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ISO/TS 28660 — Plastics — Determination of J-R Curve
A Technical Specification for Fracture Toughness Evaluation of Plastic Materials Using the J-Integral Resistance Curve Method
ISO/TS 28660 provides a standardized methodology for determining the J-integral resistance curve (J-R curve) of plastic materials. Fracture toughness is a critical design parameter for load-bearing plastic components in applications ranging from automotive under-hood parts and medical device housings to pipeline systems and structural composites. Unlike metals, plastics exhibit significant viscoelastic and viscoplastic deformation, rate-dependent mechanical behavior, and ductile-to-brittle transition characteristics that complicate conventional fracture toughness measurement. The J-R curve method captures the material’s resistance to crack initiation and stable crack propagation under elastic-plastic conditions, making it particularly suitable for tough polymers that do not satisfy the small-scale yielding requirements of linear-elastic fracture mechanics (LEFM).
ISO/TS 28660 is closely aligned with ASTM D6068 and ISO 17281 but extends the methodology specifically for the unique time-dependent deformation behavior of polymeric materials. The specification provides explicit guidance on loading rate selection, data acquisition frequency, and creep correction procedures that are not addressed in standards developed primarily for metallic materials.
Test Methodology and Specimen Configuration
The specification defines the J-R curve determination procedure using either compact tension (CT) or single-edge notch bending (SENB) specimen configurations. Both configurations employ a fatigue pre-crack introduced under cyclic loading at the notch root, followed by monotonic loading to propagate the crack while simultaneously recording load, load-line displacement, and crack extension. The J-integral value at each point is calculated from the area under the load-displacement curve using the ASTM E1820-consistent formulation, with correction terms for the plastic component of deformation.
A significant contribution of ISO/TS 28660 is its treatment of specimen size effects specific to plastics. The standard requires that the initial ligament length (the remaining uncracked cross-section) be at least 25 times the average spherulite or domain size for semicrystalline polymers, and at least 10 times the average filler particle diameter for filled systems. These requirements ensure that the measured fracture resistance represents bulk material behavior rather than local morphological features. For fiber-reinforced plastics, the crack plane orientation relative to the fiber direction must be specified, and testing in both 0° and 90° orientations is recommended for anisotropic materials.
| Parameter | CT Specimen | SENB Specimen | Notes |
|---|---|---|---|
| Specimen width (W) | 25-50 mm | 20-40 mm | Proportional to material toughness |
| Initial crack ratio (a₀/W) | 0.45-0.55 | 0.45-0.55 | Fatigue pre-crack required |
| Specimen thickness (B) | 10-25 mm | 10-20 mm | Full thickness for plane strain |
| Loading rate | 0.1-10 mm/min | 0.1-10 mm/min | Lower rates for creep-prone polymers |
| Data acquisition rate | ≥50 Hz | ≥50 Hz | Higher rates for brittle materials |
| Temperature | 23 ± 2°C | 23 ± 2°C | Other temperatures by agreement |
A key innovation in ISO/TS 28660 is the multi-specimen versus single-specimen flexibility. The standard allows both the multi-specimen method (testing multiple specimens to different crack extensions) and the single-specimen normalization method (using a single specimen with elastic compliance or potential drop crack monitoring). The normalization method significantly reduces material requirements and testing time while achieving comparable accuracy for tough polymers.
Engineering Challenges in J-R Curve Measurement for Plastics
The viscoelastic nature of polymers introduces several measurement challenges that ISO/TS 28660 specifically addresses. The most significant is load relaxation during crack propagation. Unlike metals that reach a steady-state crack growth resistance quickly, many engineering plastics (particularly polyamides, polycarbonates, and polypropylenes) exhibit increasing crack growth resistance with crack extension — the so-called rising R-curve behavior — due to the development of a process zone ahead of the crack tip involving crazing, shear yielding, and cavitation. The specification requires that the test be conducted under displacement control at a rate that minimizes viscoelastic relaxation effects while maintaining a stable crack front.
Another critical issue is crack tip blunting correction. Prior to stable crack propagation, the crack tip blunts elastically, producing an apparent crack extension that must be distinguished from true crack growth. ISO/TS 28660 provides a blunting line construction procedure based on the material’s elastic modulus and yield strength, with an offset of 0.2 mm (similar to the ASTM E1820 approach) to define the engineering crack initiation toughness Jᵢc. The standard also provides an alternative construction using the intersection of the blunting line with a power-law fit to the J-R data, which gives the initiation toughness J₀.₂.
A frequently encountered problem in plastics J-R testing is specimen buckling under compressive loading in the CT configuration. The low elastic modulus of plastics (typically 1-5 GPa versus 200 GPa for steel) means that standard CT specimen dimensions designed for metals may be too slender for plastics. ISO/TS 28660 provides modified geometry guidelines including reduced ligament lengths and anti-buckling guides to ensure valid fracture data.
