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D6435-99 – Standard Test Method Technical Guide
📐 Scope and Significance of D6435-99
This test method, under the jurisdiction of ASTM Subcommittee D20.20 on Plastic Products, establishes a standardized procedure for determining the mechanical properties of plastic lumber and plastic lumber shapes when loaded in shear at relatively low uniform rates of straining or loading (Section 1.1). Developed primarily for recycled plastics exhibiting a non-homogeneous cross section, the method is also fully applicable to similarly manufactured products made from virgin resins (Section 1.2). According to Section 1.3, values stated in inch-pound units are regarded as the standard, with SI units provided in parentheses for informational purposes only.
Section 4.1 emphasizes that shear tests conducted under this method provide specific information about shear properties under conditions approximating those of the test. Users must refer to the relevant material specification, such as those listed in Table 1 of Classification D 4000, prior to testing, as procedural modifications in the specification take precedence. Furthermore, Note 1 in the scope explicitly states that there is no similar or equivalent ISO standard for this method, making it the definitive international reference for the shear testing of plastic lumber.
⚠️ Material Variability Warning: Because plastic lumber is frequently non-homogeneous in the cross-section, the standard dictates that careful specimen selection is required. Test results may be significantly influenced by the distribution of skins, cores, and voids within a given profile.
⚙️ Key Terminology, Definitions, and Test Principles
Understanding the specific terminology in Section 3 is critical for accurate application of the standard. Plastic lumber is defined as a product composed of more than 50 weight percent resin, generally rectangular in cross section, and supplied in sizes corresponding to traditional lumber. Plastic lumber shapes are defined as plastic lumber which is generally not rectangular in cross section (Sections 3.1.1 and 3.1.2). The primary quantity derived from the test is the nominal shear stress, defined in Section 3.1.4 as the shear force per unit area of the shear surface of the test specimen, expressed in force per unit area.
Section 4.2 explicitly limits the properties derived from this test method to shear strength only. For a material that fails in shear by a fracture, this value is very definite. The calculation procedure involves dividing the maximum load recorded during the test by the original shear area. The standard references key supporting standards to ensure uniform practice across laboratories.
| 🟦 Standard Designation | 📏 Purpose and Application in D6435 |
|---|---|
| D 618 (Conditioning) | Specifies standard pretreatment and conditioning atmospheres for test specimens prior to shear testing. |
| D 4000 (Classification) | Provides the material classification system and lists specifications that may modify this shear test method. |
| E 4 (Force Verification) | Mandates practices for verifying the accuracy of the universal testing machine’s loading system. |
| D 883 / D 5033 | Provide foundational terminology for plastics and guides for the proper use of recycled plastics. |
📊 Material Systems, Conditioning, and Measured Properties
Proper conditioning per Practice D 618 is essential to obtain reproducible results, as plastic lumber properties can be influenced by moisture and thermal history. The standard requires a relatively low and uniform loading rate to ensure a valid determination of the failure mechanism. The nominal shear stress is calculated using the maximum force the specimen sustains divided by the original shear area (specimen width multiplied by the depth of the shear plane).
| 📐 Calculated Property | 🎯 Definition per D6435 | ⚡ Standard Units |
|---|---|---|
| Shear Stress (Nominal) | Shear force divided by the shear area of the test specimen (Section 3.1.4). | psi (lb/in²) |
| Shear Strength | The maximum shear stress a material is capable of sustaining before shear fracture (Section 4.2). | psi (lb/in²) |
💡 Reporting Best Practice: Always report the conditioning method, crosshead speed, and orientation of the shear plane relative to the manufactured shape. As per Section 1.3, results must be reported primarily in inch-pound units. Document any visible failure modes, as the non-homogeneous nature of the material can lead to complex fracture surfaces.
❓ Frequently Asked Questions
🔍 What materials does this test method apply to?
It applies to plastic lumber (more than 50 wt% resin, generally rectangular) and plastic lumber shapes (non-rectangular cross sections). The method was developed primarily for recycled plastics with non-homogeneous cross sections but is also applicable to virgin resin products (Section 1.2).
💡 What is the primary property measured by this method?
The primary property is shear strength. The standard explicitly states in Section 4.2 that the shear properties derived from this method are limited to shear strength only, which has a very definite value when the material fails in shear by a fracture.
⚡ What are the standard units for reporting results?
Per Section 1.3, the values stated in inch-pound units are to be regarded as the standard. SI units provided in parentheses in the text are for information only.
📌 Is there an equivalent ISO standard for this test method?
© 2026 TNLab — This article is a technical interpretation for reference only. The original standard as published by ASTM International takes precedence.