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Tear resistance
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Tear resistance (or tear strength) is a measure of how well a material can withstand the effects of tearing.[1] It is a useful engineering measurement for a wide variety of materials by many different test methods.
Discussion
[edit]For example, with rubber, tear resistance measures how the test specimen resists the growth of any cuts when under tension, it is usually expressed in kN/m.[2] Tear resistance can be gauged via the same ASTM D 412 apparatus used to measure tensile strength, modulus and elongation. ASTM D 624 can be applied to measure the resistance to the formation of a tear (tear initiation) and the resistance to the expansion of a tear (tear propagation). Regardless of which of these two is being measured, the sample is held between two holders and a uniform pulling force applied until the aforementioned deformation occurs. Tear resistance is then calculated by dividing the force applied by the thickness of the material.[2]
Materials with low tear resistance sometimes have poor resistance to abrasion and when damaged will quickly fail (this includes hard materials, since hardness is not related to tear resistance).[2]
Tear resistance of different materials
[edit]Substances with high tear resistance include epichlorohydrin, natural rubber and polyurethane. In contrast, materials such as silicone and fluorosilicone have low tear resistance.[2]
Tear-yield ratio
[edit]The ratio of tear resistance to the yield strength is called the tear-yield ratio. It is a measure of notch toughness.[3]
Standards by ASTM International
[edit]- C1681-14 Standard Test Method for Evaluating the Tear Resistance of a Sealant Under Constant Strain
- D1004-13 Standard Test Method for Tear Resistance (Graves Tear) of Plastic Film and Sheeting
- D1922-15 Standard Test Method for Propagation Tear Resistance of Plastic Film and Thin Sheeting by Pendulum Method
- D1938-14 Standard Test Method for Tear-Propagation Resistance (Trouser Tear) of Plastic Film and Thin Sheeting by a Single-Tear Method
- D2212-00(2015) Standard Test Method for Slit Tear Resistance of Leather
- D2582-16 Standard Test Method for Puncture-Propagation Tear Resistance of Plastic Film and Thin Sheeting
- D4932/D4932M-89(2014)e1 Standard Test Method for Fastener Rupture and Tear Resistance of Roofing and Waterproofing Sheets, Roll Roofing, and Shingles
- E604-15 Standard Test Method for Dynamic Tear Testing of Metallic Materials
See also
[edit]References
[edit]- ^ NAHAD GLOSSARY, http://www.nahad.org/aws/NAHAD/pt/sp/hsi_glossaries, 15 June 2012
- ^ a b c d Tear Resistance., "Physical Properties: Tear Resistance". Archived from the original on 25 May 2012. Retrieved 15 June 2012., 15 June 2012
- ^ John Gilbert Kaufman (2001). Fracture Resistance of Aluminum Alloys: Notch Toughness, Tear Resistance. ASM International. p. 38. ISBN 9780871707321.
