How to Measure Tensile Strength - Professional Methods Guide

Measurement Methodology

Tensile strength measurement requires applying controlled uniaxial force to a standardized specimen until failure occurs. The maximum force divided by original cross-sectional area determines ultimate tensile strength (UTS).

Equipment Requirements

Universal Testing Machine Specifications

Parameter	Requirement	Tolerance	Standard Reference
Load accuracy	Class 1	±1% of reading	ISO 7500-1
Load resolution	1/1000 full scale	-	ASTM E4
Crosshead speed	0.1-500 mm/min	±1% set value	ISO 6892-1
Position accuracy	±0.01mm	-	ASTM E83

Load Cell Configuration

Capacity Selection

• Required capacity = Expected UTS × Cross-sectional area × 2.5 safety factor
• Load range utilization: 10-90% of full scale capacity
• Resolution: Minimum 1/50,000 of applied load

ITM-LAB Load Cell Specifications

Capacity	Accuracy Class	Creep (30 min)	Temperature Effect	Calibration Interval
50kN	0.5	<0.02%	<0.01%/°C	12 months
100kN	0.5	<0.02%	<0.01%/°C	12 months
300kN	1.0	<0.03%	<0.02%/°C	12 months

Specimen Preparation Methods

Metallic Materials

Machining Requirements (ASTM E8)

• Surface finish: Ra ≤ 1.6 μm
• Dimensional tolerance: ±0.1mm
• Gauge length: 4 × diameter (round) or 50mm (rectangular)
• Parallel section: Minimum 60mm length

Specimen Dimensions

Type	Diameter/Width	Thickness	Gauge Length	Parallel Length
Round	12.5mm ±0.1mm	-	50mm	60mm
Rectangular	12.5mm ±0.1mm	3-20mm	50mm	57mm
Sub-size	6.25mm ±0.05mm	-	25mm	32mm

Plastic Materials

Injection Molding Parameters (ASTM D638)

• Mold temperature: Per material specification
• Injection pressure: 80-120 MPa typical
• Cooling time: Minimum 30 seconds
• Gate location: End tab area only

Type I Specimen Dimensions

• Overall length: 165mm ±0.5mm
• Gauge length: 50mm ±0.1mm
• Width (narrow): 13mm ±0.5mm
• Width (grip): 19mm ±0.5mm
• Thickness: 3.2mm ±0.4mm

Composite Materials

Cutting Methods

• Water jet cutting: ±0.1mm tolerance
• Diamond blade sawing: Coolant required
• Ultrasonic cutting: For thin laminates
• CNC milling: Carbide tooling

Tab Installation (ASTM D3039)

• Tab material: Glass/epoxy, 1.5mm thickness
• Adhesive: Room temperature cure epoxy
• Tab angle: 0° or 90° to loading direction
• Overlap length: 25mm minimum

Test Procedure Implementation

Environmental Controls

Standard Conditions (ISO 291)

• Temperature: 23°C ±2°C
• Relative humidity: 50% ±5%
• Pressure: 86-106 kPa
• Air velocity: <0.2 m/s around specimen

Conditioning Requirements

Material	Conditioning Time	Temperature	Humidity
Metals	None required	Ambient	Ambient
Plastics	40 hours minimum	23°C ±2°C	50% ±5% RH
Composites	48 hours minimum	23°C ±2°C	50% ±5% RH

Measurement Setup

Dimensional Measurement

• Width measurement: 3 locations minimum
• Thickness measurement: 3 locations minimum
• Measurement tool: Micrometer, 0.01mm resolution
• Cross-sectional area: Average of measurements

Grip Installation

1. Center specimen in grips
2. Apply hand-tight pressure
3. Additional 1/4 turn maximum
4. Verify specimen alignment
5. Check for grip marks (none visible)

Test Execution Parameters

Loading Rates (ASTM E8)

Material Type	Stress Rate (MPa/s)	Strain Rate (s⁻¹)	Crosshead Speed
Steel	1.15-11.5	-	1-10 mm/min
Aluminum	1.15-11.5	-	1-10 mm/min
Copper	1.15-11.5	-	1-10 mm/min
High strength	2.3-23	-	2-20 mm/min

Data Acquisition Settings

• Sampling rate: 10-50 Hz
• Force resolution: 1N minimum
• Position resolution: 0.01mm minimum
• Time stamping: Synchronized force-displacement

Calculation Methods

Ultimate Tensile Strength

Formula

UTS = Fmax / A0

Where:
UTS = Ultimate tensile strength (MPa)
Fmax = Maximum force during test (N)  
A0 = Original cross-sectional area (mm²)

Example Calculation

• Maximum force: 25,000 N
• Original diameter: 12.5mm
• Cross-sectional area: π × (12.5/2)² = 122.7 mm²
• UTS = 25,000 N / 122.7 mm² = 204 MPa

Measurement Uncertainty

Uncertainty Components

Source	Type	Value	Distribution
Force measurement	B	±0.5%	Rectangular
Diameter measurement	B	±0.01mm	Rectangular
Cross-sectional calculation	B	±0.1%	Rectangular
Temperature effect	B	±0.1%	Rectangular

Combined Uncertainty

• Standard uncertainty: uc = 1.2% typical
• Expanded uncertainty: U = 2.4% (k=2, 95% confidence)

Material-Specific Procedures

Steel Testing (ASTM A370)

