Tensile load testing of plastic woven sacks is the core mechanical evaluation used to verify the warp, weft, and seam strength of polypropylene and polyethylene woven packaging before it is filled with cement, fertilizer, grain, or chemical products. This article explains the applicable standards such as GB/T 8946, GB/T 8947, ISO 527, ISO 13934-1, and ASTM D5034, describes specimen preparation and test procedures, and answers common questions about equipment, acceptance criteria, and reporting.
Technical Overview
Plastic woven sacks are manufactured from polypropylene (PP) or high-density polyethylene (HDPE) flat or split tapes that are woven into fabric, optionally coated with a thin polyolefin film layer, and then converted into bags by cutting, sewing, or heat sealing. Because of their high strength-to-weight ratio, moisture resistance, and low cost, these sacks dominate the packaging of cement, fertilizer, flour, rice, sugar, salt, animal feed, and granular or powdery chemicals.
The single most important mechanical property of a woven sack is its tensile load, defined as the maximum force a specimen of specified width withstands before rupture when stretched at a constant rate of extension. Tensile load is determined on strip specimens cut in the warp (machine) direction and the weft (cross) direction, as well as on specimens containing sewn or heat-sealed seams. The result is expressed in newtons, typically normalized to a 50 mm specimen width, and directly indicates whether the sack can survive filling impact, stacking pressure, palletized storage, transport vibration, and repeated handling.
The product performance requirements are specified in standards such as GB/T 8946 for plastic woven sacks and GB/T 8947 for composite plastic woven bags. The test procedures themselves draw on general method standards including the GB/T 1040 series (aligned with ISO 527) for plastic materials, GB/T 3923.1 and ISO 13934-1 for textile strip methods, and ASTM D5034 or ASTM D5035 as widely used North American equivalents. Testing is performed in manufacturer laboratories for routine quality control, by buyers for incoming inspection, and by accredited third-party laboratories for trade compliance, certification, and dispute resolution.
- Material scope: PP and HDPE woven fabric, coated or uncoated
- Key measurand: maximum tensile load at break, in N per 50 mm width
- Test directions: warp, weft, and seam orientation
- Governing product standards: GB/T 8946, GB/T 8947
- Supporting method standards: ISO 527, ISO 13934-1, GB/T 3923.1, ASTM D5034, ASTM D5035
Testing Samples
Base Woven Fabric (Uncoated)
Strip specimens are cut from the body fabric of finished sacks or directly from production rolls. Specimens must be free of visible defects such as broken tapes, creases, oil stains, or weave distortions, and any creases are smoothed without stretching the fabric. Cut-strip or ravel-strip techniques are used to obtain the specified 50 mm width with an exact and reproducible number of tapes.
Coated and Laminated Woven Fabric
For film-coated or laminated sacks, the coating is not removed, and the specimen is tested as an integral laminate. Failure modes are recorded carefully, since partial delamination or coating splitting before fabric rupture can indicate bonding problems that reduce effective strength in service.
Finished Sacks and Seam Specimens
Seam specimens are taken across the bottom seam or side seam of finished sacks so that the seam line lies perpendicular to the loading axis and centered in the gauge region. These specimens evaluate the combined performance of fabric, sewing thread, stitch density, and seam construction, which is frequently the weakest link in a filled sack.
Sampling and Batch Representation
Sampling follows the lot-sampling rules of the applicable product standard so that tested specimens represent the production batch. As a general practice, at least five valid specimens are prepared for each direction or seam type. All specimens are conditioned in a standard laboratory atmosphere, commonly 23 ± 2 °C and 50 ± 10% relative humidity per ISO 291, for a minimum of 24 hours before testing.
Testing Items
A complete tensile evaluation of plastic woven sacks typically covers the following items, each reported as the arithmetic mean of valid specimen results together with the individual values and measures of scatter.
- Warp-direction tensile load (N/50 mm): strength along the machine direction of the woven fabric
- Weft-direction tensile load (N/50 mm): strength across the fabric, governing hoop stress in a filled sack
- Seam or joint tensile load: strength of the sewn bottom, sewn side, or heat-sealed closure
- Heat-seal strength: required for composite and laminated bags per GB/T 8947, referencing methods such as ASTM F88
- Elongation at break: recorded as supporting data to assess ductility and energy absorption
- Failure mode description: fabric rupture, thread breakage, seam slippage, or delamination
Warp and weft tensile loads verify the intrinsic quality of the woven tape and the weaving process, including tape tenacity, draw ratio, and weave density. Seam tensile load verifies the conversion process, including thread selection, stitch type, stitch density, and seam allowance. Together these items establish whether a sack will maintain containment under the static and dynamic loads experienced throughout the packaging, storage, and distribution cycle.