External links
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Tear resistance
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Fundamentals
Definition and Mechanisms
Tear resistance, also known as tear strength, refers to a material's capacity to resist the initiation and propagation of a crack or tear under applied mechanical forces, typically quantified as the force or energy required per unit length or thickness of the tear front.[7] This property is crucial in assessing a material's durability against localized failure modes, with common measurement units including newtons per millimeter (N/mm) for thin sheets or kilonewtons per meter (kN/m) for broader applications in engineering contexts.[7] The mechanisms of tearing involve distinct stages of crack initiation and propagation, influenced by stress concentrations at preexisting flaws or notches. Crack initiation occurs when localized stresses exceed the material's yield strength, often at sharp defects where stress intensity factors amplify the applied load, leading to initial deformation or microcrack formation.[8] Propagation follows as the crack advances through the material, driven by the release of stored elastic energy that overcomes the resistance from atomic bonds or microstructural features. Fracture modes during tearing can be brittle, characterized by rapid, low-energy cleavage along planes with minimal plastic deformation, or ductile, involving significant energy absorption through mechanisms like void coalescence, necking, and plastic flow that blunt the crack tip and slow advancement.[9] In ductile tearing, the process consumes substantial energy via shear deformation, enhancing overall resistance compared to brittle modes where cracks propagate unstably once a critical stress threshold is met.[10] The basic physics of tear resistance draws from fracture mechanics, particularly Griffith's criterion, which posits that tearing initiates when the energy released by crack growth equals the energy needed to create new fracture surfaces. Adapted to tearing, this involves the energy release rate , defined for a through-crack in plane stress as where is the applied tensile stress, is the half-length of the crack, and is the material's elastic modulus; fracture occurs when reaches a critical value related to surface energy.[8] This criterion highlights how tear resistance depends on balancing elastic energy release against dissipative processes at the crack tip, such as viscoelastic deformation in polymers.[11] Tears in materials are classified by geometry and propagation path, with key types including the Elmendorf tear, which simulates a dynamic, pendulum-initiated propagation in a pre-slit specimen to measure average resistance over a short distance, and the tongue tear, where a tensile load pulls apart a bifurcated sample resembling a tongue shape, allowing steady-state crack growth along a defined path.[7] Conceptually, the Elmendorf method captures impulsive tearing akin to sudden impacts, while the tongue tear emphasizes controlled extension, illustrating how tear resistance varies with loading rate and crack front geometry in fibrous or sheet-like materials.[12]Importance and Applications
Tear resistance plays a pivotal role in ensuring the durability and safety of materials across various industries, where premature tearing can lead to catastrophic failures. In critical applications such as medical gloves, high tear resistance maintains the integrity of protective barriers against microbial transmission during surgical procedures, reducing infection risks for both patients and healthcare workers.[13] Similarly, in parachutes, materials with superior tear resistance prevent propagation of defects under high stress, ensuring reliable deployment and user safety during descent.[14] By enhancing product longevity, tear resistance also contributes to cost reduction in manufacturing through minimized rework and material waste. Key applications of tear resistance span multiple sectors, underscoring its practical value. In packaging, particularly plastic films used for food preservation, adequate tear resistance safeguards product integrity against handling stresses, preventing contamination and extending shelf life.[15] Automotive tires rely on tear-resistant rubber compounds in sidewalls to withstand impacts and abrasions, thereby improving vehicle safety and reducing the risk of blowouts.[16] In apparel, such as industrial workwear, fabrics with strong tear resistance endure rigorous use in hazardous environments, protecting workers from injury.[15] Biomedical fields benefit from tear-resistant materials in sutures, which resist cutting through soft tissues during wound closure, and in artificial skin simulants for surgical training, where they mimic human tissue durability to support realistic practice.