ASTM D7615 Open-Hole Fatigue Test | Composite Laminate Fatigue Response

Information on the most widely used ASTM standards within the materials testing industry

Descriptions of the most prominent lSO standards within the materials testing industry

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Material strength is a course that most engineering fields are required to study in university education. This interesting course is very practical and makes use of testing equipment in the materials laboratory.

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Mechanical testing in the transportation industry concerns the load-bearing and fatigue resistance performance of core components such as vehicles, tracks, and containers, directly ensuring transport safety and equipment durability. UnitedTest can cover mechanical testing for all types of transportation materials, complying with national and international standards, and provides customized testing solutions including tensile, fatigue, and impact tests.

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UnitedTest offer world-class testing solution in testing for polymeric materials, including elastomers, thermoplastics, composites, coatings, and structural adhesives in a variety of industries.

Testing of rubber and elastomers is used to assess the various properties and performance of materials under different conditions. This is crucial for ensuring the safety, functionality, and quality of rubber and elastomer products used in industries such as automotive, construction, consumer goods, and aerospace.

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Tensile test evaluates material resistance to stretching, measuring strength/elongation critical for quality control. UnitedTest offers universal tensile testers (10-600KN) compliant with ASTM/ISO standards, providing tailored solutions for metals, polymers, medical & industrial materials.

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Flexural & Bend tests evaluate material resistance to bending forces, measuring strength, stiffness and ductility for quality assurance. UnitedTest provides flexural testing machines (3-point/4-point bend) compliant with ASTM/ISO standards, offering tailored solutions for composites, plastics, metals and construction materials.

Torsion Test evaluates material resistance to rotational forces, measuring torsional strength/ductility critical for fasteners, insulators, shafts. UnitedTest offers torsion testers compliant with ASTM A938/ISO 16047, providing tailored solutions for automotive, aerospace, fastener industries.

Fatigue Test assesses material durability under cyclic loading, critical for predicting service life of industrial components. UnitedTest offers fatigue testing systems (axial/bending/torsion) compliant with ASTM E466/ISO 12106, providing tailored solutions for automotive, aerospace and manufacturing sectors.

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ASTM D7615 Open-Hole Fatigue Tester | Polymer Composite Laminate Fatigue Testing | UnitedTest

ASTM D7615 evaluates open-hole fatigue response of symmetric balanced polymer matrix composite laminates via cyclic OHT and OHC fatigue loading. UnitedTest manufactures ASTM D7615 compliant composite fatigue testing machines for structural damage tolerance and lifecycle performance analysis.

ASTM D7615 is a professional standard practice dedicated to characterizing the open-hole fatigue response of continuous-fiber-reinforced polymer matrix composite laminates with symmetric and balanced layup structures. This standard transforms static open-hole tensile (OHT) and open-hole compressive (OHC) strength testing methodologies into standardized constant-amplitude uniaxial cyclic fatigue tests, simulating real-world repeated structural loading conditions on notched composite components with central drilled holes.

The ASTM D7615 test procedure supports multiple cyclic loading modes, including tension-tension, compression-compression, and reversed tension-compression fatigue loading. Testing can be controlled by constant engineering stress or constant applied force amplitude, delivering comprehensive fatigue performance data such as composite fatigue life, progressive stiffness degradation, cumulative damage accumulation, and final fatigue failure modes. It effectively evaluates notch sensitivity and long-term structural durability of perforated composite laminates under cyclic mechanical stress.

Widely applied in aerospace structural validation, wind energy component qualification, automotive lightweight composite R&D and advanced material durability certification, ASTM D7615 complements static open-hole test standards including ASTM D5766 for full-spectrum composite structural performance assessment. UnitedTest designs and manufactures high-precision ASTM D7615 open-hole fatigue testing machines, supporting accurate cyclic loading control, real-time stiffness monitoring and reliable fatigue data recording for industrial and laboratory composite fatigue analysis.

Test Principle

A flat coupon with a centrally located hole is cycled axially between a minimum and maximum force (stress) at a specified frequency and waveform.

The hole is a deliberate stress concentrator/notch — it reproduces the condition of a fastener hole, access cut-out or any penetration in real structure.

Fatigue damage (matrix microcracking, fibre fracture, splitting, delamination) accumulates progressively and shows up macroscopically as loss of axial stiffness. Because stiffness can be measured without destroying the coupon, it is used as a non-destructive damage metric.

