Tensile testing is the core mechanical evaluation for plastic woven sacks made from polypropylene (PP) or polyethylene (PE) tape yarns, verifying that the fabric, seams, and joints can withstand filling, handling, stacking, and transport loads. This article explains the sample types, testing items, test procedures, and applicable standards such as GB/T 8946, ASTM D5035, and ISO 13934-1 that are used by manufacturers and accredited third-party laboratories for quality control and product compliance.
Technical Overview
Plastic woven sacks are manufactured by circular or flat weaving of extruded polypropylene (PP) or high-density polyethylene (HDPE) tape yarns, and are frequently laminated with a polyethylene film to improve moisture resistance and printability. Because these sacks are filled with heavy, dense, or abrasive goods such as cement, fertilizer, grain, sugar, resin, and minerals, the mechanical integrity of the woven fabric and of its stitched or heat-sealed joints directly determines whether the package survives filling lines, palletizing, warehouse stacking, and multimodal transport.
Tensile testing, also referred to as tension or pull testing, is therefore the most important laboratory evaluation in woven sack quality control. It measures the maximum force that the woven fabric, the seams, and the joints can resist before rupture, together with the elongation behavior of the tape yarns. The resulting data support factory quality release, buyer acceptance, supplier qualification, failure analysis, and compliance with product standards such as GB/T 8946 for plastic woven sacks and GB/T 9774 for cement packaging bags.
- Verification of warp and weft breaking force of the woven fabric
- Verification of seam and bottom joint strength
- Confirmation of the declared safe working load of the package
- Support for root-cause analysis of sack failures in the field
Testing Samples
Woven Fabric Strip Specimens
Strip specimens are cut from the body of finished sacks or from production rolls of woven fabric. Specimens are prepared in two orientations: parallel to the warp (machine direction) and parallel to the weft (cross direction). A typical specimen is 50 mm wide with sufficient length to allow a gauge length of about 200 mm between the grips. Depending on the governing method, either a raveled strip or a cut strip technique is applied, following standards such as ASTM D5035 or ISO 13934-1.
Seam and Joint Specimens
Seam specimens are cut so that the stitched bottom seam, the top hem, or the side fold line lies perpendicular to the direction of the applied force, with the joint located midway between the grips. The specimen width follows the specified method, commonly in the range of 50 mm to 100 mm. These specimens are tested to determine whether the stitched or heat-sealed joint, rather than the base fabric, becomes the weak point of the sack. Seam tensile properties can be evaluated with reference to ISO 13935-1 or ISO 13935-2.
Complete Sack Samples
Whole sacks are retained for confirmation of identity, dimensions, fabric construction, grammage, and lamination condition. While full sacks are generally evaluated by drop impact and stacking tests rather than tensile methods, they serve as the source material for all strip and seam specimens and ensure that the test program reflects the actual commercial product.
Testing Items
A typical tensile test program for plastic woven sacks includes the following items:
- Warp-direction breaking force of the woven fabric, expressed in newtons per 50 mm width
- Weft-direction breaking force of the woven fabric, expressed in newtons per 50 mm width
- Elongation at break of the fabric in both directions, expressed as a percentage
- Seam breaking strength of stitched joints, including bottom seams and top hems
- Joint strength of heat-sealed or heat-cut edges
- Optional related items, such as tensile properties of the laminating film tested in accordance with ISO 527 or ASTM D882, and peel adhesion of the lamination
Each item addresses a distinct failure mode. Fabric breaking force reflects the tenacity and density of the tape yarns; elongation reflects the ductility that allows the sack to absorb shock loads during dropping and stacking; and seam strength reflects the thread quality, stitch density, and sewing workmanship that hold the package together under internal pressure from the fill material.
Testing Methods
Specimen Conditioning
Prior to testing, specimens are conditioned to equilibrium in a standard laboratory atmosphere. For textile-based methods, the standard atmosphere is typically 20 ± 2 °C with 65 ± 4 % relative humidity in accordance with ISO 139 or GB/T 6529. When plastics-based methods are applied to film or laminate components, conditioning is usually performed at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291. A conditioning period of at least 24 hours is common practice.