[17][18] The economic implications of inadequate tear resistance are substantial, often resulting in direct financial losses from product damage and indirect costs like increased returns and labor. For instance, failures in stretch films due to low tear strength have caused pallet collapses leading to $30,000 in product losses over two months for a distributor.[19] Torn mailers in e-commerce can elevate return rates by 25%, amplifying handling expenses and reputational harm.[19] Historically, the importance of tear resistance gained prominence in the 20th century amid polymer advancements, particularly during World War II when synthetic fabrics like nylon were developed for military applications. Nylon's high tear resistance made it ideal for parachutes, ropes, and uniforms, replacing silk and enabling reliable performance under combat conditions.[20] This era marked a shift toward engineering materials for enhanced mechanical integrity in high-stakes scenarios.Testing and Measurement
Standard Test Methods
Standard test methods for measuring tear resistance in materials such as plastics, films, and sheeting primarily rely on standardized protocols developed by organizations like ASTM International and the International Organization for Standardization (ISO). These methods ensure reproducible results by specifying sample preparation, equipment, procedures, and data analysis. Key ASTM standards include D1004 for the Graves tear method, which evaluates tear resistance at low loading rates for flexible plastic films and sheeting, and D1922 for the Elmendorf tear method, which assesses propagation tear resistance using a pendulum impact. Equivalent ISO standards are ISO 6383-1 for trouser tear propagation (similar but not identical to D1004 in specimen size and procedure) and ISO 6383-2 for the Elmendorf method, which is technically equivalent to D1922.[4][21][22] Sample preparation is critical for accuracy and varies by method. For the Elmendorf tear (ASTM D1922), specimens are typically cut to dimensions of 63 mm width by 100 mm length using a constant radius template to ensure uniformity, with a 20 mm precut slit made from one edge to initiate the tear. In the Graves or trouser tear method (ASTM D1004), rectangular specimens measuring approximately 100 mm by 63 mm are prepared, featuring a small nick or initial cut (about 2.5 mm deep) at one end to start propagation, though trouser-style specimens under related standards like ASTM D1938 may use wider dimensions such as 75 mm by 150 mm with a central precut to form "legs." Specimens must be conditioned at 23°C and 50% relative humidity for at least 40 hours to standardize environmental effects.[23][24][22] The Elmendorf tear procedure (ASTM D1922) uses a pendulum-based method to measure dynamic tear propagation. First, the specimen is clamped securely in the tester with the precut edge aligned, ensuring the slit is positioned correctly for the pendulum's path. A sharp blade or knife then extends the initial slit to 43 mm. The pendulum, weighted to deliver a specific energy (e.g., 10,000 or 64,000 g·cm depending on material thickness), is released from a horizontal position, swinging through the specimen to propagate the tear. The pendulum's reduced swing angle is recorded, which correlates to the energy absorbed by the material. Multiple specimens (at least five per direction) are tested, with the average value calculated after correcting for any initial cut energy.[21][25][26] In contrast, the trouser tear procedure (ASTM D1004 or D1938) employs a constant-rate tensile method for quasi-static tear propagation. The specimen's "legs" (formed by the initial cut) are clamped into the upper and lower jaws of a tensile testing machine, with the nick or precut aligned at the jaw line. A steady force is applied by separating the jaws at a controlled rate, typically 51 mm/min for Graves tear under D1004 or up to 500 mm/min for some trouser configurations in related standards, until the tear propagates across the full width. The maximum force required to maintain propagation is recorded, often using a load cell for precision. At least five replicates are performed in both machine and transverse directions to account for anisotropy.[24][4][27] Equipment for these tests includes specialized tear testers. The Elmendorf tester is a pendulum impactor device with a pivoting arm, clamping mechanism, and scale for measuring swing deflection; it must be calibrated regularly using known weights to account for friction, windage, and leveling errors, often via an automatic function that verifies pendulum energy against standards like 10 J. Universal testing machines (UTMs), such as those from Instron or ZwickRoell, are used for trouser or Graves methods, featuring programmable crosshead speeds (1–500 mm/min), high-resolution load cells (up to 0.02 N sensitivity), and grips to hold specimens without slippage. UTMs require calibration of load cells and extensometers per ISO 7500-1 to ensure accuracy within ±1% of indicated force.[25][28][29] Data reporting focuses on tear strength, normalized for material thickness to enable comparisons. For both methods, tear strength is calculated as the average force (or energy) divided by specimen thickness, yielding units such as N/mm for force-based results or mN for Elmendorf energy equivalents. In the Elmendorf method, the propagation tear resistance is reported as the average tearing load in grams or millinewtons, derived from the pendulum's energy loss formula: tear value = (pendulum energy - residual energy) / tear length. For trouser tear, results are expressed as maximum load per unit thickness (e.g., N/mm), with the average of replicates and standard deviation included; thickness is measured via micrometer to ±0.01 mm. Reports often specify test direction and any corrections for specimen weight.[21][7][30]Factors Influencing Test Results
