Environment Stress Cracking

Environment Stress Cracking

SUBJECT: confidential information rev. 0 Pag. 1 di 21

Environment Stress

Cracking

Federico GUIDI



INDEX

FIGURES 4

TABLES 4

1 ABSTRACT 5

2 SUBJECT 6

3 FAILURE TYPE 6

4 CRAZING AND CRACKING IN AIR 6

5 STRESS ANALYSIS IN SECONDARY COIL BOBBIN 9

5.1 CURRENT MATERIAL (NORYL GFN3 PPE + PS) 9 5.2 DATA SHEET OF NORYL GFN3 PPE + PS 10 5.3 FEA ANALYSIS 11 5.3.1 FEA MODEL 11 5.3.2 FEA DESCRIPTION OF CURRENT MATERIAL 12 5.3.3 FEA CONSTRAINT CONDITION @ LOW TEMPERATURE 12 5.3.4 FEA CONSTRAINT CONDITION @ HIGH TEMPERATURE 13 5.3.5 FEA LOAD CONDITION 13 5.3.6 FEA RESULTS @ LOW TEMPERATURE (-30°C) 14 5.3.7 FEA RESULTS @ HIGH TEMPERATURE (+100 °C) 16

6 MATERIAL SELECTION 16



6.1 POLYMER MOLECULAR WEIGHT 16 6.2 CRYSTALLINE VS. AMORPHOUS MATERIAL 18 6.3 TECHNICAL PROPOSAL 19 6.4 NEW MATERIAL (POCAN T 739I PBT) 19 6.5 DATA SHEET OF POCAN T 739I PBT VS. NORYL GFN3 PPE + PS 20

7 NEW ACTIVITIES 21



FIGURES

Figure 1: sample ............................................................................................................... 6

Figure 2: the sequence leading to slow crack growth ....................................................... 7

Figure 3: ESC result ......................................................................................................... 8

Figure 4: FEA model ...................................................................................................... 11

Figure 5: FEA description of current material................................................................ 12

Figure 6: FEA constraint condition @ low temperature ................................................ 12

Figure 7: FEA constraint condition @ high temperature ............................................... 13

Figure 8: FEA results @ low temperature - minimum CTE (30⋅10-6 1/°C) ................... 14

Figure 9: FEA results @ low temperature (fillet detail) – minimum CTE ..................... 15

Figure 10: FEA results @ low temperature (fillet detail) – maximum CTE .................. 15

Figure 11: FEA results @ high temperature - minimum CTE (30⋅10-6 1/°C) ................ 16

Figure 12: crystalline Vs. amorphous ............................................................................. 18

TABLES

Table 1: summary of NORYL GFN3 PPE + PS data sheet ........................................... 10

Table 2: thermal cycles Nr.1 (specs.) ............................................................................. 13

Table 3: thermal cycles Nr.2 (specs.) ............................................................................. 14

Table 4: significance of the molecular weight................................................................ 17

Table 5: summary of POCAN T 739I PBT Vs. NORYL GFN3 PPE + PS ................... 20

Table 6: thermal cycle (compressive load) ..................................................................... 21

Table 7: thermal cycle (tensile load) .............................................................................. 21



1 ABSTRACT

Environment Stress Cracking (ESC) is one of the main causes of failure of injection

moulded plastic components. Failure occurs as a result of the accelerated fracture of

polymeric materials due to the combined action of environmental exposure and stress.

A low temperature condition combined with a particular constraint condition produces a

critical tensile stress distribution that culminates in a slow crack growth: the material

used for moulded plastic component is not suitable for a creep condition.



2 SUBJECT

The subject of this document is the high tension coil made by “confidential

informations”: the analysis will investigate the material of high tension coil secondary

coil bobbin.

3 FAILURE TYPE

Thermal cycles produce a crack in high tension side of secondary coil bobbin: through

this crack occurs a spark that reduces high tension coil performances.

