Safety Factors - A363

 

Safety Factors
Foundational knowledge article
Document Type	Article
Document Identifier	363
Themes	Residual stress and dimensional control management Quality/inspection management
Tags	Mechanical properties Outcome parameters Part parameters Structural analysis Functional Requirements
Prerequisites	Composite properties

Introduction

This page explores the selection and application of safety factors, also known as strength ratios, in composite material design.

Background

The methodology for determining safety factors when designing with composites differs from that used for traditional materials like metals for a number of reasons. Composites are orthotropic, consist of different constituent materials, and has different stress and strain limits depending on the fibre orientation and direction of loading (see A144). Additionally, the failure modes of composites are different than other materials (see A361).

Various failure criteria have been developed that can be used to determine safety factors for composites. Criteria such as maximum stress, and maximum strain consider the applied stress and strain in only a single direction at a time, whereas criteria such as Hoffman or Tsai-Hill consider the interaction between the different stress components. Strength ratios or safety factors for composites can still be interpreted in the same way as safety factors for other material types. Namely, in the sense that a safety factor of 2, for example, indicates that the design can withstand twice the design load before failure.

Application

When determining suitable safety factors to apply in the design of a composite component, regulations and guidelines must be adhered to. If none exist, ultimately the engineering authority responsible for the design must use their professional judgment and experience. In many cases, a governing standard for the particular application will recommend minimum safety factors. Governing standards may provide factors to be applied based on individual design parameters, or they may provide a general minimum safety factor which must be met or exceeded in all situations. Parameters often considered include material type (glass versus carbon for example, or polyester versus epoxy), fabric type, fabrication method, and factors related to the operational environment. Also taken into account are uncertainties in the material properties and other unknowns.

Factors to Consider when Calculating Safety Factors

Material Properties

Uncertainty in the material properties must be identified, quantified, and considered. If the material properties are estimated or obtained from supplier datasheets, there is typically a lower level of certainty, requiring the selection of a higher safety factor. The safety factor that accounts for uncertainty in material properties may be lowered when physical testing of the materials is introduced. Care must be taken that the test specimens are fabricated using the same process and under the same conditions as the final product. Testing of a small sample size still leaves a level of uncertainty in the final results so safety factors should still be applied. If the material properties have been qualified through rigorous A or B-basis testing programs, the design values are calculated to meet a 99% or 90% probability with a 95% confidence level and a lower safety factor can be used. A-Basis refers to a strength value where only 1 out of 100 specimens is expected to fail, with a 95% confidence level. B-Basis represents a strength value where 10 out of 100 specimens are expected to fail, also with a 95% confidence level. As the number of tested specimens increases, a higher strength value can be used as a valid allowable . For more information on A/B testing, see A372.

Fabrication Methods

The part fabrication method should be considered when selecting a safety factor. A higher safety factor is recommended for laminates produced through less controllable processes, such as hand layup or spray-up. Lower safety factors are required for processes such as prepreg layup or infusion, as these are typically better controlled processes which produce less variability in final part thickness and material properties.

Additionally, curing/post-curing of parts should be considered when selecting safety factors. Post-curing generally improves final material properties, reducing uncertainty and variability, and allows for lower safety factors to be used than with non-post cured parts.

Environmental Factors

Environmental factors including temperature, moisture, ultraviolet light, and exposure to chemicals may affect the final performance of a composite part. If a part is expected to be used in hot, cold, or wet conditions, and testing was only performed at room temperature, then possible variation in the material properties should be accounted for when selecting a safety factor. Additionally, properties may degrade over time when exposed to ultraviolet light or certain types of chemicals. Once again, the selection of the safety factor should take this into consideration if other protection methods (e.g. the use of UV resistant gel coats or sacrificial plies) are not employed.

Fatigue Factors

When selecting safety factors for fatigue load cases, special considerations must be made. Factors such as fabric type and required fatigue life must be taken into account.