Environmental factors play a critical role in tear resistance measurements, as they alter the material's molecular structure and mechanical response. Temperature influences tear strength significantly, with elastomers exhibiting varied behavior depending on their glass transition temperature (Tg); for instance, fluorocarbon elastomers show marked differences in tear properties at -40°C compared to +49°C due to Tg-related changes in flexibility. Above Tg, polymers soften, leading to reduced tear resistance, sometimes by up to 50% or more in susceptible materials like certain silicone rubbers under high-temperature exposure. Humidity also affects results, particularly in fiber-based materials, where tear strength peaks at around 50% relative humidity (RH) and decreases at higher levels due to increased moisture softening fiber bonds and reducing interfiber friction; for example, tensile force in paper sheets drops from 7.56 N at 50% RH to 6.6 N at 90% RH. Aging from UV exposure causes degradation, with tear strength reductions of 40-80% observed in high-performance fibers like meta-aramids after 7-14 days of exposure at 0.24 W/m² and 40°C, 50% RH, due to chain scission and loss of molecular integrity.[31][32][33][34] Material variables such as thickness, orientation, and notch geometry directly impact test outcomes by influencing stress distribution and crack initiation. Thicker specimens generally exhibit higher tear strength, as the force required scales with thickness; for example, in blown films, machine-direction tear strength can double from 238 g at 1 mil thickness to 826 g at 3 mil. In anisotropic composites and laminates, orientation leads to directional variations, causing crack paths to deviate up to 12° in polyethylene terephthalate films and increased delamination in laminates, resulting in higher tear force anisotropy compared to isotropic materials. Notch geometry affects crack propagation, with sharp notches providing consistent fracture toughness values and lower scatter, while blunt notches (e.g., U- or V-shaped) introduce perturbations, leading to overestimation of toughness and greater data variability.[1][35][36] Testing parameters, including tear propagation speed, sample clamping, and preconditioning, must be controlled to ensure reproducibility. Higher tear propagation speeds increase resistance, with tear load rising by up to 30% at rates from 300-1000 inches per minute compared to slower speeds, mimicking dynamic impacts but potentially altering failure modes in rubbers. Sample clamping influences results through separation distance and grip design; for trapezoidal tests, tear strength remains consistent for specimens at least 55 x 110 mm in warp direction, but smaller sizes below this limit the tear distance and accuracy, while corner clamping can induce premature tearing. Preconditioning standardizes moisture content, typically at 23°C and 50% RH for at least 40 hours per ASTM D618, to approximate equilibrium and minimize humidity-induced variability in plastics and elastomers.[37][38] Statistical considerations address inherent variability from sample defects, such as inconsistencies in ply multiplicity or manufacturing flaws, which increase scatter in thicker or multi-ply materials. To mitigate this, standards recommend a minimum of five replicate specimens per direction, reporting the median value to reduce outlier influence, with error analysis via repeatability studies showing 5-9% variation attributable to testing conditions versus material differences.[39][40]Material Comparisons
Polymers and Elastomers