Figure 1: sample

4 CRAZING 1 AND CRACKING IN AIR

Modest levels of stress applied over long periods of time induce purely “mechanical

degradation” in the form of crazes and cracks. This is the underlying cause of the long-

term transition from ductile to brittle behaviour.

The sequence that culminates in slow crack growth is illustrated in Figure 2.

1 A network of fine cracks on or under the surface of a material.



Figure 2: the sequence leading to slow crack growth At the microscopic level, polymer surfaces unavoidably contain a high density of stress

concentrating defects. Typically these range in stress concentration factor from 1 to ~

50. Under modest tensile stresses (a few MPa), the yield strength (typically < 100 MPa)

will be locally exceeded. The micro-yielded zones blunt the defect and the feature is

initially stabilised. The size and shape of a yielded zone are determined by the yield

stress contour of the local stress field. However the yield strengths of all polymers

decline with time under stress, and the rate of decline is increased with increasing

temperature. Therefore with time under stress, the size of the micro-yielded zones

increases. Thin plate like zones grow in area in a plane normal to the principal tensile

stress direction. As they do so the zones increase in thickness. Eventually volume

expansion induces cavitation within the zone. The feature can now be described as a

craze.

With further growth of the zone and zone cavities, the inter-cavity material stretches to

form highly oriented, load-bearing fibrils. Voids and fibrils are typically of the order of

tens of nanometres in diameter. With further growth of the craze, a point is reached

when the most highly stretched fibrils (generally those nearest the polymer surface) will

rupture. This will lead to an increase in stress at the craze tip, a transient increase in

craze growth rate, and an increase in the extension of adjacent fibrils. The sequence is



repeated. The now unsupported portion of the craze is called a crack. The crack

advances behind a craze of reasonably constant length. The part fractures when the

crack reaches a critical length. On examination of fracture surfaces the slow crack

growth region (the initial stage up to the critical length) will be macroscopically flat and

microscopically 'pimpled'. The pimples are the remnants of ruptured fibrils. The fast

crack growth region will be macroscopically uneven (hackle marks, bifurcation etc.) but

microscopically smooth.

The progress of slow crack growth (and consequently the durability of the stressed part)

depends primarily upon the strength and stability of the craze material, and therefore

upon the ultimate properties of the bridging fibrils. Semi-crystalline plastics generally

form higher tenacity fibrils than amorphous plastics. Higher molecular weight plastics

are also superior in this respect. Therefore high molecular weight grades of semi-

crystalline thermoplastics generally offer the best resistance to slow crack growth and

such associated phenomena as ESC, dynamic fatigue fracture, and fretting wear.

Figure 3: ESC result

START POINT



5 STRESS ANALYSIS IN SECONDARY COIL BOBBIN

The key point in the sequence leading to a slow crack growth is a tensile load condition.

During thermal cycles the secondary coil bobbin it could be loaded by a tensile load?

In order to answer this question, it is necessary a Finite Element Analysis (FEA) of

secondary coil bobbin: the first step is a very simple FEA in which we do not consider

potting resin (the CTE ratio between NORYL and potting resin is ~ 1÷2.5)2 and copper.

Anyway, the presence of potting resin and copper does not introduce a tensile load but

only a compressive load if CTE ratio is higher than 1. Regarding the magnet inside the

secondary coil bobbin, we consider it a rigid solid (the CTE ratio between NORYL and

ferrite is ~ 10): this is one safety hypothesis.

5.1 CURRENT MATERIAL (NORYL GFN3 PPE + PS)3

General Class: PPE/PPO (Polyphenylene Ether Blends).

Company: GE Plastics.

Trade Name: NORYL GFN3 PPE + PS, Grade with 30% glass fibers.

Application: household appliances, electrical applications such as control housings,

fibre-optic connector, etc.