Specimen Requirements

• Product form: As-received condition
• Heat treatment: Per specification
• Machining: Remove 1mm minimum from surfaces
• Identification: Permanent marking in grip area

Test Parameters

• Speed: 1-10 mm/min to yield, then increase allowable
• Temperature: 23°C ±3°C
• Measurement: Load-displacement or load-strain
• Rounding: To nearest 5 MPa

Aluminum Testing (ASTM B557)

Special Considerations

• Work hardening: Minimize during machining
• Surface condition: Remove oxidation layer
• Grip pressure: Prevent indentation
• Speed control: Constant throughout test

Plastic Testing (ASTM D638)

Test Speed Selection

Material Category	Test Speed	Speed Tolerance
Rigid plastics	5 mm/min	±25%
Semi-rigid plastics	5-50 mm/min	±25%
Flexible plastics	500 mm/min	±25%

Data Analysis

• Stress calculation: Force/original area throughout test
• Strain calculation: Extension/gauge length
• Modulus: Slope between 0.05% and 0.25% strain

Quality Control Procedures

Statistical Analysis

Sample Size Requirements

• Production testing: 5 specimens minimum
• Material qualification: 10 specimens minimum
• Research applications: Statistical power analysis
• Special studies: Per test plan requirements

Acceptance Criteria

Property	Coefficient of Variation	Action if Exceeded
UTS (metals)	<5%	Investigate test procedure
UTS (plastics)	<10%	Check conditioning
UTS (composites)	<8%	Verify fiber orientation

Outlier Detection

Statistical Tests

• Grubbs test: Single outlier detection
• Dixon test: Multiple outlier detection
• Box plot analysis: Visual outlier identification
• Z-score analysis: >3 standard deviations

Documentation Requirements

Test Report Contents

• Material identification and condition
• Specimen dimensions and area calculations
• Test parameters and environmental conditions
• Force-displacement data or stress-strain curves
• Calculated properties with uncertainty
• Statistical analysis results

Equipment Maintenance

Daily Calibration Checks

Load Verification

1. Apply reference load (50% capacity)
2. Record indicated value
3. Calculate error: [(Indicated-Reference)/Reference] × 100%
4. Acceptance: ±0.5% maximum error

Alignment Verification

1. Install dummy specimen
2. Apply 10% maximum load
3. Check load distribution on both grips
4. Tolerance: ±2% load difference

Weekly Maintenance

Component	Task	Duration	Acceptance Criteria
Crosshead	Lubrication and movement	15 min	Smooth operation
Grips	Cleaning and inspection	20 min	No wear or damage
Load frame	Alignment check	10 min	<0.1mm deviation
Software	Update and backup	15 min	Current version

Annual Calibration

Calibration Standards

• Dead weight calibrator: 0.02% accuracy
• Proving ring: 0.05% accuracy
• Reference load cell: 0.1% accuracy
• Traceability: NIST or equivalent national standard

Calibration Points

• Minimum 5 points: 10%, 25%, 50%, 75%, 100% capacity
• Loading sequence: Increasing then decreasing
• Acceptance: ±0.5% maximum error at each point
• Certificate validity: 12 months maximum

Troubleshooting Common Issues

Grip Slippage

Symptoms

• Sudden load drop during test
• Specimen movement in grips
• Marks outside gauge length

Solutions

• Increase grip pressure (check material limits)
• Use serrated grip faces
• Apply grip aids (rosin, emery cloth)
• Verify specimen surface condition

Premature Failure

Causes and Solutions

Cause	Symptom	Solution
Stress concentration	Failure at radius	Improve specimen geometry
Surface defects	Failure at scratch	Better surface preparation
Misalignment	Failure at angle	Check crosshead alignment
Excessive speed	Brittle failure	Reduce test speed

Load Cell Drift

Monitoring

• Zero reading variation: ±0.1% FS maximum
• Temperature coefficient: <0.02%/°C
• Long-term stability: <0.05% per year

Corrective Actions

• Temperature stabilization: 2 hours minimum
• Electrical connections: Clean and tighten
• Environmental isolation: Reduce vibration/EMI
• Recalibration: If drift exceeds limits

ITM-LAB Technical Support

Installation Services

Site Preparation Checklist

• Foundation: Reinforced concrete, 500mm depth
• Vibration isolation: <5 μm displacement
• Environmental: 23°C ±5°C, <70% RH
• Power quality: <3% THD, ±5% voltage

Commissioning Process

1. Mechanical installation and leveling
2. Electrical connections and verification
3. Software installation and configuration
4. Performance verification per ISO 7500-1
5. Operator training (16 hours standard)
6. Documentation delivery

Service Support

Response Times

Support Level	Phone Support	On-site Response	Parts Delivery
Standard	4 hours	72 hours	5 business days
Enhanced	2 hours	48 hours	3 business days
Premium	1 hour	24 hours	Next business day

Remote Capabilities

• System diagnostics and monitoring
• Parameter adjustment assistance
• Software updates and patches
• Training session delivery
• Preventive maintenance scheduling

Quality Assurance

Manufacturing Standards

• ISO 9001:2015 quality management
• Load frame stiffness: >200 kN/mm
• Machine compliance: <0.1mm at rated load
• Load cell linearity: ±0.1% full scale

Performance Verification

• Load accuracy: ±0.3% maximum error
• Speed accuracy: ±1% set value
• Position accuracy: ±0.01mm
• Repeatability: <0.1% coefficient of variation

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