At prescribed cycle intervals, fatigue loading is paused, the specimen is taken through one slow quasi-static cycle, and the chord stiffness is extracted from the force–deflection hysteresis loop.

The stiffness-versus-cycles record is "S"-shaped (rapid early change, long plateau, rapid drop near failure). The test ends at fracture, at a predefined stiffness loss agreed beforehand, or at run-out (a specified cycle limit with no failure).

Repeating this at several stress levels produces an S–N (stress–life) curve, and replicate tests at each level give a fatigue-life distribution (log-normal per E739, or two-parameter Weibull).

ASTM D7615 Open-Hole Fatigue Testing for Polymer Matrix Composite Laminates

Standard Test Specimen Information

Base configuration: As defined in ASTM D3039 — flat, rectangular coupon with end tabs.

Mandatory specimen preparation extra rules for fatigue testing:

All cutting edges must be polished to optical microscope observable smoothness to eliminate edge delamination initiation points

Tabbing adhesive must possess high cyclic fatigue resistance to avoid tab premature failure before gauge section fatigue damage

Laminate stacking sequence must be documented; multi-angle laminates require pre-test assessment of free-edge delamination risks

Minimum specimen quantity requirements:

Exploratory preliminary test: Minimum 6 specimens

R&D material performance testing: Minimum 12 specimens

Design allowable & reliability certification data: Minimum 24 specimens per load/strain level

At least 3 distinct maximum load/strain gradient levels required to complete valid S-N curve plotting.

Specific test methods covered

By loading mode / parent static method

Fatigue mode	Governing specimen & apparatus configuration
Tension–tension (T–T)	D5766 Configuration A (open-hole tensile)
Tension–compression (T–C, reversed)	D6484 Procedure A (open-hole compression)
Compression–compression (C–C)	D6484 Procedure A

Two ways of transitioning into fatigue loading

Method A — Amplitude Loading: quasi-statically ramp force to the desired mean force (set point), then slowly open the amplitude (span) until the target peak and valley are reached. The number of cycles used in this transition must be recorded and reported.

Method B — Direct Loading: quasi-statically ramp to the maximum (or minimum) force, then cycle immediately between peak and valley with a haversine waveform (valley never drops below the minimum). This eliminates the transition phase and requires modern signal generators/controllers.

Two end-point definitions: fracture (with failure mode/location recorded) or a designated degree of stiffness degradation; plus run-out as a censored result.

Test Equipment for ASTM D7615 Open-Hole Fatigue Testing for Polymer Matrix Composite Laminates

Dynamic Fatigue Testing Machine

Recommend UnitedTest UTDS series electronic dynamic fatigue testing machine.

Complies with ASTM E467 for dynamic force verification; force control stability; test amplitude controller monitoring load at least every three cycles; minimize load drift away from peak/valley set‑points.

Dynamic force verification within 1 % of true applied force; fatigue rating higher than maximum test force

Auxiliary Supplies

Gauge length 25 mm [1.0 in.]; minimum Practice E83 Class B-1 over the strain range of interest, calibrated per E83; essentially free of inertia lag at the test speed.

Hydraulic wedge grips recommended for fatigue; per D5766 (T–T) or D6484 Procedure A (T–C/C–C); must have sufficient fatigue rating.

ASTM D3479 Tension-Tension Fatigue of Polymer Matrix Composites

Support fixture: Required whenever compressive force is applied (during fatigue or during quasi-static stiffness/residual-strength checks) — per D6484/D6484M; bolts retorqued at each stiffness stop.

Test Specimen Information

Sample‑size guidance for S‑N data:

Preliminary / exploratory: minimum 6 specimens

R&D testing: minimum 12 specimens

At least three distinct stress levels are required to construct S‑N curves.

Material limitation: Continuous‑fiber reinforced polymer‑matrix composite laminates, symmetric and balanced relative to the test loading direction.