Note: PPO is a poly (2,6 dimethyl p-phenylene) oxide. The ether linkages offer easier

processibility. Copolymers are referred to as PPEs (Polyphenylene Ethers). Typically,

the commercially available PPOs (PPEs) are blended with other thermoplastic materials

such as PS (or HIPS), Nylon, etc. These blends are still referred to as PPOs or PPes. The

blends offer superior processibility compared to pure PPOs. Their viscosities are lower.

2 It depends from the direction that we consider.

3 “Dr.C-Mold user’s Guide Molding Intelligence for Plastics Professionals” C-Mold, Ithaca, New York,

USA, 1996.*



Blends with Nylons are crystalline. They offer improved chemical resistance and

perform well at high temperatures. The water absorption is low and the moulded

products have excellent dimensional stability.

Blends with PS are amorphous. The addition of glass fibres reduces shrinkage levels to

0.2%. These materials have excellent dielectric properties and a low coefficient of

thermal expansion. The viscosity level depends on the ratio of the components in the

blend: higher PPO levels increase the viscosity.

5.2 DATA SHEET OF NORYL GFN3 PPE + PS

Information provided by GE Plastics for their European product line. This product is

also a part of their North American product line.

PHYSICAL PROPERTIES METRIC COMMENTS

Density 1.3 g/cm3 ISO 1183

Tensile modulus (E) 8 GPa ISO 527

Poisson’s ratio 0.365 ASTM D 368

Ultimate tensile strength 100 MPa ISO 527

Elongation at break 1.5 % ISO 527

CTE, linear 20°C in flow direction 30⋅10-6 1/°C ISO 11359-2

CTE, linear 20°C transverse to flow 70⋅10-6 1/°C ISO 11359-2

Thermal conductivity 0.28 W/m°C ISO 8302

Dielectric strength 22 kV/mm Short time, 3.2 mm - ASTM D 149

Table 1: summary of NORYL GFN3 PPE + PS data sheet



5.3 FEA ANALYSIS

The FEA analysis has been performed with the software ProMechanica integrated with

the design software ProEngineer.

In the following paragraphs they will be described all elements necessary to perform

FEA analysis.

5.3.1 FEA MODEL

The FEA model is in agreement with “confidential informations”: the model has been

simplified where the details are not important for the analysis.

Figure 4: FEA model

The FEA model is referred to a cylindrical coordinate system (CS1) in which the “z”

direction corresponds to the axis of FEA model (positive direction is from low voltage

side to high voltage side). The plane z = 0 corresponds to the bottom plane of secondary

coil bobbin (low voltage side).



5.3.2 FEA DESCRIPTION OF CURRENT MATERIAL

The FEA analysis has been performed using the following material model.

Figure 5: FEA description of current material

5.3.3 FEA CONSTRAINT CONDITION @ LOW TEMPERATURE

At low temperature (under the reference temperature) it is a good hypothesis to suppose

that only the inner surface of secondary coil

bobbin will be constrained: as shown in the

picture (Fig.6), on the red surface they have

been applied the constraints in “r” direction and

on the blue surface they have been applied the

constraints in “z” direction. The constraint

condition is doubly symmetrical, like the load

condition, and therefore the FEA model can be

simplified: on the green surfaces they have been

applied the symmetric constraints (mirror).

Figure 6: FEA constraint condition @ low temperature

MINIMUM CTE



5.3.4 FEA CONSTRAINT CONDITION @ HIGH TEMPERATURE

At high temperature (over the reference temperature) it is a good hypothesis to suppose

a constraint distribution as shown in the picture (Fig.7): on the red surfaces they have

been applied constraints in “r” direction, on the blue surfaces they have been applied

constraints in “z” direction and on the magenta surfaces they have been applied

constraints in “r” and “z”. The constraint condition is doubly symmetrical, like the load

condition, and therefore the FEA model can be simplified: on the green surfaces they

have been applied the symmetric constraints (mirror).