Apparatus and Setup
Testing is performed on a calibrated constant-rate-of-extension (CRE) universal testing machine equipped with a load cell matched to the expected breaking force, typically in the range of 1 kN to 10 kN for woven sack applications. Wedge or pneumatic grips with serrated or rubber-faced jaw inserts are used to secure the specimen without slippage and without pre-damaging the tape yarns. Data acquisition software records the force-extension curve, the maximum force, and the elongation at break.
Test Procedure
- Verify specimen width, gauge length, and freedom from visible weaving defects at the test section
- Mount the specimen centrally in the grips, ensuring the yarns are parallel to the direction of force
- Apply a small pre-tension where required by the method to remove slack
- Run the test at the crosshead speed specified by the standard, commonly in the range of 20 mm/min to 300 mm/min depending on the method applied
- Record the maximum force, the elongation at break, and the exact location and mode of failure
- Test at least five valid specimens per direction or per joint type, and repeat any specimen that slips in the jaws or breaks at the jaw line
Data Evaluation and Acceptance Criteria
The individual breaking forces are averaged and the coefficient of variation is checked to confirm consistency of the woven construction. Acceptance criteria are defined by the governing product standard according to the sack type and nominal filling capacity; for example, GB/T 8946 assigns minimum breaking force requirements for fabric and seams according to the designated sack grade. In general, the seam or joint is expected to retain a specified proportion of the base fabric strength so that the joint does not become the primary weak point of the package. Results significantly below the specified minimum, or excessive scatter between specimens, indicate deficiencies in tape yarn quality, weaving density, or sewing parameters that must be corrected before release.
Applications
Tensile testing of plastic woven sacks is applied across the full life cycle of the product, from raw material development to incoming inspection at the buyer's warehouse:
- Factory quality control, with routine testing of each production batch or shift to support release decisions
- Packaging of cement, mortar, and other construction materials, where stacking loads and rough handling are severe
- Fertilizer and agrochemical packaging, which combines heavy loads with moisture-sensitive contents
- Food and feed packaging, including flour, rice, sugar, starch, and animal feed sacks
- Chemical, resin, and mineral packaging requiring verified joint integrity
- Buyer and importer verification programs, product certification, and customs compliance support
- Research and development of new tape yarn formulations, weave constructions, and laminate structures
Independent verification by ISO/IEC 17025 accredited third-party laboratories is commonly requested for export shipments, supplier audits, and dispute resolution, since laboratory reports traceable to recognized international standards are widely accepted by buyers and regulatory authorities.
FAQ
Which standards apply to plastic woven sack tensile testing?
The product-level requirements for plastic woven sacks are defined in GB/T 8946, while cement packaging bags are covered by GB/T 9774. The test methods themselves typically reference ISO 13934-1, ISO 13935-1, ASTM D5034, or ASTM D5035 for fabric and seam testing, and ISO 527 or GB/T 1040 for plastic film and laminate components. Japanese-market products may be evaluated with reference to JIS L 1096 methods.
What are the typical specimen dimensions?
Fabric strip specimens are commonly 50 mm wide with a gauge length of approximately 200 mm, cut separately in the warp and weft directions. Seam specimens are cut so the joint lies midway between the grips, with widths of 50 mm to 100 mm depending on the selected method.
Why are warp and weft directions tested separately?
The warp and weft tape yarns differ in draw ratio, linear density, and weaving density, so their breaking forces are usually unequal. Product standards set independent minimum requirements for each direction, and the lower of the two values effectively limits the safe working load of the sack.
What are the common causes of tensile test failures?
Frequent causes include low tenacity or insufficient draw ratio of the tape yarn, low fabric grammage or weave density, degraded or mismatched sewing thread, incorrect stitch density, and damage to the fabric during lamination or printing. Failures concentrated at the seams indicate workmanship issues, while failures distributed across the fabric indicate material or weaving deficiencies.
How frequently should tensile testing be performed?
Manufacturers typically test each production batch, and at minimum whenever the raw material, tape yarn specification, weaving setup, or sewing parameters change. Buyers and importers commonly commission periodic verification through accredited third-party laboratories, for example annually or on a per-contract basis.
Can tensile testing be combined with other evaluations?
Yes. Tensile testing is usually performed together with grammage determination, dimensional checks, drop impact tests, stacking or compression tests, and lamination peel testing to form a complete mechanical qualification package for the sack.