Key Test Parameters

1. Force / stress ratio R‑ratio

2. Fatigue loading frequency and waveform (haversine is typical for Method B)

3. Intervals for stiffness‑measurement (suggested log‑spaced cycle intervals covering early damage and near‑failure cycles)

4. Quasi‑static peak/valley loads for stiffness measurement

5. Termination criteria: failure definition (fracture or threshold stiffness drop), run‑out cycle limit

Standard Test Procedures of ASTM D7615 Open-Hole Fatigue Testing for Polymer Matrix Composite Laminates

Pre‑define all required test parameters; prepare and inspect open‑hole specimens. Perform conditioning if specified. Measure specimen physical dimensions and hole diameter.
Install specimen into grips or anti‑buckling fixture (for compression‑containing loading). Attach thermocouple for temperature logging.
Mount extensometer for deformation measurement; execute initial quasi‑static loading cycle to record baseline hysteresis and initial chord stiffness Ki; remove extensometer before starting cyclic fatigue.
Apply fatigue loading via either Method A or Method B transition approach, start cyclic force‑controlled constant‑amplitude fatigue.
Upon reaching each predefined cycle‑count interval: halt fatigue, return specimen to zero force, retorque fixture bolts if applicable, re‑attach extensometer, run quasi‑static loading cycle, record force‑deflection hysteresis curve, compute chord stiffness KN and stiffness‑change percentage. Remove extensometer and resume fatigue loading.
Continue cycling until specimen fracture, predefined stiffness‑loss threshold, or run‑out cycle limit is met. Record failure cycle number, failure location and failure mode.
Perform residual‑strength quasi‑static test if requested by test specification.

Industrial Application Fields

Aerospace (the dominant user): fatigue substantiation of composite structure containing fastener holes, access/inspection holes, and cut-outs — skins, spars, frames, control surfaces, rotor components. Results feed material specification, R&D, quality assurance, and structural design/analysis, and are used to derive design factors from small coupons that are then extrapolated to element-, subcomponent- and full-scale test articles.

Aircraft OEM / airframe supply chain and joint design: the practice is deliberately tied to Guide D8509 (test method selection and specimen design for bolted-joint-related properties), so OHT/OHC fatigue data are used alongside bearing-fatigue data when sizing bolted joints.

Wind energy, automotive lightweighting, marine, rail and sporting goods: any continuous-fiber composite part subjected to cyclic loading around holes or attachments.

Materials databases / handbooks: open-hole fatigue S–N data are a standard entry in composite allowables programmes (e.g. CMH-17-style qualification pyramids), supporting preliminary/exploratory screening through to design-allowables and reliability data sets

Related Test Standard:

ISO 9664	Adhesives - Test methods for fatigue properties of structural adhesives in tensile shear
ASTM D3166	Standard Test Method for Fatigue Properties of Adhesives in Shear by Tension Loading (Metal/Metal)
GB/T 35465.3	Test method for fatigue properties of polymer matrix composite materials -Part 3: Tension-tension fatigue
ISO 16525-5	Adhesives - Test methods for isotropic electrically conductive adhesives - Part 5: Determination of shear fatigue
GB/T 16779	Test method for tension-tension fatigue of fiber reinforced plastic laminates
ISO 13003	Fibre-reinforced plastic composites — Determination of fatigue properties under cyclic loading conditions — the international equivalent of ASTM D3479 for composite fatigue testing
GOST R 57143	Polymer composites. Standard test method for tension-tension fatigue
ASTM D3479	Composite material fatigue test, adaptable for bonded composite lap joint fatigue evaluation
ASTM D7615	Standard Practice for Open-Hole Fatigue Response of Polymer Matrix Composite Laminates
ASTM D5766	Open-Hole Tensile Strength of Polymer Matrix Composite Laminates
ASTM D6873	Bearing fatigue of composites (supplements D5961/D5961M bearing response) — the other half of the bolted-joint fatigue picture.

Industrial equivalents seen alongside it: Boeing BSS 7260 (type I OHC/OHT), Airbus AITM 1-0008, SACMA SRM 3R-94 — these describe similar open-hole coupons and fixtures

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Electronic Dynamic Testing Machine for high-precision cyclic and vibration testing of materials and components, featuring accurate frequency control, stable performance, and laboratory-grade reliability.

Hydraulic wedge test grips are a robust and versatile solution for tensile testing of high-strength materials. It can be used in both static and dynamic materials testing applications – from 20KN to 500 kN force capacity for dynamic tests, and up to 600 kN for static tests.

Tensile test fixture for plastics is based on the test standatd ASTM D638, determination of the tensile properties of unreinforced and reinforced plastics in the form of standard dumbbell-shaped test specimens when tested under defined conditions of pretreatment, temperature, speed.

Q1. What is ASTM D7615/D7615M?

