Flexural strength and Vicat softening temperature are two of the most frequently specified performance indicators for thermoplastic and thermoset materials, quantifying mechanical rigidity and short-term heat resistance respectively. This technical article explains the testing principles, international standards (ISO 178, ASTM D790, ISO 306, ASTM D1525), specimen requirements, procedural parameters, and industrial applications of these essential evaluation methods, providing engineers and quality professionals with a practical reference.
Technical Overview
Flexural strength (bending strength) is the maximum stress sustained by a specimen loaded in three-point bending, measured at the outer fiber of the midspan cross-section and expressed in megapascals (MPa). It is one of the primary mechanical indices used to compare the rigidity and load-bearing capacity of rigid plastics, filled compounds, fiber-reinforced composites, and thermoset molding materials. Related outputs include the flexural modulus (stiffness in bending), the flexural stress at 3.5% strain, and the flexural stress at break.
The Vicat softening temperature (VST) is a measure of short-term heat resistance for thermoplastics. A flat-ended needle with a bearing area of 1 mm², loaded with 10 N or 50 N, is pressed against the specimen surface while the specimen is heated in a stirred liquid bath at a controlled rate. The temperature at which the needle penetrates 1 mm into the specimen is recorded as the Vicat softening temperature. A higher VST indicates better retention of shape and dimensional stability under elevated temperature.
Evaluating flexural strength and Vicat softening temperature together provides a compact profile of how a material behaves under combined mechanical load and thermal exposure, supporting material selection, grade comparison, incoming inspection, quality control, and compliance verification in industries such as automotive, electrical and electronics, appliances, piping, and construction.
- Flexural testing answers: how stiff and how strong is the material in bending?
- Vicat testing answers: at what temperature does the material begin to soften under a localized load?
- Combined, they support safe product design near structural and thermal operating limits.
Testing Samples
Flexural Test Specimens (ISO 178 / ASTM D790)
The preferred ISO 178 specimen is a rectangular bar of 80 mm × 10 mm × 4 mm, tested at a support span of 64 mm, corresponding to a span-to-thickness ratio of 16:1. Specimens are typically cut from ISO 3167 multipurpose test pieces or injection molded directly in a designated mold. ASTM D790 commonly uses bars of 127 mm × 12.7 mm × 3.2 mm with a 50.8 mm span, also at a 16:1 ratio, while a 32:1 ratio may be applied to certain high-strength reinforced materials. At least five valid specimens are recommended for each orientation and test condition.
Vicat Test Specimens (ISO 306 / ASTM D1525)
Each specimen shall present a flat test surface of at least 10 mm × 10 mm with a thickness between 3 mm and 6 mm. For thin sheet materials, several layers may be tightly stacked to reach the minimum thickness. At least two specimens are tested; for materials suspected of anisotropy, or where the two as-molded surfaces differ, both surfaces should be evaluated.
Sources and Material Scope
- Injection-molded or compression-molded laboratory plaques and multipurpose test pieces.
- Machined or saw-cut bars taken from sheets, profiles, or finished parts, with orientation and surface condition documented.
- Typical materials: PP, PE, ABS, PS, HIPS, SAN, PMMA, rigid and flexible PVC, PA6, PA66, PBT, PET, PC, POM, modified PPO blends, talc-filled and glass-fiber-reinforced polyolefins, and thermoset compounds such as phenolic, epoxy, and unsaturated polyester moldings.
Testing Items
- Flexural strength (σfM) — the maximum flexural stress sustained before fracture or before the conventional 3.5% strain limit.
- Flexural stress at 3.5% strain (σf3.5) — applicable to ductile materials that do not fracture within the standard deflection range.
- Flexural stress at break (σfB) — applicable to brittle materials that fracture during the test.
- Flexural modulus (Ef) — the tangent modulus of the initial linear region of the stress–strain curve, conventionally taken between 0.05% and 0.25% flexural strain.
- Vicat softening temperature (VST) — reported with the method designation: A50 (10 N, 50 °C/h), B50 (50 N, 50 °C/h), A120 (10 N, 120 °C/h), or B120 (50 N, 120 °C/h).
Test conditions such as specimen orientation, conditioning state, and any annealing treatment must be reported together with the numerical results, because these factors measurably influence both flexural and Vicat data.