Figure 7: FEA constraint condition @ high temperature

5.3.5 FEA LOAD CONDITION

LOW TEMPERATURE

(°C) TIME

HIGH TEMPERATURE

(°C) TIME

-30 3 hrs +80 3hrs

x 10 times (60 hrs)

Table 2: thermal cycles Nr.1 (specs.)



LOW TEMPERATURE

(°C) TIME

HIGH TEMPERATURE

(°C) TIME

-30 1 hr +100 1hr

x 200 times (400 hrs)

Table 3: thermal cycles Nr.2 (specs.)

The reference temperature for thermal properties of materials is 20°C.

5.3.6 FEA RESULTS @ LOW TEMPERATURE (-30°C)

Figure 8: FEA results @ low temperature - minimum CTE (30⋅10-6 1/°C)

The FEA results for the low temperature condition show that there is a zone around the

fillet in the high voltage side critical regarding the stress: the Von Mises stress

distribution points out that around the fillet there is a wide zone in which the SF is about

1.76 ÷ 3. This SF in not enough first of all because typically the stress concentration

factor ranges from 1 to ~ 50, beside this because the yield strengths of all polymers

TENSILE STRESS

ZONE

TENSILE

STRESS ZONE

TENSILE STRESS

ZONE



decline with time under stress and finally because the material used for the FEA

analysis has been described with the minimum CTE (30⋅10-6 1/°C).

Figure 9: FEA results @ low temperature (fillet detail) – minimum CTE

Figure 10: FEA results @ low temperature (fillet detail) – maximum CTE

MAXIMUM STRESS (56.8 MPa)

SF = 1.76

MAXIMUM STRESS (132.6 MPa)

SF = 0.75



5.3.7 FEA RESULTS @ HIGH TEMPERATURE (+100 °C)

Figure 11: FEA results @ high temperature - minimum CTE (30⋅10-6 1/°C)

The FEA results for the high temperature condition show that the zone around the fillet

in the high voltage side is in a condition of compressive stress. Regarding the sequence

that culminates in a slow crack growth, a compressive stress is a favourable condition

because it closes the crack. However if the compressive stress exceeds the compressive

yield stress, it could produce the micro-yielded zones from which in the next low

temperature cycle it could start the sequence that culminates in a slow crack growth.

6 MATERIAL SELECTION

6.1 POLYMER MOLECULAR WEIGHT

The molecular weight is expressed in grams/mole.

The most important useful properties of polymers arise primarily from the high

molecular weight and extended chain length of the polymer molecules:

COMPRESSIVE

STRESS ZONE

COMPRESSIVE

STRESS ZONE

COMPRESSIVE

STRESS ZONE



- improves impact resistance;

- lowers the brittleness temperature;

- increases melt viscosity;

- improves long-term performance

o fatigue;

o ESC resistance;

o chemical resistance.

Lower Molecular Weight Higher

Broader Molecular Weight Distribution Narrower

Higher Crystallinity Lower

Faster Rate of Loading Slower

Lower Temperature Higher

Higher Filler Content Lower

Lower Filler Aspect Ratio Higher

Higher Applied Load Lower

Higher Internal Stresses Lower

Abrupt Wall Thickness Transition Gradual

Concentrated Chemical Exposure Dilute

Brittle Ductile

Table 4: significance of the molecular weight Causes of low molecular weight:

- specification – viscosity too low in initial material;

- specification – blended materials;



- thermal degradation – high melt temperatures and/or long residence time;

- hydrolytic degradation – excess moisture in the raw material during processing

- drying – oxidation due to aggressive drying conditions;

- environmental influences – heat aging, UV degradation, chemical exposure.

6.2 CRYSTALLINE vs. AMORPHOUS MATERIAL

Figure 12: crystalline Vs. amorphous Performance differences:

- a semi-crystalline structure provides better chemical resistance;

- a semi-crystalline structure provides better ESC resistance (ESCR);

- a semi-crystalline structure provides better resistance to fatigue failure;

- a semi-crystalline structure benefits more from the incorporation of fillers;



- a semi-crystalline structure generally has lower impact performance;

- a semi-crystalline structure generally has a good dimensional accuracy and a

good dimensional stability but not quite for achieving the same level of

amorphous structure.

