Interlaminar tensile strength in composites and how to interpret test results
Why interlaminar tensile strength matters
Interlaminar tensile strength is the resistance of a composite laminate to opening, or through-thickness tensile, loads that tend to separate adjacent plies. It matters because many fiber-reinforced laminates are very strong along the fiber direction but much less robust through the thickness, where the polymer matrix and the fiber-matrix interface carry much of the load.
For design, quality control, and material screening, the number is useful only when the test method, laminate architecture, conditioning, and failure mode are understood. ASTM D7291 flatwise tension and ASTM D6415 curved beam strength are often discussed together, but they create different stress fields. Their results generally should not be treated as interchangeable.
In industry applications, interlaminar tensile strength becomes important when a laminate sees peel-like loading, through-thickness tension, tight curvature, ply drops, bonded joints, impact damage, or local stress concentrations. A high in-plane tensile strength does not automatically mean a laminate will resist delamination well under out-of-plane loading.
What interlaminar tensile strength measures
In a laminated composite, each ply can be designed to carry load efficiently in selected directions. The weak direction is often the z-direction, or thickness direction, because conventional laminate designs usually do not place continuous reinforcement through the thickness. When an opening load acts between plies, the matrix, ply interface, void population, cure quality, and local fiber architecture strongly influence the measured strength.
The word strength is important. Interlaminar tensile strength is normally reported as a stress at failure under a defined test configuration. It is not the same as interlaminar fracture toughness, which describes the energy needed for a delamination crack to grow. A laminate may show a useful strength value in a small coupon test while still requiring fracture toughness, impact, fatigue, and compression-after-impact data for a complete damage tolerance assessment.
It is also not the same as interlaminar shear strength. Short-beam tests and double-beam shear methods are relevant to shear-dominated interlaminar behavior, but they do not replace a tensile opening test. The correct property depends on the service load path being investigated.
Main test methods and what they can show
As of October 2026, ASTM International identifies ASTM D7291/D7291M-22 and ASTM D6415/D6415M-22 as active standards commonly associated with through-thickness or interlaminar tensile behavior in polymer matrix composites. ISO 19927:2018 is also relevant to interlaminar behavior, but it addresses interlaminar strength and modulus by a double beam shear test rather than a tensile opening test. A standard title alone should not decide a test plan; the specimen, stress field, laminate type, and intended comparison all matter.
| Method or property | What it emphasizes | Useful interpretation | Key limitation |
|---|---|---|---|
| ASTM D7291/D7291M flatwise tension | Through-thickness tensile strength and modulus using bonded metal end-tabs | Useful when a relatively uniform through-thickness tensile stress field is desired | Results are sensitive to bonding, alignment, specimen preparation, and valid laminate failure |
| ASTM D6415/D6415M curved beam strength | Out-of-plane tensile stress in the curved region of a composite beam | Useful for curved features and, under appropriate unidirectional conditions, interlaminar tensile strength evaluation | The stress field is non-uniform and should not be directly compared with flatwise tension results |
| ASTM D5528/D5528M mode I fracture toughness | Opening-mode delamination growth in a double cantilever beam specimen | Useful for damage tolerance and crack growth resistance | Reports toughness, not tensile strength |
| ASTM D7905/D7905M mode II fracture toughness | Shear-mode delamination growth using end-notched flexure | Useful when delamination is driven by sliding shear | Not a substitute for opening-mode tensile strength |
| ASTM D2344/D2344M short-beam strength | Short-beam response dominated by resin and interlaminar behavior | Useful for comparative screening and quality control when failure modes are consistent | It is not a pure interlaminar tensile test |
The main practical issue is comparability. ASTM notes that D7291 and D6415 results generally are not comparable because D7291 subjects a larger material volume to an almost uniform stress field, while D6415 subjects a smaller volume to a non-uniform stress field. A report that provides only one strength value, without the method and specimen context, is incomplete for engineering use.
Variables that can change the measured value
Interlaminar tensile strength is matrix- and interface-sensitive, so it can change for reasons that may not strongly affect in-plane, fiber-dominated tensile properties. Resin chemistry, cure cycle, fiber surface treatment, sizing compatibility, void content, fiber volume fraction, ply waviness, and local compaction can all influence the result. This is why a small processing change may appear clearly in an interlaminar test even when longitudinal tensile properties remain stable.
Laminate architecture also matters. A unidirectional specimen, a woven fabric laminate, a quasi-isotropic laminate, and a thick multidirectional laminate may not fail in the same way under a nominally similar through-thickness load. Ply drops, blocked plies, tight radii, corners, and thickness transitions can create local out-of-plane stresses that are not captured by a simple in-plane coupon test.
Environmental conditioning should not be treated as an afterthought. Moisture and temperature can reduce matrix-dominated properties or change the failure behavior. If two data sets were generated under different conditioning histories, test temperatures, or moisture states, the numbers may not support a direct material ranking.