References that provide guidance for determining safety factors, such as the GL Guideline for Certification of Wind Turbines , specify different factors for fatigue loading based on the construction of the fibre mat. For example, for the determination of fatigue safety factors, unidirectional fabrics require a factor of 1.0 (no additional safety factor), whereas woven fabrics require a factor of 1.2. The higher safety factor accounts for an increase in crack initiation sites due to kinking of the fibre from the weaving process. This factor is not applied for static loading because any degradation due to fibre kinking should be accounted for in the material test properties.

The number of load cycles, N, a part must endure is specific to the given design or application. The number of load cycles specified by the application must be considered when selecting a safety factor. In general, a higher number of load cycles will require a higher safety factor. In addition, the repair or maintenance plan must be considered. For example, if the part is expected to be replaced at the end of the service life, a lower safety factor would be required than if the part is expected to require only minimal service or inspection. There are a number of models used for estimating fatigue life based on the applied loads. The GL Guideline for Certification of Wind Turbines provides one methodology for calculating fatigue safety factors based on the number of cycles. This relationship is based on a power law, \(\frac{N_1}{m}\), where \(N\) is the number of cycles and \(m\) is the slope of the stress-life curve.

Other Factors

Additional factors that should be considered when determining appropriate safety factors include the influence of ageing, loading type (static, fatigue, impact, etc.), and the severity of the end result if a failure occurs. This is especially true if human safety is potentially compromised by a failure.

Reference Standards

Many industries that use composites publish reference standards that provide guidelines, or in some cases requirements, for the selection of safety factors. The following list is not exhaustive but provides some applicable standards:

• GL Guideline for Certification of Wind Turbines
• DNV GL RU-HSLC (Rules for High speed and light craft)
  • Part 3 Structures, equipment – Chapter 4 Hull structural design, fibre composite and sandwich constructions (Section 6 Scantlings – 7. Design Rules)
• Reinforced Thermoset Plastic Corrosion Equipment (RTP-01)

Example

The Germanischer Lloyd (GL) Guideline for the Certification of Wind Turbines provides a useful methodology for determining safety factors to be used for the design of composites. Section 5.5 of the reference provides general background and a procedure for determining short-term and fatigue strength allowables for both fibreglass and carbon fibre reinforced components. The overall safety factor \(\lambda_{M_x}\) for short-term strength (\(x=a\)) or fatigue strength (\(x=b\)) is calculated using a base partial safety factor \(\lambda_{M_0}=1.35\), multiplied by a number of reduction factors, \(C_{ix}\).

\(\lambda_{M_x} = \lambda_{M_0}\times\prod_{i}C_{ix}\)

This example assumes a hand layup glass fibre component that is not post-cured. Factors are selected based on Section 5.5.2.4 of the standard. This includes a factor of 1.35 for the influence of ageing, a factor of 1.1 for temperature effects, a factor of 1.2 for hand layup fabrication, and a factor of 1.1 for a non-post-cured part. Multiplying these factors together, and including the partial safety factor of 1.35, results in a static safety factor, \(\lambda_{M_a}\) equal to 2.65. The table below summarizes the selected factors.

Fibreglass hand layup, not post-cured	\(\lambda_{M_0}\)	1.35	\(\lambda_{M_a}\) = 2.65
	\(C_{1a}\)	1.35
	\(C_{2a}\)	1.1
	\(C_{3a}\)	1.2
	\(C_{4a}\)	1.1

Conclusion

This document provides information and factors to consider when approaching the selection of safety factors for designing with composites.

References

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Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

The individual materials that combine to form the composite material. The constituent materials are separate and distinct on a macroscopic level.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Carbon fibres are composed of large aromatic sheets similar to those in graphite. These graphitic layers form the basic structural units in the shape of ribbons. The structure of carbon fibre ribbon is believed to be a columnar arrangement of disoriented graphite crystallites parallel to the ribbon length. The idealized tetragonal crystallites are stacked above one another, with slight disorientation between the crystals in the direction of fibre axis, trapping sharp needle like voids, where the boundaries between the stacks represent the disordered regions.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability