of the CMH-17 recommendations for the evaluation of A- and B-basis allowables is presented by Rust et al 21 and a deep understanding of the certification process by test evidence can be found in Feraboli. 22 King 23 outlines the existing statistical methods that can be adopted for composite material characterization. Several available certification documents and research studies 24–26 show the usage of the CMH-17 approach on real testing data.…”

Review of material test standardization status for the material qualification of laminated thermosetting composite structures

“…Moreover, other obsolete standards, such as Suppliers of Advanced Composite Materials Association (SACMA), Composites Research Advisory Group (CRAG), and Association Européenne des Constructeurs de Matériel Aérospatial (AECMA), are still in use. 21 , 22 The typical test matrices for material qualification are summarized in Tables 1 to 3, wherein the available test standards and sampling requirements are also listed. Although most testing methods described in different standards are basically equivalent, various differences exist between the geometric features of the specimens, test fixtures, test parameters and their respective requirements (i.e.…”

“…In the preliminary stage of engineering projects, the importance of material specification lies in effectively validating the structural design, and performing quality control on the selected materials. 12 , 21 To quantitatively assess the differences in the material properties and process parameters, a subset of qualification testing is subsequently required to determine the material properties of uncured prepreg and cured laminar, as presented in Tables 1 and 2. 22 Moreover, as mentioned above, the design allowables can be obtained by statistically analyzing the experimental data.…”