Flexural strength and Vicat softening temperature (VST) are two of the most widely specified properties for thermoplastics, defining how a material resists bending loads and how it behaves as temperature rises. This guide explains the standardized laboratory test methods for both properties, covering ISO, ASTM, and GB standards, specimen requirements, test procedures, calculation formulas, and practical applications in quality control. It is intended for materials engineers, quality inspectors, and product designers who need reliable, repeatable data for material selection and specification.
Technical Overview
Flexural strength testing and Vicat softening temperature testing are complementary laboratory methods used to characterize rigid and semi-rigid plastics. The flexural test, performed to ISO 178, ASTM D790, or GB/T 9341, measures the maximum stress a material sustains in bending before yielding or fracturing, together with the flexural modulus, which describes stiffness. The Vicat test, performed to ISO 306, ASTM D1525, or GB/T 8802, determines the temperature at which a standard flat-ended indenter penetrates 1 mm into the surface of a specimen under a fixed load while the specimen is heated at a controlled rate.
Together, these two tests answer two of the most common questions in plastics engineering: how strong and stiff is the material at room temperature, and up to what temperature can it retain its shape under load. For amorphous and semi-crystalline thermoplastics such as ABS, PC, PVC, PP, PE, PA, and PMMA, both values are core entries on nearly every material datasheet.
- Flexural strength standards: ISO 178, ASTM D790, GB/T 9341
- Vicat softening temperature standards: ISO 306, ASTM D1525, GB/T 8802
- Conditioning atmosphere standard: ISO 291 or ASTM D618 (typically 23 ± 2 °C and 50 ± 10% relative humidity)
Testing Samples
Flexural Test Specimens
The preferred specimen for ISO 178 is an injection-molded bar measuring 80 mm × 10 mm × 4 mm. Specimens may also be machined or sawn from compression-molded sheets, extruded profiles, or finished products, provided the surface is flat and free of visible defects. For ASTM D790, the recommended span-to-depth ratio is 16:1, and specimens should be at least 20% longer than the support span used. At least five specimens are normally tested for each direction, since molded plastics often show anisotropy between the flow and transverse directions.
Vicat Test Specimens
For ISO 306, the specimen is a flat piece measuring at least 10 mm × 10 mm in surface area, with a preferred thickness between 3 mm and 6.5 mm. Specimens may be compression molded, injection molded, or cut directly from sheet stock or finished parts. The test surface must be smooth, flat, and parallel to the opposite face, because the test measures penetration from a single defined surface. Two specimens are tested for each condition; if the results differ by more than 2 °C, additional specimens are recommended.
Sample Preparation and Conditioning
- Molding conditions (melt temperature, cooling rate, gate location) strongly influence both results and should be documented.
- Hygroscopic materials such as polyamide should be conditioned to a defined moisture state before testing.
- Internal stresses from molding can lower the measured Vicat temperature; annealing may be specified by the relevant product standard.
- Unless otherwise specified, condition specimens at 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 16 hours before testing.
Testing Items
A combined flexural and Vicat test program typically reports the following items:
- Flexural strength (MPa): the maximum flexural stress sustained, either at break or at the conventional 3.5% outer-fiber strain when the material does not break.
- Flexural stress at break (MPa): recorded for brittle or fiber-reinforced materials.
- Flexural modulus (MPa): the secant modulus determined between 0.05% and 0.25% strain in ISO 178, or from the initial linear portion of the load–deflection curve in ASTM D790.
- Flexural strain at break (%): the outer-fiber strain at fracture, where applicable.
- Vicat softening temperature, VST (°C): reported with the test designation, e.g., VST/B/50, VST/A/120, indicating load and heating rate.
Testing Methods
Flexural Strength Test Procedure (ISO 178 / ASTM D790 / GB/T 9341)
The standard configuration is three-point bending. The specimen rests on two parallel supports and is loaded at mid-span by a loading nose, all having rounded contact radii. For the preferred ISO 178 specimen, the support span is 64 mm (16 times the 4 mm thickness) and the test speed is 2 mm/min. Force and deflection are recorded continuously until the specimen breaks or the conventional deflection is reached.
The flexural stress at any point on the curve is calculated as:
σf = 3FL / (2bh²)
- F = applied force (N)
- L = support span (mm)
- b = specimen width (mm)
- h = specimen thickness (mm)
The outer-fiber strain is calculated as εf = 6sh / L², where s is the deflection at mid-span. The flexural modulus is obtained from the slope of the stress–strain curve within the prescribed strain window. Four-point bending variants, which subject the region between the loading noses to a uniform bending moment, are used where specified, for example for continuous-fiber-reinforced composites.
Vicat Softening Temperature Test Procedure (ISO 306 / ASTM D1525 / GB/T 8802)
A hardened steel indenter with a flat end of 1 mm² cross-sectional area (circular or square) is placed on the specimen surface under a defined dead load. The assembly is immersed in a temperature-controlled liquid bath (typically silicone oil for temperatures above 100 °C) starting at 20–23 °C. The bath is then heated at a constant rate, and the temperature at which the indenter has penetrated exactly 1 mm into the specimen is recorded as the Vicat softening temperature.