Specimen preparation is another common source of scatter. Bonded end-tabs in flatwise tension tests must transfer load without creating premature adhesive failure or bending. Curved beam tests depend on geometry, manufacturing quality, and the local defect population in the radius. Failure mode inspection is therefore as important as the calculated value.
How to read an interlaminar tensile test report
A useful report should allow the reader to understand not only the average strength, but also why the value is valid for the intended comparison. A single number calculated from maximum load is rarely enough for engineering interpretation.
| Report item | Why it matters |
|---|---|
| Test standard and edition | Confirms whether the result came from flatwise tension, curved beam loading, or another method |
| Material system and laminate layup | Defines resin, reinforcement, stacking sequence, thickness, and architecture that control interlaminar behavior |
| Specimen dimensions and preparation route | Helps identify geometry effects, machining effects, bonding quality, and volume of material under stress |
| Conditioning and test environment | Shows whether temperature, humidity, moisture, or aging could affect matrix-dominated strength |
| Failure mode and failure location | Separates valid laminate failure from tab, adhesive, grip, edge, or mixed failures |
| Number of specimens and scatter | Indicates whether the result is a stable material indicator or a limited screening value |
For production quality control, consistency may be more important than the absolute maximum value. For design allowables, the data requirement is usually broader: multiple batches, controlled environments, statistical treatment, and correlation with structural features. For research work, the report should also describe microstructural observations when possible, because voids, resin-rich regions, and fiber waviness can help explain unexpected scatter. See also: Application.
Common interpretation mistakes
The first mistake is treating interlaminar tensile strength as a universal delamination number. Delamination can be driven by opening, sliding shear, mixed-mode loading, fatigue, impact, thermal mismatch, or manufacturing defects. One strength value cannot describe all of these conditions.
The second mistake is comparing results from different methods as if they were equivalent. A D7291 flatwise tension result and a D6415 curved beam result may both relate to through-thickness behavior, but they do not load the same material volume in the same way. If the goal is to compare materials, batches, or processes, the same method and comparable specimen preparation should be used.
The third mistake is ignoring invalid or mixed failure modes. Adhesive failure in a bonded fixture, edge-initiated damage from machining, or premature grip-related failure can make a value look like a material property when it is actually a specimen or setup artifact. Photographs, fracture surface notes, and failure mode coding make the data more useful.
The fourth mistake is using a room-temperature dry coupon result to represent a hot-wet structure, a fatigue environment, or an impacted laminate. Matrix-dominated properties are especially sensitive to environment and damage history, so the test conditions must match the question being answered.
Practical use in material selection and quality control
Interlaminar tensile testing is most useful as part of a broader building-block approach. At the material screening stage, it can help compare resin systems, fiber surface treatments, cure cycles, and laminate processing routes. In quality control, it can reveal changes in void content, consolidation, cure, or bonding quality that may not be obvious from in-plane tensile results alone.
For structural design, the property should be connected to the actual load path. Curved composite parts, bonded assemblies, sandwich transitions, thick sections, and areas with ply drops may require more representative element or subcomponent tests. Coupon data can guide material selection and analysis, but it should not replace validation of the feature that creates the interlaminar stress.
Readers looking for related materials testing topics can browse the Testing section for more articles on composite and new material evaluation. When planning formal tests, always check the controlled versions of the relevant ASTM or ISO standards, because standard editions, scope language, and reporting requirements can change.
Frequently asked questions
Is interlaminar tensile strength the same as through-thickness tensile strength?
The terms are closely related and are sometimes used together, especially when a through-thickness tensile load causes separation between plies. However, the exact interpretation depends on the test method and the observed failure mode. A flatwise tension result may be reported as through-thickness tensile strength, but the report still needs to confirm where and how the specimen failed.
Which test method should be used for ILTS?
There is no single answer for every laminate. ASTM D7291/D7291M is relevant when a flatwise through-thickness tensile loading arrangement is appropriate. ASTM D6415/D6415M is relevant for curved beam strength and can be used for interlaminar tensile strength under the standard’s applicable laminate conditions. The choice should follow the engineering question, material form, geometry, and required comparability.
Can ILTS predict impact delamination?
Not by itself. Impact damage involves contact mechanics, bending, local crushing, matrix cracking, delamination growth, and residual strength. ILTS may provide useful matrix-dominated strength information, but impact evaluation usually requires dedicated impact and residual strength tests, along with fracture toughness data when crack growth is important.
Why do different laboratories report different values?
Differences can come from laminate quality, specimen machining, end-tab bonding, alignment, conditioning, temperature, loading rate, failure mode classification, and statistical sample size. For meaningful comparison, laboratories should use the same standard edition, specimen geometry, conditioning protocol, and reporting rules.