Testing Methods
Flexural Strength Testing — ISO 178 / ASTM D790 / GB/T 9341
The specimen rests on two supports and is loaded at midspan by a loading nose; in the ISO configuration the support and loading nose radii are 5 mm. The crosshead speed is 2 mm/min for the standard 4 mm specimen, or is calculated proportionally for other dimensions to maintain an equivalent outer-fiber strain rate. The flexural stress is calculated as σf = 3FL / (2bh²), where F is the applied force, L is the support span, and b and h are the specimen width and thickness respectively. Deflection is measured from crosshead displacement or a dedicated deflection transducer, and the modulus is derived from the slope of the initial stress–strain curve. Testing is terminated when the specimen breaks or, for materials that deform without fracture, when the outer-fiber strain reaches 3.5%.
Vicat Softening Temperature Testing — ISO 306 / ASTM D1525 / GB/T 1633
The specimen is placed in a temperature-controlled bath (commonly silicone oil for elevated temperatures) with the flat, polished end of the indenter resting on the upper surface under a dead weight of 10 N (Method A) or 50 N (Method B). After a pre-heating period at a starting temperature of 20 °C to 23 °C, the bath is raised at 50 °C/h or 120 °C/h with continuous stirring. The temperature at which the indenter has penetrated 1.0 mm below the original surface is recorded as the Vicat softening temperature, reported to the nearest degree Celsius together with the method designation, for example VST/B/120. A higher starting temperature may be used for highly crystalline engineering plastics provided it is documented.
Instrumentation, Conditioning, and Calibration
- A calibrated universal testing machine equipped with a three-point bending fixture, with force and displacement verification under a traceable calibration program.
- A Vicat apparatus comprising a stirred liquid bath with precision heating control and an LVDT or equivalent transducer capable of detecting 1.0 mm penetration with adequate resolution.
- Specimen conditioning at 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with ISO 291 or GB/T 2918, typically for at least 16 hours prior to testing, unless otherwise specified.
- Accredited third-party laboratories routinely perform these tests with traceable calibration and documented quality systems, producing data suitable for supplier declarations, customer specifications, and regulatory submissions.
Applications
- Material selection and grade comparison: ranking unfilled, mineral-filled, and glass-reinforced grades for stiffness and heat performance during early design.
- Quality control and incoming inspection: verifying production lots of compounds, sheets, and molded components against internal specifications or material data sheets.
- Automotive engineering: interior trim, air ducts, brackets, and under-hood components where rigidity and sustained exposure to elevated temperature must be balanced.
- Electrical and electronic products: housings, connectors, switches, and structural insulating parts, where Vicat data complement thermal indices used in IEC and UL polymeric-material schemes such as UL 746.
- Pipes, fittings, and construction products: thermal-endurance screening for PVC and polyolefin systems prior to long-term hydrostatic and aging programs.
- Research and development: evaluating the influence of fillers, nucleating agents, copolymer ratios, annealing, and recycled content on stiffness and softening behavior.
FAQ
What is the difference between Vicat softening temperature and heat deflection temperature (HDT)?
Vicat measures the penetration of a small loaded needle into the surface and is generally more sensitive to the onset of softening, often yielding higher values than HDT, which measures the bulk deflection of a bar under bending stress in accordance with ISO 75 or ASTM D648. Both are short-term indicators, and neither replaces long-term thermal-aging data such as relative thermal indices.
Which Vicat method — A50, B50, A120, or B120 — should be selected?
Selection follows the product standard or customer specification. Method B (50 N) applies a more severe load and gives lower, more conservative values, while the 120 °C/h rate shortens test duration. B120 is commonly cited for rigid engineering plastics, whereas specification sheets for PVC and polyolefins frequently reference B50 or A50 for comparability with historical data.
Why can ISO and ASTM flexural results differ for the same material?
Differences in specimen geometry (4 mm versus 3.2 mm thickness), support span, strain rate, and processing history lead to systematic deviations, often on the order of several percent. Data should only be compared within a single method, and the standard used must always be stated alongside the reported result.
How many specimens are required, and how are results reported?
ISO 178 recommends at least five valid specimens for flexural testing, and ISO 306 requires at least two specimens per Vicat condition. The mean value is reported together with the method designation, specimen preparation details, and conditioning state.
Can the tests be performed on samples cut from finished products?
Yes, provided the dimensions, orientation, and surface condition satisfy the standard requirements. Results from finished parts reflect actual processing history, but differences in thickness, anisotropy, and skin effects mean they may not be directly comparable with laboratory-molded specimens.
What factors commonly cause scatter in results?
Typical sources include internal voids and sink marks, weld lines, insufficient conditioning, anisotropic filler or fiber orientation, non-parallel specimen surfaces, bath temperature gradients or inadequate stirring, and inaccurate measurement of specimen width and thickness, the latter entering the stress calculation as a squared term.