Flexural strength testing service

Flexural Strength Testing Service – Accredited ISO/IEC 17025 Mechanical Performance and Material Stiffness Assessment for the Croatian Market

Flexural strength is a fundamental mechanical property that quantifies the ability of a material to resist deformation under bending loads. This parameter is essential for ensuring the structural integrity, durability, and performance of materials and components used in construction, automotive, aerospace, marine, consumer goods, medical devices, and packaging applications, where resistance to bending and flexural stress is critical. In the Croatian market, where the Hrvatski zavod za norme (HZN), the Ministarstvo gospodarstva i održivog razvoja, the Državni inspektorat, the Ministarstvo graditeljstva i prostornoga uređenja, and the Carinska uprava enforce strict quality, safety, and performance standards aligned with EU directives and HRN EN (Croatian standards based on European norms), the accurate evaluation of flexural strength is essential for product certification, supplier qualification, type testing, quality control in manufacturing, and import-export processes. Our laboratory offers a comprehensive flexural strength testing service, applying standardized methods such as ASTM D790, ISO 178, ASTM C1161, ASTM E290, ISO 7438, ASTM D7264, and HRN EN ISO 178 to measure flexural strength, flexural modulus, flexural strain, and failure characteristics under controlled loading conditions. All tests are performed under our ISO/IEC 17025 (CNAS) accreditation, and the resulting reports are fully accepted by Croatian authorities and notified bodies, making them indispensable for regulatory compliance, product validation, and market access in Croatia and the European Union.

Test Samples and Materials We Regularly Examine

Our laboratory receives a wide variety of materials and components for flexural strength testing. Typical samples include:

• Plastics and polymers – sheets, films, rods, and molded components of PE, PP, PVC, ABS, PA, PC, and thermosetting materials.
• Ceramics and refractories – tiles, bricks, crucibles, and structural ceramics for high-temperature applications.
• Composites and laminates – carbon fiber, glass fiber, and hybrid composites for aerospace, automotive, and construction.
• Metals and alloys – beams, bars, sheets, and tubes of steel, aluminum, copper, titanium, and their alloys.
• Wood and engineered wood products – timber beams, plywood, MDF, and laminated veneer lumber.
• Prototype and new material designs – submitted by manufacturers for validation of flexural performance before series production.
• Field-retrieved components – for failure analysis and remaining life assessment.

Three-Point and Four-Point Bending Tests – Standard Flexural Strength Measurement

Three-point and four-point bending tests are the most widely used methods for evaluating the flexural strength and flexural modulus of materials. These tests apply a controlled bending load to a specimen supported at two points, and the resulting stress and strain are calculated from the load-deflection data. Our procedures follow international standards and the requirements of the Croatian construction, automotive, and industrial sectors.

• Three-point bending test for plastics and composites (ASTM D790 / ISO 178 / HRN EN ISO 178 / NTC 5700) – we place a rectangular specimen on two supports with a specified span length (16 times the thickness). A load is applied at the center at a constant rate (1 to 2 mm/min) until the specimen breaks or reaches a maximum deflection of 5 % of the span. The flexural strength (σ_f), flexural modulus (E_f), and flexural strain (ε_f) are calculated. We report the flexural strength (in MPa), the flexural modulus (in GPa), the flexural strain (in %), and the load-deflection curve.
• Three-point bending test for metals (ASTM E290 / ISO 7438 / NTC 5701 – for metallic materials) – we perform a three-point bending test on a metal bar or plate specimen, measuring the bending force and the deflection. The test is performed until a specified deflection or a specified bend angle is reached. The bending strength (in MPa) and the bending angle (in °) are reported.
• Four-point bending test (ASTM D7264 / ISO 14125 / NTC 5702 – for composites and advanced materials) – we use a four-point bending configuration (with two loading points) to apply a uniform bending moment over a central section of the specimen. This reduces the effect of shear stresses and provides a more accurate measure of the flexural strength. We report the flexural strength and the flexural modulus.
• Flexural strength at different temperatures (NTC 5703 – for the thermal effect on flexural properties) – we perform the three-point bending test at elevated temperatures (e.g., 40 °C, 60 °C, 80 °C) or at low temperatures (e.g., -10 °C, -20 °C) to evaluate the effect of temperature on the flexural strength and modulus. We report the flexural properties at each temperature.
• Flexural strength after environmental aging (NTC 5704 – for the durability assessment) – we age the specimen (e.g., by thermal aging, humidity exposure, UV exposure, or salt spray) and then perform the bending test to evaluate the long-term stability of the flexural properties. We report the flexural strength after aging and the retention of strength (in %).

Flexural Modulus and Stiffness Measurement – Evaluating Resistance to Deflection

The flexural modulus is a measure of the material's resistance to bending deformation, while the flexural stiffness is a measure of the component's resistance to deflection under a given load. Our tests determine these parameters from the initial linear portion of the load-deflection curve, providing essential data for the design of beams, panels, and structural members.