Testing Methods
Specimen Preparation and Conditioning
Specimens are cut to 50 ± 0.5 mm width with an overall length sufficient for full grip engagement, typically 350 to 600 mm depending on grip design, giving a gauge length of 200 mm between the jaws. Edges must be clean and parallel; for ravel-strip specimens, yarns at the edge are removed symmetrically until the exact width is reached. Specimens are then conditioned in the standard atmosphere described above so that temperature and moisture effects on polyolefin tape strength are controlled.
Warp and Weft Tensile Load Determination
Testing is performed on a constant-rate-of-extension universal testing machine fitted with a calibrated load cell of appropriate capacity, for example 5 kN or 10 kN, meeting the accuracy classes of ISO 7500-1 or ASTM E4. Pneumatic or wedge-action grips with rubber-faced or abrasive inserts are used to prevent slippage without crushing the tapes. A small preload is applied to remove slack, the specimen is aligned with the loading axis, and the crosshead is driven at the speed specified by the applied method, commonly 200 mm/min for woven polypropylene fabric in routine practice. The machine records the force-extension curve up to rupture, and the maximum force reached before break is taken as the tensile load for that specimen.
Seam and Joint Strength Determination
Seam specimens are mounted with the fabric legs gripped on either side and the seam centered between the jaws. The specimen is extended until failure occurs by thread breakage, fabric tearing adjacent to the seam, or seam slippage, and the maximum force is recorded together with the observed failure mode. This test directly simulates the bottom-seam loading that occurs when a filled sack is lifted, dropped, or stacked.
Data Evaluation and Acceptance Criteria
Results are reported as the mean of at least five valid specimens for each direction or seam type. Specimens that slip in the jaws or break within approximately 5 mm of the grip faces with an anomalously low force are treated as jaw breaks; if such invalid results are limited in number, replacement specimens are tested. Product standards such as GB/T 8946 assign sacks to specification types with minimum required tensile loads; common commercial grades specify warp and weft minima in the approximate range of 650 N to 900 N per 50 mm, with seam requirements set at lower levels, typically several hundred newtons. The declared or contractual type governs acceptance, and conformity is judged by comparing individual and mean results against the specified minima according to the rounding and decision rules of the applicable standard.
Applications
Tensile load testing of plastic woven sacks supports quality control, product qualification, and regulatory compliance across many industries. Manufacturers use it to release production batches, optimize tape extrusion and weaving parameters, and validate new sack designs, while buyers and exporters use test reports to verify supplier claims and satisfy market-entry documentation requirements.
- Cement, concrete additives, and dry-mortar packaging subject to heavy stacking loads
- Chemical fertilizers, agrochemicals, and powdered minerals
- Food commodities including rice, flour, sugar, salt, and starch
- Animal feed, seed, and agricultural inputs
- Plastic resins, granules, and intermediate bulk packaging liners
- Sandbags and civil-engineering or flood-control applications
- Supplier qualification, incoming inspection, and factory audits
- Input data for transport simulation and packaging design validation
- Arbitration and claims settlement in domestic and international trade
FAQ
What equipment is required for tensile load testing of plastic woven sacks?
A constant-rate-of-extension universal testing machine is required, equipped with a calibrated load cell meeting ISO 7500-1 or ASTM E4 accuracy requirements, suitable pneumatic or wedge grips with non-slip inserts, and data acquisition software capable of recording maximum force and elongation at break. A controlled-atmosphere conditioning chamber is also needed to prepare specimens at standard temperature and humidity.
What specimen dimensions and how many specimens are used?
The standard specimen is a strip of 50 mm width with a gauge length of 200 mm and sufficient extra length for gripping. At least five valid specimens should be tested for each direction, warp and weft, and for each seam type, with mean values and individual values both reported.
Why do some specimens break at the grips, and how is this handled?
Grip-zone failures usually result from stress concentration or slippage at the jaw faces. They are managed by adding protective padding, adjusting clamping pressure, ensuring correct alignment, and, where allowed by the method, discarding clearly invalid jaw breaks and testing replacement specimens.
What are typical acceptance criteria?
Acceptance depends on the specification type declared for the sack. Common grades under GB/T 8946 require warp and weft tensile loads in the approximate range of 650 N to 900 N per 50 mm, while seam requirements are typically lower. The exact limiting values must always be taken from the applicable product standard or the purchase contract.
What is the difference between the strip method and the grab method?
In the strip method, the full 50 mm width of the specimen is stressed, and results are normalized per unit width, which suits woven fabric characterization under ISO 13934-1 or GB/T 3923.1. In the grab method, such as ASTM D5034, only the central fabric zone held between jaw faces is effectively stressed, which better simulates localized loading but produces results that are not directly comparable with strip values. The method used must always be stated in the report.
Do coated or composite woven bags require additional testing?
Yes. Composite plastic woven bags per GB/T 8947 typically require heat-seal strength evaluation in addition to fabric and seam tensile load, and laminated structures may be assessed for seal integrity using methods such as ASTM F88 and ASTM F2029 to ensure that closures perform reliably alongside the woven substrate.