Polymers exhibit varying degrees of tear resistance depending on their chemical structure, molecular weight, and processing conditions. Low-density polyethylene (LDPE) typically demonstrates moderate tear resistance as measured by initial tear tests on films.[41] Higher molecular weight in polyethylene enhances tear resistance by promoting greater chain entanglement and reducing crack propagation.[42] Polypropylene (PP) films show tear strengths of 40 to 80 N/mm under propagation tear conditions, benefiting from its semi-crystalline nature that distributes stress more effectively.[43] Polyvinyl chloride (PVC) extruded materials have an average tear strength of approximately 54 kN/m, with ranges from 17.5 to 105 kN/m across formulations, influenced by plasticizer content that can soften the material and lower resistance.[44] Elastomers, characterized by their viscoelastic behavior, provide unique tear resistance through mechanisms like strain-induced crystallization, particularly in natural rubber, where stretching aligns polymer chains to form crystalline domains that hinder crack growth and boost toughness.[45] Natural rubber exhibits tear strengths of 50 to 100 kN/m when tested per ASTM D624 using Die C specimens, reflecting its ability to undergo large deformations before failure.[1] Silicone rubber, in contrast, displays lower values of 10 to 30 kN/m under the same standard, due to its more flexible siloxane backbone that offers less resistance to tear propagation but superior flexibility.[46]| Material | Test Standard | Typical Tear Strength | Notes |
|---|---|---|---|
| LDPE | ASTM D1004 | Moderate (typical 20-50 N/mm) | Film thickness ~0.05-0.1 mm; initial tear. |
| Polypropylene | ASTM D1922 | 40-80 N/mm | Propagation tear; oriented films higher. |
| PVC | ASTM D1004 | 17.5-105 kN/m (avg. 54 kN/m) | Extruded; plasticized variants lower. |
| Natural Rubber | ASTM D624 (Die C) | 50-100 kN/m | Vulcanized; strain-induced crystallization key. |
| Silicone Rubber | ASTM D624 (Die B) | 10-30 kN/m | Room temperature; flexible grades. |
Textiles and Composites
Tear resistance in textiles arises from the interplay of fibrous structures, where individual yarns bear and distribute loads during propagation of a tear. Cotton fabrics, being natural fibers, generally display moderate tear strengths of 20-40 N when tested via the Elmendorf method under ASTM D1424, reflecting their relatively lower yarn tenacity compared to synthetics. In contrast, nylon fabrics achieve higher tear strengths of approximately 30-50 N for lightweight constructions like 210D variants, owing to the superior elongation and toughness of polyamide fibers that allow better energy dissipation. Polyester fabrics perform similarly to nylon, with tear values often in the 50-100 N range for comparable deniers, benefiting from high modulus and low creep under stress.[50][51][52] Weave structure significantly modulates these properties, with twill weaves outperforming plain weaves by 20-30% in tear resistance due to longer yarn floats that enhance crimp interchange and load sharing among yarns. This structural advantage reduces stress concentration at tear initiation points, allowing the fabric to resist propagation more effectively than the balanced but rigid plain weave. In apparel applications, such as durable workwear, twill-constructed synthetics provide reliable tear resistance for repeated flexing and abrasion.[53][11] Yarn twist and density further influence tear behavior in textiles; higher twist multipliers increase yarn compactness and inter-yarn friction, boosting tear strength by up to 20-30% through improved load transfer, while denser weaves can either enhance or diminish resistance depending on crimp levels—looser densities promote yarn mobility for better tear absorption.[54][55] In fiber-reinforced polymer composites, tear resistance is predominantly governed by interlaminar fracture toughness, often assessed via ASTM D5528 for mode I delamination, with glass-fiber epoxy systems typically yielding G1c values of 0.3-1.5 kJ/m², reflecting the energy required for crack advance between plies. Delamination poses a critical risk, as it initiates from manufacturing voids or impact and propagates under shear or tension, drastically reducing overall structural integrity by decoupling load-bearing fibers. The matrix-fiber interface plays a pivotal role, where strong adhesion via chemical bonding or sizing enhances toughness by promoting fiber bridging and matrix plasticity, whereas weak interfaces lead to premature debonding and lower energy absorption.[56][57][58] Hybrid composites, such as Kevlar-carbon fiber mixes embedded in epoxy, leverage complementary properties for elevated tear and impact resistance in ballistic applications, where Kevlar's ductility mitigates carbon's brittleness, achieving up to 40% higher energy absorption before delamination. Fiber alignment introduces anisotropy, with unidirectional orientations boosting tear resistance along the fiber direction by 2-5 times compared to transverse loading, due to efficient stress alignment but vulnerability perpendicular to fibers.[59][60]| Material Type | Test Standard | Representative Tear Resistance | Key Trend |
|---|---|---|---|
| Cotton Fabric | ASTM D1424 | 20-40 N (Elmendorf) | Lower due to natural fiber limits; weave-dependent |
| Nylon/Polyester Fabric | ASTM D1424 | 30-100 N (Elmendorf) | Higher tenacity; twill boosts by 20-30% |
| Glass-Fiber Epoxy Composite | ASTM D5528 | 0.3-1.5 kJ/m² (G1c) | Anisotropic; alignment enhances in-plane resistance |
| Kevlar-Carbon Hybrid Composite | ASTM D5528 (adapted) | 1.0-2.0 kJ/m² (G1c) | Improved delamination resistance for ballistic use |