• Flexural modulus measurement (ASTM D790 / ISO 178 / NTC 5710 – for the elastic bending behavior) – we calculate the flexural modulus (E_f) from the slope of the load-deflection curve in the linear elastic region, using the formula appropriate for the test geometry (three-point or four-point bending). We report the flexural modulus (in GPa or MPa).
• Flexural stiffness measurement (NTC 5711 – for the component-level stiffness) – we calculate the flexural stiffness (EI) of the component from the load-deflection curve, using the formula for the beam deflection. The stiffness is expressed in N·mm². We report the flexural stiffness and the load-deflection curve.
• Effect of geometry on flexural stiffness (NTC 5712 – for the design optimization) – we test specimens with different geometries (e.g., different thicknesses, widths, or spans) to evaluate the effect of the geometry on the flexural stiffness. We report the stiffness for each geometry.
• Flexural modulus at different temperatures (NTC 5713 – for the temperature dependence) – we measure the flexural modulus at elevated and low temperatures to evaluate the effect of temperature on the stiffness. We report the flexural modulus at each temperature.
• Flexural modulus of anisotropic materials (NTC 5714 – for composites and laminates) – we test composite and laminated specimens in different orientations (e.g., 0°, 90°, 45°) to determine the directional dependence of the flexural modulus. We report the modulus for each orientation.

Flexural Strain and Deformation Analysis – Evaluating Ductility and Failure Behavior

Flexural strain is a measure of the deformation of the material under bending, and it is a critical parameter for evaluating the ductility and the failure behavior. Our tests measure the flexural strain at the maximum load and at fracture, providing data for the design of flexible and impact-resistant components.

• Flexural strain at maximum load (NTC 5720 – for the maximum deformation capacity) – we measure the deflection at the maximum load and calculate the flexural strain (ε_f) using the formula appropriate for the bending test geometry. We report the flexural strain at maximum load (in %).
• Flexural strain at break (NTC 5721 – for the fracture strain) – we measure the deflection at the point of fracture and calculate the flexural strain at break. We report the flexural strain at break (in %).
• Load-deflection curve and ductility assessment (NTC 5722 – for the ductile and brittle behavior) – we analyze the load-deflection curve to identify the ductile or brittle nature of the material. A ductile material exhibits a large plastic deformation region, while a brittle material fails abruptly. We report the curve and the ductility rating.
• Flexural strain at different temperatures (NTC 5723 – for the temperature dependence) – we measure the flexural strain at elevated and low temperatures to evaluate the effect of temperature on the ductility. We report the flexural strain at each temperature.
• Deformation and fracture analysis (NTC 5724 – for the failure mode identification) – we examine the fracture surface and the deformation pattern of the bent specimen to identify the failure mode (e.g., tensile failure, compressive failure, shear failure, or delamination). We report the failure mode and the fractographic analysis.

Flexural Fatigue and Cyclic Bending Testing – Evaluating Resistance to Repeated Flexing

Flexural fatigue and cyclic bending testing evaluate the resistance of a material to repeated bending loads, which is essential for components that are subjected to cyclic flexing, such as springs, flexible circuits, and folding structures.

• Flexural fatigue test (ASTM D7791 / ISO 13003 / NTC 5730 – for plastics and composites) – we apply a cyclic bending load (with a constant amplitude or a variable amplitude) to the specimen at a specified frequency (e.g., 1‑5 Hz) until the specimen fails or until a specified number of cycles (e.g., 10⁶ cycles) is reached. The S‑N curve (stress vs. cycles to failure) is constructed. We report the S‑N curve and the fatigue limit.
• Cyclic bending test for flexible materials (NTC 5731 – for the fold endurance) – we subject the specimen (e.g., a film, a textile, or a paper) to repeated bending over a specified angle (e.g., ±90°) at a specified frequency (e.g., 1 Hz) for a specified number of cycles. The number of cycles to failure is recorded. We report the number of cycles to failure and the failure mode.
• Bending fatigue at different temperatures (NTC 5732 – for the thermal effect) – we perform the bending fatigue test at elevated temperatures (e.g., 40 °C, 60 °C) to evaluate the effect of temperature on the fatigue life. We report the S‑N curve at each temperature.
• Bending fatigue with crack propagation monitoring (NTC 5733 – for the damage evolution) – during the bending fatigue test, we monitor the crack initiation and propagation using an optical microscope or a video camera. The crack length vs. cycles curve is plotted. We report the crack initiation time and the crack growth rate.
• Bending fatigue under corrosive environment (NTC 5734 – for the corrosion‑fatigue assessment) – we perform the bending fatigue test in a corrosive environment (e.g., salt spray or acidic solution) to evaluate the combined effect of cyclic bending and corrosion. We report the corrosion‑fatigue life and the failure mechanism.

Environmental and Aging Effects on Flexural Strength – Evaluating Long‑Term Durability

The flexural strength and the flexural modulus of materials can change over time due to environmental exposure, thermal aging, moisture absorption, and UV degradation. Our environmental and aging tests evaluate the long‑term stability of the flexural properties, ensuring the reliability of the product over its service life in the diverse Croatian climate (coastal, continental, and mountainous).

• Thermal aging effect on flexural properties (ASTM D573 / ISO 188 / NTC 5740 – for the heat‑aged materials) – we age the material in an oven at a specified temperature (e.g., 70 °C, 100 °C) for a specified duration (e.g., 7, 14, or 28 days), and then we re‑measure the flexural strength and the flexural modulus. We report the flexural properties after aging and the retention of properties (in %).
• Humidity and moisture effect (ASTM D570 / NTC 5741 – for the moisture‑exposed materials) – we condition the material at a high‑humidity environment (e.g., 40 °C, 95 % RH) for a specified duration (e.g., 7 days), and then we re‑measure the flexural properties. We report the flexural properties after humidity exposure and the moisture uptake.
• UV and weathering effect (ASTM G154 / NTC 5742 – for the outdoor‑exposed materials) – we expose the material to UV radiation (UVA‑340) and condensation cycles for a specified duration (e.g., 500 hours), and then we re‑measure the flexural properties. We report the flexural properties after UV exposure and the change.
• Chemical exposure effect (ASTM D543 / NTC 5743 – for the chemically exposed materials) – we immerse the material in various chemicals (e.g., mineral oil, 10 % HCl, 10 % NaOH, or a solvent) for a specified duration (e.g., 7 days), and then we re‑measure the flexural properties. We report the flexural properties after chemical exposure and the compatibility.
• Freeze‑thaw effect (NTC 5744 – for the cold‑climate applications) – we subject the material to repeated freeze‑thaw cycles (e.g., -20 °C to +20 °C) and then we re‑measure the flexural properties. We report the flexural properties after the freeze‑thaw cycles and the effect.

Complementary Tests – Hardness, Tensile, and Microstructure for Flexural Correlation

To fully understand the flexural behavior and to correlate it with the material's properties, we perform complementary tests, including hardness testing, tensile testing, and microstructural examination.

• Hardness testing (ASTM D2240 / NTC 5750 – Shore hardness for plastics; ASTM E18 / NTC 5751 – Rockwell for metals) – we measure the hardness of the material. The hardness is correlated with the flexural strength and the flexural modulus. We report the hardness and the correlation.
• Tensile testing (ASTM D638 / ISO 527 / NTC 5752 – for plastics; ASTM E8 / ISO 6892 / NTC 5753 – for metals) – we perform a tensile test on the material to measure the tensile strength, the elongation, and the tensile modulus. The tensile properties are correlated with the flexural properties. We report the tensile properties and the correlation.
• Metallographic examination (ASTM E3 / NTC 5754 – for the grain size and the phase distribution) – we examine the microstructure of the material (the grain size, the phase distribution, and the inclusion content) using optical microscopy and SEM. The microstructure is correlated with the flexural properties. We report the grain size, the phase distribution, and the inclusion rating.
• Density measurement (ASTM D792 / NTC 5755 – for the material density) – we measure the density of the material (in g/cm³) using the Archimedes method. The density is correlated with the flexural properties. We report the density.
• FTIR spectroscopy for polymer identification (ASTM E168 / NTC 5756 – for the chemical composition) – we use FTIR spectroscopy to identify the chemical composition of the polymer material and to detect any degradation. We report the FTIR spectra and the chemical identification.

Test Report and Recognition in the Croatian Construction, Automotive, and Industrial Sector

All procedures described are within the scope of our ISO/IEC 17025 accreditation, with equipment calibrated periodically (universal testing machines, extensometers, displacement sensors, and thermal chambers) and traceability to international standards (NIST, PTB). Our test reports are issued in English (with a Croatian summary available on request) and include:

• Full identification of the test sample (material, dimensions, orientation, and conditioning history).
• Detailed description of the test methods applied (ASTM/ISO/HRN EN/NTC standards, test conditions, temperature, and test speed).
• Numerical results: flexural strength (MPa), flexural modulus (GPa), flexural strain (%), deflection (mm), flexural stiffness (N·mm²), fatigue limit (MPa), and property retention after aging (%).
• Graphical data: load‑deflection curves, stress‑strain curves, S‑N curves, and flexural modulus vs. temperature curves.
• Comparative tables against the values specified by the client or against the limits of the relevant standards (ASTM D790, ISO 178, HRN EN ISO 178, ASTM C1161, and the requirements of the HZN, Ministarstvo graditeljstva, and Državni inspektorat).
• Photographs of the test specimens before and after the test, and the fracture surfaces.
• Recommendations for material selection, design optimization, and quality control measures to achieve the required flexural performance.
• Expanded uncertainty (k=2) for all key measurements, calculated according to the ISO/IEC 98‑3 Guide.

These reports are fully accepted by the Hrvatski zavod za norme (HZN) for the verification of product conformity, by the Ministarstvo gospodarstva i održivog razvoja for industrial and energy compliance, by the Državni inspektorat for market surveillance, by the Ministarstvo graditeljstva i prostornoga uređenja for building materials approval, and by the Carinska uprava (Croatian Customs) for tariff classification and quality verification in the import of materials and components. Additionally, we offer consulting services for the selection of materials with optimal flexural properties, the design of flexural members and structures, and the implementation of quality control programs for flexural performance, contributing to the safety, reliability, and performance of products in the diverse and growing Croatian market, from the automotive and aerospace sectors to the construction, packaging, and consumer goods industries.