The specification also addresses the crack extension measurement challenge. For plastics, the commonly used elastic compliance method (where crack length is inferred from the unloading compliance) is complicated by the material’s time-dependent deformation that superimposes creep compliance on the elastic response. ISO/TS 28660 recommends the direct current potential drop (DCPD) or alternating current potential drop (ACPD) method as the primary technique for in situ crack monitoring, with optical measurement of the fracture surface post-test as a backup. The standard specifies correction procedures for the temperature coefficient of resistivity and the geometric factor for potential drop to crack length conversion specific to polymeric materials.
Data Analysis and Engineering Applications
The J-R curve obtained from ISO/TS 28660 testing is typically represented by a power-law relationship: J = Jᵢc + C(Δa)ⁿ, where Jᵢc is the initiation toughness, Δa is the stable crack extension, and C and n are fitting constants describing the tearing modulus of the material. The slope of the J-R curve at a given crack extension (dJ/da) is the tearing modulus, which characterizes the material’s resistance to unstable crack propagation. Materials with higher tearing modulus values are more tolerant of manufacturing defects and service-induced damage — a critical parameter for fail-safe engineering design.
From an engineering design perspective, the J-R curve provides essential input for leak-before-break (LBB) assessments of plastic piping systems and pressure vessels. For a given applied stress and initial flaw size, the J-R curve determines whether crack growth will be stable (allowing detection before catastrophic failure) or unstable (leading to rapid fracture). The specification provides worked examples of LBB calculations for polyethylene (PE) gas distribution pipes and polyvinyl chloride (PVC) water mains, demonstrating how the J-R curve parameters integrate with stress analysis and defect assessment procedures such as the Failure Assessment Diagram (FAD) method.
One of the most significant design risks when using J-R curve data for plastic components is the loading rate effect. The J-R curve measured at quasi-static rates (0.1-10 mm/min) may not be conservative for impact or rapid pressurization scenarios. ISO/TS 28660 recommends supplementary dynamic J-R curve testing at loading rates representative of the intended service conditions, particularly for components subject to pressure surge, water hammer, or impact loading.
Q1: How does ISO/TS 28660 differ from ASTM D6068 for plastics J-R curve testing?
A: ISO/TS 28660 provides more comprehensive guidance on viscoelastic corrections, creep compliance compensation, and specimen size requirements specific to polymeric materials. ASTM D6068 is largely adapted from metallic testing standards and does not address the unique time-dependent behaviors of plastics as thoroughly.
A: ISO/TS 28660 provides more comprehensive guidance on viscoelastic corrections, creep compliance compensation, and specimen size requirements specific to polymeric materials. ASTM D6068 is largely adapted from metallic testing standards and does not address the unique time-dependent behaviors of plastics as thoroughly.
Q2: Can ISO/TS 28660 be applied to rubber-toughened plastics and polymer blends?
A: Yes, but with caution. The presence of a discrete rubber phase creates additional energy dissipation mechanisms (cavitation, debonding, matrix shear yielding) that can lead to extremely steep J-R curves. The standard recommends reduced test rates and increased data acquisition frequency for these materials to capture the full process zone development.
A: Yes, but with caution. The presence of a discrete rubber phase creates additional energy dissipation mechanisms (cavitation, debonding, matrix shear yielding) that can lead to extremely steep J-R curves. The standard recommends reduced test rates and increased data acquisition frequency for these materials to capture the full process zone development.
Q3: What is the minimum number of valid data points required for a J-R curve under ISO/TS 28660?
A: The specification requires a minimum of 8 data points within the valid crack extension range (0.1 mm to 0.25W) for the multi-specimen method, or 20 data points for the single-specimen normalization method. At least 3 replicate tests are required for statistical validity.
A: The specification requires a minimum of 8 data points within the valid crack extension range (0.1 mm to 0.25W) for the multi-specimen method, or 20 data points for the single-specimen normalization method. At least 3 replicate tests are required for statistical validity.
Q4: Does the standard address environmental effects such as moisture content on J-R curve behavior?
A: Yes. ISO/TS 28660 requires that the moisture content of hygroscopic polymers (e.g., polyamides, polycarbonates) be measured and reported before and after testing. The standard provides conditioning guidance at 50% RH and immersion conditions, with a note that the J-R curve can shift significantly — up to 50% reduction in initiation toughness for nylon 6 at saturation compared to dry-as-molded condition.
A: Yes. ISO/TS 28660 requires that the moisture content of hygroscopic polymers (e.g., polyamides, polycarbonates) be measured and reported before and after testing. The standard provides conditioning guidance at 50% RH and immersion conditions, with a note that the J-R curve can shift significantly — up to 50% reduction in initiation toughness for nylon 6 at saturation compared to dry-as-molded condition.
