Adjustable spanTJ-124
Three Point Bend Fixture
A three-point bend fixture with adjustable span and the roller diameters the method specifies — span-to-depth ratio is set on the fixture, not assumed.
SpecificationsTesting standard
ASTM D5045
Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D5045 determines the plane-strain fracture toughness KIc and the critical strain energy release rate GIc of plastics, using either a single-edge-notch bend or a compact tension specimen. The result is valid only if size and sharpness criteria are satisfied, and those are checked after the test rather than before.
At a glance
- Test type
- Fracture toughness
- Published by
- ASTM
- Edition
- D5045-14(2022)
- Material
- Plastics, polymers & films
- Runs on
- Series 7200 and Series 9000
What the test does
A specimen is machined to either the single-edge-notch bend or the compact tension geometry, notched, and given a sharp crack at the notch root. After conditioning it is loaded at a constant rate to fracture — in three-point bending for SENB, or through clevises and pins for CT — while force and displacement are recorded. The specimen is then opened and the crack length measured on the fracture surface. KIc is calculated using the geometry function for the specimen type, and GIc is derived from it using the elastic modulus and Poisson's ratio.
What it measures, and why it matters
The resistance of a plastic to a crack that already exists — a different and often more relevant question than how strong it is. A tensile strength describes an unblemished specimen; real parts contain weld lines, gate marks, scratches and moulded-in stress concentrations, and what governs whether they survive is how readily a crack runs from them. The distinguishing feature of this method is that it does not simply return a number: it returns a number plus a question about whether that number is a material property at all, and the answer is only available after the test.
Validity is not assumed
A number always comes out. Whether it is a material property depends on criteria you check afterwards.
- Geometries
- Single-edge-notch bending (SENB) and compact tension (CT)
- Plane strain
- Required at the crack tipToo thin a specimen is in plane stress, which gives a higher apparent toughness that is not a material constant.
- Thickness
- Must be sufficient to ensure plane strainChecked against the measured toughness after the test — the criterion depends on the answer.
- Crack sharpness
- Sharp enough to give a minimum value of toughnessA machined notch alone reads high. A natural or razor-initiated crack is what the method requires.
- Outputs
- KIc and GIc
- Record whether the validity criteria were met
- On every specimenDakAn invalid result is not a wrong number, it is a number that is not a material property — and it must not be quoted as one.
This is a linear elastic method. If the material yields substantially before the crack runs, the assumptions fail and the result is not a valid KIc, however cleanly the specimen broke.
Test speed
- Rate
- Constant, and slow enough to stay quasi-static
- Reported
- KIc and GIc, with validity stated
- Crack length
- Measured on the fracture surface after the testThe pre-crack is not measured before the test — you cannot see it accurately until the specimen is open.
- Photograph the fracture surface
- With the crack front visibleDak
Calculations
Plane-strain fracture toughnessKIc
KIc = (F / (B × W^½)) × f(a/W)
- F
- force at fracture
- B
- specimen thickness
- W
- specimen width
- a
- crack length
- f(a/W)
- a geometry function tabulated for SENB and CT
The geometry function differs between the two specimen types, so the correct one must be used for the geometry actually tested.
Critical strain energy release rateGIc
GIc = KIc² × (1 − ν²) / E
- E
- elastic modulus
- ν
- Poisson's ratio
Derived from KIc, so it inherits every error in it and adds the uncertainty in E and ν. Both must be for the same material and condition.
Validity—
Specimen dimensions checked against the measured toughness and the yield stress
Circular by design: the size needed depends on the answer, so validity can only be confirmed afterwards.
How the test runs
- 01Select the geometry — SENB or CT — and machine specimens to it.
- 02Machine the notch to the specified depth.
- 03Introduce a sharp crack at the notch root, by the method specified.
- 04Condition the specimens.
- 05Measure thickness and width.
- 06Fit the three-point bend fixture or the clevises, as the geometry requires.
- 07Load at a constant rate to fracture, recording force and displacement.
- 08Check the trace for linearity — substantial non-linearity invalidates a KIc.
- 09Open the specimen and measure the crack length on the fracture surface.
- 10Calculate KIc using the geometry function for the specimen type, then GIc.
- 11Apply the size and sharpness criteria and state whether the result is valid.
Grips and fixtures for this method
Uniform momentTJ-165
Four Point Bend Fixture
Four-point loading where the specification calls for it, putting a length of the specimen under constant moment rather than concentrating it under one nose.