A: ASTM D7615/D7615M-23, Standard Practice for Open-Hole Fatigue Response of Polymer Matrix Composite Laminates, is an ASTM practice (currently the 2023 edition, originally approved 2011) that converts the static open-hole tensile and open-hole compressive strength test methods into constant-amplitude fatigue tests. It is under the jurisdiction of ASTM Committee D30 on Composite Materials, Subcommittee D30.05 on Structural Test Methods. It is a practice, i.e. it supplements existing static test methods rather than standing alone.

Q2. Why is open-hole fatigue testing important for composite materials?

A: Because real composite structures always contain holes — fastener holes, bolted joints, access panels, inspection openings, plumbing and wiring penetrations. Composites are notch-sensitive: a hole costs far more strength than the material it removes, because the fibres cannot plastically redistribute load the way metals do. Under cyclic loading, the hole edge is where matrix cracking, splitting and delamination initiate, so the hole governs durability. Static open-hole strength alone cannot predict how long a notched laminate survives in service; D7615 generates the fatigue life and stiffness-degradation data that durability design, damage tolerance and certification actually require.

Q3. What is the main result (property) obtained from this test?

A: The fatigue life of the open-hole specimen under a specified loading and environmental condition, expressed as cycles to fracture, cycles to a predefined stiffness loss, or run-out. Secondary results are stiffness-versus-cycles curves, normal stress versus stiffness curves, and S–N curves at selected stress ratios.

Q4. How is it different from simply repeating the static open-hole test?

A: The static method (D5766 or D6484) gives a single strength value in one monotonically increasing load. D7615 adds cyclic loading between defined minimum and maximum forces at a chosen frequency and waveform, periodic non-destructive stiffness measurement, fatigue-rated grips and load verification to E467, temperature monitoring, statistical life distributions and S–N curve generation. It answers "how many cycles?" instead of "how much load?".

Q5. Who uses this test and in which industries?

A: Aerospace (primary user — fatigue substantiation of composite airframe structure with fastener holes and cut-outs), wind energy (blade root and bolted joints), automotive and EV lightweight structures, marine, rail, sporting goods, and any composite materials database or design-allowables programme. Outputs are used for material specifications, R&D, design allowables, reliability data and quality assurance.

Q6. Which loading modes does D7615 cover?

A: Three: tension–tension (T–T) using D5766/D5766M Configuration A; tension–compression (reversed, T–C) and compression–compression (C–C) using D6484/D6484M Procedure A. Either engineering stress or applied force may be used as the constant-amplitude fatigue variable.

Q7. What materials and laminates are acceptable?

A: Continuous-fibre reinforced polymer matrix composites in which the laminate is symmetric and balanced with respect to the test direction. Acceptable laminate and thickness ranges are defined in Section 8.2 (referenced back to D5766/D6484).

Q8. Is variable-amplitude or spectrum loading covered?

A: No. The practice is limited to constant-amplitude uniaxial loading. It notes that it may be used as a guide for variable-amplitude loading, but spectrum loading is not covered by the standard.

Q9. How critical is hole quality?

A: Critical — it is the dominant variable. Holes must be drilled/reamed consistently (with backing to prevent breakout) with no delamination, fibre pull-out or heat damage, and each hole should be inspected before testing because a damaged hole has already decided the result.

Q10. Why is load control accuracy emphasised so strongly?

A: Because fatigue life is extremely sensitive to load error — the standard states plainly that "small errors in loading may result in significant errors in fatigue life". Hence the requirements for E467 dynamic verification, ≤1 % force verification, amplitude monitoring every three cycles, and mandatory reporting of any period when loading was outside 2 % of the desired peak and valley.

Q11. Can one machine do both open-hole tension fatigue and open-hole compression fatigue?

A: Yes, provided it is a fatigue-rated frame with a reversible load train (tension–compression capability through zero), fatigue-rated hydraulic wedge grips, and the D6484 support fixture with bolt-retorque access for the compression modes. This is the configuration normally supplied for a combined D7615 capability.

Q12. Which parameters must be fixed before testing?

A: Specimen sampling method, type and geometry; minimum and maximum test forces (σ_min, σ_max) and the force (stress) ratio; test frequency and waveform; cycle counts at which stiffness is measured; method of stiffness measurement; quasi-static peak/valley forces for stiffness measurement; the stiffness level at which fatigue loading ceases; and run-out cycles. Note that the stress ratio is not mandated by the standard but must be specified and reported for repeatable results.

Q13. How often should stiffness be measured?

A: Damage accumulation curves are "S"-shaped, so more points are needed early and again near failure. A worked example in the standard for a 2-million-cycle test: N = 1, 2, 5, 10, 20, 50, 100, 200, 500, 1000, 2000, 5000, 10 000, 20 000, 50 000 and every 100 000 cycles thereafter; the final interval should be about one order of magnitude below the anticipated life.

Q14. Which standards is D7615 directly tied to?

A: D5766/D5766M (open-hole tensile strength) and D6484/D6484M (open-hole compressive strength) are the parent static methods it modifies; D8509 (guide for bolted-joint-related test selection and specimen design) supplies terminology, significance/use and interferences; D5229/D5229M governs conditioning; E4, E83, E467, E122, E177/E456, E739, E1823 cover calibration, verification, sampling and analysis.

Q15. What are the closest related or "sibling" fatigue standards?

A: ASTM D3479/D3479M — tension–tension fatigue of unnotched composites (Procedure A load control, Procedure B strain control). ASTM D6873/D6873M — bearing fatigue response (supplements D5961/D5961M), the other half of bolted-joint fatigue. ASTM D6115 — Mode I fatigue delamination onset. ISO 13003 — the international generic fatigue practice for fibre-reinforced plastics under constant-amplitude, constant-frequency loading. Industrial equivalents seen alongside D7615 coupons include Boeing BSS 7260, Airbus AITM 1-0008 and SACMA SRM 3R-94.

Q16. How do D7615 and D3479 differ, and do I need both?

A: D3479 uses unnotched coupons and covers tension–tension only; D7615 uses notched (open-hole) coupons and covers tension–tension, compression–compression and reversed tension–compression. They answer different questions: D3479 characterises the material's intrinsic fatigue response, D7615 characterises the fatigue response of a structural detail. A complete composite fatigue qualification programme normally needs both, together with bearing fatigue (D6873) for bolted joints.

Q17. Does UnitedTest supply complete ASTM D7615-compliant open-hole fatigue test systems?

A: Yes. UnitedTest (Beijing United Test Co., Ltd.) builds turnkey open-hole fatigue solutions around its UTDS electronic dynamic/fatigue series and electro-hydraulic servo fatigue frames, configured for ASTM D7615/D7615M with fatigue-rated hydraulic wedge grips, an ASTM D6484 open-hole compression support fixture, 25 mm-gauge dynamic extensometers (E83 Class B-1), E467-capable dynamic force verification, and optional temperature-humidity chambers for D5229 conditioning.

Q18. Can one UnitedTest machine cover D7615, D3479, D5766 and D6484?

A: Yes — that is the recommended configuration. A single fatigue-rated frame with reversible load train, interchangeable hydraulic wedge grips, OHC support fixture and dynamic extensometer can run static OHT (D5766) and OHC (D6484), open-hole fatigue (D7615), unnotched tension–tension fatigue (D3479 Procedure A/B) and ISO 13003, so one investment covers the whole open-hole and fatigue workflow.

Q19. Does the UnitedTest software handle the stiffness-degradation part of D7615?

A: Yes. The control software can automatically pause cyclic loading at user-defined cycle intervals, run the quasi-static stiffness cycle, remove/re-apply the extensometer sequence, compute chord stiffness K_N = ΔP/ΔS and percent stiffness change Δ_N, plot stiffness-versus-cycles curves, and generate S–N curves with log-normal/Weibull statistics per E739.

Q20. What about the ±1 % dynamic force accuracy and drift limits the standard demands?

A: UnitedTest fatigue frames use closed-loop servo control with fatigue-rated load cells (E4-compliant calibration) and support E467 dynamic force verification. Peak/valley values are monitored continuously and logged, so any excursion beyond the ±2 % band required by D7615 is flagged in the report.

Q21. Can UnitedTest provide the fixtures and grips, including the D6484 support fixture?

A: Yes. UnitedTest manufactures and customises ASTM/ISO-compliant fixtures — hydraulic wedge grips, the ASTM D6484 OHC support fixture with bolt-retorque access, alignment aids, and specimen/hole measurement tooling. Fixtures and accessories are also supplied compatible with other brands' frames (e.g. Instron, MTS, Zwick).

Q22. What support comes with a D7615 system?

A: Load-frame sizing for your coupon size and stress levels, fixture/custom grip design, installation, on-site calibration, operator training on D7615 parameter setup (stress ratio, frequency, stiffness intervals, run-out criteria), and export documentation. UnitedTest serves composite laboratories in aerospace, wind energy, EV/automotive, rail and research institutes worldwide.

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