6.3 TECHNICAL PROPOSAL

In the previous paragraphs it has been shown that the best solution for the new material

will be a compromise between an amorphous structure and a semi-crystalline structure.

For the semi-crystalline structure it will be chosen a polyester like PBT and for the

amorphous structure it will be chosen a polycarbonate (PC): the new material will be a

PBT/PC blend. Polycarbonate has been chosen first of all because it shows a good creep

resistance, beside this it is fine for all precision parts and finally it has a temperature-

independent dielectric constant, as well as good insulating properties.

Regarding stress analysis in secondary coil bobbin, the FEA results point out that the

new material must have first of all a lower CTE, beside this a higher ultimate tensile

stress and finally a good creep behavior in order to have a lower yield strengths

reduction with time under stress and therefore to support a higher number of thermal

cycles (specs.).

6.4 NEW MATERIAL (POCAN T 739I PBT)

General Class: PBT + PC blend.

Company: LANXESS.

Trade Name: POCAN T 739I PBT, Grade with 45% glass fibers.

Application: automotive sector.

Note: very good surface finish, low warpage, very high strength and stiffness, good

creep behavior, low thermal expansion.



6.5 DATA SHEET OF POCAN T 739I PBT Vs. NORYL GFN3 PPE + PS

Information provided by Lanxess for POCAN T 739I PBT and GE Plastics for NORYL

GFN3 PPE + PS.

PHYSICAL PROPERTIES

METRIC

COMMENTS CURRENT MATERIAL

NEW MATERIAL

Density 1.3 g/cm3 1.67 g/cm3 ISO 1183

Tensile modulus (E) 8 GPa 17 GPa ISO 527

Poisson’s ratio 0.365 0.365 ASTM D 368

Tensile Creep modulus (E)

1hr n.a. 16.5 GPa n.a.

Tensile Creep modulus (E)

1000 hrs n.a. 15 GPa n.a.

Ultimate tensile strength 100 MPa 165 MPa ISO 527

Elongation at break 1.5 % 1.7 % ISO 527

CTE, linear 20°C

in flow direction 30⋅10-6 1/°C 20⋅10-6 1/°C ISO 11359-2

CTE, linear 20°C

transverse to flow 70⋅10-6 1/°C 50⋅10-6 1/°C ISO 11359-2

Thermal conductivity 0.28 W/m°C n.a. ISO 8302

Maximum service temperature, Air n.a. 210 °C n.a.

Dielectric strength 22 kV/mm 27 kV/mm Short time, 3.2 mm

ASTM D 149

Table 5: summary of POCAN T 739I PBT Vs. NORYL GFN3 PPE + PS



7 NEW ACTIVITIES

It has been verified, only with a FEA analysis, that thermal cycles are able to generate a

tensile load in high voltage side of secondary coil bobbin: the FEA analysis results point

out that it is the low temperature the responsible of the sequence that culminates in slow

crack growth. In order to verify this hypothesis, a very easy activity it could be to

perform two different thermal cycles:

LOW TEMPERATURE

(°C) TIME

HIGH TEMPERATURE

(°C) TIME

+20 3 hrs +100 3hrs

x 10 times (60 hrs)

Table 6: thermal cycle (compressive load)

LOW TEMPERATURE

(°C) TIME

HIGH TEMPERATURE

(°C) TIME

-30 3 hrs +20 3hrs

x 10 times (60 hrs)

Table 7: thermal cycle (tensile load) if the hypothesis is correct, the high tension coils that have been loaded by the first

thermal cycle (table 6) will not be damaged whereas the others (table 7) will be

damaged.

Environment Stress Cracking

Technology

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