SpecificationsWhat the report has to contain
- Reference to ASTM D5045 and the edition
- Material identification and processing history
- Specimen geometry — SENB or CT
- Specimen thickness, width and span
- How the sharp crack was introduced
- Crack length measured on the fracture surface
- Conditioning and test temperature
- Rate of loading
- KIc and GIc, with E and ν used for the conversion
- Whether the validity criteria were satisfied, specimen by specimen
What the machine must be capable of
Moderate force, a steady quasi-static rate, and the right fixture for the geometry — a three-point bend rig at the specified span for SENB, or clevises and pins that let a compact tension specimen rotate freely as the crack opens. Where crack opening displacement is required, a clip gauge across the notch is needed. Data capture matters more than capacity here, because the trace has to be examined for linearity: substantial non-linearity means the material yielded before the crack ran, and no valid KIc can be extracted from it.
What goes wrong in practice
Quoting an invalid result as a material property. The size criterion is computed from the measured toughness and the yield stress, so it can only be applied retrospectively — which means every specimen needs checking rather than the set being assumed valid because the first one was. Testing from a machined notch without introducing a sharp crack, which reads high. Ignoring a non-linear trace. And reporting GIc without the modulus and Poisson's ratio used to derive it, which makes the second figure unverifiable. Measuring the crack from outside the specimen belongs on the list too: a sharp pre-crack is very fine and its front is rarely straight, so it can only be measured honestly once the specimen is open.
ASTM D5045 or ISO 13586
| ASTM D5045 | ISO 13586 | |
|---|---|---|
| Outputs | KIc and GIc | KIc and GIc |
| Geometries | SENB and CT | SENB and CT |
| Basis | Linear elastic fracture mechanics | Linear elastic fracture mechanics |
| Cite | Whichever the specification names | Whichever the specification names |
Closely aligned methods for the same quantities. Differences lie in the detail of specimen preparation and validity checking, so results should not be pooled across the two without confirming those match.
Questions we are asked about this test
What is ASTM D5045?
It is the ASTM method for the plane-strain fracture toughness KIc and the critical strain energy release rate GIc of plastics. Two geometries are covered — single-edge-notch bending and compact tension — and a pre-cracked specimen is loaded to fracture in the linear elastic range. The current designation is ASTM D5045-14(2022).
Why does the specimen have to be thick enough?
To keep the crack tip in plane strain. A thin specimen deforms through its thickness and sits in plane stress, where the apparent toughness is higher and depends on the thickness — so it is not a material property at all. Enough thickness constrains that deformation and produces a value that belongs to the material rather than to the specimen. This is why a KIc measured on too thin a specimen reads high and cannot be used in design.
Why can't validity be checked before the test?
Because the size criterion depends on the toughness, and the toughness is what you are measuring. The required dimensions are computed from the measured toughness and the material's yield stress, so the check is necessarily retrospective. In practice you estimate, machine generously, test, and then confirm — and a specimen that turns out to have been too small yields a number that must not be quoted as a KIc.
Why does the crack have to be sharp?
Because a blunt notch distributes stress over a larger radius and needs more load to propagate, so the measured toughness reads high. The method requires a crack sharp enough to give a minimum value of toughness — in practice a natural crack initiated at the machined notch root, often by a razor. A machined notch alone is one of the commonest ways an invalid, flattering result is produced.
What if the force-displacement trace is not linear?
Then the linear elastic assumptions behind KIc may not hold. This is a linear elastic fracture mechanics method: it assumes the plastic zone at the crack tip is small compared with the specimen. Substantial non-linearity means the material yielded appreciably before the crack ran, and the calculated KIc is not valid — a different framework, such as a J-integral method, is needed for that behaviour.
How are KIc and GIc related?
GIc is calculated from KIc using the elastic modulus and Poisson's ratio. That means it inherits every error in the KIc and adds the uncertainty in the two elastic constants, which must be for the same material in the same condition. It is not an independent measurement, and reporting both without saying which values of E and ν were used leaves the second unverifiable.
Why is the crack measured after the test?
Because you cannot see it accurately until the specimen is open. A sharp pre-crack is very fine and its front is rarely straight, so measuring it from the outside underestimates it and does so inconsistently. Opening the specimen and measuring the crack length on the fracture surface, across the front, is the only reliable way — which is why crack length is a post-test measurement in this method.
Running ASTM D5045 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Low to moderate — plastics fracture specimens commonly fail between tens of newtons and a few kilonewtons | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 over the working range | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | A three-point bend fixture for SENB, or clevises and pins for the compact tension geometry | Our bend fixtures, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.