The standard test designations are combinations of load and heating rate:
- VST A/50: 10 N load, 50 °C/h heating rate
- VST B/50: 50 N load, 50 °C/h heating rate (most commonly specified)
- VST A/120: 10 N load, 120 °C/h heating rate
- VST B/120: 50 N load, 120 °C/h heating rate
Because the load and heating rate significantly affect the result, the exact designation must always accompany the reported temperature. A higher load or a lower heating rate produces a lower VST value, and results obtained under different designations are not directly comparable.
Equipment and Measurement Requirements
- Universal testing machine with calibrated load cell and bend fixture for flexural testing.
- Vicat apparatus with a temperature-controlled oil bath, 1 mm² indenter, 10 N and 50 N weights, and penetration transducer with resolution better than 0.01 mm.
- Bath temperature uniformity and heating-rate accuracy are critical; the bath medium must not swell, dissolve, or otherwise affect the specimen.
- All instruments must be calibrated and traceable, and testing is normally performed in accredited third-party laboratories or in calibrated in-house laboratories.
Applications
Flexural strength and Vicat softening data are used across the entire plastics value chain, from resin production to end-product qualification:
- Quality control of resins: incoming inspection of ABS, PP, PE, PVC, PC, PS, PMMA, and PA compounds against datasheet limits.
- Pipes, fittings, and profiles: verifying PVC and PE grades for pressure and non-pressure applications, where both stiffness and softening point are acceptance criteria.
- Cable insulation and sheathing: Vicat temperature is a key requirement in cable material specifications, indicating resistance to deformation near heat sources.
- Automotive components: interior trims, dashboards, and under-hood parts are often qualified by flexural modulus and thermal indices that correlate with Vicat behavior.
- Material selection and design: comparing candidate materials for housings, appliance parts, connectors, and structural components exposed to elevated service temperatures.
- Research and development: evaluating the effect of fillers, glass fiber, plasticizers, nucleating agents, and new formulations on stiffness and heat resistance.
- Failure analysis: retesting suspect parts to determine whether substandard material or improper molding contributed to field failures.
For orientation, typical published ranges (highly grade- and method-dependent) are approximately: rigid PVC VST/B/50 around 75–85 °C; ABS VST/B/50 around 90–105 °C; PP homopolymer VST/A/50 around 150–155 °C; polycarbonate VST/B/50 around 145–150 °C. Flexural strength values similarly range from roughly 30–50 MPa for polyolefins up to 130–150 MPa or higher for glass-fiber-reinforced engineering plastics.
FAQ
Q1: What is the difference between the Vicat softening temperature and the heat deflection temperature (HDT)?
Vicat (ISO 306 / ASTM D1525) measures penetration of a 1 mm² indenter under 10 N or 50 N, while HDT (ISO 75 / ASTM D648) measures the temperature at which a bending specimen deflects a set amount under a defined surface stress of 1.80 MPa or 0.45 MPa. The two values are correlated but not interchangeable; Vicat generally gives a higher temperature and reflects surface softening, whereas HDT reflects bulk stiffness retention in bending.
Q2: Should I specify the 10 N or the 50 N load for the Vicat test?
The 50 N load (methods B/50 or B/120) is the most widely specified and the more conservative choice, giving lower values suitable for general quality control. The 10 N load is often used for softer materials or for closer alignment with short-term heat performance near the glass transition. Always report the designation together with the result.
Q3: Why are the 50 °C/h and 120 °C/h heating rates both permitted?
The 50 °C/h rate is the classic reference condition with better repeatability and closer approach to thermal equilibrium inside the specimen. The 120 °C/h rate shortens test time and is used for high-throughput screening, but it yields slightly higher values and greater scatter. Results from the two rates should not be mixed in comparisons.
Q4: How many specimens are required?
ISO 178 flexural testing normally uses at least five specimens per direction and condition. ISO 306 requires at least two Vicat specimens, with additional testing if the two values differ by more than 2 °C. Destructive tests mean each specimen is used only once.
Q5: Can reinforced and filled plastics be tested with these methods?
Yes. Glass-fiber and mineral-filled compounds are routinely tested for flexural strength and modulus, with the fiber orientation noted since it strongly affects results. Vicat testing also applies to filled thermoplastics, provided the surface is properly finished and the indenter does not land directly on an exposed fiber bundle.
Q6: What factors most often cause inconsistent results?
Common causes include non-uniform specimen thickness, unrecorded molding conditions, residual internal stress, moisture in hygroscopic resins, bath temperature gradients, incorrect heating rate, and mixing of test designations. Careful sample preparation, standardized conditioning, and calibrated equipment in accredited laboratories minimize these errors.